A SIDE-BY-SIDE COMPARISON OF PERVIOUS CONCRETE AND POROUS ASPHALT1
Andrea L. Welker, James D. Barbis, and Patrick A. Jeffers2
ABSTRACT: This article compares the performance of two permeable pavements, pervious concrete and porousasphalt, that were installed side-by-side in fall 2007. Because the pavements are located directly adjacent to oneanother, they experience the same vehicle loads, precipitation, and pollution loads. These permeable pavementsare part of an infiltration stormwater control measure (SCM). This article focuses on the comparison of waterquality parameters, maintenance and durability, and user perception. Eleven different water quality parameterswere analyzed at this site for 19 different storm events over a one year period: pH, conductivity, total suspendedsolids, chlorides, total nitrogen, total phosphorus, total dissolved copper, total dissolved lead, total dissolved cad-mium, total dissolved chromium, and total dissolved zinc. Results from the two pavement types were comparedusing the MannWhitney U-test. The only parameter that was found to be statistically different between thetwo pavements was pH. Periodic inspection of the two pavement types indicated that after two years of use bothpavements were wearing well. However, there was some evidence of clogging of both pavements and some evi-dence of surface wear. A survey of users of the lot indicated that the perception of these permeable pavementswas favorable.
(KEY TERMS: best management practices; nonpoint source pollution; stormwater management; infiltration;urbanization; permeable pavements.)
Welker, Andrea L., James D. Barbis, and Patrick A. Jeffers, 2012. A Side-by-Side Comparison of PerviousConcrete and Porous Asphalt. Journal of the American Water Resources Association (JAWRA) 48(4): 809-819.DOI: 10.1111 j.1752-1688.2012.00654.x
INTRODUCTION AND BACKGROUND
A shift in the methods used to manage stormwater(National Resource Council, 2008) has increasedthe use of permeable pavements as a means topromote infiltration. The goal of these stormwatercontrol measures (SCMs), which are also calledstormwater best management practices (BMPs), is toalleviate the detrimental effects of development byrestoring the hydrologic cycle. Permeable pavements
include pervious concrete, porous asphalt, permeablepavers, and proprietary products manufactured fromrecycled materials such as tires and glass. This arti-cle focuses on a comparison of two of the most com-monly used permeable pavements: pervious concreteand porous asphalt.
Pervious concrete and porous asphalt are similarto their relatively impermeable counterparts. Themain difference between permeable and traditionalpavements is the screening of aggregate toremove the fines (Pennsylvania Department of
1Paper No. JAWRA-11-0081-P of the Journal of the American Water Resources Association (JAWRA). Received June 22, 2011; acceptedFebruary 2, 2012. 2012 American Water Resources Association. Discussions are open until six months from print publication.
2Respectively, Associate Professor, CEE Department, Villanova University, 800 Lancaster Avenue, Villanova, Pennsylvania 19085; WaterResources Professional, AMEC Earth & Environmental, Plymouth Meeting, Pennsylvania 19462; and Graduate Engineer, SSM Group, Inc.,Reading, Pennsylvania 19611 (E-Mail Welker: firstname.lastname@example.org).
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Environmental Protection, 2006). Although both per-meable pavement types were developed in the 1970s,their use has only recently become more widespread(Tennis et al., 2004; Ferguson, 2005). Pervious con-crete typically has a porosity between 20 and 30%and an infiltration rate of 7-20 m h (Tennis et al.,2004). The porosity of porous asphalt generallyranges between 16 and 25% and a typical infiltrationrate is 35 m h (Schaus, 2007). There is a tradeoffbetween strength and porosity and it is up to thedesigner to determine which parameter takes prece-dence (Delatte et al., 2007).
The impermeability of traditional asphalt pave-ments contributes to the movement of pollutants fromthe traditional to the permeable pavements (Gilbertand Clausen, 2006). The exported pollutants aredependent upon the pavement material used, thelocation of the permeable pavement, and the vehicu-lar traffic (if any) found on the site. The sources ofroadway and parking lot pollutants come from thepavements themselves, vehicles, litter, and spills ontothe roadway surface. Vehicles provide a large per-centage of the pollutants through tire wear, fuellosses, lubrication losses, and exhaust emissions. Theland environment surrounding the pavements willalso convey pollutants to the pavements. These pollu-tants come in the form of nutrients, pesticides, anddeposits from the atmosphere (Barrett et al., 1995;National Resource Council, 2008). The U.S. Environ-mental Protection Agency (USEPA) (1983) studiedurban runoff from locations across the nation, andfound that metals such as copper, lead, and zinc weredetected in more than 90% of the stormwater sam-ples. Organic chemicals were found in more than 10%of the samples.
