Stamping is quite possibly the most popular method of coloring, imprinting and providing texture to concrete. Its use began to grow in the early 1950’s due to Brad Bowman and the founding of the Bomanite Company. Today, stamped concrete is widely used in all 50 states, and much of the world is following suit by utilizing stamped concrete as a sustainable building material. Manipulating the texture of concrete was originally done with metal tools, which quickly evolved to rubber and textured stamping mats, which are now used daily in stamped concrete projects across the world.
STAMPED CONCRETE WITH INTEGRAL COLORING SYSTEMS
The addition of color is used to enhance the effects of stamped concrete. Introducing color to concrete was first achieved through broadcast color hardeners. A powdered releasing agent (usually a different color) is used to provide a contrasting or “antique” look. Today’s methods involve using both integral colors and broadcast colors (color hardeners). These chemicals should meet ASTM C979, otherwise, even effective curing methods may lead to uneven looks in the colored/textured concrete. Curing of stamped concrete should be done as quickly as possible. The use of a membrane compound that meets ASTM C1315 or ASTM C309 will allow for curing soon after stamping is complete. These compounds also need to be compatible with any sealers or secondary colors that may be used. The use of dissipating cures may be needed when other colors are to be later applied. Some curing papers may also be used. Several companies offer a waxed or water-based curing compound of the same color. These cures should be of high quality materials that keep color true. Note: The use of dissipating cures requires a longer period for applying secondary colors or sealers. Be sure that both owners and other contractors know this while they are effective methods they will slow the construction process. The use of sodium silicates does not meet the requirements of ASTM C 309. They are a chemical reactive and not membrane; however, contractors have used this method with no negative or visible results for years. ( ASTM 309 1991 STATEMENT) The use of some solvent borne cures containing small amounts of colored release powders can provide both a cure and antiquing effect on stamped concrete. When this method is used, the manufacture should specify the guidelines and coverage rates. Some of these methods may require thinning of the curing compounds. If this method is used, it should be with materials that meet both local and federal standards.
STAMPED CONCRETE WITH AN ANTIQUING AFFECT
Several companies now offer antiquing solutions. These can be applied shortly after stamping. Curing compounds may then be applied after these materials have sufficiently dried. If these antiquing methods are to be used, be sure they are compatible with the curing agent. These methods have been field proven with very good results. There are, however, some concerns with these products. These types of compounds cannot be applied if a powdered release agent has been applied to the concrete. These compounds can be easily applied to concrete, but if stains, dyes or other treatments will be applied, some curing compounds should not be used or they will need to be stripped completely before application of these materials. As of now, no liquid or membrane curing compounds can be applied over either stamped or stenciled concrete where a powdered release agent has been used. The use of impervious paper or non-wrinkling staining, Kraft paper or curing blankets may be used with caution as some can promote efflorescence. These methods should always be done with mock-ups and be approved before construction begins. Use of the one-time rolls of paper is the most effective concerning both labor and cost. These papers should meet ASTM C309 for moisture retention in compliance with ASTM C156. This material should also meet the requirements of ASTM C171. The use of one-time roll papers can keep hydration water in for extended time but each job should be evaluated and approved in mock-ups. Care will need to be taken that these materials are as wrinkle free as possible to avoid a blotchy look on the concretes surface. If curing blankets and paper are to be re-used, they should also be inspected for any organic impurities. It is important for the papers to be installed properly, for the paper to be as wrinkle free as possible and to ensure that the edges are properly covered to make these materials as effective as possible. If curing paper is used, consult the manufacturer. Curing in colder weather or wet weather can lead to issues with efflorescence in the concrete. Carefully decide if curing papers are the best for the job. Several contractors have successfully laid curing papers over stamped concrete when release powder has been used with no negative results. This guide does not address issues with saw cutting of fresh concrete when a curing paper is used. Each job that a curing paper is used should be considered on a case-by-case basis. All procedures and curing methods should be the same on the mock-up as on the project. All work should be done by the same personal that will also be on the job. Saw cuts should be planned out at pre-job meetings before placement. Note: If saw cutting is done before application of cures or sealers, any dust MUST be removed before the curing agent is applied. Traditional methods of cutting of control joints for the most part cannot be done in decorative jobs. All excess must be removed if cuts are done after curing. Have procedures in place to remove all dust immediately. Joints should be always preplanned and work within the period of any curing method. Avoid excess water when cutting colored concrete as this can affect the final appearance.
