The performance assessment of patch repaired and CFRP strengthened RC T-beams under transverse impact loading

Abstract

Includes bibliographical references.The collision of abnormally loaded vehicles into bridge beams is a frequent occurrence on many highways in South Africa. Structural damage due to such loading conditions requires repair and strengthening procedures that can return the structure to its original load bearing capacity. The use of patch repair mortar is common practice for the repair of impact damaged bridge beams and the use of carbon fibre reinforced polymers (CFRP) has become an established alternative to traditional strengthening materials. However, rehabilitated bridge beams may still be susceptible to the same adverse loading conditions. The focus of the dissertation was to provide an in depth experimental investigation to determine the performance of patch repaired, CFRP strengthened T-beams subjected to consecutive, transverse, impact loading. The impact loading was applied in the transverse direction to simulate vehicular impact into repaired and strengthened bridge beams. The effect of the repair and strengthening systems on the dynamic response and damage progression was analysed. The possibility of enhancing the impact performance of the T-beams through the application of additional CFRP strengthening was also investigated. Finally, T-beams with varying stirrup spacing were selected in order to investigate the effects of stirrup spacing on the dynamic response and failure mechanisms under consecutive impact loading. A total of five T-beams were tested. The beams were 1.9m long with identical cross-sectional dimensions and longitudinal reinforcement. The shear reinforcement, however, varied according to stirrup spacing. Four of the test specimens were damaged by exposing the tensile reinforcement and mechanically reducing the cross-sectional area of the reinforcement by approximately 25%. The damaged beams were repaired using patch repair mortar and subsequently strengthened for flexure with externally bonded CFRP laminates applied along the bottom of the T-beams. The remaining T-beam was used as an undamaged control specimen. Additional horizontal strengthening was applied to the side of one of the repaired and strengthened beams to provide additional resistance to the transverse impact loading. A custom support system was manufactured to secure the T-beams horizontally so that the impact loading could be applied to the webs of the T-beam specimens. The impact loading was induced at midspan, using a drop hammer impact machine. The beams were impacted consecutively from varying drop heights in order to analyze their behaviour as damage intensified. The contact force response, midspan deflection response and the progression of damage were recorded after each drop test. The results indicated that the damage mechanisms varied according to stirrup spacing. High stirrup spacing resulted in low composite action between the flange and the web, which led to excessive cracking at the flange-web interface and a larger proportion of damage induced in the web. Conversely the damage observed in beams with low stirrup spacing showed a larger transfer of damage between the web and the flange, thus indicating a high degree of composite action. Increased stirrup spacing was also observed to result in an earlier deterioration of stiffness due to the consecutive impact tests. The repaired and strengthened beams showed a greater capacity to withstand consecutive impact loading, although this improvement was considered minor. The slight increase in capacity was attributed to the combined effect of the patch repair concrete (which has superior tensile and compressive strengths than the substrate) and the transverse stiffness of the CFRP laminates. No cracking was observed to form along the interface between the repair mortar and the substrate thus indicating that the patch repair provided a continuous bond and did not have a noticeable effect on the damage progression. The T-beam strengthened in the horizontal direction with additional CFRP laminates showed an increase in capacity to withstand transverse impact and an increase in transverse stiffness. The additional strengthening and stiffening also prevented delamination of the CFRP laminate applied to the bottom of the T-beam and minimised damage progression into the web, thus indicating the potential use of such a strengthening system as a means of energy absorption in bridge beams susceptible to transverse vehicular impact