Highway Materials MCQ with answer and solution

Concept

CBR test calculation and standard penetration loads

Formula

CBR (%) = (Test Load / Standard Load) × 100

Step-by-step solution

1. Standard loads for crushed aggregate in CBR test: - At 2.5 mm penetration = 1370 kg (13.44 kN) - At 5.0 mm penetration = 2055 kg (20.16 kN) 2. Compute CBR for 2.5 mm penetration: CBR₂.5 = (60 / 1370) × 100 = 4.3796% ≈ 4.38% 3. Compute CBR for 5.0 mm penetration: CBR₅.0 = (80 / 2055) × 100 = 3.8929% ≈ 3.89% 4. The design CBR value is the higher of the two, which is 4.38%. Answer: B

Explanation

The California Bearing Ratio (CBR) evaluates the subgrade soil strength relative to standard crushed aggregate. Standard loads are 1370 kg at 2.5 mm penetration and 2055 kg at 5.0 mm penetration. For this sample: CBR₂.5 = (60 / 1370) × 100 = 4.38% and CBR₅.0 = (80 / 2055) × 100 = 3.89%. The CBR of the sample is taken as the maximum value between the two penetrations, which is 4.38%.

Why the other options are wrong

A) Incorrect: 8.76% is double the correct value. C) Incorrect: 2.19% is half the correct value. D) Incorrect: 0.43% represents the decimal fraction without multiplying by 100.

Shortcut

CBR = (60 / 1370) × 100 ≈ 4.38% (since 60/1370 > 80/2055).

Common mistake

Confusing standard loads (using 2055 kg for 2.5 mm or 1370 kg for 5.0 mm), or forgetting to take the maximum of the two.