AASHTO M 297

AASHTO M 297 is the material specification governing preformed polychloroprene (neoprene) compression seals used in bridge deck expansion joints. The standard sets minimum requirements for the elastomer compound, dimensional tolerances, physical properties, and the compression-recovery behavior that makes a preformed seal viable as a watertight, traffic-rated joint closure.

Note on placement: M 297 is an AASHTO standard, not an ASTM standard. It lives inside the ASTM/UL General resource section as a pragmatic grouping with the adjacent joint and sealant standards — ASTM C920, ASTM D5893, and ASTM D6690 — that bridge specifiers reference together. The M 297 specification itself references several ASTM test methods, including ASTM D5973 for the finished compression seal.

Designers select preformed compression seals when bridge deck movement is small and predictable (typically under 2 inches), when initial cost matters, and when the deck geometry suits a fixed-width joint pocket. For a comparison across joint system families (strip seal, modular, finger plate, asphaltic plug, and compression seal), see our Bridge Expansion Joint Selection Guide.

AASHTO M 297 covers preformed polychloroprene compression seals intended to be installed in armored or unarmored bridge deck expansion joints. The seals are extruded as continuous lengths with a multi-cell or web cross-section that allows them to compress horizontally while resisting set.

AASHTO LRFD Bridge Construction Specifications §20 ("Joints") references M 297 as the controlling material standard for preformed polychloroprene seals in bridge deck applications. State DOT bridge design manuals typically cite both M 297 and the LRFD section together when calling for compression seals.

M 297 limits the seal compound to virgin polychloroprene (CR / neoprene) with no reclaimed rubber. The standard sets minimum and maximum values for the physical properties of the cured elastomer, measured on test specimens taken from finished extrusions or from the same compound batch.

The specific limit values above reflect the long-standing M 297 requirements; designers should confirm the current published edition of M 297 against project specifications because AASHTO updates these standards periodically.

The behavior that defines a compression seal — its ability to push back against the joint walls after years of repeated cycling — is measured through compression and recovery tests performed on full cross-section finished seal specimens, not just compound buttons.

A preformed compression seal works only if it stays compressed across the full range of joint movement. The seal must be wider than the maximum design joint opening at the coldest expected temperature, and narrow enough to fit through the minimum joint opening at the hottest expected temperature without buckling or extruding above the deck.

Values above are illustrative; refer to the seal manufacturer's certified sizing chart and the controlling state DOT bridge design manual for the project. AASHTO LRFD §20 requires that joint movement calculations include thermal movement, creep, shrinkage, live load rotation, and an appropriate safety factor.

AASHTO M 220 is the parallel material specification for preformed polychloroprene compression seals used in PCC pavement joints. The two specs are easy to confuse because the elastomer is the same family and the seal cross-sections look similar, but the application, geometry, sizing rules, and test methods differ. Substituting one for the other is a documented cause of premature joint failure.

Both specifications require the seal to be installed with a lubricant-adhesive that wets the joint walls, allows the compressed seal to slide into position without tearing, and then bonds to the polychloroprene as it cures. The lubricant-adhesive is not optional — a dry-installed compression seal will lose contact within months.

State DOT bridge design manuals typically include their own compression seal sizing tables; when the project specification cites a state DOT manual, those tables override generic manufacturer charts. Always verify the controlling document hierarchy in the project front-end specs before sizing.

A preformed compression seal is the right call when joint movement is small, deck geometry is regular, and lifecycle cost matters more than maximum movement capacity. It is the wrong call when movement exceeds the seal's compression range, when the joint pocket is irregular or non-armored, or when the deck has significant skew or torsional movement.

For the broader decision among joint system families, see the Bridge Expansion Joint Selection Guide. For inspection criteria, failure modes, and repair-vs-replacement decision logic on existing compression seals, see the Bridge Deck Joint Maintenance & Replacement Guide.