How Does Temperature Affect Concrete During Casting?

Question:
How does temperature affect my concrete countertops during casting, and what are some measures I can take to help control temperature?

Answer:
Temperature plays a significant role during concrete casting and curing. Like most chemical reactions, the hydration rate of cement is affected by how hot or how cold it is. Warmer temperatures speed up the reaction, and cooler temperatures slow it down.

Once mixed, cement and water start reacting immediately. Depending upon the temperature, the initial set can occur within 30 minutes to as long as several hours. Cooler temperatures permit longer work times, but early strength suffers. Hot temperatures shorten work time but yield higher early strengths.

After the concrete sets and begins to gain strength, the rate at which its strength increases depends upon the concrete’s temperature. For example, concrete placed outdoors in the winter at 50° F gains strength very slowly, while in the summer, concrete placed at 90° F gains strength much quicker.

Similarly, when you can safely strip your concrete countertops and start handling them largely depends on how strong they are at a particular time. If you normally let your concrete cure in the mold for three days, but colder temperatures have retarded the strength, the concrete won’t be as strong as you expect. As a result, the concrete could crack from being handled while it’s still weak, and honing or polishing might not produce the desired results. For example, if the cement paste is weak, polishing may cause tear-out as the diamond discs grab the sand grains and pull them out of the weak paste.

Typically there are two ways to deal with hot or cold temperature extremes. One way is thermally, the other is through chemical admixtures. If curing conditions are too cold and you want to accelerate set time and strength gain, using hot water instead of cold water will speed up the initial reaction. The hot water will warm the ingredients, temporarily making them warmer than the ambient surrounding temperatures. As the concrete cools down, the effects of the hot water are reduced. But strength gain will still be higher than if cold water was used. Another method is to use insulation or even heating blankets (or steam) to warm the concrete after it’s cast.

Acelerating admixtures like calcium chloride, calcium nitrate and calcium diformate all speed up the set time, and some influence the early strength gain. However, you should never use calcium chloride in concrete countertops (or other decorative concrete) because the admixture can alter the color.

If it’s too hot and you want to slow down the set time, using cold water or even ice as part of the mix water can cool the ingredients, slowing set time. You can use the ice pound for pound as mix water, but be sure it’s fully melted before discharging the concrete from the mixer. Ice is much more effective at cooling than cold water because the phase change from solid to liquid absorbs 80 times more heat than simply warming an equal amount of cold water.

Retarding admixtures can help delay the set time of concrete too. While table sugar can be used (with extreme care), commercial retarders are much more reliable, easy to use and have far fewer negative consequences if accidentally overdosed.