Accurate measurement and monitoring of vibration characteristics is critical for proper detection of the location and severity of damage. However, experimentally measured modal properties are inevitably corrupted by measurement noise and errors, which could render most vibration-based structural damage identification algorithms unreliable for civil structural health monitoring (SHM). This article, through computer simulation and experimental investigation of a simply supported beam, comparatively evaluates the performance of these techniques for practical civil SHM by using displacement modes from accelerometers and long-gage distributed strain measurements. Most of all the techniques proved unreliable for damage identification using noisy measurements from accelerometers, while successful with distributed strain measurements. The findings reveal that long-gage distributed strain measurements are much more efficient choice over the traditional measurement techniques for reliable civil SHM. It may therefore be concluded that the performance of some algorithms might be improved for application to civil infrastructure by using distributed strain fiber optic sensing measurement techniques.

This paper presents an experimental investigation on the bond behavior between basalt fiber–reinforced polymer (BFRP) sheet and concrete substrate under the coupled effects of freeze-thaw cycling and sustained load. Test variables were freeze-thaw cycles, level of sustained load, and adhesive type. Double-lap shear specimens were used in the tests, and a specially designed reaction-loading system was used to apply the sustained load during freeze-thaw cycles. Specimens with or without sustained load were exposed to up to 300 freeze-thaw cycles. A modified epoxy resin, made by adding a toughening agent to the original epoxy resin, was used in the test to study the effect of adhesive type on the durability of the BFRP–concrete interface. Coupon tests were also conducted to determine the freeze-thaw resistance of the constituent materials of the BFRP–concrete interface. After exposure, double-lap shear tests were carried out to investigate the residual bond capacity of the BFRP–concrete interface. Digital image correlation measurement was applied to capture the full-field deformation of the FRP sheet and the concrete block during the double-lap shear tests. A nonlinear bond-slip relationship of the BFRP–concrete interface was determined based on the analysis of displacement data. Test results show that (1) the bond capacity of the BFRP–concrete interface decreases with increasing freeze-thaw cycles, (2) the failure mode changes from debonding in the concrete layer to debonding in the adhesive layer, (3) extra degradation of the bond-slip relationship could be caused by the coupled effects, and (4) the durability improvement of the adhesive may result in a better durability of the BFRP–concrete bond capacity in a freeze-thaw environment. Finally, the coupled effects and evaluation of freeze-thaw procedures on the bond degradation of FRP–concrete interface are discussed.

To predict temperature-dependent tensile strength of unidirectional CFRP composites, numerical analysis and model simulation were conducted in terms of experimental results. The effect of elevated temperatures on tensile strength of CFRP sheets was evaluated between 20^°C and 120^°C, and the storage moduli of two types of epoxy resins, with glass transition temperatures (T _g) of 42^°C and 45^°C, were tested by DMA. The tensile experiments of CFRP sheets show that the tensile strength exhibits stable behavior at the low-temperature range (below glass transition temperature), but drops rapidly during the glass transition region, and then reaches a plateau. Analysis results indicate that the degradation of tensile strength is mainly due to matrix softening and loss of fiber—matrix adhesion, based on which a semi-empirical model is proposed to precisely describe tensile strength reduction as the temperature increases. The parameters of the model consist of the glass transition temperature of polymer matrix, the polymer glass transition region, and the residual strength after the glass transition region. Model prediction of tensile strength vs. temperature shows good agreement with the experimental results.

Debonding failure due to intermediate crack-induced (IC) fracture is one of the most dominant failure modes associated with the fiber-reinforced polymer (FRP) bonding technique. To date, extensive efforts have been paid by many researchers worldwide to study the debonding phenomenon for effective applications of FRP composites and rational design of FRP-strengthened structures. Based on these efforts and various relevant field applications, different models and code provisions have been proposed to predict IC debonding failure. Out of all the existing code provisions and models, five typical ones are investigated in the current paper. A comprehensive comparison among these code provisions and models is carried out in order to evaluate their performance and accuracy. Test results of 200 flexural specimens with IC debonding failures collected from the existing literature are used in the current comparison. The effectiveness and accuracy of each model have been evaluated based on these experimental results. Finally, based on a statistical analysis, a simple and more effective model for predicting the load-carrying capacity of FRP-strengthened flexural members due to IC debonding failure is proposed.

This paper presents the seismic performance of concrete bridge columns reinforced with both steel and fiber-reinforced polymer (FRP). A bond-based parametric experimental study was conducted on five FRP steel–reinforced concrete (FSRC) bridge columns—using basalt FRP (BFRP) bars and BFRP sheets—and two steel-reinforced concrete (SRC) bridge columns served as references to investigate the fundamental characteristics of the proposed reinforcement. The investigated bond parameters included the texture of the FRP bars (smooth and ribbed), diameter of the FRP bars, location of the FRP bars with respect to the steel bars, and application of external FRP confinement. All columns were tested under the combined effect of constant axial load and reversed cyclic loading. The experimental results showed that the FSRC bridge column could realize the existence of a stable postyield stiffness (hardening behavior) as well as a reasonable displacement ductility of up to 10 before encountering strength degradation. Moreover, the FRP bars added for column longitudinal reinforcement did not have a substantial impact on the column elastic stiffness. The bond condition of the FRP bars to the surrounding concrete had pronounced effects on the column failure mode, postyield stiffness, residual displacement, and ductility; thus, it could be adopted as a design parameter.