It has been observed that large percentage of strength gain is achieved near 28 days and subsequent increase is very slow.
I was reading recently as to why 28 days and not 29 or 30 days. The reason was quite interesting as you may be casting cubes on any working day and when you test it on 28th day the day will be working day only and not Sunday whereas for other days it can fall on Sunday.
concrete gains 16 percent strength in one day, 40 percent in 3 days, 65% in 7 days, 90% in 14 days and 99% strength in 28 days.
Thus, it is clear that concrete gains its strength rapidly in the initial days after casting, i.e. 90% in only 14 days. When, its strength have reached 99% in 28 days, still concrete continues to gain strength after that period, but that rate of gain in compressive strength is very less compared to that in 28 days.
The concrete develops strength with continued hydration, the rate of gain of strength is faster to start with and then gets decreasing with age. The increase in strength beyond 28 days used to get immersed with factor of safety.
Earlier only one type of cement was in use. For OPC increase in strength after 28 days was considerable, but now a days particularly 53 grade cement ,being ground finer, the increase in strength after 28 days is nominal, most of strength gained in respect of cured concrete takes place by 28 days.
It has been observed that practically on construction site curing will be done at the maximum 28 days. At the time of gaining strength as a exothermic process heat gets evolved, that heat taken care of curing, up to 28 days. after 28 days this chemical reaction becomes slower and slower on other hand curing is also not provided in such case gain of strength is very marginal.
it is also observed that 7 days strength is approximately 66 % of 28 days strength.
To ensure ourselves as a site engineer 7 days strength is taken. from 7 days strength one can predict 28 days strength.
in case desired results are not getting ,then after 7days of concrete itself engineer can think fresh concreting-thus avoiding poor quality concrete and save valuable construction days also.
In terms of the quality assurance and control for nuclear engineering construction, this paper introduced the determination, origin analysis, remediation, disposal and close of the major non conformity of crack of concrete baseplate.