Journal of Composites for Construction

This paper presents an analytical study to investigate the ability of low-modulus ($<150\mathrm{GPa}$) carbon fiber–reinforced polymer (CFRP) laminates to strengthen steel beams. Although the need for retrofitting structural elements is high in South Asian countries, the nonavailability of high-modulus CFRP led to the use of low-modulus CFRP in this investigation. The aim of this study was twofold: (1) to understand the behavior of structural members strengthened with low-modulus CFRP; and (2) to suggest an appropriate limit state to predict the design strength of the CFRP-strengthened member. Four-point bending experiments were carried out to examine the behavior of CFRP-strengthened steel beams. In general, the governing failure mode of steel beams strengthened with low-modulus CFRP was intermediate debonding. The occurrence of intermediate debonding was due to the large difference in moduli of steel and low-modulus CFRP. The design predictions indicated that the use of the ultimate strain design limit state suggested by current design provisions is unconservative compared with the experimental results. Therefore, based on the failure mode observed in the experiments, two new design limit states were introduced to determine the ultimate design strain; the first is conservative, up to the elastic strain of steel beam (elastic strain limit state), and the other is up to transition point of low-modulus CFRP (transition strain limit state). Such conservative design limit states are suggested to prevent the debonding failure between CFRP and steel beams. The design and reliability analysis results indicated that the newly introduced limit states (elastic strain and transition strain) are suitable for the design of steel beams strengthened with low modulus CFRP.

An important advantage of the near surface mounted (NSM) technique for concrete beam strengthening over the externally bonded reinforcement technique is its higher resistance to debonding failures. Despite the increased resistance, debonding failures do still occur on NSM strengthened beams. The aim of this paper is to investigate the possibility of using a global energy balance based fracture mechanics model for NSM strengthened beams. The methodology includes determining the available energy for the propagation of the interface flaw. Debonding failure occurs when the available energy for interface flaw propagation reaches the Mode I fracture energy of the concrete, as this is the weakest material of the concrete-fiber reinforced plastic (FRP) composite system. The validation using published experimental results demonstrates that the model is capable of predicting possible modes of debonding failure for NSM FRP strengthened reinforced concrete beams and for any material and geometric properties of concrete beams, adhesive, and FRP. Validation against published experimental results yields a satisfactory performance of the model. The mean ratio between simulated to experimental failure loads for NSM strengthened beams is 0.99 with a standard deviation of 0.09.

Hollow concrete columns (HCCs) are one of the preferred construction systems for bridge piers, piles, and poles because they require less material and have a high strength-to-weight ratio. While spiral spacing and concrete compressive strength are two critical design parameters that control HCC behavior, the deterioration of steel reinforcement is becoming an issue for HCCs. This study explored the use of glass fiber-reinforced polymer (GFRP) bars as longitudinal and lateral reinforcement for hollow concrete columns and investigated the effect of various spiral spacing and different concrete compressive strengths ($f_c^{\prime }$). Seven HCCs with inner and outer diameters of 90 and 250 mm, respectively, and reinforced with six longitudinal GFRP bars, were prepared and tested. The spiral spacing was no spirals, 50, 100, and 150 mm; the $f_c^{\prime }$ varied from 21 to 44 MPa. Test results show that reducing the spiral spacing resulted in increased HCC uniaxial compression capacity, ductility, and confined strength due to the high lateral confining efficiency. Increasing $f_c^{\prime }$, on the other hand, increased the axial-load capacity but reduced the ductility and confinement efficiency due to the brittle behavior of high compressive-strength concrete. The analytical models considering the axial load contribution of the GFRP bars and the confined concrete core accurately predicted the behavior of the HCCs after the spalling of the concrete cover or at the post-loading behavior.

The results of an experimental investigation into the behavior of fiber-reinforced cementitious matrix (FRCM) composites made of basalt textile coupled to different inorganic matrices are presented and compared. Single- and double-lap shear tests have been carried out to assess the bond behavior of the FRCM systems adopting four different inorganic matrices: a commercial lime mortar, specifically designed for the basalt textile, and three conventional mortars mixed in the laboratory. Experimental tests on the four types of FRCM systems confirmed that the debonding mechanism always occurs at the textile-matrix interface, and single-lap shear test, easier to be carried out, provides results with low variability. Moreover, in the framework of Mode-II fracture mechanics, analytical modeling was carried out exploiting four different local laws that were calibrated to the experimental results. Good agreement was found between some of the adopted local laws and experimental tests.

The sensitivity of the joint performance of pultruded fiber reinforced polymer (FRP) to elevated temperatures has been known to limit the widespread application of FRP composites in civil construction. This paper presents a study of the effect of elevated temperatures and mechanical inserts on the joint strength and failure mechanism of a square hollow section (SHS) of pultruded glass FRP (GFRP) composites. Three pultruded GFRP bolted joint configurations were implemented— namely, a joint without mechanical insert (N), a joint with mechanical insert with tight-fit attachment (I), and a joint with mechanical insert bonded with epoxy adhesive (G). Sixty (60) square pultruded GFRPs with a single all-threaded bolt connection each were tested up to failure at room temperature 40°C, 60°C, and 80°C. Specimen G exhibited the highest joint strength, with twice more than that of Specimen N across the temperature range. Specimens N and I failed by shear-out at high temperatures mainly due to the deterioration of the interfacial bond between the fibers and the matrix. The strength reduction factor, $k$, and modification factor, $m$, which incorporate the increasing temperatures and the designed joint configurations showed excellent agreement when compared to the experimental results.

