Computational modeling, experimental and theoretical study on bond behaviors of hybrid-bonded FRP strengthened concrete structures

Abstract

External bonding of fiber-reinforced polymer (FRP) to reinforced concrete members has become a popular method of retrofitting/strengthening concrete structures in recent years. This technology allows the high tensile strength of FRP to be transmitted to concrete structures through surface adhesion. However, surface adhesion between FRP and concrete is known to be weak and unreliable; it offers poor bonding between the FRP and the concrete members and often induces premature debonding at the bond interface. Such premature debonding has greatly reduced the usage rate of FRP. To solve this problem, the author's supervisor has developed a new technology, namely, the hybrid bonded FRP (HB-FRP), that combines adhesive bonding with mechanical fastening. In this research, the mechanism of the HB-FRP system was studied by using FE analysis; a FE model for HB-FRP strengthened beams was developed to simulate the responses of the new structural system. Bond behaviors for externally bonded FRP (EB-FRP) and HB-FRP joints were experimentally investigated by employing the single shear pull-off test on ten EB-FRP and HB-FRP specimens with different bond lengths. Based on the analytical bond-slip model for HB-FRP joints proposed in this work, bond behaviors of adhesive and HB-FRP joints were studied theoretically. Based on the experimental results, performance of the proposed bond-slip model for HB-FRP joints was verified. Finally, a standard test protocol for HB-FRP joints was established for design purposes. An FE model for numerical simulation of the HB-FRP strengthened beam under static loading is developed to study the mechanism of the HB-FRP system. A combination of the shear friction model with the traditional bond-slip model for the adhesive bond interface and the dowel model for dowel action of concrete nails render structural analysis of the very complicated HB-FRP strengthening scheme feasible. The FE model is valid and capable of studying the debonding mechanism and the failure mode, and of simulating overall responses of the complicated structural system. Mechanical fasteners alter the failure mechanism from adhesive debonding to pull-out failure of mechanical fasteners, which increases the interfacial bond and the load carrying capacity of the beam, and results in better use of strength of the FRP strip. The frictional shear resistance caused by mechanical fasteners can be several times greater than the adhesive bond strength. The additional frictional resistance depends on the pull-out resistance of the anchors and on the number and spacing of mechanical fasteners. The single shear pull-off test was employed on ten EB-FRP and HB-FRP specimens with different bond lengths. The test results indicate the importance of careful specimen preparation as the failure mode can be significantly affected. In addition to further verification of effectiveness on HB-FRP system, test results were used to verify the new proposed analytical bond-slip model for HB-FRP joints. Adopting the bond-slip relationship for adhesive joints proposed by Zhou et al., the local bond-slip relationship for HB-FRP joints is developed by superimposing three mechanisms in the HB-FRP system. Theoretical relationship between the pull-off load and the slip at the free and loaded end for HB-FRP joints is obtained by homogenizing the bond behavior induced by the mechanical mechanism. Expressions for load capacity are theoretically established for adhesive and homogenized HB-FRP joints with an infinite bond length. Based on the indirect analytical method, identification procedures were developed to calibrate the parameters in the proposed bond-slip model for HB-FRP joints. The unknown parameters were determined by computational minimization of the difference between experimental and theoretical results. Adopting the identified parameters, good performances of the proposed bond-slip models for EB-FRP and HB-FRP joints were obtained by comparison between numerical and experimental results. The calibrated models were then used to calculate the load capacity and the effective bond length for the particular EB-FRP and HB-FRP joints. Finally, a standard test protocol for HB-FRP joints was proposed for design purposes.

Date of Award	15 Jul 2011
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yufei WU (Supervisor)