Water is an important ingredient of concrete as it actively participates in the chemical reaction with cement. Since it helps to form the strength giving cement gel; the quantity and quality of water required to look into very carefully. Here quality of water has been discuss. In practice, very often great control on properties of cement and aggregate exercised. But the control on the quality of water is often neglect. Since quality of water affects the strength. So it is necessary for us to go into the purity and quality of water.
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Qualities of Water
A popular suitability of water for mixing concrete is that, if water is fit for drinking; it is fit for making concrete. This does not appear to be a true statement for all conditions. Some waters containing a small amount of sugar would be suitable for drinking but not for mixing concrete. Conversely water suitable for making concrete may not necessarily fit for drinking. Few specifications require that if the water not obtained from source that has proved satisfactory; the strength of concrete/mortar made with questionable water should then compared with similar concrete/mortar made with pure water.
Some specification also accept water for making concrete if the pH value of water lies between 6 and 8. Also the water is free from organic matter. Instead of depending upon pH value and other chemical composition; the best course to find out whether a particular source of water is suitable for concrete making or not; is to make concrete with this water and compare its 7 days’ and 28 days’ strength with cubes made with distilled water. If the compressive strength is upto 90 percent, the source of water may be accepted.
This criteria may safely adopted in places like coastal area of marshy area. Or in other places where the available water is brackish in nature and of doubtful quality. However, it is logical to know what harm the impurities in water do to the concrete. And what degree of impurity is permissible is mixing concrete and curing concrete.
Carbonates and bi-carbonates of sodium and potassium effect the setting time of cement. While sodium carbonate may cause quick setting, the bi-carbonates may either accelerate or retard the setting. The other higher concentrations of these salts will materially reduce the concrete strength. If some of these salts exceeds 1000 ppm, tests for setting time and 28 days strength should be carried out. In lower concentrations they may be accepted.
Brackish water contains chlorides and sulphates. When chloride does not exceed 10,000 ppm and sulphate does not exceed 3,000 ppm; the water is harmless, but water with even higher salt content has been used satisfactorily.
Salts of Manganese, Tin, Zinc, Copper and Lead cause a marked reduction in strength of concrete. Sodium iodate, sodium phosphate, and sodium borate reduce the initial strength of concrete to an extra-ordinarily high degree. Another salt that is detrimental to concrete is sodium sulphide and even a sulphide content of 100 ppm warrants testing.
Silts and suspended particles are undesirable as they interfere with setting, hardening and bond characteristics. A turbidity limit of 2,000 ppm has been suggested. Following table shows the tolerable concentration of some impurities in mixing water.
The initial setting time of the test block made with a cement and the water proposed to be used shall not differ by +-30 minutes from the initial setting time of the test block made with same cement and distilled water.
The following guidelines should also be taken into consideration regarding the quality of water.
- To neutralize 100 ml sample of water using phenoplhaline as an indicator, it should not require more than 5 ml of 0.02 normal NaOH.
- To neutralize 100 ml of sample of water, using mixed indicator, it should not require more than 25 ml of 0.02 normal H2SO4.
- Permissible limits for solids is as given below in table.
Algae in mixing water may cause a marked reduction in strength of concrete either by combining with cement to reduce the bond or by causing large amount of air entrainment in concrete. Algae which are present on the surface of the aggregate have the same effect as in that of mixing water.
Use of Sea Water for Mixing Concrete
Sea water has a salinity of about 3.5 percent. In that about 78% is sodium chloride and 15% is chloride and sulphate of magnesium. Sea water also contain small quantities of sodium and potassium salts. This can react with reactive aggregates in the same manner as alkalies in cement. Therefore sea water should not used even for PCC if aggregates are_known to be potentially alkali reactive. Its reported that the use of sea water for mixing concrete does not appreciably reduce the strength of concrete; although it may lead to corrosion of reinforcement in certain cases.
Research workers are unanimous in their opinion, that sea water can be used in un-reinforced concrete or mass concrete. Sea water slightly accelerates the early strength of concrete. But it reduces the 28 days strength of concrete by about 10 to 15 percent. However, this loss of strength could be made up by redesigning the mix. Water containing large quantities of chlorides in sea water may cause efflorescence and persistent
dampness. When the appearance of concrete is important sea water may be avoided. The use of sea water is also not advisable for plastering purpose-which is subsequently going to be painted.
Divergent opinion exists on the question of corrosion of reinforcement due to the use of sea water. Some research workers cautioned about the risk of corrosion of reinforcement particularly in tropical climatic regions; whereas some research workers did not find the risk of corrosion due to the use of sea water. Experiments have_shown that corrosion of reinforcement occurred when concrete was_made with pure water and immersed in pure water. When the concrete was comparatively porous, whereas, no corrosion of reinforcement was_found when sea water was_used for mixing. Also the specimen was_immersed in salt water when the concrete was dense and enough cover to reinforcement was_given.
Conclusion of research
From this it is_concluded that factor for corrosion is; not the use of sea water or the quality of water where the concrete is placed. The factors effecting corrosion is permeability of concrete and lack of cover. However, since these factors cannot be adequately taken care of always at the site of work. It may be wise that sea water be_avoided for making reinforced concrete. For economical or other passing reasons, if sea water cannot be avoided for making reinforced concrete; particular precautions should be take to make the concrete dense. This_is done by using low water/cement ratio and to give an adequate cover of at least 7.5 cm. The use of sea water must be_avoided in prestressed concrete work. Because of stress corrosion and undue loss of cross section of small diameter wires.
The latest IS 456 : 2000 prohibits use of sea-water for mixing and curing of reinforced concrete and prestressed concrete work. This specification permits the use of Sea Water for mixing and curing of plain cement concrete (PCC) under unavoidable situation.
It is pertinent at this point to consider the suitability of water for curing. Water that contains impurities which caused staining, is objectionable for curing concrete members whose look is important. The most common cause of staining is usually high concentration of iron or organic matter in the water. Water that contains more than 0.08 ppm. Of iron may be avoided for curing if the appearance of concrete is important. Similarly the use of sea water may also avoided in such cases. In other cases, the water, normally fit for mixing can also used for curing.