The shear strength of reinforced concrete (RC) elements can be improved by applying externally bonded (EB) fiber-reinforced cementitious matrix (FRCM) composites. FRCM composites are generally U-wrapped around the cross-section of RC beams and completely wrapped around the cross-section of RC columns. When the U-wrapped layout is employed, composite debonding usually occurs before the tensile strength of the composite can be attained. However, depending on the specific FRCM adopted, different failure modes can be observed. Although the use of FRCM composites to strengthen existing RC members is gaining popularity, limited work has been done to formulate a reliable design procedure for FRCM shear strengthening of RC members. In this paper, a model is proposed to compute the shear strength contribution of FRCM composite U-wrapped around RC members. The model, which is an extension of the model used for fiber-reinforced polymer (FRP) shear strengthened beams, is based on mechanical considerations, does not contain empirical coefficients, and can be applied to any FRCM composite provided that the bond behavior and tensile strength are identified. The model is validated by comparing the analytical predictions with the experimental results of RC beams strengthened in shear with polyparaphenylene benzobisoxazole (PBO) and carbon FRCM composites found in the literature. Finally, an example of the evaluation of the shear capacity associated with the FRCM reinforcement is provided to illustrate the use of the model.

This study presents an experimental investigation of the effect of environmental exposure on the fiber-reinforced polymer (FRP) to clay brick masonry bond when the reinforcement is near-surface-mounted. Three alternate types of exposure are considered: (1) thermal loading with single or multiple thermal exposure cycles, (2) water immersion, and (3) hygrothermal cycling. The latter two treatments were used to condition the samples over a long-term period with testing undertaken after 3, 6, 15, and 24 months from initial exposure. Since previous tests on externally bonded FRP have shown that environmental conditioning can often cause failure in the adhesive, both an epoxy and cementitious adhesive are investigated. The results of the study indicate that although environmental conditioning caused some degradation in the mechanical properties of the various constituents, the strength of the FRP-to-masonry bond remained highly durable regardless of the adhesive used.

This paper presents results from tests on seven reinforced concrete square columns under simulated seismic loading and demonstrates their satisfactory behavior even when axial load is high. All seven specimens were confined by glass fiber–reinforced polymer reinforcement (GFRP) rectilinear ties, while five specimens were reinforced with steel longitudinal bars and two with GFRP longitudinal bars. The results showed that irrespective of the longitudinal bar type, GFRP confinement effectively improved the seismic resistance of columns. The difference in the type of the longitudinal bars resulted in significant changes in column behavior. A comparison was made between these test specimens and earlier circular columns from literature, and the confinement effectiveness of the GFRP rectilinear ties and circular spirals was evaluated. Lateral GFRP was found to have a greater influence on the flexural strength enhancement of columns with GFRP longitudinal bars versus steel longitudinal bars. Results indicate that available methods for predicting flexural strength enhancement due to confinement invariably miscalculated the true strength enhancement by a substantial margin, particularly when GFRP longitudinal bars are used. A new model for predicting the flexural strength enhancement of square and circular columns confined by lateral GFRP is proposed.

This paper provides a framework for the calibration of partial safety factors in prestressed concrete (PC) girders strengthened in flexure with carbon fiber–reinforced polymer (CFRP) laminates. A hybrid approach was proposed to take advantage of comprehensive nonlinear numerical models in reliability analysis using a first-order reliability method (FORM) in conjunction with the response surface method (RSM). The PC girders selected for analyses were taken from real structures designed and built in the 1980s based on old standards that now require strengthening and upgrade due to partial corrosion of the prestressing strands. Using the proposed approach, a sensitivity analysis was performed to identify the most relevant variables and assess the area of CFRP laminates needed to restore capacity up to new design standards. A partial safety factor was proposed for strengthening PC girders using CFRP laminates. Sensitivity analysis showed that traffic loads and model uncertainties are the most important variables for calibration.

Fabric-reinforced cementitious matrix (FRCM) composites consist of a dry fiber fabric embedded in an inorganic mortar that may be enriched with short fibers. These composites are particularly well-suited for the strengthening of historical buildings due to their high compatibility with the substrate, vapor permeability, and durability. One of the most critical factors influencing the effectiveness of a composite applied externally to masonry or concrete structures is debonding of the system from the substrate. In FRCM systems, the failure is often localized at the mortar–fabric interface. This paper presents a summary of experimental investigations of the bond properties of six different FRCM systems in various configurations of bond length and substrate material. Finally, some considerations and indications for inclusion of the test procedure in guidelines and acceptance criteria are presented.

To analyze the seismic performance of reinforced concrete columns strengthened with textile-reinforced concrete (TRC) and subjected to a chloride environment, ten TRC-strengthened columns were prepared, and cyclic loading tests were carried out after exposing the columns to chloride dry–wet cycles. The effect of the number of dry–wet cycles, axial compression ratio, shear span ratio, and stirrup spacing was studied. The results showed that chloride dry–wet cycles had little effect on the seismic performance of the TRC-strengthened columns and that a certain number of dry–wet cycles (not more than 180) may be beneficial to seismic performance. With an increase in the axial compression ratio, the stiffness of the strengthened columns degraded rapidly, and the energy dissipation and deformation capacity also decreased. Strengthened columns with higher shear span ratios performed better in terms of ductility and cumulative energy consumption than those with lower shear span ratios. However, TRC effectively improved the deformation capacity of strengthened columns with low shear span ratios. The higher the stirrup ratio was, the better the behavior of the strengthened columns was in terms of ductility, deformability, and energy dissipation capacity. Therefore, the seismic performance of RC columns in a chloride environment can be improved by strengthening with TRC.