Design of Reinforced Concrete Structures

INTRODUCTION TO
1
REINFORCED CONCRETE
1.1 INTRODUCTION TABLE 1.1 Annual consumption of major structural materials in the world
Material Unit Weight Million Tonnes/Person
The word ‘concrete’ comes from the Latin word concretus (kg/m3) Tonnes
(meaning compact or condensed), the perfect passive Structural steel 7850 1244 0.18
participle of concrescere, from con- (together) and crescere
Cement 1440 3400 0.48
(to grow). This name was chosen perhaps due to the fact that
this material grows together, due to the process of hydration, Concrete 2400 ∼18,000 2.4 (990 litres)
from a visco-elastic, moldable liquid into a hard, rigid, solid Timber 700 277 0.04
+
rock-like substance. The Romans first invented what is today Drinking water 1000 5132 0.73 (730 litres)
called as hydraulic cement-based concrete or simply concrete. Notes: The estimated world population as of August 2012 is 7.031 billion.
They built numerous concrete structures, including the 43.3 m + Assumed as two litres/day/person
Source: US Geological Survey, International Iron and Steel Institute, NIST, US
diameter concrete dome, the Pantheon, in Rome, which Department of Agriculture
is now over 2000 years old but still in use and remains the
world’s largest non-reinforced concrete dome (see case study Concrete technology has advanced considerably since its
in Chapter 2 for more details about the Pantheon). discovery by the Romans. Now, concrete is truly an engineered
Concrete is used in nearly every type of construction. material, with a number of ingredients, which include a host
Traditionally, concrete is primarily composed of cement, of mineral and chemical admixtures. These ingredients should
water, and aggregates (aggregates include both coarse and be precisely determined, properly mixed, carefully placed,
fine aggregates). Although aggregates make up the bulk of the vibrated (not required in self-compacting concretes), and
mix, it is the hardened cement paste that binds the aggregates properly cured so that the desired properties are obtained; they
together and contributes to the strength of concrete, with the should also be inspected at regular intervals and maintained
aggregates serving largely as low-cost fillers (though their adequately until their intended life. Even the cement
strength also is important). currently being used has undergone a number of changes.
Concrete is not a homogeneous material, and its strength A variety of concretes are also being used, some tailored
and structural properties may vary greatly depending upon its for their intended use and many with improved properties.
ingredients and its method of manufacture. However, concrete Few specialized concretes have compressive strength and
is normally treated in design as a homogeneous material. ductility matching that of steel. Even though this is a book
Steel reinforcements are often included to increase the tensile on RC design, it is important for the designers to know about
strength of concrete; such concrete is called reinforced cement the nature and properties of the materials they are going to
concrete (RCC) or simply reinforced concrete (RC). specify for the structures designed by them. As concrete
As of 2006, about 7.5 billion cubic metres of concrete technology has grown in parallel with concrete design, it is
were produced each year—this equals about one cubic metre impossible to describe all the ingredients, their chemistry,
per year for every person each on the earth (see Table 1.1). the different kinds of concretes, and their properties in this
The National Ready Mixed Concrete Association (NRMCA) chapter. Hence, only a brief introduction is given about them,
estimates that ready-mixed concrete production in 2005 was and the interested reader should consult a book on concrete
about 349 million cubic metres in the USA alone, which is technology (many references are given at the end) for more
estimated to have about 6000 ready-mixed concrete plants. details.
2 Design of Reinforced Concrete Structures
1.1.1 Brief History bricklayer and mason in Leeds, England, who, in 1824, patented
Many researchers believe that the first use of a truly the first ‘Portland’ cement, so named since it resembled the
cementitious binding agent (as opposed to the ordinary lime stone quarried on the Isle of Portland off the British coast (Reed,
commonly used in ancient mortars) occurred in southern et al. 2008). Aspdin was the first to use high temperatures to
Italy around the second century BC. Volcanic ash (called heat alumina and silica materials, so that cement was formed.
pozzuolana, found near Pozzouli, by the Bay of Naples) It is interesting to note that cement is still made in this way.
was a key ingredient in the Roman cement used during the I.K. Brunel was the first to use Portland cement in an
days of the Roman empire. Roman concrete bears little engineering application in 1828; it was used to fill a breach
resemblance to modern Portland cement concrete. It was in the Thames Tunnel. During 1959–67, Portland cement was
never put into a mould or formwork in a plastic state and made used in the construction of the London sewer system.
to harden, as is being done today. Instead, Roman concrete The small rowboats built by Jean-Louis Lambot in the early
was constructed in layers by packing mortar by hand in and 1850s are cited as the first successful use of reinforcements
around stones of various sizes. The Pantheon, constructed in in concrete. During 1850–1880, a French builder, Francois
AD 126, is one of the structural marvels of all times (Shaeffer Coignet, built several large houses of concrete in England and
1992). France (Reed, et al. 2008). Joseph Monier of France, who is
During the Middle Ages, the use of concrete declined, considered to be the first builder of RC, built RC reservoirs in
although isolated instances of its use have been documented 1872. In 1861, Monier published a small book, Das System
and some examples have survived. Concrete was more Monier, in which he presented the applications of RC. During
extensively used again during the Renaissance (14th–17th 1871–75, William E. Ward built the first landmark building in
centuries) in structures such as bridge piers. Pozzolanic RC in Port Chester, NY, USA. In 1892, François Hennebique,
materials were added to the lime, as done by the Romans, to of France, patented a system of steel-reinforced beams,
increase its hydraulic properties (Reed, et al. 2008). slabs, and columns, which was used in the construction of
In the eighteenth century, with the advent of new technical various structures built in England between 1897 and 1919.
innovations, a greater interest was shown in concrete. In 1756, In Hennebique’s system, steel reinforcement was placed
John Smeaton, a British Engineer, rediscovered hydraulic correctly in the tension zone of the concrete; this was backed
cement through repeated testing of mortar in both fresh and by a theoretical understanding of the tensile and compressive
salt water. Smeaton’s work was followed by Joseph Aspdin, a forces, which was developed by Cottançin in France in 1892
(Reed, et al. 2008).
CASE STUDY
The Ingalls Building
The Ingalls Building, built in 1903 in Cincinnati, Ohio, is the
world’s first RC skyscraper. This 15-storey building was designed
by the Cincinnati architectural firm Elzner & Anderson and
engineer Henry N. Hooper. Prior to 1902, the tallest RC structure
in the world was only six storeys high. Since concrete possesses
very low tensile strength, many at that time believed that a concrete
tower as tall as the Ingalls Building would collapse under wind
loads or even its own weight. When the building was completed
and the supports removed, one reporter allegedly stayed awake
through the night in order to be the first to report on the building’s
failure.
Hooper designed a monolithic concrete box of 200 mm walls,
with the floors, roof, beams, columns, and stairs all made of
concrete. Columns measured 760 mm by 860 mm for the first 10
floors and 300 mm2 for the rest. It was completed in eight months,
and the finished building measured 15 m by 30 m at its base and
was 64 m tall.
Still in use today, the building was designated a National
Historic Civil Engineering Landmark in 1974 by the American
The 15-storey Ingalls Building in Cincinnati, Ohio
Society of Civil Engineers; in 1975, it was added to the American
(Source: http://en.wikipedia.org/wiki/Ingalls_Building)
National Register of Historic Places.
Introduction to Reinforced Concrete 3
Earnest L. Ransome patented a reinforcing system using Availability of materials The materials required for concrete
twisted rods in 1884; he also built the first RC framed building (sand, gravel, and water) are often locally available and are
in Pennsylvania, USA, in 1903. In 1889, the first concrete relatively inexpensive. Only small amounts of cement (about
reinforced bridge was built. The Ingalls building, which is 14 per cent by weight) and reinforcing steel (about 2–4 per cent
the first concrete skyscraper, was built in 1904 using the by volume) are required for the production of RC, which may
Ransome system and is still in use. have to be shipped from other parts of the country. Moreover,
By the 1900s, concrete was generally used in conjunction reinforcing steel can be transported to most construction sites
with some form of reinforcement, and steel began to more easily than structural steel sections. Hence, RC is the
replace wrought iron as the predominant tensile material. material of choice in remote areas.
A significant advance in the development of RC was the
Low maintenance Concrete members require less mainte-
pre-stressing of steel reinforcing, which was developed by
nance compared to structural steel or timber members.
Eugène Freyssinet, in the 1920s, but the technique was not
widely used until the 1940s. Victoria in Montreal, constructed Water and fire resistance RC offers great resistance to the
in 1964, with a height of 190 m and utilizing 41 MPa concrete actions of fire and water. A concrete member having sufficient
in the columns, paved way for high-strength concretes (HSCs) cover can have one to three hours of fire resistance rating
(Shaeffer 1992). without any special fire proofing material. It has to be noted
During the 1980s, Prof. Mörsch and Bach of the University that steel and wood need to be fireproofed to obtain similar
of Stuttgart conducted a large number of tests to study the rating—steel members are often enclosed by concrete for fire
behaviour of RC elements. Prof. Mörsch’s work can be resistance. If constructed and cured properly, concrete surfaces
considered to be the starting point of modern theory of RC could provide better resistance to water than steel sections,
design. Thaddeus Hyatt, an American, was probably the first which require expensive corrosion-resistant coatings.
to correctly analyse the stresses in an RC beam and in 1877 Good rigidity RC members are very rigid. Due to the
published a small book. In 1895, A. Considére of France greater stiffness and mass, vibrations are seldom a problem in
tested RC beams and columns and in 1897 published the concrete structures.
book Experimental Researches on Reinforced Concrete.
Several early studies of RC members were based on ultimate Compressive strength Concrete has considerable compres-
strength theories, for example, flexure theory of Thullie in sive strength compared to most other materials.
1897 and the parabolic stress distribution theory of Ritter in Economical It is economical, especially for footings,
1899. However, the straight line (elastic) theory of Coignet basement walls, and slabs.
and Tedesco, developed in 1900, was accepted universally
because of its simplicity. The ultimate strength design was Low-skilled labour Comparatively lower grade of skilled
adopted as an alternative to the working stress method only in labour is required for the fabrication, erection, and construction
1956–57. Ecole des Ponts et Chaussees in France offered the of concrete structures than for steel or wooden structures.
first teaching course in RC design in 1897. The first British In order to use concrete efficiently, the designer should
code was published in 1906 and the first US code in 1916. The also know the weakness of the material. The disadvantages of
first Indian code was published in 1953 and revised during concrete include the following:
1957, 1964, 1978, and 2000. Low tensile strength Concrete has a very low tensile
strength, which is about one-tenth of its compressive strength
1.1.2 Advantages and Disadvantages of Concrete and, hence, cracks when subjected to tensile stresses.
Reinforced concrete has been used in a variety of applications, Reinforcements are, therefore, often provided in the tension
such as buildings, bridges, roads and pavements, dams, zones to carry tensile forces and to limit crack widths. If
retaining walls, tunnels, arches, domes, shells, tanks, pipes, proper care is not taken in design and detailing and also during
chimneys, cooling towers, and poles, because of the following construction, wide cracks may occur, which will subsequently
advantages: lead to the corrosion of reinforcement bars (which are also
Moulded to any shape It can be poured and moulded termed as rebars in the USA) and even failure of structures.
into any shape varying from simple slabs, beams, columns Requires forms and shoring Cast in situ concrete construction
to any complicated shells and domes, by using formwork. involves the following three stages of construction, which are
Thus, it allows the designer to combine the architectural and not required in steel or wooden structures: (a) Construction of
structural functions. This also gives freedom to the designer formwork over which concrete will be poured—the formwork
to select any size or shape, unlike steel sections where the holds the concrete in place until it hardens sufficiently,
designer is constrained by the standard manufactured member (b) removal of these forms, and (c) propping or shoring of
sizes. new concrete members until they gain sufficient strength to
support themselves. Each of these stages involves labour and to about 1450°C by burning fossil fuels, and it accounts for
material and will add to the total cost of the structure. The about 5–7% of CO2 emissions globally. Production of one ton
formwork may be expensive and may be in the range of one- of cement results in the emission of approximately one ton of
third the total cost of RC structure. Hence, it is important for CO2. Hence, the designer should specify cements containing
the designer to make efforts to reduce the formwork cost, by cementitious and waste materials such as fly ash and slags,
reusing or reducing formwork. wherever possible. Use of fly ash and other such materials not
only reduces CO2 emissions but also results in economy as
Relatively low strength Concrete has relatively low strength
well as improvement of properties such as reduction in heat
per unit weight or volume. (The compressive strength of
of hydration, enhancement of strength and/or workability,
normal concrete is about 5–10 per cent steel, and its unit
and durability of concrete (Neville 2012; Subramanian 2007;
density is about 31 per cent steel; see Table 1.2.) Hence,
Subramanian 2012).
larger members may be required compared to structural steel.
This aspect may be important for tall buildings or long-span
structures. 1.2 CONCRETE-MAKING MATERIALS
TABLE 1.2 Physical properties of major structural materials
As mentioned already, the present-day concrete is made up
of cement, coarse and fine aggregates, water, and a host of
Item Mild Steel Concretea Wood
M20 Grade
mineral and chemical admixtures. When mixed with water,
the cement becomes adhesive and capable of bonding the
Unit mass, 7850 (100) c 2400 (31)c 290–900
kg/m3 (4–11)c aggregates into a hard mass, called concrete. These ingredients
are briefly discussed in the following sections.
Maximum
stress in MPa
Compression 250 (100) 20 (8) 5.2–23b (2–9) 1.2.1 Cement (Portland Cement and Other Cements)
Tension 250 (100) 3.13 (1) 2.5–13.8 (1–5) The use of naturally occurring limestone will result in
Shear 144 (100) 2.8 (1.9) 0.6–2.6 (0.4–1.8) natural cement (hydraulic lime), whereas carefully controlled
Young’s 2 × 105 (100) 22,360 (11) 4600–18,000 computerized mixing of components can be used to make
modulus, MPa (2–9) manufactured cements (Portland cement). Portland cements
Coefficient of 12 10–14 4.5 are also referred to as hydraulic cements, as they not only
linear thermal harden by reacting with water but also form a water-resistant
expansion, product. The raw materials used for the manufacture of
°C × 10−6
cement consist of limestone, chalk, seashells, shale, clay,
Poisson’s ratio 0.3 0.2 0.2 slate, silica sand, alumina and iron ore; lime (calcium) and
Notes: silica constitute about 85 per cent of the mass.
a
Characteristic compressive strength of 150 mm cubes at 28 days
b
Parallel to grain
The process of manufacture of cement consists of grinding
c
The values in brackets are relative value as compared to steel. the raw materials finely, mixing them thoroughly in certain
proportions, and then heating them to about 1480°C in huge
Time-dependent volume changes Concrete undergoes
cylindrical steel rotary kilns 3.7–10 m in diameter and 50–
drying shrinkage and, if restrained, will result in cracking or
150 m long and lined with special firebrick. (The rotary kilns
deflection. Moreover, deflections will tend to increase with
are inclined from the horizontal by about 3° and rotate on
time due to creep of the concrete under sustained compressive
its longitudinal axis at a slow and constant speed of about
stress (the deflection may possibly double, especially in
1–4 revolutions/minute.) The heated materials sinter and
cantilevers). It has to be noted that both concrete and steel
partially fuse to form nodular shaped and marble- to fist-sized
undergo approximately the same amount of thermal expansion
material called clinker. (It has to be noted that at a temperature
or contraction; see Table 1.2.
range of 600–900°C, calcination takes place, which results
Variable properties The properties of concrete may widely in the release of environmentally harmful CO2). The clinker
vary due to the variation in its proportioning, mixing, pacing, is cooled (the strength properties of cement are considerably
and curing. Since cast in situ concrete is site-controlled, its influenced by the cooling rate of clinker) and ground into fine
quality may not be uniform when compared to materials such powder after mixing with 3–5% gypsum (gypsum is added
as structural steel and laminated wood, which are produced in to regulate the setting time of the concrete) to form Portland
the factory. cement. (In modern plants, the heated air from the coolers is
returned to the kilns, to save fuel and to increase the burning
CO2 emission Cement, commonly composed of calcium
efficiency). It is then loaded into bulk carriers or packaged
silicates, is produced by heating limestone and other ingredients
into bags; in India, typically 50 kg bags are used.
Storing in cement silos

10
and packaging
9 Grinding of cement
8
Blending with gypsum
Kiln flame
7 Cooling and storing of clinker

6 Production of clinker in rotary kiln
5 Pre-calcinator
Electro static precipitator
4 Pre-heating in cyclones by
hot gases from the kiln and
pre-calcinator
3 Pre-homogenization
and grinding in raw mill
2 Crushing of
raw materials
1 Quarrying raw materials

such as clay, shale and
limestone
FIG. 1.1 Dry process of cement manufacture (a) Schematic representation (b) View of MCL Cement plant, Thangskai, Meghalaya
Sources: www.cement.org/basics/images/flashtour.html and http://en.wikipedia.org/wiki/File:Cement_Plant_MCL.jpg (adapted)
Two different processes, known as dry and wet, are used in TABLE 1.3 Types of Portland cements
the manufacture of Portland cement, depending on whether India/UK USA Typical Compounds3
the mixing and grinding of raw materials are done in wet (ASTM)
or dry conditions. In addition, semi-dry process is also OPC (IS 269, Type I1 C3S 55%, C2S 19%, C3A 10%,
sometimes employed in which the raw materials are ground IS 8112 and C4AF 7%, MgO 2.8%, SO3 2.9%,
dry, mixed with water, and then burnt in the kilns. Most of the IS 12269) Ignition loss 1.0%, and free CaO1.0%
(C3A < 15%)
modern cement factories use either dry or semi-dry process.
The schematic representation of the dry process of cement Type II1 C3S 51%, C2S 24%, C3A 6%, C4AF
11%, MgO 2.9%, SO3 2.5%, Ignition
manufacture is shown in Fig. 1.1.
loss 1.0%, and free CaO 1.0%
Portland Cement (C3A < 8%)
Portland cement (often referred to as ordinary Portland cement Rapid Type III1 C3S 57%, C2S 19%, C3A 10%,
hardening C4AF 7%, MgO 3.0%, SO3 3.1%,
or OPC) is the most common type of cement in general use
Portland Ignition loss 0.9%, and free CaO
around the world. The different types of cements covered by cement (IS 1.3%
the Indian and US standards and their chemical compounds are 8041:1990) Its seven day compressive strength is
shown in Table 1.3. Cement production in India consists mainly almost equal to types I and II 28 day
of the following three types (see Fig. 1.2): OPC ∼39 per cent, compressive strengths.
Portland pozzolana cement (PPC) ∼52 per cent, and Portland Low heat Type IV C3S 28%, C2S 49%, C3A 4%, C4AF
slag cement (PSC) ∼8 per cent. All other varieties put together Portland 12%, MgO 1.8%, SO3 1.9%, Ignition
comprise only 1 per cent of the total production (Mullick 2007). cement (IS loss 0.9%, and free CaO 0.8% (C3A <
12600: 7% and C3S < 35%)
Others, 1per cent PSC, 8 per cent 1989)
PPC, 52
Sulphate Type V C3S 38%, C2S 43%, C3A 4%, C4AF 9%,
per cent
resisting MgO 1.9%, SO3 1.8%, Ignition loss
Portland 0.9%, and free CaO 0.8% [C3A < 5%
cement (IS and (C4AF) + 2(C3A) < 25%]
OPC, 39 12330:
per cent 1988)
FIG. 1.2 Production trend of different varieties of cement in India (Continued)
Source: Mullick 2007
TABLE 1.3 (Continued) 1. 33 grade OPC, IS 269:1989

India/UK USA Typical Compounds 3 2. 43 grade OPC, IS 8112:1989
(ASTM) 3. 53 grade OPC, IS 12269:1987
PSC (IS Type IS Made by grinding granulated high-
455:1989, IS quality slag with Portland cement clinker
The number in the grade indicates the compressive strength of
12089:1987) the cement in N/mm2 at 28 days. The 33 grade cement is suitable
PPC [IS Type IP A blended cement made by inter-
for producing concrete up to M25. Both 43 grade and 53 grade
1489- grinding Portland cement and pozzolanic cement are suitable for producing higher grades of concrete. The
Part 1:1991 materials without burning important physical properties of the three grades of OPC and
(fly ash other types of cements are compared in Table 1.4. The chemical
based), IS composition of OPC is given in Table 1.5 and Fig. 1.3.
1489-Part
2:1991 TABLE 1.5 Chemical composition of OPC (Bogue’s Compounds)
(calcined (Moir 2003)
clay based)]
S. No. Compound Cement Typical Mineral
Ternary Type A blended cement made by inter- Chemist Composition Phase
blended IT(SX) grinding Portland cement, slag, and Notation as %
cement (PY)2 pozzolana without burning (CCN)*
Notes: 1. Tricalcium silicate C3S 45–65 Alite

