For the production of cement concrete, water is one of the most important ingredients as hydration of cement is possible only in the presence of water. The hydration of cement forms gel which on hardening gives strength to the concrete. Thus concrete of any type cannot be prepared without water. The properties of water have been found to influence the properties of concrete to a great extent.
For concrete production water is used for the following purposes:
1. Water for preparing concrete i.e., for mixing concrete ingredients.
2. Water for washing aggregates.
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3. Water for curing concrete.
1. Quality of Water for Preparing Concrete (Mixing Water):
The common criteria or yard stick to the suitability of water for preparing concrete is that water fit for human consumption (drinking) is fit for concrete making also. But this yard stick is not true for all conditions. Water containing 0.05% sugar by weight of cement is quite fit for drinking, but it retards cement’s initial setting time by 4 hours. Thus water to be used for concrete production should not contain substances which may have appreciable harmful effect on the initial setting time, strength, and durability of the concrete.
Substances like oils, acids, carbonates and bi-carbonates, alkalis, sugar, silt and organic materials have been found to have harmful effect on the properties of the fresh and hardened concrete. Hence concrete mixing water should be free from these impurities. The pH value of concrete mixing water should be between 6 and 8.
Streams carrying large concentration of suspended solids, industrial and domestic waste, streams and wells in mining and arid alkali areas should be viewed with suspension and the effect of such waters should be determined before the use in actual construction. The effluents from paint, textile, fertilizer and sugar factories, sewage works, gas works have been found to have harmful effect on concrete.
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Effect of Water Impurities on Properties of Concrete:
1. Carbonates and Bicarbonates of Potassium and Sodium:
The carbonates and bicarbonates of potassium and sodium affect the setting time of cement. The presence of sodium carbonate accelerates the setting time, while bicarbonate may either accelerate or retard the setting of the cement. The higher concentrations of these salts will reduce the concrete strength sufficiently. Salts of Manganese, Tin, Zinc, Copper and lead reduce the concrete strength to a great extent.
Sodium iodate, sodium borate, sodium phosphate reduce the initial strength of concrete to an extra ordinarily high degree. Sodium sulphide also deteoriates the strength of concrete. Even very small quantity of sodium sulphide of the order of 100 ppm needs testing. Brackish water contains chlorides and sulphates. Chlorides upto 10,000 ppm and sulphates upto 3000 ppm are harm less.
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The presence of zinc chloride retards the rate of setting to such an extent that strength does not develop upto 72 hours. The effect of lead nitrate is completely destructive. Carbonates of sodium and potassium may cause very high rapid setting and if present in large concentration, reduce the strength of concrete. The presence of calcium chloride accelerates the setting and hardening of concrete. The quantity of calcium chloride is restricted to 1.5% by weight of cement.
Algee:
It may be present on the surface of aggregate or in mixing or washing water. It combines with cement forming a layer on the surface of aggregate and reduces the bond between the cement paste and aggregate. Algee also has the air entraining effect in large quantities in the concrete, resulting in lowering the strength of concrete.
Mineral Oils:
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Mineral oils not mixed with vegetable or animal oils have no adverse effect on the concrete strength. Concentration of mineral oils upto 2% by weight of cement have been found to increase the strength of concrete. 8% Concentration of mineral oil reduces the strength slightly. The vegetable oils have adverse effect on the strength of concrete at later ages.
The tolerance of concentration of some impurities and their effect is shown below:
i. Chloride – 10,000 ppm
ii. Calcium chloride ― 2% by weight of cement in non-pre-stressed concrete.
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iii. Sodium and potassium carbonates and bicarbonates ― 1000 ppm (total). If this limit exceeds, test for initial and setting time and 28 days strength should be carried out.
iv. Sulphuric anhydride ― 3000 ppm.
v. Sodium sulphide ― less than 100 ppm, even for 100 ppm test should be carried out.
vi. Sodium hydroxide ― 0.5% by weight of cement, if quick setting is not induced.
vii. Sodium iodates, sodium sulphate, sodium borate and arsenate ― very low
viii. Salts and suspended particles ― 2,000 ppm, with high contents of suspended solids the mixing water should be allowed to stand in a settling tank before use.
ix. Total dissolved salt ― 15,000 ppm.
x. Organic material ― 3,000 ppm. The presence of humic acid or such other organic acids affect the hardening of concrete adversely.
xi. pH ― it should be less than 6, preferably between 6 and 8.
Practical Approach:
The practical recourse for evaluating the effect of use of doubtful quality water is to carry out comparative test for setting time and soundness of cement and for strength and durability of concrete.
For comparing the compressive strength of concrete at least three 15 cms cubes be prepared using the available water at site and three cubes using distilled water and cured for 28 days under similar conditions. The cubes should be prepared and tested as per IS 516-1959.
The average compressive strength of 3 cubes prepared with available water should not be less than 90% of the average compressive strength of similar cubes prepared with distilled water. Tests have shown that water containing excessive amount of dissolved salts reduces the compressive strength from 10 to 30% of that obtained using fresh or distilled water.
The effects on compressive strength of concrete due to various dissolved salts are shown in Table 5.2 below:
The initial setting time of test samples made with appropriate cement and water proposed to be used should not be less than 30 minutes and should not differ by ± .30 minutes from the initial setting time of controlled samples prepared with the same cement and distilled water. The test should be carried as per IS 4031-1961.
The permissible limits of solids in water as per revised IS 456-2000 should be as shown in Table 5.3 below:
Effect of Acids and Alkalis:
The effect of acidity in water as gauged on the basis of total acidity, the extent of which should satisfy the following requirements.
