Performance of strengthened concrete deep beams predamaged in shear

Frequency of strengthening or repair work has increased tremendously in recent years, and it has now become part of a multibillion dollar opportunity, as well as a problem. So, it is urgent to find suitable procedures for strengthening or repairing concrete members. In the experiment described in this paper, 18 prestressed and nonprestressed concrete deep beams were tested to failure, strengthened, and then retested to failure for a second time. On the occurrence of the first (shear) failures, the failed shear spans of the beams were strengthened by using steel clamping units that acted as external stirrups. By using the concept of the strut-and-tie approach it can be shown that on retesting, the clamped or strengthened shear span would receive less severe loads and thus would seem to become stronger. This is due to changes in the load transfer mechanism and in the direction of the compression strut that occur in the failed shear span following the introduction of a clamping unit. Fourteen of the externally strengthened deep beams, when tested for the second time, failed at higher failure loads than the first failure loads. This indicates that the full capacities of the damaged deep beams can be restored to the fullest following a strengthening technique using simple clamping units. Comparisons with the ACI Code and other prediction methods show that the strengthened deep beams failed at conservative ultimate loads.