Construction Material
Construction Material
Construction Material
The factors which form the basis of various systems of classifications of materials
in material science and engineering are:
A. . Metallic material
Ferrous
Wrought iron
Cast iron
Steel
Non Ferrous
Aluminum
Copper
Lead
Zinc
Tin
a) solid
b) Liquid
c) Gaseous
1. Physical properties:
Density, specific gravity, porosity, water absorption, etc....
2. Mechanical properties:
Tensile strength, compressive strength, rigidity, hardness. Creep,
fatigue......
3. Thermal properties:
Thermal conductivity, thermal expansion and other.......
4. Chemical properties:
Resistance to acids, alkalis, brines and oxidation.
Density: - Density is defined as mass per unit volume for a material. The derived
unit usually used by engineers is the kg/m3. Relative density is the density of
the material compared with the density of the water at 4˚C. Table gives densities
for some materials in kg/m3.
Unit weight γ
𝛾=𝑊/𝑉 or 𝛾=𝜌.𝑔
Porosity (n)
It is the ratio of the volume of the spaces in the material to the overall volume.
It is the ratio between the sizes of voids to the volume of solid material.
Water absorption
It denotes the ability of the material to absorb and retain water. It is expressed
as percentage in weight or of the volume of dry material:
Weathering resistance
It is the ability of a material to endure alternate wet and dry conditions for a long
period without considerable deformation and loss of mechanical strength.
Water permeability
Frost Resistance
2. Mechanical Properties
When forces are applied to a solid body, two results are produced
There are several types of stress which depend on types of applied load. These
stresses can be classified as:
1- Compression stress
2- Tension stress
3- Shear stress
4- Bending stress
5- Torsion stress
Loading causes materials to deform and, if high enough, to break down and fail.
All loading on materials can be considered combinations of three basic types:-
Flexible materials on the other hand have low modulus of elasticity and bend
considerably without breakdown. Tough materials withstand heavy shocks.
Toughness depends upon strength and flexibility. Malleable materials can be
hammered into sheets without rupture. It depends upon ductility and softness
of material. Copper is the most malleable material. Hard materials resist
scratching and denting, for example cast iron and chrome steel. Materials
resistant to abrasion such as manganese are also known as hard materials.
Cementitious Materials
In a general sense, these are materials with adhesive and cohesive properties
and are capable of uniting or bonding solid particles together. This definition
covers a large number of very different substances.
Binders are substances which are used to bind inorganic and organic particles
and fibers to form strong, hard and/or flexible components. Cementing materials
that are widely used for construction are materials that exhibit characteristic
properties of setting and hardening when mixed to a paste with water
Mineral binders
Bituminous binders
Natural binders
Synthetic binders.
Mineral Binders
Hydraulic binders
The most common hydraulic binder is cement. Hydraulic binders are usually
available in the form of a fine powder: the finer they are ground (usually in a ball
mill), the larger is the specific surface area (of the sum of the particles) per unit
weight. And the larger the surface area, the more effective and complete is the
chemical reaction with the water that it comes into contact with.
Non-hydraulic binders
The most common non-hydraulic binder is lime (CaO). Hardening depends in its
combination with carbon dioxide from the air (carbonation), by which it again
becomes calcium carbonate (limestone).
Thermoplastic binders
Bituminous Binders
When bitumen is used, it must be either heated or mixed with solvents like
gasoline, kerosene or naphtha, which is called "bitumen cutback"; or dispersed
in water, which is called "bitumen emulsion".
Natural Binders
Varieties of binders are obtained from plants and animals, and can be used in
their natural form or after processing.
Examples of natural binders are plant juices (e.g. juice of banana leaves; latex of
certain trees). Natural binders have played an important role in traditional
constructions since ancient times, but, nowadays face low social acceptance.
Synthetic Binders
Cementitious materials may be classified in several different ways. One way often
used is by the chemical constituent responsible for setting or hardening the
cement.
Gypsum cements are widely used in interior plaster and for fabrication of boards
and blocks; but the solubility of gypsum prevents its use in construction exposed
to any but extremely dry climates.
