Two-lift paving makes a come - Roads & Bridges

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strategies and specifications used todesign long-life concrete pavements.The team included members fromthe Federal Highway Administration(FHWA), state DOTs and the NationalConcrete Pavement TechnologyCenter (National CP Tech Center),as well as contractors and materialssuppliers.After the scan tour, the team analyzedthe techniques they had seental pavement thickness. The bottomlift generally contains locally availableor recycled aggregates that maynot be suitable as a wearing surface.These aggregates can be obtained ata lower cost than aggregates used ina traditional paving project.The top layer, on the other hand,is relatively thin and contains dense,wear-resistant aggregates that provideenhanced durability, reducedA Kansas DOTworker lookson as the beltspreader placesthe top-liftconcrete. (Photocourtesy of PeterTaylor).in Europe and selectedtwo-lift paving asthe strategy with thegreatest potential forproducing a cost-effective,sustainable,long-lasting pavementin the U.S.Laying it onthick—and thinSustainable solutions are thosethat balance economic, environmentaland social benefits.“Sustainability is not about perfection,”said Tom Van Dam, programdirector for materials and sustainabilityat Applied Pavement Technology.“It’s about balancing competing, andoften contradictory, interests.”The two-lift process has great potentialas a sustainable paving solution,said Van Dam. It reduces thecost of materials and materials transportation,reduces the environmentalimpact caused by quarrying andimporting aggregates and increasessocietal benefits by lowering noiselevels and providing a long-lastingpavement that does not need frequentreconstruction.In two-lift paving, the bottom layeris thick, typically 80-90% of the to-noise and increased friction. Whilethese aggregates are more costly andfrequently have to be imported, theoverall cost and environmental impactof the pavement system is reducedbecause only a small amountof aggregates is required.“Two-lift is optimized for recyclingand reuse,” said Joep Meijer, presidentof the Right Environment, anenvironmental consulting firm inAustin, Texas. “Recycling is basicallyembedded in the technology.”Since the bottom lift is not subjectedto as harsh an environmentas the top lift, a variety of recycledaggregates, including concrete or asphaltpavements, can be used in thebottom lift without sacrificing durability.“We can use the road that is alreadythere—just crush it up and useit in the new road,” explained Meijer.Two-lift paving also can be optimizedto reduce the environmentalimpact caused by cement production.“The concrete industry is wellaware of the energy intensity andCO 2emissions related to cement production,so we’re looking into optionsto reduce clinker content fromcement,” said Meijer.One way to reduce the cement contentof concrete is to use supplementarycementitious materials, such asfly ash. This strategy is commonlyused in concrete paving projects andcan be used in higher quantities intwo-lift paving.Quiet, safe, long-lasting pavementsmean happy citizens. Two-lift pavementshave the potential to meetall of these criteria through surfacetreatments such as exposed aggregatesurfaces.Creating an exposed aggregate surfacerequires using extra-hard, wearresistant,small aggregates. Whensmall aggregates are used, more cementpaste must be used, which increasesboth the cost of the concretemix and the energy and CO 2footprintof the pavement. However, thethin top lift of a two-lift pavementneeds only a relatively small amountof concrete, making exposed aggregatesurfaces a possibility withoutincreasing the cost or the environmentalimpact.ROADS & BRIDGES • JANUARY 2009 •33
aThe exposed aggregate surface was produced by using small,wear-resistant rhyolite aggregates. (Photo courtesy of KDOT)Results from Europe, where exposedaggregate surfaces are commonlyused, suggest that these surfacesreduce tire-pavement noise,leading to quieter pavements. Theteam also hopes that exposed aggregatesurfaces will increase skid resistance,potentially making the pavementsafer.American-made caseOnce the European scan teamidentified two-lift paving as its firstpriority, the next step was to set updemonstration projects in variouslocations throughout the U.S. Workingwith the FHWA, the NationalCP Tech Center coordinated the firstdemonstration project in Kansas.“In Kansas, they were strugglingbecause the local limestone in Kansasis not wear-resistant,” explained VanDam. Kansas would like to importaggregates that are denser and morewear-resistant from other states, butthese imported aggregates come at ahigher cost.While reducing costs is a plus, itwas not the primary reason Kansasvolunteered to do a demonstrationproject, said Rod Montney, a concreteresearch engineer with the KansasDOT.“Our goal was really just to provethat two-lift paving could be done inthe U.S. with American equipment,”he said.For the I-70 two-lift project, Kan-sas imported dense, wear-resistantrhyolite aggregates from Oklahomafor the top lift but used locally available,more porous limestone aggregatesfor the bottom lift, significantlylowering the cost of purchasing andtransporting materials.The team used a standard KansasDOT paving mix for the 300-mm bottomlift. They also used a combinationof admixtures to stiffen the bottomlift and prevent segregation.The rhyolite was used as a coarseaggregate in the 40-mm top lift. Toreduce permeability, the contractorsubstituted a Class F fly ash-gypsumcombination for 20% of the cementin the top lift.The Kansas DOT contracted withKoss Construction for the I-70 twoliftproject.“Koss got on board right from thebeginning,” said Montney. “Theyhad been thinking about two-liftpaving and said, ‘Let’s try it on theI-70 project.’”Koss used a dual-drum mixingplant to simultaneously produce thetwo concrete mixes. Green and redplacards were used to identify whichmix each truck was carrying: a greenplacard told the road crew the truckwas delivering to the bottom liftspreader (which was marked withgreen paint); a red placard indicatedthe concrete was for the top liftspreader (which was marked withred paint).The pavement was placed on a150-mm, cement-treated recycledconcrete base produced from thepre-existing concrete pavement.During the paving process, thebottom-lift concrete was placed inthe belt spreader to deliver concreteto the base. The first paver followedbehind the belt spreader to form thefinal pavement profile for the bottomlift. Almost immediately after thebottom lift was placed, it was stiffenough to support the weight of theworkers.The top lift was then deliveredto a second belt spreader, with thesecond paver forming the final pavementprofile. The total distance ofthe paving operation from the firstbelt spreader to the final paver wasless than 45 meters. In this wet-onwetapplication, both the bottom andtop lifts were completed in about 30-60 minutes.The Kansas DOT experimentedwith several different pavement surfacetextures as part of the I-70 twoliftproject, including longitudinaltining, grooving, Astroturf drag andan exposed aggregate surface.The exposed aggregate surfacecontained a much smaller aggregate—9.5-mmrhyolite—comparedwith the other surface test sections,which used 12.5-mm rhyolite in thetop lift. Fine mason sand was usedin the exposed aggregate section,whereas standard river sand wasused in other top-lift surface test sections.To create the exposed aggregatesurface, retarder was sprayed on thesurface of the top lift to prevent theconcrete from setting. White sheetingwas used to cure the retardedsurface.After a five-hour curing time, thewhite sheeting was removed and thesurface exposed for 15-20 minutesbefore brooming to remove the retardedpaste from the surface and exposethe aggregate. After brooming,white curing compound was appliedto provide final cure for the exposedaggregate surface. Transverse and34• JANUARY 2009 • ROADS & BRIDGES WWW.ROADSBRIDGES.COM
Page 1: ROAD CONSTRUCTIONBY SABRINA SHIELDS

Two-lift paving makes a come - Roads & Bridges

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