Details of concrete properties
Concrete properties are affected with various factors primarily because of mix proportion of cement, sand, aggregates and water. Proportions of these materials manage the different concrete properties described below:-
Properties of Concrete:
- Grades (M20, M25, M30 etc.)
- Compressive strength
- Characteristic Strength
- Tensile strength
- Unit weight
- Modular Ratio
- Poisson’s ratio
Grades of concrete:
Concrete is mainly recognized with its grade that is expressed as M15, M20 etc. Here M denotes concrete mix and numeric 15, 20 stands for the indicated compressive strength (fck) of 150mm cube at 28 days that is stated as N/mm2. Therefore, concrete is identified with its compressive strength. M20 and M25 are considered as the widely accepted grades of concrete. Exclusive of these grades, higher grades of concrete are employed for harsh, very harsh and acute atmospheres.
Compressive strength of concrete:
Similar to load, the concrete strength also fluctuates significantly for the equivalent concrete mix. So, a single representative value called characteristic strength is applied.
Characteristic strength of concrete:
It is described as the value of the strength under which not over 5% of the test results are presumed to fall (it means that there is 95% chance for obtaining this value only 5% of not attaining the same)
Characteristic strength of concrete in flexural member
The characteristic strength of concrete in flexural member remains as 0.67 times the strength of concrete cube.
Design strength (fd) and partial safety factor for material strength
Design strength is defined as the strength applicable for creating the design. It is represented as follow :-
Design strength (fd) = characteristic strength/ partial safety factor for material strength
The value of partial safety factor is based on the type of material as well as the type of limit state. As per IS code, partial safety factor for concrete is 1.5 for concrete and for steel 1.15.
Design strength of concrete in member = 0.45fck
Tensile strength of concrete:
To workout the flexural tensile strength or the modulus of rupture or the cracking strength of concrete out of cube compressive strength, the following relations are used.
fcr = 0.7 fck N/mm2
Split cylinder is used to determine the tensile strength of concrete in direct tension by experiment. It differs among 1/8 to 1/12 of cube compressive strength.
Creep in concrete:
Creep is known as the plastic deformation under sustain load. Creep strain is mainly dependent on the extent of sustained loading. Based on the code, the value of the ultimate creep coefficient remains as 1.6 at 28 days of loading.
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