Most people consider concrete to be impervious—after all, it's HHl used to make swimming pools and kitchen sinks. And most concrete is, indeed, nearly watertight. But a new kind of concrete that allows water to flow through virtually unimpeded is gaining popularity. Pervious concrete is being used mostly in the construction of parking lots, but also for sidewalks, bike paths, playgrounds, and erosion control. Pervious concrete pavement is not, however, suitable for areas subject to high traffic volumes or speeds.
The mix is everything
Pervious concrete is a pretty simple material—typically pea gravel, portland cement, and water (heavy on the cement, light on the water, and no fine aggregate). A typical cubic yard of pervious concrete would have 2650 pounds of #89 gravel, 600 pounds of portland cement, and a water-cement ratio of about 0.30. This mix will provide a compressive strength of around 2500 psi and an effective void ratio of 20%. That will allow water to drain into the base course at rates of 2 to 18 gallons per minute per square foot—275 to 400 inches of rain per hour—much more than you are ever likely to experience.
On a jobsite, though, things are seldom that simple. Various admixtures are used based on the aggregate and environmental conditions. “We use air entraining agents, water reducers, some retarders, and sometimes a viscosity modifier,” says Dale Fisher, PCI Systems, Alpharetta, Ga., a contractor who specializes in pervious concrete pavements. “The mix design is everything. We often have to tweak it a bit in the field. We specify the concrete by unit weight, which tells us the void structure. We use 15% to 20% voids for parking lots. But on a recent bus stop, with heavier loads, we went to a higher unit weight and about 12% voids for higher strength.” Using a small amount of fines is a common way to increase compressive strength, but doing so will reduce the drainage rate. Often the fines are fly ash or slag cement. “We run unit weight tests because they tell us what's going on with the mix,” says Fisher. “If it's heavy, that can mean more fines, so we change the mix to keep it a little drier. If it's light, then we can pour it slightly wetter.”
Pervious concrete typically is produced in a ready-mix plant and delivered with mixer trucks, but it is not pumpable. Ohio Concrete, published by the Ohio Ready Mixed Concrete Association, sponsored a demonstration pervious pavement job recently that was installed by Ball Brothers Foundations, Monroe, Ohio. Moraine Materials provided the concrete. Ohio Concrete reports that, “For batching a maximum 8-cubic-yard load, they added air-entraining admixtures up front with half of the water, then the coarse aggregate, then the cement, and then the balance of the water.” This produced a successful mix.
A pervious pavement, however, is not just the pavement but rather a system of soil, subbase, and pavement (see sidebar). Each part of the system is essential to success.
Design and construction
A sample specification is available on the Web site of Charger Enterprises, a concrete contractor in Seminole, Fla. (www.chargerconcrete.com). Pervious pavements have been used in Florida for many years, and Charger has gained a lot of experience. It noted that an important first consideration is the soil beneath the subbase—some may be unsuitable since the soil must be able to support the subbase, the pavement, and the loads that will be applied. Get a geo-technical engineer to evaluate the soil.
The first layer against the soil is typically a nonwoven geotextile fabric (one that drains easily). This is topped with a subbase constructed with a minimum of 6 inches of No. 57 stone, well-compacted so that it “will not be rutted by a fully-loaded ready-mix truck,” according to Charger's Web site.
The thickness of the pavement will vary according to its design load and how much water the pavement and sub-base are expected to store, but 6 inches is a typical parking lot thickness. The contractor sets the screed rails or side forms, often shimming them during strike-off ½ to ¾ inch higher than the final surface elevation to allow for compaction later with rollers.
During a recent PCI project constructing a Safeway parking lot in Denver (see sidebar), one worker directed the chute and pushed the nearly zero-slump concrete down with his hand. Others moved the concrete into position with come-alongs. When a new truckload arrived, workers sprayed the 15-minute cold joint with water. Once the concrete was in place and roughly struck off, the surface was compacted and finished with a truss screed followed by a hand-operated steel pipe roller that was sprayed regularly with form release oil to prevent sticking. Finally, workers popped chalk lines and cut contraction joints at a standard spacing using a roller with a 1½-inch fin. The edges were then tooled, and final troweling was completed.
Vibration is not effective for pervious concrete. “As vibration contacts normal concrete, it ‘melts,' or sinks into a consolidated mass caused by both push and pull from the vibration energy,” says David Mitchell with Bunyan. “In perv, the energy delivered by vibration has only push and builds a crumbling mass rather than consolidating it. A roller screed* uses shear applied to the surface from the face of a spinning tube. This compresses the material, consolidating it as it advances over the surface. The spinning tube travels easily in both directions, enabling operators to seed more material on the surface and strike it again.”
* Fisher uses a Bunyan Striker.
Surface profile is not critical with pervious concrete—you're not going to get any bird baths—and a typical specification will call for a maximum 3/8-inch deviation in 10 feet. “Flatness is more important for the bottom of the subbase than for the pavement,” Fisher points out. “The flatter the ground is, the more water the pavement and sub-base will hold. Even a 2% slope would allow the water to run out fairly quickly and defeat some of the purpose.” On sloping sites, the soil surface is some-times terraced to help the system retain water. At a recent Atlanta parking lot project for the city library, PCI installed a 4000-cubic-foot “pond” within the subbase (a system of slotted and solid PVC pipe). The water stored within this system will be used for irrigation of the library's landscaping.
