How is pervious concrete made?
Pervious concrete is made from carefully controlled amounts of water and cementitious materials used to create a paste that forms a thick coating around aggregate particles. Unlike conventional concrete, the mixture contains little or no sand, creating a substantial void content – between 15 to 25 percent.
Using sufficient paste to coat and bind the aggregate particles together creates a system of highly permeable, interconnected voids which drain quickly. Both the low mortar content and the high porosity reduce strength compared to conventional concrete, but sufficient strength is readily achieved for many applications.
Pervious concrete allows 3 to 8 gallons of water per minute to pass through each square foot of the material. By allowing rainwater to seep into the ground, pervious concrete can be instrumental in recharging groundwater and reducing stormwater runoff. This capability can reduce the need for retention ponds, swales, and other stormwater management devices. Pervious pavement integrates hardscape surfaces with stormwater management.
Residential roads, alleys, and driveways
Sidewalks and pathways
Swimming pool decks
Pavement edge drains
Foundations/floors for greenhouses, fish hatcheries, aquatic amusement centers, and zoos
Tree grates in sidewalks
Groins and seawalls
Sustainability Benefits of Pervious Concrete
Pervious concrete has many environmental benefits. See associated sustainability solutions and technical briefs for more detail.
Stormwater Management. By allowing water to soak through and infiltrate, pervious paving reduces stormwater flow and pollutant loads. Can contribute to LEED Credit 6.
Minimize Site Disturbance. By integrating paving and drainage, less site area may need to be used to manage stormwater, allowing a more compact site development footprint. May contribute to LEED v4 Credit Sustainable Sites - Rainwater Management.
Local. Materials are usually extracted and manufactured locally. May contribute to LEED Credit M 5.
Recycled content. Fly ash, slag cement, or silica fume can substitute partially for cement, and recycled aggregates can replace newly mined gravel. Recycled content can contribute to LEED Credit M 4.
Cool. The voids reduce mass reducing the heat build-up associated with heat islands. Lighter colored cements can increase reflectivity.
General Guidelines for Specifications
The properties of pervious concrete vary with design and depend on the materials used and the compaction procedures. General guidelines for specifications are provided below.
Permeability. Typical flow rates for water through pervious concrete are 3 to 8 gallons per square foot per minute, but can be double that amount if desired.
Compressive Strength. Pervious concretes can develop compressive strengths in the range of 500 to 4000 pounds per square inch (psi) – suitable for a wide range of applications.
Flexural Strength. Flexural strength of pervious concrete ranges between 150 and 550 psi.
Shrinkage. Drying shrinkage of pervious concrete is faster but much less than that experienced with conventional concrete. Many pervious concretes are made without control joints and are allowed to crack randomly.
Freeze-Thaw Resistance. Freeze-thaw resistance depends on the saturation level of the voids in the concrete at the time of freezing. In the field, it appears that the rapid draining characteristics of pervious concrete prevent saturation from occurring. Where substantial moisture and freeze-thaw conditions are anticipated, pervious concrete should be placed on a 6 to 18-inches-thick layer of drainable rock base such as 1-inch crushed stone. Abrasion resistance.
Because of the rougher surface texture and open structure of pervious concrete, abrasion and raveling of aggregate particles can be a problem, particularly where snowplows are used to clear pavements. Surface raveling in new pervious concrete can occur when rocks loosely bound to the surface pop out under traffic loads.
Click here to view or print Rainwater Runoff - What's the Problem? by PCA Director of Sustainability David Shepherd.
"Pervious Concrete Mixtures an Properties," PCA Concrete Technology Today, CT043, December 2004, 2 pages, PDF
Pervious concrete is ideally suited as a solution to stormwater management issues with added environmental benefits. The large void content designed into this specialty concrete allows water to pass through rapidly, minimizing runoff and recharging groundwater supplies. Also known as permeable concrete, porous concrete, gap-graded concrete, no-fines concrete, and enhanced porosity concrete, pervious concrete can be used in a wide range of applications, although its primary use is in pavements.
"Pervious Concrete Pavement: A Win-Win System," D. Brown, PCA Concrete Technology Today, CT032, August 2003, 3 pages, PDF
Use of pervious concrete pavements helps owners and the environment