Journal of Composites for Construction

This paper presents the results of an experimental study on the use of carbon fiber–reinforced polymer sheets as reinforcement for the rotational behavior of bolted glulam beam-to-column connections. Monotonic loading was applied on connection specimens to identify their stiffness, ductility, and maximum moment resistance values. Material property tests were conducted and used to provide an unbiased judgment of the reinforcing efficiency considering varied specimen qualities. Different sheet wrapping patterns and layers of sheets were examined, and the results were compared with each other and results from previous studies. It was found that carbon fiber–reinforced polymer sheets could improve the moment capacity of unreinforced connections by up to 59%, which is very close to the reinforcing ratio (65%) achieved by using locally cross-laminated glulam members. A linear relationship was found between the increased perpendicular-to-grain glulam tensile strength and the increased moment resistance by use of the reinforcing materials, and the moment resistance increase corresponding to a unit glulam tensile strength increase, designated as the reinforcing efficiency, from the internally wrapped carbon fiber–reinforced polymer sheets was higher than the other methods studied. The results of this study can be used as reference for reinforcement of existing glulam timber frame structures.

This paper presents the results of an experimental study on the compressive behavior of circular and noncircular concrete columns confined by polyethylene terephthalate (PET) fiber-reinforced polymer (FRP). The main objective of this study is to observe the compressive behavior of PET FRP–confined noncircular concrete columns, in which the cross-sectional geometry significantly influences the effectiveness of FRP confinement. For this purpose, 54 specimens were tested under monotonic axial compression. The parameters considered were the cross-sectional shape (i.e., circular, square, and rectangular), the corner radius in case of square and rectangular specimens, and the number of FRP layers. The results of this study showed that, unlike the bilinear stress-strain response of square and rectangular concrete columns confined by other FRP materials, PET FRP–confined square and rectangular concrete columns exhibit a distinct trilinear relationship. Owing to the large rupture strain (LRS) characteristics of PET FRP, all the confined specimens exhibit highly ductile behavior, and the knife action of sharp corners is efficiently resisted in noncircular columns. The results also indicate that the effectiveness of PET FRP confinement decreases with an increase in sectional aspect ratio and increases with an increase in corner radius. In addition, to evaluate the applicability of existing strength and strain models developed for other FRPs to PET FRP–confined concrete, the experimental results are compared with the predictions of five existing models. The results indicate that the average compressive strength of PET FRP–confined circular, square, and rectangular concrete columns is predicted well, but the average ultimate axial strain is highly underestimated by all the five models.

Recently, basalt fiber–reinforced polymer (BFRP) reinforcement emerged as a new FRP type. Research is needed, however, to understand the behavior of BFRP bars in concrete members. This paper presents an experimental study aimed at investigating the flexural behavior and serviceability performance of concrete beams reinforced with ribbed BFRP bars. A total of eight concrete beams measuring 3,100 mm in $\mathrm{length}\times 200\mathrm{mm}$ in $\mathrm{width}\times 300\mathrm{mm}$ in depth were constructed and tested up to failure. Six beams were reinforced with 8-, 12-, and 16-mm BFRP bars with ribbed surfaces and two reference beams were reinforced with 10M and 15M steel bars. The beam specimens were designed in accordance with Canadian standards, and tested under four-point bending over a clear span of 2,700 mm until failure. The test results are presented and discussed in terms of cracking behavior, deflection, and failure modes. The test results yielded an average bond-dependent coefficient ($k_b$) of 0.83, which is lower than 1.0 recommended by Canadian standards for ribbed FRP bars. Moreover, comparing the test results to the design provisions showed that American Concrete Institute (ACI) Committee 440 Guide underestimated the deflections at service load for the beams reinforced with ribbed BFRP bars, while Canadian standards for design and construction of building structures with FRP yielded reasonable and conservative deflections.