To neutralize 100 ml (c.c.) sample of water using phenolphthalein as an indicator, it should not require more than 1 ml (c.c.) of 0.1 normal NaOH. This acidity is equivalent to 49 ppm of H2SO4 or 36 ppm of HCl.
The limit of alkalinity is guided on the basis of total alkalinity, the extent of which should satisfy the following requirement.
To neutralize 100 ml of sample of water using methyl orange as an indicator it should not require more than 5 ml (c.c.) of 0.1 normal HC. This alkalinity is equivalent to 265 ppm as carbonate (Na2CO3), 420 ppm as bicarbonate (NaHCO3) and 685 ppm as the sum of both carbonates and bicarbonates. (Both the tests should be carried out as per IS 3025).
Use of Sea Water for Mixing Concrete:
Sea water contains about 3.5% salinity. This salinity contains about 78% sodium chloride and 15% chlorides and sulphates of magnesium. Sea water also contains small quantities of sodium and potassium salts as shown in table 5.4. These salts can react with reactive aggregates in the same way as alkalies in the cement. Thus if aggregates are found alkali reactive, then sea water should not be used even for the production of plain cement concrete.
It has been reported that the use of sea water for mixing concrete does not reduce the strength of concrete appreciably, but it may lead to corrosion of reinforcement in certain conditions. Researchers are unanimous in their opinion about the use of sea water in plain cement concrete or mass concrete. Sea water has been found to accelerate the early strength of concrete slightly, but it reduces the 28 days strength by 10 to 15%.
Sea water containing large quantities of chlorides may cause efflorescence and constant dampness in the structure. Thus where appearance is important, sea water should not be used for concrete mixing. The use of sea water is also not advisable in plaster work, where the surface is likely to be painted on a later date.
Researchers differ in their opinions about the corrosion of reinforcement by the use of sea water. Some researchers have expressed concern about the risk of corrosion of reinforcement specially in tropical climatic regions, whereas others did not find any risk of corrosion due to the use of sea water.
Experiments have shown that corrosion of reinforcement occurred even when concrete was made and cured immersed in pure water but the concrete was comparatively porous. On the other side no corrosion of reinforcement was observed when concrete was made and cured immersed in sea or salty water, but the concrete was dense with adequate provision of cover to the reinforcement.
From this fact it can be concluded that the cause of corrosion of reinforcement is not the use of sea water or the quality of water where concrete is placed. The reason of reinforcement corrosion is the permeability of concrete and lack of proper cover to the reinforcement.
However, it will be wise to avoid the use of sea water for the production of reinforced concrete. For economical or other reasons if sea water has to be used then the concrete should be made dense using low water/cement ratio and vibration for compaction with the provision of 7.5 cm cover to the reinforcement.
The IS 456-2000 totally prohibits the use of sea water for mixing and curing reinforced and pre-stressed concrete works. However the specifications of this code permit the use of sea water for the production and curing of plain cement concrete under unavoidable circumstances. Sea water contains following salts. The salt composition of different sea waters in gram/lit is shown in Table 5.4.
Effect of Sugar on the Properties of Concrete:
Sugar has a marked effect on setting of cement. 0.05% sugar by weight of cement retards the setting time by 4 hours. The quantity of sugar upto 0.1 % at normal temperature has little adverse effect on the rate of hydration, but 0.2% sugar by weight has been found to retard the final setting time upto 72 hours or more. Sugar more than 1% by weight of cement has very destructive effect on setting and strength of concrete.
While some researchers say that sugar upto 0.15% by weight of cement retard setting time of cement and reduce early strength, while 28 days strength improves. 0.2% sugar by weight of cement accelerates the setting of cement. If the percentage of sugar is increased more than 0.2%, very rapid setting may occur and the 28 days strength reduces. Thus before use, test should be carried out to find out the effect of sugar contents on the properties of concrete.
2. Water for Washing of Aggregates:
The most important effect of the use of impure water for washing aggregate is the deposition of coating of salts and silt, organic matter etc. on the surface of the aggregate particles. The coating of the impurities form a layer between the gel and aggregate surface resulting poor bond between them, poor bond between aggregate and cement paste reduces the compressive strength of concrete to a great extent.
Thus the concentration of impurities in water which cause deleterious coatings on particles are more harmful than those present in mixing water. However no limits have been laid. Thus to judge the harmful effect, a comparative study has to be made. However water used to wash the truck mixer is satisfactory as mixing water as the solids in this water are proper cement ingredients.
3. Water for Curing Concrete:
Generally water suitable for mixing concrete, is also suitable for curing of concrete. However following points should be noted regarding the use of water for curing of concrete.
Generally curing water should be free from following impurities:
i. Iron or organic matter. The presence of these matters may cause staining of concrete particularly if the flow of water over the concrete is slow and evaporation is rapid.
ii. Water should be free from carbon dioxide (CO2) as it attacks hardened concrete.
iii. Water formed by melting ice or by condensation should not be used for curing as it contains little CO2. This CO2 in water dissolves forming Ca(OH)2 and cause surface erosion. Sea water also is prohibited for curing purposes.
Effect of Some Other Impurities:
1. Pure Water:
Pure water has been found to dissolve free lime from cement concrete. This reaction accelerates in the presence of CO2 as it forms calcium bicarbonate, which is soluble. Thus concrete deteriorate, if pure water is used with ordinary Portland cement. However this reaction does not take place if pure water is used with high alumina cement.
2. Marshy Water:
This water contains free carbonic acid or humic acid. Hence it is not good for good concrete production.
3. Water having Tannic Acid:
Tannic acid retards the setting time of cement as 0.05% of sugar.