Other cementitious materials, such as polymers, fly ash, and silica fume, may
be used as a cement replacement in concrete. Polymers are plastics with long-
chain molecules. Concretes made with them have many qualities much superior
to those of ordinary concrete.
2.2 Lime
It involves burning of the raw material and then slaking. Depending on the
amount of heat and the method of slaking, the product is hydraulic or non-
hydraulic lime. The raw material is burnt in a vertical kiln.
B. Hydrated/Slaked lime
Quick lime can never be used as such for construction purposes but must be
mixed with water. This process is called slaking or hydration of lime. The
resulting product is calcium hydroxide [Ca (OH) 2] and is called slacked or
hydrated lime.
It is ready to be made into plaster or mortar by adding water and sand to form a
temporary plastic mass. There are two types of slaking: Wet-slaking and Dry-
slaking, depending on the amount of water added.
Wet-slaking
Lime is slaked or hydrated at the building site by mixing quick lime with an
excess of water and the resulting slaked lime should be passed through a fine
sieve to remove slow slaking particles and then left to mature for several days.
This can be made in a watertight box or a hole dug in the ground. The lime most
be continually stirred by a shovel or a stick during the slaking process to reduce
all a hydrated particles which might hydrate later in the building and cause
popping, pitting and disintegration.
Dry-Slaking
Quicklime has so much affinity for water and will quickly absorb moisture from
the atmosphere and becomes air-slaked, and loses its cementing qualities. It
must therefore be kept in dry storage and carefully protected from dampness
until used.
Slaked lime hardens or sets by gradually losing its water through evaporation
and absorbing CO2 from the air, thus changing back from Ca (OH) 2 to CaCO3
or limestone.
1. When burnt, the limestone loses its carbon dioxide and becomes oxide of lime
or quicklime.
2. When slacked, the oxide of lime combine with water and becomes hydroxide,
slacked, or hydrated lime.
3. When setting, the calcium hydroxide loses its water through evaporation and
absorbs carbon dioxide from the air, becoming Ca C03 or limestone once more.
The limes are classified as fat lime, hydraulic lime and poor lime:
(ii) Hydraulic lime: It contains clay and ferrous oxide. Depending upon
the percentage of clay present, the hydraulic lime is divided into the
following three types:
(iii) Poor lime: It contains more than 30% clay. Its color is muddy. It has
poor binding property. The mortar made with such lime is used for
inferior works.
(i) For white washing, it gives a sparkling white finished at a very low cost.
(ii) For making mortar for masonry works and plastering.
(iii) To produce lime sand bricks.
(iv) Lime is used as a stabilizer in soil constructions with clayey soils,
because the lime reacts with clay to form a binder
(v) As a refractory material for lining open hearth furnaces.
(vi) For making cement
(vii) As a raw material for the manufacture of glasses.
Problems
Soil stabilization with lime requires more than twice the curing time
needed for soils stabilized with cement.
If quicklime is stored in moist conditions (even humid air), it will hydrate.
Hydrated lime, stored for long periods, gradually reacts with the carbon
dioxide in the air and becomes useless.
2.2 Gypsum
Gypsum Plasters
Gypsum plasters are used in the arts and in building construction. In all these
powders, gypsum in a more or less dehydrated state is the essential element.
When water is added to these substances they become dehydrated, forming
compounds similar to those existing before calcinations.
Manufacture of plasters
Gypsum plasters are manufactured by heating the raw material gypsum at either
moderate or high temperatures the results being plaster of Paris or hard-finish
plaster.
Plaster of Paris
Plaster of Paris is a white powder having a specific gravity of 2.57. This plaster
is also known as low-temperature gypsum derivative or semi-hydrated plasters
(semi hydrate). When mixed with sufficient water to form a plastic paste it sets
very rapidly, the whole process taking only 5 to 10 minutes. The setting of
gypsum derivative is not a chemical change as in the setting of carbonate of lime
but is due to the recombination of the dehydrated lime sulfate, CaSO4 or
CaSO4.1/2 H2O with water to form the original hydrated sulfate CaSO4.2H2O.
Re-crystallization takes place, the dehydrate precipitates from the solution to
form a solid mass of fine interlocking crystals, the material hardening back to its
original state and attaining its ultimate strength on drying.
