Porous Paving

Also called pervious or permeable, porous paving allows rainwater to infiltrate into the ground. Although some porous pavement types are not new, pavements that are engineered to allow infiltration while also withstanding regular vehicle use are a more recent innovation.

Porous concrete is similar to conventional concrete, but uniform aggregate is used, and fines (smaller particles) are left out. Porous concrete is cast in place over a deep sub-base, with care taken not to float the concrete too much during setting, as that can reduce permeability. Porous asphalt, like porous concrete, is produced by eliminating the fines typically used in asphalt. Experience is required to maintain a porous surface during rolling. Unit pavers such as concrete, brick, or stone can be installed with joints filled with uniform aggregate to allow drainage. Pavers are also available as open-celled blocks that are filled with permeable, uniform aggregate or planting media and turf. Plastic geocell unit pavers, like open-celled masonry pavers, are filled with permeable, uniform aggregate or permeable soil planted with grass. Loose aggregate such as uniformly sized crushed stone can provide porous paving, although it is only suitable in light-traffic applications where it won’t quickly be displaced, ground down, or mixed with organic matter. Soft materials such as chipped bark or crushed seashells may be used as porous paving in pathways. Conventional dirt roads use mixed aggregate, including fines, and are largely impermeable.

Porous pavement depends on a subsurface that can absorb water. This system of multiple layers, which is usually deeper and more carefully engineered than the average roadbed, supports loading and provides reservoir capacity for stormwater.