Pervious Concrete Pavement
By Matt McPharlin and John Seith
Pervious Concrete is a special type of concrete that allows rainwater to pass through it, thereby reducing the runoff from a site and surrounding areas. Pervious Concrete has seen more widespread use in parking lot pavements in Florida than in any other area because of government regulations on storm water retention. Here in California it is becoming more commonplace. The California Costal Commission is starting to recommend pervious materials as a Best Management Practice for managing storm water runoff.
Storm water runoff has become a significant issue in land development and construction. According to the EPA, 90% of pollutants are typically carried off by the first 1 ½ inches of rainfall into traditional storm water collection facilities. This captured runoff flows into nearby streams eventually flowing into the ocean. This runoff is carried away and not allowed to recharge underground aquifers.
The voids in pervious concrete are due to the absence of fine aggregates. The voids allow for rainwater infiltration into the pavement. The rainwater may be stored in the pavement or allowed to percolate into the subgrade, offering improved filtration and an enormous amount of surface area to catch oils and chemical pollutants. Some experts believe that the bacteria living in these spaces break down pollutants preventing much of the polluted runoff that normally occurs with traditional pavements. Pervious concrete offers several advantages:
- Groundwater is recharged.
- Water resources are preserved.
- Storm water runoff is reduced.
- Storm water quality is improved.
Pervious Concrete is one of the newest technologies to allow for maximum land development, while adhering to storm water regulations. Pervious Concrete provides an environmentally sound alternative that meets the challenges of governmental regulations, as well as design professionals and owners.
Pervious concrete is a no-slump concrete mixture containing a single coarse aggregate size. Typically the coarse aggregate conforms to ASTM C-33 size #8 (3⁄8 inch to No. 8), although larger sizes have been used. The void content of the concrete is typically 15% to 25%. The content of coarse aggregate per cubic yard is based on the dry rodded unit weight of the aggregate times 27. This works out to be around 2,500 lbs per cubic yard (pcy). The cement contents range from 400 to 600 pcy and compressive strengths average 2500 to 3,500 psi. The strength can be improved with small additions of fine aggregate, but this will reduce the permeability of the mixture.
The water content of pervious concrete is critical and is confined to a narrow range. If the water content is too low, a poor paste-to-aggregate bond and a loss of strength occurs. Mixing also becomes difficult to achieve. If the water content is too high the paste will not adhere to the aggregate and will flow downward, binding the void structure and also causing a loss in strength. The correct water content gives the mixture a wet-metallic sheen, with all the coarse aggregate particles coated with paste. Normally, the water-to-cement ratio of pervious concrete will range from 0.35 to 0.45.
Due to the low water content it is recommended that placement not exceed one hour from time of batching. Graniterock can extend this time if needed by adding a hydration stabilizer to the concrete during batching. This can add up to another hour of work time.
Prior to placing the pervious concrete the sub grade should be compacted to a minimum density of 90–98%. The sub grade should be damp but have no standing water; any ruts or grooves should be repaired. If the subgrade material is of poor absorption like clay, 3–4 inches of ¼″ drain rock should be placed. This additional material will allow the pavement to hold more water until it is absorbed into the ground. Pervious concrete is typically placed with a bucket loader and a vibratory screed. Concrete is spread across the forms and screeded off ¼ inch above the finish grade. This ¼ inch allows for compaction of materials with a roller. Curing is critical. Mixes with low water contents can self desiccate, so covering with plastic sheeting should be done before rolling. The concrete should be kept damp and covered for at least 7 days for the concrete to attain enough strength to handle traffic loads.