Although expansion jointsare among the smallest ele-ments of a bridge, whenthey fail to function prop-erly, they can create prob-

lems out of proportion to their size.Leaking joints can promote corro-sion of underlying structural ele-ments, and embedded debris canprevent movement of joints, causingstructural distress. Many transporta-tion departments consider expan-sion joints the Achilles’ heel of abridge.

Many engineers believe that pre-mature deterioration of expansionjoints is due in part to neglect dur-ing construction. “Expansion jointsare usually the last thing installedon a bridge, so they don’t get the re-spect they deserve,” says John VanLund of the Washington State De-partment of Transportation.

“You spend a year building abridge, but you spend only a dayputting in expansion joints,” saysWSDOT maintenance superinten-dent Doss Roberts. “The bridge willstand for decades, but we’ll be re-pairing the expansion joints the fol-lowing year.”

Expansion joint maintenance issuch a costly problem that many en-gineers now design jointless decks inwhich flexible piles, deformation ofslender substructure elements, orhinged columns accommodate su-perstructure movements. Elimina-

tion of existing joints is also becom-ing more common. “If we’re doing afairly major retrofit, we prefer toeliminate joints by placing a 41⁄2-inch-thick concrete overlay with onemat of rebar and live with the hair-line cracks that develop,” says EdWasserman, director of the struc-tures division of the Tennessee DOT.

The demand for sealed joints

Years ago, many bridges had un-sealed joints. A simple gap in thedeck accommodated small move-ments, while finger-plate joints or

similar systems, such as sliding-plateand sawtooth-plate joints, accom-modated larger movements (Fig. 1).Water and debris simply fell throughthe joints.

But transportation departmentshave paid a price for this joint sim-plicity. Water and deicing salts driponto substructure elements, causingmajor rebar corrosion. Even if corro-sion is not an issue, many states re-quire leakproof joints for environ-mental reasons.

To catch water and debris thatfalls through them, many openjoints are fitted with a neoprene

A closer look at bridgeexpansion joints

BY MARTIN S. MCGOVERN

Expansion joints often deteriorate faster than any other bridge element.Here are the pros and cons of the most common systems.

Most states require sealed joints to prevent leaks onto substructure elements andto prevent environmental contamination.

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trough and drainage system. But thetroughs quickly fill with debris, espe-cially in states that use sand andgravel to add traction to bridgedecks during the winter. Therefore,many states are replacing their re-maining open joints with sealedones.

Although there are many differenttypes of sealed expansion joints, themost common systems are field-poured sealants, compression seals,strip seals, sheet seals, plank sealsand modular joints. The suitabilityof each expansion joint for a partic-ular application depends on theamount of movement expected atthe joint.

Field-poured sealantsCommonly used in pavements,

field-poured sealants are installedover backer rod and bonded to thejoint walls (Fig. 2). These sealants areusually recommended for joint

movements of 1 inch or less, butsome states have installed them injoints that move up to 2 inches. Themost successful have been two-com-ponent silicones.

An advantage of field-pouredsealants is that, unlike preformedseals, their performance is generallyunaffected by joint walls that are notperfectly parallel. They are also easyto maintain. “If a small sectionfails,” says Roberts, “you can takeout a pocket knife, cut it out, cleanthe walls and refill the joint.”

One downside to field-pouredsealants is that they cure slowly atlow temperatures and don’t bondwell to moist concrete. Sealants re-quire careful cleaning of the jointedges to ensure a strong bond. It’salso important to place the backerrod at the appropriate depth to en-sure the right depth-to-width ratioof the sealant. “It can be hard toconvince a crew that more sealant is

not necessarily better,” says Roberts.

Compression seals and strip seals

Compression seals are made ofpreformed, closed-cell plastic or,more commonly, hollow extrudedneoprene shapes (Fig. 3). The seal isinstalled by squeezing and insertingit into a preformed joint opening.Properly sized seals remain in com-pression under all anticipated deckjoint movements. Most compressionseals are recommended for jointmovements of 3 inches or less.

Strip seals consist of a gland ofspecially shaped elastomeric materialthat spans a deck joint opening (Fig.4). The gland is mechanically lockedinto a pair of rolled or extrudedmetal shapes that in turn are an-chored to the edges of deck slabs.Strip seals are available in a numberof configurations and movement rat-ings. Some strip seals can provide upto 5 inches of total movement.

Both compression seals and stripseals have successful performancerecords over several years. Accordingto some engineers, an advantage ofcompression seals is that they do notrequire steel armoring angles. “If armoring angles aren’t installed cor-rectly, they can get beat up by snow-plows,” says Roberts. Vince Kaza-kavich of the New York DOT addsthat compression seals don’t collectas much debris as strip seals becausethey are installed closer to the roadsurface.