1. Types Ia, IIa, and IIIa have the same composition as types I, II, and III, but 3(CaO)· \SiO2
have an air-entraining agent ground into the mix.
2. Dicalcium silicate C2S 15–30 Belite
2. The letters X and Y stands for the percentage of supplementary cementitious
2(CaO)·SiO2
material (SCM) included in the blended cement, and S and P are the types
of SCMs, where S is for slag and P for pozzolan. For example, Type IT(S25) 3. Tricalcium aluminate C3A 5–10 Aluminate
(P15) contains 25 per cent slag and 15 per cent pozzolans. 3(CaO)·Al2O3
3. See Table 1.6 for explanation of these compounds.
4. Tetracalcium C4AF 5–15 Ferrite
alumino ferrite
There are other types, such as high alumina cement (IS 4(CaO)·Al2O3·Fe2O3
6452:1989), super sulphated cement (IS 6909:1990), 5. Gypsum CaSO4·2 2–10
hydrophobic Portland cement (IS 8043: 1991), white cement H2O
(IS 8042:1989), concrete sleeper grade cement (IRS-T * Cement chemists use the following shorthand notation:
40:1985), expanding cements, and masonry cement (IS C = CaO, S = SiO2, A = Al2O3, F = Fe2O3, M = MgO,
3466:1988), which are used only in some special situations. H = H2O, N = Na2O, K = K2O, S = SO3.
(Refer Mehta and Monteiro (2006) and Shetty (2005) for the Approximately 95 per cent of cement particles are smaller
details of these cements.) Geopolymer cements are inorganic than 45 micrometres, with the average particle being around
hydraulic cements that are based on the polymerization of 15 micrometres. The overall particle size distribution of
minerals (see Section 4.4.7 of Chapter 4). cement is called fineness. Fineness affects the heat released
Ordinary Portland cement is the most important cement and the rate of hydration; greater fineness causes greater
and is often used, though the current trend is to use PPC (see early strength (especially during the first seven days) and
Fig. 1.2). Most of the discussions to follow in this chapter more rapid generation of heat. Soundness refers to the ability
pertain to this type of cement. The Bureau of Indian Standards of the cement paste to retain its volume after setting and is
(BIS) has classified OPC into the following three grades:
TABLE 1.4 Physical properties of various types of cements
S. No. Type of IS Code Fineness Setting Time in Soundness Compressive Strength in MPa
Cement m2/kg Minutes
(min.) Initial Final Le Chatelier Auto Clave, for 3 days 7 days 28 days
(min.) (max.) (max.) (mm) MgO, (max.) (%)
1. OPC 33 269:1989 225 30 600 10 0.8 16 22 33
2. OPC 43 8112:1989 225 30 600 10 0.8 23 33 43
3. OPC 53 12269:1987 225 30 600 10 0.8 27 37 53
4. PPC (fly 1489:1991 300 30 600 10 0.8 16 22 33
ash based) (Part 1)
5. PSC (slag) 455:2002 225 30 600 10 0.8 16 22 33
6. SRC 12330:1988 225 30 600 10 0.8 10 16 33
CaCO3 (Limestone) TABLE 1.6 Role of different compounds on properties of cement

2SiO2 • Al2O3 (Clay, Shale) Characteristic Different Compounds in Cement
Fe2O3 (Iron oxide) C3S C 2S C3A C4AF

Setting Quick Slow Rapid –
SiO2 (Silica sand)
Hydration Rapid Slow Rapid –
Heat Higher Lower Higher Higher
liberation
∼ 1450°C (Cal/g) 7 days
Early strength High up to Low up to Not much Insignificant
Kiln
14 days 14 days beyond 1
3CaO • SiO2 day
CaO • SO3 • 2H2O Later strength Moderate High at – –
2CaO • SiO2
Gypsum + Clinker at later later stage
3CaO • Al2O3 stage after 14
days
4CaO • Al2O3 • Fe2O3
Interground
Finished
a range of materials both natural and artificial. [A pozzolan may
cement
be defined as a siliceous or siliceous and aluminous material,
FIG. 1.3 Chemical compounds of cement
which in itself possesses little or no cementitious value. However,
related to the presence of excessive amounts of free lime or in finely divided form and in the presence of water, it reacts
magnesia in the cement. Consistency indicates the degree of chemically with calcium hydroxide released by the hydration
density or stiffness of cement. Initial setting of cement is that of Portland cement, at ordinary temperature, to form calcium
stage when the paste starts to lose its plasticity. Final setting silicate hydrate and other cementitious compounds possessing
is the stage when the paste completely loses its plasticity and cementitious properties (Mehta 1987)]. Fly ash, ground
attains sufficient strength and hardness. The specific gravity granulated blast furnace slag (GGBS), silica fume, and natural
of Portland cement is approximately 3.15. pozzolans, such as calcined shale, calcined clay or metakaolin,
As seen in Table 1.5 and Fig.1.3, there are four major are used in conjunction with Portland cement to improve the
compounds in cement and these are known as tricalcium silicate properties of the hardened concrete. The latest amendment (No.
(C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A), 3) to IS 1489 requires that PPC be manufactured by the inter-
and tetracalcium alumino ferrite (C4AF). Their composition grinding of OPC clinker with 15–35 per cent of pozzolanic
varies from cement to cement and plant to plant. (The levels material. The generally used pozzolanic materials in India are fly
of the four clinker minerals can be estimated using a method ash (IS 1489-Part 1) or calcined clay (IS 1489-Part 1). Mixtures
of calculation first proposed by Bogue in 1929 or by the X-ray using three cementitious materials, called ternary mixtures, are
diffraction analysis, which gives the exactly measurement.) In becoming common, but no Indian specification regarding this has
addition, to these compounds, there are other minor compounds been developed yet. UltraTech PPC, Suraksha, Jaypee Cement
such as MgO, Na2O, K2O, SO3, fluorine, chloride, and trace (PPC) are some of the brand names of PPC in India. Until now,
metals, which are present in small quantities (Moir 2003). Of in India, PPC is considered equivalent to 33 grade OPC.
these K2O and Na2O are called as alkalis and are found to react PPC offers the following advantages:
with some aggregates, resulting in alkali–silica reaction (ASR),
1. Economical than OPC as the costly clinker is replaced by
which causes disintegration of concrete at a later date.
cheaper pozzolanic material
The silicates C3S and C2S are the most important compounds
2. Converts soluble calcium hydroxide into insoluble cementitious
and are mainly responsible for the strength of the cement paste.
products, thus improving permeability and durability
They constitute the bulk of the composition. C3A and C4AF do
3. Consumes calcium hydroxide and does not produce as
not contribute much to the strength, but in the manufacturing
much calcium hydroxide as OPC
process they facilitate the combination of lime and silica and
4. Improves the pore size distribution and reduces the micro-
act as a flux. The role of the different compounds on different
cracks at the transition zone due to the finer particles than OPC
properties of cement is shown in Table 1.6.
5. Reduces heat of hydration and thermal cracking
Portland Pozzolana Cement 6. Has high degree of cohesion and workability in concrete
As mentioned already, the Romans and Greeks were aware that and mortar
the addition of volcanic ash results in better performance of The main disadvantage is that the rate of development of
concrete. The name pozzolan is now frequently used to describe strength is initially slightly slower than OPC. In addition, its
effect of reducing the alkalinity may reduce the resistance Storage of Cement
to corrosion of steel reinforcement. However, as PPC Cement is very finely ground and readily absorbs moisture;
significantly improves the permeability, the risk of corrosion hence, care should be taken to ensure that the cement bags are
is reduced. The setting time is slightly longer. not in contact with moisture. They should be stored in airtight
Portland Slag Cement and watertight sheds and used in such a way that the bags
that come in first are the first to go out. Cement stored for a
Blast furnace slag is a non-metallic product consisting
long time tends to lose its strength (loss of strength ranges
essentially of silicates and alumino-silicates of calcium
from 5–10 per cent in three months to 30–40 per cent in
developed in a molten condition simultaneously with iron
one year). It is better to use the cement within 90 days of its
in a blast furnace. GGBS is obtained by rapidly cooling the
production. In case it is used at a later date, it should be tested
molten slag, which is at a temperature of about 1500°C, by
before use.
quenching in water or air to form a glassy sand-like granulated
material. Every year about nine million tons of blast furnace Tests on Cement
slag is produced in India. The GGBS should conform to IS The usual tests carried out for cement are for chemical and
I2089:1987. PSC is obtained either by intimate inter-grinding physical requirements. They are given in IS 4031 (different
of a mixture of Portland cement clinker and granulated slag parts) and IS 4032. Most of these tests are conducted at a
with the addition of gypsum or calcium sulphate or by an laboratory (Neville 2012).
intimate and uniform blending of Portland cement and finely Fineness is measured by the Blaine air permeability test,
ground granulated slag. Amendment No. 4 of IS 455 requires which indirectly measures the surface area of the cement
that the slag constituent not be less than 25 per cent or more particles per unit mass (m2/kg), or by actual sieving (IS
than 70 per cent of the PSC. It has to be noted that PSC has 4031-Part 1:1996 and Part 2:1999). Most cement standards have
physical properties similar to those of OPC. a minimum limit on fineness (in the range 225–500 m2/kg).
The following are some advantages of PSC: Soundness of cement is determined by Le-Chatelier and
1. Utilization of slag cement in concrete not only lessens the autoclave tests, as per IS 4031-Part 3:1988. Consistency is
burden on landfills; it also conserves a virgin manufactured measured by Vicat apparatus, as per IS 4031-Part 4:1988. The
product (OPC) and decreases the embodied energy paste is said to be of standard consistency when the penetration of
required to produce the cementitious materials in concrete. plunger, attached to the Vicat apparatus, is 33–35 mm. The initial
Embodied energy can be reduced by 390–886 million and final setting times of cement are measured using the Vicat
Joules with 50 per cent slag cement substitution. This is a apparatus with different penetrating attachments, as per IS 4031-
30–48 per cent reduction in the embodied energy per cubic Part 5:1988. It has to be noted that the setting time decreases
metre of concrete (http://www.slagcement.org). with increase in temperature; the setting time of cement can be
2. By using a 50 per cent slag cement substitution less CO2 is increased by adding some admixtures. The compressive strength
emitted (amounting to about 98 to 222 kg per cubic meter of cement is the most important of all the properties. It is found
of concrete, a 42–46 per cent reduction in greenhouse gas using a cement–sand mortar (ratio of cement to sand is 1:3) cube
emissions) (http://www.slagcement.org). of size 70.6 mm, as per IS 4031-Part 6:1988. The compressive
3. Using slag cement to replace a portion of Portland cement strength is taken as the average of strengths of three cubes. The
in a concrete mixture is a useful method to make concrete heat of hydration is tested in accordance with IS 4031-Part 9:1988
better and more consistent. PSC has a lighter colour, using vacuum flask methods or by conduction calorimetry.
better concrete workability, easier finishability, higher A web-based computer software called Virtual Cement
compressive and flexural strength, lower permeability, and Concrete Testing Laboratory (eVCCTL) has been
improved resistance to aggressive chemicals, and more developed by scientists at the National Institute of Standards
plastic and hardened consistency. and Technology (NIST), USA, which can be used to explore
4. The lighter colour of slag cement concrete also helps the properties of cement paste and concrete materials. This
reduce the heat island effect in large metropolitan areas. software may be found at http://www.nist.gov/el/building_
5. It has low heat of hydration and is relatively better materials/evcctl.cfm.
resistant to soils and water containing excessive amounts
of sulphates and hence used for marine works, retaining 1.2.2 Aggregates
walls, and foundations. The fine and coarse aggregates occupy about 60–75 per cent
of the concrete volume (70–85 per cent by mass) and hence
Both PPC and PSC will give more strength than OPC at the strongly influence the properties of fresh as well as hardened
end of 12 months. UltraTech Premium, Super Steel (Madras concrete, its mixture proportions, and the economy. Aggregates
Cement), and S 53 (L&T) are some of the brand names of used in concrete should comply with the requirement of IS
PSC available in India.
383:1970. Aggregates are commonly classified into fine and S. No. Factors Influence on Concrete Property
coarse aggregates. Fine aggregates generally consist of natural 6. Gradation or particle size Water demand (strength),
sand or crushed stone with particle size smaller than about 5 mm distribution cohesion, bleeding, and
(materials passing through 4.75 mm IS sieve). Coarse aggregates segregation
consist of one or a combination of gravels or crushed stone 7. Maximum size of aggregate Strength and water demand
with particle size larger than 5 mm (usually between 10 mm 8. Presence of deleterious Water demand (strength),
and 40 mm). Aggregates can also be classified in two more materials such as dust, clay, cohesion, bond, and durability
ways. Depending on the source, they could be either naturally silt, or mud
occurring (gravel, pebbles, sand, etc.)
or synthetically manufactured (bloated Sieve Rounded Irrugular Agg.
Crushed
clay aggregates, sintered fly ash size size
aggregate, etc.). Moreover, depending
on the bulk density, aggregates can
either be normal weight (1520–1680
80
kg/m3), lightweight (less than 1220 mm
kg/m3), or heavyweight (more than
2000 kg/m3). The normal weight 40
aggregates—sand, gravel, crushed mm
rock (e.g., granite, basalt, and sand
stone), and blast furnace slag— 40
are used to produce normal weight mm
concrete with a density of 2200–2400 20
mm
kg/m3. Aggregates such as expanded
shale, clay, slate, slag, pumice, perlite, 20
10 mm
vermiculite, and diatomite are used
mm
to produce structural lightweight
concrete (SLWC) with density ranging 4.8 10
3 3 mm mm
from about 1350 kg/m to 1850 kg/m .
Heavy weight aggregates consists of
hematite, steel, or iron and are used in Fine
special applications such as providing Sieve
radiation shielding and abrasion 2.4 1.2 0.6 0.3 0.15 2.4 1.2 0.6 0.3 0.15
size
resistance (ACI 301M:10 2010, ACI FIG. 1.4 Different shapes and sizes of aggregates
Committee E-701 2007).
Source: Ambuja technical booklet 125:2007
The factors of aggregates that
may directly or indirectly influence the properties of concrete TABLE 1.8 Properties of aggregates
are given in Table 1.7 (Ambuja technical booklets 5:1996, Aggregate Property Aggregate Property
125:2007). Only normal weight aggregates are discussed here
Specific Gravity Minimum
and should confirm to IS 383:1970. The coarse aggregates Voids (%)
form the main matrix of the concrete and hence provide Gravel 2.67 River sand
strength to the concrete, whereas the fine aggregates form the
Granite 2.80 Fine 43
filler matrix and hence reduce the porosity of concrete. Some
Sand 2.65 Coarse 35
properties of aggregates are shown in Table 1.8.
Basalt 2.85 Mixed and moist 38
TABLE 1.7 Factors of aggregates that may affect properties of concrete Bottom ash 1.57 Mixed and dry 30
S. no. Factors Influence on Concrete Property Bulk density (kg/l)
1. Specific gravity/Porosity Strength/Absorption of water Broken granite 1.68 Broken stone,
2. Crushing strength Strength graded
3. Chemical stability Durability Broken stone 1.60 Maximum 46
4. Surface texture Bond grip size: 25 mm
5. Shape (see Fig. 1.4) Water demand (strength) Stone screening 1.45 Maximum 45
size: 50 mm
(Continued)
TABLE 1.8 (Continued) workability, pumpability, shrinkage, and other properties of

