Stamping Techniques Still the leading method for decorating concrete, stamping encompasses multiple ways to impar t color, pattern, and texture Stamping Patterns in Concrete By Joe Nasvik n the late 1940s, Bradford Bowman began exploring ways to stamp patterns in fresh concrete to simulate brick, stone, and quarry tile. First developing wooden tools, he then switched to large cast-aluminum platform stamps in the mid-1950s, which led to the decorative concrete methods of today. Although one could argue that others produced decorative concrete long before, Bowman’s invention provided the tools that enabled the creativity of large numbers of contractors that now drives this industry. Now, more than 50 years after Bowman’s invention, the largest segment of the flourishing decorative concrete industry is stamping patterns and textures in concrete. Recently, the original concept has been expanded to include texturing the surface of a stamped pattern and using mats to imprint only texture. Today, a large number of patterns, textures, and designs are further enhanced with diamond-saw-cut borders and infill techniques. I Stamping equipment Bowman’s original aluminum stamps were heavy and not durable—they could stamp only 50,000 to 100,000 square feet before cracking and breaking. They were often referred to as giant “cookie cutters” that imprinted a joint between the units of a pattern. In contrast, today’s stamps are made from long-lasting two-part urethane resins. The tops of the stamp are closed and have texture that simulates rock or brick patterns, and they include the pattern joint. Stamps vary in flexibility, but they should be rigid enough to support a worker’s weight without causing the concrete surface to compress under the worker’s foot. However, they must also be flexible enough to let air esBrad Bowman’s original produccape from under the stamp tion tool was made with cast aluwhen it is lifted and moved, minum. Typically, a set of 12 and strong enough to prostamps was used to set up a line vide a good impression as of pattern across the slab width, the concrete starts to harden. with at least one stamp to start In order to reach small 1 the next row. Impressions were ⁄4 areas that a platform stamp to 3⁄4 inch in depth and were often can’t reach, one stamp is grouted. usually made from very flexible urethanes so that it can be bent and folded into small spaces. In addition, hand-stamp tools usually BRAD BOWMAN Stamping Techniques Left: The manufacturers of stamps and concrete coloring materials provide seminars on a regular basis for concrete contractors who want to add this decorative technique to their offerings to customers. Below: Mats that produce texture only are often used today. Sawed borders and infills and chemical stains are frequently added to provide increased interest in the finished product. GRAF ARCHITECTURAL CONCRETE TRADEMARK Some texture mats include joints as deep as 11 ⁄ 2 inches, re-creating the natural cracking and fissures found on the top of rock outcrops. SCOFIELD INCRETE SYSTEMS Dinosaur footprints for unique applications are also created by stamping tools. made of cast aluminum are needed to complete pattern lines that the flexible stamp can’t reach. Texture for these small areas is provided by small “texture skins,” also made of urethane. A “beater,” or tamper, used to press platform tools into the concrete, completes the list of special equipment needed for stamping impressions. The beater usually has a flat bottom surface and is approximately 8x8 inches. Concrete mix design considerations The performance of all decorative concrete depends on the performance of the concrete. Contractors should practice the proper methods of placing concrete, though there isn’t anything special about the mixes. Choose a durable mix design that is appropriate to your region of the country. For impressions not deeper than 1⁄4 inch, a 3⁄4-inch topsized aggregate works well; for deeper impressions, the blades of the stamp can move 1⁄ 2-inch top-sized aggregate without causing a “dimpled” appearance on the surface of the concrete. In order to provide more time for imprinting patterns when concrete temperatures exceed 80° F, a portion of the portland cement in the mix design can be replaced with fly ash. Retarding admixtures are also helpful (see “Stamping Concrete: Managing Initial Set Times,” CONCRETE CONSTRUCTION, July 2002, pp. 46–48). And when concrete temperatures are less than 60° F, accelerating admixtures become necessary. Methods for coloring There are two primary ways to color concrete: mixing integral colors into ready-mixed concrete—metallic oxide pigments placed in the ready-mix truck either at the batch plant or on the jobsite—(see “Integrally Coloring Concrete,” CONCRETE CONSTRUCTION, December 2000, pp. 45–49) and broadcasting color hardeners onto the surface of freshly placed and bull-floated concrete (see “Using Color Hardeners,” CONCRETE CONSTRUCTION, September 2002, p. 52). Another way to add color is to add powdered or liquid release agents, which color the depressions in patterns and textures, providing a highlighted appearance. Release agents are needed to keep concrete from sticking Stamping Techniques to the stamps when they are lifted after making an impression (see “Release Agents for Stamping Concrete,” CONCRETE CONSTRUCTION, December 2001, pp. 65–68). Chemical stains are also increasingly being used to add color highlights to patterns after the