PROPERTIES OF CONCRETE
COMPRESSIVE STRENGTH GRADE OF CONCRETE
The compressive strength of concrete is the characteristic compressive strength (/cel of 150 mm concrete cubes at the age of 28 days in N/mm².
IS 456-2000 specifies the characteristic compressive strength of 150 mm cube at the age of 28 days in N/mm², as the Grades of concrete varying from 15 to 80 N/mm² designated as M 15 to M 80. Based on the characteristic compressive strength (f). concrete is graded as given below:In the designation of concrete mix, M refers to the mix and the number to the characteristic compressive strength of 150 mm cubes at 28 days in N/mm².For reinforced concrete, the minimum grade of concrete to be used is M 20. For various weather conditions, the minimum grade of concrete to be used for reinforced concrete is given
TENSILE STRENGTH
The tensile strength of concrete is very low and hence it is not taken in to account in the design of reinforced concrete.
Fcr = 0.7 √fck N/mm²
MODULUS OF ELASTICITY
Modulus of elasticity of concrete is an important property required for computation of deflections of structural concrete membersorat properte of test data, the modules of elasticity of concrete is related
Ec=5000 √FckN/mm²
Where Ec is the short term static modules of elasticity in N/mm²
POISSON RATIO
Poisson ratio varies between 0.1 for high strength concrete and 0.2 for weak concrete. Usually it is taken as 0.15 for strength design and 0.2 for serviceability calculations.
SHRINKAGE
The property of decreasing in volume during the process of drying and hardening of concrete is called shrinkage, Shrinkage in concrete may results in surface cracks Shrinkage of concrete also influences the deflections of reinforced concrete members.It depends up on the ingredients of concrete and environmental conditions like temygature and humidity The IS 456-2000 recommends the total shrinkage strain of 0.0003 for design purpose
CREEP
The creep coefficient which is defined as the ratio of ultimate creep strain to the elastic strain at various ages of loadings as recommended by IS 456-2000 is given below
Creep of concrete considerably increases the deflections of reinforced concrete flexural members. The value of creep coefficient is useful in the calculation of time dependent deflections in reinforced concrete members.
WORKABILITY
A workable concrete should not show any segregation (separation of constituents of concrete in its plastic stage) or bleeding (rise of water in the mix to the surface when it is compacted).
A workable concrete possesses required lubricatión for handling concrete with out segregation, can be placed in the forms with out loss of homogeneity, compacted with minimum effort and shall be finished easily
The factors which influence the workability are
(1) Water-Cement ratio
(u) Type and grading of aggregate.
(iii) Ratio of fine and coarse aggregate.
(iv) Use of admixtures
(v) Efficiency of mixing
Recommended tests to measure workability, are
(i) Slump test (ii) Compacting factor test
(iii) Vee Bee consistometer test
(iv) flow test
A Suggested ranges of workability of concrete in accordance of IS : 1199 are given
UNIT WEIGHT
unit weight depends upon the aggregates and amount of voids. The unit weight as specified by the IS: 456-2000 for plain concrete and reinforced concrete are 24 kN/m³ and 25 kN/m³ respectively.
