Reason for conducting concrete compressive strength after 28 days
The concrete strength is normally checked after 28 days like concrete cube strength or concrete cylinder strength. Given below, the purposes of examining the concrete strength after 28 days :-
Once the casting is completed, the concrete attains its strength in due course. Long time is required for concrete to attain 100% strength and the required time period can’t be identified. Throughout the first 28 days of casting, the rate of gain of concrete compressive strength remains higher then it reduces gradually.
The table given above demonstrates the compressive strength attained by concrete after 1, 3, 7, 14 and 28 days relating to the grade of concrete applied.
From above table, it is found that, concrete attains 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.
Therefore, it can be said that concrete attains its strength quickly in the preliminary days as soon as casting is completed, i.e. 90% in only 14 days. When, the concrete achieves its strength up to 99% in 28 days, it still carried on to attain strength after that period, but that rate of gain in compressive strength becomes very less with reference to that in 28 days.
Once the 14 days of casting concrete is completed, concrete attains only 9% in next 14 days. Therefore, rate of gain of strength is reduced. Though, it can’t be said clearly when concrete attains its final strength but it is assumed that concrete may attain its final strength after 1 year.
Therefore, as the concrete strength is 99% at 28 days, it’s almost bordering on its final strength. Consequently, the results of compressive strength test after 28 days become a vital factor and this strength is applied as the base for our design and evaluation.
Some rapid methods exist for testing concrete compressive strength which makes relation among rapid test methods and 28 day strength. The rapid test is accomplished where time is short for construction and strength of structural member is recognized to conduct further construction work.