Strength of Concrete MCQ with answer and solution

Concept

Different tests measure different aspects of concrete strength: cube compression yields the highest value, standard cylinder compression yields a slightly lower value, and flexural tension (modulus of rupture) is the lowest.

Formula

f_cr = 0.7 √f_ck, f_cyl ≈ 0.8 f_cube

Step-by-step solution

1. Understand the types of concrete strength tests: Cube strength (f_cube) is tested on 150mm cubes and gives the highest compressive strength. 2. Cylinder strength (f_cyl) is tested on 150mm dia x 300mm height cylinders and is about 80-85% of the cube strength. 3. Modulus of rupture (f_cr) measures the flexural tensile strength and is roughly 10-15% of the compressive strength. 4. Therefore, arranging in descending order of magnitude gives: Cube strength (1) > Cylinder strength (2) > Modulus of rupture (3), which corresponds to sequence 1 -> 2 -> 3.

Explanation

The characteristic cube strength of concrete is the highest standard measure of compressive strength. The cylinder strength is typically about 0.8 to 0.85 times the cube strength due to slenderness effects. The modulus of rupture (flexural tensile strength) is much lower, typically about 10% to 15% of the compressive strength. Thus, the descending order of magnitude is Cube strength (1) > Modulus of rupture (3) > Cylinder strength (2)... wait, let's check: Cube strength (~25 MPa) > Cylinder strength (~20 MPa) > Modulus of rupture (~3.5 MPa)? Wait, let's verify standard values: Cube strength is highest (1), followed by Cylinder strength (2), and Modulus of Rupture (3) is the lowest. Therefore, the sequence in decreasing order is 1 > 2 > 3. Let's look at the options: (A) 1 -> 2 -> 3.

Why the other options are wrong

B: Cylinder strength is lower than cube strength, not higher. C: Modulus of rupture is the lowest, not between cube and cylinder. D: Cylinder strength is not greater than cube strength.