by Dan Huffman

While pervious concrete pavement has beenaround for more than 20 years, it has onlyrecently garnered much attention, dueto increasingly stringent stormwatermanagement guidelines that now position the

product as a sustainable building material. Perviousconcrete provides the potential for environmentallyresponsible site use and lowered construction costs inprojects ranging from simple sidewalks, driveways, andpatios, to major pedestrian plazas and full-blown multi-acreparking lots for national commercial big box builders.

Pervious concrete has been specified as pavement inFlorida for more than 20 years, earning a fine record ofdurability and high performance. Still, the history of thematerial also known as 'no fines concrete' stretches even

42 The Construction Specifier December 2005

rVlous

further back abroad-substantial amounts were producedin Europe immediately following the Second World War,some still in service today.

Pervious concrete is a performance-engineered structuralmaterial using the usual constituents of conventionalportland cement concrete, only with little or no sand in themixture, allowing for a 15- to 30-percent air void factor.Taking advantage of the corresponding decreased density,the concrete is incredibly permeable while still able toprovide a quality structural pavement. Instead of moisture(e.g. rain/snow melt) running off the surface horizontally,virtually all stormwater falling onto pervious concreteimmediately drains directly down through the pavement tothe subgrade, eliminating runoff while providing filtrationand ground water recharge.A pervious concrete system can

be designed with an appropriate porous aggregatebase layerto function as a stormwater storage basin thataccommodates precipitation for a design storm event.

Pervious concrete is an open-cell material with anappearance sometimes described as that of a RiceKrispies" treat. Nevertheless, the product can be integrallycolored, painted, or otherwise modified to be aestheticallyin tune with the project environment in the same ways asconventional concrete. It can even be made acceptablysmooth for good shopping cart mobility by the means ofrapidly advancing placing techniques, equipment, andconcrete mix design technology while still maintaining anon -slip surface for Americans with Disabilities Act(ADA)-compliance.

At the University of California, Berkeley's botanicalgardens entryway, pervious concrete (the darker material)provides functional drainage and highly complementaryarchitectural design with conventional concrete.

-,

Site optimization and cost controlFederal guidelines supporting the Clean Water Act haveincreasingly stimulated facility owners to consider perviousconcrete, especially as it relates to commercial and lightindustrial construction. U.S. Environmental Protection

Agency (EPA)Phase II regulations require owners of newlydeveloped (or redeveloped) sites of 0.4 ha (1 acre) or moreto have an on-site management system for treating allstormwater before it leavesfor conveyanceby the respectivelocal agency.To do this, owners frequently find themselvesdedicating 10 to 20 percent of the overall site to non-revenue-generating detention/retention ponds, swales, orother surface treatment devices.Their other option usuallyinvolves specifying potentially expensive underground

--- ---

December 2005 The Construction Specifier 43

treatment systems-for big box builders, these can cost inthe millions per project.

As opposed to such heavy investment in space and/orstormwater treatment devices, by employing infiltrationtechnology acceptable to EPA, pervious concretepavement can be used as both a parking facility and thestormwater treatment system itself. Pervious concreteeliminates the runoff on the parking lot surfaces or otherpaved areas where it might be applied (e.g. sidewalks,plazas) and, via its filtering action, significantly improvesthe quality of the water passing through. Its use is arecommended Best Management Practice (BMP) of theEPA for first-flush pollution mitigation within the realmof stormwater management.l

In early 2005, the designers of a light industrial facilityproject for a wood structural building componentmanufacturer in Westminster, Maryland, had anapproximately 5-ha (l2-acre) site, which included a 3.2-ha(8-acre) parking lot. Through the specification of perviousconcrete for the entire lot, the 0.6-ha (1.5-acre) retentionpond and underground drainage system were eliminatedfrom the original design plans (which called for an asphaltparking lot), allowing the team to recover about 13 percentof the site. This move saved $400,000 in undergrounddrainage construction costs alone.

