ost people consider concrete tobe impervious—after all, it’sused to make swimming pools

and kitchen sinks. And most concreteis, indeed, nearly watertight. But a newkind of concrete that allows water toflow through virtually unimpeded isgaining popularity. Pervious concrete isbeing used mostly in the constructionof parking lots, but also for sidewalks,bike paths, playgrounds, and erosioncontrol. Pervious concrete pavement isnot, however, suitable for areas subjectto high traffic volumes or speeds.

The mix is everythingPervious concrete is a pretty sim-

ple material—typically pea gravel, port-land cement, and water (heavy on thecement, light on the water, and no fineaggregate). A typical cubic yard of per-vious concrete would have 2650 poundsof #89 gravel, 600 pounds of portlandcement, and a water-cement ratio ofabout 0.30. This mix will provide acompressive strength of around 2500psi and an effective void ratio of 20%.That will allow water to drain into thebase course at rates of 2 to 18 gallonsper minute per square foot—275 to 400inches of rain per hour—much morethan you are ever likely to experience.

On a jobsite, though, things areseldom that simple. Various admixturesare used based on the aggregate and

MBy William D. Palmer Jr.

ConcretethatDrains

The concrete is placed and followedimmediately with a truss screed.

The concrete issmoothed with awell-oiled rollerimmediately afterplacement.

BILL PALMER

Pervious concrete’s abilityto store rainwater is makingit a popular pavement

environmental conditions. “We use airentraining agents, water reducers, someretarders, and sometimes a viscositymodifier,” says Dale Fisher, PCI Sys-tems, Alpharetta, Ga., a contractor whospecializes in pervious concrete pave-ments. “The mix design is everything.We often have to tweak it a bit in thefield. We specify the concrete by unitweight, which tells us the void struc-ture. We use 15% to 20% voids forparking lots. But on a recent bus stop,with heavier loads, we went to a higherunit weight and about 12% voids forhigher strength.” Using a small amountof fines is a common way to increasecompressive strength, but doing so willreduce the drainage rate. Often the finesare fly ash or slag cement. “We run unitweight tests because they tell us what’sgoing on with the mix,” says Fisher. “Ifit’s heavy, that can mean more fines, sowe change the mix to keep it a littledrier. If it’s light, then we can pour itslightly wetter.”

Pervious concrete typically is pro-duced in a ready-mix plant and deliv-ered with mixer trucks, but it is notpumpable. Ohio Concrete, publishedby the Ohio Ready Mixed Concrete As-sociation, sponsored a demonstrationpervious pavement job recently that wasinstalled by Ball Brothers Foundations,Monroe, Ohio. Moraine Materials pro-vided the concrete. Ohio Concrete re-ports that, “For batching a maximum8-cubic-yard load, they added air-en-

training admixtures up front with halfof the water, then the coarse aggregate,then the cement, and then the balanceof the water.” This produced a suc-cessful mix.

A pervious pavement, however, isnot just the pavement but rather a sys-tem of soil, subbase, and pavement (seesidebar). Each part of the system is es-sential to success.

Design and constructionA sample specification is available

on the Web site of Charger Enterprises,a concrete contractor in Seminole, Fla.(www.chargerconcrete.com). Perviouspavements have been used in Floridafor many years, and Charger has gaineda lot of experience. It noted that an im-portant first consideration is the soilbeneath the subbase—some may be un-suitable since the soil must be able tosupport the subbase, the pavement, andthe loads that will be applied. Get a geo-technical engineer to evaluate the soil.

The first layer against the soil istypically a nonwoven geotextile fabric(one that drains easily). This is toppedwith a subbase constructed with a min-imum of 6 inches of No. 57 stone, well-compacted so that it “will not be rut-ted by a fully-loaded ready-mix truck,”according to Charger’s Web site.

