Findings of the Fatigue Study
Three beams were tested in the fatigue study. Beam FAT-CONT remained unstrengthened to serve as a control beam for the fatigue study while beams FAT-1 and FAT-1b were strengthened with a reinforcement ratio of CFRP of 4.3 percent using two different bonding techniques. The two strengthened beams were tested with a 20 percent increase of the applied load range to simulate the effect of increasing the allowable live load for the strengthened beams. All three beams survived a three million-cycle fatigue loading course without exhibiting any indication of failure. Figures 5 (a) and (b) present the degradation of the stiffness and mean deflection respectively of the three beams throughout the three million-cycle loading course. Figure 5 presents the stiffnesses and mean deflections of the beams normalized with respect to the initial values at the beginning of the fatigue loading program.
All three beams exhibited a minimal degradation of the elastic stiffness of less than 5 percent throughout the three million fatigue loading cycles as shown in Figure 5(a). However, beam FAT – CONT exhibited a nearly 30 percent increase of the mean deflection due to the applied fatigue cycles as shown in Figure 5(b). This was likely due to the fatigue-creep behavior of the concrete deck slab. Both of the strengthened beams exhibited superior performance with an increase of only 10 percent in the measured mean deflection. The observed degradation of the two strengthened beams throughout the three million fatigue cycles was similar which indicates that the bonding technique did not affect the fatigue behavior of the strengthening system.
Figure 5: Degradation of (a) stiffness and (b) mean deflection for the fatigue beams
At the completion of the fatigue program, the three beams were loaded monotonically to failure. The load deflection behavior of the beams followed a similar trend to the observed load – deflection envelope of the three beams that were tested in the overloading study. The ultimate capacity of the two strengthened beams, FAT-1 and FAT-1b, was governed by rupture of the CFRP at a load of 250 kN. After rupture of the CFRP, the load deflection behavior of the two beams followed a similar trend to the load-deflection behavior of the unstrengthened beam, FAT-CONT. Crushing of the concrete for all three beams occurred at a measured load of between 200 kN and 215 kN. The findings of the fatigue study demonstrate the durability of the strengthening system when loaded at an increased live-load level. The allowable increase of the live load should be selected in accordance with the proposed design guidelines outlined later in this paper.