But compression seals have signif-icant drawbacks too. Over time, the

Figure 1. In open joints, small joint movements are accommodated with a simplegap in the deck (left), while larger movements are accommodated by finger-platejoints (right). To catch water and debris that fall through them, many open jointsare fitted with a neoprene trough and drainage system.

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Figure 2. Field-poured sealants areeasy to install and maintain, but theyrequire careful joint preparation.

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Figure 3. Compression seals do notrequire armoring angles, but they losetheir resiliency over time.

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Figure 4. Strip seals provide good leakresistance but can fill up with debris.

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Field-Poured Sealant

Neoprene Compression Seal

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BackerRod

neoprene loses its resiliency andmay lose contact with joint edges.“In cold weather, when the joint isopen to its fullest,” says Roberts,“sand and grit get in the joint andthe seal works itself out of the joint.Then snowplows come along and ripit out.” Another problem with com-pression seals is that joint edgesmust be parallel or the seal may leak.Van Lund says that sawcutting thejoint slot helps reduce this problem.

Two expansion joints that havemovement ratings similar to stripseals are shown in Figure 5. One po-tential advantage of these systemsover strip seals is that they use anadhesive rather than steel extrusions

to hold the seals in place. But someengineers are wary of relying on anadhesive bond.

Sheet seals and plankseals

Sheet seals consist of a sheet offiber-reinforced elastomeric mem-brane with a center corrugation thatbridges the joint opening (Fig. 6).The membrane is anchored to thedeck by steel retainer bars and an-chor bolts. Sheet seals can accom-modate maximum joint movementsof about 4 inches.

The performance of sheet seals de-pends on achieving firm, even con-tact between the deck, sheet seal and

retainer bars. Loosening of anchorbolts under repeated live-load im-pact is a common problem.

Plank seals are molded neoprenesections of varying widths. The sealspans the deck opening and is boltedto each side (Fig. 7). Grooves in theplank surface accommodate jointmovements. Depending on thewidth of the plank and the numberof grooves, plank seals can accom-modate movements from 11⁄2 to 13inches.

Plank seals have not performed aswell as compression seals or stripseals. Like sheet seals, loosening ofanchorages has been their mostcommon problem.

Modular jointsModular joint systems represent

the state-of-the-art approach to ac-commodating the large movementsof long-span bridges. They consist ofthree main components: sealers, sep-arator beams and support bars (Fig.8). Sealers and separator beams forma watertight joint at the riding sur-face. Separator beams often are ex-truded or rolled metal shapes andallow joining of the seals in series.The separator beams are supportedon support bars at frequent inter-vals.

Many states install modular sys-tems for joint movements greaterthan 4 inches. The modular systemson the market today can provide

Figure 5. These systems do not require armoring angles, but some engineers arereluctant to rely on an adhesive bond to keep the seals in place.

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Silicone Adhesive

SiliconeSealNeoprene SealEpoxy Adhesive

Consider plug joints when placing asphalt overlays If a bridge deck is to receive an asphalt overlay, expansion joints that

move less than 2 inches can be retrofit with plug joints. Plug joints con-sist of a metal flashing installed over the existing joint and covered with

concrete containing an asphaltic or other elastomeric binder. These sys-tems have the flexibility to accommodate joint movement and thestrength to carry traffic.

Plug joints work well with asphalt overlays because they have similarflexibility. If portland-cement-concrete headers are placed within an as-phalt overlay, they often are damaged by traffic impacting the edge.

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Closure plate

Asphaltic or elastomeric concrete

BinderBacker rod

Watson Bowman Acme

Figure 6. Sheet seals are typically usedfor joint movements up to 4 inches.Loosening of anchor bolts under re-peated impact has been their mostcommon problem.

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maximum movements of about 4feet.

Kazakavich is impressed by theperformance of modular joints. “Forlarge movements, I think modularjoints are the way to go. In my expe-rience, they have required very littlemaintenance,” he says. However,Roberts says that modular joints re-quire frequent inspections: “If anypart of them starts to fail, you wantto get on them right away. Theyhave Teflon or stainless steel slider

plates that allow the support bars tomove. If the slider plates don’t func-tion right, the joints hammer them-selves apart.”

The Illinois DOT installs modularsystems for joint movements greaterthan 8 inches but opts for finger-plate joints with drainage troughsfor movements of 4 to 8 inches. “Ifwe place an overlay on the bridge,finger plates are easy to raise by sim-ply welding another plate on top,”says Ralph Anderson, engineer of

bridges and structures. “Modularjoints don’t lend themselves to rais-ing very well.” Reference

Parsons Brinckerhoff, Bridge Inspectionand Rehabilitation, John Wiley & SonsInc., 1993.

Publication #C99C064Copyright© 1999, The Aberdeen Group

a division of Hanley-Wood, Inc.All rights reserved