The thickness of the pavement willvary according to its design load andhow much water the pavement and sub-base are expected to store, but 6 inches

Advantagesof per v iousconcre ten Retains stormwater so thatretention ponds aren’t needed forparking lots.n Allows stormwater to infiltrateinto the ground to replenishgroundwater aquifers.n Keeps pavement surfaces dryeven in wet situations like green-houses.n Allows parking lots to be ice-free in freeze/thaw areas sincesnow melt immediately drains offthe surface.n Allows water and air to get toroots of trees within a parking area.n Aerobic bacteria that developwithin the pavement matrix canbreak down oil and remove otherpollutants from water that wash-es off the surface.n Light reflectivity higher thanwith asphalt surfaces reduces heatisland effect and saves lighting cost.n Allows a project to claimLEED points.n Can collect and retain water tobe used for irrigation.

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Joints are cut witha finned roller.

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is a typical parking lot thickness. Thecontractor sets the screed rails or sideforms, often shimming them duringstrike-off 1⁄2 to 3⁄4 inch higher than thefinal surface elevation to allow for com-paction later with rollers.

During a recent PCI project con-structing a Safeway parking lot in Den-ver (see sidebar), one worker directedthe chute and pushed the nearly zero-slump concrete down with his hand.Others moved the concrete into posi-tion with come-alongs. When a newtruckload arrived, workers sprayed the15-minute cold joint with water. Oncethe concrete was in place and roughlystruck off, the surface was compactedand finished with a truss screed fol-lowed by a hand-operated steel piperoller that was sprayed regularly withform release oil to prevent sticking. Fi-nally, workers popped chalk lines andcut contraction joints at a standard spac-ing using a roller with a 11⁄2-inch fin.The edges were then tooled, and finaltroweling was completed.

Vibration is not effective for per-vious concrete. “As vibration contactsnormal concrete, it ‘melts,’ or sinks intoa consolidated mass caused by bothpush and pull from the vibration en-ergy,” says David Mitchell with Bun-yan. “In perv, the energy delivered byvibration has only push and builds acrumbling mass rather than consoli-dating it. A roller screed* uses shearapplied to the surface from the face ofa spinning tube. This compresses thematerial, consolidating it as it advances

over the surface. The spinning tube trav-els easily in both directions, enablingoperators to seed more material on thesurface and strike it again.”

Surface profile is not critical withpervious concrete—you’re not going toget any bird baths—and a typical spec-ification will call for a maximum 3⁄8-

inch deviation in 10 feet. “Flatness ismore important for the bottom of thesubbase than for the pavement,” Fisherpoints out. “The flatter the ground is,the more water the pavement and sub-base will hold. Even a 2% slope wouldallow the water to run out fairly quicklyand defeat some of the purpose.” Onsloping sites, the soil surface is some-times terraced to help the system retainwater. At a recent Atlanta parking lotproject for the city library, PCI installeda 4000-cubic-foot “pond” within thesubbase (a system of slotted and solidPVC pipe). The water stored within thissystem will be used for irrigation of thelibrary’s landscaping.

Because of the very low water-ce-ment ratio and the open matrix that al-lows air movement through the con-crete matrix, pervious concrete can dryout very quickly. Curing is therefore

Per v ious par k ing lo t

In December 2004, PCI Systems installed a landmark pervious con-crete parking lot at a Denver Safeway store. “The parking lot was a

replacement, and pervious was used at the strong encouragement ofthe city to offset the need for a stormwater retention system,” saidNRMCA’s pervious expert Dan Huffman. With the cold weather, cur-ing was followed with insulation blankets. This lot will be the ultimatetest of freeze/thaw durability since, with Denver’s sunny days and coldnights, it will cycle frequently under moist conditions. One of thispavement’s first major tests was a late April snowstorm. By the follow-ing afternoon, the advantages of pervious concrete were obvious.

The day after a major snowstorm, plowedand with a little sun, the pervious con-crete parking lot was nearly dry, while anasphalt lot across the street was wet andfroze into a sheet of ice at sunset.

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Placed and screeded, the pervious is sealed with plastic sheeting to cure for seven days.

