Concrete Curing Tips for Achieving Long Service-Life Performance Goals

 

The curing of freshly placed concrete plays an important role in increasing the ultimate strength and decreasing the permeability of hardened concrete, as well as mitigating thermal and plastic cracks that can severely impact the aesthetics and durability of structures.

Proper curing requires satisfactory and uninterrupted moisture content in the concrete after it is placed and finished to allow continued hydration for a reasonable amount of time. If curing of the concrete is inadequate, strength, durability, appearance, and resistance to freeze-thaw damage will suffer.

There are three important considerations with curing—the type of cure, when you need to cure, and how much curing is required. The best cured concrete is concrete that is cured slowly, uniformly, and evenly from top to bottom.

 

Types of cure

There are a variety of different approaches for curing concrete. Each one has advantages and disadvantages. The method used may depend on availability, size and shape of the concrete, the location, environment, or economics.

The wet curing method requires a continuous sprinkling of water over the concrete and keeping it moist by applying saturated wet coverings, such as burlap. By keeping the surface wet, you are keeping the concrete temperature low and preventing the concrete surface from drying faster than the bottom. While wet curing may be the best method, it may not be the most practical as most people cannot keep their sprinklers running for a week and do not have the time to continuously rewet the burlap coverings. Wet curing also makes it difficult for other trades to work on the project while curing takes place. The cost and consumption of precious water resources are additional considerations.

A second curing option is to wrap a moisture-retaining covering, such as polyethylene plastic sheeting or a curing insulation blanket, over the concrete so that it seals the evaporation of water. Curing with plastic sheeting requires covering the concrete without damaging the finish. Polystyrene insulation blankets maintain a 100% relative humidity condition. While moisture-retaining coverings are a practical and efficient curing practice, they can be challenging to work with, must remain securely in place for the entire 7-day curing period, and can leave stains or marks on the concrete surface, like wet curing, if placed improperly.

The third and most common curing method is the application of a liquid membrane-forming compound to lessen moisture loss from the concrete surface. Some curing compounds are described as dissipating resins that are designed to last on the surface for a minimum of 28 days. Most hydration of the cement is completed by the time the curing compound wears off. Advantages of using a curing compound include ease of application, cost effectiveness, and the extended curing action over 30 days. Not all curing compounds are equal though: wax-based products prove to be the most efficient with regards to curing but require grinding prior to gluing tiles or carpet. Remember to ask your vendor for advice depending on your application.

 

When to cure

Regardless of which method you use, the best practice is to start the curing process as quickly as possible after placement and finishing. You should never leave the job site without curing the concrete.

Concrete should not be allowed to dry fast in any situation. Avoid the wind and protect the concrete from sun rays, especially during the hot summer months. Water evaporating too quickly from the surface will weaken the finished product with stresses and cracking.

In frigid conditions, be careful not to let the concrete get too cold. Consider using hot water and accelerators in the mix to set up the concrete more quickly and cover the new concrete with insulating blankets. The application of a curing compound is the best approach in cold climates because the curing compound will not freeze. Apply it immediately after finishing, when the sheen has disappeared from the concrete and application of the cure will not mar the surface.

 

How much to cure

For wet curing to be effective, the concrete surface must be kept continuously moist for at least 7 days after final finishing. Alternate wetting and drying will end up damaging the concrete. It is important to not let wet coverings, such as burlap, dry out. The edges should be lapped, and the materials weighted down so they are not blown away. 

Moisture-retaining coverings, such as polyethylene plastic sheeting, need to remain in place over all exposed areas of the concrete for at least 7 days. After placing the sheet on the surface, water should be allowed to flow between the concrete surface and the sheet. When plastic sheeting is used over flat surfaces, it should be firmly secured and extended beyond the edges of the slab by a length of at least twice the thickness of the pavement.

Spray equipment with a fan tip nozzle is the preferred method for applying curing compounds, but rollers may also be used. No matter how the cure is applied, sufficient and continuous coverage must be created for optimal results. It is important to apply a uniform spread of the compound across the entire surface or it will be very noticeable when you come back to inspect it a year later.

A lot of concrete construction professionals point the spray nozzle down and swing it in an arc when applying curing compounds. A better practice is to point the nozzle up and let the chemical mist down to get a much more even spread across the entire pavement. If you have a windy day, you can spray down and use rollers to get uniform coverage across the pavement.

Some products labeled cure and seal do not mean they do both at the same time. After applying these products as a concrete curing agent, you need to come back after 30 days to seal the pavement with the same product.

Contact your local representative to learn more on how we can help you succeed with your next concrete placement project. These field specialists can share with you a wealth of information on curing and sealing best practices to ensure outstanding results and long-term customer satisfaction.