3. Introduction
What is Concrete?
Concrete:- is stone like material obtained artificially
by hardening of the mixture of cement, inert-
aggregate materials (fine & coarse) and water in
predetermined proportions.
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5. Introduction
When these ingredients are mixed, they form a
plastic mass which can be poured in suitable
moulds and set-on standing into hard solid mass, as
a result of exothermic chemical reaction between
cement and water.
Concrete has been the construction material used in
the largest quantity for several decades.
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6. Introduction
The reason for its popularity can be:
1) Durability under hostile environments (including
resistance to water),
2) Ease with which it can be cast into a variety of
shapes and sizes,
3) Its relative economy and easy availability.
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7. Properties of Concrete
The properties of concrete ingredients have a major
influence on the fresh as well as hardened concrete.
Therefore, the selection of concrete-making materials
for a given purpose is quite important.
In order to make this selection intelligently, the
selecting person should be able to assess concrete-
making materials, and should know what to select, how
to select it, and why to select it in a particular way.
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8. Properties of Concrete
Properties in the Hardened State
a) Compressive Strength
A wide range of strength properties can be obtained for
concrete by appropriate adjustment of the proportions
of the constituent materials, using different degree of
the compaction and the conditions of temperature and
moisture under which it is placed and cured.
Based on this, it is possible to produce different grade
of concrete.
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10. Properties of Concrete
For complete hydration of a given amount of cement, an
amount of water equal to about 25 percent of that of
cement, by weight-i.e., a water-cement ratio of 0.25 is
needed chemically.
For normal concretes, the water-cement ratio is
generally in the range of about 0.40 to 0.60, although
for high-strength concretes, ratios as low as 0.21 have
been used. In this case, the needed workability is
obtained through the use of admixtures.
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11. Properties of Concrete
Tests for Compressive Strength of Concrete
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12. Properties of Concrete
i. Cubic test
Standard test specimens of 150mm cube are taken at
the age of 28days to determine the compressive
strength of concrete according to Ethiopian standard
agency (ESA).
At age of 7days, concrete may attain approximately
about 2
3 of the full compressive strength of
concrete.
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14. Properties of Concrete
ii. Cylindrical test
In some national standard (example ACI code),
cylinder specimens of 150mm diameter by 300mm
high are taken.
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16. Properties of Concrete
Although the load is applied uni-axially, the friction between
the loading plate and the contact faces of the test specimen
has more effect on cube strength than the cylinder strength.
Because of this, the cube strength gives more strength than
the true compressive strength of concrete, whereas cylinder
strength gives reasonably the true compressive strength.
On average, cube strength is taken as 1.25 times cylinder
strength.
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18. Properties of Concrete
The curves are somewhat linear in the very initial phase
of loading.
The non-linearity begins to gain significance when the
stress level exceeds about one-third to one-half of the
maximum.
The maximum stress is reached at a strain
approximately equal to 0.002; beyond this point, an
increase in strain is accompanied by a decrease in stress.
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19. Properties of Concrete
For the usual range of concrete strengths, the strain
at failure is in the range of 0.003 to 0.005.
The higher the concrete grade, the steeper is the
initial portion of the stress-strain curve, the sharper
the peak of the curve, and the less the failure strain.
For low-strength concrete, the curve has a relatively
flat top, and a high failure strain.
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20. Properties of Concrete
b) Tensile strength
Even though concrete is weak in tension, its tensile
strength is important in a variety of items.
Shear and torsion resistance of RC members primarily
depend on tensile strength of concrete.
Further, the conditions under which cracks form and
propagate on tension zone of RC flexural members
depend strongly on the tensile strength of concrete.
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21. Properties of Concrete
Two methods are used to determine tensile strength
of concrete.
i. Flexural test (Beam-test) and
ii. Split-cylinder test method.
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24. Properties of Concrete
Tensile strength of concrete is obtained by loading
plain concrete test-beam laterally by two point loads at
the third points of test-beam until the tension zone of
the beam fracture.
Tensile strength of concrete is then computed using
flexural stress formula in terms of modulus of rupture
concrete.
