Rehabilitation and Flexural Strengthening of Reinforced Concrete Beams using External Steel Reinforcement

Abstract

Rehabilitation of existing structures in the form of structural strengthening may be required due to decrease of load carrying capacity with aging, improper design or to accommodate with increased load requirements with time or codal change. Structural demolition can be reduced by applying strengthening technique to improve the capacity. This research investigated the flexural behavior of reinforced concrete (RC) beams rehabilitated with different strengthening techniques involving external steel reinforcement. The main focus of the study was to apply those strengthening techniques under service load condition. Twelve half-scaled beams were prepared and divided into six groups. The first group was used as control specimens while the other five groups were strengthened with different strengthening techniques. The control specimens were tested by 3rd point loading to find the ultimate load carrying capacity in flexure. Then all other beams from each of the five groups were preloaded with 65%-75% of the ultimate load to simulate the service load condition. Some initial cracks were formed in each beam due to preload before strengthening. After observation of the crack patterns, the preloading was released and the beams were ready for strengthening. Two groups were strengthened with 3mm thick external steel plate bonded with epoxy adhesive, of which the steel plate in one group was anchored by steel bolts in addition to the adhesive. Two different types of epoxy adhesive were used in two separate beams of each group. The 4th and 5th groups were strengthened with near surface mounted (NSM) external steel bars. The NSM bars in the 4th group were attached by epoxy adhesives while those in the 6th group were welded with the original bottom stirrups after removing the bottom concrete cover. The remaining group was strengthened by using external steel angle welded with the bottom stirrups after removal of the required concrete cover. Finally, the bottom concrete cover was cast again. The average ultimate load carrying capacity of control beams in flexure was found to be 49.1kN. The ultimate capacity of strengthened beams with steel plate was observed to be 92.4kN for a specific adhesive type, which was as much as 88% higher than the control beams. The capacity was greatly influenced by the type of adhesives and the bond strength between steel plate and concrete. Anchoring the steel plate by steel bolts at both ends in addition to the epoxy adhesive further increased the capacity to 104% higher than the control beams and mode of failure switched from a brittle to a ductile nature. The capacity of beams strengthened with NSM bars varied from 101.9kN to 115.0kN depending on the type of the adhesives. Flexural failure occurred either by the separation of NSM steel bars by bond failure or by pure bending. The beams strengthened with external steel angles welded with bottom stirrups showed an ultimate flexural load capacity of 124.4kN and 116.8kN, which were 153% and 138% higher, respectively than the control beams. A ductile behavior was obtained when the welded connections in this type of strengthening did not fail. The ultimate capacity was 127.5kN and 134kN for the beams strengthened with external steel bars welded with bottom stirrups which were 160% and 173% higher than the control beams and 95% and 100% of their designed strength. The initial flexural failure pattern in the unstrengthened beams was transformed to an obvious shear failure pattern in that case. After normalizing the experimental results with respect to external steel ratio and grade, it could be concluded that the NSM method of flexural strengthening was more effective in comparison to the other methods. On the other hand, strengthening with external steel plate was more convenient and easier to apply although its capacity is slightly lower than the NSM method.