Selecting the right product to cure and/or seal new concrete, or re-seal older concrete, can be an overwhelming task. Not only is the decision of whether to use a curing compound or a cure & seal a challenge, but there are hundreds of commercially available product options in each category. This final installment in the Demystifying Concrete Curing & Sealing Series will provide some guidance to help make those decisions a little easier.
Curing Compound or Cure & Seal?
In Part 2 of this series, the performance differences between concrete curing compounds and cure & seals were described. Deciding which type of product to use is simple if there is a specification governing the concrete work on a project. In this case the approved product(s) may be called out by brand name, or in the case of a performance specification, the required ASTM standards will be listed. Recall that ASTM C309 is the standard for concrete curing compounds, and ASTM C1315 is for concrete cure & seals. If either one of these is listed in the construction specification, a simple review of product literature will point you in the right direction, as most curing compound and cure & seal manufacturers will list which ASTM standard their product complies with on its technical data sheet.
If the concrete curing method or materials are not listed in the specification – or if a specification doesn’t exist – then the curing compound vs. cure & seal decision requires some questioning of the construction manager or owner. The chart below provides a few questions that should be asked in order to steer the curing product choice in the right direction.
Cure & Seal
Will the concrete later receive a densifier, penetrating sealer, coating, flooring, or topping of any kind?
If yes – the best choice is a dissipating curing compound that can be easily removed before another product is applied. Reactive silicate curing materials are also an option in certain conditions.
If yes – cure & seals are not recommended. The presence of a cure & seal may prevent proper adhesion of coatings, coverings, and toppings.
Is the concrete to be a cured floor in a heavy-duty or industrial environment?
If yes – use a dissipating or reactive curing compound…and apply a liquid densifier when the curing period is over.
Cure & seals are not recommended on heavy-duty floors where traffic and chemical exposure will quickly deteriorate them.
Is the concrete decorative, where a deepening of color and glossy appearance are desired?
Curing compounds do not provide color enhancement or gloss, so they are typically not recommended for decorative concrete.
If a gloss appearance and color enrichment are goals, cure & seals are the best choice.
Is material cost the primary deciding factor?
Typically cost less than cure & seals.
Cure & seals cost more than most curing compounds, but the additional benefits and longevity may be worth it.
Selecting a Curing Compound
If using a curing compound is the right choice for a project, narrowing it down to a specific product type is fairly easy. If the project specification calls for a curing compound that meets ASTM C309, the general product options are 1) a dissipating curing compound or 2) a white pigmented curing compound. Dissipating curing compounds are most often used on interior, hard-troweled floors when a liquid densifier will be applied later. White pigmented curing compounds are common on concrete roads and bridge decks, since the white pigment reflects sunlight (keeping the concrete cooler) and provides visual verification that the product has been applied.
If there is no specification or C 309 requirement, a reactive cure may be appropriate. Refer back to Part 3 of this series for more information on using silicate solutions as a curing method on fresh concrete.
Concrete Cure & Seal Selection
There are several types of concrete cure & seals available, so choosing the right product requires some thought about project specifics, safety, and the expectations of the owner. Here are some factors to consider when deciding on which cure & seal product to use.
Water Based or Solvent Based? Acrylic cure & seals are formulated with either water based polymer emulsions or solvent based polymer solutions. Both types produce a continuous film that retains moisture and seals the concrete, yet the mechanism of drying is different and poses unique considerations pertaining to application conditions and appearance.
In water based cure & seals, the acrylic polymer particles are dispersed in water. Water based cure & seals appear milky-white in the pail because of the way light reflects off these acrylic particles. When applied to concrete, the water evaporates and the polymer particles move closer together. As the evaporation of water continues, the polymer particles begin to deform and fuse together, eventually forming a continuous film on the concrete surface.
With a solvent-based cure & seal, the acrylic polymer is not present as separate particles. Instead, the polymer chains and solvent form a continuous, clear solution. After application of the cure & seal, the solvent evaporates and the polymer chains are drawn closer together and eventually entangle, and the film of cure & seal is formed on the surface.
Once applied and fully dry, water and solvent based cure & seals function in a similar fashion. All other factors being equal (solids content, proper application, and condition of the concrete), the toughness and durability of water based and solvent based cure & seals are comparable.
One key difference between water based and solvent based cure & seals is their appearance after application and cure. Compared to water based products, solvent based cure & seals tend to wet out and penetrate concrete surfaces to a greater degree, resulting in a glossier finish that deepens the color of the underlying concrete. Water based cure & seals tend to provide a lower gloss, matte finish.