Because of the very low water-cement ratio and the open matrix that allows air movement through the concrete matrix, pervious concrete can dry out very quickly. Curing is therefore critical. “I've seen poorly cured pervious concrete that you could dig out with your hands,” says Fisher. Improper curing may be the single most common cause of pervious concrete failures. Charger recommends that curing start within 20 minutes of concrete placement. Standard wet curing, though, such as with sprinklers is not an option, since any excess water would drain through, washing paste to the bottom of the pavement and destroying the permeability. Therefore, the pavement is simply covered with 6-mil polyethylene sheeting that is sealed down tightly, sometimes preceded by a light fogging in dry conditions.
Durability in service
There have been pervious installations that did not perform as expected due to lack of experience in the design, materials, or construction. If there are too many fines and too much water, the paste can settle out and form an impermeable layer at the bottom of the pavement. If the mix is too dry, it can be unworkable. If the contractor over-finishes the surface, an impermeable layer can form. If curing isn't done properly, the concrete won't gain strength. But all of these problems are easily overcome with experience.
Objections typically raised about pervious concrete in service are that it will be destroyed by freeze/thaw action or that it will clog with dirt. The National Ready Mixed Concrete Association (www.nrmca.org) has produced an excellent brochure on pervious that responds to these charges. The bottom line on freeze/thaw is that there have been many successful pervious pavements placed in cold climates. NRMCA states that these pavements “have two common design features—the cement paste is air-entrained and the pervious concrete is placed on 6 to 12 inches of drainable aggregate base.”
Plugging is also not really an issue, as long as soil isn't actually eroding onto the pavement surface. “Plugging shouldn't be a problem with a good pervious pavement,” says Fisher. “You can power wash and vacuum it if you need to, and that will restore 90% of the original drainage. If it won't drain, it's more likely that it's a bad pavement than that it's plugged.”
Pervious concrete pavement is on its way to a parking lot near you. “There aren't too many good pervious contractors out there right now,” says Fisher, “simply because they don't have the experience. It will come as more is placed. NRMCA is creating a craftsman certification for pervious that should help a lot.”
Advantages of pervious concrete
- Retains stormwater so that retention ponds aren't needed for parking lots.
- Allows stormwater to infiltrate into the ground to replenish groundwater aquifers.
- Keeps pavement surfaces dry even in wet situations like greenhouses.
- Allows parking lots to be ice-free in freeze/thaw areas since snow melt immediately drains off the surface.
- Allows water and air to get to roots of trees within a parking area.
- Aerobic bacteria that develop within the pavement matrix can break down oil and remove other pollutants from water that washes off the surface.
- Light reflectivity higher than with asphalt surfaces reduces heat island effect and saves lighting cost.
- Allows a project to claim LEED points.
- Can collect and retain water to be used for irrigation.
Pervious parking lot
In December 2004, PCI Systems installed a landmark pervious concrete parking lot at a Denver Safeway store. “The parking lot was a replacement, and pervious was used at the strong encouragement of the city to offset the need for a stormwater retention system,” said NRMCA's pervious expert Dan Huffman. With the cold weather, curing was followed with insulation blankets. This lot will be the ultimate test of freeze/thaw durability since, with Denver's sunny days and cold nights, it will cycle frequently under moist conditions. One of this pavement's first major tests was a late April snowstorm. By the following afternoon, the advantages of pervious concrete were obvious.
Design of pervious pavement
- Slope of ground typically should not exceed 5%.
- Do not use in areas of potential contamination (for example, gas stations).
- Water table must be a minimum of 3 feet below the base.
- Bedrock must be a minimum of 2 feet below the base.
- Soil must be uncompacted and level.
- Permeability of underlying soil must be tested and found acceptable.
- There must be no organic material that could degrade and allow settlement.
- Expansive clays are not suitable.
- With poorly percolating soils, the system can be designed to retain all rainwater.
- With very sandy soils, the pervious concrete can be placed directly atop the sand.
- The subgrade and sides of the excavation must be covered by a nonwoven geotextile fabric; the Pennsylvania Stormwater Management Manual lists Mirafi 140N, Amoco 4547, and Geotex 451 as acceptable types.
- The depth of the subbase depends on runoff volume required to be retained.
- Ensure a void space of 40%.
- Suggested stone is 6 to 12 inches of #57 stone.
- Mix with #89 washed stone, 600 pounds portland cement per cubic yard, 0.30 w/c ration, but no sand.
- Typically has 20% to 25% voids.
- Built to allow a water infiltration rate of 20 inches per hour.
- Is six inches or more in depth.
- Struck off with a vibratory screed and rolled with a steel pipe roller, or roughly struck off and compacted with a roller screed.
- Covered with plastic and cured for a minimum of seven days.