GraniteCrete – For Natural-Looking Porous Walkways
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GraniteCrete was used at Asilomar State Beach for durable, but natural-looking pathways and steps. Photo: GraniteCrete. Click on photo to enlarge. GraniteCrete is a portland-cement-based aggregate binder used with locally sourced, ground, decomposed... During installation, the dry-mix material, which is sold in bags and referred to as either an aggregate binder or aggregate admixture, is mixed at a 1:11 ratio with decomposed granite aggregate (a pre-consumer recycled waste material from granite quarrying and processing) that can be either 1/2" minus (material that passes through a 1/2" screen) or 3/8" minus. A small amount of water is added, creating a moist but not-flowing mix that is spread on the area being paved, raked even, and power-tamped. A key advantage during installation is that you can walk on it during installation. Full strength is reached after 28 days. Spreading GraniteCrete after mixing at a residential installation in Carmel. Photo: CJ Floriani. Click on photo to enlarge. For residential applications, a three-inch layer is typically installed on top of a carefully prepared and tamped base. For commercial applications and light vehicular use, a four-inch layer is recommended. The GraniteCrete mix includes pigment (11 colors, earth-tone colors are available), but the aggregate itself also adds color. With some installations, aggregate is derived from the local rock. I wasn't able to find out what the percent portland cement is (proprietary information), but it has to be less than 9%, since GraniteCrete is mixed with aggregate at a 1:11 ratio, and the material contains pigments and other components; I'm guessing that it's about 5%. (Most concrete is 10-12% portland cement.) I have been assured by Smith that there are no coal combustion products, such as fly ash, that could result in the leaching of mercury and other heavy metals. The company claims that the product can achieve 13 points in LEED, but some of these are a stretch. (I rarely pay much attention to manufacturers' claims about how many LEED points their products achieve.) You end up with a fairly low-grade, pervious concrete that is suitable for walking and light vehicle use (such as off-street parking), but not for roadway use. If needs change, the material can be easily broken up with a pick-axe. (Two potential LEED credits are listed for and "Materials Reuse," with the dubious ideas that GraniteCrete "left over from a construction site or removed later can be re-constituted and used on other sites" (Construction Waste Management credit) or "removed from one site and reused at another site" (Materials Reuse credit). Power-tamping of GraniteCrete. Photo: CJ Floriani. Click on photo to enlarge. The installed cost is higher than loose decomposed granite aggregate but less than standard concrete or concrete pavers, according to the company. Not including labor and base preparation, the GraniteCrete website lists the average price at $1.14 per square foot at a 3" thickness, compared with $0.72 for decomposed granite, $0.96 for asphalt, $1.72 for porous concrete, and $2.55 for pavers. In landscape applications, an important advantage is that it doesn't erode and wash away, like loose decomposed granite. At Asilomar State Beach and Conference Center in Monterey, pathways are made from GraniteCrete, as are steps that lead down to the beach. Some of those steps are below the high-tide level, and they have held up beautifully, according to Rasmussen. Depending on the pigment, the reflectivity is high enough that it helps to control the urban heat island effect--thus potentially earning this LEED credit, according to company literature. Designer CJ Floriani, a principal at Holland Hill Gardens in Carmel, California, has used GraniteCrete on at least eight projects and loves it. "It has a beautiful, natural look," she told me. The product is popular in Carmel, because it doesn't count as hardscape. Due to runoff concerns, the amount of hardscape (impermeable surface) on properties is strictly regulated. Tamping walkway edges. Photo: CJ Floriani. Click on photo to enlarge. In most of her installations, which are residential but often quite large, a ready-mix concrete truck is used to mix the ingredients onsite then emptied into wheelbarrows for spreading. With installation, the most important thing is getting the consistency right and dealing with the edges, she said. Another notable recent installation is at Crissy Field in San Francisco, where 6,000 square feet of the surfacing was installed by the Golden Gates National Parks Conservancy at the recently completed Crissy Field Center . For more information: GraniteCrete Carmel Valley, California 800-670-0849 www.granitecrete.com See more on this product in the GreenSpec Guide Alex Wilson is the executive editor of Environmental Building News and founder of BuildingGreen, LLC . In addition to this product-of-the week blog, he writes the weekly Energy Solutions blog . To keep up with his latest articles and musings, you can sign up for his Twitter feeds .