Curing Concrete in Hot Weather

Have you seen a slab of concrete that was cracked, but you knew that it was just poured about five days ago? How can a new slab already be cracked in such short time? The answer is poor technique and a poor mixing process.

Once you have mixed the ingredients to make concrete, it will start the hardening process right away, so once you have laid the concrete, you have to cover it for it to cure. The drying process should be a gradual process, and to keep it gradual (even on a hot day), you should cover the slab with plastic while it is curing/drying. This will allow the slab to stay moist while curing, which is the key to reduce cracking.

Part of the mixing process includes adding water, which concrete loves, so you have to keep water in the process. While the concrete is curing, it is also building its strength, which is quick at first, then tapers off, and as long as there is water present, the concrete will continue to cure (making it even stronger). Once all the water is gone, the concrete is done curing (so a longer curing process makes for stronger concrete).

If it is hot outside, and the water is evaporating quickly, you can lift up the plastic covering, gently spray the concrete with a water hose, put the plastic back over it, and let it continue to cure. You can keep doing this for several days without harming the slab.

What about slabs of concrete to be poured against an older surface of concrete that has already cured? This process would include moistening the older surface first, then adding the new concrete to it. If you do not initially moisten the older surface, a “cold joint” will be created, and the bond will not be strong, thus allowing a weak area, causing cracking in that area possible separation between the two slabs.

Can you have too much water in the mix? Yes, but you want to have enough that the concrete is allowed to cure properly. A good rule of thumb is to look for a shimmer while mixing. If you have a shimmer, then you have too much water, which is easily fixed by adding more dry ingredients (mostly cement mix). It may make the mix a little richer, but in the long run will only add strength, and reduce cracking.

After the concrete has set in about two to three hours, it will start showing its hardness, but keep it moist (using the spray nozzle gently) for the next two to three days allowing it to cure properly. Remember, to keep the concrete surface covered with plastic to keep in the moisture. After about the third day, you can uncover and allow the concrete to dry.

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