Few professional concrete organizations would suggest that concrete should not be cured, though some exceptions exist. For example, conventional wisdom suggests concrete placed in freeze/thaw climates during the late fall should be allowed to dry out before slabs freeze in order to diminish early freezing damage. There is also some evidence that hard-troweled and burnished finished warehouse floors are nearly impermeable to the movement of moisture either into or out of slabs, with the result that curing has little effect.
The purpose of curing is to retain water content in a slab during the time when initial hydration occurs. The usual methods to achieve this include wet-curing (usually for the first seven days), covering a slab with an impermeable cover such as poly plastic, installing a chemical membrane forming curing product, or applying a sodium or other silicate product thought by some to reduce moisture permeability at the surface (but not recognized by ACI as a curing membrane).
Efflorescence is a cause for so many of the problems affecting decorative concrete. It's one of the byproduct compounds resulting from the cement hydration process when calcium hydroxide (CH) and other hydroxides after dissolving in water migrate to the surface of a slab through pores and capillaries. At the surface, the water evaporates and the dissolved CH (Ca(OH)2) combines with carbon dioxide (CO2) from the air to form calcium carbonate (CaCO3)—a very difficult-to-remove white deposit or whitish film that sometimes covers an entire slab. The + chemical reaction for this is Ca(OH)2+ CO2 ? CaCO3 + H2O. As little as 0.2 ounce of calcium carbonate per square yard of concrete surface can cause a significant whitish shift in color. There is 140 pounds of CH produced in the average concrete mix.
The reaction that produces calcium carbonate—referred to as carbonation—lowers the pH of the concrete surface. Carbonation is a normal and expected reaction and results in a lighter surface paste color—acceptable for most uncolored concrete but not a good outcome for colored concrete.
Bleed water brings CH to the surface of fresh concrete so any mechanism that prolongs bleed time ultimately increases the amount of calcium carbonate formed and this is part of the curing dilemma. The problem for decorative concrete contractors is this efflorescent whitish color cast is nearly impossible to remove—diamond grinding, acid washing (not successful unless done immediately after it first appears), or sandblasting are the most effective means of removal.
Curing and sealing issues
An important step for many or most decorative processes includes the application of a sealer: a membrane forming coating or penetrating compound that absorbs a short distance into the surface. Acrylic sealers placed on exterior work or on indoor slabs on ground without a vapor retarding membrane under them allow water vapor to pass through. Owners also like the enriched color appearance these coating type sealers provide, but more important to the contractor is their ability to close concrete pores and capillaries just below the surface, preventing CH from reaching the visible surface of a slab.
There can be serious compatibility problems between membrane forming curing compounds and sealers. Curing membranes are surface coatings designed to prevent water-of-convenience in concrete from escaping. Acrylic sealers allow water vapor to pass through. When these sealers are applied over curing membranes, water vapor is still trapped in the slab, which can result in “blushing” or the formation of blemishes over time. Compatibility issues between sealers and curing products can result in the loss of bond between them, resulting in delamination of the sealer. However, there isn't an issue for colored wax curing products, but using waxes commits owners to maintain their work with them from that time forward.
The solution to these problems is removal. Strippers and solvents can be used to remove some but not all material. If sealers are installed again, the same bond problems can redevelop. Mechanical means of removal, such as grinding or sandblasting to get below the depth of these products, becomes the effective solution. But owners have to agree to this method and decorative contractors usually end up paying for it.
Some curing products are advertised as self-dissipating but this may not happen fast enough to suit construction schedules and part of these products always remain in concrete pores, which can cause problems when sealers are applied over them.
A number of problems can result from curing colored concrete, including:
- Wet curing traps moisture laden with CH at the surface that quickly effloresces when curing covers are removed.
- As the length of bleed time is increased more CH comes to the surface, resulting in more efflorescence. For example, when concrete is placed in the fall under low temperature conditions, efflorescence can almost completely mask the color of the concrete.
- When there is too much water in the pores or capillaries, curing, cure-seal, or sealer products don't bond properly, causing blemishing and possible delamination of the sealer.
- When chemical membranes lose bond, small air voids develop between them and the concrete. Light is diffused and reflected in these voids causing a milky or hazy color appearance, camouflaging the color beneath.
