Q.: Why are strengths related to time periods of 7 days and 28 days?
A.: One could perhaps say that the reasons are historical--that these time periods were established in the early days of cement and concrete testing and have persisted ever since.However, there seem to be good reasons that these time periods were chosen. One of the practical considerations in selecting testing ages is to avoid ages that fall on weekends. By selecting ages such as 7 and 28 days the test age will fall on the same day of the week as the day on which the test specimens were made--usually not a weekend.
On this basis, starting with multiples of 7 as being good ages, the question becomes
The curve in which compressive strength is plotted against age is generally of the form shown in the figure. As can be seen, enough strength has usually been attained by age 7 days to give some indication of whether the concrete is gaining strength at a satisfactory rate; so 7 days is a useful age at which to test. (Age 14 days is also sometimes used when an accurate record of the shape of the strength curve is needed.) Age 28 days gives a good clue to the strength that will be achieved by the end of the period of most rapid strength gain (see figure), so 28 days is a useful multiple of 7 days at which to test. Consequently, concrete mixes are usually proportioned in terms of the strength they can develop at 28 days.
Interestingly enough, now that high-strength concretes (6000 to 10,000 psi) have come into use, high-strength mixes are commonly proportioned to meet their design strengths at 56 or 91 days instead of 28 days, since the full strength is ordinarily not needed as early as 28 days.One of the problems with testing at 28 days or later is that if the concrete does not meet the specified 28-, 56-, or 91-day strength it may have been a part of the structure for a long time before its deficiency becomes known. This problem has led to some reliance on accelerated methods of strength testing.