In concrete, how do strength and stiffness differ?

• A. Strength is deformation response under load; stiffness is load-bearing capacity.
• B. Strength is load-bearing capacity; stiffness is deformation response under load.
• C. They are the same property.
• D. Stiffness relates to surface finish.

Answer: (B)

The main idea here is the difference between how a material resists breaking versus how much it deforms under load. Strength is about load-bearing capacity—the maximum stress a concrete member can withstand before it fails. In practice, that means tests that push the material to its ultimate strength (like compressive strength tests) to determine how much load it can carry safely.

Stiffness, on the other hand, is about deformation under load. It describes how resistant the material is to bending or shortening—how much it deflects when a given load is applied. This is tied to the modulus of elasticity and tells you about serviceability (deflections, crack widths) rather than ultimate failure.

So the best description is that strength equals load-bearing capacity, while stiffness equals the deformation response under load. The other options mix up these ideas or bring in unrelated factors like surface finish.