The historical, economic and social importance of strengthening and rehabilitation of reinforced concrete (RC) structures has become an important part of civil engineering. It is not only the need to protect our built heritage but also the demand to strengthen significant numbers of ‘new’ concrete structures. Achieving this successfully represents a great challenge, considering that these structures are deteriorating while simultaneously being asked to carry higher and higher loads. Fibre-reinforced polymer (FRP) has become popular as a retrofitting and reinforcing material, with many research studies being focused primarily on flexural strengthening of RC beams.
Shear strengthening, however, is much more complicated, particularly in continuous RC T-beams which are indeed the norm in reality, rather than simply-supported rectangular beams. A lack of structural ductility can lead to a brittle and catastrophic failure of a structure. Dissimilar to old codes that didn’t have strict rules for increasing ductility (irregular stirrups, spacing and lack of concrete cover), current codes require a high amount of shear reinforcement. Externally Bonded Reinforcement (EBR) or Near Surface Mounted (NSM) techniques have proved to be very effective in enhancing the shear capacity of RC beams. However, these techniques have many issues associated with the inability to ‘tie’ the tension chord to the compression chord in such T-beams, thereby leaving the possibility of a truss action unachievable. Similarly, debonding of FRP laminates at relatively low strain raises additional problems. Finally, simply supported beams and continuous beams behave differently. Continuous structures develop large shear forces co-existent with large bending moments at the same location. See Figure 1.
Fig.1. Bending moment and shear force diagrams in continuous beam
An altogether more reliable technique to achieve shear strengthening, even in cases where the webs of the beams are inaccessible, is the Deep Embedment (DE) or Embedded Through Section (ETS) technique, Figure 2 (Left). In this technique, DE bars are epoxy bonded into previously drilled holes (vertical or inclined) through the cross section of the RC beams, see Figure 2 (Right). In this way, the top chord of the beam is tied to the bottom chord through the additional DE bars. Unlike EB and NSM methods where the FRP relies on the concrete cover of the beams, in this method the FRP relies on the concrete core of the beams, which gives a better confinement and thus improves bond strength.
Fig.2. Deep Embedment technique (Left) and inserting an epoxy-covered bar into a previously drilled hole (Right)
This research project consists of three essentially connected parts focused on different facts of the study. First step is focused on bond characteristics and failure mechanisms of RC push-off specimens strengthened with DE bars across a shear plane. An emphasis was put on the minimum bar anchorage lengths required for effective shear transfer. Second phase is focused on the extension of the Deep Embedment technique to consider T sections, GFRP and steel bars besides CFRP bars, angles of drilling other than vertical, and the effects of continuity on shear strengthening. For this purpose, ten tests were performed on 3840 mm long RC continuous T-beams designed to fail in shear, see Figure 3. This experimental program is followed by the development of an analytical model for the calculation of the shear contribution of DE bars. Therefore, the primary focus of this research is to deepen knowledge of the behaviour of such strengthened continuous structures where large shear forces are combined with large bending moments. It is important to be able to implement this into proposed design guidelines to satisfy the majority of concrete structures in terms of continuity.
Fig.3. Test Setup