Prestressed CFRP sheets for strengthening concrete beams at room and low temperatures

The use of fibre reinforced polymer (FRP) composite sheets in civil engineering applications is growing. FRP sheets bonded to the tensile face of an under-strength or structurally damaged concrete beams provide an efficient, lightweight, non-corrosive alternative to other rehabilitation and strengthening methods. FRPs supplement the flexural reinforcement of the deficient beam and increase its strength. To improve the efficiency of this strengthening technique, FRP sheets may be prestressed. One of the areas where knowledge is limited is in the long-term behaviour of concrete beams strengthened with prestressed carbon fibre reinforced polymer (CFRP) sheets exposed to low temperatures. To address this deficiency, the effect of short and long-term exposure was investigated, with particular emphasis on identifying any material or prestress changes due to temperature variations that would affect and cause a change in flexural behaviour.; The research work of this thesis comprised both experimental and analytical investigations that examined the effectiveness and feasibility of using prestressed CFRP sheets to increase the flexural capacity, and simultaneously improve the serviceability of a structurally damaged concrete bridge girder model. The behaviour of beams strengthened with prestressed CFRP sheets tested at low temperature was studied. A new practical mechanical anchorage system was developed to directly prestress the CFRP sheets by jacking against the strengthened concrete beam.; The experimental program consisted of testing nine, T-section, simply supported post-tensioned concrete beams. The beams were divided into two groups; group “I” consisted of five beams tested to failure at room temperature (+22°C), while group “II” consisted of four beams tested to failure at low temperature (−28°C). In each group, one beam served as an unstrengthened control beam, one beam was strengthened by prestressed sheets and tested for short-term behaviour, and two beams were strengthened and tested for long-term behaviour. The fifth beam in group “I” was an understrength representing a beam that suffered loss in internal prestressed reinforcement. All beams were precracked before being strengthened with the prestressed CFRP sheets.; The developed anchorage/prestressing system was easy to apply, and the anchors were effective at preventing peeling failure of the prestressed sheets. No damage in the bond between the sheets and the concrete was observed at low temperature under long-term exposure. The prestressing levels in the CFRP sheets were sufficiently maintained when subjected to long-term exposure at both room and low temperature. The strengthened beams showed significant increases in flexural stiffness and ultimate capacity as compared to the unstrengthened control beams. Failure of the strengthened beams was by tensile rupture of the composite prestressed CFRP sheets.; Theoretical models were developed to predict the flexural behaviour of the strengthened beams during the prestressing process, and when subjected to flexural loading at serviceability and ultimate states at both room and low temperatures. The model accurately predicts the flexural beam behaviour at low temperature based on low temperature constituent material properties. Improved serviceability behaviour and higher strength were predicted for beams strengthened with the prestressed CFRP sheets.; Comparison of long-term with short-term results at both room and low temperatures showed that long-term and temperature effects did not adversely affect the strength of the beams.