Whenever concrete is to be placed under extreme weather conditions or under water its performance is affected to a great extent if appropriate measures are not taken to control these conditions. Extreme weather conditions include situations where environmental temperature during the production, and placing of concrete and subsequent curing period are markedly different from those of normal conditions i.e., either the temperature is too low or too high. Under these conditions the properties and performance of concrete are affected to a great extent unless appropriate precautions are taken to safe guard against these factors.
In general an increase in temperature accelerates the rate of hydration resulting in the accelerated development of strength. This accelerated hydration of cement may result in the development of less uniform gel than that could be produced under normal conditions. On the other hand the decreased temperature retards the rate of hydration and the strength development, but improves the quality of gel resulting in a more orderly and compact gel.
The decrease in humidity, or increase in wind velocity or their combination further aggravates the situation. This may cause rapid loss of water due to evaporation affecting the workability of fresh concrete. The rapid drying of concrete may develop plastic shrinkage and cracking. Due to evaporation, cooling of concrete may develop tensile stresses in it.
Hot Weather Concreting:
Actually it is difficult to define the hot weather condition. However for the sake of convenience any operation of concreting done at an atmospheric temperature above 40°C (94°F) or when the temperature of concrete at the time of placement is expected to be above 40°C is considered as hot weather concreting. Concreting at a temperature above 40°C is not recommended without a proper precaution as specified in IS 7861 Part-III 1975.
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The increase in the wind velocity further enhances the effect of high ambient temperature and reduced relative humidity. These are the most important climatic factors which affect the concreting in hot weather conditions.
Problems of Hot Weather Concreting:
At the temperature above 40°C, following special problems are encountered:
1. Rapid rate of hydration of cement, quick setting and early stiffening.
2. More plastic shrinkage.
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3. Rapid evaporation of mixing water.
4. Less time for finishing.
5. Absorption of water from the concrete by the form work and subgrade.
6. Difficulty in un-interrupted and continuous curing.
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7. Difficulty in incorporation of air entrainment.
The effect of the above factors on the quality of concrete should be fully investigated and care should be taken to produce a strong and durable concrete.
1. Rapid Rate of Hydration:
The rate of hydration depends upon the temperature, higher the temperature, higher the rate of hydration. At high ambient temperature the setting time of the cement is also reduced considerably. For proper setting of cement the temperature should be in the range of 27 ± 2°C. At a higher temperature, the setting time will be reduced resulting in early stiffening of the concrete, reducing the workability of concrete. The partially set concrete will have poor bond with the successive layers more than anticipated.
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The gel structure and its quality formed at higher temperature in the early period of hydration are of poor quality. Concrete placed in hot weather no doubt will develop high early strength, but its long term strength will be less as shown in Fig. 8.14. This Fig. shows the effect of curing temperature on one day and 28 days compressive strength, which reduces about 40% due to curing at higher temperature. It has been observed that an increase of 11°C temperature of concrete resulted in about 25mm decrease in its slump.
2. More Plastic Shrinkage:
In hot weather conditions, the rate of evaporation of water from the surface of the concrete will be faster than the rate of movement of water from the interior body of the concrete to the surface. This will cause the setting up of moisture gradient, resulting in the formation of surface cracks. These surface cracks are known as plastic shrinkage cracks. The plastic shrinkage cracks are more pronounced where the exposed surface area is more than the depth of the concrete layer as in the case of roads, floors and pavement etc.
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3. Rapid Evaporation of Mixing Water:
The hot weather condition normally is associated with the lower relative humidity. Due to the lower relative humidity, the water mixed in the concrete for the required workability will be lost soon turning the concrete unworkable. To compact such a concrete fully, enormous amount of compacting energy will be required. If it is not done large voids will remain in the concrete, reducing it to a very poor quality concrete, having all ill effects.
4. Less Finishing Time:
In hot weather conditions, the finishing of the concrete must be done as early as possible after placing it in position. If it is not done due to quicker evaporation of water and faster stiffening, the quality of finishing will be of poor standard. Usually for finishing in such conditions extra fresh mortar has to be applied, resulting in poor performance of the concrete.
5. Absorption of Water by Subgrade and Form Work:
In hot weather regions usually the subgrade is dry and absorptive. Thus the subgrade or the surface of form work has to be wetted before placing the concrete. If it is not done properly, the water will be absorbed by the subgrade and form work, leaving less water for proper hydration of cement. The concrete in contact with the subgrade and form work will be formed of poor quality. Thus the contact zone of concrete will be of poorer quality.
