Numerical simulation of the shear behaviour of reinforced concrete rectangular beam specimens with or without FRP-strip shear reinforcement
The successful validation of a numerical model is presented that can realistically approximate the shear behaviour of reinforced concrete (R/C) rectangular beams strengthened against shear with externally applied open hoop fibre reinforcing polymer (FRP) strips. For this purpose, the measured load-deformation response of ten (10) full-scale R/C beam specimens is utilised. These specimens were loaded monotonically in a four-point bending arrangement up to failure. Open hoop FRP strip shear reinforcement was applied externally to upgrade the shear capacity of eight (8) R/C beam specimens. Four of these specimens had these FRP strips without anchorage, whereas for the other four the FRP strips were attached together with novel anchoring devices. This successful numerical simulation predicts with a very good degree of approximation the observed load-deformation behaviour and the ultimate shear capacity of all these specimens as well as the observed modes of failure including diagonal concrete cracking, debonding of the FRP strips in the case of no anchoring, or the plastification of parts of the anchoring devices plus the adjacent crushing of the concrete.
INVESTIGATION OF NOVEL ANCHORING DEVICES FOR OPEN HOOP CFRP STRIPS BONDED ON R/C ELEMENTS UNDER MONOTONIC AND CYCLIC LOADING-UNLOADING CONDITIONS
Results obtained from research conducted in the framework of an initiative introduced by the Hellenic Earthquake Planning and Protection Organization aiming to support relevant provisions of the Greek Code for the repair and strengthening of R/C structures are presented and discussed. When trying to utilize fiber polymer sheets, as external reinforcement attached on R/C elements, their potential of high tensile strength is prevented by the FRP sheets’ debonding mode of failure thus limiting the exploitation of the material. In order to confront this limitation the effectiveness of two novel types of anchoring devices that can be incorporated together with such FRP strips was studied. For this purpose special unit beam concrete specimens were fabricated and were used to attach the open hoop carbon strips (CFRP). The novel loading arrangement developed at Laboratory of Strength of Materials and Structures was utilized to apply the necessary forces to these unit beam specimens together with instrumentation capable of capturing the behavior of these specimens up to failure. Studying in this way the transfer of forces from the open hoop FRP strips it could be demonstrated that when this type of retrofitting was accompanied with a properly designed anchoring device, a significant increase in the bearing capacity of the tested specimens was observed. The present investigation incorporates also the influence of number of cycles, in a loading-unloading way, on the overall behavior of the strengthening scheme, in terms of ultimate strength and observed mode of failure. Finally, this paper includes the most important findings of a supplementary numerical investigation, using the FEA software ABAQUS, aimed to study the behaviour of the novel anchoring device which is patented in Europe under the patent number WO2011073696.
Debonding failure along a softening FRP-to-concrete interface between two adjacent cracks in concrete members
Abstract A concrete beam can be strengthened by bonding a fibre reinforced polymer (FRP) plate to the tension face, and a common failure mode for such beams involves the debonding of the FRP plate that initiates at a major flexural crack, which is widely referred to as intermediate crack (IC) debonding. To understand IC and other debonding failures, the bond behaviour between FRP and concrete has been studied extensively using simple pull-off tests, in which a plate is bonded to a concrete prism and is subject to tension. However, the behaviour of the FRP-to-concrete interface in a beam can be significantly different from that captured in a pull-off test as, in a beam, whether debonding along the FRP-to-concrete interface occurs at a major flexural crack or not depends on the conditions at this crack as well as at the adjacent crack on the path of the debonding propagation. This paper is therefore concerned with the debonding process of an FRP-to-concrete bonded joint where the FRP plate is subject to tension at both ends, which closely approximates the IC debonding process in a flexurally strengthened RC member. The same problem has been the topic of a previous study by the authors, where a bilinear local bond–slip model was employed for the FRP-to-concrete interface. However, that solution is rather complex and difficult to apply in practice. The aim of this study is to produce a simplified solution by employing the simple linearly softening local bond–slip law for the interface. Results from this simplified analytical solution are compared with those from the previous solution, showing little loss of accuracy in predicting the load–displacement response and the ultimate load. The most significant outcome of the new solution is a simple expression for the ultimate load of the bonded joint which offers the potential for direct practical application. While the emphasis of the paper is on FRP-to-concrete joints, the solution and methodology are applicable to similar joints between other materials such as FRP-to-steel or steel-to-concrete bonded joints.