Strengthening of distresssed RC beams by external prestressing: Experimental Studies

摘要

It is well known that distressed existing concrete bridges are generally strengthened by external prestressing over decades. This technique has been used successfully for strengthening of some bridges in India too. However, some bridges have shown signs of distress even after strengthening. It is necessary to predict the behaviour of members, which were strengthened by external prestressing up to ultimate behaviour. Although different studies were carried out to understand the behaviour of strengthened members, there are issues still to be addressed. In case of strengthening of bridges, mostly longitudinal girders of bridge are strengthened while dead weight of a bridge acting on it (load from deck slab, cross girder and other loadings). In view of this, dead weight of a real bridge girder was simulated in the experimental investigations. New RC beams were loaded to induce cracks in concrete to a limit of around 85 % of yield strain in steel, such that the beams will be induced number of cracks Then, the cracked RC beams were strengthened by external prestressing, while 25% of the calculated ultimate load for the RC beam is acting on the beam (simulating dead weight of a bridge girder). The cracks were closed and the flexural capacity that was lost due to cracks was restored. Further, the strengthened beams were loaded till failure to study the ultimate behaviour. It is observed that flexural load carrying capacity of the strengthened members increased by about 48 %, when compare to the calculated ultimate load capacity of the Reinforced Concrete member. Compression due to the external prestressing shifts the neutral axis in the strengthened members and increased the flexural capacity. Strengthened members failed, when concrete crushing occurs in the extreme compressive fibre. For beams prestressed with internal unbonded tendons, the stress increase in unbonded tendons at ultimate state is directly related to the equivalent plastic hinge length. In case of beams strengthened by external prestressing, plastic hinge was less pronounced at the ultimate flexural behaviour.