An experimental investigation on initially strengthened and repaired 2-span continuous RC beams with carbon fiber–reinforced polymer (CFRP) strips was conducted. Fourteen full-scale beam specimens were tested using a four-point bending test with a shear span to effective depth ratio $(a_v/d)$ of 2.5:3.5. This study presents and discusses several influences on the behavior of strengthened and repaired beams and shear strength prediction: the influence of shear span to effective depth ratio, CFRP contribution to shear, CFRP wrapping schemes, number of layers of CFRP strips, and CFRP orientation angle. At failure, the CFRP strips were observed to rupture and debond along the critical shear crack line of the beam. The shear strength prediction was adopted from the design provisions of the American Concrete Institute (ACI) and was compared with the experimental results. The ACI code model was observed to have underestimated the experimental results from 32 to 44% for Beam Group 1 $(a_v/d=2.5)$, 34 to 44% for Beam Group 2 $(a_v/d=2.5)$, and 14 to 18% for Beam Group 3 $(a_v/d=3.5)$. The findings verify that the ACI code model gave an acceptable conservative prediction of the shear strength capacity of an initially strengthened or repaired 2-span continuous RC beam.

As traffic loads increase and bridge infrastructure ages, it is becoming increasingly necessary to strengthen existing bridge members. Externally applied carbon fiber–reinforced polymer (CFRP) materials are often used to strengthen reinforced concrete structures. Four full-scale I-girder tests showed differences in the shear-strength contributions of anchored CFRP strips when they are applied in unidirectional U-wrap applications as opposed to bidirectional applications. Because the number of tests was so limited, a series of panel tests was developed to attempt to explain the role of unidirectional and bidirectional CFRP layouts. Concrete panels were constructed and tested under compressive forces generating bottle-shaped compression struts to evaluate effectiveness of CFRP strengthening. Although the results cannot be directly compared with the conditions in the web of an I-girder, they allowed consideration of a number of parameters that could not have been studied by using expensive full-scale I-girders. The effects of the inclination of CFRP strips relative to the direction of cracking, the layout of CFRP strips, and the amount of CFRP strip and anchor were studied through the panel tests. To prevent premature debonding of CFRP strips from the panel surface, CFRP anchors were used. The tests highlighted the most critical parameters that influence the effectiveness of CFRP materials in strengthening applications, especially for bidirectional CFRP layouts. Qualitative comparisons were presented on the basis of the panel tests. Bidirectional CFRP layout showed better crack control and strength gain than unidirectionally strengthened panels before unexpected CFRP anchor failure.

A method of improving the capacity of reinforced concrete T-beams using profiles made of hybrid basalt-carbon fiber sheet–reinforced polymer (HFRP) bonded to concrete using three different methods was researched. Various basalt-carbon ratios were considered, along with three types of different bonding interfaces. The flexural performance of the HFRP-RC (reinforced concrete) composite T-beams with different interfaces was studied by both experimental and theoretical analysis methods, the main areas of focus including the feasibility of the interface type, failure mode of these beams, load capacity, flexural stiffness, and ductility, among others. Three types of interface were studied by both the double-lap shear test and the four-point bending test of beam specimens. Good bond behavior can be achieved with the three methods and the additional anchorage system. The hybrid effect of hybrid fiber sheets made of basalt fiber (BFRP) and carbon fiber (CFRP) was studied both by a tensile test of hybrid fiber sheets and by a four-point bending test. A constitutive relationship of hybrid fiber sheets was put forward in which an improvement factor of the CFRP ultimate strain value was introduced according to the result of the material and checked by the beam test results. Meanwhile, the HFRP-RC beams were found to have an obvious postyield stiffness, which can control the structure deformation after the yield of steel bars. The postyield stiffness formula for the HFRP-RC beams is given, and its precision was checked by using both the test results and the literature results. Finally, the ductility of these beams was studied and results show that these kinds of beams can have adequate energy ductility.

Pull-out experiments were performed on four carbon fiber–reinforced polymer (CFRP) ground anchors simulating their applications in rock and soil. The CFRP tendons used in these anchors comprise a single-strap end on both ground and air sides. On the ground side, the single-strap end is embedded in a prefabricated cylinder of high-strength grout that fits into the borehole. In rock applications, the ground-side anchor can be used without additional confinement, while in the case of soil, an additional CFRP confinement ring is needed to prevent premature grout failure in the strap region. The load transfer from the strap to the grout cylinder occurs progressively; approximately 50% of the load is transferred at the semicircular strap end, while the remaining 50% is transferred at the curved transition from the strap to the rod segment of the free length.

