Some vital properties of concrete for mix design
While designing the reinforced concrete structure, the designer should concentrate on the following properties of concrete in its harden state.
COMPRESSIVE STRENGTH: At the time of making the design of the buildings and other structures, the compressive strength of concrete is applied as the most common performance measurement.
Compressive strength of concrete is affected by several factors like water-cement ratio, cement strength, quality of concrete material, quality control throughout production of concrete etc.
Concrete compressive strength for general construction fluctuates from 15 MPa (2200 psi) to 30 MPa (4400 psi) and elevated in commercial and industrial structures.
Compressive strength is a vital factor to evaluate the performance of the material throughout service conditions. The compressive strength of concrete is decided in batching plant laboratories for each batch to retain the optimal quality of concrete throughout casting. The strength of concrete is essential to work out the strength of the members. Concrete samples are casted and tested under the action of compressive loads to find out the strength of concrete.
TENSILE STRENGTH: Tensile strength is an important property of concrete since concrete structures prone to tensile cracking because of different types of effects and applied loading itself. Tensile strength of concrete is very low with regard to its compressive strength.
Because of the complexity in employing uniaxial tension to a concrete specimen, the tensile strength of the concrete is obtained by indirect test methods like split cylinder test and flexure test.
Tensile strength of concrete = 1/10 times of compressive strength
With adherence to IS: 456, the tensile strength of concrete is measured from the compressive strength with empirical relation provided by:
Flexural strength: fcr=0.7√fck N/mm2
MODULUS ELASTICITY: The modulus of elasticity of concrete belongs to a function of the modulus of elasticity of the aggregates and the cement matrix and their relative proportions. The elastic modulus of the hardened paste may come in the order of 10-30 GPa and aggregates about 45 to 85 GPa.
The strength of concrete is mainly based on the relative proportion and modulus of elasticity of the aggregate.
To determine the exact value of elastic modulus of a concrete batch, laboratory test should be carried out. There also exist some empirical formulas which are arranged with different code to get the elastic modulus of Concrete. These formulas are derived on the basis of the relationship among modulus of elasticity and concrete compressive strength. One can easily get an rough value of modulus of elasticity of concrete by applying 28 days concrete strength (f'c) with these formulas.
It can be measured with following formula for normal density concrete:
E = 57,000 (fc’)^0.5
The result will be produced in psi.
fc’ stands for the 28 days cylinder crushing strength in psi.
Modulus of elasticity of concrete is affected by the various factors like type of the aggregates used, type of cement and Mix proportions.
SHRINKAGE OF CONCRETE: It stands for a physical property of concrete. The volumetric variations of concrete structures occur because of the loss of moisture by evaporation is defined as concrete shrinkage or shrinkage of concrete. It is a time-dependent deformation that decreases the volume of concrete devoid of the influence of external forces.
This shrinkage leads to surge in tensile stress, that results in cracking, internal warping, and external deflection, before the concrete has to undergo any type of loading.
Water content in concrete considerably impacts the shrinkage. The IS: 456-2000 suggests the total shrinkage strain as 0.0003 when there is no test data. Drying shrinkage in plain concrete leads to surface cracks. The deflections of reinforced concrete members is also influenced by the shrinkage of concrete.
CREEP OF CONCRETE: Creep is one of the fundamental property of concrete. It is very crucial for designing of concrete structure because in concrete the microstrains of creep fluctuates from 400 to 1000 x 10 -6
Creep is described as the elastic and long-standing deformation of concrete under a continuous load. Normally, a long term pressure alters the shape of concrete structure and the deformation is found along the direction of the applied load.
Creep stands for the time dependent deformations of concrete under permanent loads (self weight), PT forces and permanent displacement.
The creep is affected by the several factors like creep concrete mix proportion, aggregate properties, age at loading, curing conditions, cement properties, temperature, stress level.
COEFFICIENT OF THERMAL EXPANSION: The coefficient of thermal expansion of concrete is impacted by the type of aggregate applied in concrete and it is necessary for making the design of structures like chimneys, water tanks, silos etc. The values provided in IS:456-2000 are as follow :-
Type of Aggregate - Coefficient of Thermal Expansion for Concrete
Quartzite - 1.2 to 1.3 x 10-5
Sandstone - 0.9 to 1.2 x 10-5
Granite - 0.7 to 0.95 x 10-5
Basalt - 0.8 to 0.95 x 10-5
Lime stone - 0.6 to 0.9 x 10-5