In many instances, in order that the workman may properly handle Plaster of
Paris (stucco), it is necessary to delay the time of setting. This is accomplished
by adding a fraction of 1 percent of a retardant like glue and sawdust after the
plaster has cooled. Plaster of Paris while setting under water does not gain
strength and ultimately, on continued water exposure, will disintegrate. In
hardening, plaster of Paris first shrinks and then expands. The latter property
makes the material valuable in making casts, since a sharp impression of the
mold can be secured. Owing to the rapidity of set and difficulty in working its
use in structures is limited to ornamental work. It produces hard surfaces, sharp
contours, and is sufficiently strong.
Hard-Finish Plaster
This Plaster is less soluble with consequent reluctance to absorb water in the
process of re-crystallization, which must occur in order to develop a plaster
comparable with the original gypsum formula. The result is a plaster too slow in
setting action for practical purpose.
2.3 Cement
Raw materials
Modern Portland cement is made from materials which must contain the proper
proportions of lime (CaO), silica (SiO2), alumina (Al2O3), iron (Fe203) with miner
amounts of magnesia and sulfur trioxide.
The methods of manufacture depend on the hardness of the raw materials used
and on their moisture content.
The mixing and grinding of the raw materials can be done either in water or in
dry conditions and named wet and dry processes respectively.
Dry Process:
The raw materials are subjected to processes as: crushing, drying, grinding and
proportioning & blending (mixing) before they are fed into the kiln for burning.
The crushing stage involves breaking the raw materials to small fragments that
vary in size between 6-19mm. Primary crusher machines are used for this
purpose. The drying stage is typical of the dry process. Drying of the crushed
materials is essential and is achieved by heating the materials separately at
temperatures sufficiently high to drive out uncombined water. Heating is done
in a rotary drying kiln.
The grinding of each material as obtained from the dryers is done in two stages.
First preliminary grinding in which the materials are reduced to 0.3mm and then
the fine grinding in which the size of the materials are reduced to 0.075mm.
Each raw material is thus reduced to a required degree of fineness and is stored
in suitable storage tanks called silos or bins where from it can be drawn out
conveniently in requisite quantities.
2. Burning or calcinations:
The well- proportioned finely powdered mixture (raw meal) is charged into long
steel cylinder, called rotary kiln. Rotary kilns differ in design and dimensions in
accordance with the production requirements. Thus, these may be 100-180m in
length, 3- 5m in diameter and have rotation of 60-90 revolutions per hour.
The raw mixture is burnt in the kiln till the proper burning is achieved. This is
indicated by its taking a greenish black color and virtuous luster. This burnt
material called clinker is cement in composition but not in size: it is in about
walnut sized lumps when it comes out of the kiln.
During the burning process water is completely driven off at a very initial
stage of burning at a temperature as low as 400 0C and dissociation of
carbonates in to calcium oxide and carbon dioxide takes place at 800-900 0C
temperature range. Then finally compound formations will proceed and it
starts at 1200 0C and requires a temperature as high as 1500 0C
The completely burnt or calcined raw materials of cement are converted to lump
shaped product called clinker, which is drawn out from the lower end of the
rotary kiln. It is extremely hot when discharged, and is therefore first cooled and
both (clinker and gypsum) are sent for pulverizing. The mixture is reduced to an
extremely fine powder by grinding it.
Wet process:
1. Preparation of slurry
2. Burning or calcinations and
3. Treatment of clinker
1. Preparation of Slurry
In wet process, raw materials are supplied to the kiln in the form of an intimate
mixture with a lot of water in it. This is called slurry. To obtain the slurry of
standard composition, the raw materials are first crushed separately using
crushers for limestone and grinding mills (wet) for clays. These crushed materials
are stored in separate tanks or silos. They are drawn from the silos in prefixed
proportions in to the wet grinding mills where, in the presence of a lot of water,
thus ground to fine thin paste. This is slurry, which is stored in a silo (storage
tower). Its composition is tested once again and corrected by adding limestone
slurry in required proportions such corrected slurry is then fed into the rotary
kiln
2. Burning:
For burning of slurry, a rotary kiln of almost similar type is used as described
under dry process. All the moisture is driven off the slurry as it passes through
the drying zone. Chemical compounds will be produced in the burning process.