This paper aims to analyze the results of compression tests conducted on fabric-reinforced cementitious matrix (FRCM)–confined concrete elements. The analysis is developed using a database of 152 compression tests on FRCM-wrapped, plain concrete cylindrical specimens assembled from the literature. The database is first used to analyze the influence of the relevant mechanical and geometric parameters on the structural response of the confined specimens. Then, empirical design–oriented models to predict the peak strength and the corresponding strain of the FRCM-wrapped specimens are proposed and verified with the experimental database through a best-fit analysis of the collected data. The performance of the proposed models is also compared to that of some existing models (which have been proposed in both the literature and codes) by using the database.

Glass fiber–reinforced polymer (GFRP) pultruded profiles are prone to web buckling and/or crushing when subjected to concentrated loads in the direction transverse to the pultrusion axis due to their low elastic and strength properties. Based on a recent work in which it was concluded that the Tsai-Hill criterion does not succeed in providing reasonable estimates of the web-crippling capacity of GFRP profiles, in the present work another progressive damage model is implemented into a finite-element (FE) model for web-crippling analysis. First, previous web crippling experiments on I-section GFRP profiles are summarized. Then the basis of the FE model is presented (element types, failure initiation criteria, and damage model) and results of preliminary analyses (mesh size and viscous regularization) are discussed. Subsequently, the load versus displacement curves, damage zones, and failure modes of GFRP profiles under different load configurations and bearing lengths are presented. Finally, the model sensitivity to different parameters (transverse compressive strength, in-plane shear strength, matrix compressive fracture energy, modeling of web-flange rounded corner) is analyzed. The proposed model is shown to be much more accurate than those based on Tsai-Hill criterion. It is also shown that the in-plane shear strength governs failure initiation, while the transverse compressive strength is more influential to the profiles’ ultimate behavior. This study also highlights the major impact of fracture energy in the behavioral response of GFRP profiles subjected to transverse concentrated loads.

This paper presents the behavior of multistory buildings with fiber-reinforced polymer (FRP) brace members for new construction and for retrofit applications, including a comparative assessment against conventional steel brace systems. The El Centro earthquake load with a magnitude of 7.1 is employed to examine the performance of three brace types (hollow structural steel sections, and pultruded carbon FRP (CFRP) and glass FRP (GFRP) tubes for new buildings; and CFRP-confined and aramid FRP (AFRP)-confined concrete elements for existing buildings: seismic strengthening with FRP wrapping) and three shapes frequently used in practice (single-bay X-brace, super X-brace, and chevron brace). Nonlinear pushover and time-history models are developed to predict the seismic response of the retrofitted buildings with an emphasis on base shear force, ductility, mode shapes and frequencies, and strain energy. The brace materials and confinement schemes of the buildings control the effectiveness of the seismic-retrofit rather than the brace shapes. The FRP-braced buildings show strain energy characteristics comparable with those of the steel-braced buildings, although the ductility index of the latter is higher than the index of the former. The FRP-based braces reduce the maximum kinetic energy of the buildings up to 64% in comparison with the steel braces. However, the $P$-delta effect is a concern when pultruded FRP tubes are used because of their low elastic modulus. The FRP-confined concrete braces are effective in terms of lowering the dynamic amplification of the buildings relative to their steel and pultruded FRP counterparts.

Redesigning buildings to improve their space efficiency and allow changes in use is often essential during their service lives to comply with shifts in living standards and functional demands. This may require the introduction of new openings in elements such as beams, walls, and slabs, which inevitably reduces their structural performance and hence requires repair or strengthening. However, there are uncertainties regarding both the effects of openings and the best remedial options for them. Here, the authors report on an experimental investigation of the effectiveness of fiber-reinforced polymer (FRP)–based strengthening for restoring the axial capacity of a solid RC wall after cutting openings. Nine half-scale specimens, designed to represent typical wall panels in residential buildings with and without door-type openings, were tested to failure. It was found that FRP-confinement and mechanical anchorages increased the axial capacity of walls with small and large openings (which had 25 and 50% reductions in cross-sectional area, respectively) by 34–50% and 13–27%, to 85–94.8% and 56.5–63.4% of their precutting capacity, respectively.