Aggregate Property Aggregate Property concrete (Kosmatka, et al. 2011). In general, aggregates that
Specific Gravity Minimum do not have a large deficiency or excess of any size and give
Voids (%) a smooth grading curve will produce the most satisfactory
Beach or river 1.60 Maximum 41 results (Kosmatka, et al. 2011). Coarse and fine aggregates
Shingle size: 63 mm should be batched separately.
River sand Stone 48
TABLE 1.9 Grading requirements for fine aggregates
screening
IS Sieve Percentage Passing by Weight for ASTM
Fine 1.44 Fineness
Designation Grading Zone Standard
Modulus C 33
I II III IV
Medium 1.52 Sand 2.70
10 mm 100 100 100 100 100
Coarse 1.60 Bottom ash 2.08
4.75 mm 90–100 90–100 90–100 95–100 95–100
In several countries including India, natural course aggregates 2.36 mm 60–95 75–100 85–100 95–100 80–100
and river sand are scarce; at the same time, the waste from 1.18 mm 30–70 55–90 75–100 90–100 50–85
the demolition of buildings is escalating. The amount of 600 µm 15–34 35–59 60–79 80–100 25–60
construction waste in India alone is estimated to be around 300 µm 5–20 8–30 12–40 15–50 5–30
12–14.7 million tons per annum (Rao, et al. 2011). In such
150 µm 0–10 0–10 0–10 0–15 0–10
places, recycled coarse aggregates (RCA) could be used
profitably. More details about RCA and their use in concrete The fineness modulus (FM) of either fine or coarse aggregate
may be found in the works of Dhir and Paine (2010), Rao, is calculated by adding the cumulative percentages by
et al. (2011), and Subramanian (2012). In general, the mass retained on each of the series of sieves and dividing
mechanical properties such as compressive strength, split the sum by 100. The higher the FM, the coarser will be the
and tensile strengths, and modulus of elasticity are reduced aggregate. The maximum size of coarse aggregate should not
with increasing percentage of RCA. It is suggested that 25 per be greater than the following: one-fourth of the maximum
cent of RCA may be used in concrete, as it will not affect the size of member, 5 mm less than the maximum clear distance
properties significantly (Rao, et al. 2011). Other substitutes for between the main bars, or 5 mm less than the minimum cover
coarse aggregate include incinerator bottom ash aggregate and of the reinforcement. For RCC works, 20 mm aggregates
sintered fly ash pellets. Recycled glass aggregates, bottom ash are preferred. In thin concrete members with closely spaced
from thermal power plants, and quarry dust have significant reinforcement or small cover and in HSC, Clause 5.3.3 of IS
potential for use as fine aggregates in concrete (Dhir and Paine 456 allows the use of 10 mm nominal maximum size. Rounded
2010; Mullick 2012). Clause 5.3.1 of IS 456 stipulates that aggregates are preferable to angular or flaky aggregates, as
such aggregates should not contain more than 0.5 per cent of they require minimum cement paste for bond and demand less
sulphates as SO3 and should not absorb more than 10 per cent water. Flaky and elongated aggregates are also susceptible to
of their own mass of water. Before using these materials, it segregation and low strength.
is better to study their effect on the properties of concrete. It should be noted that the amount of water added to make
For example, manufactured sand, often referred as M-sand, concrete must be adjusted for the moisture conditions of the
from crushed gravel or rock is cubical in shape and results in aggregates to accurately meet the water requirement of the mix
increased water demand of the concrete mix. design. Various testing methods for aggregates to concrete are
Aggregates must be clean, hard, strong, and durable; they described in IS 2386-Parts 1 to 8:1963.
should be free of coatings of clay, absorbed chemicals, and
other fine materials that could affect the hydration and bond 1.2.3 Water
of the cement paste. Aggregates are usually washed to remove Water plays an important role in the workability, strength, and
impurities and graded at the site or plant. Grading or particle durability of concrete. Too much water reduces the concrete
size distribution of aggregates is a major factor determining strength, whereas too little will make the concrete unworkable.
the workability, segregation, bleeding, placing, and finishing The water used for mixing and curing should be clean and free
characteristics of concrete. The grading of fine aggregates from injurious amounts of oils, acids, alkalis, salts, sugars, or
has been found to influence the properties of green (fresh) organic materials, which may affect the concrete or steel. As per
concrete more than those of coarse aggregates. The grading Clause 5.4 of IS 456, potable water is considered satisfactory
requirements recommended by the Indian and US standards for mixing as well as curing concrete; otherwise, the water to
for fine aggregates is given in Table 1.9. Combined gradation be used should be tested as per IS 3025-Parts 1 to 32 (1984 to
of fine and coarse aggregate may result in better control of
1988). In general, sea water should not be used for mixing or during mixing in order to improve the properties of concrete.
curing concrete. The permissible limits for impurities as per They should comply with the requirements of IS 9103:1999.
Clause 5.4 of IS 456 are given in Table 1.10. The pH value of Admixtures are used for several purposes, such as to increase
water used for mixing should be greater than six. flowability or pumpability of fresh concrete, obtain high
strength through lowering of w/c ratio, retard or accelerate time
TABLE 1.10 Permissible limits for impurities in mixing water
of initial setting, increase freeze–thaw resistance, and inhibit
Impurity Maximum Permissible Limit corrosion (Krishnamurthy 1997). Normal admixture dosage
IS 456 (mg/l) ASTM C 94 is about 2–5 per cent by mass of cement. The effectiveness
(ppm)
of an admixture depends upon factors such as type, brand,
Organic 200 –
and amount of cementing materials; water content; aggregate
Inorganic 3000 – shape, gradation, and proportions; mixing time; slump; and
Sulphates 400 3000 temperature of the concrete (Kosmatka, et al. 2011).
(such as SO3) The common types of admixtures are as follows (Kosmatka,
Chlorides 2000 (for plain concrete 10001 et al. 2011; Krishnamurthy 1997; IS 9103: 1999):
(such as Cl) work)
500 (for RCC) 1. Accelerators enhance the rate of hydration of the concrete
Suspended matter 2000 50,000 and, hence, result in higher early strength of concrete and
early removal of formwork. Typical materials used are
Alkalis – 600
(such as Na2O + calcium chloride, triethenolamine, sodium thiocyanate,
0.658K2O) calcium formate, calcium nitrite, and calcium thiosulphate.
Note: Typical commercial products are Mc-Schnell OC and
1
Prestressed concrete or concrete in bridge decks 500 ppm (ppm and mg/l are Mc-Schnell SDS. Typical dosage is 2–3% by weight of
approximately equal) cement. As the use of chlorides causes corrosion in steel
In general, the amount of water required to be added for reinforcing, they are not used now.
cement hydration is very less compared to that required for 2. Retarders slow down the initial rate of hydration of cement
workability. For complete hydration of Portland cement, only and are used more frequently than accelerators. They are
about 36 per cent water (this is represented by the water/ often combined with other types of admixtures such as
cement or water/cementitious ratio, usually denoted by w/c water reducers. Typical retarders are sugars, hydroxides of
ratio or w/cm ratio), that is, w/c of 0.36, is needed. If a w/c zinc and lead, calcium, and tartaric acid. Typical dosage
ratio greater than about 0.36 is used, the excess water, which is is 0.05 per cent to 0.10 per cent by weight of cement.
not required for cement hydration, will remain in the capillary Commonly used retarders are lignosulphonic acids and
pores or may evaporate in due course. This process leads to hydroxylated carboxylic acids, which act as water-reducing
drying shrinkage (drying shrinkage is destructive as it leads and water-retarding admixtures; they delay the initial
to micro-cracking and may eventually weaken concrete). setting time by three to four hours when used at normal
Similarly, when a w/c ratio of less than about 0.36 is used, ambient temperatures.
some cement will remain unhydrated. The space initially 3. Water-reducing admixtures are used to reduce the
taken up by water in a cementitious mixture will be partially quantity of mixing water required to produce concrete.
or completely replaced over time by the hydration products. Water-reducing admixtures are available as ordinary
If a w/c ratio of more than 0.36 is used, then porosity in the water-reducing admixtures (WRA) and high-range
hardened material will remain, even after complete hydration. water-reducing admixtures (HRWRA). WRA enable
This is called capillary porosity and will lead to corrosion of up to 15 per cent water reduction, whereas HRWRA
reinforcement. enable up to 30 per cent. Popularly, the former are called
plasticizers and the latter superplasticizers. In modern
1.2.4 Admixtures day concreting, the distinction seems to be disappearing.
It is interesting to note that the Romans were the first to Compounds used in India as superplasticizers include
use admixtures in concrete in the form of blood, milk, and sulphonated naphthalene formaldehyde condensates
lard (pig fat). Present-day admixtures may be classified as (SNF), sulphonated melamine formaldehyde condensates
chemical and mineral admixtures. (SMF), and modified lignosulphonates (MLS). Some new
generation superplasticizers include acrylic polymer based
Chemical Admixtures (AP) superplasticizers, copolymers of carboxylic acid with
Chemical admixtures are materials in the form of powder or acrylic ether (CAE), polycarboxylate ethers (PCs), and
fluids that are added to the concrete immediately before or multi-polycarboxylate ethers (MCEs). The naphthalene
and melamine types of superplasticizers or HRWRA are 1. Fly ash is a by-product of coal-fired thermal power
typically used in the range 0.7–2.5 per cent by weight of plants. In India, more than 120 million tons of fly ash is
cement and give water reductions of 16–30 per cent. The produced every year, the disposal of which poses a serious
PCs are more powerful and are used at 0.3–1.0 per cent environmental problem. Any coal-based thermal power
by weight of cement to typically give 20 per cent to over station may produce four kinds of ash: fly ash, bottom ash,
40 per cent water reduction. Use of superplasticizers with pond ash, and mound ash. The quality of fly ash to be used
reduced water content and w/c ratio can produce concretes in concrete is governed by IS 3812 (Parts 1 and 2):2003,
with (a) high workability (in fresh concretes), with increased which groups all these types of ash as pulverized fuel ash
slump, allowing them to be placed more easily, with less (PFA). PFA is available in two grades: Grade I and grade
consolidating effort, (b) high compressive strengths, (c) II (Class F—siliceous fly ash and Class C—calcareous
increased early strength gain, (d) reduced chloride ion fly ash, as per ASTM). Both these grades can be used as
penetration, and (e) high durability. It has to be noted that it is admixtures. Up to 35 per cent replacement of cement by fly
important to consider the compatibility of superplasticizers ash is permitted by the Indian codes. Fly ash is extracted
with certain cements (Jayasree, et al. 2011; Mullick 2008). from flue gases through electrostatic precipitator in dry
4. Air entraining admixtures are used to entrain tiny air bubbles form. It is a fine material and possesses good pozzolanic
in the concrete, which will reduce damage during freeze– properties. The properties of fly ash depend on the type
thaw cycles, thereby increasing the concrete’s durability. of coal burnt. The lower the loss on ignition (LOI), the
Furthermore, the workability of fresh concrete is improved better will be the fly ash. The fineness of individual fly ash
significantly, and segregation and bleeding are reduced or particles rage from 1 micron to 1 mm in size. The specific
eliminated. However, entrained air entails a trade off with gravity of fly ash varies over a wide range of 1.9 to 2.55. For
strength, as each 1 per cent of air may result in 5 per cent a majority of site-mixed concrete, fly ash-based blended
decrease in compressive strength. The materials used in such cement is the best option. Fly ash particles are generally
admixtures include salts of wood resins, some synthetic spherical in shape and reduce the water requirement for a
detergents, salts of petroleum acids, fatty and resinous acids given slump. The use of fly ash will also result in reduced
and their salts, and salts of sulphonated hydrocarbons. heat of hydration, bleeding, and drying shrinkage.
5. Corrosion inhibitors are used to minimize the corrosion of 2. Ground granulated blast furnace slag is a by-product
steel and steel bars in concrete. of steel production and has been used as a cementitious
material since the eighteenth century. It is currently inter-
The other chemical admixtures include foaming agents (to
ground with Portland cement to form blended cement,
produce lightweight foamed concrete with low density),
thus partially replacing Portland cement. It reduces the
alkali–aggregate reactivity inhibitors, bonding admixtures
temperature in mass concrete, permeability, and expansion
(to increase bond strength), colouring admixtures, shrinkage
due to alkali–aggregate reaction and improves sulphate
reducers, and pumping aids. It is important to test all chemical
resistance. See Section 1.2.1 for more details on PSC.
admixtures adequately for their desired performance. It is also
3. Silica fume is also referred to as micosilica or condensed
desirable to prepare trial mixes of concrete with chemical
silica fume. It is a by-product of the production of silicon
admixtures and test their performance before using them in
and ferrosilicon alloys. Silica fume used in concrete should
any large construction activity (see also Clause 5.5 of IS 456).
conform to IS 15388:2003; as per Clause 5.2.1.2 of IS 456,
They should not be used in excess of the prescribed dosages,
its proportion is 5–10 per cent of cement content of a mix.
as they may be detrimental to the concrete.
Silica fume is similar to fly ash, with spherical shape, but
Mineral Admixtures has an average particle size of about 0.1 micron, that is, it
Mineral admixtures are inorganic materials that also have is 100 times smaller than an average cement particle. This
pozzolanic properties. These very fine-grained materials results in a higher surface to volume ratio and a much faster
are added to the concrete mix to improve the properties pozzolanic reaction. Silica fume addition benefits concrete
of concrete (mineral admixtures) or as a replacement for in two ways: (a) The minute particles physically decrease
Portland cement (blended cements). Pozzolanic materials the void space in the cement matrix—this phenomenon is
react with the calcium hydroxide (lime) released during the known as packing. (b) Silica fume is an extremely reactive
hydration process of cement to form additional C-S-H gel. pozzolan; it increases the compressive strength and improves
This can reduce the size of the pores of crystalline hydration the durability of concrete. Silica fume for use in concrete
products, make the microstructure of concrete more uniform, is available in wet or dry form. It is usually added during
and improve the impermeability and durability of concrete. concrete production at a concrete plant. However, it generally
These improvements can lead to an increase in strength and requires the use of superplasticizers for workability.
service life of concrete. Some of the mineral admixtures are 4. Rice husk ash (RHA) is produced by burning rice husk
briefly described here: in controlled temperature, without causing environmental
pollution. (India produces about 125 million tons of 5. Fulfils durability requirements to resist the environment in
paddy and 30 million tons of rice husk.) It exhibits high which the structure is expected to serve
pozzolanic characteristics and its use in concrete results
in high strength and impermeability. Water demand and Changes in Procedure for Mix Proportioning in IS 10262:2009
drying shrinkage should be studied before using rice husk. As per Clause 9.1.1 of IS 456, the minimum grade of concrete
5. High-reactivity Metakaolin (HRM) is obtained by calci- to be used in an RCC should not be less than M20. Moreover,
nation of pure or refined kaolinitic clay at a temperature all concretes above M20 grade for RCC work must be design
between 650°C and 850°C followed by grinding to achieve mixes. Concrete grades above M60 fall under the category of
a fineness of 700–900 m2/kg. The strength and durability of HSC and, hence, should be proportioned using the guidelines
concrete produced with the use of HRM is similar to that given in specialist literature, such as ACI 211.4-93 and the
produced with silica fume. Whereas silica fume is usually work of Krishna Raju (2002) and de Larrard (1999).
dark grey or black in colour, HRM is usually bright white The 2009 version of the code does not contain the graph
in colour, making it the preferred choice for architectural of w/c ratio versus 28-day compressive strength. Now, the
concrete, where appearance is important. relationship between w/c ratio and the compressive strength
of concrete needs to be established for the materials actually
More details about mineral admixtures may be found in the
used or by using any other available relationship based on
work of Bapat (2012).
experiments. The maximum w/c ratio given in IS 456:2000
for various environmental conditions may be used as a
1.3 PROPORTIONING OF CONCRETE MIXES starting point. The water content per cubic metre of concrete
Concrete mix design is the process of proportioning in the earlier version of the standard was a constant value for
various ingredients such as cement, cementitious materials, various nominal maximum sizes of aggregates. However, in
aggregates, water, and admixtures optimally in order to the revised version, the maximum water content per cubic
produce a concrete at minimal cost and will have specified metre of concrete is suggested. Another major inclusion in
properties of workability and homogeneity in the green state the revised standard is the estimation of volume of coarse
and strength and durability in the hardened state (SP 23:1982). aggregate per unit volume of total aggregate for different
Earlier mix design procedures such as minimum voids zones of fine aggregate. As air content in normal (non-air
method, Fuller’s maximum density method, Talbot–Richart entrained) concrete will not affect the mix proportioning
method, and fineness modulus method are based on the significantly, it is not considered in the revised version; it is
principles of minimum voids and maximum density (Krishna also not considered in IS 456-2000.
Raju 2002). The modern mix design methods include the Road
Note No. 4 method, the ACI (American Concrete Institute) Data for Mix Proportioning
method, the USBR (United States Bureau of Reclamation) The following basic data is required for concrete mix
method, the Bolomeya model, the British mix design method, proportioning of a particular grade of concrete:
and the BIS method (Krishna Raju 2002; Nataraja and Reddy 1. Exposure condition of the structure under consideration
2007). All these methods are mostly based on empirical (see Table 3 of IS 456:2000 and Table 4.4 in Chapter 4 of
relations, charts, graphs, and tables developed through this book for guidance)
extensive experiments using locally available materials. 2. Grade designation—The minimum grade of concrete
Although the older BIS code (IS 10262:1982) differed from to be designed for the type of exposure condition under
the ACI method (ACI 211.1, 1991) in some aspects, the consideration (see Tables 3 and 5 of IS 456:2000 and
present the BIS code (IS 10262:2009) is in line with the Table 4.4 in Chapter 4 and Table 1.11 of this book for
ACI code method (Nataraja and Das 2010). In all these mix guidance)
proportioning methods, the ingredients are proportioned by 3. Type of cement (OPC, PPC, PSC, etc.)
weight per unit volume of concrete. 4. Minimum and maximum cement content (see Tables 3, 4,
The main objective of any concrete mix proportioning 5, and 6 of IS 456:2000 and Tables 4.4 and 4.5 in Chapter
method is to make a concrete that has the following features: 4 of this book for guidance)
1. Satisfies workability requirements in terms of slump for 5. Type of aggregate (basalt, granite, natural river sand,
easy placing and consolidating crushed stone sand, etc.)
2. Meets the strength requirements as measured by the 6. Maximum nominal size of aggregate to be used (40 mm,
compressive strength 20 mm, or 12.5 mm)
3. Can be mixed, transported, placed and compacted as 7. Maximum w/c ratio (see Tables 3 and 5 of IS 456:2000
efficiently as possible and Tables 4.4 and 4.5 in Chapter 4 of this book for
4. Will be economical to produce guidance)
8. Desired degree of workability (see Table 1.12, which is Standard deviation should be calculated for each grade of
based on Clause 7 of IS 456) concrete using at least 30 test strength of samples (taken from
9. Use of admixture, its type, and conditions of its use site), when a mix is used for the first time. In case sufficient
10. Maximum temperature of concrete at the time of placing test results are not available, the values of standard deviation
11. Method of transporting and placing as given in Table 1.13 may be assumed for proportioning the
12. Early age strength requirements, if required mix in the first instance. As soon as sufficient test results are
available, actual standard deviation shall be calculated and
TABLE 1.11 Grades of concrete
used to proportion the mix properly.
Group Grade Designation Specified Characteristic
28-day Compressive TABLE 1.13 Assumed standard deviation
Strength of 150 mm
S. no. Grade of Concrete Assumed Standard Deviation, N/mm2
cube, N/mm2
Ordinary M10–M20 10–20 1. M10
3.5
concrete 2. M15
Standard M25–M60 25–60 3. M20
concrete 4.0
4. M25
High-strength M65–M100 65–100 5. M30
concrete
6. M35
TABLE 1.12 Workability of concrete 7. M40
5.0
Placing Conditions Degree of Workability Slump, mm 8. M45
Mud mat, shallow Very low 0.70–0.80 9. M50
section, pavement (compacting factor) 10. M55
using pavers
Note: These values correspond to strict site control of storage of cement,
Mass concrete; Low 25–75 weigh batching of materials, controlled addition of water, and so on. The values
lightly reinforced given in this table should be increased by 1 N/mm2 when the above are not
slabs, beams, walls, practised.
columns; strip
footings Step 2 Select the w/c ratio. The concrete made today has
Heavily reinforced Medium 50–100 more than four basic ingredients. We now use both chemical
slabs, beams, walls, and mineral admixtures to obtain concretes with improved
columns properties both in fresh and hardened states. Even the qualities
Slip formwork, Medium 75–100 of both coarse and fine aggregates in terms of grading, shape,
pumped concrete size, and texture have improved due to the improvement in
In situ piling, trench High 100–150 crushing technologies. As all these variables will play a
fill role, concretes produced with the same w/c ratio may have
Tremie concrete Very high 150–200 different compressive strengths. Therefore, for a given set
(flow test as per of materials, it is preferable to establish the relationship
IS 9103:1999) between the compressive strength and free w/c ratio. If such
Note: Internal (needle) vibrators are suitable for most of the placing conditions. a relationship is not available, maximum w/c ratio for various
The diameter of the needle should be determined based on the density and environmental exposure conditions as given in Table 5 of IS
spacing of reinforcements and the thickness of sections. Vibrators are not
required for tremie concrete. 456 (Table 4.5 in Chapter 4 of this book) may be taken as a
starting point. Any w/c ratio assumed based on the previous
The step-by-step mix proportioning procedure as per IS 10269 experience for a particular grade of concrete should be
is as given as follows (IS 10262; Nagendra 2010): checked against the maximum values permitted from the point
Step 1 Calculate the target mean compressive strength for of view of durability, and the lesser of the two values should be
mix proportioning. The 28-day target mean compressive adopted.
strength as per Clause 3.2 of IS 10262 is Step 3 Select the water content. The quality of water
fck′ = fck + 1 65 × s (1.1) considered per cubic metre of concrete decides the workability
of the mix. The use of water-reducing chemical admixtures in
where fck′ is the target mean compressive strength at 28 days the mix helps to achieve increased workability at lower water
(N/mm2), fck is the characteristic compressive strength at contents. The water content given in Table 1.14 (Table 2 of
28 days (N/mm2), and s is the standard deviation (N/mm2). IS 10262) is the maximum value for a particular nominal
TABLE 1.14 Maximum water content per cubic metre of concrete for TABLE 1.15 Volume of course aggregate per unit volume of total
nominal maximum size of (angular) aggregate aggregate for different zones of fine aggregate
S. No. Nominal Maximum Size Maximum Water Content*, kg Nominal Volume of Coarse Aggregate* Per Unit Volume of Total
of Aggregate, mm Maximum Aggregate for Different Zones of Fine Aggregate (for w/c
Size of Ratio = 0.5)
1. 10 208
Aggregate, Zone IV Zone III Zone II Zone I
2. 20 186 mm
3. 40 165 10 0.50 0.48 0.46 0.44

