Concrete strength testing: Why is the cylinder compressive strength of concrete typically lower than the cube strength?

Introduction / Context:
Concrete compressive strength depends not only on material quality but also on specimen geometry and boundary conditions during testing. Standard cube and cylinder specimens yield systematically different strengths, with cubes generally reporting higher values than cylinders from the same batch.

Given Data / Assumptions:

• Standard cube (e.g., 150 mm) and cylinder (e.g., 150 × 300 mm) specimens.
• Both tested axially between steel platens.
• Comparable curing and age.

Concept / Approach:

End friction between the specimen and machine platens restrains lateral expansion near the loaded faces. This end restraint produces a triaxial compression state near the ends, artificially increasing apparent strength. The effect is stronger in cubes (shorter height) than in cylinders (slenderer), hence cubes show higher strengths. Shape and slenderness contribute, but the root cause is end restraint due to friction.

Step-by-Step Solution:

Verification / Alternative check:

Capping or use of lubricated/ball-seated platens reduces uneven stress distribution but does not reverse the inherent trend. Empirical conversion factors between cube and cylinder strengths are widely used.

Why Other Options Are Wrong:

• Cross-section shape and slenderness describe geometry but are secondary to end friction restraint.
• Capping reduces end imperfections; it is not the main cause of lower cylinder strength.

Common Pitfalls:

• Comparing design strengths without applying cube-to-cylinder conversion when code requires one basis (e.g., fck vs. f'c).

Final Answer:

the friction between the concrete specimens and the steel plate of the testing machine