This paper presents a reliability-based methodology for calibration of the fiber-reinforced polymer (FRP) resistance reduction factor used in the design of FRP-confined RC columns designed based on current guidelines. The Monte Carlo simulation technique was employed to generate a large population of design cases based on frequency of the design variables. Within the code calibration procedure, a comprehensive probabilistic study on variability of resistance, i.e., compressive strength of FRP-confined RC columns considering uncertainties in material properties, section dimension, and the model error, was carried out. It was shown that statistics of resistance and consequently reliability of the ultimate limit state are highly dependent on the variability of confinement model and the FRP strain efficiency factor. Furthermore, for RC columns with spiral confinement, the current FRP resistance reduction factor advised by current guidelines should be reduced. Sensitivity of the FRP resistance reduction factor to some selected target reliability indexes was also studied. In addition, a confinement-dependent formula for determining the FRP resistance reduction factor was proposed.

An experimental study was conducted to investigate the fatigue behavior of RC beams strengthened with posttensioned prestressed carbon fiber-reinforced polymer (CFRP) sheets. The experimental program consisted of nine rectangular-section simply supported RC beams: four beams were statically tested to failure to determine the values of the fatigue loads to apply, and the remaining five beams were tested under fatigue load. The main purpose of the fatigue tests was to gain a better understanding of the fatigue performance and failure modes of RC beams strengthened with posttensioned prestressed CFRP sheets. The experimental results indicated that the fatigue failure mode of the prestressed CFRP sheet-strengthened RC beams was the tensile steel reinforcement rupture at the main cracked section. Moreover, the fatigue performance of the prestressed CFRP sheet-strengthened RC beams was significantly better than that of both unstrengthened and nonprestressed CFRP sheet-strengthened beams. Finally, a fatigue life prediction model that considers the gradual deterioration of performance of the component materials and partial debonding of the FRP was presented and applied to predict the fatigue life of 28 tested beams with two extreme FRP-to-concrete interfacial states. The results showed that the predicted fatigue life was close to the experimentally measured fatigue life for the fully bonded state. Thus, the effectiveness of the proposed model was verified, and the effect of fatigue load-induced FRP debonding along the beam substrate on fatigue life prediction was found to be insignificant.

This study presents the results of an experimental program and analytical modeling for axial-flexural interactions of concrete-filled glass fiber–reinforced polymer tube (GFRP-CFFT) columns with and without reinforcing glass fiber–reinforced polymer (GFRP) bars. The experimental program included four steel RC specimens, four GFRP-CFFT specimens, and four GFRP bar–reinforced GFRP-CFFT specimens with an outer diameter of 205–206 mm and a height of 800–812 mm. The specimens were tested under concentric and 25- and 50-mm eccentric axial loads and four-point load. The experimental results showed that GFRP bar–reinforced GFRP-CFFT specimens sustained higher peak axial loads, axial and lateral deformations at peak axial load, and flexural loads than GFRP-CFFT specimens without reinforcing GFRP bars and steel RC specimens. Axial load and bending moment ($P\text{-}M$) interactions of GFRP-CFFT specimens with and without reinforcing GFRP bars and steel RC specimens were analytically modeled. A parametric study was conducted to evaluate the effects of actual confinement ratio and GFRP bar reinforcement ratio on $P\text{-}M$ interactions of GFRP-CFFT specimens. The $P\text{-}M$ interactions were found to be significantly affected by both actual confinement ratio and GFRP bar reinforcement ratio.

Existing unreinforced masonry buildings in seismically active regions are in urgent need of consolidation and preservation against seismic action to prevent damage and loss of financial resources. In this research, an experimental study of externally confined brick masonry piers, which are frequently preferred as load-bearing elements in historical buildings, was conducted. The confinement system included a combination of open-grid basalt textile and mortar. Eighteen masonry pier specimens were produced using solid bricks collected from a historical building constructed in approximately the 1930s and a local mortar with substandard mechanical characteristics to simulate mortar properties in existing heritage buildings. All the square/rectangular pier specimens were tested under concentric compressive loads. In general, confinement of the tested textile-reinforced mortar (TRM) improved the energy dissipation of the masonry piers significantly. A comparison was made between the experimental results and theoretical predictions using the available analytical models. The compressive strengths predicted by the models are satisfactory.

