The quantity and quality of water used determine the strength of concrete, says A.R. Santhakumar
Water is the most important material for construction, especially for making concrete.
It is also the least expensive on site.
Hence careless use of more water has lead to poor quality work.
The purpose of water in concrete is three-fold:
It distributes the cement evenly
It reacts with cement chemically and produces calcium silicate hydrate (C-S-H) gel which gives the strength to concrete.
It provides for workability, i.e., it lubricates the mix.
Hence, for construction, quantity and quality of water is as important as cement.
Amount of water
The reaction products of hydration consist of 20 to 30 per cent of calcium hydroxide which is crystalline.
These crystalline products are surrounded by calcium silicates and aluminates which are in colloidal form.
The entire composition of CaOH2 and C-S-H is called the ‘gel'. When this gel hardens, we get adequate strength to cement paste and thus to concrete.
Creation of pores
Only a small amount of water is needed to hydrate cement. Additional water is required to lubricate the mix. Too much of water will create pores.
With the given concrete materials and other conditions remaining the same, the quantity of water mixed also determines the strength of concrete so long as the mix is workable. This law, though developed by Abrams in the U.S., has now been adopted universally by all countries.
There are two sources of water in concrete:
Intentionally added water, known as mix water
Aggregate moisture: this can act in two ways - either it can add water to mixture or can absorb water from the mixture
A thumb rule is to use not more than 0.5:1 as water-to-cement-ratio for making a dense concrete. In coastal areas this ratio may be limited to 0.4:1.
As water quantity goes up in a mix, the following are the effects: strength decreases, durability decreases, workability increases, cohesion decreases and economy may increase at the expense of quality and reliability.
Since workability increases with increase in water content, the general tendency of site workers will be to increase the amount of water in the mix. This should be avoided at any cost by proper supervision.
Water has to be potable
Water used for mixing and curing should be free from oil, acid and alkali, salts and organic material. It should be potable and concreting generally requires a value purer than that of drinking.
Uncertainty in quality
Whenever there is uncertainty in quality, water should be tested before use. Even chlorine added for city water supply will affect concrete if used carelessly without proper testing and treatment.
Chlorides can cause corrosion of steel reinforcement and can accelerate setting. The water used may be contaminated with chlorides because of seawater, some admixtures, salts or deliberate chlorination for disinfections.
Sulphates reduce long-term strength levels.
Organic matter effects on concrete are variable. If algae is present, water should not be used. It will affect the setting and strength development.
Sugar will retard setting time. Too much of it may ‘kill' the concrete (the concrete will never set).
Wastewater should never be used in construction. Water for curing should be pure.