In the public sector, agencies devote huge financialresources to stormwater conveyance-where treatment isactually part of an agency's wastewater system, suchfacilities can be massive and extremely expensive.Additionally,a great deal of public inconvenience and trafficcontrol problems can result from streets and roads torn upas ever-increasingstorm sewersand drains (due to the largevolume of runoff) are being deployed because of newdevelopment (or existing ones are in need of repair orreplacement). As such, some government agencies haveimplemented stormwater impact fees on all impervious

44 The Construction Specifier December 2005

In this Palm Beach Gardens, Floridd~ gated community,a pervious concrete street provides myriad safety

benefits, including reduced slips andfalls, thanks to

a virtual elimination of waterponding. II'

" II III"

outdoor areas. Limiting the volume of stormwater runofftakes pressure off existing infrastructure, thereby savingtaxpayer dollars and improving overall water quality.

Environmental considerations

The greatest concern about stormwater runoff from existinghardscape and continuing development relates to theamount of inadequately treated pollutants (mostlyhydrocarbons) runoff carries with it into waterways, lakes,and oceans. Impervious parking lots, roadways, androoftops cause more stormwater runoff and pollutant loadsthan any other type ofland use.2

Not only are there concerns about the obvious chemicalpollutants (often including the carcinogen, polycyclicaromatic hydrocarbon [PAH]) reaching the human foodchain, but runoff has alsobeen deemed a major contributorto thermal pollution caused by the effects of dark,impervious surfaces such as streets and roofs. According toEPA, runoff increases the ambient temperature of thestormwater such that it negatively affects the existence ofvegetation, fish, and other aquatic life.3

The U.S. Green Building Council's Leadership in Energyand Environmental Design<&(LEED<&)rating system givescredit to the effective use of pervious concrete inSustainable Sites (SS) Credit 6, Stormwater Management.Due to its potential to be used for slope protection, perviousconcrete pavement could alsohave positive implications forSS Prerequisite 1, Erosionand Sedimentation Control.Otherenvironmentally responsible implications associated withthe use of pervious concrete pavement are explored below.

Water filtration

As previously stated, a major positive attribute of pervious

concrete paving is its exceptionally high porosity. When the

material is designed with typical thicknesses and mix design

constituents (as explored later in this article), and supported

with an appropriate base design such that nothing beneathimpedes the moisture's passage,its permeability rate exceedsthe needs of almost all natural events.With its void factor,pervious concrete handles moisture in excess of 5080 mm(200 in.) of water hourly per 0.09 m2 (1 sf)-this translatesto 1104to 19 L (3 to 5 gal) a minute.

When rain or snow lands on pervious concrete, themoisture goes directly through the pavement and into theground in much the same way it would if there was nohardscape in the first place.In addition to improvingthe overall water qualityby reducing the amountof the pollutant-carryingrunoff, pervious concretesignificantly improves thequality of the water thatpasses through it via thereduction of the negativeeffectsof hydrocarbon-basedmaterials (e.g. grease, oil)the first flush of stormwater

would otherwise carry away.On a parking lot, there is alarge surface area thatcomprises a matrix of smallcoarse aggregate by whichthe pervious concretecaptures and aerobicallydegrades the hydrocarbonresidue passing throughwhile the pollutants arefurther converted by theattached microbial growth.The sun also boils off

volatile components withinthe matrix of the open-celled concrete, and theremaining carbon is thenabsorbed by the native soilwhere it is digested byplants, fungi, or microbes.

Trees almost alwaysflourish in the presence ofpervious concrete. Thenatural infiltration processallows both moisture and

oxygen to reach the roots oftrees in a way usuallylimited by conventional

pavements. (This kind of infiltration can be particularlyuseful in the dry Western United States, where there is astrong desire to better use what little moisture falls to theearth.) After water passesthrough this pavement, it can evenbe harvested for various uses including agriculture.

