Water fit for drinking; test on water

"Water should be fit for drinking"

IS 456:2000 cl. 5.4 says that water used for mixing and curing should be clean and free from injurious amounts of oils, acids, alkalis, sugar, salts, organic materials and other substances harmful to concrete or steel. Potable (drinking) water is regarded as suitable because long experience has shown that it contains only acceptable amounts of these impurities, so it needs no special testing. But the reverse is not always true: some water that is not fit to drink (e.g. slightly salty or with a bad smell) may still be fine for concrete, while sweet-tasting water containing sugar, or water with high sulphates, may not be suitable. The clause is therefore a simple, practical guide. Sea water is not allowed in RCC, and water with a pH below 6 is unsuitable.

Standard tests on water (IS 456 cl. 5.4, IS 3025)

• Chemical tests: pH (not less than 6), organic solids (max 200 mg/L), inorganic solids (3000 mg/L), sulphates as SO$_3$ (400 mg/L), chlorides (2000 mg/L for plain concrete; 500 mg/L for RCC), suspended matter (2000 mg/L).
• Physical comparison tests: the average 28-day compressive strength of three 150 mm cubes made with the test water should be not less than 90% of that of cubes made with distilled water, and the initial setting time should not differ by more than ± 30 min and should not be less than 30 min.
• Acid test: 200 mL of the sample should need not more than 2 mL of 0.1 N NaOH to neutralise it, using phenolphthalein; alkalinity test: not more than 10 mL of 0.1 N HCl, using methyl orange.

Functions of water in a concrete mix

1. Reacts with cement (hydration) to form the binding gel.
2. Acts as a lubricant, giving workability for mixing, placing and compaction.
3. Used in curing to keep hydration going and to control shrinkage and heat.
4. Washes the aggregates and helps the bonding of paste and aggregate.
5. Excess water, however, evaporates and leaves pores that decrease strength, so the w/c ratio must be controlled.