Previous research has shown that permeablepavements are effective at reducing the pollutantconcentrations found in runoff. For example, the con-centrations of nitrogen, copper, and phosphorus werereduced by more than 90% from inlet to outlet at anSCM that utilized pervious concrete in a pedestrianarea (Kwiatkowski et al., 2007; Horst et al., 2011).Legret and Colandini (1999) and Rushton (2001)reported a reduction in metals concentration for run-off that infiltrated porous asphalt. Chlorides, ofcourse, present a problem for all SCMs as they areconservative and are flushed through the system.Kadurupokune and Jayasuriya (2009) attribute muchof the pollutant reduction to the trapping of sedi-ments, to which the pollutants are attached, in thepore spaces of the permeable pavements. However,pollutants are also likely to sorb to the aggregate inthe infiltration beds beneath the pavements and inthe natural soils found beneath the infiltration beds(e.g., Pitt et al., 1994; Prakash, 1996; Mikkelsenet al., 1997; Kwiatkowski et al., 2007).
The overarching goal of this research was to holis-tically compare two permeable pavements, perviousconcrete and porous asphalt. To achieve this goal anexisting traditionally paved parking area for facultyon Villanova Universitys campus was demolishedand replaced with an infiltration bed that was over-lain by the two pavement types. The two pavementtypes were evaluated by comparing water qualityparameters, maintenance requirements, durability,and public perception. Eleven different water qualityparameters were analyzed at this site for 19 differentstorm events over a one year period: pH, conductiv-ity, total suspended solids, chlorides, total nitrogen,total phosphorus, total dissolved copper, total dis-solved lead, total dissolved cadmium, total dissolvedchromium, and total dissolved zinc. The maintenancerequirements and durability were assessed by per-forming periodic inspections. The faculty using thelot were asked to participate in an on-line survey toascertain their perceptions of the pavements.
The infiltration SCM that is the focus of this studyis part of a research and demonstration park thathas been created on Villanovas campus as part of theresearch efforts of the Villanova Urban StormwaterPartnership (VUSP). Villanova University is locatedin southeastern Pennsylvania and is about 15 mileswest of Philadelphia. The site was selected primarilybecause it was not slated for development under theuniversitys master plan and there were no knownutilities under the lot. A secondary reason was that itwas a faculty parking area and, as such, would be inuse year round.
The drainage area for the site is divided into twosections, one that drains to the pervious concrete andone that drains to the porous asphalt. The drainageareas are roughly equal and consist of conventionalasphalt parking areas that are essentially 100%impervious. All planted areas surrounding the studysite are separated from the drainage area by curbs,thus limiting the amount of pore clogging sedimentreaching the permeable pavements.
The soil underlying the area was classified accord-ing to the Unified Soil Classification System as ML:silt with sand (ASTM D2487). No variation in soilproperties was found over the test area. Generally,infiltration SCMs are not built on this type ofmaterial because it typically has a low hydraulic
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conductivity; however, it was not possible to place theSCM elsewhere. It is important to note that despitethe low hydraulic conductivity of the material, thesite is infiltrating water. The geometric design of theinfiltration basins for the given project was governedprimarily by site and financial constraints. In theparking lot used for the study, an area between twoplanted traffic islands provided the best area for theplacement of permeable pavements. This location dic-tated the available surface area, 9.1 m by 30.5 m.Half of this area was allotted for pervious concreteand half for porous asphalt. The depth for each infil-tration bed ranges from the minimum of 0.5 m (theminimum recommended depth for permeable pave-ment infiltration beds) to 1.5 m because of the slopeof the site, and the desire to keep the bottom of thebeds level. Additionally, because of the slope acrossthe site, the pervious concrete bed bottom is located0.5 m below the porous asphalt bed bottom. The bedgeometry and drainage area was dictated by site andfinancial constraints, not the volume of water to bedetained. However, the amount of runoff that can bestored by the infiltration beds is consistent with mostdesigns in the southeastern Pennsylvania area. Theinfiltration bed geometry provides a volume ofapproximately 140 m3. This volume is filled withAASHTO #2 stone (approximately 102 mm in diame-ter) which has a porosity of 40%. Thus, the storagevolume for water is approximately 56 m3, which islarge enough to store the runoff generated from a84 mm rain event that falls on the 0.07 hectare site.A bed of this size should capture over 90% of theannual runoff. The storage bed was underlain by ageotextile to separate the stone bed from the underly-ing original soil.
