Use of adhesives to retrofit out-of-plane distortion induced fatigue cracks

Multi-girder steel bridges are in wide use in the U.S. state and interstate highway systems. Prior to 1985, it was common practice to avoid welding the connection plates to the tension flange of the girders. However, extensive fatigue cracking has developed in the unstiffened web gaps because of unexpected out-of-plane distortion.; Among all the retrofit methods, positive attachment combined with stop holes is the most widely applicable and accepted. A new retrofit option was investigated that uses a room-temperature-cured two-part epoxy (3M Adhesive DP460-NS) to join a small length of 3/4-inch thick steel angle shape to the tension flange and the connection plate. Unlike bolting, this retrofit does not require access to the outer surface of the tension flange, thus partial closure to traffic due to removal of the concrete slab in the negative moment region can be avoided. Also, unlike welding, this retrofit operation is not sensitive to vibration caused by traffic and can be conducted without closing the bridge.; A field test on two skewed bridges showed that the adhesive-angle retrofit system decreased the out-of-plane strain range by 40 to 50% when the original strain range was more than 50 microstrains. The ten adhesive-angle retrofits remained in place and were in good condition after three and a half years, suggesting that the chosen adhesive had good environmental durability.; A laboratory large-scale specimen test with 8 web gaps showed that the adhesive-angle retrofit system stopped or retarded most cracks even without stop holes at 12 ksi and 6 ksi in-plane stress ranges. The corresponding measured out-of-plane strains for the above test were approximately 600 microstrains. The adhesive-angle retrofit system stopped all cracks at 4 ksi and 2 ksi in-plane stress ranges.; Coupon tests were conducted to investigate the environmental durability of the chosen adhesive, which was not directly correlated to the initial strength. The relative aggressiveness of the environment with respect to stiffness and strength is ranked. It was found that the chosen adhesive is suitable for applications at room or low temperature, even with high relative humidity.