Pouring concrete in hot weather creates issues that can not only affect the final product's quality, but can also decrease workability. Even though pouring concrete in dry and hot conditions increases the chances of problems, there are proven steps that can be taken to produce high-quality slabs.
Concrete Pouring and Setting
Concrete must set after it's poured, meaning that it undergoes chemical reactions that change it from slurry to a solid form. This process happens best at temperatures between 50°and 60° Fahrenheit. Concrete poured in temperatures above 80° F can set too quickly, though, causing the slab to lose its strength.
Besides atmospheric temperature, there are other environmental factors that affect how hot concrete gets. Solar radiation and low humidity cause moisture in the slurry to evaporate, increasing dry time while reducing the structural integrity from improper setting. High humidity levels also affect concrete's compressive strength and durability, and moisture can harbor mold, mildew and bacteria that weakens concrete over time. Wind is also a factor to consider when pouring concrete in hot weather since it causes evaporation, so pour on days with little to no wind if possible.
In ideal conditions, concrete can set in 8 to 48 hours and will usually take up to 28 days to cure to its full strength. On hot days, it can start to set in as little as 30 minutes and will be relatively weak as a result. Keep in mind that concrete sets best in a constant ambient temperature, meaning that a slab poured on a hot day will shrink if nighttime temperatures fall significantly. This temperature variance can cause random cracks and fissures called crazing.
Tips for Pouring Concrete in Hot Weather
Ambient temperature, solar radiation and high wind speeds can cause the plastic compounds within cement to shrink and crack, causing flaking and discoloration. This evaporation from environmental conditions causes concrete to crack and split, but keep in mind that rehydrating a slab can cause crazing. Quicker dry times affect the slurry's slump (or workability), which means crews must work at a fast pace when pouring concrete in hot weather. There are also plenty of other steps that can be taken to reduce problems.
Use Additives
There are plenty of chemicals that can be used to make hot-weather concrete jobs more manageable. One of the cheapest and easiest ways to keep concrete cool is through the use of cold water and ice. Simply add cold water when mixing, and remember that the friction from mixing will melt the ice and keep things cool. It's also a good idea to keep aggregates cool by periodically spraying them with water to keep them wet.
Liquid nitrogen (also known as dry ice) can be a better option than ice and water since its constant temperature makes it more predictable. Liquid nitrogen can be added directly to the slurry mixture, aggregates and even finished concrete itself.
There are also several types of admixtures that can be added to concrete before or during mixing. These substances increase slump, strength and durability to produce a high-grade finish. There are three main types of admixtures:
- Air entraining admixtures create a network of microscopic air bubbles that increase concrete's performance and workability in cold weather.
- Set retarding admixtures increase the time that concrete takes to set without reducing its strength.
- Hydration stabilizing admixtures allow precise control of hydration events.
Colloidal silica finishing aids are also a great option when pouring concrete in hot weather. These compounds are trowled directly into concrete during placement and finishing and are a great option for reducing moisture loss through evaporation. Finishing aids also act as a lubricant that aids in troweling and has other benefits including:
- Greater surface strength
- Greater abrasion resistance
- Reduced curling
- Reduced surface permeability
After concrete is poured, evaporation retardants can be added to form a hydrophobic barrier that prevents moisture loss. These chemicals form a thin membrane that is peeled off after setting, so it's important to remember that evaporation retardants must not be worked into the concrete. Besides being relatively inexpensive, evaporation retardants provide many benefits including reduced cracking, decreased slump loss and greater slab strength.
Plan Ahead
When planning a job or drafting a bid, check weather reports and be aware of the local climate. Remember that the ideal conditions for concrete jobs are:
- Temperatures of 50° to 60° F
- Little to no wind (check speed and direction)
- No direct sunlight (cloudy days are ideal)
- Little to no chance of rain
- Normal humidity levels
If you must pour concrete in temperatures over 80° F, it's crucial that you use cold water (or liquid nitrogen), admixtures, finishing aids and evaporation retardants. Don't pour on days that are 100° F or hotter, and avoid pouring and working on concrete during the hottest parts of the day.
Having all equipment set up and ready as early in the morning as possible will ensure that concrete can be poured before the heat of the day sets in, and working when it's cooler will reduce worker fatigue. Remember also that workers should be kept well hydrated and provided with electrolytes to increase efficiency and reduce health risks. Read more in our guide on how to work safely in the heat.
Be ready to work as soon as the truck arrives, or when the slurry is mixed and ready to be poured. Mixing concrete increases heat through friction, so it's important to reduce mixing time with thorough planning and keen awareness. Making sure that everyone is ready to work as soon as the slurry is ready will reduce work hours, increase worker safety and reduce complications from pouring concrete in hot weather.
When purchasing supplies, remember that larger aggregates can reduce the probability of the slab's shrinkage. It's a good idea to purchase items to control environmental conditions such as sun shades, windblockers and waterproof tarps to protect aggregates, slabs and other concrete materials.
Pouring concrete in hot weather is not ideal, but if you're on a schedule, it's not impossible. With the right material and a little bit of know-how, you can ensure that your building project continues and gets finished on time, and that the concrete you pour lasts for years to come.
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