Article No: 162
By: Cindy Rizzo with Jan Bloom
What is curing and why does
concrete need to be cured?
Curing is the treatment of newly placed concrete during the period in which it is hardening so that it retains enough moisture to minimize shrinkage and resist cracking. In fact, curing is essential to installing decorative concrete correctly. Properly cured decorative concrete is stronger, more durable, watertight and wear-resistant.
Technically, when you cure concrete you assist the chemical reaction called hydration. So that it can be placed more easily, most freshly mixed concrete contains more water than it needs. But if the excess water evaporates too quickly, the hydration process can be delayed or even prevented. If the temperature of the newly placed concrete remains favorable, the concrete will hydrate relatively quickly in the first few days. The resulting surface will be more stable and resistant to freeze/thaw cycles and deicing salts.
Decorative concrete performs best when cured with a compound
Concrete can be cured several ways. Some contractors cure concrete by ponding the slab with water. Others use burlap or straw, or even moist earth. But using water or air curing for integrally colored or color-hardened decorative concrete can leach soluble salts from it, causing efflorescence or surface lightening. Plastic or paper curing compounds can leave marks on the concrete or, even worse, discolor it.
Membrane-forming curing compounds
Because they're the most practical, the majority of experienced contractors use liquid membrane-forming curing compounds to cure decorative concrete. These compounds are available in a clear, white-pigmented or color-matched finish. White pigmented compounds are recommended for use on hot, sunny days because their reflective quality helps to keep the temperature of the concrete lower. Some clear compounds contain a dye that fades quickly to help the installer check for complete coverage. Color-matched compounds are recommended for integrally colored and color hardened concrete where a uniform appearance is desired.
Sealers are critical for long-term performance
Weather, water stains, oil and petroleum-based products, deicing salts and abrasion can discolor or otherwise mar a decorative concrete surface. Sealers help protect the concrete by resisting stains and making it easier to clean and maintain the surface. That's why it's so important to seal every decorative concrete surface. On interior applications, sealers not only protect the concrete, they support the final floor finish.
Apply a sealer to clean, dry concrete with a surface temperature of no less than 60 degrees Fahrenheit. Sealers should not be applied to newly placed concrete surfaces; the concrete should be at least 28 days old to allow completion of the hydration process. To get the best long-term performance on interior applications, apply the sealer in several thin coats. After the sealer dries, protect the surface with the appropriate floor finish. Then maintain a cleaning schedule that meets the floor finish manufacturer's directions. You, or your client, will need to replace the floor finish based on the surface's use. For most residential applications, that typically means maintaining the floor finish every year or two. By comparison, high traffic commercial areas, like grocery stores, require monthly maintenance. The frequency with which the floor finish should be replaced depends on the amount of foot traffic.
Exterior concrete surfaces need several thin coats of sealer, too. Based on the area's use, the sealer will need to be reapplied regularly. On a home patio or pool deck, that might be every two years. In hard-use situations, the sealer will need to be replaced more often.
Choosing cures and sealers for decorative concrete
All decorative concrete surfaces require maintenance. Exterior surfaces should be power-washed at least twice a year. Interior floors should be cleaned regularly and commercial floor finishes reapplied. The sealer protects the concrete and supports the floor finish.
ASTM International is the primary source of technical standards for materials, products, systems and services. For curing concrete, the ASTM C-309 Liquid Membrane- Forming Compounds for Curing Concrete and C1315-00 Liquid Membrane-Forming Compounds Having Special Properties for Curing and Sealing Concrete test methods and outline the minimum performance values a product must have. Cures that are best for decorative concrete are in the acrylic families and are broken into two types: solvent-borne and water-borne.
The curing method and sealer you use must be compatible. In fact, the final floor treatment and its sealer should dictate the curing method you use. In other words, you should choose a sealer based on the way in which the floor will be used and how it will be maintained. For example, on exterior, broom-finished colored concrete, a color-matched curing compound is recommended. But for a monochromatic interior or exterior concrete surface, then a color-matched cure and seal are ideal. For curing and sealing a surface that needs to be extremely durable, a solvent-based material may be applied (check local regulations prior to use).
If you know in advance that an interior concrete floor won't be maintained regularly, use a higher-performing sealer. It's much easier to remove and replace a floor finish than it is to replace the sealer. Once the sealer begins to wear, the underlying concrete substrate is at risk for damage.
Exterior sealers must be able to breathe in order to allow moisture to pass through them. Non-breathable sealers that are applied to exterior decorative concrete applications are likely to discolor and delaminate. Even worse, a non-breathable sealer can cause the concrete to deteriorate. That's why it's very risky to use a sealer that traps moisture in concrete that is subject to freeze/thaw cycles.
When you are certain that the concrete you're curing will be a reactive stain, either water- or acid-based, cure it with new, unwrinkled, high quality non-staining curing paper. Non-staining curing paper has the permeability that will permit the concrete to cure at an even rate, and it contains no dyes. It is not rosin paper. Do not use liquid curing materials on concrete that will have a topping placed over it or will be treated with reactive stains. The type of project determines your sealer choice.
Most water-borne cures and sealers are VOC compliant and can be used anywhere in the country. Solvent-borne cures and sealers are generally used on exterior applications or in uninhabited, well-ventilated interiors. Most interior applications are cured and sealed with VOC compliant water-borne products. No matter whether it's an interior or exterior, you'll need to decide whether to use a clear sealer or one that has been color-matched to the concrete's color. You also must choose between gloss, semi-gloss and matte finishes.
Before you select a curing and sealing compound for your decorative concrete project, consult with the decorative concrete manufacturer. Make sure to choose compatible products that work as a system. Most quality decorative concrete manufacturers offer a system approach for their complete line of products. Your finished project is likely to be more long-lasting when you use a complete system to create it.