Journal of Composites for Construction

Textile reinforced mortars (TRM), also known in the international literature as textile reinforced concrete (TRC) or fabric reinforced cementitious matrix (FRCM) materials, have been widely studied during the last two decades as they constitute a promising alternative to the fiber reinforced polymer (FRP) retrofitting solution for strengthening of reinforced concrete members. This paper presents a state-of-the-art review on the strengthening of concrete structures with TRM. First, the tensile and bond behavior of TRM is described. Next, an overview of studies on the use of TRM for flexural, shear, confinement, and seismic retrofitting of concrete or RC members is included, and the key parameters are investigated.

This paper presents the details of a reliability analysis of the debonding in steel beams strengthened with externally-bonded carbon fiber-reinforced polymer (CFRP) composites. A comprehensive experimental database of CFRP-strengthened steel beams and the corresponding double-lap shear joints was compiled. The experimental database was used to quantify the modeling uncertainty for debonding failures based on the analytical formulations that are most commonly adopted in design guidelines. A Monte Carlo simulation (MCS), was used to calculate the resistance factors to achieve a target reliability index of 3.5, for different design scenarios, considering debonding of the FRP as a limit state that would constitute member failure. A sensitivity study was carried out to assess the influence of individual design variables on the reliability index. It was found that the resistance factors vary with adhesives and surface preparation techniques, and the resistance of the representative specimens and modeling uncertainty play the most important roles in achieving a reliable bond.

Adhesives are fundamental materials in reinforced concrete (RC) members. Depending on the type of adhesive, the fire resistance of these members will vary. However, previous studies have focused on organic adhesives, although inorganic adhesive and hybrid organic-inorganic configurations may provide certain advantages. In the research presented in this paper, the mechanical properties of carbon fiber-reinforced polymer (CFRP) bars with an inorganic mortar cladding layer were studied experimentally. Then, fire tests were conducted on near-surface-mounted (NSM) CFRP-strengthened RC beams. The results indicated that the bars still exhibited satisfactory mechanical properties after high-temperature exposure due to the protection of the inorganic mortar cladding layer. The fire resistance of RC beams filled with fiber polymer mortar was better than that of RC beams filled with modified acrylic emulsion mortar. The hybrid organic-inorganic configurations in the NSM FRP-strengthened RC structures were feasible. Considerable fire resistance may be achieved without applying fire protection along the entire length of the beam.

To overcome the drawbacks of bond-type anchors for carbon fiber–reinforced polymer (CFRP) cables, an innovative bonded anchor with steel wedges at the free end is developed in this study. The new anchor features high compressive stresses in the free end zone and relatively low radial stresses in the loaded end zone. Theoretical methods for assessing the carrying capacity and mechanical behaviors of this new bond-type anchor are established after analyzing the working mechanism of the new anchor. Furthermore, an optimal design method for the new anchor is developed. The optimal design and mechanical behavior for different cable specifications of the new anchorage system are analyzed, and the optimal design parameters are provided. Experimental studies were conducted on the static and fatigue properties of the new anchor, which is designed with 19 CFRP wires. The results show that the cone angle of the barrel has a significant effect on the mechanical behaviors of the anchor. The theoretical results match the test results well. The experiments show that the new anchor with 19 CFRP wires reached a failure load of up to 97% of the CFRP cable’s total capacity, and it is capable of resisting bear over 2 million load cycles with a stress amplitude of 161 MPa. In addition, the static load-carrying capacity of the new anchor increases after the fatigue test.

This study aims to investigate the behavior of slender RC columns retrofitted with fiber-reinforced polymer (FRP) composites by means of the grooving method (GM) in an attempt to lessen the slenderness effects observed in other retrofitting methods. For this purpose, nine circular concrete columns with slenderness ratios ranging from 15.4 to 27.7 were tested under constant eccentric loading. Six of the specimens were strengthened with longitudinal FRP composites by either externally bonded reinforcement (EBR) or externally bonded reinforcement on grooves (EBROG) methods. Results showed that with increasing slenderness, load-carrying capacity decreases. However, compared with the EBR and control columns, those strengthened through the EBROG method exhibited lower reductions; this was evidenced by the 12.4%, 12.9%, and 4.2% reduced load-carrying capacities in the control, EBR, and EBROG columns with increasing slenderness ratio. In addition, the EBROG method was observed to enhance the flexural stiffness of the columns and decrease the secondary moments compared with EBR and reference columns. Ductility was also observed to improve as a result of applying the EBROG method to strengthen the columns in the longitudinal direction compared with the EBR and control columns. Also, a semianalytical approach was used to evaluate the theoretical procedure for prediction of experimental results.

