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38 Specimen S2R (Proposed End Zone Reinforcement, Unrepaired) The specimen contained four pairs of #6 bars and three pairs of #4 bars, as shown in Figure 3.48(a). The specimen failed at a load of 154.2 kips and reached a maximum deflec- tion of 0.246 in. Figures 3.48(b) and 3.48(c) show the spec- imen after failure. Once again, the cracks formed in the same direction as the rebar. 3.3.5 Discussion and Conclusions Calculations were performed to estimate the cracking and failure moments of the five specimens. Table 3.4 gives the calculated cracking and failure load, and the test results (a) Setup with Wider Bearing Plates of the five specimens. Cracking load is measured as the load at the intersection between the steep and flat lines on the load-deflection diagram. None of the specimens exhibited a discernible "kink" in the load-displacement curve, imply- ing that there was practically no cracking load capacity. Another less accurate method of measuring cracking is by visual inspection as the load is gradually applied. The computer-aided data acquisition system is more accurate, because micro cracks are impossible to detect visually. These observations led the team to conclude that (1) all speci- mens became cracked transverse to the prestressing direc- tion at the time of prestress release, and (2) epoxy injection for these specimens was ineffective in restoring them to a pre-cracked condition. The epoxy injection testing demonstrated the following: 1. Cutting coupons from the web end of a pretensioned I-beam (b) At Failure was not an effective method of testing for structural tensile Figure 3.45. Specimen S1R before and after testing. capacity; Figure 3.46. Specimen S1MS after failure.

OCR for page 38
39 (a) Pre-Existing Crack across Bottom Layer of Reinforcement (b) View at Bottom Flange Junction (c) View at Top Flange Junction (d) Splitting Occurred Right through Epoxy Injection (e) Splitting Occurred Right through Pre-Existing Crack at Lower Layer of Reinforcement Figure 3.47. Specimen S2L (LRFD end zone reinforcement, repaired specimen). 2. Prestressing release causes end zone cracking some of injection totally repairs the individual cracks being which cannot be epoxy injected or even seen with the injected; naked eyes; 5. The AASHTO LRFD method was effective in controlling 3. The epoxy injection used on the specimens, even though end zone cracks; it was applied by experienced professionals in a precast 6. The proposed reinforcement was more effective than the concrete plant, was not a reliable method of totally filling AASHTO LRFD method; and the injected cracks across the entire web width; 7. The bottom flange confinement reinforcement and the base 4. The tested specimens had no concrete tensile capac- plate should be treated as an integral part in crack control of ity, indicating that epoxy injection does not restore con- the end zone (they are highly recommended in all stemmed crete tensile capacity of repaired end zones even if the prestressed concrete girders).