This research assesses the effects of the presence of water during the carbon fiber reinforced polymer (CFRP) application and after the CFRP cure on the bond between the CFRP and the concrete. Modified double cantilever beam (MDCB) specimens are used to determine the interfacial energy release rate, G, of the CFRP—concrete bond. A CFRP fabric is applied to the specimens with three different initial levels of water/moisture presence. Other specimens, with a CFRP fabric applied in dry conditions and allowed to cure, are conditioned in a saturated environment for different lengths of time. The test results indicate that the presence of water during the CFRP application decreases the bond quality significantly and most of the resulting failures are adhesive failures along the primer/concrete interface. The use of a specially formulated primer results in a slightly higher bond capacity but the same undesirable failure. High quality CFRP applications, conditioned in a saturated environment for a relatively short period of time, from 3 to 8 weeks, demonstrate that exposure to water degrades the bond between CFRP and concrete after the epoxy has cured.

A novel experimental method using modified double cantilever beam specimens and a customized test frame are introduced to evaluate bond characteristics and toughness of fiber reinforced polymer (FRP) composite overlays and a concrete substrate under mixed mode loading. A computer vision system is used to measure the crack location, near-tip deformations and crack opening displacement during the crack growth process. Digital image correlation is used to determine the crack opening displacement (COD) for flaws growing in the vicinity of the FRP–concrete interface. Results from this study indicate that during crack growth, (1) the Mode I component of COD is dominant for all angles of specimen loading, (2), the magnitude of the local Mode I component of COD is maximized when good bond quality is present and crack extension occurs within the mortar/concrete near the FRP–concrete interface and (3) good agreement exists between independent energy release rate estimates based upon both an approximate elastic doub...

An experimental and numerical program to investigate the behavior of intermediate crack (IC) debonding failure and the bond-slip relationship between the fiber-reinforced polymer (FRP) plates and concrete in FRP-strengthened concrete beams is presented. Simple notched-beam specimens were used to represent an existing flexural-shear crack that triggers IC debonding. In all experimental cases, it was observed that the IC debonding initiated at the tip of a diagonal crack close to the notch or a flexural crack at the beam midspan. To study the sensitivity of the debonding behavior to the location of the cracks along the beam span, the notch position was placed at different locations along the shear span. The combination of the concrete damaged plasticity model and the extended finite-element method (XFEM) based cohesive method is proposed in this paper to model the complete concrete response, and to examine the stress state of the FRP–concrete interface in the presence of IC debonding failure. The numerical results show that the diagonal crack close to the notch results from a mixed-mode stress state prior to FRP debonding. Once the diagonal crack is initiated close to the flexure-shear crack, IC debonding propagates in pure shear stress condition at the FRP–concrete interface regardless of the moment shear ratio at the location of the flexure-shear crack.

This study experimentally investigates the mechanism of deterioration for the bond between CFRP plate and concrete due to moisture attack. The debonding failure in the concrete structures with externally bonded FRP has two major modes: cohesive failure in the concrete surface layer and adhesion failure at the adhesive/concrete interface. A modified double cantilever beam (MDCB) test was used to measure the interfacial fracture energy for the CFRP plate debonding from concrete substrate under Mode I loading. A simple method was developed to measure the value of interface region relative humidity (IRRH). The relation between IRRH and the bond fracture energy was then obtained. By quantitatively measuring the residual thickness of concrete (RTC) on the detached plate, it was found that RTC was directly related to the value of IRRH. Based on the experimental data, a constitutive relation between RTC and IRRH was obtained. Using this relation and virtual crack closure technique (VCCT), the bond fracture energy was numerically determined as a function of IRRH for the FRP/concrete bond joints. It was found that when the IRRH value was greater than 55%, the deterioration of fracture energy was accelerated. When the IRRH was greater than 75%, the fracture energy tended to become steady, and did not change significantly with the increases of IRRH.

Modeling the movement and distribution of moisture in the fiber-reinforced polymer (FRP) composites strengthened concrete structure is important because the interfacial adhesion between FRP and concrete is susceptible to moisture attack. Using relative humidity as the global variable, the moisture diffusion governing equation was derived for the multilayered system in this study. The moisture diffusivity (diffusion coefficient) and the isotherm curve, which correlates the moisture content to environmental relative humidity, of each constitutive material (concrete, epoxy, and FRP) were experimentally determined. A multilinear diffusivity model was developed for concrete based on desorption test, and a linear diffusivity model was proposed for epoxy adhesive based on absorption test. A simple method was developed to directly measure the FRP/concrete interface region relative humidity (IRRH). Finite-element analysis was performed to study the moisture diffusion in the FRP-adhesive-concrete system. The IRRH v...

This study proposed a bond mechanism based deterioration model of bond interfacial fracture energy for fiber-reinforced polymer (FRP)-concrete joints in moist environments. The bond interface region relative humidity (IRRH) in a moist environment was correlated to the bond fracture energy in this deterioration model. The IRRH-dependent interface separation tractions were derived in the frame of a cohesive zone model. Such an IRRH-dependent interface separation-traction law was simulated by a series of nonlinear interface elements attached to the bond interface to calculate the macroscopic load-displacement curves for the modified double cantilever beam (MDCB) specimens. Through moisture diffusion analysis, IRRH was determined as a function of the moisture exposure time for given specimen dimension and environmental RH. Using IRRH as the bridge, the time-dependent load-displacement curves of the MDCB specimens were obtained. The good agreement with the experimental data indicated that the model worked well...