6. Difficulty in Curing:
In hot weather concreting early curing becomes necessary particularly if 53 grades cement is used. Hot weather needs a continuous curing. If there is any discontinuity in the curing, the concrete surface will dry up fast, resulting interruption in the continuous hydration. Once the interruption in curing takes place, the subsequent wetting will not contribute to the development of full strength. Continuous curing in hot weather costs more due to the high cost of water and labour.
7. Difficulty in Air Entrainment:
Usually air entrained concrete is not used in hot-weather conditions. However, if used for better workability, the proportion of air entraining agent will be required more to compensate the loss of air entrainment due to higher temperature. The normal set for standard temperature in respect of percent air entrainment does not apply for hot weather conditions.
8. Conveyance of Concrete:
Conveyance of readymade concrete over a long distance is likely to pose a serious problem due to faster loss of slump. In such situations the transit mixer drum may be covered with insulating material.
9. Use of Curing Compounds:
In hot weather concreting the use of curing compounds has not been found satisfactory as it leads to lower strengths than continuous water cured concrete strength. The reduction in strength is of the order of 30%.
Influence of Reduction in Relative Humidity on Concrete:
The reduction in relative humidity has a marked influence on the strength of the concrete. It has been observed that specimens moulded and cured in air at 23°C and 60% relative humidity developed 73% strength of the specimens moist cured at 23°C for 28 days. The specimens moulded and cured in air at 38°C and 25% relative humidity attained only 62% strength of the moist cured specimens at 23°C for 28 days. The reduction of 35% in relative humidity has been found to decrease the strength by about 11%.
Thus due to these difficulties concreting in important structures be stopped during hot periods. The temperature of concrete delivered at site in hot weather should be as low as possible at about 16°C (60°F). The upper limit of 29°C (85°F) should never be crossed.
Temperature of Fresh Concrete:
The temperature ‘T’ of freshly mixed concrete can be calculated easily from the temperatures of ingredients by the following relation.
where,
Ta = temperature of aggregate
Tc = temperature of cement
Tw = temperature of water
Wa = weight of aggregate
We = weight of cement
Ww = weight of water
The actual temperature of the concrete how ever has been found somewhat higher than given by the above relation (18.1) due to the mechanical work done in mixing and due to heat of wetting and hydration of cement etc. Experimentally it has been found that for a mix having aggregate/cement ratio of 5.6 and water/cement ratio as 0.5, a drop of 1°C or 1°F in the temperature of freshly mixed concrete can be obtained by lowering the temperature either of cement by 9°C (9°F) or of water by 3.6°C (3.6°F) or of aggregate by 1.6°C (1.6°F) respectively.
The quantity of cement in a given mix being small, its temperature is not important. Though the use of hot cement is not detrimental to the strength, but cement at temperature about 75°C (170°F) should not be used. (The specific heat of common aggregate is 0.22, while that of water is 1.0 i.e., the specific heat of water is five times that of aggregate. Due this reason term 0.22 has been used in relation (18.1). Hence it is much easier to cool water than cement or aggregate.
Measures to Minimize the Temperature Difference or Gradient of Concrete:
Several measures can be taken to minimize the temperature gradient as follows:
1. Cooling of the mix ingredients by any method as to reduce the temperature of the fresh concrete to about 7°C (45°F). By this measure the difference between the peak temperature and ambient temperature on cooling will be reduced.
2. Cooling the surface of the concrete, but only for those sections whose thickness is less than 50 cm. For such sections steel form work may be used which offers little insulation. Thus cooling of the concrete surface reduces the temperature rise of the core without causing harmful temperature gradient inducing internal restraint.
3. Insulating of the entire surface of the concrete more than 50 cms in thickness including the upper surface. This can be achieved by using a suitable material for form work. In this way the temperature gradient may be minimized. Now the concrete will be allowed to expand and contract freely, provided there is no external restraint.
4. Selecting mix ingredients carefully.
The choice of the mix ingredients partly is dependent upon other factors also which influence cracking besides the temperature. A suitable aggregate can help to reduce the coefficient of thermal expansion of concrete and increase its tensile strain capacity. For example, concrete made with angular aggregate has a greater tensile strain capacity than concrete made with rounded aggregate.
Similarly light weight aggregate leads to a greater tensile strain capacity than normal weight aggregate. However this advantage is reduced to some extent by the higher requirements of cement content when light weight aggregate is used for the same strength and workability.
The use of low heat cement, pozzolanic cement, part replacement of cement by pozzolanic materials, use of low cement content and of water reducing admixtures have been found beneficial in reducing the peak temperature. The choice of the type of cement is governed by the heat evolution characteristics which affect the temperature rise i.e., rate at which the heat is evolved and the total heat generated.
