Journal of Composites for Construction

This paper presents the results of an experimental study on the fatigue performance of RC beams strengthened with different externally bonded carbon fiber-reinforced polymer (CFRP) systems. Seven specimens were fabricated; three had no CFRP, while the remaining four had one of two CFRP systems. The objective of the research reported in this paper was to determine the effect the CFRP repair had on the growth rate of a fatigue crack initiating from a notch in the tensile rebars. The steel reinforcement was selected for focus because similar studies have found this reinforcement to be the limiting fatigue component in RC beams. The experimental results reported in this paper showed an extended fatigue life and a slowed crack growth rate in specimens repaired with both CFRP systems. The crack growth rates were used to determine material constants for the Paris law, which describes growth of a stable fatigue crack. These results are then used to propose recommendations for design of CFRP repair systems for a specific fatigue life.

Studying interfacial behavior is important for a composite structure involving relative movement or slip at its joint interface. Recent experimental and theoretical studies on bond interfaces have found that snap-back phenomenon may occur, in certain scenarios, in monotonic pull-off tests where interfacial slip in some local region is opposite to that in other areas. Little is known about the snap-back phenomenon at present, such as the conditions under which it occurs, distribution and development of reverse slip fields, and whether and how it affects global load-deformation responses. Further and more in depth investigations into the snap-back problem are necessary for better understanding of joint interfaces; e.g., in experimental bond tests with digital image correlation (DIC) technology where detailed interfacial movements on a whole bond interface are recorded for identification of interfacial properties. Therefore, this work attempts to study the snap-back problem in detail through analytical study. Furthermore, the results from this work provide a theoretical explanation of an interesting interfacial phenomenon—strain softening toward the loaded end that has puzzled researchers in the past. This paper facilitates a better understanding of the interfacial behavior for composite structures.

The use of basalt fiber–reinforced polymer (BFRP) bars as a reinforcing material has gained increasing interest worldwide. However, few studies have reported on these bars’ performance in concrete when exposed to harsh environments. This paper investigates the effect of five different accelerated environments, namely (1) tap water, (2) seawater, (3) elevated temperature, (4) elevated temperature followed by tap water, and (5) elevated temperature followed by seawater, on the bond stress-slip response, adhesion to concrete, and bond strength [of two types of BFRP bars and one type of glass fiber–reinforced polymer (GFRP) bar]. The bond-slip responses of all specimens were governed by the surface treatment of each bar and its manufacturing quality, regardless of the fiber type. Sand-coated BFRP bars showed higher bond characteristics than helically grooved bars after conditioning. Moister environments caused enhanced adhesion at the early loading stages for all specimens. Nevertheless, such environments had a detrimental effect on the bond strength at later stages depending on the bar material’s moisture absorption. Finally, exposure to elevated temperatures caused insignificant variation in the bond strength of all tested specimens.

In the present study, application of carbon-fiber-reinforced polymer (CFRP) composites with fibers aligned along the column’s axis to improve compressive strength of reinforced concrete (RC) columns has been investigated. Global buckling of fiber-reinforce polymer (FRP) composite with fibers along the column’s axis may result in debonding of FRP from the column’s surface, and thus, the compressive load carrying capacity of the strengthened column would not be increased considerably. To limit the global buckling of composite under compression, a newly introduced strengthening method, named as grooving method (GM), was utilized in the present study and compared with externally bonded reinforcement (EBR) method using conventional surface preparation and near surface mounted (NSM) method. For this purpose, 22 circular RC columns with 150-mm diameter and 500-mm height were tested under uniaxial compression. Two techniques of GM named as externally bonded reinforcement on grooves (EBROG, to be pronounced as $/ebr\mathrm{\Lambda }g/$) and externally bonded reinforcement in grooves (EBRIG, to be pronounced as $/ebrIg/$) were used in the present study. Experimental results showed that grooving method considerably enhanced the columns’ maximum loads. It was also observed that the maximum compressive stress capacity of CFRP based on average compressive stresses of fibers in the composite was impressively increased when grooving method was used. The results showed that in the columns strengthened by EBROG method, nearly 80% of the fibers’ tensile strength in direct shear test could be utilized as compressive stresses in fibers of longitudinal composite; this value was about 13% and 16% when EBR and NSM techniques were used, respectively.