STAMPED CONCRETE CURING METHODS
One successful method used by many contractors is adding an additional layer of release powder when the texturing is complete and then covering with plastic, or curing blankets. If this method is used, the excess release should be swept up before any release is washed off. Follow all manufacturers’ recommendations for washing and disposing of excess release. Take note that washing off release powder and applying cure and seal products 24-48 hours later is not defined as curing. When this method is used, the concrete surface should be completely dry and all construction joints should be checked for water. A leaf blower can be used to dry these joints out before application. With the use of liquid release agents, curing on stamped concrete can be done much more effectively. Most of these products leave behind no adverse residue, which will affect cures. Contractor experience can play a great role in the success of the contrasting colors. Ask the supplier beforehand if any residue will need to be removed as other curing methods may have to be applied. If rinsing is required on any new decorative application, avoid power washers. Power washers force too much water into the new concrete surface, and added pressure on the new concrete may negatively affect the hydration process. A rinse and scrub method is best for the concrete. Note: Liquid release is known as “bubble gum” because of an added mixture that smells like bubble gum. Liquid releases evaporate quickly. One advantage of using a powdered release is the antique effect may be added during the stamping operation to the liquid release. This will allow for curing compounds to be applied as soon as the release has dried. Always be sure that the liquid release has dried. Neither water-based nor the solvent-lacquer is compatible with the liquid release so curing will be slightly delayed until the material is dry.
NEVER USE THE SAME SPRAYER FOR RELEASE AND CURES
When using membrane-forming compounds on stamped concrete, it is highly recommended that experienced concrete finishers perform this stage of the finishing operation. When application is done with pump-up sprayers, use quality sprayers. Keep a backup sprayer and extra replacement parts. Never place pump-up sprayers on the fresh concrete, as the pressure may cause a small ring look. Also, take care in refilling. Have a workstation to contain any spills and use a funnel to pour. On larger projects, use a floor mat to wipe boots before entering the work area. When using solvent cures always wear protective gloves and eyewear. Always use a clean sprayer for curing. Do not pour out release and then refill with a curing agent. Properly mark all sprayers and containers to avoid mix-ups on jobs. Consider color-coding sprayers to avoid any problems during application. When using solvent cures inside, provide proper ventilation, use a respirator, tightly close, and seal all air ducts during installation. Whether spraying or rolling, protect the area from over-spray or splashing from the roller. Use only lint-free solvent resistant rollers. Each cure may have different coverage rates. The use of a small rope with the approximate S/F coverage per 1 gallon can help estimate the coverage. Even coverage is vital for an even, consistent look when using membrane compounds. On textured concrete, care will need to be taken so that application does not run off the higher areas and accumulate in the grooves. Application of a cure and seal (most are solvent borne; that meet ASTM C1315) will darken the appearance of the concrete. If the same material is then used as a sealer, it will darken even more. Follow the manufacturer’s guidelines for time of application when these materials are used for sealing. Note that neither liquid nor powdered release agents meet the requirements of a curing compound at this time. Liquid release agents do form a membrane but evaporate quickly, therefore, they do hold in hydration water, but not enough to meet a standard for curing compounds. Powder release does cover the entire surface but does meet the curing specifications. Note that release agents are classified as bond breakers not cures. Tests have shown the release powders do aid in moisture retention. One of the most effective curing methods is the use of water curing either by ponding, wet burlap (or other like materials) or fog spraying. Water curing by ponding or wet burlap should not be used on colored or stamped concrete. These methods, while effective on normal concrete, can have negative effects on decorative concrete. The wet surface can effectively lighten the color and give a blotchy appearance (much like plastic sheeting). Water curing of colored concrete may also contribute to efflorescence issues. Fog spraying or misting may be used if care is taken not to allow any moisture to be in contact with colored concretes surface. This method requires that a high humidity be maintained just above the concrete. Note: Any colored or textured concrete surface involving decorative concrete should be evenly compacted, well drained and provide adequate slope for drainage. Even grading is necessary for decorative concrete slabs to insure that hydration and curing are as even as possible. While ACI 310 stresses the importance of mock-ups, it also realizes that on many jobs this may not be possible. When this occurs, pre-plan with careful written documentation of both curing and sealing procedures that outline periods for the stated cures and sealers.