Journal of Composites for Construction

This paper examines the effect of depth on the shear strength and behavior of high-strength concrete beams reinforced with glass-fiber-reinforced polymer (GFRP) and carbon-fiber-reinforced polymer (CFRP) bars in the longitudinal direction only without stirrups. Three beams, for each reinforcement type, with depths approximately equal to 300, 450, and 600 mm were tested to determine their shear strength and behavior before and after cracking. The targeted concrete strength was 70 MPa. The tests were carried out using two-point monotonic loading. The test results are presented in terms of crack patterns, load-deflection behavior, and failure modes. It was observed that the shear strength decreased with the increase in the depth of the beams. These results were compared with Bažant size-effect law and a good agreement was observed. The test results were also compared with the predictions using the Canadian Standard Association (CSA) and American Concrete Institute (ACI) shear design equations. The predicted results using the CSA equation were in better agreement with the experimental results than those obtained using the ACI equation.

An experimental study was conducted to investigate the fatigue behavior of artificially notched steel beams strengthened with four different types of materials tested under equivalent tensile stiffness. These materials include high-modulus carbon-fiber-reinforced polymer (HM-CFRP) plate, high-strength CFRP (HS-CFRP) plate, steel-wire basalt-fiber-reinforced polymer (SW-BFRP) plate, and welded steel plate. Some key parameters, such as material type, the number of HS-CFRP layers, the configuration of HS-CFRP, and the interface treatment of SW-BFRP, are discussed. Compared to the traditional welded steel-plate method, the test results show that the application of a fiber-reinforced composite plate can not only delay crack initiation, decrease the crack growth rate, and prolong the fatigue life, but also reduce the stiffness decay and residual deflection. HM-CFRP exhibited the best strengthening performance; however, SW-BFRP is the optimal strengthening material on the basis of the cost–performance ratio. The fatigue behavior of steel beams can be improved significantly by increasing the layers of strengthening material. SW-BFRP with a rough surface can prolong the fatigue life of steel beams more effectively than SW-BFRP with a smooth surface. The plate configuration has certain effects on the fatigue life.

Carbon-fiber reinforced polymer (CFRP)-overlay elements were developed with the purpose of enhancing the fatigue performance of welded connections in steel bridge girders. Fatigue tests of seven specimens, including four CFRP-strengthened specimens and three control specimens, were performed to quantify the effect of the CFRP overlays on the fatigue crack initiation lives of the welded connections. Results showed that bonding of CFRP overlays significantly reduced the stress demand on welded connections tested at high stress ranges, leading to a large increase in fatigue crack initiation life. The level of effectiveness of the CFRP-overlay elements in extending the fatigue crack initiation lives of the tested connections was found to be affected primarily by bond strength under cyclic loading; bond strength was found to be dependent on the composition and thickness of the resin layer used to bond the CFRP to the steel. With the AASHTO fatigue design curves as a frame of reference, it was found that when an optimal bond composition was employed, reinforcing the welded connections with CFRP overlays led to a change in fatigue performance category from that consistent with Category E to runout at high stress ranges. An optimal bond composition was identified that resulted in excellent performance under fatigue loading.

The majority of experimental studies investigating the stress-strain behavior of square concrete columns confined with carbon fiber-reinforced polymer (CFRP) composites have focused largely on unreinforced columns of small size. Research on the influence of larger cross section, height, internal steel reinforcement, and initial damage on the axial strength and compressive behavior of CFRP-confined square reinforced concrete (RC) columns is, however, limited. To address such knowledge gaps, this paper presents the results of an experimental investigation on the axial stress-strain behavior of 34 larger-sized square-sectioned RC columns confined with CFRP composite wraps. The primary test variables were (1) cross-sectional dimensions, (2) volumetric ratio of internal hoop steel reinforcement, (3) number of layers of CFRP wrap, (4) nature of loading (i.e., monotonic and cyclic), and (5) damage level before CFRP wrapping. The experiments showed the CFRP wrap to considerably enhance the axial strain capacity but to only slightly increase the axial stress capacity. The experiments also showed the internal reinforcement to influence the shape of the axial stress-strain envelope curve and unloading path and the ultimate axial strain and plastic strain values. Predamage was, however, found to have a small influence. A new confinement pressure model for fiber-reinforced polymer (FRP) confined square RC columns is finally proposed.

For the seismic design of fiber reinforcement polymer (FRP) confined reinforced concrete (RC) columns, the development of an accurate axial stress-strain model that considers cyclic compression is necessary. In light of such a demand, this paper presents a cyclic axial stress-strain model for FRP-confined RC square columns. The model is informed from physical observations and test measurements obtained from an experimental investigation reported in the companion paper, in which FRP-confined square unreinforced and reinforced concrete columns of larger size under varying cyclic axial compression patterns were tested. In the current paper, the proposed stress-strain model is presented and it consists of three main components, namely (1) a monotonic stress-strain model to describe the envelope curve, (2) a polynomial expression for the unloading path, and (3) a straight line for the reloading path. The influence of internal longitudinal and hoop steel reinforcement is also considered in the proposed model, in addition to their influence on the ultimate stress and strain. The accuracy of the model is finally validated with an experimental database compiled of tests reported in the companion paper and other relevant tests extracted from the open literature

