HOT WEATHER CONCRETING
Any concreting operation carried out at an atmospheric temperature higher than 40 ° C or in which the temperature of the concrete at the time of placement is expected to exceed 40 ° C can be classified as concreting in hot weather.
It is not recommended to place concrete at a temperature higher than 40 ° C without proper precautions, as specified in IS: 7861 (Part-I) –1975. The climatic factors that affect concrete in hot weather are high ambient temperature and low relative humidity, the effects of which can be more pronounced with increasing wind speed.
The effects of hot weather may be summarized as follows:
1. Accelerated setting :
Higher temperature results in faster hydration leading to accelerated set, reducing concrete handling time, and also decreasing the strength of hardened concrete.
The workability of concrete decreases and therefore the demand for water increases with increasing concrete temperature. The addition of water without proper adjustments in mixed proportions adversely affects the final quality of the concrete.
It has been reported that an approximately 25 mm decrease in slump resulted from an 11 ° C increase in concrete temperature.
2. Reduction in strength :
Concrete produced and cured at an elevated temperature generally develops a higher initial strength than normally produced concrete, but eventual strengths are lower. Regarding the influence of the simultaneous reduction of the relative humidity, it is seen that the samples
molded and cured in the air at 23 ° C and 60 percent relative humidity.
You may also like: Transportation of concrete
And at 38 ° C and 25%, the relative humidity achieved strengths of only 73 and 62%, respectively, compared to samples that were wet cured at 23 ° C for 28 days. The high-temperature results in more evaporation and therefore requires an increase in mixing water, which consequently reduces resistance.
3. Increased tendency to cracking :
Rapid evaporation leads to plastic shrinkage cracking, and subsequent cooling of the hardened concrete introduces tensile stresses. The rate of evaporation depends on the ambient temperature, relative humidity, wind speed, and the temperature of the concrete.
4. Rapid evaporation during curing :
As the hydration of the cement can only take place in capillaries filled with water, it is imperative to avoid the loss of water by evaporation of the capillaries. In addition, the water that is lost internally through self-drying has to be replaced by water from the outside.
Rapid initial hydration results in poor gel microstructure that is likely to be more porous, resulting in a large proportion of the pores being unfilled. This leads to less force.
5. Difficulty in controlling the air content :
At higher temperatures, it is more difficult to control the air content in air-entrained concrete. This adds to the difficulty of controlling workability. For a given amount of air-entraining agent, hot concrete entrains less air than concrete at normal temperatures.
Recommended Practices and Precautions
Temperature Control of Concrete Ingredients :
The temperature of the concrete can be kept low by controlling the temperature of the ingredients as shown in Fig. 12.1.
Aggregates can be protected from direct sunlight by building temporary sheds or shelters over aggregate piles. Water can also be sprayed on the aggregate before using it on concrete.
Mixing water has the greatest effect in reducing the temperature of concrete because the specific heat of water (1.0) is almost five times that of common aggregate (0.22).
In addition, controlling the temperature of the water is easier than controlling the temperature of other components. Under certain conditions, the water temperature can be further controlled by mechanical cooling or mixing with crushed ice.
Precooling of the aggregates can be achieved in the mixing stage by adding calculated amounts of broken ice chunks as part of the mixing water, provided the ice has completely melted by the time the mixing is complete. Cooling of concrete can also be achieved with nitrogen gas.
Proportioning of Concrete Mix :
The mixture should be designed to have a minimum cement content consistent with other functional requirements. as far as possible,
Cement with a lower water temperature should be preferred over those with a higher temperature of hydration. The use of water-reducing agents or stabilizing inhibitors is beneficial. Accelerators must not be used under these conditions.
Production and Delivery :
The temperature of aggregate, water, and cement should be kept at the lowest practical levels so that the temperature of the concrete is maintained below 40°C placement time.
The concrete temperature should be measured at the time of leaving the batching plant with a suitable metal-clad thermometer.
The period between mixing and delivery should be kept to a minimum by coordinating the delivery of concrete with the rate of its setting.
Placement and Curing of Concrete :
Formwork, reinforcement, and substrate should be sprayed with cold water just before concrete is poured. The area around the work should be kept wet as much as possible to cool and increase the humidity of the surrounding air. Quick placement and installation help reduce problems with pouring concrete in hot weather.
hot weather concreting
Immediately after compaction, concrete must be protected to prevent moisture evaporation by wet (not dripping) burlap bags, burlap, etc. After the concrete has reached a sufficient degree of hardening to withstand surface damage, wet curing should begin.
You may also like: Cold weather concreting
Continuous treatment is important because changes in volume due to alternate wetting and drying promote the development of surface cracking.
On hardened concrete, the curing should not be any colder than concrete due to the potential for thermal stresses and resulting cracking. High-velocity winds cause a higher rate of evaporation, so protective suits should be provided as much as possible. If possible, concrete pouring can be done during night shifts.