Note: These quantities of mixing water are for use in computing cementitious 20 0.66 0.64 0.62 0.60
material contents for trial batches.
40 0.75 0.73 0.71 0.69
* Water content corresponding to saturated surface dry aggregate
Note: The volume of coarse aggregate per unit volume of total aggregate needs
maximum size of (angular) aggregate, which will achieve a to be changed at the rate of ±0.01 for every ±0.05 change in w/c ratio.
slump in the range of 25 mm to 50 mm. The water content * Volumes are based on aggregate in saturated surface dry condition.
per unit volume of concrete can be reduced when increased requires to be suitably adjusted for other w/c ratios. This table
size of aggregate or rounded aggregates are used. On the other is based on ACI 211.1:1991. Aggregates of essentially the
hand, the water content per unit volume of concrete has to same nominal maximum size, type, and grading will produce
be increased when there is increased temperature, cement concrete of satisfactory workability when a given volume of
content, and fine aggregate content. coarse aggregate per unit volume of total aggregate is used. It
In the following cases, a reduction in water content is can be seen that for equal workability, the volume of coarse
suggested by IS 10262: aggregate in a unit volume of concrete is dependent only
1. For sub-angular aggregates, a reduction of 10 kg on its nominal maximum size and the grading zone of fine
2. For gravel with crushed particles, a reduction of 20 kg aggregate.
3. For rounded gravel, a reduction of 25 kg Step 6 Identify the combination of different sizes of coarse
For higher workability (greater than 50 mm slump), the aggregate fractions. Coarse aggregates from stone crushes are
required water content may be established by trial, an increase normally available in two sizes, namely 20 mm and 12.5 mm.
by about 3 per cent for every additional 25 mm slump, or Coarse aggregates of different sizes can be suitably combined
alternatively by the use of chemical admixtures conforming to satisfy the gradation requirements (cumulative per cent
to IS 9103:1999. passing) of Table 2 in IS 383:1970 for the given nominal
maximum size of aggregate.
Use of water reducing admixture Depending on the per-
formance of the admixture (conforming to IS 9103:1999) that Step 7 Estimate the proportion of fine aggregate. The
is proposed to be used in the mix, a reduction in the assumed absolute volume of cementitious material, water, and the
water content can be made. Water-reducing admixtures will chemical admixture is found by dividing their mass by
usually decrease water content by 5–10 per cent and super- their respective specific gravity, and multiplying by 1/1000.
plasticizers decrease water content by 20 per cent and above The volume of all aggregates is obtained by subtracting the
at appropriate dosages. As mentioned earlier, the use PC- summation of the volumes of these materials from the unit
based superplasticizers results in water reduction up to 30–40 volume. From this, the total volume of aggregates, the weight
per cent. of coarse and fine aggregate, is obtained by multiplying their
fraction of volumes (already obtained in Step 5) with the
Step 4 Calculate the content of cementitious material. The respective specific gravities and then multiplying by 1000.
cement and supplementary cementitious material content
per unit volume can be calculated from the free w/c ratio of Step 8 Perform trial mixes. The calculated mix proportions
step 2. The total cementitious content so calculated should be should always be checked by means of trial batches. The
checked against the minimum content for the requirements concrete for trial mixes shall be produced by means of actual
of durability and the greater of the two values adopted. materials and production methods. The trial mixes may
The maximum cement content alone (excluding mineral be made by varying the free w/c ratio by ±10 per cent of
admixtures such as fly ash and GGBS) should not exceed the pre-selected value and a suitable mix selected based on the
450 kg/m3 as per Clause 8.2.4.2 of IS 456. workability and target compressive strength obtained. Ribbon-
type mixers or pan mixers are to be used to simulate the site
Step 5 Estimate the proportion of coarse aggregate. Table conditions where automatic batching and pan mixers are used
1.15 (Table 3 of IS 12062) gives the volume of coarse aggregate for the production of concrete. After successful laboratory
for unit volume of total aggregate for different zones of fine trials, confirmatory field trials are also necessary.
aggregate (as per IS 383:1970) for a w/c ratio of 0.5, which
The guidelines for mix proportioning for HSC are provided formed in hardened cement pastes are more complicated,
by ACI 211.4R:93, for concrete with quarry dust by Nataraja, and the chemical equations are shown in Table 1.16. More
et al. (2001), and for concrete with internal curing by Bentz, details of the chemical reactions may be found in the works
et al. (2005). Rajamane (2004) explains a procedure of mix of Johansen, et al. (2002), Lea (1971), Powers (1961), and
proportioning using the provisions of IS 456:2000. Optimal Taylor (1997).
mixture proportioning for concrete may also be performed As shown in Fig. 1.5, tricalcium silicate (C3S) hydrates
using online tools such as COST (Concrete Optimization and hardens rapidly and is mainly responsible for the initial
Software Tool) developed by NIST, USA (http://ciks.cbt.nist. set and early strength of concrete. Thus, OPC containing
gov/cost/). increased percentage of C3S will have high early strength. On
the other hand, dicalcium silicate (C2S) hydrates and hardens
1.4 HYDRATION OF CEMENT slowly and contributes to strength increase only after about
seven days. Tricalcium aluminate (C3A) is responsible for the
When Portland cement is mixed with water, a series of
large amount of heat of hydration during the first few days
chemical reactions takes place, which results in the formation
of hydration and hardening. It also contributes slightly to
of new compounds and progressive setting, hardening of the
the strength development in the first few days. Cements with
cement paste, and finally in the development of strength. The
low percentages of C3A are more resistant to soils and waters
overall process is referred to as cement hydration. Hydration
containing sulphates. Tetracalcium aluminoferrite (C4AF)
involves many different reactions, often occurring at the
contributes little to strength. The grey colour of cement is
same time. When the paste (cement and water) is added to
due to C4AF and its hydrates. As mentioned earlier, gypsum
aggregates (course and fine), it acts as an adhesive and binds
(calcium sulphate dihydrate) is added to cement during final
the aggregates together to form concrete. Most of the hydration
grinding to regulate the setting time of concrete and it reacts
and about 90 per cent strength development take place within
with C3A to form ettringite (calcium trisulphoaluminate or
28 days; however, the hydration and strength development
AFt). In addition to controlling setting and early strength
continues, though more slowly, for a long time with adequate
gain, gypsum also helps control drying shrinkage (Kosmatka,
moisture and temperature (50 per cent of the heat is liberated
et al. 2003). Figure 1.5 shows the relative reactivity of cement
between one and three days, 75 per cent in seven days,
compounds. The ‘overall curve’ has a composition of 55 per
and about 90 per cent in six months). Hydration products
cent C3S, 18 per cent C2S, 10 per cent C3A, and 8 per cent
TABLE 1.16 Portland cement compound hydration reactions C4AF.
Basic Cement Compounds Hydrated Compounds 100%
2(C3S) +11H = C3S2H8 +3 (CH)
Tricalcium Water Calcium Calcium
silicate silicate hydrate hydroxide 80%
Degree of reaction, % by mass
(C-S-H)
2(C2S) +9H = C3S2H8 +CH
Dicalcium Water Calcium Calcium 60%
silicate silicate hydrate hydroxide
(C-S-H)
C3 S
C3A +3(C S H2) +26H = C6A S3 H32 C2S
40%
Tricalcium Gypsum Water Ettringite (AFt)
C3A
aluminate
C4AF
2(C3A) +C6A S3 H32 +4H = 3(C4A S H12) Overall
20%
Tricalcium Ettringite Water Calcium mono-
aluminate (AFt) sulphoaluminate
(AFm) 0%
0 20 40 60 80 100
C3A +CH +12H = C4A13H Age, days
Tricalcium Calcium Water Tetracalcium
FIG. 1.5 Relative reactivity of cement compounds
aluminate hydroxide aluminate
hydrate Source: Tennis and Jennings 2000
C4AF +10H +2(CH) = 6CAF12H Heat of hydration When Portland cement is mixed with
Tetracalcium Water Calcium Calcium alumino- water, heat is liberated as a result of the exothermic chemical
aluminoferrite hydroxide ferrite hydrate
reaction. This heat is called the heat of hydration. The heat
S = SO3 (Sulfur trioxide) generated by the cement’s hydration raises the temperature of
Source: Lea 1971; Tennis and Jennings 2000
concrete; temperature rises of 55°C have been observed with designed to resist these tensile forces and are often provided
mixes having high cement content. Such a temperature rise in the tension zones. Hence, only RCC is used in structures.
will result in the cracking of the concrete. As a rule of thumb, Depending on the strength it may attain in 28 days, concrete
the maximum temperature differential between the interior may be designated as ordinary concrete, standard or normal
and exterior concretes should not exceed 20°C to avoid crack strength concrete (NSC), HSC, and ultra-high-strength
development. ACI 211.1:91 states that as a rough guide, concrete (UHSC). In IS 456, the grades of concrete are
hydration of cement will generate a concrete temperature rise designed as per Table 1.11. Clause 6.1.1 of IS 456 defines
of about 4.7–7.0°C per 50 kg of cement per cubic metre of the characteristic strength as the strength of the concrete
concrete. Usually, the greatest rate of heat of hydration occurs below which not more than five per cent of the test results will
within the first 24 hours and a large amount of heat evolves fall (see also Section 4.7.3 and Fig. 4.25 of chapter 4). The
within the first three days. Factors influencing heat development minimum grade for RC as per IS 456 is M20; it should be noted
in concrete include the cement content (cements with higher that other international codes specify M25 as the minimum
contents of tricalcium silicate (C3S) and tricalcium aluminate grade. In general, the usual concretes fall in the M20 to M50
(C3A) and higher fineness have higher rates of heat generation), range. In normal buildings M20 to M30 concretes are used,
w/c ratio, placing and curing temperature, the presence of whereas in bridges and prestressed concrete construction,
mineral and chemical admixtures, and the dimensions of the strengths in the range of M35 to M50 are common. Very high
structural element. Higher temperatures greatly accelerate strength concretes in the range of M60 to M70 have been
the rate of hydration and the rate of heat liberation at early used in columns of tall buildings and are normally supplied
AQ1 ages (less than seven days). Kulkarni (2012) observed that by ready-mix concrete companies (Kumar and Kaushik
over the years there is a large increase in the C3S content and 2003).
fineness of cement, both of which speed up the hydration Concrete with enhanced performance characteristics is
reaction and provide high early strength and accompanying called high-performance concrete (HPC). Self-compacting
side effect of higher heat of hydration (for example, in 1920s, concrete (SCC) is a type of HPC, in which maximum
the cement in the USA contained 21 per cent of C3S and 48 compaction is achieved using special admixtures and without
per cent of C2S; now their proportion is completely reversed using vibrators. Structural engineers should aim to achieve
and it is 56 per cent of C3S and 17 per cent of C2S). In view of the HPC through suitable mix proportioning and the use of
these changes in the cement characteristics, design strengths chemical and mineral admixtures.
could be achieved with low cement content and higher When fibres are used in concrete, it is called fibre-
w/c ratio. reinforced concrete (FRC). (Fibres are usually used in
Mineral admixtures, such as fly ash, can significantly concrete to control cracking due to plastic shrinkage and
reduce the rate and amount of heat development. The methods drying shrinkage.) High-performance FRCs are called ductile
to minimize the rise in concrete temperature include cooling fibre-reinforced cementitious composites (DFRCCs); they are
the mixing water, using ice as part of the mixing water, using also called as ultra-high-performance concretes (UHPCs) or
a moderate-heat Portland cement or moderate- or low-heat engineered cementitious composites (ECCs). Because of the
blended cement, using chemical admixtures (retarder, water- non-availability of standard aggregates or to reduce the self-
reducer, or water-retarder), keeping cement contents to a weight, lightweight aggregates may be used; such concretes
minimum level, or cooling the aggregate. Moreover, curing are called SLWCs or autoclaved aerated concretes (AACs). A
with water helps to control temperature increases and is better brief description of these concretes is given in the following
than other curing methods. sections.
1.5 TYPES OF CONCRETE 1.5.1 Ready-mixed Concrete

Ready-mixed concrete is a type of concrete that is manufactured
Depending on where it is mixed, concrete may be classified as
in a factory or batching plant, based on standardized mix
site-mixed concrete or factory-mixed (ready-mixed) concrete
designs, and then delivered to the work site by truck-mounted
(RMC). Site mixing is not always recommended as the
transit mixers. This type of concrete results in more precise
mixing may not be thorough and the control on the w/c or
mixtures, with strict quality control, which are difficult to
w/cm ratio cannot be strictly maintained. Hence, it is used
make on construction sites. Although the concept of RMC was
only in locations where RMC is not readily available. Concrete
known in the 1930s, this industry expanded only during the
without reinforcement is called plain concrete and with
1960s. The first RMC plant started operating in Pune, India, in
reinforcement is called RCC or RC. Even though concrete
1987, but the growth of RMC picked up only after 1997. Most of
is strong in compression, it is weak in tension and tends to
the RMC plants are located in seven large cities of India, and
crack when subjected to tensile forces; reinforcements are
they contribute to about 30–60% of total concrete used in TABLE 1.17 Desired characteristics of HPCs
these cities. (Even today, a substantial proportion of concrete Property Criteria that may be specified
produced in India is volumetrically batched and site-mixed, High strength 70–140 MPa at 28–91 days
involving a large number of unskilled labourers in various
High early compressive strength 20–28 MPa at 3–12 hours or 1–3
operations.) The percentage of RMC to total concrete being days
used is 28.5 per cent. RMC is being used for bridges, flyovers,
High early flexural strength 2– 4 MPa at 3–12 hours or 1–3
and large commercial and residential buildings (Alimchandani days
2007).
High modulus of elasticity More than 40 GPa
The RMC plants should be equipped with up-to-date
Abrasion resistance 0–1 mm depth of wear
equipment, such as transit mixer, concrete pump, and
concrete batching plant. RMC is manufactured under Low permeability 500–2000 Coulombs
computer-controlled operations and transported and placed Chloride penetration Less than 0.07% Cl at 6 months
at site using sophisticated equipment and methods. The Sulphate attack 0.10% or 0.5% maximum
major disadvantage of RMC is that since the materials are expansion at 6 months for
batched and mixed at a central plant, travelling time from moderate or severe sulphate
exposures
the plant to the site is critical over longer distances. It is
better to have the ready mix be placed within 90 minutes Low absorption 2–5%
of batching at the plant. (The average transit time at Low diffusion coefficient 1000 × 10−14 m/s
Mumbai is four hours during the daytime.). Though modern Resistance to chemical attack No deterioration after 1 year
admixtures can modify that time span, the amount and type Low shrinkage Shrinkage strain less than 0.04%
of admixture added to the mix may affect the properties of in 90 days
concrete. Low creep Less than normal concrete
1.5.2 High-performance Concrete TABLE 1.18 Typical HPC mixtures used in some structures
High-performance concrete may be defined as any concrete Structure
that provides enhanced performance characteristics for a Two Great Kaiga Petronas Urban
given application. It is difficult to provide a unique definition Union Belt Link, Atomic Tower, Viaduct,
Ingredients Square, East Project Malaysia, Mumbai,
of HPC without considering the performance requirements
Seattle, Bridge, Unit 2, 1999 India,
of the intended use. ACI has adopted the following broad 1988 Denmark, India, 2002
definition of HPC: ‘A concrete meeting special combinations 1996 1998
of performance and uniformity requirements that cannot Water
130 130 136 152 148
always be achieved routinely by using only conventional kg/m3
materials and normal mixing, placing, and curing Portland
513 315 400 186 500
practices. The requirements may involve enhancements of cement, kg/m3
characteristics such as easy placement, compaction without Fly ash, kg/m3 – 40 – 345* –
segregation, long-term mechanical properties, early-age Slag,
strength, permeability, density, heat of hydration, toughness, – – – – –
kg/m3
volume stability, and long service life in severe environments’ Silica fume,
(ACI 363 R-10). Table 1.17 lists a few of these characteristics. 43 23 25 35 50
kg/m3
Concretes possessing many of these characteristics often Coarse 762
achieve higher strength (HPCs have usually strengths aggregates, (20mm)
1080 1140 1069 1000
greater than 50–60 MPa). Therefore, HPCs will often have kg/m3 + 384 (10
high strength, but a HSC need not necessarily be called mm)
as HPC (Mullick 2005; Muthukumar and Subramanian Fine
1999). aggregates, 685 710 827 725 682
The HPCs are made with carefully selected high-quality kg/m3
ingredients and optimized mixture designs (see Table 1.18). Water reducer,
– 1.5 – – –
These ingredients are to be batched, mixed, placed, L/m3
compacted, and cured with superior quality control to get the Retarder, – – – – –
desired characteristics. Typically, such concretes will have a L/m3
low water–cementing materials ratio of 0.22 to 0.40.
Structure should be adopted to control plastic and autogenous shrinkage

Two Great Kaiga Petronas Urban cracking.
Union Belt Link, Atomic Tower, Viaduct, The HPC has been primarily used in tunnels, bridges, pipes
Ingredients Square, East Project Malaysia, Mumbai, carrying sewage, offshore structures, tall buildings, chimneys,
Seattle, Bridge, Unit 2, 1999 India,
1988 Denmark, India, 2002
and foundations and piles in aggressive environments for its
1996 1998 strength, durability, and high modulus of elasticity. It has also
Air content % – 5.5 2 − 1.5 been used in shotcrete repair, poles, parking garages, and
Superplasti- 15.7 5.0 5.82 9.29 8.25
agricultural applications. It should be noted that in severe
cizer, fires, HPC results in bursting of the cement paste and spalling
L/m3 of concrete. More information on HPC may be had from ACI
W/cm ratio 0.25 0.34 0.32 0.25– 0.269 363R-10 and IS 9103:1999 codes and the works of Zia, et al.
0.27 (1991), Zia, et al. (1993), Aïtcin and Neville (1993), Aïtcin
Slump, mm – – 175 + 180–220 130–180 (1998), Aïtcin, et al. (1994), Ramachandran (1995), and
25 (at plant) Rixom and Mailvaganam (1999).
80–120
(at site) Self-compacting Concrete
Strength at 28 119 – 75.9 80 79.6– Self-compacting concrete, also called as high-workability con-
days, MPa 81.3 crete, self-consolidating concrete, or self-levelling concrete, is
Strength at 91 145 – 81.4 100 87.2– a HPC, developed by Prof. Okamura and associates at the Uni-
days, MPa (180 (56 days) 87.4 versity of Tokyo (now Kochi Institute of Technology), Japan,
days) in 1988 (Okamura and Ouchi 2003). SCC is a highly workable
* Mascrete, which is a cement–fly ash compound in the ratio 20:80 concrete that can flow through densely reinforced and complex
Superplasticizers are usually used to make these concretes fluid structural elements under its own weight and adequately fill
and workable. It should be noted that without superplasticizers, all voids without segregation, excessive bleeding, excessive
the w/cm ratio cannot be reduced below a value of about air migration, and the need for vibration or other mechanical
0.40. Typically, 5–15 L/m3 of superplasticizer can effectively consolidation. The highly flowable nature of SCC is due to
replace 45–75 L/m3 of water (Aïtcin and Neville 1993). This very careful mix proportioning, usually replacing much of the
drastic reduction in mixing water reduces the distance between coarse aggregate with fines and cement, and adding chemical
cement particles, resulting in much denser cement matrix than admixtures (EFNARC 2005). SCC may be manufactured at a
NSC. The optimal particle-packing mixture design approach site batching plant or in an RMC plant and delivered to site by
may be used to develop a workable and highly durable design truck mixer. It may then be placed by either pumping or pour-
mixture (with cement content less than 300 kg/m3), having ing into horizontal or vertical forms. To achieve fluidity, a new
compressive strength of 70–80 MPa (Kumar and Santhanam generation superplasticizers based on polycarboxylic esters
2004). (PCE) is used nowadays, as it provides better water reduc-
As the crushing process takes place along any potential tion and slower slump loss than traditional superplasticizers.
zones of weakness within the parent rock, and thus removes The stability of a fluid mix may be achieved either by using
them, smaller particles of coarse aggregates are likely to be high fines content or by using viscosity-modifying agents
stronger than the large ones. Hence, for strengths in excess (VMA).
of 100 MPa, the maximum size of aggregates should be Several new tests have been evolved for testing the
limited to 10–12 mm; for lesser strengths, 20 mm aggregates suitability of SCC (see Fig. 1.6). They essentially involve
can be used (Aïtcin and Neville 1993; Aïtcin, 1998). Strong testing the (a) flowability (slump flow test), (b) filling ability
and clean crushed aggregates from fine-grained rocks, mostly (slump flow test, V-funnel, and Orimet) (It may be noted that
cubic in shape, with minimal flaky and elongated shapes are in the slump flow test, the average spread of flattened concrete
suitable for HPC. In order to have good packing of the fine is measured horizontally, unlike the conventional slump test,
particles in the mixture, as the cement content increases, the where vertical slump is measured.), (c) passing ability (L-box,
fine aggregates should be coarsely graded and have fineness J-ring which is a simpler substitute for U-box), (d) robustness,
modulus of 2.7–3.0. and (e) segregation resistance or stability (simple column box
As the HPC has very low water content, it is important to test, sieve stability test). The details of these test methods
effectively cure HPC as early as possible. Membrane curing may be found in the works of Okamura and Ouchi (2003) and
is not suitable for HPC, and hence fogging or wet curing Hwang, et al. (2006).
f 100
Abrams cone 300 Adapter
f 200 Base plate
Smooth bars 3×
(or 2×) f12 mm gap
f 500 41 (or 59) mm f 300
f 200
00 Δh
≥9
≥900 H
Fresh SCC sample
(a) (b) Bar diameter:

515 37.5 mm 16 mm
75 Open the center
gate
25
450
680 mm
Obstacle 100
200
225
150
Hinged Height
65 280
trapdoor
(d) (e) (c)
FIG. 1.6 Tests on self-consolidating concrete (a) Slump flow test (b) L-box (c) J-ring (d) V-funnel (e) U-flow test
The SCC has been used in a number of bridges and precast elements, thus resulting in overall economy. In most cases, the
projects in Japan, Europe, and USA (Ouchi 2003). Recently, slightly higher cost of SLWC is offset by reductions in the
SCC has been used in a flyover construction in Mumbai, volume of concrete and steel used. Seismic performance is
AQ2 India (ICJ, August 2009). The various developments in SCC also improved because the lateral and horizontal forces acting
undertaken in India may be found in ICJ (2004, 2009). An on a structure during an earthquake are directly proportional
amendment (No. 3, August 2007) in the form of Annex J was to the inertia or the mass of a structure. Companies such as
added to IS 456, which prescribes the following for SCC: Lafarge produce varieties of industrial lightweight aggregates;
examples include Aglite™, Haydite™, Leca™, Litex™,
1. Minimum slump flow: 600 mm
Lytag™, True Lite™, and Vitrex™ (www.escsi.org). As a
2. Amount of fines (< 0.125 mm) in the range of 400–600
result of these advantages, SLWC has been used in a variety
kg/m3, which may be achieved by having sand content
of applications in the past 80 years. The reduced strength
more than 38 per cent and using mineral admixture to the
of SLWC is considered in the design by ACI code by the
order of 25–50 per cent by mass of cementitious materials
factor l.
3. Use of HRWRA and VMA
An effective technique developed to help mitigate and
1.5.3 Structural Lightweight Concrete overcome the issues of autogenous shrinkage and self-
desiccation is internal curing; Autogenous shrinkage is defined
Some of the early structures from the Roman Empire that still
as a concrete volume change occurring without moisture
survive today, including the Pantheon, have elements that were
transfer to the environment, as a result of the internal chemical
constructed with lightweight concrete. The use of lightweight
and structural reactions (Holt 2001). Autogenous shrinkage
concrete in modern times started when Steven J. Hayde, a
is accompanied by self-desiccation during hardening of the
brick-maker from Kansas City, Missouri, developed a rotary
concrete, which is characterized by internal drying. Self-
kiln method for expanding clays, shales, and slates in the early
desiccation, or internal drying, is a phenomenon caused by the
1900s. SLWC is made with lightweight coarse aggregates such
chemical reaction of cement with water. The reaction leads to
as natural pumice or scoria aggregates and expanded slags;
a net reduction in the total volume of water and solid (Persson,
sintering-grate expanded shale, clay, or fly ash; and rotary-
et al. 2005). The porosity of lightweight aggregates provides
kiln expanded shale, clay, or slate (ACI E1-07). The in-place
a source of water for internal curing, resulting in continued
density (unit weight) of such SLWC will be of the order of
enhancement of the strength and durability of concrete.
1360–1920 kg/m3, compared to the density of normal weight
However, this does not prevent the need for external curing.
concrete of 2240–2400 kg/m3. For structural applications, the
More details about the mix design, production techniques,
strength of such SLWC should be greater than 20 MPa. The
properties, and so on may be found in ACI 213R-03 manual
use of SLWC allows us to reduce the deadweight of concrete
and the works of Neville (1996), Clarke (1993), and Chandra

