# Fatigue Testing of Shear Reinforcement in Prestressed Concrete T-Beams of Bridges
## 3 Test Results  
Prior to the cyclic loading, an initial static load was applied to create a shear crack pattern, which could occur at the serviceability limit state. The cyclic loading led to an elongation and widening of the initial cracks as well as the development of further shear cracks. Consequently, the stirrups in the web were activated to carry shear forces, causing a gradual fatigue damage of the reinforcement. After the failure of each stirrup the shear forces were redistributed to adjacent stirrups and/or a direct concrete compression strut. At the same time, the vertical deflection of the beam also increased. As a typical example for the tests, the behavior of specimen T-33-2 is described in more detail hereafter. At the beginning of the cyclic loading, the deformation increased significantly because new cracks occurred, and existing cracks widened (Figure 4a). After some thousand load cycles, the deflection gradient decreased, and an internal equilibrium was found by activating a truss model consisting of stirrups and concrete struts. At this stage, the stirrups in the area of the cracks exhibited an almost constant stress range. After about 550,000 load cycles, a disproportional increase in crack width was measured by an inductive displacement transducer (R1D, Figure 4b). The increase can be explained in more detail by means of strain gauge measurements on the stirrups. With the stress-strain relation of the reinforcement the corresponding stirrup stresses were determined from the steel strains at upper and lower load. Figure 3: Crack pattern in the web and upper flange after failure (test specimen T-33-2, first part of test). A pronounced yielding, as it is known from static tests, was not observed. Figure 5 shows the measured steel stresses of selected stirrups under upper load. At about 375,000 load cycles, a sudden decrease in steel stress under the upper load was observed for the measuring point S2U, which was induced by fatigue failure of stirrup S2. At 550,000 cycles, the stirrup S3 also broke (Figure 5; measuring point S3U), explaining the increase in crack width. The forces of the internal model were redistributed, so that the adjacent stirrups exhibited higher tensile stresses. The highest increase was measured in strain gauge S4 OR on stirrup S4. Almost simultaneously, the steel stress in stirrups S1 (S1U) and S2 (S2M) increased. This indicates an upcoming rearrangement of the truss model into a direct compression strut from load introduction to support. At this time, the truss model in the range of stirrup S5 and S6, also with increased stirrup stresses, was still intact. This chain reaction proceeded faster with increasing fatigue fractures during cyclic loading, until the crack clearly propagated into the upper flange at about 1.01 million cycles. At the same time, the stirrup stress of stirrup S5 decreased abruptly. Consequently, the crack widths and vertical deformations increased significantly until the end of the test. At about 1.07 million cycles, the final failure of the compression zone occurred (Figure 3) and the first part of the test was stopped. The second part of the test was continued with the unimpaired shear span of the beam as a three-point bending test with a span of 4.0 m. The shear span with fatigue failure was positioned beyond the shifted support and acted as a cantilever (Figure 1). For the second part of the test, the same cyclic shear forces and the same a/d ratio (shear slenderness) were applied until fatigue failure after a total of 1.37 million load cycles. After the second part of test, the concrete of the webs was removed at the intersection of cracks and stirrup reinforcement in order to visually check the stirrups for fractures. In Figure 5a the positions where stirrup fractures were detected are marked by \"x\". The corresponding number indicates whether one or both legs of the closed stirrups were broken. In the shear span shown here, almost all stirrups were broken at both legs.