In a 1:3:6 concrete mix, the actual quantity of sand per unit volume of cement, if bulking of sand is 15%, is:

Detailed Explanation: Bulking Correction for 1:3:6 Mix

Why B (3.45) is correct: The mix specifies 3 parts dry sand per 1 part cement. With 15% bulking, the moist sand occupies more volume. To deliver 3 dry-volume units of sand, we must measure an increased (bulked) volume. Specified dry sand per unit cement = 3 volumes
Bulking = 15% = 0.15

Measured moist sand volume = 3 × (1 + 0.15)
= 3 × 1.15 = 3.45 volumes

General Bulking Correction Table

Mix	Specified Sand	Bulking	Measured Volume
1:2:4	2	20%	2 × 1.20 = 2.40
1:3:6	3	15%	3 × 1.15 = 3.45
1:1.5:3	1.5	25%	1.5 × 1.25 = 1.875

• For 1:3:6 with 15% bulking: measure 3.45 volumes of moist sand per unit cement.