This paper investigates the flexural behavior of engineered cementitious composite (ECC)–concrete hybrid composite beams reinforced with fiber-reinforced polymer (FRP) bars and steel bars. Thirty-two hybrid reinforced composite beams with various ECC height replacement ratios and combinations of FRP and steel reinforcements are experimentally tested to failure in flexure. Test results show that cracking, yield, ultimate moments, and the stiffness of hybrid and ECC beams are improved compared with traditional concrete beams with the same reinforcement, due to the excellent tensile properties of ECC materials. The average crack spacing and width decrease with the increase of ECC height replacement ratio. The ductility of hybrid reinforced composite beams is higher than that of traditional RC beams, whereas their practical reinforcement ratios are similar. Reinforced ECC beams show considerable energy dissipation capacity due to the ECC’s excellent deformation ability. Considering the constitutive models of materials, compatibility and equilibrium conditions and formulas for the prediction of cracking, yield, and ultimate moments as well as deflections of hybrid reinforced ECC–concrete composite beams are developed. The proposed formulas are in good agreement with the experimental results. A comprehensive parametric analysis is conducted to illustrate the effect of reinforcement and ECC and concrete properties on the moment capacity, curvature, ductility, and energy dissipation of composite beams.

This investigation defines the problem and proposes guidelines for seismically upgrading masonry buildings using textile-reinforced mortar (TRM). The TRM technique in the form of externally applied jackets is appropriate for protecting masonry structures because it can provide sufficient strength and deformation capacity while satisfying the compatibility, reversibility, and durability requirements. Theoretical models are developed based on analytical equations using the material properties of masonry and TRM. In-plane flexure and shear failure modes are treated separately for biaxial stress. The capacity against out-of-plane loads is estimated for the overturning, horizontal, and vertical flexural collapse mechanisms. The proposed design methods apply to the ultimate limit state design. The theoretical models are validated using experimental data and the models are found to be reliable and reasonably conservative. Results of this study will improve the understanding of the performance and mechanisms of TRM under seismic loading. Recommendations for structural design and a series of guidelines for designers are also provided.

Brittle behavior of substandard columns during earthquakes is among the most common reasons of structural failures. Because capacity design principles and ductile detailing concepts have not been properly adopted during the construction of a large portion of existing buildings, many of them require seismic retrofitting. Among other deficiencies, lack of sufficient ductility is generally the major problem. External fiber-reinforced polymer (FRP) jacketing of columns is a feasible and promising method to overcome the problem of insufficient column ductility in existing substandard buildings. However, experimental data on the behavior of full-scale substandard columns is scarce, particularly in the case of columns with extended rectangular cross sections. Consequently, current standards do not allow FRP jacketing for the enhancement of ductility against seismic actions when the cross-sectional aspect ratio is higher than 1.5. Therefore, in this study, the seismic performance of full-scale substandard columns with extended rectangular cross sections is investigated both experimentally and theoretically. The columns are tested before or after retrofitting with carbon fiber-reinforced polymer (CFRP) jacketing. The findings of the study clearly demonstrate the efficiency of FRP jacketing for substandard columns, even for the case of extended rectangular cross sections. Furthermore, nonlinear analyses of reference and FRP-retrofitted columns, and comparison of the results of analytical study with experimental data, revealed that FRP retrofit design methods recommended by the American Concrete Institute (ACI) and Turkish codes are quite conservative, leading to unfeasible retrofit options, whereas the Eurocode approach leads to more realistic and economical retrofit solutions.

Confinement of concrete columns with fiber-reinforced polymer (FRP) laminates has been extensively studied in recent decades due to their important applications for strengthening purposes or construction of hybrid elements. Significant differences were observed in confinement effects obtained for normal-, high-, and ultrahigh-strength concrete when wrapped by FRP laminates. There are few studies explaining these differences, especially when ultrahigh-strength concrete (UHSC) is taken into consideration. This paper reports on an analysis of confinement effects generated by carbon and glass fiber-reinforced polymer (CFRP and GFRP) wraps in normal-, high-, and ultrahigh-strength concrete (NSC, HSC and UHSC) cylinders. In total, 102 concrete cylinders confined with GFRP and CFRP wraps were tested, including 7 concrete mixes and different number of layers. The results showed that, different from NSC, both HSC and UHSC confined with FRP have a stress-strain behavior with a considerable strength enhancement in the first branch of the curve and a softening or hardening in the transitional zone depending on the confinement pressure.

