Journal of Composites for Construction

This paper addresses the strengthening of T-joints of rectangular hollow steel section (HSS) chords against web buckling induced by transverse compression loading of welded brace members. Twelve T-joints of 1,200 mm long, $203\times 76\times 3.09$ (or 5.92) mm HSS chords, welded to 400 mm long, $76\times 76\times 8.9\mathrm{mm}$, HSS braces were tested under brace axial compression. Glass fiber reinforced polymer (GFRP) plates, 9.4 mm thick, of different configurations, with and without edge tapering, and 2 mm thick high-modulus carbon-FRP plates of equivalent stiffness, were adhesively bonded to the webs of the chord near the brace. Strength gain increases significantly, from 9–38%, as the web height-to-thickness ($h/t$) ratio of the HSS chord increases from 34 to 65. In thin-walled HSS ($h/t=65$), retrofitting provided significant gains in strength but not in ductility. Fiber-reinforced polymer (FRP) plates delayed and shifted local buckling away from brace location but did not prevent it. Square-edged and tapered plates behaved similarly. Plates as small as half the chord depth and of width equal to the brace provided a 35% strength gain. Very wide plates, 0.4 of the span, provided a 53% gain. In thick-walled HSS ($h/t=34$), retrofitting provided little strength gain but enhanced ductility, especially with properly bonded plates extending on the brace. Generally, plates fractured under local bending or delaminated within plate layers while the bond was fully intact.

A design method is developed for determining the required thickness of fiber-reinforced polymer (FRP) composite jackets in rehabilitating RC columns to achieve a target displacement ductility. The method is applicable to RC columns with a rectangular, square, or circular cross section. The FRP jacket thickness is determined from the target ultimate compressive strain of confined concrete within the plastic hinge region. The design is based on the strain ductility increase provided by the jacket. Relationships are introduced to determine the minimum FRP jacket thickness required to preclude strain-softening compressive behavior of rectangular, square, or circular columns. This minimum jacket stiffness can prevent premature failure of the FRP jacket due to uncontrolled dilation of the confined concrete. The design procedure compares favorably with experimental results from the literature for columns upgraded with FRP jackets, which displayed a substantial displacement ductility increase.

Strengthening of RC columns through lateral confinement using fiber-reinforced polymer (FRP) jackets is now a widely accepted technique. While extensive research has been conducted on the behavior of such strengthened columns, only a very limited amount of work has examined the slenderness effect in these columns. To correct this deficiency, this paper presents a systematic study on the subject and proposes the first ever design method for slender FRP-confined circular RC columns. A simple theoretical column model for such columns is first described in the paper. Numerical results obtained from this theoretical model are then presented to examine the behavior of these columns to identify the key parameters of influence. A design method based on numerical results from the column model and following the nominal curvature approach is next formulated. The accuracy and safety of the proposed design method is demonstrated through comparison with existing experimental results.

The comparatively weak performance of fiber-reinforced polymer (FRP) strengthened members under fire exposure is a primary factor hindering the application of FRP strengthening in buildings in which fire risk is not negligible. As part of a larger ongoing study investigating the behavior of FRP strengthening systems under fire exposure, an investigation was set up to examine and document the performance of six full-scale near-surface mounted (NSM) FRP-reinforced concrete beams exposed to 1 h of fire. All the specimens were preloaded to the service load of the strengthened member. Tests results indicated that, if appropriately insulated, the NSM FRP-strengthened beams can achieve a satisfactory fire endurance of 1 h as per fire resistance test specifications. Moreover, this paper also presents a study on the residual performance of fire-tested beams. Results of this study suggest that if the insulation system is able to maintain the adhesive temperature at relatively low value ($t_{\text{adhesive}}\le 1.6t_g$ for the beam configuration in this test program), the FRP concrete bond degradation under fire is limited, and the FRP-strengthened beam can retain a large part (up to 92% in this test program) of its original strength.

This paper addresses the buckling behavior and strength of hybrid pultruded short profiles subjected to uniform compression. Since the design of glass fiber-reinforced polymer (GFRP) pultruded profiles is often governed by deformations and buckling phenomena, the introduction of carbon fibers in GFRP (hybrid) profiles has been proved to increase both strength and stiffness of flexural members. In the context of short compression members (columns), experimental, numerical, and analytical studies were carried out, with particular attention being given to the local buckling. Two series of 660-mm-long I-section profiles ($200\times 100\times 10\mathrm{mm}$) were tested under uniform compression. In order to evaluate the advantages of introducing carbon fiber reinforcement in the flanges, two profile types were tested: bare GFRP profile (reference column) and carbon fiber reinforced polymer (CFRP) strengthened GFRP profile (hybrid column). Shell finite-element (SFE) models were developed and validated, based on experimental data. After that, and in order to predict the critical buckling loads of the tested columns, a study was performed using analytical solutions based on the orthotropic plate theory. Both numerical and analytical results presented a good agreement with experimental data. Results obtained show that the critical load, the ultimate load, and the axial stiffness considerably increase with the introduction of CFRP sheets in the flanges of the GFRP profile.

