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RESEARCH PAYS OFF

U.S. AND EUROPEANASPHALT TECHNOLOGIESA Perfect MarriageHAROLD R. PAUL AND LOUAY N. MOHAMMAD

he Hale Boggs Bridge over theMississippi River at Luling, Louisiana,is a cable-stayed bridge with a steel

deck. It is part of an important Interstate loopthat carries a large traffic volume around theNew Orleans metropolitan area. The bridge hasfour lanes—two in each direction. The designof the wearing surface for the steel deck stipu-lated an epoxy asphalt layer less than 100 mmthick because of that material's lower dead loadweight, impermeability, and resistance to per-manent deformation and fatigue. In early 1983the wearing surface was constructed, and laterthat year the bridge was opened to local traffic.The area of the asphalt-covered deck was12,240 m2.

PROBLEM

Before construction had been completed, blis-ters started to form on the epoxy asphaltwearing surface. These blisters were the resultof the entrapment of water vapor from mois-ture in the mix during construction. More than3,000 blisters had been repaired by the time thebridge was opened to local traffic. Yet break-down of the epoxy system continued, and re-sulted in distress of the wearing surface in theform of shoving and rutting.

By 1986, shoving and rutting of the mixwere limited to three areas. With the connec-tion of the southbound approach to a majorhighway in 1988, however, traffic volumeincreased to 9,000 average daily traffic(ADT), and the distress increased. In 1992 amaintenance overlay was placed on the threeworst areas, approximately 280 m2, using apolymer modified asphalt concrete (PMAC).The cost of this maintenance patching was$40,000. In 1993 the north approach was con-nected to the 1-310 loop, and traffic increasedto over 28,000 ADT. Many new distressedareas formed in the wearing surface. A

new mix design that would resist permanentdeformation and provide extended fatigue lifewas therefore needed.

SOLUTION

Investigation indicated that the steel deckwas reaching a temperature as high as 77°Cinside the box girders. The high temperaturewas causing a loss of bond between the as-phalt binder and the steel deck, resulting inthe observed shoving. At that time Louisianadid not have SuperpaveTM mix design capa-bilities, so a latex modified concrete (LMC)or a PMAC overlay was proposed. LMC waseliminated after an off-site trial. The ob-served performance of PMAC under serviceconditions was better, but not satisfactory.Therefore, the Louisiana Transportation Re-search Center recommended a merging oftwo technologies: use of the SuperpaveTM

method to specify a performance graded(PG) asphalt binder and incorporation of theselected binder in a European stone masticasphalt (SMA).

Several polymer modified asphalt binderswere tested using the SuperpaveTM method. Alaboratory program was established to test as-phalt mixes containing the selected asphaltbinders. Each mix was tested for indirect ten-sile strength, strain at failure, indirect tensilecreep, and fatigue. The test results were thenstatistically evaluated. On the basis of the testresults, field implementation of a modifiedasphalt SMA containing the selected PG as-phalt binder was recommended.

APPLICATION

To evaluate production, handling, and compactionissues before deck construction, an off-site testwas conducted. The bridge deck construction

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sequence consisted of removing the existingepoxy asphalt, replacing the primer coat wherenecessary, and applying the tack coat and themodified asphalt SMA. The PG asphalt binderwas also used as the tack coat material becauseof the high-temperature concerns.

During the actual paving operation, a mate-rial transfer device was used to facilitate acontinuous paving operation, which was es-sential for smooth placement of the stiff mix.The bridge was closed in one direction for thepaving operation, which was limited to Sun-days.

To maintain a continuous operation on thebridge deck, the plant production rate wasgreatly reduced, which resulted in severalunscheduled construction stoppages. In ad-dition, the viscous asphalt with its highpolymer content plugged the asphalt lines,and the fine-sized material needed for theSMA sometimes had problems with the coldfeed. Other than these problems, the con-struction of the modified asphalt SMA over-lay was a success.

After 2 years the mix is performing well andshows only minor distress. The distress is lim-ited, however, to four areas that are directly re-lated to the construction problems noted earlier.

BENEFITS

The design load limitations on the Hale BoggsBridge dictated that a wearing surface of lessthan 100 mm be used. The epoxy asphaltmix originally used for the wearing surfacebroke down under the extreme service condi-tions imposed by the Louisiana environment.The cost of the epoxy asphalt mix was$400/Mg in 1983. Monthly repairs wereneeded before the construction was com-pleted, and major areas of the bridge required

maintenance patching at a cost of $40,000within a few years after placement.

The SMA mix designed using the Super-paveTM PG asphalt binder has satisfactorily car-ried the high traffic volume on the bridge with-out requiring any maintenance for 2 years. Thecost of the SMA mix was $94/Mg in 1995, and atotal of 2,000 Mg of the mix was used. Use ofthe SMA mix resulted in a significant savings oninitial costs as compared with the epoxy asphaltmix: the Louisiana Department of Transporta-tion and Development estimates savings of atleast $500,000. Additional savings have accruedduring the last 2 years because no maintenancework has been needed. This marriage of U.S.and European technologies has therefore provento yield significant benefits.

For further information contact Harold R.Paul, Louisiana Department of Transportation,and associate director, Research, LouisianaTransportation Research Center, 4101 GourrierAvenue, Baton Rouge, LA 70808-4443 (tele-phone 504/767-9102; fax 504/767-9108), orLouay N. Mohammad, assistant professor, Lou-isiana State University, and manager, Engi-neering Materials Characterization and Re-search Facility, Louisiana Transportation Re-search Center (telephone 504/767-9126; fax504/767-9108).

EDITOR'S NOTE: Special appreciation is ex-pressed to Frederick Hejl and G. P. Jayapra-kash, Transportation Research Board, for theirefforts in developing this article.

Suggestions for "Research Pays off" Topicsare welcome. Contact G. P. Jayaprakash,Transportation Research Board, 2101 Con-stitution Avenue, N.W., Washington, D.C.20418 (telephone 202-334-2952; e-mailgiayapra@nas.edu).

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