A growing number of architects and owners are opting for concrete flatworks that resemble slate, brick, tile, stone, or even wood. Stamped concrete creates an illusion of expensive slate or tiles at a lower cost. It can brighten a poolside, enhance an entrance, and glamorize a driveway with colors and patterns. Unlimited color capabilities offer architects and designers total flexibility to match any design, theme or specification. If texture is desired on patterned surfaces, stamped concrete is applied using stamping mats that simulate slate, granite pavers, used bricks, and wood. Yet, results require the use of the right tools, good concrete, and correct finishing and stamping procedures.
Concrete requirements for stamping
Generally, stamped concrete proportions depend on depth of stamped pattern. For example, if an area is intended primarily for vehicular traffic or if the grooves are to be grouted, the desired pattern penetration may be as much as 20mm. In such cases, concrete with 20mm maximum size aggregate works best and enables workers to achieve uniform imprint depth; the slump should not exceed 120 to 150 mm. In contrast, embedding a stamping tool to a depth hovering 5mm requires minimal interference from coarse aggregates, thereby requiring reduced maximum aggregate size.
Entrained air is required for concrete that will be exposed to freezing and thawing; this can be as high as 7.5% for 20mm gravel concrete. When deeply grooved patterns are cut without using thin polyethylene films between the tool and concrete, tools must be cleaned frequently with water to prevent concrete from sticking to them. Spraying tools with a release agent may also help.
Retarders are sometimes needed to extend the period during which concrete can be stamped and increase daily production. This slows the setting long enough to permit stamping. However, if too much retarder is used, the stamped surface may crust over and even crack. Some contractors try to slow setting, especially in hot weather, by using less cement and increasing the slump. Concrete quality might suffer when this is done. No admixtures containing calcium chloride should be used for colored stamped concrete, as these could cause discoloration.
Procedures for stamping concrete
Good quality stamped concrete requires dedicated craftsmen who consistently follow proper concrete placing and stamping procedures.
Planning and preparation
Ideally, stamping should be the last job done on-site so the concrete is not damaged or stained during other construction operations. Plan the layout to provide a pleasing appearance, including the location of contraction and construction joints to minimize cracking and control pour size. Pours should be scheduled about 2 1/2 hours apart. Experienced crews can stamp as much as 60 to 70 m² / hr. and achieve daily rates of 150 m². The ready-mix concrete producer should be notified of special mix requirements.
After placement, concrete should be struck-off and bull-floated. Vibrating screeds or hand tampers may be needed if aggregates larger than 20mm sizes are used in the concrete to embed coarse aggregates below the concrete surface, making it easier to stamp deep patterns. No troweling should be done after completion of the coloring process.
Stamped concrete coloring may be achieved by two alternative methods, the integral color procedure, where the entire volume of concrete is colored by adding the color in liquid or powder form to the concrete mixing or cast on color procedure, where the color hardener is spread onto the surface of the wet concrete and the colored layer floated onto the top layer of the wet concrete. After the bleed water has disappeared, about half the color hardener is spread on the surface and floated on the concrete. When no surface bleed water is visible, more color is spread and floated. At least two spreading and floating operations are required, but some contractors spread hardener and float three or four times to achieve a surface of uniform color. The integral color procedure offers the advantage of the entire volume being colored while the cast on color increases the wear resistance of the surface treated. A stamped concrete system may combine the above two methods.
Judgment is required to decide when to start stamping. Typically, it is recommended pushing a finger into the plastic concrete to determine when to start; if the hole keeps its shape and does not fill with water, stamping can begin. Concrete is frequently stamped through plastic sheeting laid on the surface. Thicker sheets will wrinkle and mar the surface. When placing plastic sheeting, use a soft broom or trowel to smooth it over the concrete surface. When texturing is done with rubber mats, the powdered release agent is dusted by-hand. Note that the powder is extremely fine, and workers should avoid breathing it by wearing double-layered painter masks.
One way to joint stamped concrete is with leave-in-place wood strips. Another way is to use cold chisels, hand stamping joints that conform to the pattern. However, when this is done, the owner should be alerted to the possibility that cracks would not follow the joints. Leave-in-place headers can be used as construction joints between adjacent pours. When there are continuous straight lines in the pattern, such as with running bond bricks, simply run the stamp up to the irregular edge of the pour.
Wet surface brooming should eliminate most stamping imperfections. Some contractors mix 20 parts of water with one part color hardener and dip the broom in this mixture before gently brooming the surface (check with the color hardener manufacturer for recommendations concerning use of product). Workers may have to walk on the concrete and should wrap their boots with polyethylene film so they don’t mar the surface. Care must be taken not to over-broom or to start brooming when the surface is too soft.
Curing and sealing
Colored wax curing agents must be used because other curing methods are likely to cause surface color variations. Use a color curing membrane that matches the color of concrete. Applying wax produces a shiny surface, promotes cement hydration, and reduces shrinkage cracks. Periodic waxing is required to maintain a shiny surface, but many owners opt for the appearance of naturally weathered surface after the initial coating has worn off. When powdered release agent is used, concrete-curing operations must be delayed because the wax-curing compound cannot be applied until the release agent is removed.
To beautify existing or new concrete surfaces, a highly durable wear resistant mortar overlay in thickness of 0.5 to 1 cm may be colored and stamped to match various design schemes and specifications using procedures outlined herein above. It is commonly used in applications ranging from pool decks, patios and driveways to hotel lobbies, shopping centers and retail stores. It may also be applied, without stamping on it a pattern, if the design so requires.
A popular alternative to covering existing concrete with thin mortar that is gaining in popularity is to spray or trowel, over new or existing concrete surfaces, a two-part deck product that includes a liquid resin and cement-based powder product. In many instances, old surfaces may be also cleaned and polished with special cleaning and polishing products to extend their service lives and improve their appearance.
Exposed aggregate finishes to create texture can be used in conjunction with stamped concrete techniques to create various decorative options on regular or stamped concrete surfaces. The product reacts with the upper concrete surface to delay its setting, thus exposing the aggregate particles with distinctive permanent color together with high durability surface.
When properly applied, stamped concrete yields high durability surfaces with colors and shapes similar to natural building materials. As in all decorative systems, it is always recommended a concrete sample be produced first to ensure it meets the designer’s specifications and expectation.