Phased nonlinear finite-element (FE) analyses were carried out to predict the behavior of precracked reinforced concrete (RC) T-beams repaired in shear with externally bonded (EB) carbon fiber–reinforced polymer (CFRP) sheets and subjected to two loading patterns (LPs). Appropriate constitutive relationships were employed to model the behavior of concrete, internal steel reinforcement, EB CFRP reinforcement, and CFRP-to-concrete interface and consequently predict the structural behavior and capture the failure modes of the strengthened beams. Three constitutive models for the behavior of concrete in shear were evaluated, namely, a total strain rotating crack model and two fixed-angle crack models with either constant or variable shear retention factors. The majority of published FE studies have considered rectangular sections that were strengthened before testing. The key feature of the FE models presented in this paper is the use of the phased-analysis technique to model realistically the process of strengthening RC T-beams under load and predict the structural response of the beams to different loading patterns. Furthermore, the paper provides insight into and evaluates the accuracy of the three concrete shear models named above. A detailed comparison between the numerical and experimental results included the shear forces at failure, shear force-deflection curves, crack patterns, failure modes, and strains in the internal steel and external CFRP shear reinforcement. The FE models predicted the experimental shear force capacities and crack patterns with sufficient accuracy but underestimated the postrepair stiffness for the beams subjected to Loading Pattern 1 and overestimated the strain in the CFRP sheets.

Strengthening of both aging and modern infrastructure has become necessary as a result of increased load demand and/or to restore the capacity of a member. For example, flat slabs (commonly utilized in parking garages) are subjected to an aggressive environment and increased traffic loads and additionally require strengthening and/or repair. Fiber-reinforced polymer (FRP) sheets and laminates externally bonded to concrete slabs around the slab-column joint are widely used to enhance the strength of flat slabs. Slab-column connections are most often subjected to eccentric loading; however, most reported studies have focused on retrofitted slab-column connections subjected to concentric loads. This paper aims to fill this knowledge gap and experimentally investigate the effect of eccentric loading on the behavior of slab-column connections retrofitted by externally bonded carbon fiber-reinforced polymer (CFRP) sheets. Six full-scale ($\mathrm{2,000}\times \mathrm{1,000}\times 150\text{-}\mathrm{mm}$) interior slab-column connections subjected to eccentric loading were constructed and tested to failure. Three control test specimens were subjected to applied loads with different eccentricities. The remaining three test specimens were externally strengthened with different configurations of carbon FRP sheets before being subjected to eccentric loading. The test results are presented as comparisons of the load-deflection and load-strain behavior. Eccentric loading reduces the ultimate load capacity and ductility of slab-column connections. The increase of the ultimate strength of carbon FRP-strengthened slab-column connections decreases with the increase of applied load eccentricity.

Experimental and numerical investigations were conducted to evaluate the performance of a newly developed bridge system. Through the investigation, a decked bulb T-beam bridge model was constructed, instrumented, and tested under service and ultimate loads. The bridge model had a width of 2.59 m (8.5 ft), an effective span of 9.45 m (31 ft), a depth of 356 mm (14 in.), and was composed of five adjacent decked bulb T-beams. The T-beams were interconnected at their top flanges using 76-mm (3-in.)-wide ultra-high-performance concrete (UHPC) shear key joints and five full-depth equally spaced transverse diaphragms along the span. Each diaphragm was posttensioned with two nonbonded transverse carbon fiber composite cable (CFCC) strands. The investigation revealed that the developed decked bulb T-beam bridge system maintained its structural integrity under service loads with signs of distress in neither the shear key joints nor top flanges. UHPC shear keys with the transverse diaphragms were adequate to achieve monolithic action across the width of the bridge model. In addition, transverse posttensioning forces were effective in restoring the structural integrity of the bridge model when cracks were artificially induced in the shear key joints. At the ultimate limit state the bridge model exhibited compression failure by crushing of the concrete in the top flange. The compression failure was associated with low ductility, a dense cracking pattern, and excessive deflection.

