Environmental factors, particularly hot weather conditions, affect concrete properties and the construction operations of mixing, transporting, and placing of the concrete materials.
By understanding how these factors—heat, humidity, and even wind—affect the curing of concrete, producers can adjust mix designs and compensate in a variety of other ways to maintain high quality standards and avoid issues with the finished product.
What is Hot Weather Concreting?
The ACI defines hot weather concreting as, “one or a combination of the following conditions that tends to impair the quality of freshly mixed or hardened concrete by accelerating the rate of moisture loss and rate of cement hydration, or otherwise causing detrimental results: high ambient temperature; high concrete temperature; low relative humidity; and high wind speed.”
While hot weather problems are most frequently encountered in the summer, climatic conditions of high winds, low relative humidity, and solar radiation can occur any time, especially in warmer climates. As such, hot weather concreting is any period of high temperature in which special precautions need to be taken to ensure proper handling, placing, finishing, and curing of concrete.
Hot weather concreting is any period of high temperature in which special precautions need to be taken to ensure proper handling, placing, finishing, and curing of concrete.
The exact temperature where special precautions should be taken varies. According to Jack Holley, a concrete veteran of 45 years, 80°F (27°C) is a good benchmark to use. This is generally the temperature that starts to affect the efficiency of the cementitious system.
Evaporation rate is a more accurate indicator of hot weather conditions for concrete—when the evaporation rate exceeds 0.20 psf/hr (1kg/m2/hr), precautionary measures are required.
Identifying Problems with Hot Weather Concrete
Hot weather conditions can lead to problems in mixing, placing, and curing hydraulic cement concrete that can adversely affect the properties and serviceability of the concrete. If precautions are not effectively implemented during hot weather, the concrete may be damaged through plastic-shrinkage cracking, thermal cracking and decreased 28-day strengths.
In the freshly mixed state, the main problems encountered in hot weather conditions include:
- Increased water demand which reduces the water-cementitious ratio.
- Increased slump loss leading to the requirement to add water at the job site.
- Accelerated setting time which creates issues with handling, compacting, and finishing, and a greater risk of cold joints.
- Increased potential for plastic shrinkage and thermal cracking, mainly due to the evaporation of water on the surface of the placement.
- Difficulty controlling air content—depending on the cementitious system and admixture package, a gel may form and causes air bubbles to break, decreasing the air content.
- Need for the concrete to be promptly cured and cut early. Concrete that is cured at high temperatures at an early age will not be as strong at a later age as concrete cured at more favorable temperatures.
Once the concrete is hardened, problems caused by hot weather can never be fully rectified. As such, the hardened concrete deficiencies often include:
- Increased potential for drying shrinkage and differential thermal cracking from either cooling of the overall structure or from temperature differentials within the cross-section of the slab.
- Decreased compressive strength resulting from higher water demand.
- Increased potential for cold joints, color differences, or other variations in surface appearance. This is due to different rates of hydration or different water-cementitious material ratios (w/cm).
- Decreased water-tightness and durability due to cracking.
Minimize the Impact of Hot Weather
The key to successful hot weather concreting is the planning for both internal processes at the plant and external processes at the job site.
Ensure teams are adequately trained to handle concrete in hot weather conditions. If your team is unaware of the different hydration control admixtures or the effects of the admixtures in hot weather, check with your admixture supplier and with your cement supplier for best practices for your mix scenario.
It’s recommended that you take a collaborative approach to hot weather concreting. Hold meetings with contractors and the end customer so everyone is aware of the risks including the potential for plastic shrinkage, and so on. That way, if problems arise down the line, expectations have already been set.
Best Practices at the Plant
The most important precaution to take in hot weather is to the dampen the concrete aggregates that are dry and absorptive. You can use typical sprinklers. If you have the bin capacity and space at the yard, it’s best to dampen the aggregate the night before use so you have the ability to drain it before use. This will bring it back to a controllable moisture content and avoid issues with slump control.
Use a trusted formula to monitor the temperature continuously, keeping the concrete temperature low by cooling the aggregates and mixing water—you can even use ice for the mixing water. In very warm climates, consider investing in liquid nitrogen cooling. This is mainly recommended for high volume production—if you're only doing a couple of placements a week, it may not be cost effective.
Monitor drum revolutions to avoid excessive mixing, but turn the drum at least 70 to 100 revolutions at the mixing speed designated by the manufacturer.
Best Practices at the Job Site
Firstly, limit the travel time of the concrete—that includes the time the concrete is actually on site waiting to be used. Work with the contractor to coordinate delivery times to ensure a consistent supply is available and avoid having trucks sitting on site with their drums agitating in the heat of the sun.
Retarding agents or hydration stabilizing admixtures are great but they require you to do your own testing so you can be confident of what they are capable of. Remember, the goal is not to retard the concrete but to control the set until the concrete is in place.
Practice prompt discharge from the trucks so there’s less time for the concrete to react before placement. Dampen the discharge area for extra cooling and ensure the area is shaded. If the contractor can put the roof on first, that’s an ideal scenario. If not, erect temporary sunshades to reduce concrete surface temperatures. Protect the concrete with temporary coverings, such as polyethylene sheeting, during any appreciable delay between placing and finishing. Also erect temporary windbreaks to reduce wind velocity over the concrete surface.
Cure the slab immediately. On bridge decks or overpasses where there is a high potential for evaporation, the white curing compound takes a while to become a membrane. Spray it on right after the finishing process and cover it with wet burlap. Using white-pigmented curing compounds helps with coverage and also helps reflect heat from the concrete surface.
Finally, protect your test cylinders. They should be continually moist and in a 60°F (16°C) to 80°F (27°C) condition for both acceptance or rejection. Water baths are very effective for this but ensure you monitor the water temperature and don’t put too many cylinders in one water bath.
Summary of Best Practices
|At the Plant||At the Job Site|
|• Dampen aggregates |
• Monitor temperature
• Use ice for mix water
• In very warm climates, consider
investing in liquid nitrogen cooling
• Monitor drum revolutions
• Specialty mixes may require
|• Limit travel time |
• Use retarding agents
• Practice prompt discharge
• Dampen discharge area
• Use shade and wind breaks
• Cure ASAP
• Protect cylinders
While you will not likely need to take all of the recommended precautions described in this article, each hot-weather scenario should be analyzed individually by qualified personnel, who should find the optimum mix of quality, practicability, and cost-efficiency.
CarbonCure and Hot Weather
CarbonCure has been used for many years in hot weather conditions. It won't contribute to any of the issues described above, nor will it prevent any of them. While CO2 is a cooling material, the CarbonCure technology only uses a very small amount of CO2—not significant enough to alter the temperature of the concrete.
Want to learn more? Ask us to send you the recording of our recent webinar on this topic featuring CarbonCure’s Jack Holley and Diane Praught.