Articles

Water is an important ingredient of concrete as it actively participates in the chemical reaction with cement. Unfortunately, most of the construction engineers are cautious about the quality of cement and aggregate but not water. As a result any water available near the work site is used for making cement mortar and cement concrete. This will often results in poor quality of concrete. Such a concrete may have less compressive strength and less durability.

A popular yard stick to the suitability of water for mixing with cement and aggregates is that, if water is fit for drinking it is fit for making concrete. This assumption is not correct many times. Water having moderately high alkalinity (> 250 mg/l) is used for drinking purpose but not for concreting. Hence, it is absolutely necessary to test water sample in a well established laboratory before using the same for construction

Water pollution is a problem in India and in many parts of the world. Both surface and ground waters are polluted by industrial, agricultural and various other commercial activities. The untreated sewage from urban areas is also contributing for water pollution. The construction engineers should be aware that polluted waters are not suitable for making concrete. The ground water from great depths normally contains high concentration of dissolved inorganic salts and hence not suitable for making concrete.

Carbonates and bicarbonates of Sodium and Potassium affect the setting time of cement. Salts of Manganese, Tin, Zinc, Copper and Lead cause a marked reduction in strength of concrete. Algae in mixing water not only reduce the strength of concrete but also affect durability. Concrete will become porous and pervious. Chlorides in mixing water accelerate corrosion process and sulfates attack cement.

Hence, in Plain and Reinforced Concrete Code of Practice IS: 456- 2000, standards have been specified for the quality of mixing water to make concrete. These standards although look stringent, absolutely necessary to produce quality concrete. The permissible limits for various water quality parameters as per the above code are shown in Table 1. Although, the limits have not been specified in the code for hardness and Iron concentrations, they have been added in the list of parameters for testing purposes. High concentration of iron (>0.08 mg/l) in curing water causes staining on concrete surfaces. High hardness in curing water causes white deposits of Calcium carbonate on concrete surface.

Table 1 Water Quality Parameters and Permissible Limit

^*For Plain concrete works the permissible limit for Chlorides is 2000 mg/l

In case of doubt regarding development of strength, the suitability of water for making concrete shall be ascertained by the compressive strength and initial setting time tests specified in the Clauses 5.4.1.2 and 5.4.1.3 of IS: 456-2000. They are,

i) Average 28 days compressive strength of at least three 150 mm concrete cubes prepared with water proposed to be used shall not be less than 90 percent of the average of strength of three similar concrete cubes prepared with distilled water.

ii) The initial setting time of test block made with appropriate cement and the water proposed to be used shall not be less than 30 min and shall not differ by ±30 min from the initial setting time of control test block prepared with the same cement and distilled water.

The water suitable for mixing is also suitable for curing concrete. However, water used for curing should not produce any objectionable stain or unsightly deposit on the concrete surface. The presence of iron compounds or high hardness is objectionable.

Several water samples have been tested to check the suitability for plain and reinforced to cement concrete constructions at Environmental Engineering Laboratory, Department of Civil Engineering, NIE, Mysore. Water samples from surface sources such as streams and rivers have normally satisfied the requirements for construction purposes. But, majority of bore well water samples from Mysore and adjoining districts have failed to satisfy the requirements for construction purposes. This is mainly due to the presence of inorganic salts, in particular alkaline compounds. While the permissible limit for alkalinity is 250 mg/l the bore well water samples have shown alkalinity values in the range of 300 to 400 mg/l.

The construction engineers are advised against using such waters which do not satisfy the requirements as per the IS code to produce quality concrete. To produce high strength concrete low water cement ratio has to be maintained. I.e. a small quantity of water is required to produce a cubic metre of concrete. Hence, if good quality of water is not available near the work site it is always safe to get water from a good source rather than compromising with quality of water. Many times, representatives of the contractors or construction companies bring tampered water samples to the water testing laboratories just to get a certificate. The engineers and contractors should be aware that such activities will not benefit the construction industry in the long run.