What constitutes "hot weather" as it applies to concreting requires definition. ACI 305R-10 identifies it as one or more conditions that adversely impact concrete placement by accelerating moisture loss and the hydration rates:

• High ambient temperature
• Low relative humidity
• Excessive wind speed
• High concrete temperature

In general, hot weather concreting precautions become relevant when the temperature of freshly mixed concrete rises above 77 degrees F.

Although concreting in hot weather is even more of a challenge in the Southern states, Pennsylvania is no stranger to weather extremes. The commonwealth's highest temperature was recorded, quite appropriately, in Phoenixville. On July 10, 1936, the temperature reached a scorching 111 degrees.

Of course, you don't need triple-digit temperatures to encounter significant challenges with concrete placement. In addition to high temperatures, low relative humidity and/or high winds adversely impact projects as well.

So, why even place concrete in hot weather? In part because successful hot weather concreting simultaneously delivers quality and increased margins. Owners and developers appreciate projects completed on time, and crews appreciate steady work.

However, to reap the benefits of hot weather concreting, contractors must track multiple variables. Hydration, evaporation and water absorption are just some of the factors that require consideration.

The challenges associated with high temperatures include, but are not limited to:

• Higher water demand
• Increased rate of slump loss
• Higher concrete temperatures
• Decreased setting time
• Increased plastic shrinkage
• Cracking

A failure to address the demands of concreting in hot weather can compromise durability. Optimal durability is vital for many reasons. For example, it is crucial in Pennsylvania where repetitive freeze-thaw cycles promote scaling and spalling.

Concrete hydration is a heat generating, exothermic process. The hotter the concrete, the faster the exothermic reaction. During the hydration process, crystallization occurs around aggregates. In excessively warm concrete, hydration happens so quickly that the crystals may lack strength. For example, concrete's 28-day compressive strength may decrease as much as 10 percent when temperatures are 18 degrees F warmer than usual.

Slump is another challenge. Extra mixing water counters warm weather's adverse impact on slump. However, compressive strength suffers when too much extra water is used. Also, when the concrete is colored, the added water may lead to unacceptable color variations between pours.

Although condensation counteracts the impact of surface drying, cracking and plastic shrinkage are remain concerns when humidity drops and ambient temperatures soar. Therefore, it is important to monitor surface evaporation rates by calculating the combined effect of humidity, ambient temperature, wind velocity and concrete temperature. Agents that control surface evaporation should always be available to crews.

In hot and dry conditions, water absorption by formwork and subgrades is another challenge. It is also important to maintain air content in warm concrete. Finally, hot weather may increase cracking due to excessive temperature differences in adjacent pours.

High concrete temperatures affect curing rates. High curing temperatures at the start may reduce 28-day strengths by as much as 10 percent. When corrective actions are lacking, concrete strength can suffer.

Contractors capable of precise, quick concrete placement are in a better position to maintain quality in hot weather. Therefore, have adequate crews available to ensure prompt placement.

Adverse conditions impact all aspects of a project, including planning, mixing, transporting, placing and finishing.

Excellent communication is a key to success in hot weather concreting. Proactive collaboration among engineers, contractors, producers and testing labs prevents avoidable problems. The end result is the high quality, durable concrete that owners and developers expect and deserve.

Dealing with hot weather begins at the plant. It is possible to add chilled water, ice or liquid nitrogen to keep concrete temperatures under control. Producers can also wet and shade stocked aggregates to keep concrete temperatures under control. They can also add retardants and other helpful compounds.

In adverse summer conditions, consideration should be given to the length of the haul from plant to project. Tighten up concrete delivery schedules for best results.

To facilitate prompt placement, concrete contractors should have their forms in place in advance. Onsite facilities should expedite concrete handling. Contractors should have sufficient labor available to speed placement and finishing. Temporary windbreaks and sunshades mitigate the effects of high winds and hot temperatures. To minimize water absorption, wet down subgrades prior to concrete placement.

Concrete mixtures at 80 degrees F may reach final set in half the time of mixtures at 60 degrees F. Fortunately, the right admixtures extend set times. Water reducers and hydration stabilizers extend work times without compromising performance. Finally, it is important to maintain concrete moisture levels with evaporation retarders or plastic sheeting.

More information about hot weather concreting is available in a publication of the American Concrete Institute titled "305R-10 Guide to Hot Weather Concreting."