The compressive strength of concrete tested with a 4-inch cube is always which relation to testing with a 6-inch cube?

• A. Less
• B. Equal
• C. Variable
• D. More

Answer: (D)

Smaller test specimens tend to show higher measured compressive strength because of the size effect in concrete. Strength isn’t just a single number for the material; it depends on the presence and distribution of flaws and the paths cracks must follow. In a 4-inch cube, the region under load is smaller, so there are fewer or smaller critical flaws in that volume and a crack has a shorter route to grow to failure. In a 6-inch cube, there’s more volume and a greater chance of encountering a larger flaw, plus the crack has a longer path to propagate, which lowers the load needed to fail. So, under the same concrete mix and curing, the 4-inch cube typically records a higher compressive strength than the 6-inch cube. This is a general tendency, though exact values depend on curing, mix quality, and testing conditions.