Table of Contents
Hot weather concreting is any duration of raised temperature in which special protection must be carried to assure correct handling, placing, and transporting of concrete.
In India, most of the areas are in tropical regions and some areas are in extremely cold-weather regions. Therefore, it is necessary for us to be aware of the special problems faced in making concrete structures in both not weather and cold-weather regions.
1. Hot Weather Concreting
The procedure of concreting in such a situation is set out in IS 7861 (Part I) and IS 7861 (Part II).
(a) Rapid rate of hydration of cement, quick setting, and early stiffening
(b) Rapid evaporation of mixing water.
(c) Greater plastic shrinkage.
(d) Less time for finishing.
(e) Reduced relative humidity
(f) Absorption of water from the concrete by the subgrade and formwork
(g) Difficulty in continuous and uninterrupted curing
(h) Difficulty in the incorporation of air entrainment The effect of the above situations on the production of quality concrete is required to be fully identified and care must be taken to make the concrete strong and durable.
a. Rapid rate of hydration
At high ambient temperature, the setting time of the cement is reduced considerably. It must be remembered that the setting time as discussed earlier pertains to a temperature range of 27 ± 2°C.
At a higher temperature, naturally setting time will be reduced with the result that early stiffening takes place which makes the concrete lose its workability Partially set concrete may reduce the bond between the successive lifts more than anticipated.
This is also pointed out that the quality of gel and gel structure formed at higher .mperature in the early period of hydration is of poor quality. Concrete placed in hot weather io doubt develops high early strength, but it will suffer a certain loss of long term strength Fig below shows the effect of curing temperature on one day and 28 days compressive strength.
Rapid evaporation of mixing of water;
Hot weather condition is normally associated with relatively lower relative humidity. On account of this, the water mixed with the concrete to give the required workability will be lost. The concrete turns out to be unworkable as the result, an inordinate amount of compacting effort is required to compact concrete fully. If this is not forthcoming, large voids will remain in the concrete, which ae responsible for all the ills in concrete.
b. Greater Plastic Shrinkage
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 to the surface. As a result, a moisture gradient will be set up which results in surface cracks known as plastic shrinkage cracks. The plastic shrinkage is more pronounced, in the case of floors, roads, and pavement concrete where the surface area is exposed is more when compared to depth.
c. Finishing Time
In hot weather, finishing must be done early as possible after placing. In certain cases, if early finishing is not possible due to faster stiffening and quicker evaporation of water, the quality of finishing will be of poor standard. Usually, extra fresh mortar is required to be used for finishing which results in poor performance.
d. Absorption of water by subgrade
In hot weather regions, the subgrade is – orally dry and absorptive- The subgrade or surface of formwork is required to be wetted placing the concrete. If this is not done carefully with proper considerations, the water zone poorer in quality 6tre concrete may be lost by absorption by the surface in contact with concrete making the contact zone poorer in quality.
In hot Weather, comparatively early curing becomes necessary, particularly I. woe 53-grade cement is used. Hot weather requires a continuous effort for curing. If there, is any lapse, the Concrete surface dries the interruption takes place the fast and interrupts the continuous hydration. Once, subsequence wetting doesn’t fully contribute to the development of full strength. No doubt continuous curing in hot weather entails greater cost and does not fully contribute to the water and labor.
Air-entrained concrete is rarely used in hot weather conditions. 43wever if used from the consideration of better workability, a greater proportion of air-entraining agents are required to compensate for the loss of air-entrainment due to higher temperature.
The norms given for standard temperature with respect to the percent air-entrainment cannot be taken as a guide.
g. Ready Mixed Concrete
Conveyance of concrete over long distances in the case of Ready Mixed Concrete is likely to pose a serious problem on account of faster loss of slump. The transit mixer drum may be covered with insulating material.
2. Precautions Taken
To improve the quality of concrete it is necessary that the temperature of the concrete should be as low as possible.
To obtain such a condition attempts should be made to keep the temperature of the ingredients of concrete as low as possible.
The following precautions could be taken.
Aggregates should be stockpiled in shade. A sprinkling of water over the stockpile and the evaporation of this water will result in lowering the temperature of the aggregate. If possible heavy spraying of cold air over the aggregate just before it is batched is desirable.
The temperature of the mixing water has the greatest effect on the temperature of concrete. In practice, the temperature of the water is easier t control than that of the other ingredients.
Even though the weight of water used is lesser than the other ingredients, the use of cold mixing water will affect god reduction or concrete temperature, The effect of cold water at 5ºC on concrete temperature.
If the ambient temperature is very high, the use of cooled water may not fully effective. The use of ice may be made as a part of the mixing of water. Crushed ice can be in co-operated directly into the mixer. It shall be ensured that ice crystals should be completely melted by the time mixing is completed.
c. Production and Delivery
The temperature of aggregates, water, and cement shall be maintained at the lowest practical level so that me temperature of concrete is below 40ºC at the time of placement.
Some works demand the maximum placement temperature to be much less than 40°C. The concrete is to the minimum required time. When ice is used it must be mixed to such an extent that all the ice gets melted. the etc mixer must be positioned as close to the final place lion to reduce the length of delivery to the minimum.
Reinforcement, formwork, and subgrade should be sprinkled with cooled water just prior to placing the concrete. Reinforcement projecting out of the concrete should be covered or kept cool by any practical means. More masons are required to be employed to finish the concrete at the same rate as the concrete.
The concrete is placed in comparatively thin layers so that the time interval between the successive lifts is reduced. It should be noted that the layer should not be so thin as to get dried up very early. Concrete on finishing must be covered effectively to prevent loss of moisture from the concrete.
Covering the top of concrete with a wet gunny bag or hessian cloth is desirable. Precautions should be taken to squeeze the gunny bag fully so that water does not drip into the fresh concrete.
Covering of finished floor with wet gunny bags may not be very suitable for flooring where surface finish is of primary importance. Wet curing must be commenced at the earliest possible time.
If the concrete is effectively moist covered, the ponding may be commenced after 24 hours in the case of floor, roof, or pavements. It must be remembered that concrete should not be allowed to become dry. At the same time application of water should not be commenced before the final set of cement also. The appearance of some dry particles on the surface of concrete does not mean the final set has taken place.
As said above, the best practice is to cover the concrete with moist covering for tout 24 hours and then apply water by spraying, or by ponding. When the day temperature is very high, it is better to do the concreting operation during the evening time, leaving the young concrete to undergo early hydration during relatively cooler nights.
A temperature record of the air, the concrete that comes from the mixer, the concrete placed, and the temperature of the concrete during the early period of hydration will be of help.
|Read More: Cold Weather Concreting|