Hot Weather Concreting – Effect of Hot Weather on Concrete

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Hot weather has a significant effect on all stages of concrete production and placement procedures such as increasing rate of hydration and movement of moisture from within and surface of the concrete. It affects long term strength and durability. Hot weather along with relative humidity and wind speed also have a significant influence.

Effect of Hot Weather on Concrete

Following table shows the effect of hot weather on different stages of concrete production and placement :

Stage	Effect
Production	Increased water demand for given workability, Increased difficulty in controlling entrained air content
Transit	Loss of water by evaporation, Increased rate of loss of workability
Placing, finishing and curing	Loss of water by evaporation, Increased rate of loss of workability, Increased rate of setting, Increased tendency to plastic shrinkage cracking, Higher peak temperature during hydration leading to increased tendency to cracking and lower long-term strength
Long-term	Lower strength, Decreased durability, Variable appearance

1. Higher Water Demand

Loss of moisture occurs due to hot weather which results in reduction in workability of fresh concrete. To maintain the required workability, the demand for water increases.

2. Rapid Loss of Workability

During hot weather, the rate of hydration, as well as the rate of loss of moisture, is high which results in loss of workability of concrete.

3. Decrease in Concrete Setting Time

Due to the high rate of hydration reaction, the initial setting time of concrete is considerably decreased which also shortens the time available for transit, placing and finishing.

4. Plastic Shrinkage Cracks

The rate of evaporation increases with increasing temperature and wind speed and decreasing relative humidity. When freshly placed concrete bleeds, i.e the movement of water from the inner surface of the concrete member, this moisture gets dried up quicker than the speed with which it is replaced by bleeding action. This leads to the development of plastic shrinkage cracks on concrete surface.

5. Strength of Concrete

With the increase in temperature, the rate of hydration reaction increases, this leads to greater early strength, but the long-term strength of the concrete gets reduced. This happens due to loss of moisture and a decrease in workability which affects compaction.

6. Durability of Concrete

As discussed above, the workability of concrete is reduced by hot weather and to make up for the same, more amount of water is added, which leads to the generation of pores inside concrete as moisture gets dried up quickly. This makes the concrete porous and its durability is impacted.

Placing Concrete in Hot Weather

At some point, usually between 75ºF and 100ºF, hot weather problems for concrete may begin. The combination usually causing the most problems is low relative humidity and high wind velocity. These conditions, when added to the harsh sun and high temperature, create a very high potential for problems.

There are several methods for cooling concrete. The most efficient way is to cool the aggregates, which may be done simply by sprinkling them with water and allowing the evaporation process to cool them.

Other methods of cooling the concrete include using ice or injecting liquid nitrogen into the mixer. However, both methods add cost to the concrete. The contractor should also be prepared with sunshades, windbreaks and other means to prevent rapid drying.

Basic Guides for Hot Weather Concreting

1. Plan in advance. Have equipment and materials ready before the hot weather arrives.
2. Keep the subgrade and forms moist so they do not absorb water from the mix.
3. Keep sunshades and windbreaks available and use them whenever possible.
4. Have everything prepared before the ready-mix truck arrives. Don't make the truck wait for you.
5. Keep in constant communication with the ready-mix concrete provider. Coordination between the contractor and the producer is key.
6. Concrete should be placed, struck off and darby immediately.
7. Use evaporation retardants, fogging or misting with water, or cover with a vapor-proof sheet after screeding. This will help prevent rapid drying, crusting, plastic shrinkage and rubber sets.
8. Temporary covers, like continuously moistened burlap, may be placed over the fresh concrete and removed in small sections immediately ahead of the finishers.
9. Substituting a burlap drag or broom finish will eliminate other high-risk finishing practices, such as smooth trowel finish.
10. Curing should take place when the surfaces are hard enough to resist marring.
11. Seal with a good, quality sealer for a minimum of 30 days after placing and curing.
12. Remember to protect the crew in high temperatures. Drink plenty of fluids and be careful with long exposure to the sun.