and Berntsson (2002).
Autoclaved Aerated Concrete

Autoclaved aerated concrete, also known as autoclaved
cellular concrete (ACC) or autoclaved lightweight concrete
(ALC) with commercial names Siporex, e-crete, and Ytong,
was invented in the mid-1920s by the Swedish architect Johan
Axel Eriksson. It is a lightweight, strong, inorganic, and non-
toxic precast building material that simultaneously provides
strength, insulation, and fire, mould, and termite resistance.
Though relatively unknown in countries such as the USA,
India, Australia and China, AAC now accounts for over 40 per
cent of all construction in the UK and more than 60 per cent
of construction in Germany.
Autoclaved aerated concrete is a precast product FIG. 1.7 Fibres used in concrete
manufactured by combining silica (either in the form Source: www.tgrmcc.com
of quartz/silica sand or recycled fly ash), cement, lime,
The amount of fibres added to a concrete mix is expressed as a
water, and an expansion agent—aluminium powder—at
percentage of the total volume of the composite (concrete and
the rate of 0.05–0.08% (it has to be noted that no coarse
fibres) and termed volume fraction, which is denoted by Vf and
aggregates are used). Aluminium powder reacts with calcium
typically ranges from 0.25 per cent to 2.5 per cent (0.75–1.0 per
hydroxide and water to form numerous hydrogen bubbles,
cent is the most common fraction). The aspect ratio of a fibre is
resulting in the expansion of concrete to roughly two to
the ratio of its length to its diameter. Typical aspect ratio ranges
five times its original volume. The hydrogen subsequently
from 30 to 150. The diameter of steel fibres may vary from
evaporates, leaving a highly closed-cell aerated concrete.
0.25 mm to 0.75 mm. Increasing the aspect ratio of the fibre
When the forms are removed from the material, it is solid
usually increases the flexural strength and toughness of
but still soft. It is then cut into either blocks or panels and
the matrix. However, fibres that are too long tend to ‘ball’
placed in an autoclave chamber for 12 hours. AAC blocks
in the mix and create workability problems (Subramanian
(typically 600 mm long, 200 mm high, and 150–300 mm
1976b). To obtain adequate workability, it is necessary to use
thick) are stacked one over the other using thin-set mortar,
superplasticizers. The ultimate tensile strength of steel fibres
as opposed to the traditional concrete masonry units (CMU)
should exceed 350 MPa. More information on FRC may be had
construction.
from the works of Parameswaran and Balasubramanian (1993)
and Bentur and Mindess (2007) and ACI 544.1R-96 report.
1.5.4 Fibre-reinforced Concrete
Fibres are added to concrete to control cracking caused by 1.5.5 Ductile Fibre-reinforced Cementitious Composites
plastic shrinkage and drying shrinkage. The addition of
Ductile fibre-reinforced cementitious composite is a broader
small closely spaced and uniformly dispersed fibres will act
class of materials that has properties and superior perfor-
as crack arresters and enhance the tensile, fatigue, impact,
mance characteristics compared to conventional cementitious
and abrasion resistance of concrete. They also reduce the
materials such as concrete and FRC. DFRCCs have unique
permeability of concrete. Though the flexural strength may
properties including damage reduction, damage tolerance,
increase marginally, fibres cannot totally replace flexural steel
energy absorption, crack distribution, deformation compat-
reinforcement (the concept of using fibres as reinforcement is
ibility, and delamination resistance (delamination is a mode
not new; fibres have been used as reinforcement since ancient
of failure in composite materials—splitting or separating a
times, for example, horsehair in mortar and asbestos fibres in
laminate into layers) (Matsumoto and Mihashi 2003). The
concrete).
various subgroups of DFRCC are shown in Fig. 1.8 and
Clause 5.7 (Amendment No. 3) of IS 456:2000 permits
Table 1.19 (Matsumoto and Mihashi 2003). It should be noted
the use fibres in concrete for special applications to enhance
that DFRCC encompasses a group of high-performance fibre-
its properties. Steel, glass, polypropylene, carbon, and basalt
reinforced cementitious composites (HPFRCC). UHPC, also
fibres have been used successfully; steel fibres are the most
known as ultra-high performance fibre-reinforced concrete
common (see Fig. 1.7). Steel fibres may be crimped, hooked,
(UHPFRC) or reactive powder concrete (RPC), developed in
or flat. This type of concrete is known as FRC.
France in the late 1990s, is a new class of DFRCCs that have Engineered Cementitious Composites
ultra-strength and ultra-performance characteristics. Engineered cementitious composites is a special type of
HPFRCC that has been micro-structurally tailored based
DFRCC on micro-mechanics. ECC is systematically engineered to
achieve high ductility under tensile and shear loading. By
HPFRCC employing material design based on micro-mechanics, it
Ductal can achieve maximum ductility in excess of three per cent
ECC
under uniaxial tensile loading with only two per cent fibre
SIFCON UHPFRC SIMCON
content by volume. Experiments have shown that even
Concrete, at the ultimate load (five per cent strain), the crack width
mortar,
cement remains at about 60 µm and is even lower at strain below one
FRC, FRM
per cent.
FRCC
As shown in Fig. 1.10, extensive experimental studies have
demonstrated superior seismic response as well as minimum
Cementitious material
post-earthquake repair (Fischer and Li 2002). It should be
FIG. 1.8 Classification of cementitious materials noted that even at the high drift level of 10 per cent, no spalling
Source: Matsumoto and Mihashi 2003 of the reinforced ECC was observed; in contrast, the RCC
column lost the concrete cover after bond splitting and being
TABLE 1.19 Characteristics of different cementitious materials
subjected to heavy spalling. The test results also illustrated the
Characteristics Cement, Concrete, DFRCC HPFRCC
potential reduction or elimination of steel stirrups by taking
Mortar FRC
advantage of the shear ductility of ECC. The tensile ductility
Material Brittle Quasi Quasi- Ductile
in ECC also translates into shear ductility since the material
response brittle brittle
(tension) undergoes diagonal tensile multiple cracking when subjected
or ductile to shear (Li, et al. 1994).
(flexure)
Strain – Strain Strain Strain
softening or softening softening hardening
hardening (tension)
(see Fig. 1.9) or
hardening
(flexure)
Cracking Localized Localized Multiple Multiple
behaviour cracking cracking cracking cracking
(flexure)*
Cracking Localized Localized Localized Multiple
behaviour cracking cracking cracking cracking
(tension)
* Cracking behaviour in flexure is dependent on specimen size. This comparison
is based on specimen size of 100 × 100 × 400 mm
Source: Matsumoto and Mihashi 2003
Tensile stress (a) (b)

FIG. 1.10 Damage of column at 10 per cent drift after reverse cyclic AQ3
Tensile strength C
Ductile loading (a) RCC (b) ECC without stirrups
First cracking Strain hardening Source: Fischer and Li 2002
strength Strain softening
Brittle
Life cycle cost comparison showed that ECC slab systems
Strain softening
A B provide 37 per cent cost efficiency, consume 40 per cent less
Strain
total primary energy, and produce 39 per cent less carbon
First cracking strain Tensile ultimate dioxide compared to conventional RCC systems (Li 2003).
strain More details about the behaviour and application of ECC may
FIG. 1.9 Definitions of brittle, ductile, strain softening, and strain be found in the study of Li (2003).
hardening under uniaxial tensile loading
Source: Matsumoto and Mihashi 2003
Ultra-high-performance Concrete 400
Ultra-high-performance concrete is a high-strength, high- 350

stiffness, self-consolidating, and ductile material, formulated Confined and
300 Confined and Confined
by combining Portland cement, silica fume, quartz flour, fine RPC
pressed RPC
pressed RPC without fibres
silica sand, high-range water reducer, water, and steel or organic 250 with fibres
Stress, MPa
with fibres
fibres. Originally it was developed by the Laboratoire Central
des Ponts et Chaussées (LCPC), France, containing a mixture 200
Confined RPC
of short and long metal fibres and known as multi-scale fibre- 150 without fibres
reinforced concrete (Rossi 2001). It has to be noted that there RPC with fibres
100 High
are no course aggregates, and a low w/cm ratio of about 0.2 is
performance
used in UHPC compared to about 0.4–0.5 in NSC. The material 50 concrete
provides compressive strengths of 120–240 MPa, flexural Normal strength concrete
strengths of 15–50 MPa, and post-cracking tensile strength of 0
0 0.005 0.01 0.015 0.02
7.0–10.3 MPa and has modulus of elasticity from 45 GPa to 59 Longitudinal strain, mm/mm
GPa [Ductal® (Lafarge, France), CoreTUFF® (US Army Corps FIG. 1.11 Comparison of stress–strain curves of NSC, HPC, and
of Engineers), BSI®, Densit® (Denmark), and Ceracem® RPC
(France and Switzerland) are some examples of commercial Source: Blais and Couture 1999
products]. The enhanced strength and durability properties
of UHPC are mainly due to optimized particle gradation that (made of cement and sand in the proportion 1:1, 1:1.5, or 1:2,
produces a very tightly packed mix, use of steel fibres, and with fly ash and silica fume equal to 10–15 per cent by weight
extremely low water to powder ratio (Nematollahi, et al. 2012). of cement, w/cm ratio of 0.3–0.4, and superplasticizer equal
Some of the potential applications of UHPC are in to 2–5 per cent by weight of cement) into pre-placed steel
prestressed girders and precast deck panels in bridges, columns, fibres (single plain or deformed fibres) in a formwork. It has
piles, claddings, overlays, and noise barriers in highways. The to be noted that it does not contain any coarse aggregates but
60 m span Sherbrooke pedestrian bridge, constructed in 1997 has a high cementitious content. Due to the pre-placement
at Quebec, Canada, is the world’s first UHPC bridge without of fibres, its fibre volume fraction may be as high as 6–20
any bar reinforcement. More details of this bridge may be had per cent. The confining effect of numerous fibres yields high
from the works of Blais and Couture (1999) and Subramanian compressive strengths from 90 MPa to 210 MPa, and the
(1999). The 15 m span Shepherds Creek Road Bridge, New strong fibre bridging leads to tensile stain hardening behaviour
South Wales, Australia, built in 2005 is the world’s first UHPC in some SIFCONs. Slurry infiltrated mat concrete (SIMCON)
bridge for normal highway traffic. Since then, a number of is similar to SIFCON, but uses pre-placed fibre mat instead of
bridges and other structures have been built utilizing UHPC steel fibres. SIFCON and SIMCON are extremely ductile and
all over the world (see www.fhwa.dot.gov). hence ideally suitable for seismic retrofit of structures (Dogan
The materials for UHPC are usually supplied by the and Krstulovic-Opara 2003). They also have improved
manufacturers in a three-component premix: powders uniaxial tensile strength, flexural, shear, and impact strengths,
(Portland cement, silica fume, quartz flour, and fine silica and abrasion resistance (Parameswaran 1996). They are best
sand) pre-blended in bulk bags; superplasticizers; and organic suited for the following applications: Pavement rehabilitation,
fibres. Care should be exercised during mixing, placing, and safety vaults, strong rooms, refractory applications, precast
curing. The ductile nature of this material makes concrete to concrete products, bridge decks and overlays, repair and
deform and support flexural and tensile loads, even after initial rehabilitation of structures, especially in seismic zones,
cracking. The use of this material for construction is simplified military applications, and concrete mega-structures, such
by the elimination of reinforcing steel and its ability to be as offshore platforms and solar towers. More details about
virtually self-placing. More details about UHPC may be found the SIFCON and SIMCON may be found in the works of
in the works of Schmidt, et al. (2004), Fehling, et al. (2008), and Parameswaran, et al. (1990), Parameswaran (1996), Lankard
Schmidt, et al. (2012). A comparison of stress–strain curves in (1984), Naaman, et al. (1992), Sashidhar, et al. (2010, 2011)
concretes is provided in Fig. 1.11. The influence of fibres and and Hackman, et al. (1992).
confinement on the ductility of RPC should be noted.
1.5.6 Ferrocement
Slurry Infiltrated Fibrous Concrete and Slurry Infiltrated Mat Ferrocement also known as ferrocrete, invented by Jean Louis
Concrete Lambot of France, in 1848, is a composite material like RCC.
Slurry infiltrated fibrous concrete (SIFCON), invented by In RCC, the reinforcement consists of steel bars placed in
Lankard in 1979, is produced by infiltrating cement slurry the tension zone, whereas ferrocement is a thin RC made
of rich cement mortar (cement to sand ratio of 1:3) based post-tensioning method), which applies pre-compression to
matrix reinforced with closely spaced layers of relatively the member. The design of prestressed concrete members
small diameter wire mesh, welded mesh, or chicken mesh. should conform to IS 1343:1980.
(The diameter of wires range from 4.20 mm to 9.5 mm and
are spaced up to 300 mm apart.) The mesh may be metallic 1.6 REINFORCING STEEL
or synthetic (Naaman 2000). The mortar matrix should have
As stated earlier, steel reinforcements are provided in RCC to
excellent flow characteristics and high durability. The use of
resist tensile stresses. The quality of steel used in RCC work
pozzolanic mineral admixtures such as fly ash (50 per cent
is as important as that of concrete. Steel bars used in concrete
cement replacement with fly ash is recommended) and use of
should be clean and free from loose mill scales, dust, loose rust
superplasticizers will not only permit the use of water–binder
and any oily materials, which will reduce bond. Sand blasting
ratio of 0.40–0.45 by mass but also will enhance the durability
or similar treatment may be done to get clean reinforcement.
of the matrix. A mortar compressive strength of 40–50 MPa
As per Clause 5.6 of IS 456, steel reinforcement used in
is recommended.
concrete may be of the following types (see Table 1.1 of SP
During the 1940s, Pier Luigi Nervi, an Italian engineer,
34 (S&T):1987 for the physical and mechanical properties of AQ4
architect, and contractor, has used ferrocement for the
these different types of bars):
construction of aircraft hangars, boats and buildings. It has
to be noted that though Nervi used a large number of meshes 1. Mild steel and medium tensile steel bars (MS bars)
in his structures, in many present-day applications, only conforming to IS 432 (Part 1):1982
two layers of mesh reinforcement are used. Applications 2. High strength deformed steel bars (HYSD bars) conforming
of ferrocement include boats, barges, water tanks, pipes, to IS 1786:2008
biogas digesters, septic tanks, toilet blocks, and monolithic 3. Hard drawn steel wire fabric conforming to IS 1566:1982
or prefabricated housing (Subramanian 1976a). Recently, 4. Structural steel conforming to Grade A of IS 2062:2006
Spanos, et al. (2012) have studied the use of ferrocement
It should be noted that different types of rebars, such as plain
panels as permanent load bearing formwork for one-way and
and deformed bars of various grades, say grade Fe 415 and
two-way slabs. Such panels provide economic advantages
Fe 500, should not be used side by side, as this may lead to
and the slabs incorporating them will provide superior
confusion and error at site. Mild steel bars, which are produced
serviceability performance. At the new Sydney Opera House,
by hot rolling, are not generally used in RCC as they have
the sail-shaped roofs (built of conventional RC) have been
smooth surface and hence their bond strength is less compared
covered with tile-surfaced panels of ferrocement, which serve
to deformed bars (when they are used they should be hooked
as waterproofs for the concrete underneath. More information
at their ends). They are used only as ties in columns or stirrups
about the design and construction of ferrocement may be had
in beams. Mild steel bars have characteristic yield strength
from the study of Naaman (2000) and ACI 549.1R-93 manual.
ranging from 240 N/mm2 (grade I) to 350 N/mm2 (medium
Polymer concrete Polymer concrete is obtained by tensile steel) and percentage elongation of 20–23% over a
impregnating ordinary concrete with a monomer material gauge length of 5.65√area.
and then polymerizing it by radiation, by heat and catalytic Hot rolled high yield strength deformed bars (HYSD bars)
ingredients, or by a combination of these two techniques. were introduced in India in 1967; they completely replaced
Depending on the process by which the polymeric materials mild steel bars except in a few situations where acute bending
are incorporated, they are classified as (a) polymer concrete was required in bars greater than 30 mm in diameter. They
(PC), (b) polymer impregnated concrete (PIC), and (c) were produced initially by cold twisting (CTD bars) and later
polymer modified concrete (PMC). Due to polymerization, by heat treatment (TMT bars) and micro-alloying. They were
the properties are much enhanced and polymer concrete is introduced in India by Tata Steel as Tistrong bars and later as
also used to repair damaged concrete structural members Tiscon/Torsteel bars. Cold twisted deformed bars (CTD bars
(Subramanian and Gnana Sambanthan 1979). or Torsteel bars) are first made by hot rolling the bars from
In addition to these types of concrete, prestressed concrete high-strength mild steel, with two or three parallel straight ribs
is often used in bridges and long-span structures; however, it is and other indentations on it. After cooling, these bars are cold
outside the scope of this book. A prestressed concrete member twisted by a separate operation, so that the steel is strained
is one in which internal stresses (compressive in nature) are beyond the elastic limit and then released. As the increase in
introduced, which counteract the tensile stresses resulting strength is due to cold-working, this steel should not be normally
from the given external service level loads. The prestress is welded. In CTD bars, the projections will form a helix around
commonly introduced by tensioning the high-strength steel the bars; if they are over-twisted, the pitch of the helixes will
reinforcement (either by using the pre-tensioning or the be too close. Cold twisting introduces residual stresses in steel,
Rolling mill Quenching Shear Cooling corrosion resistant steel bars (TMT CRS bars) are produced,
final stand box bed which have better corrosion resistance than ordinary TMT
bars. It is better to adopt precautions against corrosion
even while using such bars, as they are not 100 per cent
corrosion-resistant. Though IS 1786 specifies four grades
for these HYSD bars, namely Fe 415, Fe 500, Fe 550, and
Fe 600, and additional three grades with a suffix D, denoting
that they are ductile, the availability of Fe 550, Fe 600, Fe
415D, Fe 500D, and Fe 550D grades are limited (the numbers
after Fe denoting the 0.2 per cent proof or yield stress,
in N/mm2).
Austenite Martensite Tempered Tempered
Martensite Martensite The most important characteristic of the reinforcing
Austenite Austenite Austeninte Ferrite-pearlite bar is its stress–strain curve; the important property is its
AQ5 FIG. 1.12 Manufacturing process of TMT bars characteristic yield strength or 0.2 per cent proof stress as
Source: Tata Steel Catalog, 2004 the case may be (see Fig. 1.13 and Table 1.20), and as per
Reference: Tata steel-TISCON CRS-Corrosion Resistant Steel Bars, Tata Clause 5.6.3 of IS 456, the modulus of elasticity Es for these
Steel, Kolkata, August 2004. steels may be taken as 200 kN/mm2. The design stress–strain
and as a result, these bars corrode much faster than other bars; curves for steel reinforcements (both mild steel and HYSD
hence, these are not recommended in many advanced countries. bars) are given in Fig. 5.5 of Chapter 5. The inelastic strains
Thermo-mechanically treated reinforcement bars (TMT in HYSD bars for some design stress values, as per IS
Bars) are a class of hot rolled HYSD bars, which are rapidly 456, are given in Table 5.1 (see Section 5.4). The chemical
cooled to about 450°C by a controlled quenching process using composition of various grades of steel is given in IS 1786:2008
water when they are leaving the last specifications.
stand of the rolling mill at a temperature 600
Strain hardening
of about 950°C. This sudden partial
Yield Ultimate 500 Fe 500
quenching, along with the final cooling, plateau Stress
transforms the surface layer of the bars Stress N/mm2Fe 415
400
from austenite to tempered martensite,
Breaking
stress
Stress
with a semi-tempered middle ring of 300