Heat reflectivityand cold-weatheradvantagesOther benefits of using pervious concrete are similar to thegeneral environmental benefits of its conventional

De<:ember200STheConstructionSpecifier 45

Pedestriansafety at Portland'sOregonZoo is increasedwithpervious concretewalkwaysproviding a non-slip surfaceinall weather conditions without supplemental drainage.

counterpart. With plain gray portland cement as itsaggregate binder, concrete has a significant reflectivityadvantage over asphaltic products, which are usually verydark in color. Concrete has much higher albedo, whichcontributes to its ability to earn points within LEEDrelativeto heat island mitigation (SSCredit 7, Heat Island Effect).Italso offers the potential for lowering the number of lightpoles in various night illumination environments, savingboth money and energy. In addition to its reflectancebenefits, pervious concrete's high void factor and lowerdensity means it absorbs less solar heat and cools morerapidly in hot environments.

In cold weather, anecdotal evidence suggests both snowand ice more rapidly leavethe surface of pervious concretethan traditional pavements because the moisture haspathways to drain down immediately upon melting.Additionally,since there is no ponding of moisture, there isnot the same propensity for re-freezing as withconventional pavements.

In some parts of the country, the dynamics of dealingwith traditional non-porous pavement are increasinglycostly for owners as taxes and levies are applied in anattempt to limit impervious surfaces.As communities seekthe means of compliance with the EPAPhase II stormwaterregulations, mandatory applications of pervious surfacesolutions should be expected to increase.

Location concerns

Design considerations for pervious concrete immediatelyfocus on the qualities of the native soil over which it is to beplaced. For every sizable or otherwise important project, a

46 The Construction Specifier December 2005

Vibratory screedsarefrequentlyusedin theplacementofpervious concrete,such as in this Denver, Colorado,Safeway supermarket project.

geotechnical evaluation is highly recommended. Whereexpansive soils are known to exist, strategies should beapproached with extreme caution. Nevertheless, the vastmajority of potential sites in the United States and Canadashould be good. environments for the effective use ofpervious concrete. In areas of high clay content, should thepercolation rate be suitable for septic tank systems, it alsoshould be acceptable for pervious concrete.

The design of the aggregatebase, an area sometimes alsoreferred to as the 'recharge bed: is very critical to perviousconcrete performance and durability. Directly beneath theslab, this section ensures moisture flowsunimpeded thoughthe pavement. It stores the water and allows it to percolateover time into the subgrade. (When necessity dictates, itcould also be engineered to move water laterally anddispensed otherwise.) However, in Florida, where thepercolation rate of the sandy native soil is quite high,pervious concrete placement directly atop the soil is verycommon-and supported by more than 20 years ofperformance. In all cases, the percolation of the soilscombined with infiltration rate dynamics is critical,especiallyin areaswhere freeze-thaw durability is a concern.

The factors determining the design thickness ofpervious concrete include its desired hydraulic(e.g. permeability and voids content) and mechanicalproperties (e.g.strength and stiffness). The material needsto complement the site-specific stormwater managementstrategy, the intended traffic loads, and otherconsiderations, such as the amount of rainfall, pavementcharacteristics, underlying soil properties, tolerableintensity of surface runoff, and rate of infiltration.

Typically, retail-focused parking lots are designed with152-mm (6-in.) sections of pervious concrete, as opposedto the 102 mm (4 in.) thicknesses most commonly used forconventional concrete lots. Due to pervious concrete'sdecreased density (resulting from its high air void factor),the simple design mechanism of increasing the slabthickness by 50 percent (conventional to pervious) hasproven highly successful in practice. On a national basis,152 mm of pervious concrete on 152- to 203-mm (6- to8-in.) aggregate base is the most commonly used equationfor parking lots.

With pervious concrete, the use of a flat-grade designotherwise following the contour of the land is all that isnecessary-since there is no runoff, there is no reason tograde it.4 Successful placements have taken place inCalifornia on as much as a 16-percent grade, but in suchcases, concerns about the performance of the recharge bedhave to be taken into consideration. One of the

characteristics of most high-quality pervious concrete isthat the material is made with extremely low water-to-cement (W/C) ratios (typically 0.25 to 0.35). Due to thevery low W/C ratio and the way the aggregate interlocktakes place, the shrinkage of pervious concrete is so greatlyreduced that jointing intervals of every 6 m (20 ft) arecommon in many parts of the country. (In California, asubstantial amount has even been placed with joint spacinggreatly exceeding6 m.)

