Tips and Recommendations for Pouring Concrete in Hot Weather

Concrete poured in hot weather, low humidity, or high wind can suffer adverse effects if proper adjustments are not made to the process. Any time hot and/or dry conditions are present when pouring concrete, it is important to schedule the work during the coolest part of the day, if possible, and to have plans in place to keep the concrete cool. Among other things, this can include using shade to block direct sunlight or spraying aggregates with cold water to keep them cool.

Concrete sets by hydrating. In other words, it sucks up water and forms crystals around the particles in the concrete. The cooler the concrete, the longer this process takes and the more time the crystals have to strengthen. When the concrete is hotter, the crystallization process happens more quickly, giving the crystals less time to strengthen. Evaporation also can have a negative effect on the surface layer of the concrete. The lack of water there will lead to weaker concrete at the top of the slab, which means the concrete will be more susceptible to cracking.

These terms sometimes are used interchangeably, but cement is an ingredient in concrete. Cement is made up of water and Portland cement, which is primarily limestone in the form of a fine powder. The paste it forms is combined with larger aggregates to make concrete.

While hot weather is an important and notable obstacle, it's necessary to understand that maintaining an appropriate level of moisture in the concrete is the highest priority. While temperatures in excess of about 85 degrees can make this difficult, low humidity and high wind speeds also can increase the rate of evaporation, even at lower temperatures. So, what often is described as "hot" weather when pouring concrete is often a combination of factors that combine to make it more challenging to retain moisture in the concrete.

According to the Penn State University College of Engineering, concrete will set in anywhere from about two to 19 hours, depending on the temperature. It will set in slightly less than two hours at 100 degrees Fahrenheit, but at 30 degrees Fahrenheit, it will take 19 hours. It will not set in temperatures as cold as 20 degrees Fahrenheit.

Keep in mind that setting is not the same thing as curing. Setting simply means the concrete has reached a completely solid state, but it still needs additional time to reach its full strength. According to the American Concrete Institute, concrete typically reaches 70 percent of its strength within seven days, and after 28 days concrete is generally considered to be fully cured and at 100 percent strength.

Concrete placed in hot weather sets more quickly and produces higher early strength, but its ultimate strength through the curing process will be lower than expected. Proper mix design can compensate for these conditions, and in combination with protective measures to prevent rapid evaporation, quality concrete can be poured in hot temperatures when you consider these recommendations:

• Have sufficient manpower to manage the concrete when it is being poured and for the finishing process.
• Use a large size and amount of coarse aggregate particles if hot weather is likely to occur during the concrete placement. Larger aggregates will minimize the probability of having concrete shrink due to environmental conditions.
• If possible, avoid placing concrete at noon or during the afternoon.
• Plan with the batch plant an acceptable delivery temperature so that materials can be cooled by the supplier as needed. Aggregates can be cooled down by spraying water over the stockpile.

When pouring concrete in hot weather, you must be prepared for some possible problems if you don't follow the right steps:

• Difficulties in finishing the concrete could increase.
• Cold joints could be formed due to hot weather decreasing the setting time.
• Strength and durability characteristics may be reduced.
• Concrete compression tests could yield lower strength results.
• The drying shrinkage of the hardened concrete could increase.
• Placing concrete in hot weather produces an increased rate of slump. This means a slab of concrete will not be consistent throughout.
• The risk of thermal cracking could increase.
• The heat of hydration raises the temperature of the interior of the concrete.