The following measures suggested by IS 7861-Part-II 1981 should be adopted to avoid freezing of concrete: 1. Preservation and Utilization of Heat of Hydration 2. Selection of Suitable Type of Cement 3. Pre-Heating of Concrete Ingredients (Materials) 4. Aggregate Heating 5. Cement Heating 6. Use of Accelerating Admixtures 6. Use of Accelerating Admixtures 7. Use of Air Entrainment and a Few Others.
Measure # 1. Preservation and Utilization of Heat of Hydration:
During the process of cement hydration, enormous amount of heat is generated. If this heat is preserved with in the body of the concrete for a duration equal to pre hardening period, it can offset the harmful effect of low temperature. The time interval after placing the concrete till it attains a certain amount of strength is known as pre-hardening period. This period is shown in Table 18.1.
To conserve the heat, the concrete can be insulated by a membrane, saw dust, burlap or Hession cloth. In beams and columns the form work should not be stripped till the concrete has attained sufficient strength. Strength from 5 MPa to 14 MPa has been suggested but there is no reliable data available on the strength at which concrete can successfully resist temperature below freezing point.
An alternative approach is to consider the minimum age of concrete stored at a given temperature when the exposure to frost will not cause damage (Table 18.1). In case of very low temperature the insulation of concrete should be efficient enough so that the surface temperature of concrete remains much higher than 0°C during the pre-hardening period.
Measure # 2. Selection of Suitable Type of Cement:
Certain types of cement hydrates fast producing much larger quantity of heat and developing early strength. Such Cements contain higher percentage of tri calcium silicate (C3S) and tri calcium aluminate (C3A) and comparatively lower percentage of di-calcium silicate (C2S). The pre-hardening period for such cement is about 40 to 50% of the normal port-land cement. For low temperature concreting rapid hardening cement and extra rapid hardening cements can be used. High alumina cement or mixture of port-land and high alumina cement in certain proportions can also be used.
Measure # 3. Pre-Heating of Concrete Ingredients (Materials):
Pre heating of the ingredients of concrete is one of the very common methods employed to raise the temperature of the concrete at the time of casting in subzero temperature concreting.
Heating of Water:
Heating of water is the easiest to be adopted. If the ambient temperature is not very low, heating of water alone will be sufficient in case cement and aggregates are not in frozen condition. While using hot water, it should be remembered that the temperature of the water should not exceed 60°C to 80°C (140°F to 180°F). Usually water should not be heated above 65°C (149 to 150°F) as the flash set of cement will take place when hot water comes in contact with cement in the mixer. Therefore heated water should come in direct contact with the aggregate first and not the cement.
The Figs. 18.9 (a), (b) & (c) show the temperature to which the water should be heated to maintain the temperature of the resultant concrete for different aggregate/cement ratio and cement content. It will be seen from these Figs, that heating of water alone is sufficient provided the aggregate is free from ice and the form work and top surface of concrete is properly insulated. In situations where the temperature is very low or good amount of heat is likely to be lost during transportation and placing, heating of water alone will be inadequate to maintain the required temperature of the concrete. In such cases heating of aggregate also becomes necessary.
Measure # 4. Aggregate Heating:
Aggregate should never be heated directly as it will break during heating forming small pieces of coarse aggregate and disturbing it’s grading. The heating of aggregate preferably be done by passing steam through coils embedded under the stock piles. Heating of aggregate may also be done by injecting live steam into the stock piles or by hot air blowers, but these methods lead to a variable moisture content of the aggregate. Fine aggregate may also be heated on hot plates. Overheating of aggregate should be avoided. The aggregate should not be heated above 52°C (125°F).
Measure # 5. Cement Heating:
Cement should never be heated.
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The temperature of mix ingredients must be controlled. In order to make sure that setting of concrete does not take place at too high a temperature, the temperature of the resulting concrete should be calculated in advance by equation 18.1. The temperature of resulting concrete should remain between 10°C (50°F) to 20°C (68°F) and during the period of 3 days after placement of concrete the temperature should not fall below about 10°C (50°F).
However temperature upto 20°C to 21°C (68 to 70°F) and a longer period of controlled temperature is preferable. The curing period required before the concrete is exposed to frost is shown in Table 18.1. This period may also be calculated from Maturity equation.
The pre-hardening temperature of concrete may be maintained by electrical heating where ever economically feasible. For this purpose only low alternating current should be used. The evaporation of mixing water has to be restricted by covering the entire surface area effectively with vapour tight membrane.
