What Is Pervious Concrete?

What is pervious concrete?

Pervious concrete is an open-graded concrete with connected pores that lets water pass through the pavement. The American Concrete Institute (ACI) report 522R describes it as a mix of portland cement, coarse aggregate, little or no fine aggregate, admixtures and water.

The hardened material has pores that are 2 to 8 mm across. The void content ranges from 15 to 35 percent of the volume. Typical compressive strengths are 2.8 to 28 MPa (400 to 4,000 psi). The drainage rate depends on the aggregate size and the density of the mix. It generally falls between 81 and 730 L/min per square metre (2 to 18 gal/min per square foot).

ACI lists these benefits: less stormwater runoff, better stormwater quality, possible groundwater recharge and less urban heat island effect.

Base layer

Pervious concrete normally lies on a layer of open-graded stone. The National Ready Mixed Concrete Association (NRMCA) says this base rock holds stormwater for a short time. The pavement also filters sediment and pollution from the water.

Clogging and cleaning

Pervious concrete works like a filter, and a filter must be cleaned. Sediment that washes into the voids reduces the porosity, and the pavement drains less.

The NRMCA guide sets this plan:

• Baseline test. Measure the infiltration rate with ASTM C1701 on the day the crew removes the plastic curing cover. The guide calls for three tests for every 25,000 square feet (about 2,300 square metres) and an average of the three. Repeat the test later at the same marked places.
• Routine cleaning. Remove debris with a blower, a sweeper truck or a dry vacuum, at least monthly or as needed. Ponding after rain shows that the pavement needs cleaning.
• Deep cleaning. Use pressure washing together with vacuuming. The trigger is an average drop of 25 percent from the initial infiltration rate. A rate below 100 inches per hour (2,540 mm per hour) is also a trigger. Extremely high washer pressure can damage the cement paste and cause raveling.
• Winter. Clean the voids before winter. Use coarse sand or small crushed stone as anti-skid material, not fine sand, and vacuum it after the winter.
• Sealers. Do not seal the pavement with impervious sealers such as asphalt or tar sealers.

Pervious concrete on airfields

The airport standards that we checked give no design rules for pervious concrete as a surface course. FAA AC 150/5320-6G has no pervious concrete surface course. It lists a drainable base, Item P-307, which is a cement-treated permeable base of aggregate, cement and water. The ICAO Annex 14 text and the EASA Easy Access Rules for Aerodromes also do not mention pervious concrete.

An inspector who finds pervious concrete on an aerodrome can use the checks above. These are ponding after rain, sediment on the surface and deterioration or spalling of the surface, which the NRMCA guide lists as inspection items. TarmacView does not offer a permeability measurement.