3. Grinding of Clinker:
The relative amounts of these four chemicals in the final product depend on the
desired properties of the cement concrete such as rate of hardening, amount of
heat given off, and resistance to chemical attack.
The main product, 3CaO.2SiO2.3H2O is calcium silicate hydrate gives cement its
strength. The total amount of water required to complete the hydration of the
cement is about 25% of the mass of the cement. The proportion of C3S ranges
from 25-60%.
In some applications, a very low early strength may be a disadvantage for this
reason-modified cement was developed in the U.S. This cement has a higher rate
of heat development than Type IV cement and a rate of gain of strength similar
to that of type I cement. Type II cement is recommended for structures where a
moderately low heat generation is desirable or where moderate sulphate attack
may occur. Modified (Type II) cement is an example of a ‘compromise’ all round
cement
Improved workability
No increase in dry shrinkage
Improved resistance to sulfate attack etc.
A variety of special cements exist that are limited for specific uses and purposes.
The following are few examples:
Setting is the term used to describe the stiffening of the cement paste, although
the definition of the stiffness of the paste, which is considered, set is somewhat
arbitrary. Broadly speaking, setting refers to a change from a fluid to a rigid
stage. Although, during setting, the paste acquires some strength, for practical
purposes it is important to distinguish setting from hardening, which refers to
the gain of strength of set cement paste. Some cement set quickly, within few
minutes, others may take comparatively longer time. Moreover, setting is not an
abrupt process, which may complete immediately after its start; it is rather a
progressive phenomenon, which has beginning, full development and an end. It
is on this latter basis, setting is distinguished into initial and final setting
qualified by the time required in each case.
Heat of hydration is the quantity of heat in joules per gram generated when
cement and water react. The most common method of determining the heat of
hydration is by measuring the heats of solution of anhydrate and hydrated
cement in mixture of nitric and hydrofluoric acids: the difference between the
two values represents the heat of hydration.
For practical purpose, it is not necessarily the total heat of hydration that
matters but the rate of heat evolution. The amount of heat generated is
dependent chiefly on the chemical composition, fineness of the cement, and the
temperature of curing time.
The fineness of the cement also influences the rate of heat development, an
increase in. fineness speeding up the reaction of hydration and therefore the heat
evolved. It is reasonable to assume that the early rate of hydration of each
compound in cement is proportional to the surface area of the cement. However,
at later stages, the effect of the surface area is negligible and the total amount of
heat evolved is not affected by the fineness of cement.
For massive structures such as dams and retaining walls cement, such as
low heat cement is used which generates lesser rate of heat of hydration
Cement Composition
Fineness of the cement
Water/cement ratio
Age of paste
Ambient condition
b) Chemical Properties
Loss on ignition:
Insoluble Residue
The mass of insoluble residue shall not exceed 1.5 percent. The insoluble residue
is that part of a cement sample which is insoluble in hydrochloric acid (HCl). It
derives from the clay minerals of the raw materials, which have not reacted to
form the cement compound, which are soluble in this acid. Hence the amount of
insoluble residue indicates the efficiency of the burning process the
completeness of the reactions in the kiln.
2.4 MORTAR
Workability
Strength
Water tightness
Workability
For the same proportions, lime-sand mortar invariably gives better workability
than Portland cement-sand mortar
At times plasticizers and air-entraining agents are used in order to improve the
workability of cement-sand mortars, especially when they are lean (i.e.
containing less amount of cement) mixes.
Strength
Water tightness
With the cement content, materials, and workability all constant, strength and
degree of water-tightness increase with the density of the mix.
50 Kg cement=35 liters
Box size:40cm X 35cmX 25cm
For masonry:
For bricklaying:
Concrete
The strength and quality of concrete depend not only on the quality and
quantity of the materials, but on the procedures used in combining these
materials and the skill involved in the placing and curing of concrete.
a) Cement
b) Water
Water serves two purposes in making concrete. First of all, it causes the
hydration of cement and secondly, it makes the mix fluid and workable. Clean
water is important for the same reasons, as is clean aggregate; any impurities
present will affect bond strength between the paste and aggregate.