Traditional methods used to repair prestressed concrete (PC) girders subjected to impact damage due to overheight vehicles include strand splicing and external posttensioning. These methods have proven to be partially satisfactory in restoring the damaged girder’s ultimate strength. The use of composite materials in the strengthening, rehabilitation, and repair industry has been gaining popularity due to their excellent material behavior and ease of application. Composites such as fiber-reinforced polymer (FRP) and fabric-reinforced cementitious matrix (FRCM) systems are presently available alternatives for restoring the integrity of a damaged girder that eliminate the need for girder replacement. The use of externally bonded carbon FRP (CFRP) materials for the repair of damaged bridge components has been successfully deployed and has been shown to reduce repair costs and traffic closure time. FRCM composites represent an alternative material to FRP for structural retrofit/rehabilitation. However, the use of FRCMs is rather new and technical literature is limited in the United States and does not include design provisions for the repair of PC structures. The objective of this study is to evaluate both experimentally and analytically the effectiveness of FRP and FRCM as repair systems to impact-damaged precast PC girders. The specimens consisted of three PC girders retrieved from a bridge after 55 years of service. One specimen was load tested undamaged and used as a benchmark. The other two were damaged (four cut strands) and repaired with FRP and FRCM prior to being load tested. A cross-sectional analysis was performed to predict strength and behavior of all specimens using provisions of current design specifications and guides. Theoretical values were compared to experimental values and the effectiveness of the strengthening methods and respective design approaches were evaluated.

The dual-tube self-centering buckling-restrained brace (SC-BRB) with pretensioned basalt fiber-reinforced polymer (BFRP) composite tendons is a novel bracing system that consists of an energy-dissipative BRB and self-centering system that is used to overcome the residual drift. In this study, a finite-element model of the dual-tube SC-BRB is constructed to analyze its hysteretic characteristics. By analyzing and comparing the model of the BRB and the self-centering system, respectively, the accuracy of the model parameters, such as material, contact, and constraint, was verified. The tube length tolerance was introduced to the model of the SC-BRB, which effectively reduced the initial stiffness of the brace and corresponded with the test results. Based on the finite-element model of the SC-BRB, the influence of four parameters, including the material of the pretension tendons, initial pretension force, area of the pretension tendons, and core plates, on the hysteretic performance of the brace is discussed. This paper improved the configuration of the SC-BRB by adding a rubber cushion between the end plate and tubes. The finite-element analysis results indicated that the rubber cushion could not only decrease the initial stiffness of the brace but also reduce the influence of the tube length tolerance on the initial stiffness of the brace.

This study was conducted to investigate the durability of two types of vinyl-ester/glass fiber-reinforced polymer (GFRP) rock bolts [solid and hollow (tubular) GFRP bars] that were subsequently used for ground control of the Jurong rock caverns in Singapore. The testing program aimed to assess the long-term durability characteristics of unstressed hollow and solid GFRP rock bolts subjected to a saline solution (simulating the subsea cavern water of the Jurong rock caverns in Singapore) and predicting their long-term tensile strength in such environments. The study simulated rather aggressive conditions by immersing the hollow and solid GFRP bars (rock bolts) in the solution at different elevated temperatures (20, 40, and 50°C) in climate chambers for different exposure times of 1,000, 3,000, and 5,000 h. Arrhenius plots were applied to the test results, extrapolating the project’s life to 50 years at a mean annual temperature (MAT) of 32°C. The predicted tensile strength at a MAT of 32°C is 96.8 and 97.3% for a service life of 50 years for solid and hollow GFRP bars, respectively. Based on the obtained results, the tested GFRP rock bolts were recommended as alternatives to stainless-steel rock bolts and successfully used as ground control in the Jurong rock caverns in Singapore.