What’s the Application Environment? An attractive benefit of water based cure & seals is their safety; they are nonflammable, have no strong solvent odor, and allow for simpler clean-up once the application is complete. In many environments, such as curing and sealing of indoor concrete, or exterior use around sensitive facilities such as hospitals and schools, the use of a solvent based cure & seal is not recommended due to the solvent odor during application and drying. Similarly, there are environmental factors to consider when considering a water based cure & seal. The drying process for water based products is sensitive to the temperature and humidity of the surroundings; use during cool and damp conditions can result in poor film formation during drying, leading to a white, chalky appearance. Always carefully read the cure & seal product data sheet and follow the recommendations provided by the manufacturer for optimal application conditions.
Does Solids Content Matter? ASTM C 1315, the industry specification for concrete cure & seals, requires products to have a minimum 25% solids content; that “solids” can include the acrylic polymer and additives such as UV absorbers, leveling agents, and pigments. Many cure & seals are formulated to have right around 25% solids, but some are at 30% or higher. In general, a higher solids content will produce a thicker film on the concrete surface. To illustrate this, the dry film thickness of three cure & seal products with different solids contents is shown below, assuming application in one coat at 300 ft2/gal (7.4 m2/L). One mil equals 0.001 inch (0.0245 mm).
- 15% solids cure & seal: 0.8 mil dry film thickness
- 25% solids cure & seal: 1.3 mils dry film thickness
- 30% solids cure & seal: 1.6 mils dry film thickness
As a point of reference, a sheet of copy paper is about 1 mil thick, and a kitchen trash bag ranges between 1.2 mils and 1.7 mils thick. Based on this comparison, do you think a 30% solids cure & seal has performance and durability benefits over and above the 25% solids product? It’s hard to imagine that the extra 0.3 mil (0.0003 inch) of acrylic on the concrete provides much at all.
Since more solids means a thicker/more viscous materials, higher percentage solids cure & seals can be more difficult to apply. In the “old days” when cure & seals were all around 15% solids, application was easy. Successful application of high-solids materials requires a sprayer and spray tip built to handle a thicker liquid, and contractors need training and experience for best results.
Are There Strict VOC Regulations? The VOC content of concrete cure & seals has been regulated across the entire U.S. since 1999. In addition, several regional VOC regulations that further lowered the allowable VOC content in many products, including curing compounds and cure & seals, went into effect in 2005 and 2009, and these regulations continue to evolve today. So what exactly is a VOC? VOC stands for Volatile Organic Compound, carbon-based materials that evaporate under normal conditions and have the ability to chemically react when exposed to sunlight. This reaction creates low-level ozone, a major component of smog. In an effort to prevent this type of air pollution, VOC regulations limit the level of VOC in many products such as architectural and industrial coatings, a category that includes concrete construction liquids.
Make certain the product you intend to use is VOC compliant within the state (or in some cases, the county and municipality) it’s intended to be used. This is particularly important if you’re traveling to a remote jobsite in a state whose VOC laws you may not know. Also, keep in mind that low-VOC products often require different application techniques. Product manufacturers are a great resource for information on VOC regulations, and will be happy to help you decipher the laws, choose the right product, and apply it successfully.
Cure & Seal Troubleshooting
When concrete cure & seals don’t perform properly, or their finished appearance is not what was expected, the cause can often be traced back to a handful of common reasons: product selection, the product was applied too thick, was applied in non-ideal conditions, or was applied in too many coats. All of these circumstances can be avoided by contacting the product manufacturer or sales representative before selecting and using the cure & seal to ensure you’re selecting the best product for your project. Also, be sure to carefully read and follow the instructions on the product Technical Data Sheet and Safety Data Sheet before use.
The information that follows is supplied as a general guide to identifying and remediating concrete cure & seal issues. Each situation is different, and results may vary. Whatever remediation method is chosen should be performed on a small test section before addressing the entire area to determine if the results are acceptable.
ISSUE: CURE & SEAL HAS BUBBLED
Cause: Product was applied too heavily, or in hot weather/direct sun.
Prevention: Carefully follow manufacturer’s recommended coverage rate and apply during the coolest part of the day when concrete is not in direct sun. Two thin coats should be applied rather than one heavy coat.
Solution: Perform a solvent wash and allow to dry. Re-application is not recommended.
