MATH: Reinforced Concrete Design 01

COMPRESSIVE STRENGTH
Depending on the mix (specially the water-cement ratio) and the time and quality of curing, compressive strengths of concrete can be obtained up to 97 MPa or more. Commercial production of concrete with ordinary aggregates is usually in the 21 to 83 MPa range with the most common ranges for cast-in-place buildings from 21 to 41 MPa. On the other hand, precast and prestressed applications often expect strengths of 27.6 to 55.1 MPa
The 28-day compressive strength of concrete can be estimated from the 7-day strength by a formula proposed by W.A. Sater:
S28 = S7 + 2.5 S7 Eq. 1-2
Where S28 = 28-day compressive strength, MPa
S7 = 7-day compressive strength, MPa


Concrete strength may increase significantly after 28 days, particularly when cement is mixed with fly ash.

The stress drain diagram for concrete of a specialized compressive strength is a curved line as shown in Figure 1.1. Maximum stress is reached at a strain of 0.002 mm/mm, after which the curve descends.