Some of the impurities in mixing water that cause these undesirable effects in
the final concrete are
1. Dissolved Chemicals.
2. Seawater
3. Sugar
4. Algae Water
Dissolved chemicals may either accelerate or retard the set and can substantially
reduce the concrete strength further such dissolved chemicals can actively
attack the cement sand bond leading to early disintegration of the concrete.
Sea water containing less than 3% salt is generally acceptable for plain concrete
but not for reinforced concrete. The presence of salt can lead to corrosion of the
reinforcing bars and a decrease in concrete strength by some 10-15%.
If sugar is present in even small amounts, it can cause rapid setting and reduced
concrete strength. Algae can cause a reduction in the strength of concrete by
increasing the amount of air captured in the paste and reducing the bond
strength between the paste and the aggregate.
It should be pointed out that the total amount of water required per unit volume
of fresh concrete depends on a number of factors that are:
Rounded grains will move more easily as the concrete is placed. Long and thin
aggregate will weaken concrete. The aggregates used in concrete may be natural
aggregates, such as sand and gravels taken directly from riverbank or gravel
deposits, or they may be byproducts of an industrial process (e.g. blast-furnace
slag).
Workability when fresh for which the size and gradation of the aggregate
should be such that undue labor in mixing and placing will not be
required.
Strength and durability when hardened - for which the aggregate should:
a) Be stronger than the required concrete strength
b) Contain no impurities which affect strength and durability
c) Contain no silt which affect the adhesive strength between aggregate
and cement paste (this is mainly a problem in relation to fine aggregate)
d) Be resistant to weathering action
Economy of the mixture- meaning to say that the aggregate should be:
a) Available from local and easily accessible deposit or quarry
b) Well graded in order to minimize cement paste, hence cement, requirement
Classification of Aggregates
There are three main classes of aggregates differing in their chemical composition
and these are derived from argillaceous (composed primarily of Al2O3), siliceous
PROPERTIES OF AGGREGATES
Gradation
In general, it is desirable that the size increases uniformly from fine sand to the
maximum allowed for a given job.
Most specifications for concrete require a grain size distribution that will provide
a dense, strong mixture.
Aggregates may be dense, gap-graded, uniform, well graded, or open- graded. "
For sieve analysis, a sample of aggregate is first surface dried and then sieved
through the series, starting with the largest. The weight retained on each sieve
is recorded and the percentage computed. The summation of the cumulative
percentage of the material retained on the sieves (not including the intermediate
sieves) divided by 100 is called Fineness modulus (FM).
Smaller sieves are not included but coarse ones are used if necessary. The
smaller the value of fineness modulus, the finer is the sand. The finesse modulus
for good sand should range between 2.25 -3.25.
Very fine sand and very coarse sand are objectionable, fine sand is uneconomical
and coarse sand give harsh unworkable mixes. Fineness modulus of sand varies
as under:
Note: 2", 1”, ½ " sieves are called "Intermediate" & are not included for the
fineness modulus calculations.
Fine aggregate has a nominal maximum size of 4.75 (No.4 sieve) Therefore
specifications will require that 100% of the aggregate pass the 9.5mm (3/6")
sieve, and 90 (or 95%) pass 4.75mm.
The maximum size and grading are important because they affect:
The particle shape and the surface texture of aggregates influence the properties
of fresh concrete more than those of hardened concrete. Sharp, angular, and
rough aggregate particles require more paste to make good concrete than do
rounded ones.
Specific Gravity
Apparent specific gravity: is the ratio of the weight in air of a material of given
volume (solid matter plus impermeable pores or voids) to the weight in air of an
equal volume of distilled water.
Bulk specific gravity: is defined as the ratio of the weight in air of a given volume
of a permeable material (including both its permeable and impermeable voids) to
the weight in air of an equal volume of water.
Bulk specific gravity (SSD basis): is defined as the ratio of the weight in air of a
permeable material in a saturated surface dry condition to the weight in air of
an equal volume of water.
The specific gravity of most normal weight aggregate will range from 2.4 to 2.9.
