WATER

INTRODUCTION OF water The purpose of using water with cement is to cause hydration of the cement. Water in excess of that required for hydration acts as a lubricant between coarse and fine aggregates and produces a workable and economical concrete. As discussed later in Chapters 10, 11 and 12, there is a definite optimum water requirement for a particular concrete and mortar. In case of excess water, the cement along with water comes to the surface by capillary action and forms a thin layer over surface known as laitance. This weakens bond between the successive lifts of concrete. The excess water may leak through the form work, resulting in honeycombed concrete and on evaporation makes the concrete porous.

The excess water may leak through the form work, resulting in honeycombed concrete and on evaporation makes the concrete porous. On the other hand lesser water makes it difficult to work with concrete and because of no uniform mixing the resultant concrete is weaker in strength. On the other hand lesser water makes it difficult to work with concrete and because of nonuniform mixing the resultant concrete is weaker in strength. The amount of water must therefore be limited to produce concrete of the quality required for a job. Water is also used for washing aggregates and curing.

Quality of mixing water Almost any natural potable water that has no pronounced taste or odour is acceptable for the concrete mix. In case of a doubt, water samples should be tested for suitability. Excessive impurities may affect setting time, strength, durability and may cause efflorescence, surface discolouration, and corrosion of steel.

Suspended Particles: Water containing less than 2000 ppm (parts per million) of dissolved solids can generally be used satisfactorily for making concrete. A higher concentration affects certain cements adversely. Inorganic Salts: The presence of salts of zinc, manganese, tin, copper and lead considerably reduce the concrete strength. Sodium phosphate, sodium borate and sodium iodate act as retarders and cause a marked reduction in the strength of concrete.

Acids and Alkalis: Water containing acids or alkalis (industrial waste water) is supposed to be unsuitable for making concrete. Water with pH value between 6-8 should only be used. Sugar: Sugar up to 0.05 per cent by weight of water is harmless. Sugar up to 0.15 per cent by weight of cement retard the setting time, reduce the early strength and increase the 28 day strength. When used up to 0.2 per cent by weight of cement rapid setting with reduced 28 day strength is resulted.

Oil Contamination: Various kinds of oil are occasionally present in the mixing water. Mineral oil (petroleum), not mixed with animal or vegetable oils, probably has less effect on development of strength than other oils. Algae:- Algae, present in mixing water or on the surface of aggregate either reduces bond by combining with the cement or reduces strength by entraining a large amount of air in the concrete.

Effect of mixing water from different sources Ground Water: Natural ground waters seldom contain more than 20 to 30 ppm of iron. However, acid mine waters may carry rather large quantities of iron. Iron salts in concentrations up to 40,000 ppm do not usually affect mortar strengths adversely.

Sea Water: Sea water may be used if suitable fresh water is not available. The sea water generally contains 3.5 per cent of salts with about 75 per cent of sodium chloride, about 15 per cent of chloride and sulphate of magnesium. It has been found to reduce the strength of concrete by 10-20 per cent and slightly accelerate the setting time.

Industrial waste Water:- Most waters carrying industrial waste have less than 3,000 ppm of total solids. When such water is used as mixing water in concrete, the reduction in compressive strength is generally less than about 10 per cent. Waste waters from paint factories, coke plants chemical and galvanizing plants may contain harmful impurities. It is advisable to test any waste water that contains even few hundred parts per million of unusual solids before using it for mixing concrete.

Water for washing aggregate When aggregates are washed with water containing impurities, they get coated with layers of silt, salts and organic matters. These reduce the bond between the aggregates and cement and markedly affect the strength.

Curing Water Water fit for making concrete can be used for curing. Waters containing impurities and leading to stains is objectionable. When concrete is subjected to prolonged wetting, even a very low concentration of iron and organic matter may cause staining. Water containing more than 0.08 ppm of iron is not recommended for curing.

Water-Cement rRatio The water-cement ratio (w/c) is one of the major factors but not the only one influencing the strength of concrete. It is responsible mainly for the porosity of the hardened cement paste. Water-cement ratio is the water used to the quantum of cement in the mixture by weight. For proper workability the w/c ratio varies from 0.4–0.6.