The total heat generated is greater for higher cement content per unit volume of concrete. In small sections, the rate of heat evolution is more important with regard to temperature rise as the heat is dissipated steadily, where as in massive sections the temperature is more dependent on the total heat evolved due to greater self-insulation.
Thus the temperature rise in concrete depends on a number of factors such as type and quantity of cement, size of the section, the insulating characteristics of the form work, and the placing temperature of the concrete, With respect to placing temperature of concrete it can be seen that higher the placing temperature, the faster the hydration of the cement and greater the rise of temperature.
In practice the lowest temperature rise may be obtained by blending the cement suitability.
The best blending combinations of cements are given below in order of best performance:
1. Blend of sulphate resistant cement and ground granulated blast furnace slag cement.
2. Ordinary port-land cement and slag cement.
3. Part replacement of port-land cement by fly ash.
In case of massive sections, the quantity of cementitious materials i.e. cement plus slag or fly ash is governed more by impermeability and durability requirements (max. water/cement ratio) than by a specified 28 days compressive strength, which should not exceed 14 MPa (140 kg/cm2). However in structural reinforced concrete, a higher early strength may be critical so that ordinary port-land cement alone in larger quantities have to be used. Thus it is necessary to adopt alternative measures to minimise the ill effects of temperature rise.
The temperature rise may be controlled by measuring the temperature at several points by thermocouples. The insulation must control the loss of heat by evaporation, as well as by conduction and radiation. To control the heat loss by evaporation a plastic membrane or curing compound may be used, while the use of soft boards will insulate against the conduction and radiation loss. Plastic coated quilts have been found useful in all respects.
For minimizing the temperature gradient or temperature differential, the time of form work striking or removal is very important. For thin sections i.e., sections less than 50 cms in thickness, the early removal of form work is useful as it will allow the concrete surface to cool more rapidly. However for massive and isolated sections the insulation i.e. from work should remain in position till the whole section has cooled sufficiently, so that when the form work is removed finally, the drop in temperature does not exceed more than 10°C (50°F) for concrete made with flint gravel aggregate.
The reason for the lower values of tolerable gradient is that when the insulation is removed, the cooling of concrete surface is more rapid and the creep cannot help in increasing the tensile strain capacity of the concrete. For this reason, the form work and insulation of large sections may have to remain in position for upto at least two weeks, before the concrete has cooled to a safe level of temperature. In case the section is subjected to external restraint, this measure will not be helpful in preventing the cracking of the concrete and other remedial measures have to be adopted.
Cold Weather Concreting:
Any concreting operation under taken at a temperature below 5°C is called cold weather concreting. Many codes do not advocate concreting operations at an atmospheric temperature below 5°C without adopting special precautions. In India such areas are very small compared to fair weather regions. The production of concrete in cold weather presents peculiar problems such as delay in setting and hardening, damage to concrete in plastic stage when exposed to below freezing point due to the formation of ice crystals or lenses. Therefore it is essential that concrete temperature is maintained above 5°C, if possible at much higher temperature.
Effects of Cold Weather on Concrete:
(a) Delay in Setting and Hardening:
The rate of hydration of cement depends upon the temperature. If the temperature is low, concrete takes a long time to set and a longer time to harden. Thus the development of strength is very slow. The delay in setting time makes the concrete prone to frost attack and other disturbances. The delay in hardening period causes delay in the removal of form work, resulting in the slow rate of progress of the work. All these factors affect the economy of the work.
(b) Freezing of Concrete at Early Age:
If the temperature falls down to below freezing point, the free water in the plastic concrete freezes. Freezing of water not only slow down or prevent the hydration of cement but also causes expansion in the concrete. Water on freezing expands in its volume causing expansion in the concrete. This expansion causes disruption, resulting irreparable loss in strength and quality of concrete.
(c) Freezing and Thawing:
In the cold weather regions, the fresh or hardened concrete may be subjected to freezing and thawing cycles due to varied climatic conditions. This alternate cycle of freezing and thawing adversely affects the durability of concrete and also exerts fatigue in the concrete.
Conditions in Cold Weather Concreting:
In cold weather concreting following conditions may arise:
1. Low but above 0°C temperature at the time of concreting and hardening period.
2. Low temperature at the time of concreting, but below 0°C during hardening period.
3. Below 0°C temperature at the time of concreting and during hardening period.
4. Hardened concrete is subjected to alternate freezing and thawing.
To understand the behaviour of concrete well and to take suitable action to neutralize the effects of such condition for the successful placing of concrete, it is necessary to deal effectively with the above four conditions.
1. Low Temperature but above 0°C:
We know that the strength of concrete is a function of time and temperature. From maturity concept, Maturity = ∑ time x temperature. The low temperature, but always above freezing degree, develops no other bad effect in the concrete, except retarding the rate of hydration i.e., development of strength. Low temperature has no bad effect on fresh concrete or on hardened concrete, but produces a high quality gel structure.