- Unsealed colored concrete shifts color due to efflorescence.
- When acid stains are specified for coloring concrete, surfaces must be free of all curing and sealing products in order for the stain to penetrate.
- Curing with covers, even if composed of synthetic fabric, can result in an uneven curing method and color blemishes. Uneven curing equals uneven color.
In every region of the country, many (or most) decorative contractors air cure their projects. Concrete surfaces are left clean and free of any covering or objects, such as lumber, plywood, buckets, or other construction materials that could cause ghost imaging until they are sealed.
Efflorescence occurs on all concrete slabs. The process begins when initial set starts to occur (primary efflorescence) and continues throughout the entire life of a slab (secondary efflorescence). It's entirely dependant on the movement of moisture through a slab. Therefore, reducing the amount of efflorescence requires limiting the amount of moisture at the surface through a number of ways.
- Place vapor retarding membranes under slabs for indoor applications. This doesn't help with primary efflorescence but does prevent moisture from moving from the subgrade through a slab on a continual basis.
- Design concrete mixes with good aggregate gradations to reduce the amount of cementitious and total water required. Highly durable, low permeable dense mixes restrict the movement of moisture in concrete.
- Where possible, incorporate some fly ash as a replacement for portland cement. Fly ash, slag, and metakaolin react with and consume some CH in a mix. They also may lighten the surface color.
- Use superplasticizers to reduce the water-of-convenience (water needed to place and strike-off concrete) and maintain low water-cement ratios.
- Use accelerating admixtures when ambient temperatures are cool or when surface crusting conditions are present. Accelerators reduce bleed time before initial set occurs.
- Place a breathable sealer as soon as possible after initial set to confine the movement of CH to nonvisible space below the surface
Communicating with owners
Prepare large mock-ups, a 6x6-foot square for example, and demonstrate the results of the specified curing system. These should consist of the approved concrete mix design placed over the required subgrade section. Blemishes that occur due to the curing method will be evident and should allow for a productive meeting to resolve curing issues. The contractor may want to cast an identical mockup at the same time without a curing membrane to illustrate the difference. Larger mock-up samples are costly, especially for a residential project, but mock-ups help to establish the color, texture, and jointing requirements and should always be considered. They are as much for the contractor's benefit as they are for the owner or a project representative.
A curing system for decorative concrete must yield a finished product that is suitable to the owner. In many instances, the result may be omitting the use of curing compounds or covers.
Deciding what's important
Typically, most decorative concrete isn't cured, but sealed. There are very few problems reported due to uncured decorative concrete—curing is only one of the criteria owners might consider to be important. Some decorative finishes can't be cured: stamped concrete with colored release powders that add color variation; uncolored or colored concrete to receive acid-etch staining; exposed aggregate finishes; and dye- and water-stained concrete, are just a few.
The first priority in most owners' minds is aesthetics. They want colors to look like what they chose and although contractors carry the responsibility to educate them about the variability of concrete, they don't want to see white blushes or color significantly lighter than their choice. Few are concerned about the unseen characteristics of the work, such as mix design, placement techniques, ambient weather conditions at the time of placement, curing, or when to seal.
ACI guideline recommendations
Should a decorative contractor be forced to assume all the risks for using a specified curing system when there are so many potential problems? ACI guidelines shouldn't require all decorative to be cured, setting the scene for court rulings that go against contractors. Owners, design professionals, and contractors should discuss together the risks of using curing and sealing products at preconstruction meetings and agree on the best approach to take based on the owners' expectations.
Most decorative concrete installed in the U.S. isn't cured. Many contractors install sealers as soon as ambient conditions permit and shortly after initial set occurs in an effort to control efflorescence. Sealers restrict the movement of moisture through slabs but do not qualify as curing membranes.
When setting guidelines for curing decorative concrete, ACI must consider the contractor's risk as well as the overall durability of a colored concrete slab. ACI 310 Decorative Concrete guidelines should neither require nor not require curing for decorative work. Curing is only one of many considerations for the installation of good work and should be treated as a negotiable item. Owners should be included in the discussion about final color appearance and the types of products that will be used.