Curing & Sealing

Curing is an important step to ensuring durable crack-resistant concrete. Start curing as soon as possible by maintaining the proper temperature (neither too hot nor too cold) and the dampness for at least seven days or until concrete reaches 70% of its specified strength. If you skip the curing process, it will have a major impact on the quality of your finished work. While curing is important for all concrete, the problems that arise from not curing are most obvious with horizontal surfaces. An uncured slab, whether decorative or plain gray, is likely to develop a pattern of fine cracks (called crazing) and once it’s in use, the surface will have low strength that can result in a dusting surface that has little resistance to abrasion.

When most people think of curing, they think only of maintaining moisture on the surface of the concrete. But curing is more than that, it is giving the concrete what it needs to gain strength properly. Concrete strength depends on the growth of crystals within the matrix of the concrete. These crystals grow from a reaction between Portland cement and water, a reaction known as hydration. If there is not enough water, the crystals cannot grow and the concrete does not develop the strength it should.

The other important aspect of curing is temperature, the concrete cannot be too cold or too hot. As fresh concrete gets cooler, the hydration reaction slows down. The temperature of the concrete is what is very important, not necessarily the air temperature. Below about 10°C (50°F), hydration slows down a lot; below about 4°C (40°F), it almost stops.

After Finishing

After final finishing, the concrete surface must be kept continuously wet or sealed to prevent evaporation.

The following can be used to keep the concrete wet:

• Burlap or cotton mats and rugs used with a soaker hose or sprinkler. Care must be taken not to let the covering dry out and absorb water from the concrete.
• Sprinkling on a continuous basis is suitable provided the air temperature is well above freezing. The concrete should not be allowed to dry out between soakings.
• Liquid membrane-forming curing compounds which should be applied to the concrete surface about one hour after finishing. Do not apply to concrete that is still bleeding or has a visible water sheen on the surface. A single coat may be adequate but a second application of the curing compound the next day is a good quality assurance step.

Sealing

Concrete is inherently a porous material, which is susceptible to staining from liquids seeping into the surface. It can also be attacked by harsh chemicals such as oils, antifreeze, or even drinks seeping into the porous surface. Applying a liquid concrete sealer to the finished surface will protect your concrete from surface contamination, ensuring your concrete stays beautiful for years to come. This sealer can be applied either with a roller or by using a simple garden sprayer. There are many sealers available on the market that will give different final finishes. Most hardware or concrete supply stores will have a selection of sealers. Your concrete finisher would likely be willing to do this job for you. With a few suggestions from a sealer supplier, it is an easy task for you as a homeowner to achieve. Sealing should be performed several weeks after curing has finished and after several days of dry weather to allow the concrete to dry.