Absorption
Over a 24-hr period lightweight aggregates may absorb water in the amount of 5
to 20 percent of their own dry weight, depending on the type of aggregate and its
pore structure. A tendency of this sort must be taken into account when concrete
is made with lightweight aggregate. To make light weight mixtures as uniform as
Moisture Content
Large shrinkage: fine grained sandstones, slate, basalt, trap rock, clay-
containing
Low shrinkage: quartz, limestone, granite, feldspar
d) Admixtures
Admixtures are ingredients other than water, aggregates, hydraulic cement, and
fibers that are added to the concrete batch immediately before or during mixing.
A proper use of admixtures offers certain beneficial effects to concrete, including
improved quality, acceleration or retardation of setting time, enhanced frost and
sulfate resistance, control of strength development, improved workability, and
enhanced finish ability.
a) Water - reducers
b) Set – retarders
c) Accelerators
Accelerating admixtures are added to concrete either to increase the rate of early
strength development or to shorten the time of setting, or both. Chemical
compositions of accelerators include some of inorganic compounds such as
soluble chlorides, carbonates, silicates, fluosilicates, and some organic
compounds such as triethanolamine.
d) Superplasticizers
The use of superplasticizers (high range water reducer) has become a quite
common practice. These classes of water reducers were originally developed in
Japan and Germany in the early 1960s; they were introduced in the United
States in the mid-1970s.
a) Workability
It is the property of fresh concrete that determines the ease with which a material
can be used to give a product of the required properties or it is the property of
fresh concrete that determines the amount of work required for placement and
compaction that determines the resistance to segregation.
b) Consistency
Slump is the subsidence of concrete cone after mold is lifted up. Slump test is
made in laboratory and on site to measure subsidence of a pile of concrete in a
mold (slump test apparatus of dimensions: base diameter = 20 cm, top diameter
= 10cm, and height =30 cm.) compacted with a steel rod (16 mm diameter and
60cm long).
The volume of fresh concrete is equal to the sum of the absolute volumes of
its components, including the naturally entrapped or purposely entrained air.
If VA = Volume of air
Vw = volume of water
Then the total volume of the fresh compacted concrete will be:
From the point of view of concrete technology it would be best to prescribe mix
proportions by the "absolute volume" of the ingredients, because the volume of
the resulting concrete and its properties are dependent on the absolute volume
and not on their weight or bulk volume. But this is an impractical way to
proportion materials, because the absolute volume of the ingredients cannot be
𝑊
V= ………….….. (2)
1000𝐺
Where: V is the absolute volume in m3
W is the weight of the material in kg
G is the specific gravity of the material & 1000 is the density or unit
weight of fresh water in kg/m3
The specific gravity of cement may be taken, for all practical purposes, equal to
3.15. For calculating the volumes of the aggregates we use their specific gravity
(bulk, saturated surface dry basis), which is defined by " the ratio of the weight
in air or the S.S.D. aggregates (i.e., including their voids) to the weight of an
equal volume of water:
Substituting weight and specific gravities in equation (2) for absolute volumes
in equation (1) we get the volume of concrete in m3 as follows
𝑊𝑤 𝑊𝑐 𝑊𝑓𝑎 𝑊𝑐𝑎
V=Va + + + + ………………… (3)
1000 1000𝐺𝑐 1000𝐺𝑓𝑎 1000𝐺𝑐𝑎
If the cement, water, and air contents per m3 of fresh concrete are known, then
the required weight of the aggregates for a cubic meter of fresh concrete can
easily be calculated from Eq. (3).
𝑊𝑒𝑖𝑔ℎ𝑡𝑜𝑓𝑚𝑎𝑡𝑒𝑟𝑖𝑎 𝐾𝑔
Volume of Bulk material =
𝑈𝑛𝑖𝑡𝑤𝑒𝑖𝑔ℎ𝑡 𝐾𝑔/𝑚3
The desired characteristics of concrete vary from one construction to the other
and as such, they should be considered in relation to the quality required.