Experiments were conducted to assess the effect of using bonded and unbonded glass and carbon fiber–reinforced polymer (FRP) wraps to strengthen corrosion-damaged, reinforced-concrete (RC) columns. The effect of wrapping on the rate of corrosion and strain concentration on the FRP wrap was investigated through an accelerated corrosion test. The results indicated that wrapping reduced the mass loss in the reinforcing bars by 33–46% after 130 days of accelerated testing. Both glass and carbon wraps were equally effective in slowing down corrosion. Although unbonded wraps reduced stress concentrations in the FRP, they were slightly less effective in reducing the corrosion rate than bonded wraps. Finite-element (FE) analysis of the experimental specimens was performed to verify the effect of bonded and unbonded FRP wraps on the strain concentration obtained from experimental tests. The FE results confirmed that the strain concentration around a corroding bar reduced when the wrap was unbonded. Because the strain concentration on the bonded wraps at the laboratory-scale FE model was of concern, a large-scale RC column was modeled to assess the effect of strain concentration around a corroded bar in real bridges.

This paper investigates the structural behavior of uncorroded as well as corroded RC T-beams strengthened in shear with either externally bonded (EB) carbon fiber–reinforced polymer (CFRP) sheets or embedded CFRP rods. Nine tests were carried out on RC T-beams having an effective depth of 295 mm and a shear span to effective depth ratio of 3.05. The investigated parameters are the shear link corrosion level (uncorroded, 7% corroded, or 12% corroded) and type of CFRP strengthening system (EB CFRP sheets or embedded CFRP rods). The unstrengthened beams with shear link corrosion levels of 7 and 12% had shear strengths that were 11 and 14%, respectively, less than the shear strength of the uncorroded unstrengthened beam. Both the embedded CFRP rods and EB CFRP sheets were effective in enhancing the shear strength of tested beams but the effectiveness of both strengthening systems decreased with increasing shear link corrosion level. The shear strength enhancement provided by the embedded CFRP rods and EB CFRP sheets decreased from 19 and 15%, respectively, to 12 and 11%, respectively, with an increase in shear link corrosion level from 7 to 12%. Corrosion of the shear links did not have a significant effect on the beam stiffness. Premature debonding limited the effectiveness of the EB CFRP sheets whereas the embedded CFRP rods did not exhibit signs of debonding and therefore showed higher effectiveness.

This paper proposes an analytical approach for modeling the behavior of textile-reinforced mortar (TRM)-strengthened masonry-infilled reinforced-concrete (RC) frames under seismic loading. The model falls into the discrete diagonal-element type and is based on the use of single-strut and single-tie elements to represent the infill panel. It builds on the results of past experimental studies by the writers, in which the application of TRM jacketing was effective for seismic retrofitting of masonry-infilled RC frames. The model is implemented in a nonlinear finite-element code, with the parameters of the diagonal elements being determined from a series of tests on TRM coupons and masonry specimens. The results of the numerical analyses are compared with the experimental data of cyclic tests on 3-story masonry infilled RC frames (as-built and after retrofitting). The model developed in this paper adequately accounts for the TRM-strengthening contribution to the global response of masonry-infilled frames.

This paper investigates the composite action of 46 segments representing precast concrete sandwich panels (PCSPs) using a fiber-reinforced polymer [FRP; specifically, a carbon fiber–reinforced polymer (CFRP)] grid/rigid foam as a shear mechanism. The experimental aspect of the research reported in this paper examined the effect of various parameters believed to affect the shear flow strength for this CFRP grid/foam system. The parameters that were considered are the spacing between vertical lines of CFRP grids and the thickness of the rigid foam. Results of the experimental aspect of the research reported in this paper indicated that increasing the spacing between vertical lines of CFRP grid increase the overall shear flow strengths due to the increase of the bonded contact area of the rigid foam to the concrete surface. However, the overall shear stresses were decreased due to the increase of this interface surface area. Test results also indicated that increasing the rigid foam thickness decreases the overall shear flow strength when compared with the same quantity of CFRP grid spacing. A nonlinear three-dimensional (3D) FEM analysis was performed to model the behavior of the tested segments and to study the behavior of PCSPs. Results of FEM analysis were in good agreement with the experimental results. A design equation was developed to determine the shear flow strengths for the given CFRP grid/foam systems. The parametric study of the research reported in this paper was performed to predict shear flow strength of different FRP materials, rigid foam thickness, and spacing between vertical lines of the grid.

