How bridges get built · Precast concrete decks · Epoxy and dry joints, shear keys and temporary PT
Also called: epoxy joint, dry joint, match-cast joint, temporary post-tensioning
How it is done
- Select an epoxy qualified for segmental joints, with a temperature class matching the expected surface temperatures, and pre-test each batch.
- Prepare the faces: remove bond breaker and laitance by light grit or water blasting, keep the faces dry and protect them from rain.
- Dry-fit the segment to check alignment and shear-key fit, then draw it back slightly.
- Mix the epoxy and apply a thin, even coat to both faces, keeping it out of duct openings (or using duct seals).
- Close the joint and apply the temporary bars to about 0.3 MPa average compression within the open time; check for squeeze-out around the whole perimeter and swab the ducts clear.
- Hold the temporary compression until the epoxy has cured and the permanent tendons are stressed, recording temperatures, times and bar forces.
- Test companion samples for compressive and bond strength.
Key numbers
- Temporary compressionabout 0.3 MPa average across the joint
- Joint thickness after closingwell under 1.5 mm (Linn Cove joints under 1/16 in)
- Joint types (AASHTO)Type A epoxy joints required where internal tendons cross; Type B dry joints only with external tendons
- Timeabout an hour per joint once the cycle runs
Where it fits
- All precast segmental decks.
Where it does not
- Epoxy in rain or outside temperature limits; dry joints in aggressive environments or with internal tendons.
Choosing it
- Use epoxy (Type A) joints whenever internal tendons cross the joint or the environment is aggressive.
- Accept dry (Type B) joints only with external tendons in benign environments; most owners now specify epoxy everywhere.
- Choose a wet (cast-in-place) joint instead of a match-cast joint where geometry must be corrected or tolerances absorbed, accepting the slower cycle.
- Choose the epoxy temperature class for the actual surface temperatures, and heat or shelter the joint in cold weather.
Plant, pace and money
PlantEpoxy kits, temporary bars and jacks.
ProductivityAn hour per joint.
CostSmall per joint; the durability of the bridge rests on it.
Risks and controls
What goes wrong
- Poor squeeze-out leaving voids, epoxy exceeding open time, water in the joint.
Quality assurance
Epoxy batch tests, temperature logs, squeeze-out inspection, temporary bar force.
Origins
Epoxy-glued match-cast joints were introduced at Choisy-le-Roi in the early 1960s; dry joints were allowed for some externally post-tensioned spans, but durability concerns, sharpened by the 1985 collapse of Ynys-y-Gwas, whose mortar joints let water reach the tendons, led codes to require epoxy wherever internal tendons cross a joint.
Examples
AASHTO now requires epoxy joints for segmental bridges with internal tendons.
Case studies
Choisy-le-Roi Bridge, SeineFrance · early 1960sFirst use of epoxy-glued match-cast joints.
Ynys-y-Gwas Bridge, WalesUK · 1985A 1953 post-tensioned segmental span with mortar joints collapsed without warning after tendons corroded at the joints.
Linn Cove Viaduct, North CarolinaUSA · 1981–82A heating system cured the epoxy joints in winter, allowing more than 20 segments to be erected between November and March.
Related methods
Segment casting: short-line and long-line match castingSpan-by-span precast segmental with gantryPost-tensioning: tendons, stressing and groutingProgressive placement of precast segments
Further reading
AASHTO Guide Specifications for Segmental Bridges · ASBI Handbook · ASTM C881 Standard Specification for Epoxy-Resin-Base Bonding Systems for Concrete