The heating of concrete should be such that no part of it is heated excessively and the concrete does not dry out rapidly. Also no high concentration of carbon dioxide CO2 in the atmosphere should take place. For these reasons the exhaust steam heating is the best.
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In spite of all these precautions, the strength of electrical heated concrete is found about 80% of the normally cured concrete due to stresses developed during cooling of the heated concrete. Electrical heating is disadvantageous due to the high cost of installation, power consumption and loss of strength. However in Russia, Japan and Sweden electrical curing has been used quite extensively for protecting concrete from frost.
Measure # 6. Use of Accelerating Admixtures:
In cold weather concreting the use of accelerating admixtures has been widely adopted, which incidentally work as anti-freezer. The most commonly used material is calcium chloride (CaCl2). Some researchers do not permit the use of calcium chloride CaCl2 more than 3.0% of the weight of cement due to developing of flash set and loss of long term strength. On the other hand others do not find any harm in the use of much higher quantity of calcium chloride where the temperature is very low.
However the use of calcium chloride may cause some increase in volume change, greater alkali aggregate reaction and lower resistance to sulphate attack.
It is reported that in Russia, Calcium, and sodium chlorides and potassium have been widely used for concreting at subzero temperature. They have tried CaCI2 as high as 20% by weight of mixing water in the construction of Gorky-Hydro Power Project. Russian practice is to use the combination of CaCl2 and NaCl to neutralize the effect of temperature and also to give maximum benefits to the concrete at the plastic stage as well as hardened stage.
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In order to lower the freezing point of the mixing water and to develop some strength, potash (K2CO3) has been tried in Russia. As potash accelerates setting of concrete, a retarder also has been used along with it. It is also has been reported that the use of potash neither adversely affects bond with steel nor it encourages corrosion of steel. Thus the use of potash in winter concreting has good potential.
Generally it is believed that the use of sodium chloride NaCl reduces the 28 days strength of concrete by 12%, though it accelerates the early strength. Therefore it has never been accepted as one of the standard accelerating agent.
All methods of winter concreting are based on maintaining positive temperature during the early age of concrete, till it develops strength which is not adversely affected by the freezing temperature.
The addition of CaCl2 and NaCl regulate the freezing point of water in concrete and help to promote the continued hydration. CaCl2 makes C3A more active for accelerating the hydration. If NaCl alone is used more or less same condition will result as with CaCl2 except that NaCl exhibits a better plasticizing properties. In case a combination of CaCl2 and NaCl is used the concrete is found to develop quicker hardening property, much higher frost resistance property and also placeability characteristics.
The suggested quantities of the salt mixture are shown in Table 18.3 below:
It has been observed that if the concrete is well compacted, addition even of large quantity of salts result in very little corrosion of reinforcement. The corrosion problem becomes serious in the case of insufficient compaction and lack of cover to the reinforcing steel.
Measure # 7. Use of Air Entrainment:
Till recently it was considered that the durability of concrete depends only upon the compressive strength of concrete. But now it has been shown that even a weak concrete with air entrainment is more durable under freezing conditions than that of strong concrete without air entrainment. The superiority of air entrained concrete over plain concrete is shown in Fig. 18.10.
Fig. 18.10 illustrates the influence of the w/c ratio on the frost resistance of concrete moist cured for 28 days. The quantity of entrained air of about 5% is sufficient. Generally it is observed that cement paste contains about 9% voids. Thus the thickness of cement paste between air bubbles should not be more than 0.25 mm.
Fig. 18.11 illustrates the effect of w/c ratio on the frost resistance of concrete moist cured for 14 days and then stored in air at 50% relative humidity for 76 days before exposure to freezing and thawing. In general it can be said that air entrainment increases the durability of concrete to the extent of about 3 to 7 times that of ordinary concrete.
Measure # 8. Use of Light Weight Aggregate in Cold Weather Concreting:
In cold weather concreting light weight aggregate is advantageous as the light weight aggregate concrete has a lower thermal conductivity than normal concrete and thus acts as a self-insulator. Light weight aggregate concrete has a low specific heat. Thus the heat of hydration of cement keeps this concrete more effectively against freezing than the normal weight aggregate concrete.
Measure # 9. Reduction in Water/Cement Ratio:
The increase in cement content may generate more heat of hydration, resulting higher strength in concrete. Also by the use of air entrainment the water-cement ratio may be reduced which will produce greater resistance to freezing and thawing action. Thus it can be said that the use of air entraining admixtures in concrete is positively useful in increasing the durability of concrete in very cold regions.