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*Fisher uses a Bunyan Striker.

critical. “I’ve seen poorly cured pervi-ous concrete that you could dig outwith your hands,” says Fisher. Impropercuring may be the single most commoncause of pervious concrete failures.Charger recommends that curing startwithin 20 minutes of concrete place-ment. Standard wet curing, though, suchas with sprinklers is not an option, sinceany excess water would drain through,washing paste to the bottom of thepavement and destroying the perme-ability. Therefore, the pavement is sim-ply covered with 6-mil polyethylenesheeting that is sealed down tightly,sometimes preceded by a light foggingin dry conditions.

Durability in serviceThere have been pervious installa-

tions that did not perform as expecteddue to lack of experience in the design,materials, or construction. If there aretoo many fines and too much water,the paste can settle out and form animpermeable layer at the bottom of thepavement. If the mix is too dry, it canbe unworkable. If the contractor over-finishes the surface, an impermeablelayer can form. If curing isn’t done prop-erly, the concrete won’t gain strength.But all of these problems are easily over-come with experience.

Objections typically raised aboutpervious concrete in service are that itwill be destroyed by freeze/thaw actionor that it will clog with dirt. The Na-tional Ready Mixed Concrete Associ-ation (www.nrmca.org) has producedan excellent brochure on pervious thatresponds to these charges. The bottomline on freeze/thaw is that there havebeen many successful pervious pave-ments placed in cold climates. NRMCAstates that these pavements “have twocommon design features—the cementpaste is air-entrained and the perviousconcrete is placed on 6 to 12 inches ofdrainable aggregate base.”

Plugging is also not really an issue,as long as soil isn’t actually erodingonto the pavement surface. “Pluggingshouldn’t be a problem with a goodpervious pavement,” says Fisher. “You

can power wash and vacuum it if youneed to, and that will restore 90% ofthe original drainage. If it won’t drain,it’s more likely that it’s a bad pavementthan that it’s plugged.”

Pervious concrete pavement is onits way to a parking lot near you.“There aren’t too many good pervi-ous contractors out there right now,”

Design o f per v ious pavementSite considerationsn Slope of groundtypically should notexceed 5%.n Do not use in areasof potential contami-nation (for example,gas stations).n Water table must bea minimum of 3 feetbelow the base.n Bedrock must be aminimum of 2 feetbelow the base.

Soiln Soil must be uncom-pacted and level.n Permeability ofunderlying soil mustbe tested and foundacceptable.n There must be noorganic material thatcould degrade andallow settlement.n Expansive clays arenot suitable.n With poorly perco-

lating soils, the systemcan be designed toretain all rainwater.n With very sandysoils, the pervious con-crete can be placeddirectly atop the sand.

Geotextile fabricn The subgrade andsides of the excavationmust be covered by anonwoven geotextilefabric; thePennsylvaniaStormwaterManagement Manuallists Mirafi 140N,Amoco 4547, andGeotex 451 as accept-able types.

Aggregate subbasen The depth of thesubbase depends onrunoff volume requiredto be retained. n Ensure a void spaceof 40%.

n Suggested stone is 6to 12 inches of #57stone.

Pervious concrete n Mix with #89washed stone, 600pounds portlandcement per cubic yard,0.30 w/c ration, butno sand.n Typically has 20%to 25% voids.n Built to allow awater infiltration rateof 20 inches per hour.n Is six inches ormore in depth.n Struck off with avibratory screed androlled with a steel piperoller, or roughlystruck off and com-pacted with a rollerscreed.n Covered with plasticand cured for a mini-mum of seven days.

At this Atlanta library parking lot, a roller screed is used to finish the surface. Notehow closely the curing sheets follow the screed.

DAVID MITCHELL, BUNYAN USA

says Fisher, “simply because they don’thave the experience. It will come asmore is placed. NRMCA is creatinga craftsman certification for perviousthat should help a lot.”

For information on PCI Systems,go to www.pervious.com; for infor-mation about Bunyan, go to www.bunyanusa.com.

Publication #C05F044 Copyright©2005 Hanley Wood, LLC. All rights reserved.