This paper presents the results of an experimental investigation that was carried out to examine the size effect on the shear strength and behavior of concrete beams reinforced with fiber-reinforced polymer (FRP) bars. The beams were reinforced with glass FRP, carbon FRP, and steel bars in the longitudinal direction with no transverse shear reinforcement. Twelve large-scale simply supported beams with different depths, four for each reinforcement type, were tested to determine their shear capacity. The effective depth of the beams was in the range of 300–750 mm. The axial stiffness of the reinforcement was kept the same for beams with the same reinforcement type with different depths. The test results indicated that as the member depth increased, the shear strength at failure decreased for all reinforcement types. The results were compared with Bažant’s size effect law including different parameters, and a reasonably good trend was observed. The shear strength of FRP reinforced beams were also compared with the predictions using design codes and Canadian and U.S. guideline equations. The comparisons with the equations indicated that the predicted results using one of the Canadian equations were the closest to the experimental results, while one of the U.S. equations predicted results were more conservative and gave prediction results with large scatter, especially for beams with smaller depth.

This paper presents an approach for evaluating the fire response of reinforced concrete (RC) beams strengthened with near-surface-mounted (NSM) fiber-reinforced polymer (FRP) reinforcement. The model is based on a macroscopic finite-element approach and is capable of tracing the response of NSM FRP-strengthened RC beam from the preloading stage to collapse under a specified fire exposure and loading conditions. The model accounts for high-temperature properties of constitutive materials, realistic load and boundary conditions, and temperature-induced bond degradation at the FRP-concrete interface. The validity of the model is established by comparing predictions from the model with measured data from tests at both ambient and fire conditions. The validated model was applied to carry out numerical studies to compare the fire response of concrete beams with different types of strengthening systems and insulation layouts. Results from numerical studies indicate that a concrete beam strengthened with NSM FRP reinforcement yields slightly lower fire resistance as compared to a conventional RC beam but achieves higher fire resistance than that of externally bonded FRP. It is also shown that appropriate location of a NSM FRP reinforcement and insulation scheme can increase the fire resistance of concrete beams strengthened with NSM FRP reinforcement.

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.

This paper presents a numerical method to predict the ultimate load of masonry arches strengthened with carbon-fiber-reinforced polymer (CFRP) strips bonded to the intrados. The voussoirs of the arch and the CFRP strip, ideally divided into the same number of parts as the voussoirs, are modeled as rigid blocks. A finite set of stress resultants represents the stress state acting on interfaces of the rigid blocks. The local failure modes at the block interfaces are defined according to experimental evidence. The model is developed within an associated framework in such a way that the normality rule is satisfied: the upper- and lower-bound theorems of classical limit analysis apply. The ultimate load is predicted by a lower bound approach. The feasible domain is defined by the equilibrium equations and by the linear constraints imposed on the stress resultants. All the relations defining the model are linear, so that a linear programming problem is imposed. The predictions of the numerical model compare well with experimental results.

Previous studies indicated that RC flat slabs, especially without drop panels, are of high vulnerability to progressive collapse because no beams could assist in redistributing the axial force previously carried by the lost columns. In order to reduce the likelihood of progressive collapse, necessary strengthening schemes should be applied. Six specimens of similar dimensions and reinforcement details were prepared, two of which were unstrengthened and served as control specimens, while the remaining four were strengthened with two different schemes: orthogonally (Scheme 1) or diagonally (Scheme 2) bonded carbon-fiber-reinforced polymer (CFRP) laminates on the top surface of the slab. The progressive collapse performance of the strengthened specimens was studied in terms of their load-displacement relationships, first peak strength, initial stiffness, and energy dissipation capacities. The dynamic ultimate strength and corresponding dynamic effects of flat slabs after the sudden removal of a corner column was also discussed due to the dynamic nature of progressive collapse. Test results indicated that both schemes were effective in improving the performance of RC flat slabs in resisting progressive collapse.

To prevent damage caused by corroding reinforcement, fiber-reinforced polymer (FRP) reinforcing bars have been used in place of steel in a relatively small but increasing number of structures in civil infrastructure. A concern with the use of traditional FRP bars, however, is the resulting lack of ductility. This problem has been overcome with the development of a new generation of composite reinforcement, ductile hybrid FRP (DHFRP) bars. However, standards that address the design of DHFRP bars are unavailable, and appropriate resistance factors for the use of DHFRP reinforcement are unknown. In this paper, a reliability analysis is conducted on tension-controlled concrete flexural members reinforced with DHFRP with the intent to estimate potential strength-reduction factors. Flexural members considered include a selection of representative bridge decks and building beams designed to meet strength requirements and target reliability levels dictated by relevant engineering standards. Nominal moment capacity is calculated from standard analytical models and is taken as first DHFRP material failure. Statistical parameters for load and resistance random variables in the reliability model are consistent with previous code calibration efforts. The resulting resistance factors ranged from 0.61–0.64 for tension-controlled sections, which indicates a potential increase in allowed strength with respect to flexural members using nonductile bars.