If the size of the specimen used to test the compressive strength of concrete is decreased, then the compressive strength of concrete will:

📊 Effect of Specimen Size on Compressive Strength

This is the size effect in fracture mechanics. Concrete contains microscopic flaws, cracks, and weak zones distributed throughout its volume. A larger specimen has a higher probability of containing a critical flaw that initiates failure at a lower stress. A smaller specimen has fewer such flaws and therefore appears to be stronger.

This is why the standard specimen size matters: 150 mm cubes are used internationally as the benchmark. Smaller cubes (e.g., 100 mm) give higher apparent strengths, and correction factors must be applied to convert results.

Key takeaway

Smaller specimen → fewer critical flaws → higher apparent strength. The same size-effect principle applies to most brittle materials, not just concrete.