A pilot project at English Park, Atlanta, recently tested the effect of permeable interlocking concrete pavements on stormwater runoff reduction. Predictably, the initial cost of these pavements is higher than that of asphalt. However, two key owner benefits more than compensate: assistance in stormwater management and increased land utilization. These pavements, first introduced in Europe, reduce surface runoff, increasingly important to regulators and planners. The National Pollutant Discharge Elimination (NPDES) Storm Water Program, under the Clean Water Act, requires states to issue permits that set limits on pollutants for stormwater management systems. Phase I requires permits for systems serving communities of 100,000 or more and is in effect, while Phase II, for industrial facilities and stormwater management systems serving communities under 100,000, is in development. As a best management practice (BMP), these pavements can help owners retain stormwater on their properties. A community may dictate that a site's post-development stormwater volume be no higher than its pre-development volume. Other communities actually limit the amount of impermeable cover as a percentage of a site. "The market for permeable interlocking concrete pavements is wide open," says David Smith, director of the Interlocking Concrete Pavement Institute. "Many municipalities are saying that not only do you have to pay according to the percentage of impervious cover on the property, but the maintenance and depreciation of the city's stormwater disposal system" by percentage of stormwater a site contributes to the system. Although only 10% of a pavement's surface may be open area, specifiers may consider it 100% permeable due to its permeable aggregate base. A municipality may issue a 100% permeability credit for the pavement and consider only the building a runoff generator, enabling the owner to construct a larger building. These pavements consist of pavers placed on a multilayer aggregate base. The pavers must meet the minimum 8000-psi compressive strength, maximum average absorption of 5% and resistance to freeze-thaw per ASTM C 936, "Standard Specification for Solid Concrete Interlocking Paving Units." The units may be shaped to create open joints, or spacers or grid openings may create open area. These site-specific pavements can handle a given rainfall event in accordance with local requirements. For example, the aggregate storage layer at English Park is designed to process as much rainwater as a 100-year rainfall event produces. Three variables affect drainage rate: amount of fines in the base, thickness of aggregate layers and pavement slope. Because the base is crucial to system quality, "We don't treat the pavers as a commodity, but as part of a system," says Allan McLean, who heads commercial sales for Bosse Concrete, an Oldcastle company. At English Park, designer Bill Brigham chose the permeable interlocking pavement for land economy, cost and appearance. "We want to be a pioneer for this type of system," Brigham adds. "We think this will set the hydrology industry on its ear." Producers are targeting several markets for permeable interlocking concrete pavements to address runoff concerns and the scarcity of land. Keywords: paver, interlock, permeable, pavement, stormwater, runoff, NPDES, best management practice, BMP, ASTM C 936, Bosse, Oldcastle, SF Concrete Technology, Uni-Group USA, aggregate base