Decorative concrete work results from creativity and skill, learning the proper techniques, and using products that make a job even better. Believe it or not, the industry is more than 100 years old. Between 1890 and 1920, some companies began adding color to concrete and chemically staining their work, especially precast companies. Exposed aggregate concrete probably began with the construction of Meridian Hill National Park in Washington, D.C., in 1914. Stamped concrete, invented in 1950, got its formal start in 1970. Concrete overlay was invented in 1939 but didn't become a widely used decorative product until the 1990s. Other decorative histories are much more recent. All areas of decorative concrete continue to change and evolve as contractors become more creative.
In the 1970s stamping tools were made with cast aluminum “cookie cutters,” which were heavy to lift and set. Lighter and more durable plastic “open top” stamps came next, followed by “closed top” tools that made it possible to imprint surface textures as well as patterns. From there it was an easy transition to large elastomeric urethane mats that only imprinted textures. More recently the development of large texture mats with large unit patterns make it possible to stamp concrete as fast as it can be placed.
History has a way of repeating itself. In the decorative pervious article on page 35, Tom Klemens describes how cast aluminum “open top” stamps now are used to pattern pervious concrete installations.
Stamped concrete isn't considered the most exciting area of decorative concrete anymore but it still remains the biggest segment of the decorative industry and represents the largest volume of work for many contractors. New patterns continue to be developed, especially for masonry applications.
As mentioned earlier, coloring concrete is the oldest decorative treatment. Coloring for concrete involves the use of metallic oxide pigments because they don't fade and are permanent. Originally these pigments were mined, but now most colors derive from manufactured iron oxide pigments. They can be added directly to concrete during mixing (integrally colored concrete), or they can be combined with portland cement and small aggregate, and broadcast over the surface of freshly struck-off concrete. This product is referred to as “dry-shake,” “dust-on,” or “color hardener” (see “Integral Color” in the October 2008 issue of CONCRETE CONSTRUCTION).
The use of integral colors has grown significantly over the past six years. Today, many ready-mix producers have liquid dispensed systems to meter color directly into their ready-mix trucks. Additionally, many construction supply houses also have dispensing systems where contractors can pick up any number of colors for small concrete placements. But the newest offerings are dry-powder integral colors that make it easier to add color to loads based on their cement content, or to add two or more bags of different colors to make your own color.
Chemical stain technology has been around since the 1890s. At that time, contractors made up their own recipes each time they had a project they wanted to color. This changed when manufacturers offered off-the-shelf products with the same chemistry in each unit. There are only a few color choices but the possibilities become infinite when you add color on top of another. People like chemical stains because of the wide variability in coloration that results. Chemical stains are unique among decorative products because they chemically combine with portland cement hydration byproducts.
Water-based stains are an extension of the paint stain industry. They are blends of fine metallic oxide colors, not affected by UV radiation, and chemistry that helps them penetrate into concrete pores to provide translucent or opaque color appearances. They offer an infinite range of color possibility. Continued product development focuses on finer pigments that can penetrate and bond better with the concrete. Water-based stains must be sealed adequately for applications with foot traffic.
Using dyes is the newest way to color concrete. Most dyes are aniline colors dissolved in solvents, such as acetone, and aren't very resistant to UV radiation. This makes them best suited for inside applications. The resulting coloration is very translucent and provides even, gentle coloring affects. They usually are applied with sprayers to get an even appearance. The most concerning thing about solvent dye is that they can be very explosive when vapor concentrations build up in the air, causing some serious accidents and deaths as a result. For that reason several manufacturers have been working on water-based products that can achieve the same appearance; currently there are several products on the market. Dyes are becoming more important for use on diamond-polished floors.
In the mid-1900s John Earley developed the process for exposing aggregate in concrete by using wire brushes and muriatic acid. It's hard to imagine how he and his workers managed to expose the massive quantities of concrete in Meridian Hill Park, Washington, D.C., including walls as high as 20 feet. Exposed aggregate work has come a long way. Contractors discovered they could wash concrete surfaces and scrub them with brushes to do the exposing if the timing was just right. This led to spraying surface retarders on freshly finished concrete, covering it with plastic sheeting, and washing off the retarded cement paste the following morning.
Today, surface retarding agents come in several strengths, allowing contractors to profile work to several different depths. The polymers in these products form a hard vapor barrier so slabs that retain moisture don't need to be covered, offering rain resistance too. Present generation surface retarders make it possible to create low-profile sand finishes to deep rock profiles. Recycled glass aggregates have become a popular exposed aggregate finish as well.
Overlay concrete was first used as a coating over the steel decks of ships to offer corrosion resistance and traction. It originally was made with latex rubber added to cement and fine aggregate. But rubber became scarce during World War II, so it was replaced with polymers. Today, overlay concrete is used in a wide variety of applications: microtoppings troweled very thin over existing floors, self-leveling floors for stunning indoor floor treatments, stamped and textured applications over existing concrete, and vertical mixes applied a couple inches thick for carving or stamping patterns.
Overlay concrete products can be used inside or out to add color, chemical staining, stamped patterns, high-gloss walking surfaces, stenciled patterning, artificial rockwork, and handcarved artistry.
Stenciling and patterning
The first stencils for concrete application were made from paper and intended to provide patterning that competed with the look of stamped concrete for either full-depth concrete or overlay concrete applications. The look that can be achieved using stencils is infinite. Today, stencils typically are made from either paper or plastic sheet goods with an adhesive back. They are used for masking purposes, to provide elaborate patterning or graphics for overlay concrete applications, and with decorative chemical or water-based stain applications. In fact, almost all decorative techniques can use stencils. They also are used as masks for spraying surface retarding agents to add patterning to exposed aggregate finishes.
In recent years, engraving has become a major way to pattern concrete, replacing hand-grooved joints. The tool of choice is an angle grinder or saw utilizing diamond blades. Borders and patterns can be cut into slabs to provide lines where stain colors meet, sandblasted textures start and stop, and for exposed aggregate patterns.
Diamond polishing floors is the fastest-growing decorative industry of all. A significant market has developed for industrial floors, commercial big-box floors, residential floors, and concrete countertops. The rapid development of grinding equipment and diamond pads to handle all the conditions also is impressive. The technology started with the stone wet-polishing industry, but dry-polishing with efficient vacuum systems to pick up dust and keep the air clean quickly have become the primary way to polish floors. Polished concrete is easy to maintain, reduces the light requirements for commercial spaces, provides a decorative look all its own, and reduces maintenance costs. The most recent improvements include procedures for testing concrete surfaces to determine which types of diamond pads will work best, machinery that gets closer to walls to eliminate costly handwork, and strip pads with diamonds embedded in them for use on buffing machines for low-cost maintenance.