How To Pour Concrete In Hot Weather | Recommended Practices And Precautions | LCETED

How To Pour Concrete In Hot Weather | Recommended Practices And Precautions | Concrete Under Extreme Environmental Conditions

Any operation of concreting done at an atmospheric temperature above 40 °C or where the temperature of concrete at the time of placement is expected to be beyond 40 °C may be categorized as hot weather concreting. Concrete is not recommended to be placed at a temperature above 40 °C without proper precautions as specified in IS: 7861 (Part-I)–1975. The climatic factors affecting concrete in hot weather are a high ambient temperature and reduced relative humidity, the effects of which may be more pronounced with the increase in wind velocity. 

EFFECTS OF POURING CONCRETE IN HOT WEATHER

The effects of hot weather may be summarized as follows.

1. Accelerated setting A higher temperature results in more rapid hydration leading to accelerated setting, thus reducing the handling time of concrete and also lowering the strength of hardened concrete. The workability of concrete decreases and hence the water demand increases with the increase in the temperature of concrete. The addition of water without proper adjustments in mixed proportions adversely affects the ultimate quality of concrete. It has been reported that an approximately 25 mm decrease in a slump has resulted from an 11 °C increase in concrete temperature.

 

2. Reduction in strength Concrete produced and cured at an elevated temperature generally develops higher early strength than normally produced concrete, but the eventual strengths are lower. Regarding the influence of simultaneous reduction in the relative humidity, it is seen that specimens moulded and cured in the air at 23 °C and 60 per cent relative humidity, and at  38 °C and 25 per cent relative humidity attained strengths of only 73 and 62 per cent, respectively, in comparison with the specimens which are moist- cured at 23 °C for 28 days. High-temperature results in greater evaporation and hence necessitates an increase in mixing water, consequently reducing the strength.

 

3. Increased tendency to cracking Rapid evaporation leads to plastic shrinkage cracking, and subsequent cooling of hardened concrete introduces tensile stresses. The rate of evaporation depends on the ambient temperature, relative humidity, wind speed and concrete temperature.

 

4. Rapid evaporation during curing As the hydration of cement can take place only in water-filled capillaries, it is imperative that a loss of water by evaporation from the capillaries be prevented. Furthermore, water lost internally by self-desiccation has to be replaced by water from outside. A rapid initial hydration results in a poor microstructure of gel which is probably more porous, resulting in a large proportion of the pores remaining unfilled. This leads to lower strength.

 

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 does concrete at normal temperatures.

Following Table Shows The Effects Of Pouring Concrete In Hot Weather On Different Stages Of Concrete Production And Placement:

Stage	Effect
Production	Higher water demand for given workability, controlling difficulty of entrained air content
Transit	Increased rate of water loss due to evaporation, loss of work workability
Placing, finishing and curing	Water loss due to evaporation, increase in work loss rate, increase in setting rate, increase in plastic shrinkage tendency, increase in the tendency to increase high peak temperature cracking during hydration and decrease in long lasting strength
Long-term	Variable appearance, Decreased durability, Lower strength

 

Recommended Practices and Precautions

Temperature Control of Concrete Ingredients 

The temperature of the concrete can be kept down by controlling the temperature of the ingredients as shown in Fig. The aggregates may be protected from direct sun rays by erecting temporary sheds or shelters over the aggregate stockpiles. Water can also be

 

 

Fig. Temperature control of ingredients for hot-weather concreting

 

 

sprinkled onto the aggregate before using them in concrete. The mixing of water has the greatest effect on lowering the temperature of concrete because the specific heat of water (1.0) is nearly five times that of common aggregate (0.22). Moreover, the temperature of the water is easier to control than that of other ingredients. Under certain circumstances, the temperature of the water can most economically be controlled by mechanical refrigeration or mixing with crushed ice. The precooling of aggregates can be achieved at the mixing stage by adding calculated quantities of broken ice pieces as a part of mixing water, provided the ice is completely melted by the time mixing is completed. The cooling of concrete can also be achieved by nitrogen gas.

 

Proportioning of Concrete Mix 

The mix should be designed to have minimum cement content consistent with other functional requirements. As far as possible, cement with lower heat of hydration should be preferred to those having greater fineness and heat of hydration. Use of water-reducing or set-retarding admixtures is beneficial. Accelerators should not be used under these conditions.

 

Production and Delivery 

The temperature of aggregates, water and cement should be maintained at the lowest practical levels so that the temperature of concrete is below 40°C at the time of placement. The temperature of the concrete at the time of leaving the batching plant should be measured with a suitable metal-clad thermometer. The period between mixing and delivery should be kept to an absolute minimum by coordinating the delivery of concrete with its rate of placement.

 

Placement and Curing of Concrete 

The formwork, reinforcement and subgrade should be sprinkled with cool water just before the placement of concrete. The area around the work should be kept wet to the extent possible to cool the surrounding air and increase its humidity. Speed of placement and finishing helps minimize problems in hot weather concreting. Immediately after compaction, the concrete should be protected to prevent the evaporation of moisture by means of wet (not dripping) gunny bags, hessian, etc. After the concrete has attained a degree of hardening sufficient to withstand surface damage, moist-curing should begin. Continuous curing is important because the volume changes due to alternate wetting and drying promote the development of surface cracking. On the hardened concrete, the curing shall not be much cooler than the concrete because of the possibilities of thermal stresses and resultant cracking. High-velocity winds cause a higher rate of evaporation, and hence windbreakers should be provided as far as possible. If possible, the concreting can be done during night shifts.

SETTING TIME OF CONCRETE AT VARIOUS TEMPERATURE

Tips for Pouring Concrete in Hot Weather

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