Material considerations

In cold-weather climates subject to freeze-thaw cycling,cement paste in pervious concrete should be treated withair-entraining admixtures as is the case for the material's

.

conventional counterpart. While its 15- to 30-percent airvoid factor also allows for expansion of frozen moisture, itis still critical to the integrity of the slab that it should notbe totally saturated under freeze-thaw conditions. Whetherdurability issues come into play,typical thicknesses for thegranular base are 150 to 300 mm (6 to 12 in.) with amaterial such as #57 stone (with a 40-percent void factor).A complementary and adequate aggregate base design isperhaps the most critical element in pervious concrete'sfreeze-thaw durability after its ability to transfer moisturethrough the concrete's own expected porosity.

Pervious concrete mix designs vary broadly around thecountry.In most regions,rich cementitiousmixesusing272kg(600 lb) of cement and more per cubic yard are frequent.Smaller coarse aggregate sizes such as 9.5-mm (O.4-in.)rounded pea gravel are most common, but crushedmaterials and larger dimensions are sometimes also used.5Contrary to traditional concrete (where larger sizes andangular-shaped aggregate materials frequently provide thebest performance) smaller aggregate and rounded shapesappear to not only provide the best porosity for perviousconcrete, but also better strength, smoothness, andappearance characteristics.

In addition to conventional concrete admixtures, some of

the hydration-stabilizing and viscosity-modifyingadmixtures increasingly used in conventional concretingcan be selectivelyemployed in pervious concrete, along withthe same integral coloring, various fibers, andsupplementary cementitious materials (SCMs), such as flyash and slag.

As of this writing, most of the qualitypervious concreteinthe country is being placed at, or lessthan, a 13-mm (O.5-in.)

- ---

December 2005 The Construction Specifier 47

By allowing moisture to filter through into groundwater,the Rice Krispie-like structure of pervious concrete helpsreduce the environmental problems of stormwater runoff.

slump due to the extremely low W/C ratios required foracceptable strength, porosity, and durability. With theprogress in admixture technology increasingly focused onpervious concrete, the ability to increase the slump(a measure of the concrete consistency, which affectsplaceability) is likelyin the near future.

Practical use

Parking lot construction is usually accomplished with 'stripplacements: After proper compaction of the native soil, ageotextile fabric is usually placed to protect against finesmigrating up into the aggregate base. The aggregate base(recharge bed) is placed and compacted and then the ready-

mixed concrete is placed, spread and leveled with varioustypes of vibratory screeds or friction tubes. This is followedby some form of surface rollingprocess designed to compactapproximately the top 13mm (0.5 in.) of the surface,addingincreased consolidation and satisfactory smoothness.6

Very soon after pervious concrete is placed, the jointsshould be cut-most often, these are tooled joints cut witha modified rolling device within minutes behind the finalrolling process. The method of curing pervious concrete isa seven-day wet cure, and attention to this process cannotbe over-emphasized. When this type of porous, extremelylow-W/C-ratio concrete does not get proper curing, itsperformance rapidly deteriorates under normal use.

As a generalization, the amount of labor devoted toproperly placing pervious concrete can actually be less thanthat of its conventional counterpart. However, thecontractor's judgment and adherence to the basics of goodconcreting (along with an awareness of the particularmixture's appropriate water content as placed that comesonly with experience) is more critical to the success ofpervious concrete placement and in-service performancethan is the case of more conventional products.

In response to the need for a larger group of qualitycontractors to meet the rapidly expanding demand, theNational Ready Mixed Concrete Association (NRMCA)has begun rolling out a national pervious concretecontractor certification program. The initiative aims totrain, test, and certify qualified installers and otherindividuals wanting to better understand the technology.

