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Set Times

Q. We regularly seem to experience our largest number of customer complaints and callbacks on concrete placed in the late summer and early fall. What can you suggest to help keep hot weather concrete production free from these common problems?

A. To answer your question, we sought help from Gary Nielsen, regional sales manager for Axim Italcementi Group of Middlebranch, Ohio. “Many of our customers, especially in the Midwest, report that September and October are their busiest periods. Unfortunately, hot weather precautions are necessary even in the non-summer months,” said Nielsen.

A veteran of numerous concrete-placing seasons, Nielsen finds advance planning is necessary when preparing concrete to be placed at temperatures higher than 77° F (25° C). “Since hot weather has negative effects on a number of concrete properties, producers and their contractor-customers must develop a production plan during their pre-job meetings,” he said.

A production plan should suggest you prepare for the four main environmental factors contributing to most common concrete problems reported in the late summer months. These factors are high ambient temperatures, low relative humidity, solar radiation, and strong winds. When mixing and placing concrete in hot months, producers and contractors can suppress these factors by implementing a number of practices outlined in “Hot Weather Concrete,” ACI 305.

Since warm weather increases the rate of cement hydration, contractors could find their mix will yield higher early strengths and lower 28-day strengths. “Order requirements must be agreed on before the job starts to ensure that the delivered product will meet the necessary strength requirements,” said Nielsen.

Hot weather increases the rate of set, making the mix difficult to work with when placing and finishing. Accordingly, high temperatures may also increase the rate of slump loss, said Nielsen. “Producers need to remind the contractor that the addition of water to the mix results in a higher water/ cement ratio, which can cause a loss of strength and durability in the final product,” he said.

The right cure

A good pre-job discussion also includes curing procedures. “Accelerated evaporation of moisture from the fresh concrete surface contributes to plastic shrinkage cracking,” said Nielsen. He suggests producers team up with admixture specialists to customize a mix design using the appropriate admixtures to counteract hot weather effects on each application.

Chemical admixtures can help control the adverse effects hot weather may have on the properties of the concrete mix, he added. There are two types of additives used specifically to control the set time of concrete: retarding admixtures and those that stop the set.

The formulation of these additives may appear to be chemically similar. Yet in the field, the effects are quite dramatic. A retarding admixture slows down the rate of set of freshly placed concrete, offsetting the accelerating effects of high temperatures by preventing premature stiffening.

Additives that stop the set control the cement hydration process, completely delaying the setting of the concrete, and keeping plastic concrete in a fluid state for up to 72 hours. The different effects retarding admixtures and delayed time recovery systems have on concrete setting time are illustrated in the graph above.