𝒇𝒓 =
𝑴∗𝒄
𝑰
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26. Properties of Concrete
Tensile strength of concrete is obtained by loading
standard plain concrete cylinder along the side until
the cylinder splits in to two pieces.
The tensile strength of concrete is the computed by the
following equation based on the theory of elasticity for
homogeneous material in a bi-axial state of stress.
𝒇𝒕 =
𝟐𝑷
𝝅𝒅𝒍
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28. Properties of Concrete
Whatever the method, it is known that the tensile
strength of concrete is relatively low, and it is about
10 to 15% of compressive strength of concrete.
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29. Properties of Concrete
Properties of Concrete in the Plastic State
1) Workability- is an important property and concerns
the ease with which the mix can be mixed,
handled, transported and placed with little loss of
homogeneity so that after compaction it surrounds
all reinforcements completely, fills the form work
and results in concrete with the least voids.
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30. Properties of Concrete
Methods used to check the workability of Concrete:
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33. Properties of Concrete
Time dependent properties of concrete
1) Shrinkage and Thermal Movement
Concrete may under go deformations and volume changes
with out application of loading.
This phenomenon may be caused by shrinkage and
thermal-movement in fresh and hardened concrete.
Shrinkage of concrete is liable to cause cracking, but it has
the beneficial effect of strengthening the bond between the
reinforcing steel and the surrounding concrete.
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34. Properties of Concrete
Shrinkage of concrete caused initially by the
absorption of water by cement and aggregate, and
further by evaporation of water which rises to
surface as a result of capillary action.
During setting process the hydration of cement
causes a great deal of heat to be generated, and as
the concrete cools, further shrinkage takes place due
to thermal contraction.
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35. Properties of Concrete
How to reduce thermal shrinkage of concrete?
a) Using a mix-design with low cement content.
As per ES EN 1992:2015, specifies cement content
not to exceed 550 𝑘𝑔
𝑚3 of concrete.
b) Avoiding rapid hardening & finely ground cement.
Using retarder admixtures and checking the fineness
of cement by fineness test.
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36. Properties of Concrete
c) Keeping aggregate & mixing water cool, or may
be need to keep them under shade.
d) Maintaining the temperature & evaporating water
by proper curing.
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37. Properties of Concrete
2) Creep
Creep is the continuous deformation of a member
under sustained compressive stress over a
considerable length of time (under long-term
loading).
It is a phenomenon associated with brittle materials
(concrete is a brittle material).
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38. Properties of Concrete
Creep deformation depends on the stress in
concrete, duration of loading and water-cement
ratio.
This deformation grows rapidly and reaches highest
intensity in the first 3-4 months and then continues
to increase gradually approaching the limiting value
in a period of five years.
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39. Properties of Concrete
The effect of creep has to be considered in design
of reinforced concrete member subjected to
compressive stress mainly caused by long term
loading (dead load).
At any time, when the load is removed the elastic
recovery takes place immediately while part of
creep deformation recovers at a slower rate leaving
permanent deformation.
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41. Properties of Concrete
Factors influencing creep
i. Magnitude of stress: Creep grows directly
proportional.
ii. Age at loading: Loading at an earlier age causes
high creep strain.
iii. Rate of loading: Creep increases with increase in
the rate of loading.
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42. Properties of Concrete
iv. Humidity: Creep is reduced with increase in
humidity or moisture content of the surrounding
air.
v. Composition of concrete: An increase in W/C
ratio and the amount of cement per unit volume of
concrete increases creep.
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43. Properties of Concrete
The effect of creep is particularly important in beams, where
the increased deformations may cause the opening of cracks
and damage of finishes.
To reduce creep deformation, it is necessary to provide
nominal reinforcement in the compression zone of the beam.
The nominal area of compression steel required by doubly
reinforced beam is about 0.4% of the area in compression
(which may be taken as 0.2% of the whole area including
tension zone).
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44. Properties of Concrete
Grade of Concrete
Concrete is graded in terms of characteristic
compressive cube strength.
The grade of concrete to be used in design depends
on its intended use.
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45. Reinforcing Steel
Steel reinforcements are available in the form of
round bars (circular cross section) and welded
wire fabric.
