Cec 313 Lect Note
Cec 313 Lect Note
Cec 313 Lect Note
CEC 313
LECT NOTE. FOR HND1.
producing and using good concrete in construction works. Good concrete provides the
bonding properties required to transfer load from one reinforcement to the order, thereby
providing the desired resistance of the member against the applied loads, dead and
imposed loads.
• Concrete is a combination of different material (fine and coarse aggregates) held together
to form a rocklike mass with a paste of cement / water and sometimes one or more
admixtures are added to change certain properties of the concrete such as its workability,
tension, but quite good in compression. Its weakness in tension mostly call for the
introduction of steel to make it composite called reinforced concrete, very widely used in
construction material in the construction industry, because of it frequently used, but the
cost of production has negative environmental effects, from the extraction of raw materials
to the finishing point, with its high embodied energy, this has led specialists to search new
• Classification of Concrete.
Concrete can be classify into different types, depending on the followings: Type of
material used in making the concrete, (Cement concrete, Polymer concrete and Asphaltic
concrete) Nature of stress conditions and Density of the concrete. But our interest is on
cement concrete.
• Types of Concrete
Plain Concrete
The plain concrete will have no iron bar in it. The main ingredients are the cement, fine and coarse
aggregates, with water. Most commonly used mix design 1:3:6 which is the normal mix design.
The density of the plain concrete will vary between 2200 and 2500Kg/m3. The compressive
strength is 7-15KN/mm2 .These types of concrete are mainly used in the construction of the
pavement and buildings, especially in areas where there is less demand of high tensile strength.
Reinforced Concrete
The reinforced cement concrete is defined as the concrete to which reinforcement is introduced to
bear the tensile strength. Plain concrete is weak in tension and good in compression. Hence the
placement of reinforcement will take up the responsibility of bearing the tensile stresses. R.C.C
works with the combined action of the plain concrete and the reinforcement. The steel
reinforcement used in the concrete can be in the form of rods, bars or in the form of meshes. Now
fibers are also developed as reinforcement. Fiber reinforced concrete are concrete that use fibers
(steel fibers) as reinforcement for the concrete. Whatever be the type of reinforcement used in
concrete, it is very necessary to ensure proper bond between the concrete and the reinforcement.
This bond will control the strength and the durability factors of the concrete.
Pre stressed Concrete
Mega concrete projects are carried out through prestressed concrete units. This is a special
technique in which the concrete is subjected to external forces which produces a beneficial initial
stress distribution before working loads are brought into operation. The reinforcement or the
tendons used in the concrete is stressed before the actual service load application. During the
mixing and the placing of the concrete, these tensioned bars placed firmly and held from each end
of the structural unit. Once the concrete set sand harden, the structural unit will be put in
compression. This phenomenon of prestressing will make the lower section of the concrete
member to be stronger against the tension. The process of prestressing will require heavy
equipment and labor skill (jacks and equipment for tensioning). Hence the prestressing units are
made at site and assembled at site. These are used in the application of bridges, heavy loaded
The term lightweight concrete which is preferred by the American Concrete Institute (ACI, 213)
as being concrete produce with lightweight coarse aggregates and normal weight fine aggregates.
Also density lesser than 1920kg/m3 will be categorized as lightweight concrete. The use of
lightweight aggregates in concrete design will give us lightweight aggregates. Aggregates are the
important element that contributes to the density of the concrete. The examples of lightweight
Density of normal concrete is the order of about 2400kg/m3. To call the concrete, as high density
concrete, it must have unit weight ranging from 3360 to 3840 kg/m3 can be called as the heavy
weight concrete. Here heavyweight aggregates are used. The crushed rocks are used as the coarse
aggregates. The most commonly used heavyweight aggregates is Barytes. These types of
aggregates are most commonly used in the construction of atomic power plants and for similar
projects. The heavy weight aggregate will help the structure to resist all possible type of radiations.
These are concrete types into which air is intentionally entrained for an amount of 3 to 6% of the
concrete. The air entrainment in the concrete is achieved by the addition of foams or gas–foaming
agents. Some examples of air entraining agents are resins, alcohols, and fatty acids.
High-Strength Concrete
Concretes that have strength greater than 40MPa can be termed as high strength concrete. This
increased strength is achieved by decreasing the water-cement ratio even lower than 0.35. The
calcium hydroxide crystals that are the major concern product during hydration for the strength
properties is reduced by the incorporation of silica fume. In terms of performance, the high strength
concrete ought to be less performing in terms of workability which is an issue. (Marsh, 2003).
Properties of Concrete
The knowledge on properties of concrete is essential for designers before he or she begins to
design structures, this properties of concrete can be classify into the following:
Setting of Concrete: The time it take to change from plastic state to hardened state is known as
setting time of concrete or the time it take before hydration will take place. Setting of concrete is
based or related to the setting of cement paste. Thus one of the compounds that made up of concrete
is cement which can greatly affect the setting time of concrete and other factor that affect these
properties are: Water Cement ratio; Suitable Temperature; Cement content; Type of Cement;
Workability of Concrete: The term workability is broadly defined and no single method is
capable of measuring all aspects of workability. The American concrete institute (ACI) (2000)
defined workability as the property of freshly mixed concrete or mortar which determines the ease
and homogeneity with which it can be mixed, placed, consolidated and finished. Workability is
also amount of useful internal work necessary to produce full compaction. A description of
workability is based not just on the properties of the concrete, but also on the nature of the
application. The strength and durability of hardened concrete, in addition to labour costs, depend
on the concrete having appropriate workability. Determining workability is based on two methods,
i. Slump method: is where the degree of consistency of the concrete is measured in order
to determine workability. Slump can be classified to be true slump, shear slump and
ii. Compacting factor test: is defined as the weight ratio of the concrete in the cylinder to
the same concrete fully compacted in the cylinder (filled in four layers and tempered or
vibrated). The ease way of transporting, placing and consolidating without excessive bleeding
or segregation is known as workability of concrete. Because the strength of concrete is
adversely and significantly affected by the presence of voids in the compacted mass, it is vital
to achieve a maximum possible density. Also Slump Test can be used to find the workability
of concrete. The same factors that affect setting time affect the workability too.