The mechanical response of Reinforced Concrete (RC) beams strengthened by an Externally Bonded Reinforcement (EBR) made out of Fiber-Reinforced Polymers (FRPs) is deeply influenced by the interaction between the concrete substrate and the FRP system, either cured-in-place (sheets) or preformed (plates). In particular, the strength of FRP-EBR RC beams is often controlled by debonding phenomena to develop at the adhesive-to-concrete interface. The most recent theoretical formulations and some experimental results obtained in the last years pointed out the differences that characterize the debonding strength of FRP sheets and plates. According to the findings of those studies, the fracture energy is a fundamental parameter governing the debonding phenomenon. However, determining its value is not sufficient for simulating the behavior of the FRP-to-concrete interface and modeling relevant problems such as intermediate debonding in RC beams externally strengthened by FRP. Consequently, formulating and calibrating local bond-slip models, which take into account the different behavior of sheets and plates, is of fundamental importance for modeling FRP-strengthened RC members. This paper is aimed at identifying bond-laws for sheets and plates through an Indirect Identification Method (IndIM), recently implemented and validated by the authors. A wide collection of experimental results obtained by pull-out tests on FRP sheets and plates is first reported and then employed for identifying the previously noted bond-slip laws. Finally, the results of the identification procedure demonstrate that the debonding phenomenon, described as a fracture process in mode II, should be modeled by assuming different bond-slip relationships for FRP plates and sheets.

An experimental investigation has been conducted with the aim to investigate on the midspan debonding failure of reinforced concrete (RC) beams retrofitted in flexure by means of the application of a fiber-reinforced polymer (FRP) lamina externally applied to concrete substratum. Experimental tests on a series of RC beams with different geometries and type of internal steel reinforcing bars have been carried out in four-point bending up to failure to evaluate the influence of flexural/shear cracks on the debonding of FRP reinforcement from concrete substratum. It is widely known that the overall performance of a standard RC element is primarily influenced by the interaction between FRP and concrete substratum rather than by the strength of the FRP. The failure of such innovative retrofitting techniques may be attributed to an inadequate anchorage length; in other cases, when the FRP is correctly applied at the ends of beam, debonding starts in the vicinity of cracks and propagates toward the supports. The results of the experimental tests are used to investigate on the effectiveness of design procedures currently recommended for such debonding failure mode by the codes of practice. The results of the analytical calculations based on such models are presented in this paper and will provide an element of comparison and discussion with the experimental results. The codes of practice propose different analytical models based on the extensive research done in the last two decades, which involve different types of approximation and often yield quite different results.

Compared with reinforced concrete, relatively fewer experimental studies address the behavior of masonry columns under combined axial load and cyclic flexure. There exist reinforced concrete masonry (RCM) columns that are part of the moment resisting system of masonry structures that are in need of seismic upgrade. Wrapping such susceptible RCM columns with carbon fiber-reinforced polymers (CFRP) is expected to enhance the seismic behavior of reinforced masonry columns considerably. This paper focuses on assessing the seismic performance of RCM columns wrapped with CFRP. In this experimental study, six 1.4-m reinforced masonry columns were constructed and tested when subjected to constant axial force and cyclic lateral excitations. The columns had a cross-section of $390\mathrm{mm}\times 390\mathrm{mm}$ and were constructed using bull-nosed concrete units. The first column had no CFRP wraps and was used as a control specimen whereas the other five columns were wrapped using different layers of CFRP sheets or different wrapping schemes. From the tests, it was observed that wrapping the masonry columns with CFRP wraps enhanced the seismic performance of the columns by offering more ductile behavior, increasing both strength and energy dissipation capacity.

An experimental study was conducted to investigate the flexural behavior and long-term prestress losses of reinforced concrete (RC) beams strengthened with posttensioned carbon fiber-reinforced polymer (CFRP) sheets. The experimental program consisted of tensile tests of flat CFRP coupons under sustained loads and flexural tests of a total of eight RC beams: six strengthened with posttensioned CFRP sheets, one strengthened with nonprestressed CFRP sheets, and one control beam. The main objective of the tests was to gain a better understanding of the long-term prestress losses of CFRP sheets in the posttensioned system under different prestress levels and strengthening ratios. It is shown that the prestress losses of CFRP sheets in the posttension system are mainly attributable to anchorage set (approximately 12.6 to 18.2% of the initial prestress), whereas the time-dependent losses caused by creep and shrinkage of concrete and relaxation of CFRP sheets are relatively small (approximately 2.3 to 3.9% of the initial prestress).

This paper presents mechanical, microstructural, and physical characterization of aged glass fiber-reinforced polymer (GFRP) tubes used in the concrete-filled fiber-reinforced polymer (FRP) tube (CFFT) system for bridge columns and marine pile applications, or used hollow as tubular poles or in pipelines. The main objective of the study is to evaluate the durability and predict the long-term behavior of the filament-wound GFRP tubes. CFFTs were exposed to salt solution at 23, 40, and 50°C for 365 days to accelerate the environmental effect. Given the significance of confinement in a CFFT system, the measured hoop tensile strength of the tube before and after exposure was considered the primary indicator of durability performance of the specimens. In addition, Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM) were used to characterize the aging effect on GFRP tubes. Test results showed that the GFRP tubes investigated in this study, and exposed to a rather aggressive environment, performed well. The reduction in hoop tensile strength at the end of exposure ranged from 11 to 21%, depending on temperature. Using the Arrhenius theory, the predicted reduction in strength after 100 years at a mean annual temperature of 6°C, representing some northern regions, was estimated at approximately 32%.