Elastic Fe 250
martensite and bainite and a fine-grained range (Mild steel gr. I)
200
AQ6 ferrite–pearlite core (see Fig. 1.12).
TMT bars can be welded as per IS ES = 2 × 105 N/mm2 100
ES = 2 × 105 N/mm2
9417 using ordinary electrodes and no
extra precautions are required. Strength, 0
0.001
0.002
0.003
0.004
0.005
0.006
Strain
weldability, and ductility are the main
advantages of TMT bars; in addition, Strain
they are also economical. TMT bars (a) (b)
produced by SAIL or Tata are known FIG. 1.13 Stress–strain curve (a) Mild steel (b) HYSD bars AQ7
as SAIL-TMT or TISCON-TMT. Bars
produced by RINL are called REBARS. As it is visually Clause 5.3 of IS 13920 stipulates that steel reinforcements of
difficult to distinguish TMT bars from mild steel deformed grade Fe 415 or less should be used in structures situated in
bars, the following procedure is suggested in IS 1786: A small earthquake zones. However, TMT bars of grades Fe 500 and
piece (about 12 mm long) can be cut and the transverse face Fe 550, having elongation more than 14.5 per cent, are also
lightly ground flat on progressively finer emery papers up to ‘0’ allowed. For providing sufficient bond between the bars and
size. The sample can be macro-etched with nital (five per cent the concrete, the area, height, and pitch of ribs should satisfy
nitric acid in alcohol) at ambient temperature for a few seconds Clause 5 of IS 1786 (see Fig. 1.14). The nominal size (in
to reveal a darker annular region corresponding to martensite or millimetres) of the available bars as per IS 1786 are 4, 5,
bainite microstructure and a lighter core region. 6, 8, 10, 12, 16, 20, 25, 28, 32, 36, and 40. A density of
By micro-alloying with elements such as copper, 7,850 kg/m3 per metre run may be taken for calculating the
phosphorus, and chromium, thermo-mechanically treated nominal mass.
TABLE 1.20 Mechanical properties of high strength deformed bars as per IS 1786:2008
S. No. Property Fe 415 Fe 415D Fe 500 Fe 500D Fe 550 Fe 550D Fe 600
1. 0.2% proof
AQ8 stress/yield
415.0 415.0 500.0 500.0 550.0 550.0 600.0
stress, min.
N/mm2
2. Elongation,
percentage,
min. on 14.5 18.0 12.0 16.0 10.0 14.5 10.0
gauge length
5.65√A*
3. Tensile 10% more 12% more 8% more than 10% more 6% more than 8% more than 6% more than
strength, min. than the actual than the actual the actual than the actual the actual the actual the actual
0.2% proof 0.2% proof 0.2% proof/ 0.2% proof 0.2% proof 0.2% proof 0.2% proof
stress/yield stress/yield yield stress stress/yield stress/yield stress/yield stress/yield
stress but not stress but not stress but not stress but not stress but not stress but not stress but
less than 485.0 less than 500.0 less than 545.0 less than 565.0 less than 585.0 less than 600.0 not less than
N/mm2 N/mm2 N/mm2 N/mm2 N/mm2 N/mm2 660.0 N/mm2
4. *Total
elongation
at maximum
force
AQ9 – 5% – 5% – 5% –
percentage,
min. on
gauge length
5.65√A*
* A is the cross-sectional area of the test piece.
Pitch of deformation For steel bars surrounded by sound concrete, the passive
Height of deformation corrosion rate is typically 0.1 µm per year. Without the passive
film, the steel would corrode at rates at least 1000 times higher
(ACI 222R-01). The destruction of the passive layer occurs
when the alkalinity of the concrete is reduced or when the
Width of Width of chloride concentration in concrete is increased to a certain
Angel between
longitudinal longitudinal level. In many cases, exposure of RC to chloride ions is the
deformation
gap gap primary cause of premature corrosion of steel reinforcement.
and axis of
bar Although chlorides are directly responsible for the initiation
FIG. 1.14 Deformation on bars of corrosion, they appear to play only an indirect role in
Welded wire fabrics (WWF) consist of hard drawn steel the rate of corrosion after initiation. The primary factors
wire mesh made from medium tensile steel, drawn out from controlling the corrosion rate are the availability of oxygen,
higher diameter steel bars. As they undergo cold-working, electrical resistivity and relative humidity of the concrete,
their strength is higher than that of mild steel. WWF consists pH, and prevailing temperature. Carbonation is another cause
of longitudinal and transverse wires (at right angles to one for corrosion. Carbonation-induced corrosion often occurs in
another) joined by resistant spot welding using machines. building facades that are exposed to rainfall, are shaded from
They are available in different widths and rolls and as square sunlight, and have low concrete cover over the reinforcing
or oblong meshes; see Table C-1 of SP 34 (S&T):1987 and SP steel. Carbonation occurs when carbon dioxide from the air
1566:1982. Their use in India is limited to small size slabs. penetrates the concrete and reacts with hydroxides, such
as calcium hydroxide, to form carbonates. In the reaction
1.6.1 Corrosion of Rebars with calcium hydroxide, calcium carbonate is formed. This
Corrosion of steel rebars is considered as the main cause of reaction reduces the pH of the pore solution to as low as 8.5,
deterioration of numerous RCC structures throughout the destroying the passive film on steel rebars. It has to be noted
world. In fact, the alkaline environment of concrete (pH of that carbonation is generally a slow process. In high-quality
12–13) provides a thin oxide passive film over the surface concrete, carbonation is estimated to proceed at a rate up
of steel rebars and reduces the corrosion rate considerably. to 1.0 mm per year. The highest rates of carbonation occur
when the relative humidity is maintained between 50 per cent cost of these bars is high, life cycle cost is lower and they
and 75 per cent. The amount of carbonation is significantly may provide 80–125 years of maintenance-free service.
increased in concrete with a high water-to-cement ratio, low The Progresso Bridge in New Mexico, USA, was built
cement content, short curing period, low strength, and highly during 1937–41 using stainless rebar and has not required
permeable paste. Corrosion can also occur when two different maintenance until now.
metals are in contact within concrete. For example, dissimilar
Fibre-reinforced polymer bars (FRP bars) These are
metal corrosion can occur in balconies where embedded
aramid fibre (AFRP), carbon fibre (CFRP) or glass fibre
aluminium railings are in contact with the reinforcing steel.
(GFRP) reinforced polymer rods. They are non-metallic and
Conventional concrete contains pores or micro-cracks.
hence non-corrosive. Although their ultimate tensile strength
Detrimental substances or water can penetrate through these
is about 1500 MPa, their stress–strain curve is linear up to
cracks or pores, leading to corrosion of steel bars. When
failure. In addition, they have one-fourth the weight of steel
corrosion takes place, the resulting rust occupies more than
reinforcement and are expensive. The modulus of elasticity of
three times the original volume of steel from which it is
CFRP is about 65 per cent of steel bars and the bond strength
formed. This drastic expansion creates tensile stresses in the
is almost the same. As the Canadian Highway Bridge Design
concrete, which can eventually cause cracking, delamination,
Code, CSA-S6-06, has provisions for the use of GFRP rebars,
and spalling of cover concrete (see Fig. 4.5 of Chapter 4).
a number of bridges in Canada are built using them. More
The presence of corrosion also reduces the effective cross-
details about them may be had from the work of GangaRao,
sectional area of the steel reinforcement and leads to the failure
et al. (2007) and ACI 440R-07 report.
of a concrete element and subsequently the whole structure.
Mitigation measures to reduce the occurrence of corrosion Basalt bars These are manufactured from continuous basalt
include (a) decreasing the w/c or w/cm ratio of concrete and filaments and epoxy and polyester resins using a pultrusion
using pozzolans and slag to make the concrete less permeable process. It is a low-cost, high-strength, high-modulus, and
(pozzolans and slag also increase the resistivity of concrete, corrosion-resistant alternative to steel reinforcement. More
thus reducing the corrosion rate, even after it is initiated), information about these bars may be found in the study of
(b) providing dense concrete cover, as per Table 16 of IS Subramanian (2010).
456, using controlled permeability formwork (CPF), thus In addition, Zbar, a pretreated high-strength bar with both
protecting the embedded steel rebars from corrosive materials galvanizing and epoxy coating, has been recently introduced
(see Section 4.4.5 for the details of CPF), (c) including in the USA. High-strength MMFX steel bars, conforming to
the use of corrosion-inhibiting admixtures, (d) providing ASTM A1035, with yield strength of 827 MPa and having low
protective coating to reinforcement, and (e) using of sealers carbon and 8–10 per cent chromium have been introduced in
and membranes on the concrete surface. It should be noted the USA recently, which are also corrosion-resistant, similar
that the sealers and membranes, if used, have to be reapplied to TMT CRS bars (www.mmfx.com).
periodically (Kerkhoff 2007). Clause 5.6.2 of IS 456 suggests the use of coating to
As mentioned, one of the corrosion mitigation methods is reinforcement, and Amendment No. 3 of this clause states that
by using the following reinforcements: the reduction of design bond strength of coated bars should be
considered in design, but it does not elaborate. See Sections
Fusion-bonded epoxy-coated reinforcing bars Typical
7.4.2 and 7.5.3 of Chapter 7 for the reduction of design bond
coating thickness of these bars is about 130–300 µm. Damaged
strength based on the ACI code provisions.
coating on the bars, resulting from handling and fabrication
Viswanatha, et al. (2004), based on their extensive
and the cut ends, must be properly repaired with patching
experience of testing rebars, caution about the availability
material prior to placing them in the structure. These bars
of substandard rebars in India, including rerolled bars and
have been used in RC bridges from the 1970s and their
inadequately quenched or low carbon content TMT bars.
performance is found to be satisfactory (Smith and Virmani
Hence, it is important for the engineer to accept the rebars
1996). They may have reduced bond strength.
only after testing them in accordance with IS 1608:2005 and
Galvanized reinforcing bars The precautions mentioned IS 1786:2008. Basu, et al. (2004) also provide an overview of
for epoxy-coated bars are applicable to these bars as well. the important characteristics of rebars and a comparison of
The protective zinc layer in galvanized rebars does not break specifications of different countries.
easily and results in better bond.
Stainless steel bars Stainless steel is an alloy of nickel and 1.7 CONCRETE MIXING, PLACING, COMPACTING, AND CURING
chromium. Two types of stainless steel rods, namely SS304 The measurement of materials for making concrete is called
and SS316, are used as per BS 6744:2001. Though the initial as batching (see also Clause 10.2 of IS 456). Though volume
batching is used in small works, it is not a good method and of up to 46 m directly over reinforcing steel does not result in
weigh batching should always be attempted (fully automatic segregation or reduction of compressive strength (Suprenant
weigh batching equipment are used in RMC plants). The 2001).
mixing of materials should ensure that the mass becomes Concreting during hot or cold weather should conform to
homogeneous, uniform in colour and consistency. Again, the requirements of IS 7861(Part 1):1975 and IS 7861(Part
hand mixing is not desirable for obvious reasons and machine 2):1981. More guidance on hot weather concreting is given
mixing is to be adopted for better quality. Several types of in the work of Venugopal and Subramanian (1977) and ACI
mixtures are available; pan mixtures with revolving star 305R-10 manual. Guidance for underwater concreting is
blades are more efficient (Shetty 2005; IS 1791:1985; IS provided in Clause 14 of IS 456.
12119:1987). Clause 10.3 of IS 456 stipulates that if there is Right after placement, concrete contains up to 20 per cent
segregation after unloading from the mixer, the concrete should entrapped air. Vibration consolidates concrete in two stages:
be remixed. It also suggests that when using conventional first by moving the concrete particles and then by removing
tilting type drum mixtures, the mixing time should be at entrapped air. The concrete should be deposited and compacted
least two minutes and the mixture should be operated at a before the commencement of initial setting of concrete and
speed recommended by the manufacturer (normal speeds are should not be disturbed subsequently. Low-slump concrete
15–20 revolutions/minute). Clause 10.3.3 of IS 456 restricts can be consolidated easily, without adding additional water,
the dosage of retarders, plasticizers, superplasticizers, and by the use of superplasticizers. High frequency power driven
polycarboxylate-based admixtures to 0.5 per cent, 1.0 per internal or external vibrators (as per IS 2505, IS 2506, IS 2514,
cent, 2.0 per cent, and 1.0 per cent, respectively, by weight of and IS 4656) also permit easy consolidation of stiff mixes
cementitious materials. having low w/cm ratio (manual consolidation with tamping
Concrete can be transported from the mixer to the rod is suitable only for workable and flowing mixtures). The
formwork by a variety of methods and equipment such as internal vibrator or needle vibrator is immersed in concrete
mortar pans, wheel barrows, belt conveyors, truck-mixer- and the external vibrator is attached to the forms. (The radius
mounted conveyor belts, buckets used with cranes and cable of action of a needle vibrator with a diameter of 20–40 mm
ways, truck mixer and dumpers, chutes or drop chutes, skip ranges between 75 mm and 150 mm; ACI 309R:05 provides
and hoist, transit mixer (in case of RMC), tremies (for placing more data on consolidation.) Good compaction with vibrators
concrete under water) or pumping through steel pipes. As prevents honeycombing and results in impermeable and dense
there is a possibility of segregation during transportation, care concrete, better bond between concrete and reinforcement,
should be taken to avoid it. More details about the methods of and better finish. Guidance on construction joints and cold
transportation may be found in the works of Panarese (1987), joints is provided in Clause 13.4 of IS 456.
Kosmatka (2011), and Shetty (2005). All newly placed and finished concrete slabs should be
It is also important that the concrete is placed in the cured and protected from drying and from extreme changes
formwork properly to yield optimum results. Prior to placing, in temperature. Wet curing should start as soon as the final
reinforcements must be checked for their correctness (location set occurs and should be continued for a minimum period of
and size), cover, splice, and anchorage requirements, and any 7–15 days (longer curing is required in case of concretes with
loose rust must be removed. The formwork must be cleaned, fly ash). It has to be noted that in concretes without the use
its supports adequately braced, joints between planks or of retarders or accelerators, final set of cement takes place at
sheets effectively plugged, and the inside of formwork applied about six hours. Concreting in hot weather conditions requires
with mould-releasing agents for easy stripping. Details of special precautions against rapid evaporation and drying due
different kinds of formwork and their design may be found to high temperatures. More information on curing is provided
in the work of Hurd (2005) and IS 14687:1999 guidelines. It in Clause 13.5 of the IS 456 and also in Section 4.4.5 of
is necessary to clean the surface of previous lifts thoroughly Chapter 4.
with water jet and treat them properly. Concrete should be
Removal of forms It is advantageous to leave forms in
deposited continuously as near as possible to its final position
place as long as possible to continue the curing period. As
without any segregation. In general, concrete should be placed
per Clause 11.3 of IS 456, the vertical supporting members of
in thicker members in horizontal layers of uniform thickness
formwork (shoring) should not be removed until the concrete
(about 150 mm thick for reinforced members); each layer
is strong enough to carry at least twice the stresses to which
should be thoroughly consolidated before the next is placed.
the concrete may be subjected to at the time of removal of
Chutes and drop chutes may be used when the concrete is
formwork. When the ambient temperature is above 15°C and
poured from a height, to avoid segregation. Though Clause
where Portland cement is used and adequate curing is done,
13.2 of IS 456 suggests a permissible free fall of 1.5 m, it
the vertical formwork to columns, walls, and beams can be
has been found that a free fall of even high-slump concrete
removed at 16–24 hours after concreting. Beam and floor slab is the most commonly used method to measure the consistency
forms and supports (props) may be removed between 3 and 21 of the concrete, because of its simplicity. This test is carried
days, depending on the size of the member and the strength out using an open-ended cone, called the Abrams cone. This
gain of the concrete (see Clause 11.3.1 of IS 456). If high cone is placed on a hard non-absorbent surface and filled with
early strength concrete is used, these periods can be reduced. fresh concrete in three stages, and each time the concrete is
Because the minimum stripping time is a function of concrete tamped using a rod of standard dimensions. At the end of the
strength, the preferred method of determining stripping time third stage, the concrete is struck off level with a trowel at the
in other cases is to be determined based on the tests of site- top of the mould. Now, the mould is carefully lifted vertically
cured cubes or concrete in place. More details including upwards without disturbing the concrete in the cone, thereby
shoring and reshoring of multi-storey structures may be found allowing the concrete to subside. This subsidence is termed
in ACI 347-04 guide. as slump and is measured to the nearest 5 mm. Figure 1.15
shows the slump testing mould, measurement, and types of
1.8 PROPERTIES OF FRESH AND HARDENED CONCRETE slumps. If a shear slump (indicates concrete is non-cohesive)
or collapse slump (indicates a high workability mix) is
A designer needs to have a thorough knowledge of the
achieved, a fresh sample should be taken and the test repeated.
properties of concrete for the design of RC structures. As
A slump of about 50–100 mm is used for normal RC (see
seen in the previous sections, present-day concrete is much
Table 1.12)
complicated and uses several different types of materials, which
considerably affect the strength and
100 Dia.
other properties. Complete knowledge
of these materials and their use and 100
effects on concrete can be had from 25
the works of Gambhir (2004), Mehta 300 mm
and Monteiro (2006), Mindess, et al. 13
2 thick
(2003), Neville (2012), Neville and

16 thick
Brooks (2010), Santhakumar (2006),
and Shetty (2005). An introduction 2 thick
to some of the properties, which
are important for the designer and 200 Dia.
5
construction professionals, is presented
(a) (b)
in this section.
1.8.1 Workability of Concrete Slump

As discussed in Section 1.2.3, water Slump Slump
added to the concrete mix is required
not only for hydration purposes but
also for workability. Workability True Shear Collapse
may be defined as the property of the (c)
freshly mixed concrete that determines FIG. 1.15 Slump testing (a) Typical mould for slump test (b) Measuring slump (c) Types of slump
the ease and homogeneity with which
it can be mixed, placed, compacted, and finished. The desired 1.8.2 Compressive Strength
degree of workability of concrete is provided in Table 1.12. Compressive strength at a specified age, usually 28 days,
The main factor that affects workability is the water content (in measured on standard cube or cylinder specimens, has
the absence of admixtures). The other interacting factors that traditionally been used as the criterion for the acceptance of
affect workability are aggregate type and grading, aggregate/ concrete. It is very important for the designer because concrete
cement ratio, presence of admixtures, fineness of cement, properties such as stress–strain relationship, modulus of
and temperature. It has to be noted that finer particles require elasticity, tensile strength, shear strength, and bond strength
more water to wet their large specific surface, and the irregular are expressed in terms of the uniaxial compressive strength.
shape and rough texture of angular aggregate demand more The compressive strengths used in structural applications
water. Workability should be checked frequently by one of the vary from 20 N/mm2 to as high as 100 N/mm2. (In One, World
standard tests (slump, compacting factor, Vee Bee consistency, Trade Center, New York, USA, a concrete with a compressive
or flow table) as described in IS 1199:1955. Although it does strength of 96.5 MPa was used with a modulus of elasticity of
not measure all factors contributing to workability, slump test 48,265 MPa).
Cube and Cylinder Tests maximum, internal cracks are initiated in the mortar throughout
In India, the UK, and several European countries, the the concrete mass, parallel to the direction of the applied load.
characteristic compressive strength of concrete (denoted by The concrete tends to expand laterally due to Poisson’s effect,
fck) is determined by testing to failure 28-day-old concrete and the cube finally fails leaving two truncated pyramids one
cube specimens of size 150 mm × 150 mm × 150 mm, as over the other (see Fig. 1.16b). Sometimes the failure may
per IS 516:1959. When the largest nominal size of aggregate be explosive, especially in cubes of HSC; to avoid injuries,
does not exceed 20 mm, 100 mm cubes may also be used. proper precautions should be taken to contain the debris using
However, in the USA, Canada, Australia, and New Zealand, high resistance and transparent polycarbonate or steel mesh
the compressive strength of concrete shields around the testing machine.
(denoted by fc ′) is determined by P P P
testing to failure 28-day-old concrete P
cylinder specimens of size 150 mm
diameter and 300 mm long. Recently,
70 mm cube or 75 mm cylinder
HSC or UHSC specimen is being
recommended for situations in which P
P P P
machine capacity may be exceeded
Non-explosive Explosive Ideal failure
(Graybeal and Davis 2008).
(a) (b)
The concrete is poured in the cube
FIG. 1.16 Cube testing and failure of concrete cubes (a) Cubes in testing machine (b) Failure of
or cylinder mould in layers of 50 mm
concrete cubes
and compacted properly by either
hand or a vibrator so that there are no voids. The top surface Factors Affecting Compressive Strength
of these specimens should be made even and smooth by
The compressive strength of concrete is affected by the
applying cement paste and spreading smoothly on whole area
following important factors: w/c or w/cm ratio, type of
of specimen. The test specimens are then stored in moist air
cement, use of supplementary cementitious materials, type
of at least 90 per cent relative humidity and at a temperature
of aggregates, quantity and quality of mixing water, moisture
of 27°C ± 2°C for 24 hours. After this period, the specimens
and temperature conditions during curing, age of concrete,
are marked and removed from the moulds and kept submerged
rate of loading during the cube or cylinder test (the measured
in clear fresh water, maintained at a temperature of 27°C ±
compressive strength of concrete increases with increasing
2°C until they are tested (the water should be renewed every
rate of loading), and the size of specimen.
seven days). The making and curing of test specimen at site is
The w/c ratio is inversely related to concrete strength: the
similar (see also Clause 3.0 of IS 516).
lower the ratio, the greater the strength. It is also directly
These specimens are tested by compression testing machine
linked to the spacing between cement particles in the cement
after 7 days of curing or 28 days of curing. Load should be
paste. When the spacing is smaller, cement hydrates fill the
applied gradually at the rate of 140 kg/cm2 per minute until
gaps between the cement particles faster and the links created
the specimen fails. Load at the failure divided by the area
by the hydrates will be stronger, resulting in stronger concrete
of specimen gives the compressive strength of concrete.
(Bentz and Aïtcin, 2008). Various mathematical models have
A minimum of three specimens, preferably from different
been developed to link strength to the porosity of the hydrates.
batches, should be tested at each selected age. If the strength
In 1918, Abrams presented his classic law of the following
of any specimen varies by more than ±15 per cent of average
form (Shetty 2005):
strength, results of such specimen should be rejected (Clause
15.4 of IS 456). The average of three specimens gives the k1
fc,28 = (1.2a)
compressive strength of concrete. Sampling and acceptance k2wc
criteria for concrete strength, as per IS 456, are provided in
where fc,28 is the 28-day compressive strength, k1 and k2 are
Section 4.7.4 of Chapter 4. (In the USA, the evaluation of
the empirical constants, and wc is the w/c ratio by volume.
concrete strength tests is done as per ACI 214R-02.) Figure 1.16
For 28-day strength of concrete recommended by ACI
shows the cube testing and various failure modes of concrete
211.1-91, the constants k1 and k2 are 124.45 MPa and 14.36,
cubes. The ideal failure mode, with almost vertical cracks (see
respectively. Popovics (1998) observed that these values are
Fig. 1.16b) is rarely achieved due to the rough contact surface
conservative and suggested the values 187 MPa and 23.07,
between the concrete cube and the plate of testing machine.
respectively, for k1 and k2. Abrams’ w/c ratio law states that
When the stress level reaches about 75–90 per cent of the
the strength of concrete is dependent only upon the w/c ratio, Powers. It has to be noted that this relation is independent of the
provided the mix is workable. Abram’s law is a special case age of the concrete and the mix proportions. This equation is
of the following Feret formula developed in 1897 (Shetty valid for many types of cement, but the values of the numerical
2005): coefficients vary a little depending on the intrinsic strength
⎛ ⎞ of the gel. Such models that focus only on the cement paste
Vc
fc,28 k⎜ (1.2b) ignore the effects of the aggregate characteristics on strength,
⎝ Vc Vw + Va ⎟⎠ which can be significant. A comparison of these mathematical
where Vc, Vw, and Va are the absolute volumes of cement, models is provided by Popovics (1998). Based on the strength
water, and entrained air, respectively, and K is a constant. In vs w/c ratio curves provided in the earlier version of IS 10262,
essence, strength is related to the total volume of voids and Rajamane (2005) derived the following equation.
the most significant factor in this is the w/c ratio. The graph fc, 28 = 0.39 fcem [(1 / wc ) − 0.50 ] (1.2d)
showing the relationship between the strength and w/c ratio is
approximately hyperbolic in shape (see Fig. 1.17). where fcem is the 28-day compressive strength of cement tested
as per IS 4031(MPa) and wc is the w/c ratio by weight.
Many researchers have also attempted to estimate the
50 strength of concrete at 1, 3, or 7 days and correlate it to the 28-
day strength. This relationship is useful for formwork removal
and to monitor early strength gain; however, it depends on
many factors such as the chemical composition of cement,
fineness of grinding, and temperature of curing. The 7-day
40
Co strength is often estimated to be about 75 per cent of the
m
pr 28-day strength (Neville 2012). Neville, however, suggests
es
siv that if the 28-day strength is to be estimated using the
es
tre strength at 7 days, a relationship between the 28-day and
Strength, MPa
ng
30 th 7-day strengths has to be established experimentally for the
given concrete. For concrete specimens cured at 20°C, Clause
3.1.2(6) of Eurocode 2 (EN 1992-1-1:2004) provides the
following relationship.
20 ⎡ ⎛ ⎛ 28 ⎞0.5 ⎞ ⎤
fcm (t ) = exp ⎢ s ⎜ 1 − ⎜ ⎟ ⎟ ⎥ fcm (1.3a)
⎜ ⎟
⎣⎢ ⎝ ⎝ t ⎠ ⎠ ⎥
where fcm(t) is the mean compressive strength at age t days, fcm
10
Flexural strength is the mean 28-day compressive strength, and s is a coefficient
(modulus of rupture) depending on the type of cement; s = 0.2, 0.25, and 0.38 for
high early strength, normal early strength, and slow early
strength cement, respectively. ACI Committee 209.2R-08
0 recommends the relationship for moist-cured concrete made
0.4 0.5 0.6 0.7
Water – cement ratio, by weight
with normal Portland concrete as
FIG. 1.17 Relation between strength and w/c ⎛ t ⎞
fcm(t ) = ⎜ ⎟ fc 28 (1.3b)
ratio of normal concrete ⎝ a + bt ⎠
At a more fundamental level, this relation can be expressed The values of constants a and b are 4.0 and 0.85, respectively,
as a function of the gel/space ratio (x), which is the ratio for normal Portland cement and 2.3 and 0.92, respectively,
of the volume of the hydrated cement paste to the sum for high early strength cement. The 1978 version of IS 456
of the volumes of the hydrated cement and the capillary specified an ‘age factor’, based on Eq. (1.3b), using a = 4.7
voids. The data from Powers (1961) gives the following and b = 0.833, but that provision has been deleted in the 2000
relationship: version of the code.
fc,28 = 234x3 MN/m2 (1.2c) Influence of Size of Specimen
were x is the gel/space ratio and 234 is the intrinsic strength of The pronounced effect of the height/width ratio and the cross-
the gel in MPa for the type of cement and specimen used by sectional dimension of the test specimen on the compressive
In the case of cubes, the specimens are placed in the testing