ISSUE: SOLVENT BASED CURE & SEAL TURNED WHITE
Cause: Product was applied too heavily or there are too many coats of cure & seal on the concrete, and moisture trapped underneath the cure & seal has caused it to lose adhesion from the concrete.
Prevention: Follow manufacturer’s recommended coverage rate; do not re-seal concrete until previous coat(s) have worn away or have been stripped off.
Solution: Solvent wash and allow to dry. Re-application is not recommended.
ISSUE: CURE & SEAL IS PEELING OR FLAKING OFF
Cause: Product was applied too heavily, there are too many coats of cure & seal on the concrete, or the concrete was not prepared properly before application. Since concrete cure & seals last 1-3 years, some peeling and flaking should be expected as the product wears away, especially in areas of high traffic or direct sunlight.
Prevention: Follow manufacturer’s recommended coverage rate and preparation methods; do not re-seal concrete until previous coat(s) have worn away.
Solution: Pressure wash or scrub concrete to remove any loose material. Allow to dry completely. Perform a solvent wash to bring remaining product back to the surface and reestablish the seal. If solvent wash does not provide the gloss and seal desired, apply a light coat of cure & seal after solvent wash has dried.
ISSUE: WATER BASED CURE & SEAL IS MILKY-WHITE OR POWDERY
Cause: Product was applied in low temperature or high humidity conditions, where air flow is low (basement, closed garage, etc.), or product was applied too heavily.
Prevention: Follow manufacturer’s recommended coverage rate and application conditions.
Solution: The product may need to be completely removed with a chemical stripper or by mechanical means. Reapply in proper conditions.
ISSUE: CONCRETE SHOWS STREAKS, ROLLER MARKS, OR DRIPS AFTER CURE & SEAL IS APPLIED
Cause: Careless or sloppy application. Product may have been applied unevenly without keeping a “wet edge”, or wrong type of sprayer or spray tip was used.
Prevention: Carefully follow application instructions on product’s technical data sheet.
Solution: Perform a solvent wash to redistribute heavy areas of product.
ISSUE: WATER SPOTS ON SEALED CONCRETE
Cause: Hard water from landscape sprinklers dries on concrete and leaves minerals behind upon drying.
Prevention: Avoid sprinkling on concrete as much as possible.
Solution: Squeegee concrete dry in areas where hard water dwells on concrete.
ISSUE: CURE & SEAL IS STAINED FROM OIL, LEAVES, TIRES, FERTILIZER, ETC.
Cause: Cure & seals and penetrating sealers will not prevent stains.
Prevention: Prevent oil and other chemical drips from cars and equipment. Sweep tree debris and fertilizer granules from concrete as often as possible.
Solution: Use a commercial concrete cleaner or stain remover to clean stained concrete. Product may require reapplication if cleaner or stain removal process removes the cure & seal as well.
ISSUE: CONCRETE IS DARK AND BLOTCHY AFTER CURE & SEAL IS APPLIED
Cause: Uneven application or wrong product choice.
Prevention: Follow the application methods on the product technical data sheet.
Solution: Perform a solvent wash to redistribute heavy areas of product. If appearance is not acceptable after solvent wash, allow product to wear away over time, or remove and re-apply appropriate product. Only use cure & seals that are specifically listed as acceptable for sealing existing, cured concrete.
ISSUE: CARPETS, MATS, OR GARAGE DOOR WEATHERSTRIPPING IS STICKING TO CONCRETE THAT HAS A CURE & SEAL ON ITS SURFACE
Cause: Rubber additives in mats, carpet backing, or weather stripping may chemically react with concrete cure & seals, causing sticking.
Prevention: Don’t place rubber backed mats or carpet on sealed concrete. Don’t seal area of concrete where rubber items will be in contact.
Solution: Remove sources of rubber/cure and seal direct contact.
ISSUE: CURE & SEAL IS SHOWING SCRATCHES OR SCUFF MARKS FROM TRAFFIC OR FURNITURE
Cause: Concrete cure and seal products have moderate durability and should be expected to wear under abrasive traffic or moving furniture. An epoxy or urethane coating system should be considered for interior applications where more durability is desired.
Prevention: A concrete or tile floor wax/polish can be applied over sealed concrete to improve scratch and scuff resistance. Use non-scuff pads on furniture legs to prevent scratches.
Solution: Scuffed or scratched cure & seal can be repaired by lightly wiping the area with solvent and re-applying a LIGHT coat over the area.