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Effect of prestressing force on flexural behavior of unbonded prestressed concrete beams strengthened by CFRP sheets

This paper presents an experimental study on the effect of prestressing force on the flexural behavior of unbonded prestressed concrete (UPC) strengthened by Carbon fiber reinforced polymer (CFRP) sheets. The testing program was carried out on nine large-scale UPC rectangular beams. The investigated parameters included the reduction of prestressing force (0%, 15%, and 30%) and the number of CFRP layers (0, 2, and 4 layers). Experimental results showed that the strengthening effectiveness of CFRP sheets, controlling cracking, and the energy absorption capacity tended to increase with the decrease of prestressing force and decrease with the increase of the CFRP sheets ratio. The effective performance of the CFRP sheets was shown by the increase in the strain of the CFRP sheets which was proportional to the decrease in the prestressing force. The CFRP sheets strongly interacted with tendons, significantly decreased the tendon strain, and delayed the point where nominal yield strain in tendons occurred; this reduction was significant when the prestressing force was small. Besides, the reduction in prestressing force considerably increased the displacement of beams and the additional strain of the tendons (up to 164%), but this increase became smaller as the number of CFRP layers increased.