Two precast, prestressed concrete box beams, 36 in. wide x 27 in. deep x 56 ft long (914 x 686 mm x 17.1 m), were removed from a deteriorated multibeam bridge and subjected to fatigue testing. Visually, the beams appeared in generally good condition but showed signs of water leaking through the longitudinal shear keys and some corrosion of reinforcement. The beams were purposely precracked prior to fatigue loading and had periodic overloads applied. One beam, cycled to a nominal bottom tension stress level of 6sqrt(fc) psi (0.5sqrt(fc) MPa) and a stress range of 0.06 retained excellent performance after 1,500,000 cycles. Cycling to a nominal bottom tension stress level of 9sqrt(fc)psi (0.75sqrt(fc) MPa) and a stress range of 0.11f, significantly reduced the strength of the other beam and caused fatigue failure of strand wires after 145,000 cycles.