1.00 machine in such a way that the load is applied on opposite sides
Correction factor
of the cube as cast, that is, not to the top and bottom. On the
0.96
other hand, cylinders are loaded in the direction in which they
are cast. Because of this reason and also because the standard
cylinders have height/width ratio of two, the compressive
0.92 strengths predicted by cylinders are more reliable than cubes.
0.88
1.8.3 Stress–Strain Characteristics
1.0 1.2 1.4 1.6 1.8 2.0 Typical stress–strain curves of normal weight concrete of
Height/diameter ratio various grades, obtained from uniaxial compression tests, are
FIG. 1.18 Correction factor for height/diameter ratio of cylinder shown in Fig. 1.19(a) and a comparison of normal weight and
strength has been observed by several researchers. The lightweight concrete is shown in Fig. 1.19(b). (The idealized
difference in compressive strength of different sizes of stress–strain curve for concrete, and the assumed stress
specimens may be due to several factors such as St Venant’s block adopted in IS 456 are given in Fig. 5.4 in Section 5.4
effect, size effect, or lateral restraint effect due to the testing of Chapter 5). It has to be noted that, for design, the value
machine’s platen (Pillai and Menon 2003). In addition, the of maximum compressive strength of concrete in structural
AQ10
preparation of the end conditions (cappings) of the concrete elements is taken as 0.85 times the cylinder strength, fc′,
cylinder can significantly affect the measured compressive which is approximately equal to 0.67fck.
strength. When the height/diameter ratio of cylinders is less It is seen from Fig.1.19 that the curves are initially linear
than 2.0, IS 516:1959, suggests a correction factor as shown in and become non-linear when the stress level exceeds about
Fig. 1.18. Standard cubes with height/width ratio of 1.0 have 40 per cent of the maximum stress. The maximum stress is
been found to have higher compressive strength than standard reached when the strain is approximately 0.002; beyond this
cylinders with height/diameter ratio of 2.0. The ratio of standard point, the stress–strain curve descends. IS 456 limits the
cylinder strength and standard cube strength is about 0.8–0.95; maximum failure strain in concrete under direct compression
higher ratio is applicable for HSC. Similarly 100 mm × 200 mm to 0.002 (Clause 39.1a) and under flexure to 0.0035 (Clause
cylinders exhibit 2–10 per cent higher strengths than 150 mm 38.1b). The shape of the curve is due to the formation of
× 300 mm cylinders; the difference is less for higher strength micro-cracks within the structure of concrete. The descending
concrete (Graybeal and Davis 2008). It has to be noted that branch of the curve can be fully traced only with rigid testing
the ACI code formulae, which are based on standard cylinder machines. In axially flexible testing machines, the test cube
strength, fc′ have been converted to standard cube strength, fck, or cylinder will fail explosively when the maximum stress is
for easy comparison, by using the relation fc′ = 0.8fck throughout reached.
this book. A more precise coefficient R to convert cylinder Numerical approximations of stress-strain curves of
strength to cube strength is R = 0.76 + 0.2 log( fc′/20). concretes have been provided by various researchers, and a
comparison of these formulae is provided by Popovics (1998).
70
45
60
50 Normal
concrete
30
Stress, MPa
Stress, MPa
40
30
15
20 Lightweight concrete
10
0
0 0.001 0.002 0.003 0.004
0 0.001 0.002 0.003 0.004 0.005 0.006 Strain (mm/mm)
Strain (mm/mm)
(a) (b)
FIG. 1.19 Typical stress–strain curves of concrete in compression (a) Concrete with normal weight aggregates (b) Normal weight vs lightweight
aggregate concrete
Such a mathematical definition of stress–strain curve is diagonal tension failure. The design shear strength of concrete
required for non-linear analysis of concrete structures. HSCs is given in Table 19 of IS 456 as a function of percentage
exhibit more brittle behaviour, which is reflected by the shorter flexural reinforcement. The maximum shear stress in concrete
horizontal branch of stress–strain curves. with shear reinforcement is restricted in Clause 40.2.3 to the
following value:
1.8.4 Tensile Strength
As mentioned earlier, concrete is very week in tension, t c,max = 0.63 fck (1.6)
and direct tensile strength is only about 8–11 per cent of More discussions on shear strength of concrete are provided
compressive strength for concretes of grade M25 and above in Chapter 6.
(Shetty 2005). The use of pozzolanic admixtures increases the
Bond strength The common assumption in RC that plane
tensile strength of concrete. Although the tensile strength of
sections remain plane after bending will be valid only if there is
concrete increases with an increase in compressive strength,
perfect bond between concrete and steel reinforcement. Bond
the rate of increase in tensile strength is of the decreasing order
strength depends on the shear stress at the interface between
(Shetty 2005). The tensile strength of concrete is generally
the reinforcing bar and the concrete and on the geometry of
not taken into account in the design of concrete elements.
the reinforcing bar. Clause 26.2.1.1 of IS 456 provides a table
Knowledge of its value is required for the design of concrete
for design bond stress and is approximately represented by
structural elements subject to transverse shear, torsion, and
shrinkage and temperature effects. Its value is also used in tbd = 0.16(fck)2/3 (1.7)
the design of prestressed concrete structures, liquid retaining
More discussions on bond strength of concrete are provided
structures, roadways, and runway slabs. Direct tensile
in Chapter 7.
strength of concrete is difficult to determine. The splitting
(cylinder) tensile test on 150 mm × 300 mm cylinders, as per
1.8.5 Bearing Strength
IS 5816:1999, or the third-point flexural loading test on 150
mm × 150 mm × 700 mm concrete beams, as per IS 516:1959, The compressive stresses at supports, for example, at the base
is often used to find the tensile strength. The splitting tensile of column, must be transferred by bearing (Niyogi 1974).
test is easier to perform and gives more reliable results than Clause 34.4 of IS 456 stipulates that the permissible bearing
other tension tests; though splitting strength may give 5–12 stress on full area of concrete in the working stress method
per cent higher value than direct tensile strength (Shetty can be taken as 0.25fck and for limit state method it may be
2005). According to Mehta and Monteiro (2010), the third- taken as 0.45fck. According to Clause 10.4.1 of ACI 318, the
point flexural loading test tends to overestimate the tensile design bearing strength of concrete should not exceed j 0.85
strength of concrete by 50–100 per cent. fc′, where j is the strength reduction factor, which is taken as
The theoretical maximum flexural tensile stress occurring 0.65. Thus, it is approximately equal to 0.442fck.
in the extreme fibres of RC beams, which causes cracking,
1.8.6 Modulus of Elasticity and Poisson’s Ratio
is referred to as the modulus of rupture, fcr. Clause 6.2.2
of IS 456 gives the modulus of rupture or flexural tensile Concrete is not an elastic material, that is, it will not recover its
strength as original shape on unloading. In addition, it is non-linear and
exhibits a non-linear stress–strain curve. Hence, the elastic
fcr fck (1.4) constants such as modulus of elasticity and Poisson’s ratio are
not strictly applicable. However, they are used in the analysis
It should be noted that Clause 9.5.2.3 of ACI 318 code
and design of concrete structures, assuming elastic behaviour.
suggests a lower, conservative value for the modulus of
The modulus of elasticity of concrete is a key factor for
rupture, which equals l0.55√fck, where l is the modification
estimating the deformation of buildings and members as well
factor for lightweight concrete and equals 1.0 for normal
as a fundamental factor for determining the modular ratio, m.
weight concrete, 0.85 for sand-lightweight concrete, and
The use of HSC will result in higher modulus of elasticity
0.75 for all lightweight concrete. IS 456 does not provide an
and in reduced deflection and increased tensile strengths. The
empirical formula for estimating the direct tensile strength, fct.
modulus of elasticity is primarily influenced by the elastic
Clause R8.6.1 of ACI 318 suggests an average splitting tensile
properties of the aggregates and to a lesser extent by the curing
strength of
conditions, age of the concrete, mix proportions, porosity of
fct 5 fck (1.5) concrete, and the type of cement. It is normally related to the
compressive strength of concrete and may be determined by
Shear strength Pure shear is of rare occurrence; usually a means of an extensometer attached to the compression test
combination of flexural and shear stresses exists, resulting in a specimen as described in IS 516:1959.
The Young’s modulus of elasticity may be defined as the ratio Both IS 456 and ACI 318 caution that the actual measured
of axial stress to axial strain, within the elastic range. When values may differ by ±20 per cent from the values obtained
the loading is of low intensity and of short duration, the initial from Eq. (1.2). Moreover, the US code value is 16 per cent less
portion of the stress–strain curve of concrete in compression than the value specified by the Indian code. It has to be noted
is linear, justifying the use of modulus of elasticity. However, that the use of lower value of Ec will result in a conservative
when there is sustained load, inelastic creep occurs even at (higher) estimate of the short-term elastic deflection.
relatively low stresses, making the stress–strain curve non- The ACI committee report on HSC (ACI 363R-92) suggests
linear. Moreover, the effects of creep and shrinkage will make the following equation, which has been adopted by NZS 3101-
the concrete behave in a non-linear manner. Hence, the initial Part 1:2006 and CSA A23.3-04:
tangent modulus is considered to be a measure of dynamic
for Ec = (2970√fck + 6900) (rc/2300)1.5 N/mm2
modulus of elasticity (Neville and Brooks 2010).
26 MPa < fck < 104 MPa (1.8d)
When linear elastic analysis is used, one should use the
static modulus of elasticity. Various definitions of modulus Noguchi, et al. (2009) proposed the following equation which
of elasticity are available: initial tangent modulus, tangent is applicable to a wide range of aggregates and mineral
modulus (at a specified stress level), and secant modulus (at admixtures used in concrete.
a specified stress level), as shown in Fig. 1.20. Among these,
Ec = k1k2 × 3.
3 36 10 4 ( rc / 2400)2 ( fck / 75)1 3 N/mm2 (1.8e)
the secant modulus, which is the slope of a line drawn from
the origin to the point on the stress–strain curve corresponding where the correction factors k1 and k2 are given in Tables 1.21
to 40 per cent of the failure stress, is found to represent the and 1.22.
average value of Ec under service load conditions (Neville
and Brooks 2010). Clause 6.2.3.1 of IS 456 suggests that the TABLE 1.21 Values of correction factor k1
short-term static modulus of elasticity of concrete, Ec, may be Type of Coarse Aggregate Value of k1
taken as Crushed limestone, calcined bauxite 1.20
Ec = 5000√fck N/mm 2
(1.8a) Crushed quartzite aggregate, crushed andesite, crushed 0.95
basalt, crushed clay slate, crushed cobblestone
Coarse aggregate other than above 1.0
Ultimate stress TABLE 1.22 Values of correction factor k2

Type of Mineral Admixture Value of k2
Tangent
modulus Silica fume, GGPS, fly ash fume 0.95
Stress (MPa)
Specified stress Fly ash 1.10

Unloading curve Mineral admixture other than the above 1.0
The dynamic modulus of elasticity of concrete, Ecd, corresponds

Initial tangent
modulus to a small instantaneous strain. It can be determined by the
non-destructive electro-dynamic method, by measuring the
natural frequency of the fundamental mode of longitudinal
Secant modulus
vibration of concrete prisms, as described in IS 516:1959. The
dynamic modulus of elasticity has to be used when concrete is
Strain (mm/mm) used in structures subjected to dynamic loading (i.e., impact
FIG. 1.20 Various definitions of modulus of elasticity of concrete or earthquake). The value of Ecd is generally 20 per cent,
30 per cent, and 40 per cent higher than the secant modulus
As per Clause 8.5.1 of ACI 318, the modulus of elasticity for for high-, medium- and low-strength concretes, respectively
concrete may be taken as (Mehta and Monteiro 2006).
Ec = rc1 5 0.038 fck N/mm2 (1.8b) Poisson’s ratio is defined as the ratio of lateral strain to the
longitudinal strain, under uniform axial stress. Experimental
where rc is the unit weight of concrete (varies between studies have predicted widely varying values of Poisson’s
1440 kg/m3 and 2560 kg/m3). For normal weight concrete, ratio, in the range of 0.15–0.25. A value of 0.2 is usually
ACI code allows it to be taken as (assuming rc = 2300 N/mm2) suggested for design for both NSCs and HSCs. For lightweight
Ec = 4200√fck N/mm2 (1.8c) concretes, the Poisson’s ratio has to be determined from
tests.
1.8.7 Strength under Combined Stresses and additive. The total shrinkage strain in concrete is composed
Structural members are usually subjected to a combination of the following:
of forces, which may include axial force, bending moments, 1. Autogenous shrinkage, which occurs during the hardening
transverse shear, and twisting moments. Any state of combined of concrete (Holt 2001)
stress acting at any point in a member may be reduced to three 2. Drying shrinkage, which is a function of the migration of
principal stresses acting at right angles to each other on an water through hardened concrete
appropriately oriented elementary cube in the material. Any
or all of the principal stresses can be either compression or Drying shrinkage, often referred to simply as shrinkage, is
tension. When one of these three principal stresses is zero, caused by the evaporation of water from the concrete. Both
a state of bi-axial stress exists; if two of them are zero, the shrinkage and creep introduce time-dependent strains in
state of stress is uni-axial. In most of the situations, only concrete. However, shrinkage strains are independent of the
the uniaxial strength properties are known from simple tests stress conditions of concrete. Shrinkage can occur before
described in this chapter. The failure strength under combined and after the hydration of the cement is complete. It is most
stresses is usually defined by an appropriate failure criterion. important, however, to minimize it during the early stages
Until now, neither a general theory of strength of concrete of hydration in order to prevent cracking and to improve the
under combined stresses nor a universally accepted failure durability of the concrete. Shrinkage cracks in RC are due
criterion has been proposed. to the differential shrinkage between the cement paste, the
However, the strength of concrete for biaxial state of stress aggregate, and the reinforcement. Its effect can be reduced
has been established experimentally by Kupfer, et al. (1969) by the prolonged curing, which allows the tensile strength of
(see Fig. 1.21). This figure shows that under biaxial tension, the concrete to develop before evaporation occurs. The most
the strength is close to that of uniaxial tension. When one important factors that influence shrinkage in concrete are (a)
principal stress is tension and other is compressive, the concrete type and content of aggregates, (b) w/c ratio, (c) effective age
cracks at a lower stress than it would have in uniaxial tension at transfer of stress, (d) degree of compaction, (e) effective
or compression. Under biaxial compression, the strength is section thickness, (f) ambient relative humidity, and (f)
presence of reinforcement (ACI 209R-92). AQ11
greater than the uniaxial compression by about 27 per cent.
Shrinkage strain is expressed as a linear strain (mm/mm).
f1/fc'(Compression) In the absence of reliable data, Clause 6.2.4.1 of IS 456
recommends the approximate value for the total shrinkage
strain for design as 0.0003. (ACI 209R-92 suggests an average
1.0 value of 780 × 10−6 mm/mm for the ultimate shrinkage
fc'
strain, esh). Different models for the prediction of creep under
compression and shrinkage induced strains in hardened
f2
concrete are presented and compared in ACI 209.2R-08.