The storage beds under each pavement type wereseparated to eliminate the transfer of water and con-taminants from one bed to the other (Figure 1). Thisseparation was achieved by placing a Jersey barriercovered with a 2 mm geomembrane down the middleof the infiltration bed to create two equally sized infil-tration beds.
The storage beds were overlain by the permeablepavements. The mixture design and thickness of thepavements were developed in consultation withNational Asphalt Pavement Association (NAPA) andNational Ready Mixed Concrete Association(NRMCA). The pervious concrete was 152 mm thickand consisted of stone aggregate, Portland cement,water, and several modifiers. Stone aggregrate(9.5 mm diameter) comprised 78.8% of the mixture,16.9% of the mixture was Portland cement, and 4.2%was water. A high range water reducer (0.06%), vis-cosity modifier admixture (0.05%), and set retardingmixture (0.03%) were also added to the mix toimprove workability of the concrete. The thickness of
the porous asphalt was 63.5 mm and the mix con-tained a narrow gradation of stone aggregate (95% ofthe aggregate was between 12.5 and 2.38 mm indiameter), an asphalt binder, and fibers. Of the totalmix, 5.8% was a binder, PG 64-22, that is suitable fordaily average high temperatures of 64C and dailyaverage low temperature of 22C. Finally, the mix-ture consisted of 0.20% fibers to make the mixturestiffer and to prevent draindown of the asphalt bin-der. The as-built porosities of the porous asphalt andpervious concrete were 25 and 27%, respectively,which compares favorably to values typically reportedfor these pavement types (Tennis et al., 2004; Schaus,2007).
The site was extensively instrumented (Figures 2and 3). Samples for water quality testing wereobtained from first flush samplers and pore watersamplers in the natural soils under the stone bed.
GKY FirstFlush Samplers (GKY & Associates,Chantilly, VA) were employed to collect the initialrunoff from every storm. Four of these first flushsamplers were placed along the uphill edge of theproject site, two entering the pervious concrete sec-tion and two entering the porous asphalt section.
Six pore water samplers (UMS SPE20; UMS,Munich, Germany) were installed under each
FIGURE 1. Photograph of the Infiltration Beds During Construc-tion. Note the Jersey barrier and geomembrane used to separate
the infiltration beds underlying the two pavement types.
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pavement to obtain samples from the infiltratedwater. Two samplers were placed at three depthsbelow the bottom of the infiltration bed, 15, 30, and46 cm. The plastic tubes for the samplers were runthrough conduit to sample containers located nearthe observation manhole on the pervious concreteside.
A tipping bucket rain gauge, located on the roof ofan adjacent building, Mendel Hall, was used to mea-sure the amount of rainfall at the site (http://www.wunderground.com/US/PA/Villanova.html). Therain gauge measured the amount of rainfall every10 min.
Samples were obtained for water quality testingfor all rain events that exceeded 6.35 mm of rainfallin an 8-h period. The first flush samplers were pre-pared prior to any precipitation by placing a clean,acid washed, first flush insert into the sampler. Thepore water samplers were prepared for sample collec-tion after a minimum of 4.1 mm of precipitation hadfallen. To prepare the pore water samplers, 500 mlNalge-Nunc heavy-duty vacuum bottles were attached
to the filling venting caps. Using a hand vacuumpump, a vacuum of 70-82 kPa was applied to each bot-tle. The bottles were then left for 24-36 h to ensurethat a sufficient amount of sample had been obtained.
Water Quality Testing
For each stormwater sample that entered thelaboratory, approximately 50 ml were allocated fornutrient, chlorides, pH, and conductivity testing. Inaddition, 300 ml were allocated for suspended metals,total dissolved, and total suspended solids testing,while 20 ml was allocated for dissolved metals testing.
Each of the stormwater samples were analyzed forpH, conductivity, total nitrogen, total phosphorus,total dissolved solids, dissolved cadmium, dissolvedchromium, dissolved copper, and dissolved lead. Forsamples that were below the detection limit (Table 1)for the respective test a value of half of the detectionlimit was used (Smith, 1991).