2.3 Water -
Compared to other ingredients of concrete, the quality of water usually receives less attention. However, unwanted situations leading to distress of concrete, contributed among others by the mixing and curing water being not of the appropriate quality has focused the attention on quality of water as well.
Potable water is generally considered satisfactory for mixing concrete. The permissible limits for solids and impurities for mixing and curing water as specified in various specifications including IS : 456 are in excess of the requirements of potable water.
The following concentrations
represent the maximum permissible values:
a) To neutralize
100 ml sample of water, using 1 phenolphthalein as an indicator, it should not
require more than' ml of 0.02 normal NaOH. The details of test are given in 8.1
of IS 3025(Part 22).
b) To neutralize 100 ml sample of water, gains mixed indicator, it should not require more than 2 ml of 0.02 normal H2SO•. The details of test shall be as given in 8 of IS 3025 (Part 23).
c) Permissible limits for solids shall be as liven in Table 1.
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% of the average of strength of three similar concrete cubes prepared with distilled water.
The initial setting time of test block made with the appropriate cement and the water proposed to be used shall not be less than 30 min and shall not differ by ±30min from the initial setting time of control test block prepared with the same cement and distilled water.
The pH value of water shall be not less than 6.
Based
on the minimum strength ratio of 85%, the following waters were found to be
suitable for concrete-making:
bog and marsh waters; waters with a maximum concentration of 1% SO4, sea water, but not for reinforced concrete; alkali water with a maximum of 0.15% Na2 SO4 or Nacl; pumpage water from coal and gypsum mines; and waste water from slaughter house, breweries, gas plants, paint and soap factories.
The waters found unsuitable for the
purpose were acid water, lime soak water from tannery waste, carbonated mineral
water discharged from galvanizing plants, water containing over 3 % of
sodium chloride or 3.5 % of sulphates and water containing sugar or similar
compounds.
The lowest content of total
dissolved solids in these unacceptable waters was over 6000 ppm except for a
highly carbonated mineral water that contained 2140 ppm of total solids.
2.3.4 SEA WATER -
Sea water generally contains 3.5 %
of dissolved solids, about 78 % (that is, 27000 ppm) of which is sodium
chloride and 15 % (i.e, 5300 ppm) of which is chloride and sulphate of
magnesium. Opinions differ when it comes to categorically classifying sea water
as either allowable or not for use for making concrete.
Mixing or curing of concrete with
sea water is not recommended because of presence of harmful salts in water.
Under unavoidable Circumstances sea water may be used for mixing or curing in
plain concrete with no embedded steel after having given due consideration to
possible disadvantage and precaution including using of appropriate cement
system.
Water found satisfactory 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 tannic add or iron compounds is objectionable.
2.3.5 CURING WATER -
IS : 4S6 states that water found
satisfactory for mixing concrete can also be used for curing concrete but it
should not produce any objectionable stain or unsighty deposit on the surface.
Iron and organic matter in the water are chiefly responsible for staining or discolouration
and especially when concrete is subjected to prolonged wetting even a very low
concentration of these can cause staining.
According to IS: 456, the presence
of tannic acid or iron compounds in curing water is objectionable.
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