Strength of concrete Chapter 6 Properties of concrete :-
Strength of concrete is the most valuable property.
What is strength?
Strength depends on the physical structure and volume of hydration products. Therefore it gives an idea of the quality of concrete. Note :- strength depends less on the chemical composition of the paste and more on the physical structure. Fracture mechanics approach
Fracture mechanics is the formation of cracks at the interface of the paste due to the presence of flaws (in aggregates) discontinuities and pores. There are sources of weaknesses and hence cracks influence the strength of the specimen. It is therefore important to understand the process of fracture of concrete under stress. Porosity :- relative volume of pores or voids in the paste
Factors affecting strength
The most important factor is the w/c ratio
Fracture mechanics is the study of stress and stain behaviour of homogenous brittle material. Concrete can be considered as brittle even though it exhibits a small amount of apparent plasticity. Fracture takes place at a moderately low strain when concrete is loaded for short time. Limit of brittle behaviour when strain = 0.001 – 0.005
Concrete is hardly homogenous as the properties of its constituents are different and it is to some extent anisotropic. Note :- anisotropic identical properties in all directions
Tensile strength consideration
The actual tensile strength of hydrated cement paste is found to be very much lower (1000 times) than the theoretical strength. This discrepancy is caused by presence of flaws and cracks. Behaviour under tensile stress
Consider the figure below
Cracks develop due to difference in theoretical and actual strength. Stress develop at their tips under load....
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