The most commonly used bars have projected ribs
on the surface of bar. Such bars are called deformed
bars.
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46. Reinforcing Steel
The ribs of deformed bar improve the bond between
steel and the surrounding concrete in RC members
by providing mechanical keys.
A wide range of reinforcing bars is available with
nominal diameter ranging 6mm to 35mm.
Most bars except 6mm diameter are deformed one.
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47. Reinforcing Steel
Some of the common bar size with their application
in concrete works are given in table below.
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For stirrups For slabs For beams & columns
Diam.
(mm) 6 8 10 12 14 16 18 20 2
2
25 28
Area
(cm2)
0.28 0.50 0.785 1.13 1.54 2.01 2.52 3.14 3.8 4.9 6.2
Wt
(kg/m
0.222 0.395 0.617 0.888 1.21 1.57 2.0 2.47 3.0 3.9 4.8
Per.
(cm)
1.88 2.51 3.14 3.77 4.4 5.02 5.65 6.28 6.9 7.85 8.79
48. Reinforcing Steel
Strength of reinforcing steel
Reinforcing steel is capable of resisting both tension
and compression.
Compared with concrete, it is a high strength material.
For instance, the strength of ordinary reinforcing steel is
about 10 & 100 times, the compressive & tensile
strength of common structural concrete.
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49. Reinforcing Steel
Stress-Strain Curve of reinforcement
Mild steel exhibits a definite yield point where as
the higher strength steels show a smooth transition
from elastic to plastic stages of stresses.
In high strength steels, the stress corresponding to a
strain value of 0.002 (Offset method) is said to be
the proof stress, which is equivalent to the yield
stress in case of mild steel.
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51. Reinforced Concrete (as a composite material)
It is known that plain concrete is quite strong in
compression, weak in tension.
On the other hand, steel is a high cost material
which able to resist both tension & compression.
The two materials (plain concrete & reinforcing
steel) are best be utilized in logical combination if
steel bars are embedded in the plain concrete in
tension zone close to the surface.
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52. Reinforced Concrete (as a composite material)
In this case, plain concrete is made to resist the
compressive stresses and reinforcing steel resists the
tensile stresses.
Both plain concrete & reinforcing steel bar together
assumed to act as one composite unit and it is
termed as Reinforced concrete (RC).
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53. Reinforced Concrete (as a composite material)
The tensile stresses developed in the section are
transferred to reinforcing steel by the bond between
the interfaces of the two materials.
In all RC members, strength design is made on the
assumption that concrete does not resist any tensile
stresses.
All the tensile stresses are assumed to be resisted by
the reinforcing steel imbedded in tension zone.
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54. Reinforced Concrete (as a composite material)
Some times, if necessary, reinforcing steel is
provided in compression zone to assist the concrete
resisting compression in addition to reducing creep
deformation.
Reinforcing steel & concrete may work readily in
combinations due to the following reasons:
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55. Reinforced Concrete (as a composite material)
1. While setting, cement concrete grips reinforcing
bar perfectly;
To develop perfect bond b/n concrete and steel.
Enables concrete to transmit its stress to steel after it
has cracked.
2. Proper concrete mixes provide adequate
impermeability of concrete against bar corrosion.
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56. Reinforced Concrete (as a composite material)
3. Sufficiently similar rates of thermal expansion
(0.00001/0C to 0.000013/0C for concrete and
0.000012/0C for steel) introduce negligible
stresses between steel and concrete under
temperature changes.
4. The tensile strength of steel is quite high to resist
bending stress accordingly.
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57. Reinforced Concrete (as a composite material)
Advantages of Reinforced Concrete
It is monolithic. This gives it more rigidity.
It is durable. It does not deteriorate with time.
While it is plastic, it can be moldable into any desired shape.
It is fire, weather and corrosion resistant.
By proper proportioning of mix, concrete can be made
water-tight.
Its maintenance cost is practically nil.
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58. Reinforced Concrete (as a composite material)
Disadvantages of Reinforced Concrete
It is difficult to demolish in case of repair of
modification.
It is too difficult to inspect after the concrete has
been poured.
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