Under normal conditions using an ordinary cement,
A.beam soffits
B. bottom slabs of spans 4.6 m and more
C. bottom beams over 6 m spans
D. vertical sides of columns
It depend on the weather condition and country region also on wide range on type type of material and other local circumstances such as skills or specification, etc...
The problem is that concrete strength gain is a function of several things, cement quantity and activity, the surface-volume ratio, and both time and temperature. In warmer conditions, the same concrete will reach a target strength faster than the same concrete cured under cooler conditions. This has been known to cause problems when there are wide temperature changes in short periods of time.
ASTM C 31 allows use of “field cured” cylinders to assess the readiness for removal of formwork and shoring. Field cured cylinders are cast of the same concrete as the structural element, kept at under same conditions as the element , and tested when strength assessment is desired.
Several non-destructive methods might also be useful. Particularly, consider ASTM C 1074, the Maturity Method which, with a little testing on the particular mix, can make excellent predictions of strength. The Pull-out test, ASTM C 900 is also a very good test for new concrete. If the strength assessment is very important several non-destructive tests, described in ACI 228, in combination give greater confidence.
If your mix is consistent, and placement conditions are steady, with little temperature or humidity fluxuations, and the concrete is properly cured a non-destructive method, like the maturity method or field cylinders, could be used to approximate the time needed to reach sufficient strength for removal of shoring on several structures. Then apply that experience to your construction.
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