Compressive Strength of Normal-Weight Concrete

f'c is a specified strength at a specified age on a specified test: a standard cylinder, moist-cured, crushed at 28 days. Ordinary building concrete is specified at 3000–5000 psi (20–35 MPa); pavements sit near 4000 psi, high-rise columns run 8000–15 000 psi, and specialty ultra-high-performance mixes exceed 150 MPa. The 28 MPa here is a representative middle, not a property of concrete.

Two things make this the most misused number in structural work. First, concrete is roughly ten times weaker in tension than compression, about 0.6√f'c in MPa, which is why it is reinforced at all, and why every code forbids relying on concrete tension. Second, the delivered mix legitimately overshoots: producers target a mean strength well above f'c so that only a small fraction of cylinders fall below it, so a 4000 psi mix routinely breaks at 5000. Strength also keeps climbing past 28 days, and it is destroyed by adding water at the truck. Cylinder breaks tell you what the truck delivered; cores tell you what is in the structure; neither is the value you designed with.