Fiber reinforced polymer (FRP) jacketing of reinforced concrete (RC) columns is now a widely accepted strengthening/retrofit technique. The extensive existing research conducted on concrete in FRP-confined RC columns has led to a large number of stress-strain models, but most of these models are applicable only to FRP-confined RC columns with limited transverse steel reinforcement (i.e., steel hoops or spirals). In many RC columns such as those designed following modern design standards, the transverse steel reinforcement may provide significant confinement which needs to be accounted for in design. This paper presents an analysis-oriented stress-strain model for concrete in such columns where the concrete is under combined confinement from the FRP jacket and the transverse steel reinforcement (i.e., FRP-steel-confined concrete). Two alternative versions of the proposed model are proposed to cater for different considerations. Both versions of the proposed model are shown to provide accurate predictions of existing test results and to be superior to the existing models of the same type.

This paper presents the fracture characteristics of cracked wide-flange steel girders (W18–W40, W460–W1000 in metric) strengthened with carbon fiber-reinforced polymer (CFRP) sheets. A broad range of parameters are considered to study their effects on the crack-tip stress intensity of the girders: flange-to-web area ratios, crack depth-to-height ratios, CFRP-to-steel area ratios, CFRP-to-steel modular ratios, and CFRP-bond widths. A three-dimensional finite-element approach is proposed and validated with a previously conducted experimental program. A total of 1,240 models, i.e., 40 unstrengthened and 1,200 strengthened girders, are used to evaluate the crack-tip behavior of the W-shape girders. The response surface-regression analysis procedures (RSREGs) are used to develop correction factors for the stress intensity of the girders. Stochastic simulations are conducted to probabilistically understand the vulnerability of the cracked girders and evaluate the efficacy of CFRP strengthening in terms of reducing stress intensity.

Fiber-reinforced polymer (FRP)–confined concrete-steel composite columns have been recently introduced as a retrofit technique for structural steel columns. This technique involves placing a two-part FRP tube around an existing steel column and subsequently filling the void between the steel section and the FRP tube with concrete to create a composite column. The composite action occurs due to the concrete encasing the steel section and the FRP confining the concrete. Within this focus, an experimental investigation has been undertaken with the objectives of evaluating the effect of adding confined concrete to encase a steel W section, the effect of using a split-tube system as a practical encasement technique, and the effect of adding shrinkage-reducing admixtures (SRA) to the confined concrete. A total of 18 stub columns comprising six different column types were tested to achieve these objectives. The test results showed that the load-displacement response of the composite columns was similar to that of confined concrete combined with the elastic–perfectly plastic steel contribution. The experimental findings also demonstrated the enhanced composite column performance, including a significant increase in the ultimate capacity using the proposed split-tube retrofit technique. The results also indicated that the use of a SRA in confined concrete has an indistinguishable effect on the confined concrete strength.

As part of a seismic retrofit scheme, surface bonded glass fiber-reinforced polymer (GFRP) fabric was applied to two unreinforced masonry (URM) buildings located in Christchurch, New Zealand. The unreinforced stone masonry of Christchurch Girls’ High School (GHS) and the unreinforced clay brick masonry Shirley Community Centre were retrofitted using surface bonded GFRP in 2007 and 2009, respectively. Much of the knowledge on the seismic performance of GFRP retrofitted URM was previously assimilated from laboratory-based experimental studies with controlled environments and loading schemes. The 2010/2011 Canterbury earthquake sequence provided a rare opportunity to evaluate the GFRP retrofit applied to two vintage URM buildings and to document its performance when subjected to actual design-level earthquake-induced shaking. Both GFRP retrofits were found to be successful in preserving architectural features within the buildings as well as maintaining the structural integrity of the URM walls. Successful seismic performance was based on comparisons made between the GFRP retrofitted GHS building and the adjacent nonretrofitted Boys’ High School building, as well as on a comparison between the GFRP retrofitted and nonretrofitted walls of the Shirley Community Centre building. Based on detailed postearthquake observations and investigations, the GFRP retrofitted URM walls in the subject buildings exhibited negligible to minor levels of damage without delamination, whereas significant damage was observed in comparable nonretrofitted URM walls.

The ACI and CSA design codes offer no unified method for evaluating the deformability of fiber-reinforced polymer (FRP) reinforced-concrete (RC) structures, although numerous experimental results for such elements are available. This study discusses the methods for quantifying the deformability in FRP-RC structures. The methods were assessed based on the experimental results of four full-scale RC shear walls: three reinforced with FRP bars and one with steel bars. The objective was to contribute to the development of a method for verifying the deformability of FRP-RC structures. The curvature ductility index (${\mu }_{\varphi }$) and deformability index ($J$) were found to be the most appropriate methods when calculated at a concrete strain equal to 0.0035.