It has been observed that the ultimate strength of concrete cured at 9°C (48°F) developed higher strength than that of the concrete cured at higher temperature. This may be due to the superiority of the gel structure on account of slow hydration. In this case no other precautions are necessary. It has only one drawback that is the form work has to be removed after a long period due to the slow development of strength for stripping the form work. Thus putting the structure into service is delayed.
2. Temperature at the Time of Concreting Low but Below 0°C after Concreting:
In this situation following two cases may arise:
(a) Temperature falls below 0°C when the concrete is still green.
(b) Temperature falls below 0°C after the concrete has attained sufficient strength or sufficiently hardened.
(a) It might have happened that at the time of mixing and placing the concrete, the ambient temperature was above freezing point, but the ambient and concrete temperature fell below freezing point before the concrete has attained sufficient strength. In such situations the free water still available in the concrete will freeze forming ice crystals in voids of the concrete. These ice crystals formed in the capillary cavities may cause capillary suction of water from the ground if it is saturated and become bigger.
These ice crystals exert great pressure on the concrete and disintegrate it. Ice formation may also take place in the contact surface of aggregate and cement paste. After thawing these ice crystals will melt forming cavities. Thus the freezing of fresh concrete seriously damages the structural integrity of concrete and results in considerable loss of strength.
In extreme cases it may result in forming a useless mass of concrete. Fig. 18.7 shows the increase in volume of concrete during prolonged freezing as a function of age when freezing starts. Maximum expansion takes place during 1st four hours. The decrease in the magnitude of expansion of concrete allowed to harden for about 24 hours is clearly seen. During this period concrete should be protected from frost.
The resistance to alternate freezing and thawing also depends on the age of the concrete when the first cycle of freezing and thawing occurred to the concrete. Fig. 18.8 shows the increase in volume with the number of cycles of freezing at different ages. This type of exposure has been found more severe than prolonged freezing with periods of thaw. Several such cycles can cause damage even to concrete cured at 20°C for 24 hours. However there is no direct relation between the frost resistance of young concrete and durability of mature concrete subjected to many cycles of freezing and thawing.
(b) If the freezing takes place after the concrete has hardened sufficiently, there will not be much harm to the structural integrity of the concrete. At this stage, water mixed for making the concrete would have been lost either being used up in the hydration process or lost by evaporation. Due to the formation of cement gel, the capillary cavities of the concrete would have been reduced to a great extent. Thus very little free water would be available in the body of the concrete to freeze and damage the concrete. Therefore there is no immediate danger to the concrete.
(c) Temperature below 0°C at the time of concreting and during hardening period. While concreting when the temperature is below 0°C, certain precautions should be taken so that concrete does not get frozen.
Under Water Concreting:
Many a times concrete is required to be placed under water or in a trench filled with bentonite slurry. Special precautions need to be taken whenever concrete is to be placed under water.
Properties of Concrete:
As per port-land cement associations recommendations, concrete to be placed under water should possess the following qualities.
A richer mix than generally used for placing under normal conditions is required. At least 10% more cement than under normal conditions should be used. Usually 8 bags (150 kg each) i.e., 400 kg/cement per cubic metre of concrete is required. However the quantity of extra cement may vary according to the conditions of placing.
Aggregate:
The proportion of fine and coarse aggregates should be so adjusted to produce the desired workability with a somewhat higher proportion of fine aggregate than used under normal conditions. The proportion of fine aggregate should be 45 to 50% of the total aggregate depending on the grading. In other words coarse aggregate neither should be less than 1.5 times nor more than 2.0 times of the fine aggregate. To produce a plastic and cohesive mixture the aggregate should contain sufficient fine material passing through 300 and 150 micron sieve.
As per ASTM standard specifications for concrete, aggregate not less than 10% of fine aggregate should pass through 300 micron sieve and not less than 2%, pass through 150 micron. The fine aggregate should meet the minimum requirement and somewhat higher percentage of fines would be better.
The maximum size of coarse aggregate should not be more the 40 mm. For most of the works the maximum size adopted is 20mm. The slump of the concrete should be high of the order of 15 to 20 cms.
Form Work:
The importance of form work in under water concreting is much more than normal concreting as the form work for under water concreting serves the following functions:
(a) It not only imparts the required shape to concrete but also protects the mix during its placement till it matures.
(b) It also protects the mix from direct action of current and waves.
(c) After placement it protects the concrete from the impact and abrasive action of water currents
Thus form work for under water concreting must be designed and prepared carefully keeping all aspects in view.