". -, >'

~,'A d di t ion a 1.','1n fo rm at ion~">'iC",~" '-","'- , 'i, ',' ' ,

Author

Dan Huffman is the Portland, Oregon-based nationalresource director for the National ReadyMixed ConcreteAssociation (NRMCA). He has almost 30 years ofconcrete materials experience and is a voting member of

the American Concrete Institute's (ACI's)Board AdvisoryCommittee on Sustainable Development (AC-SD) andthe ACI 522 technical committee on pervious concrete.Huffman can be contacted via e-mail at

dhuffman@nrmca.org.

MasterFormat No.03 05 DO-CommonWork Results for Concrete

32 12 DO-FlexiblePaving32 14~O-Unit Paving

UniFormat No.

G2010-RoadwaysG2020-Parking LotsG2030-Pedestrian Paving

KeywordsDivisions 03, 32Heat island effectPervious concrete

Stormwater management

Abstract

Increasingly stringent stormwater management guidelinesare increasing applications for pervious concrete, whichprovides the potential for environmentally responsible siteuse and lowered construction costs in projects ranging

48 The Construction Specifier December 2005

from sidewalksto multi-acre parking lots. A performance-engineered structural material, it allows virtually allstormwater to immediately drain down through thepavement to the subgrade, eliminating runoff whileproviding filtration and ground water recharge.

The program is being sponsored onboth the state and local levels by therespective state ready-mixed concreteassociations, American ConcreteInstitute (ACI) chapters, and somecontractor organizations.

AcknowledgementsThis author thanks the followingpeople for help with drafting thisarticle: Colin Lobo, Ph.D., PE, MattOffenberg, PE, and Frank Cavaliere."

Notes

1About 90 percent of pollutants-mostly hydrocarbons contributed bygrease and oil by motor vehicles-onparking areas get carried awaybyrunoff during the first 38 mm(1.5 in.) of any rain event, describedby stormwater experts as the 'firstflush: First-flush pollution mitigationis one of pervious concrete's mostpronounced benefits. For moreinformation, visitwww.stormwatercenter.net.

2Visitwww.nrcd.org/water/pollution/ storm/ execsum.asp.3See EPA841-F-03-003,ProtectingWaterQuality from UrbanRunoff.4When pervious concrete is properlydesigned and placed, its performancecapability far exceedsnormalmaximums-it handles in excessof

5080 mm (200 in.) of hourly rain per0.09 m2 (1 sf). In many cases,runofffrom roofs and other hardscapes canbe designed to flow onto perviousconcrete sections. However, adjacentvegetative/soil areas can beproblematic as they could releaseorganics and other particulate mattercontributing to clogging.5«3/8" is exact U.S.designationper sieves.6Ride-on slip-form pavers arebeginning to appear as projects areincreasing in size,but the vastmajority of work is still done withthe screeds and rollers described.

The 50th Annual CSIShow™ & Convention

Opening General SessionBuilding Information Model

Wednesday . March29, 2006

MGMGrand Hotel.LasVegas,NV

SPEAKERS:

Frank Gehry, FAIA

James Glymph, FAIA Partner, Gehry Technologies

Dennis Shelden, Chief Technology Officer,

Gehry Technologies

Building Information Models (BIM)give designers a

faster, richer design process, contractors projects with

fewer surprises, owners more budget control through

prediaions about the projects construction process and

its prediaed maintenance costs, and facility managers a

tool to meet current tenant needs and plan for the

future, and to conduct maintenance or renovation in a

preventative, rather than reaaive, way.

Architea Frank Gehry, FAIA.and two of his colleagues

James Glymph, FAIAand Dennis R. Shelden will discuss

Building Information Models (BIMs)during the Opening

General Session of The 50th Annual CSIShow™ &

Convention.

The Construction Specifications Institute99 Canal Center Plaza, Suite 300 . Alexandria, VA22314

800-689-2900(optionII . __.th8Cs'show.__

December 2005 The Construction Specifier 49