Porous Concrete & Asphalt Launch
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pervious pavement // Grooming
Asphalt and concrete can be mixed in a manner that creates a series of voids that allow water and air to travel through. Such porous pavements are effective at mitigating stormwater runoff. While porous asphalt is more economical than porous concrete, both materials have been successfully employed in the field with installations over 20 years old. Both are conventional mixtures, from which fine particles have been graded from the aggregate mix. The void content left by this gap-grading is approximately 15 – 25%, sufficient space for rapid infiltration during most rain events. Clogging of these voids can be avoided with proper grading to prevent sources of silt from reaching the pavement unfiltered. Also street cleaners are recommended to clean the porous surfaces twice a year or when surfaces have been clogged. A stone bed, usually 18 – 36in (45.1 – 91.4cm) deep, lies underneath the porous materials and retains stormwater until it infiltrates into the soil mantle. The stone bed can be enlarged to also accept stormwater from additional sources, such as roofs and adjacent impervious paved areas. In some situations, such as areas with poor water infiltration, parts of the infiltration bed may be situated away from the porous paving, where infiltration is improved or amended, and connected with drains. The extra thick subgrade layer also serves as an enhanced foundation for the paving. The long-term endurance of some early installations of porous materials is in part attributed to its enhanced foundation. Additionally, the combination of the improved insulation provided by the extra subgrade and the porosity of the paving materials acts to melt snows faster than impervious paving, thus reducing the need for plowing and salting snow. Though porous pavement is used primarily for parking areas, it is suitable for a wide range of applications, including paths and sports areas. The absence of fine particles creates a surface that is generally rougher than regular paving. Structurally, the absence of fine particles does reduce the shear strength capability of the material and it is not recommended for sloped surfaces. Material additives such as lava rock can adjust the performance of the pavement, including increasing the shear strength and water retention capacity. Porous pavements are comparable in cost to regular pavement materials, however the extra subgrade will incur extra costs. These costs may be easily offset by the drainage infrastructure required for typical installations. Source: Portland Cement Association; National Ready Mixed Concrete Association; Cahill Associates
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