Strength
Durability
The continuous phase in concrete is the hardened cement paste (cement stone):
hence, it can rightly be said that the properties of concrete are closely related to
that of the structure of the paste, and as such stronger concretes are nearly
impermeable and consequently durable, they are also stiffer; however, they
usually exhibit higher drying shrinkage which might finally result in cracking.
a) Water/cement ratio
As far as mix proportions are concerned, this is the most important factor
affecting strength for given materials. Lower water/cement ratios lead to higher
strengths. The effect may broadly be considered as the same as that of
compaction, higher water/cement ratios resulting in more porous cement paste
and hence, lower strength. The strength water/ cement ratio relationship is in
fact, approximately logarithmic in the normal strength range the log of strength
increasing uniformly with reduction in water/cement ratio. Illustrating this
point, the strength of concrete is increased by 25% by reducing the
water/cement ratio from 0.6 to 0.5 and further 25% increases would be obtained
by further reductions to 0.4 and 0.3. Clearly, when the added advantage of high
durability is considered, there would seem to be great benefit in producing
powerful compaction methods for concrete of low workability and low
water/cement ratio, though these techniques are most suited to factory
production.
b) Age
Typical age factors are shown in the following table. For 0.6 water/cement,
concrete continuously cured, the relationship between 7 and 28 days strengths
has been the subject of particular interest, the traditional working guide being
that 28 days compressive strength is 5% greater than 7 day strength. This rule
is no longer accurate for modern cements where the hydration process occurs
more rapidly, the 28-day strength now showing a smaller percentage increase
over the 7-day value, a rise of less than 30% being more likely.
Compressive strength
Destructive tests
a) Cube test
This is currently the most common type of destructive test for concrete, owing to
the cheapness of the cube molds and the comparative simplicity of a
manufacture and testing of cubes. Carefully obtained samples of the concrete
mix are placed and compacted in steel molds. Bonding with the steel is prevented
by coating with release agent. After 24 hours the cube is removed and cured
under water until tested. The cube is ten placed centrally between the plates of
a compression-testing machine, trowel led face sideways, and the load is applied
such that the stress increases at a given constant rate until failure. The
maximum load is recorded. Cube in sizes of either 200 mm, 150 mm or 100 mm
are common.
b) Cylinder tests
These have some advantages and some disadvantages compared with the cube
test. Since only the cylinder ends are loaded, the body of the mound need not be
machined and can be formed from cheaper materials, such as plastics on the
other hand, the ends of the cylinder must be of accurate tolerance, requiring
capping of one or both ends, dependent upon weather a machined base-plate is
used. The normal height to width ratio of a cylinder is 2:1 so that platen restrain
is less than in a cube, leading to lower apparent strengths. When correlating to
cube strengths, a ratio of 1:25 is generally taken and a further factor will need
to be included if the height diameter ratio is not 2:1
Cylinder testing has been in use for many years in the form of testing of cores
cut from the concrete. This allow visible examination and strength testing of the
.3.2 Durability
a) Physical, i.e. weathering, due to the action of rain, freezing and thawing
and dimensional changes (expansion and contraction) resulting from
temperature variations and/or alternate wetting and drying,
b) Chemical, due to aggressive waters containing sulfates, leaching in
hydraulic structures and chemical corrosion, and
c) Mechanical wear, by abrasion from pedestrian or vehicular use, by wave
action in structures along the seashore or erosion from the action of
flowing water.
The resistance of concrete to the effect of weather, to salt scaling and to chemical
attack, to mechanical damage resulting from abrasion or impact are the different
aspects of durability of concrete; and the concrete that withstands the conditions
it is intended for, without deteriorating, over a long period of time, is said to be
durable.
In countries with temperate and tropical climate such as Ethiopia, the problem
of freezing and thawing does not practically exist; however, it is quite possible
that a concrete in service becomes exposed to chemical attack. Chemical attack
is brought about by the penetration of various agents of the environment (such
as reactive liquids particularly sulfate polluted air, etc.) into the mass of the
It was shown that the properties of a freshly mixed as well as the resulting
hardened concrete are closely associated with the characteristics and relative
proportions of the component materials. It is therefore obvious that by
determining the relative quantities of the component materials prior to mixing,
one can produce a concrete of desired properties. This process is known as mix
design or mix proportion.
At present there are a number of methods of mix design established and used in
different countries. Although different in few details, all mix design procedures
have the prime objective of obtaining the most economical mix proportions of
cement, water, fine and coarse aggregates and occasionally admixtures, to
produce concrete of desired properties when fresh as well as hardened.