Concrete gains strength with time after casting. It takes much time for concrete to gain 100% strength and the time for the same is still unknown. The rate of gain of concrete compressive strength is higher during the first 28 days of casting and then it slows down.
The table below shows the compressive strength gained by concrete after 1, 3, 7, 14, and 28 days with respect to the grade of concrete we use.
| Age | Strength percent |
| 1 day | 16% |
| 3 days | 40% |
| 7 days | 65% |
| 14 days | 90% |
| 28 days | 99% |
From the above table, we see that 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 has 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.
After 14 days of casting concrete, concrete gains only 9% in the next 14 days. So, the rate of gain of strength decreases. We have no clear idea up to when the concrete gains the strength, 1 year or 2 years, but it is assumed that concrete may gain its final strength after 1 year. So, since the concrete strength is 99% at 28 days, it’s almost close to its final strength, thus we rely upon the results of the compressive strength test after 28 days and use this strength as the base for our design and evaluation. Though there are also some rapid methods of testing concrete compressive strength which gives the relation between rapid test methods and 28-day strength. This rapid test is done where time is limited for construction and the strength of structural members must be known to carry out further construction work.
The acceptance criteria for the specified compressive strength according to section 26.12.3.1 of ACI 318-14 are as follows
1- Every arithmetic average of any three consecutive strength tests equals or exceeds the specified compressive strength.
2- No strength test falls below the specified compressive strength by more than 500 psi (3.5 MPa) if the specified compressive strength is 5000 psi (35 MPa) or less; or by more than 10 percent of the specified compressive strength if the specified compressive strength exceeds 5000 psi (35 MPa).
