As the weather warms up, it’s good to review best practices for hot-weather concreting. The American Concrete Institute 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.”
The Portland Cement Association indicates that when the temperature of fresh concrete approaches 77 F, adverse site conditions could affect the quality of the concrete. At that point you need to consider what mitigation measures to take.
What could go wrong in fresh concrete?
There are several things that could go wrong during hot-weather concreting. The hydration of cement, like most chemical reactions, goes faster as the temperature rises. That means you have less time when the concrete is easy to mix, place, and consolidate. If you’re not careful, you may have difficulty consolidating or finishing the concrete. You may be tempted to add water to make it easier, but that will only reduce the strength and durability of the concrete. Adding water as a finishing aid will result in weakening and/or scaling of the surface.
Most soluble materials dissolve faster and more completely in hot water, but that’s not the case with gypsum, which controls setting. Unfortunately, the aluminates that cause flash set do dissolve faster in hot water. The hotter it is, the likelier the aluminates will dissolve without sulfates in solution to control the reaction, and setting can be very fast. This situation is called incompatibility.
Another effect of high temperatures is that air entrainment is difficult to control. We entrain small air bubbles in concrete that must withstand cycles of freezing and thawing. But as the temperature goes up, it’s harder to maintain an acceptable air-void system. You may need to increase the dosage of air-entraining admixture as the temperature rises.
How to minimize cracking in hot-weather concreting
Once you’ve placed, consolidated, and finished the concrete, you need to protect it from drying. Hot, dry, windy days present the worst conditions for plastic- and drying shrinkage. As Dr. Ken Hover says, plastic shrinkage occurs when the freshly-placed concrete has the consistency of a Three Musketeers bar–semi-solid, but still conformable. Drying shrinkage occurs when the concrete has the consistency of a Butterfinger bar.
If ambient conditions promote drying of the concrete surface, it will shrink. If it shrinks enough while the underlying concrete is still wet, it will crack. Cracks often start when the concrete is still plastic and grow longer and wider after it sets.
To limit this kind of cracking, you need to protect the concrete from drying as much as possible. While you’re placing the concrete or waiting to finish it, that may mean misting or fogging to cool and moisten the ambient air, or it may mean erecting wind breaks or sun shades. Once the concrete is finished, protect it from drying with curing compound or–better still–wet burlap.
Another cracking mechanism is thermal shock. If rain falls on the newly-finished concrete surface, the sudden cooling will cause the surface to contract. That can result in cracking.
Curing at elevated temperatures
In hot weather, concrete gains strength quickly at first. You can use maturity testing to take advantage of this effect. But the ultimate strength is lower because when concrete cures at elevated temperatures, its properties change. Instead of a uniform distribution of cement-hydration products, you get dense shells of hydrated cement near the cement grains and a more open, continuous pore structure between the grains. That makes the concrete more permeable–and therefore less durable in many environments–as well as weaker.
Along the same lines, if you’re making cylinders of concrete for acceptance testing you need to take extra care to maintain the curing temperature within the range of 60 to 80 F for the first 24 hours. If you leave cylinders baking in the hot sun, you should expect low strengths. Ideally, your testing lab will use min/max thermometers to monitor the temperature of the cylinders. If possible, put them in water in an on-site trailer. Failing that, at least keep them out of direct sun. Otherwise you could end up with failed acceptance tests even if the as-placed concrete is all right.
Every time a strength test falls below the specified strength, you’ll have to stop placing concrete until you’ve investigated the strength of the in-place concrete. That usually entails coring and testing. If the core strengths are also low, the engineer of record will have to decide whether to remove the low-strength concrete. Even if he or she determines that it’s ok to leave the concrete in place, the delay itself is costly.
Prepare for hot-weather concreting
You can formulate your concrete for hot-weather placement. Fly ash, slag cement, and calcined clay all react more slowly than portland cement and evolve less heat. That means they’re less susceptible to rapid setting. They’re also less adversely affected by elevated-temperature curing. In addition, you may want to add polymer fibers to the concrete mixture to control plastic-shrinkage cracking.
Some fly ashes contain high proportions of aluminates. There’s enough gypsum in the cement to control the aluminates in the cement, but additional aluminates from the fly ash could upset the balance between them. If you have a choice about it, blended cements already have the right amount of gypsum for the combined portland cement and fly ash.
To avoid incompatibility and make sure you’ll get the properties you want, mix and test your concrete at elevated temperatures. You can heat some of your ingredients and warm the mixing pan by running hot water into it before you mix the concrete. If it works at 90 or 95 F, it should be fine.
Dosages of air-entraining, water reducing, and retarding admixtures all need to increase in hot weather. By testing your mixture at elevated temperatures, you get to see how they interact with each other and with the cementing materials. You can see whether any of them are incompatible, as well as how long the mixture will remain workable and how long it takes to set.
In the days leading up to your concrete placement, closely monitor the weather so you know what to expect. Don’t schedule the placement if it’s about to rain unless you’ll be working under a roof. Be ready to protect the concrete from drying and wind.
Consider what time of day (or night) would be best for placing the concrete. Beton can perform thermal modeling to help you keep the temperatures within reason or show you the difference between day- and nighttime placements.
Keep the placement temperature down by cooling some of the ingredients. For example, you could mist the aggregate stockpiles and shade them from the sun. Just make sure you account for any extra surface moisture in your mixture proportions. You can use chilled water, or even substitute chipped ice for some of the mixing water. The National Ready Mixed Concrete Association advises that you can reduce the concrete temperature by up to 10 F with chilled water and up to 20 F with ice. If need be, you can cool the concrete in the truck with liquid nitrogen.
Spray the forms, subgrade, and reinforcing bars with water just before placement to cool them, but don’t leave standing water. You want the surfaces damp, not soaked.
Have plenty of people on site so you can get the concrete placed, consolidated, and finished without delay.
Monitor the transport of concrete to the site so it’s not too old when you place it. Normally 60 minutes is as much time as you should allow from mixing to placement. If the concrete seems a little dry, add water-reducing admixtures, but don’t add water beyond the maximum allowed! Make sure you don’t exceed the manufacturer’s recommended dosage of admixture.
Carefully monitor the air content (and the air-void system parameters if you can). Let the ready-mix concrete plant know right away how it’s going so they can adjust admixture dosages as needed.
Make sure you get your curing compound or wet burlap on right away to minimize drying. If you do use curing compound, choose a white one that will reflect more sunlight. And protect your cylinders from getting too warm.