=
0.6 f1
f1
Long-term deflection calculations considering the effects of

f2 shrinkage and creep are covered in Chapter 12.
0.2
f2/fc' − 0.2 0.2 0.6 1.0 Temperature Effects
(Tension) ft' f2/fc'(Compression) Concrete expands with rise in temperature and contracts with
− 0.2 fall in temperature. The effects of thermal contraction are
similar to the effects of shrinkage. To limit the development
f1/fc'(Tension) of temperature stresses, expansion joints are to be provided,
FIG. 1.21 Strength of concrete in biaxial stress especially when there are marked changes in plan dimensions.
Source: Kupfer, et al. 1969 (adapted) In addition, when the length of the building exceeds 45 m,
expansion joints are to be provided, as per Clause 27 of IS
1.8.8 Shrinkage and Temperature Effects 456. Temperature stresses may be critical in the design of
concrete chimneys and cooling towers. Roof slabs may
As shrinkage and temperature effects are similar, they are
also be subjected to thermal gradient due to solar radiation.
both considered in this section.
In large and exposed surfaces of concrete such as slabs,
Shrinkage Effects nominal reinforcements are usually placed near the exposed
Shrinkage and creep are not independent phenomena. For surface to take care of temperature and shrinkage stresses.
convenience, their effects are treated as separate, independent, The coefficient of thermal expansion depends on the type of
cement and aggregate, cement content, relative humidity, and
the size of section. Clause 6.2.6 of IS 456 provides a table to loading. As per IS 456, the ultimate creep strain ecp is to be
choose the value of coefficient of thermal expansion based calculated from the creep coefficient Ct (q in IS nomenclature)
on the aggregate used. However, SP 24:1983 recommends a given in Clause 6.2.5.1. Calculation of long-term deflection
value of 11 × 10−6 mm/mm per degree Celsius for the design due to creep is provided in Section 12.4.1 of Chapter 12.
of liquid storage structures, bins and chimneys, which is close More information on creep, shrinkage, and temperature
to the thermal coefficient of steel (about 11 × 10−6 mm/mm effects may be obtained from the work of Bamforth, et al.
per degree Celsius). The calculation of deflection due (2008).
to temperature effects is discussed in Section 12.4.3 of
Chapter 12. More discussions on thermal and shrinkage 1.8.10 Non-destructive Testing
effects are provided in Section 3.9.2 of Chapter 3. Non-destructive tests are used to find the strength of existing
Fire design of concrete structures is outside the scope concrete elements. They are classified as follows:
of this book. When exposed to fire, both concrete and steel
1. Half-cell electrical potential method to detect the
reinforcement of RC members lose 60 per cent of their
corrosion potential of reinforcing bars in concrete
characteristic strength at a temperature of 500°C. Where
2. Schmidt/Rebound hammer test (IS 13311-Part 2:1992) to
HSCs are used, consideration should be given to mitigate the
evaluate the surface hardness of concrete
effects of spalling (e.g., use of fibre reinforcement, sacrificial
3. Carbonation depth measurement test to determine
concrete layers, thermal barriers, and fire-resisting concrete.).
whether moisture has reached the depth of the reinforcing
More information on fire design may be found in fib reports
bars, thereby leading to corrosion
(2007, 2008).
4. Permeability test to measure the flow of water through the
1.8.9 Creep of Concrete concrete
5. Penetration resistance or Windsor probe test to measure
Creep in concrete is the gradual increase in deformation
the surface hardness and hence the strength of the surface
(strain) with time in a member subjected to sustained loads.
and near-surface layers of the concrete
The creep strain is much larger than the elastic strain on
6. Covermeter test to measure the distance of steel reinforcing
loading (creep strain is typically two to four times the elastic
bars beneath the surface of the concrete and the diameter
strain). If the specimen is unloaded, there is an immediate
of the reinforcing bars
elastic recovery and a slower recovery in the strain due to
7. Radiographic test to detect voids in the concrete and the
creep (see Fig. 1.22). Both amounts of recovery are much less
position of prestressing ducts
than the original strains under load. If the concrete is reloaded
8. Ultrasonic pulse velocity test (IS 13311-Part 1:1992)
at a later date, instantaneous and creep strains develop again.
mainly to measure the time of travel of ultrasonic pulse
Creep occurs under both compressive and tensile stresses
passing through the concrete and hence concrete quality
and always increases with temperature. HSCs creep less
9. Sonic methods, which use an instrumented hammer
than NSCs. When the stress in concrete does not exceed one-
providing both sonic echo and transmission methods, to
third of its characteristic compressive strength, creep may be
predict the integrity of piles and bridge decks
assumed proportional to the stress (Clause 6.2.5 of IS 456).
10. Tomographic modelling, which uses the data from
It has to be noted that, unlike concrete, steel will creep only
ultrasonic transmission tests in two or more directions, to
above 700°F.
detect voids in concrete
11. Impact echo testing to detect voids, delamination, and
other anomalies in concrete
Instantaneous recovery 12. Ground penetrating radar or impulse radar testing to
Unloading
Delayed detect the position of reinforcing bars or stressing ducts
Strain
Creep strain creep 13. Infrared thermography to detect voids, delamination, and
recovery
other anomalies in concrete and also to detect water entry
Residual points in buildings
Elastic strain creep strain
The details of these tests may be found in ACI 228.1R-03
Time since application of load manual and the work of Malhotra and Carino (2003).
FIG. 1.22 Typical creep curve
The main factors affecting creep strain are the concrete mix
1.9 DURABILITY OF CONCRETE
and strength, the type of aggregate used, curing, ambient Although several unreinforced concrete structures, built
relative humidity, and the magnitude and duration of sustained 2000 years ago, such as the Pantheon in Rome and several
aqueducts in Europe, are still in excellent condition, many 4. Specific gravity of materials is as follows:
RC structures built in the twentieth century have deteriorated (a) Coarse aggregate: 2.68
within 10–20 years. In several countries such as the USA, (b) Fine aggregate: 2.65
about 40–50 per cent of the expenditure in the construction (c) Chemical admixture: 1.145
industry is spent on repair, maintenance, and rehabilitation of 5. Water absorption is as follows:
existing structures. These deteriorating concrete structures not (a) Coarse aggregate: 0.6 per cent
only affect the productivity of the society but also have a great (b) Fine aggregate: 1.0 per cent
impact on our resources, environment, and human safety. It 6. Free (surface) moisture data is as follows:
has been realized that the deterioration of concrete structures (a) Coarse aggregate: Nil (absorbed moisture also nil)
is due to the main emphasis given to mechanical properties (b) Fine aggregate: Nil
and the structural capacity and the neglect of construction 7. Sieve analysis data is as follows:
quality and life cycle management (ACI 201.2R-08). Strength (a) Coarse aggregate: Conforming to Table 2 of IS
and durability are two separate aspects of concrete; neither 383:1970
will guarantee the other. Hence, clauses on durability were (b) Fine aggregate: Conforming to grading zone 1 of Table 4
included for the first time in the fourth revision of IS 456, of IS 383:1970
published in 2000 (see Clause 8 of the code).
SOLUTION:
As per Clause 8.1 of IS 456, a durable concrete is one that
Step 1 Calculate the target strength for mix proportioning.
performs satisfactorily in the working environment of anticipated
From Eq. (1.1)
exposure conditions during its service life. The following
factors affect the durability of concrete: (a) Environment, fck′ = fck + 1 65 × s
(b) concrete cover to the embedded steel, (c) quality and type
From Table 8 of IS 456 (see Table 1.13), standard deviation
of constituent materials, (d) cement content and w/c ratio of
for M25, s = 4 N/mm2
concrete, (e) degree of compaction and curing of concrete, and
Therefore, target strength = 25 + 1.65 × 4 = 31.6 N/mm2
(f) shape and size of member. The prescriptive requirements
given in IS 456 are discussed in Section 4.4.5 of Chapter 4. The Step 2 Select the w/c ratio. From Table 5 of IS 456 (Table 4.5),
requirement of concrete exposed to sulphate attack is provided maximum water cement ratio for moderate exposure is 0.50.
in Clause 8.2.2.4 and Table 4 of IS 456. Guidance to prevent Adopt w/c ratio as 0.45 < 0.50.
alkali–aggregate reaction is given in Clause 8.2.5.4 of IS 456.
Step 3 Select of water content. From Table 2 of IS 10262,
Maximum water content = 186 kg (for 25–50 mm slump and
AQ11 EXAMPLES for 20 mm aggregate)
EXAMPLE 1.1 (Mix proportioning for M25 concrete): Estimated water content for 75 mm slump = 186 + 3/100 ×
Calculate the mix proportioning for M25 concrete if the 186 = 191.58 kg
following are the stipulations for proportioning: As superplasticizer is used, the water content can be reduced
to more than 20 per cent. Based on trials with superplasticizer,
1. Grade designation: M25 water content reduction of 20 per cent has been achieved.
2. Type of cement: OPC 43 grade conforming to IS 8112 Hence, the assumed water content = 191.58 × 0.80 = 153.2 kg.
3. Maximum nominal size of aggregate: 20 mm
4. Exposure condition: Moderate Step 4 Calculate the cement content.
5. Minimum cement content (Table 5 of IS 456): 300 kg/m3 w/c ratio = 0.45
6. Maximum w/c ratio: 0.45
7. Workability: Slump 75 mm Cement content = 153.2/0.45 = 340.4 kg/m3
8. Method of concrete placing: Pumping From Table 5 of IS 456 (Table 4.5), minimum cement
9. Degree of supervision: Good content for moderate exposure condition = 300 kg/m3. Since
10. Type of aggregate: Crushed angular aggregate 340.4 kg/m3 > 300 kg/m3, it is acceptable.
11. Maximum cement content: 450 kg/m3 Step 5 Determine the proportion of volume of coarse
12. Chemical admixture type: Superplasticizer aggregate and fine aggregate content. From Table 3 of IS 10262
The test data for materials is as follows: (Table 1.15), volume of coarse aggregate corresponding to
20 mm size aggregate and fine aggregate (Zone 1) for w/c ratio
1. Cement used: OPC 43 grade conforming to IS 8112 of 0.50 is 0.60. We now have w/c ratio as 0.45. Therefore, the
2. Specific gravity of cement: 3.15 volume of coarse aggregate has to be increased to decrease
3. Chemical admixture: Superplasticizer conforming to IS the fine aggregate content. As the w/c ratio is lower by 0.05,
9103 the proportion of volume of coarse aggregate is increased by
0.01 (at the rate of −/+0.01 for every +0.05 change in the w/c Corrected water content = 153.2 + 878 (0.01) + 1085
ratio). Therefore, corrected proportion of volume of coarse (0.006) = 168.49 kg
aggregate for the w/c ratio of 0.45 is 0.61. The estimated batch masses (after corrections) are as
follows:
Note: Even if the selected coarse aggregate is not angular, the Cement = 340.4 kg /m3
volume of coarse aggregate has to be increased suitably, based Water = 168.5 kg/m3
on experience. Fine aggregate = 878.0 kg/m3
For pumpable concrete, these values should be reduced by Coarse aggregate = 1085 kg/m3
10 per cent. Superplasticizer = 3.4 kg/m3
Therefore, volume of coarse aggregate = 0.61 × 0.09 = 0.55 Two more trial mixes with variation of ±10 per cent of w/c
Volume of fine aggregate content = 1 − 0.55 = 0.45 ratio should be carried out to achieve the required slump and
Step 6 Perform the mix calculations. The mix calculations dosage of admixtures. A graph between the three w/c ratios
per unit volume of concrete shall be as follows: and their corresponding strengths should be plotted to correctly
determine the mix proportions for the given target strength.
1. Volume of concrete = 1 m3
2. Volume of cement = Mass of cement/Specific gravity of EXAMPLE 1.2 (Mix proportioning for M25 concrete, using fly
cement × 1/1000 ash as part replacement of OPC):
Calculate the mix proportioning for M25 concrete with the
a = 340.4/3.15 × 1/1000 = 0.108 m3 same stipulations for proportioning and the same test data for
3. Volume of water = Mass of water/Specific gravity of water × materials as given in Example, 1.2, except that fly ash is used
1/1000 as part replacement of OPC.
b = 153.2/1 × 1/1000 = 0.153 m3 SOLUTION:

Considering the same data as in Example 1.1 for M25 concrete,
4. Volume of chemical admixture (superplasticizer) (at 1.0 the mix proportioning steps from 1 to 3 will remain the same.
per cent by mass of cementitious material) The procedure of using fly ash as a partial replacement to
c = Mass of chemical admixtures/Specific gravity of OPC has been explained in step 4.
admixture × 1/1000 Step 4 Calculate the cement content.
= 3.4/1.145 × 1/1000 = 0.00297m3 From Example 1.1, cement content = 340.4 kg/m3
5. Total volume of aggregate (coarse + fine) Now, to proportion a mix containing fly ash, the following
steps are suggested:
d = [1 − (a + b + c)] = 1 − (0.108 + 0.153 + 0.00297)
1. Decide percentage of fly ash to be used based on project
= 0.736 m3
requirement and quality of materials.
6. Mass of coarse aggregate = d × volume of coarse aggregate × 2. In certain situations, increase in cementitious material
specific gravity of coarse aggregate × 1000 = 0.736 × 0.55 × content may be warranted.
2.68 × 1000 = 1084.86 kg
The decision to increase cementitious material content and
7. Mass of fine aggregate = d × volume of fine aggregate ×
its percentage may be based on experience and trial. Let
specific gravity of fine aggregate × 1000 = 0.736 × 0.45 ×
us consider an increase of 10 per cent in the cementitious
2.65 × 1000 = 877.68 kg
material content.
Step 7 Determine the mix proportions for trial number 1. Cementitious material content = 340.4 × 1.1 = 374.4 kg /m3
Cement = 340.40 kg/m3 Water content = 153.2 kg/m3 (from Example 1.1)
Water = 153.2 kg/m3 Hence, w/c ratio = 153.2/374.4 = 0.41
Fine aggregate = 878 kg/m3 Fly ash at 35 per cent of total cementitious material content =
Coarse aggregate = 1085 kg/m3 374.4 × 35% = 131 kg/m3
Chemical admixture = 3.4 kg/m3 Cement (OPC) content = 374.4 − 131 = 243.4 kg/m3
W/c ratio = 0.45 Saving of cement while using fly ash = 374.4 − 243.4 =
97 kg/m3
The following are the adjustments for moisture in aggregates
Fly ash being utilized = 131 kg/m3
and water absorption of aggregates and the correction for
aggregates: Step 5 Determine the proportion of volume of coarse aggregate
Free (surface) moisture is nil in both fine and coarse and fine aggregate content. From Table 3 of IS 10262 (Table
aggregates. 1.15), the volume of coarse aggregate corresponding to 20 mm
size aggregate and fine aggregate (Zone I) for w/c ratio of 0.50 is 5. Volume of chemical admixture (superplasticizer) (at 0.8
0.60. In this example, w/c ratio is 0.41. Therefore, the volume of per cent by mass of cementitious material)
coarse aggregate is required to be increased to decrease the fine d = Mass of chemical admixture/Specific gravity of
aggregate content. As the w/c ratio is lower by approximately admixture × 1/1000
0.10, the proportion of volume of coarse aggregate is increased = 3/1.145 × 1/1000 = 0.0026 m3
by 0.02 (at the rate of −/+0.01 for every +0.05 change in the w/c
6. Total volume of aggregate (coarse + fine)
ratio). Therefore, the corrected proportion of volume of coarse
e = [1 − (a + b + c + d )]
aggregate for the w/c ratio of 0.40 is 0.62.
= 1 − (0.0773 + 0.0655 + 0.153 + 0.0026) = 0.7016 m3
Note: Even if the selected coarse aggregate is not angular, the
7. Mass of coarse aggregate
volume of coarse aggregate has to be increased suitably, based
= e × volume of coarse aggregate × Specific gravity of
on experience.
coarse aggregate × 1000 = 0.7016 × 0.56 × 2.68 × 1000 =
For pumpable concrete, these values should be reduced by
1053 kg
10 per cent.
8. Mass of coarse aggregate
Therefore, volume of coarse aggregate = 0.62 × 0.09 = 0.56
= e × volume of fine aggregate × specific gravity of fine
Volume of fine aggregate content = 1 − 0.56 = 0.44
aggregate × 1000 = 0.7016 × 0.44 × 2.65 × 1000
Step 6 Perform the mix calculations. The mix calculations = 818 kg
per unit volume of concrete shall be as follows:
Step 7 Determine the mix proportions for trial number 1.
1. Volume of concrete = 1 m3
Cement = 243.4 kg/m3
2. Volume of cement = Mass of cement/Specific gravity of
Fly ash = 131 kg/m3
cement × 1/1000
Water = 153 kg/m3
a = 243.4/3.15 × 1/1000 = 0.0773 m3 Fine aggregate = 818 kg /m3
Coarse aggregate = 1053 kg/m3
3. Volume of fly ash = Mass of fly ash/Specific gravity of fly
Chemical admixture = 3 kg/m3
ash × 1/1000
W/c ratio = 0.41
b = 131/2.0 × 1/1000 = 0.0655 m3
Note: The aggregate should be used in saturated surface dry
4. Volume of water = Mass of water/Specific gravity of condition. As mentioned in Example 1.1, three trial mixes
water × 1/1000 with slightly varying w/cm ratio has to be made to determine
experimentally the exact mix proportions that will result in the
c = 153.2/1 × 1/1000 = 0.153 m3
required workability, strength, and durability.
SUMMARY
Concrete technology has advanced considerably since the discovery of admixtures with the ingredients of cement, as they may ultimately
the material by the Romans more than 2000 years ago. A brief history affect the performance of concrete. Proportioning of concrete mixes,
of developments that resulted in the current day RC is provided. The as per the latest IS 10262:2009, is described. Hydration of cement
advantages and drawbacks of concrete as a construction material and heat of hydration are also described. In addition to the ordinary
are listed. Cement is the most important ingredient of concrete as it concrete, we now have a host of different types of concretes, such as
binds all the other ingredients such as fine and course aggregates. The RMC, HPC, SCC, SLWC, AAC, FRC, DFRC (which include ECC,
cements that are in use today include OPC, rapid hardening Portland UHPC, SIFCON and SIMCON), polymer concrete, and ferrocement.
cement, low heat Portland cement, sulphate-resisting Portland cement, They are used in some situations to achieve strength and durability.
PSC, PPC, and ternary blended cement. Making and properties of When reinforcing steel (often called as rebar) is placed inside a
these various types of cements are briefly discussed. The three grades concrete mass (they are often placed in the tension zone, as concrete
of cement and their properties are also provided. The fine and coarse is weak in tension), the solidified mass is called RC. Though
aggregates occupy about 60–75 per cent of the concrete volume (70– traditionally mild steel was used as rebar, a number of different
85 per cent by mass) and hence strongly influence the properties of types of rebars are now available and include hot rolled HYSD, hard
fresh as well as hardened concrete, its mixture proportions, and the drawn wire fabric, TMT bars, and TMT CRS bars. The mechanical
economy. Mixing water plays an important role in the workability, properties of these steel bars are also provided. A brief description
strength, and durability of concrete. Hence, their properties and use of the corrosion of steel bars, which is mainly responsible for the
in concrete are briefly discussed. deterioration of RCC Structures all over the world, is also included.
As we now use a variety of chemical and mineral admixtures to Corrosion may be mitigated by the use of fusion-bonded epoxy-
improve properties of concrete, a brief introduction to them is also coated rebars, galvanized rebars, FRP bars, basalt bars, or TMT
provided. It is important to realize the chemical interaction of these CRS bars.
In order to get quality concrete, careful mixing, placing, discussed. Expressions for finding compression strength at any day,
compacting, and curing of concrete is necessary at site. Forms should modulus of elasticity, and tensile, shear, bond, and bearing strengths,
be removed only after concrete has gained sufficient strength to carry are provided as per Indian codes and compared with the provisions
at least twice the stresses it may be subjected to at the time of removal of the US code. Discussions on strength under combined stresses and
of forms. Important properties of concrete such as workability of shrinkage, temperature, and creep effects are also included. Various
concrete (usually measured by slump test), compressive strength non-destructive tests performed on concrete to assess the strength
(measured by conducting tests on carefully made and cured cubes of existing structures are also listed. Two examples are provided to
or cylinders on the 28th day), stress–strain characteristics, tensile explain the mix proportioning of concrete.
and bearing strength, modulus of elasticity, and Poisson’s ratio are
REVIEW QUESTIONS
1. Write a short history of concrete, beginning with the Roman 24. Name any three types of concretes.
concrete. 25. Why is it better to use RMC than site-mixed concrete?
2. What are the advantages and drawbacks of concrete? 26. As per IS 456, which of the following is considered as standard
3. Compare the major properties of steel, concrete, and wood. concrete (NSC)?
4. What are the processes by which modern cement is made? (a) M25–M60, (b) M30–M50,
Explain the dry process of cement manufacture. (c) M50–M75, (d) M20–M40
5. List five different cements that are in use today. 27. As per IS 456, which of the following is considered as HSC?
6. What are the three different grades of cements used in India? (a) M50–M80, (b) M65–M100,
How is the grade of cement fixed? (c) M60–M90, (d) M50–M90
7. How does the fineness of cement affect the concrete? 28. How does HPC differ from HSC?
8. What are the four major compounds used in cement? How do 29. Write short notes on the following:
they affect the different properties of concrete? (a) HPC (b) SCC
9. How is PPC manufactured? What are its advantages? (c) FRC (d) SLWC
10. How is PSC manufactured? What are its advantages? (e) DFRCC (f) SIFCON and SIMCON
11. Name any three tests that are conducted on cement. (g) Ferrocement
12. What are the different classifications of aggregates? List five 30. How are TMT bars manufactured? How do they differ from cold
factors of aggregates that may affect the properties of concrete. twisted deformed bars?
13. The specific gravity of gravel is _______. 31. Draw the stress–stain curve for mild steel bars and HYSD bars.
(a) 2.80 (b) 2.85 32. As per IS 13920, which of the following should not be used in
(c) 2.67 (d) 3.10 earthquake zones?
14. The maximum size of coarse aggregate used in concrete is the (a) Bars of grade Fe 500 and above
lesser of _______. (b) Bars of grade Fe 550 and above
(a) one-fourth the size of member, 5 mm less than max. clear (c) Bars of grade Fe 600 and above
distance between bars, and min. cover (d) All of these
(b) one-fourth the size of member and 20 mm 33. When does corrosion of rebars take place? What are the different
(c) one-fourth the size of member, 5 mm less than max. clear methods adopted to mitigate corrosion?
distance between bars, and 10 mm less than min. cover 34. Name three types of rebars that are used in corrosive
15. Can sea water be used for mixing or curing of concrete? State environments.
the reason. 35. State the three methods by which concrete is compacted.
16. Name any three chemical admixtures used in concrete. 36. What is workability of concrete? Name and describe the test to
17. Name any two compounds used as superplasticizers in India. measure workability.
18. Name any three mineral admixtures used in concrete. 37. How is compressive strength of concrete determined?
19. Write short notes on the following: 38. Name any three factors that may affect the compressive strength
(a) Fly ash, of concrete.
(b) silica fume, 39. How does the stress–strain curve of HSC differ from NSC?
(c) GGBS, 40. Write the expressions of modulus of elasticity, tensile, shear,
20. What are the main objectives of concrete mix proportioning? and bearing strength of concrete as per IS 456.
21. How is target mean compressive strength fixed for mix 41. Write short notes on shrinkage, temperature, and creep effects
proportioning? of concrete.
22. How is initial w/c ratio assumed in mix proportioning? 42. Name any three non-destructive tests performed on concrete.
23. What is meant by hydration of cement? What is heat of hydration?
EXERCISES
1. Determine the mix proportioning for M30 concrete for the data 2. Determine the mix proportioning for M30 concrete for the data
given in Example 1.1. given in Example 1.1, with fly ash as part replacement of OPC.
REFERENCES
ACI 201.2R-08 2008, Guide to Durable Concrete, American Concrete Bamforth, P., D. Chisholm, J. Gibbs, and T. Harrison 2008, Properties of
Institute, Farmington Hills, p. 49. Concrete for Use in Eurocode 2, The Concrete Centre, Surrey, p. 53.
ACI 209R-92 1992, Prediction of Creep, Shrinkage, and Temperature Bapat, J. D. 2012, Mineral Admixtures in Cement and Concrete, CRC
Effects in Concrete Structures, American Concrete Institute, Press, Boca Raton, p. 310.
Farmington Hills, p. 47. Basu, P. C., P. Shylamoni, and A. D. Roshan 2004, ‘Characterisation of
ACI 211.1-91 1991, Standard Practice for Selecting Proportions for Steel Reinforcement for RC Structures: An Overview and Related
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INTRODUCTION TO

Storing in cement silos

7 Cooling and storing of clinker

1 Quarrying raw materials

TABLE 1.3 (Continued) 1. 33 grade OPC, IS 269:1989

Notes: 1. Tricalcium silicate C3S 45–65 Alite

CaCO3 (Limestone) TABLE 1.6 Role of different compounds on properties of cement

Fe2O3 (Iron oxide) C3S C 2S C3A C4AF

TABLE 1.8 (Continued) workability, pumpability, shrinkage, and other properties of

3. 40 165 10 0.50 0.48 0.46 0.44

1.5 TYPES OF CONCRETE 1.5.1 Ready-mixed Concrete

Structure should be adopted to control plastic and autogenous shrinkage

(a) (b) Bar diameter:

and the works of Neville (1996), Clarke (1993), and Chandra

Autoclaved Aerated Concrete

Tensile stress (a) (b)

Ultra-high-performance Concrete 400

Ultra-high-performance concrete is a high-strength, high- 350

with a semi-tempered middle ring of 300

(2003), Neville (2012), Neville and

1.8.1 Workability of Concrete Slump

In the case of cubes, the specimens are placed in the testing

Ultimate stress TABLE 1.22 Values of correction factor k2

Specified stress Fly ash 1.10

The dynamic modulus of elasticity of concrete, Ecd, corresponds

concrete are presented and compared in ACI 209.2R-08.

Long-term deflection calculations considering the effects of

b = 153.2/1 × 1/1000 = 0.153 m3 SOLUTION:

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