concrete gets hard (see “Chemical Staining,” CONCRETE CONSTRUCTION, July 2000, pp. 38–47). Planning and organization In typical summer weather conditions, it takes about 3 hours to place, finish, and stamp concrete. The job starts when the ready-mix truck arrives, and it ends with imprinting the last stamp and completing detail work. During this time, at least ten different steps must be completed, compared with four steps for exterior broom-finished concrete. Under hot weather conditions, the time available can be as little as 2 hours. When ready-mix temperatures exceed 85° F, there is no longer enough time to place 8 cubic yards of concrete and complete the stamping process. Under cold weather conditions, 6 to 8 hours can pass before initial set, so contractors will need accelerating admixtures to manage the placement in order to avoid excessive bleeding and poor impressions. If three workers are needed to place 1000 square feet of plain gray concrete in a day, plan on five or more to place and stamp 1000 square feet of imprinted concrete. How the process works Everyone has his or her idea about how the finished product should look, so the first step must be to provide a sample for the owner’s approval. Construct the sample to represent what you can actually install onsite. Next, schedule a meeting to discuss slab dimensions, pitches and drainage, pattern lines, color, coordination with other trades, and contractor responsibilities. Be sure to exclude anything beyond your scope of work. For instance, site excavation is usually completed long before decorative concrete is installed. As a contractor, you often won’t know where subgrade is “cut” or “filled” or if there was proper compaction when soils were moved. If you place concrete on uncompacted fill, there will be set- tlement for many years to come. Be clear in your communications and your contract that you are responsible only for material and compaction supplied under the terms of the contract. Also communicate the handcrafted quality of the work, that concrete can crack in unpredictable places, and that efflorescence can result in color variations (see “Efflorescence on Decorative Concrete,” CONCRETE CONSTRUCTION, October 2001, pp. 65–67). Forming and placing concrete is the same as in other concrete installations. However, when forming, be careful to create rectangular areas with parallel form lines—especially when rectangular stamping patterns are to be installed. The usual sequence for stamping concrete is to place concrete, strike off the surface, bull float and edge, placecolor (if color hardeners are used), bull float color in, edge a second time, and trowel (often with a Fresno). In most instances, if you wait long enough to get on a slab with knee boards to trowel a finish, the concrete will be too hard to complete the imprinting process. The time-honored way to determine whether the slab is ready to begin the stamping process is pushing your finger into the surface of the concrete with moderate pressure. If it penetrates the surface approximately 1⁄4 inch, the concrete is ready to stamp. When the stamping process begins, a stamp should hold your weight without sliding around or sinking too deeply into the concrete. If concrete squeezes up between the stamps, the concrete is too soft. Moving your feet around the surface of the stamp should be all that’s necessary to create the right impression. Ideally, the stamping process is like riding a surfboard on a wave—the surfboard must be located exactly on the right part of the wave. When you imprint concrete that is too fresh, the result will look “mushy,” and the imprint will be poorly defined. If you imprint too late, the impression will be light, most of the texture from the stamp will be missing, and the edges of the pattern will crack and break away. During a normal stamping sequence, the concrete will become too hard to imprint with just body weight. So workers resort to the beater to impress the platform tools. The trick is to get uniform impressions from the beginning to the end of the placement. Managing this period of time requires good planning and knowledge—including how to control the setting time of concrete. Finally, one or two workers use texture mats and hand-stamping tools to pattern areas that can’t be reached with the platform stamps. At the same time, workers using margin trowels and small texture mats go over the entire project to remove any defects that might diminish the overall appearance. Sealing Efflorescence can compromise the appearance of any colored concrete project. Most complaints by owners about color are actually the result of efflorescence lightening the right color. The problem starts as soon as water starts moving toward the surface of freshly placed concrete. For this reason, it’s advisable to place an acrylic sealer recommended by the manufacturer just after the initial set of concrete. Acrylic sealers have good water vapor transmission qualities and help to prevent efflorescence (see “Acrylic Sealers for Exterior Flatwork,” CONCRETE CONSTRUCTION, January 2001, pp. 62–66). Price Stamped concrete starts at approximately twice the price of plaingray, broom-finished concrete. Projects that require more detail work and color increase the price beyond that. For more information about materials and tools for stamped concrete, circle 7 on the reader service card. Publication #C02J054, Copyright © 2002 Hanley-Wood, LLC. All rights reserved