Durability. Durability of concrete is one of the engineering properties of concrete in terms of its
ability to resist weathering action, chemical attack, and abrasion while subjected to the service
loads. The design service life of most buildings is often 30 years, although buildings often last 50
to 100 years or longer. Also, Concrete durability has been defined by the American Concrete
Institute (ACI) as its resistance to weathering action, chemical attack, abrasion and other
degradation processes .The durability of the concrete can be affected by chemical attack through
the actions of aggressive ions such as chlorides, sulphates and many other natural or industrial
liquids and gases. Physical factors such as high temperatures and thermal expansion of the
aggregates in the hardened concrete can also lead to extensive deterioration of the concrete. Much
emphasis is placed on the strength properties of concrete than any other property as durability.
This may be because the strength properties usually provide a better picture on the quality of the
concrete. However, there may be some situations when durability and other considerations may be
of greater importance. Durable concrete is dense, water tight and able to resist, to a reasonable
extent, changes resulting from adverse effects of the elements and mechanical damage. In his own
view, proposed that the definition and concept of durability should be based on three key
parameters, namely: intended function of the material, the standardized conditions of its use and
the time the material is required to fulfill its functions. Also the bulk properties identified as likely
to have direct bearing with the investigation of the durability include: Concrete dry density
(CDD),Total water absorption (TWA),Wet compressive strength (WCS) and Dry Compressive
strength (DCS).
High-Compressive Strength
The compressive strength of concrete, known as ( fcu) is obtained by testing to failure of casted
concrete cubes in their different days of curing such as 7,14,21,28-day-old etc, the cubes are
usually submerged in water or in a room with constant temperature and 100% humidity, although
concretes are available with 28-day ultimate strengths, values obtained for the compressive
strength of concretes, as determined by testing, are to a considerable degree dependent on the sizes
and shapes of the test units and the manner in which they are loaded. In many countries, the test
specimens are cubes, 150 mm on each side. For the same batches of concrete, the testing of 6-in.
by 12-in. cylinders provides compressive strengths only equal to about 80% of the values in psi
determined with the cubes. Most codes and standards normally take compressive strength of
concrete as the ultimate axial load divided by its cross sectional area of the concrete cube. Thus,
actual ultimate strength in concrete is then much higher than the nominal compressive strength
and depends on the ratio of cross sectional area to the concrete (BS 2028:1985, BS 6073: 1981 as
amended in 2004,
Fire Resistance
The most highly fire-resistive structural compound used in construction industry in the world today
is concrete. Nonetheless, the properties of concrete and reinforcing steel change significantly at
high temperatures, strength and the modulus of elasticity are reduced, the coefficient of expansion
increases, and creep and stress relaxations are considerably higher. Other properties of concrete
Concrete is the most economical used compound material for construction of different structures
due to its structural durability, stability and strength compare to other materials such as steel,
timber, rubber etc. The cost and skill required to production quality concrete material, is more
economical compare to other construction materials. The tremendous economy advantages of this
universal construction material can be understood easily by consider its numerous advantages
1. Its compressive strength per unit cost compared with other materials is considerable.
2. Concrete has great resistance to the actions of fire and water, which make it best structural
material available for situations where water is present. During fires of average intensity, members
with a satisfactory cover of concrete over the reinforcing bars suffer only surface damage without
failure.
4. It is a low-maintenance material.
5. As compared with other materials, it has a very long service life span. Under proper conditions,
concrete structures can be used indefinitely without reduction of their load carrying abilities. This
can be explained by the fact that the strength of concrete does not decrease with time but actually
increases over a very long period, measured in years, because of the lengthy process of the
7. A special feature of concrete is its ability to be cast into an extraordinary variety of shapes from
Concrete which is solid mass of mixture of Cement, Aggregate and water involves the following
processes. This include Concrete mix design which consists process of selecting, proportioning,
Proportioning: This is the processes of measuring out by volume or weight of the various
constituent material for the concrete. The standard ratio of batching by weight is giving by 1:2:4
of cement, fine aggregate and coarse aggregate respectively for the normal concrete
Mixing: Concrete can be manually or with help of the machine (mixer). For but manual and
machine method, hard and clean surface should be used and proper mixing ensured until uniformly
of the mix attained. During mixing processes certain properties should ascertain such as
Transportation: After mixing has been taken place, the concrete is move from mixing point to
the point where it is going to be used, transportation can either be with wheel barrow, head pan, or
placing of the concrete should not fall freely more than (1m) to prevent a peaked forming
Compaction: The concrete most be well compacted after placement to secure maximum density.
This can be done by hand or using vibratos. The used of vibratos is more deferent and can be used
driers. They are three types of vibratos named. The external, internal, and surface vibratos
Curing: According to BS 8110 (1997) curing is “the process of preventing the loss of moisture
from the concrete whilst maintaining a satisfactory temperature regime.” This definition adds that
the curing should prevent the development of high temperature gradients within concrete. Other
definitions exist that make reference to hydration, durability and cost. Thus the objective of curing
is to keep the concrete saturated, or as near saturated as possible for sufficient time for the original
water-filled space to become filled to the desired extent by cement hydration products. Curing is
i. Formwork retention,
ii. Suspension of covering above the surface before the concrete sets (horizontal surface),
viii. Sunshields