Six full-sized precast, pretensioned bulb-tee girders made with high-strength concrete were tested to evaluate their behavior under fatigue loading. Each of the six bridge girders had a specified design concrete compressive strength of 10,000 psi (69 MPa). Prior to fatigue loading, four of the six girders were intentionally precracked near midspan under static loading conditions. The remaining two girders were tested in an uncracked state. Three of the six girders were tested using an upper-bound fatigue load selected to produce the current American Association of State Highway and Transportation Officials�? maximum allowable extreme fiber tensile stress (6 sqrt(f 'c ) psi [0.5 sqrt(f 'c ) MPa]). The remaining three girders were tested using upper-bound fatigue loads corresponding to higher levels of extreme fiber tensile stress of approximately 750 psi or 860 psi (5.2 MPa or 5.9 MPa). In all cases, the lower-bound fatigue load was selected to produce a stress range in the extreme fiber of the bulb-tee girder lower flange equal to that caused by the design live-load-plus-impact bending moment acting on an uncracked section. Each of the six girders was subjected to either 5 million cycles of fatigue loading or less (if fatigue fracture of the strand was detected). Findings from this testing program indicate that high-strength concrete girders incorporating midspan flexural cracks can be expected o perform adequately under fatigue loading conditions when the extreme fiber tensile stress is limited to the current allowable level of 6 sqrt(f 'c ) psi (0.5 sqrt(f 'c ) MPa). In addition, adequate fatigue performance was observed at greater extreme fiber tensile stress levels when the concrete remained uncracked throughout the duration of testing.
