Determine the exact quantity of water:

Determine the exact quantity of water to be mixed for one bag of cement to prepare a mix of 1: 1.5: 3 proportion by weight and water cement ratio as 0.5. The water content of fine aggregates is 2.5% and the coarse aggregates are in dry state (i.e. absorbed moisture is nil). Assume that water required making the coarse and fine aggregates in SSD condition is 0.5% and 1.1% respectively.

Hint: Fine Aggregates have free (surface) moisture = 2.5 - 1.1 = 1.4%. Coarse Aggregates require additional water to make them in SSD condition = 0.5%.

Solution

Quantities required in SSD state for 1 : 1.5 : 3 proportion through weight :

i.e. 1 (Cement) : 1.5 (Fine Aggregate) : 3 (Coarse Aggregate)

Weight of one bag of cement = 50 kg

Weight of Fine Aggregate required in SSD condition = 1.5 × 50 = 75 kg

Weight of Coarse Aggregate needs in SSD condition = 3 × 50 = 150 kg

For 0.5 W/C ratio the quantity of water required = 50 × 0.5 = 25 litres

Adjustment within quantity of water for fine aggregate being in wet condition: Because sand is in wet condition and having free moisture

= 2.5 - 1.1 = 1.4%.

Quantity of water to be deducted = 75 × 0.014 = 1.05 litres. Adjustment in quantity of water for coarse aggregate being in dry condition :

Coarse aggregates, being in dry state, will require additional water to achieve SSD condition = 0.5%.

Quantity of water to be added = 150 × 0.005 = 0.75 litres

Actual quantity of water required to be added = 25 - 1.05 + 0.75 = 24.7 litres.

Actual quantity of water required to be added = 24.7 litres.