In the present experimental study, six single-story, single-bay half-scale masonry-infilled reinforced concrete (RC) frames were tested to evaluate the performance of fabric-reinforced cementitious matrix (FRCM) strengthening under bidirectional loading. Five of these infilled RC frames were strengthened using different configurations, by varying the mode of fabric application, the presence of anchors, and the orientation of the fabric. Two methods of fabric application were employed in the present study: direct application and sandwich application. A unique loading protocol was used for the bidirectional loading of specimens which consisted of successive application of slow cyclic drifts for in-plane loading and shake table–generated ground motion for out-of-plane loading. The strengthened infills safely withstood story drifts of over 2.20%, preserving structural integrity without jeopardizing out-of-plane stability. The direct mode of fabric application exhibited superior performance with better bond characteristics and stress redistribution between the frame and infills. The mechanical anchors were effective in limiting the separation of infills from the frame, resulting in an enhanced bidirectional response. Orthogonal orientation of the fabric with respect to bed joints was more effective than oblique orientation.

Externally bonded (EB) fiber-reinforced polymer (FRP) composites are an effective means for the strengthening of shear-deficient reinforced concrete flexural members. Previous studies have predominantly employed epoxy-based wet layup systems. In this study, carbon FRP preimpregnated with polyurethane resin is utilized in strengthening shear-deficient RC beams and compared with an epoxy resin. Fourteen small-scale ($\mathrm{2,438}\times 152\times 305\mathrm{mm}$) and five large-scale ($\mathrm{3,353}\times 305\times 432\mathrm{mm}$) flexural specimens were tested, considering FRP type (polyurethane, epoxy), size effect, shear span-to-depth ratio, FRP configuration (U-wraps, side bonding), and FRP scheme (sheets, strips at 45° or 90°). Four-point loading tests demonstrated similar or enhanced shear capacity for the polyurethane-strengthened RC beams compared to the epoxy-strengthened beams. A mechanics-based analytical model, utilizing the principle of effective FRP stress and the upper-bound theorem of limit analysis, illustrated that the shear behavior and debonding mechanism were dependent on both FRP composite and bond characteristics.

The behavior of shear critical fiber-reinforced-polymer (FRP) RC elements is characterized by the development of comparatively large strains and crack widths, which can be strongly influenced by their relative geometrical size. This paper investigates experimentally the size effect on the shear behavior of FRP RC beams with and without shear reinforcement and with overall depth varying from 260 to 460 mm. The results confirm a considerable size effect for members without shear reinforcement, with an average reduction in normalized shear strength of about 19% and a maximum value up to 40%. It is also shown that current design provisions are overall conservative, but with nonuniform margins of safety that decrease with increasing member depth. It is anticipated that the results of this study will help improve the efficiency of future design equations for the shear strength of FRP RC.

In RC buildings, the unexpected sudden failure of a corner column is more likely to cause severe progressive collapse than the failure of an interior column or a side column due to the relatively weak tie force from the surrounding elements. Effective strengthening methods can improve the progressive collapse resistance of these structures. Fiber-reinforced polymer (FRP) has been widely used to retrofit RC structures due to its high strength, light weight, and corrosion resistance. In this study, the progressive collapse behaviors of glass FRP (GFRP)–strengthened RC beam–slab subassemblages under corner column removal were investigated. First, a quasi-static experimental test was adopted to study the progressive collapse resistance of the beam–slab subassemblages, including a control specimen and strengthened specimens. Two techniques applying externally bonded GFRP laminates and near-surface-mounted (NSM) GFRP bars were adopted to strengthen the subassemblages. The influences of the strengthening schemes on the behavior of the tested specimens were studied by comparing all the test results. The experimental results demonstrate that this strengthening technology can enhance the progressive collapse resistance of RC beam–slab structures. Finite-element (FE) analysis of the progressive collapse resistance of the beam–slab subassemblages was also conducted using the software package MSC Marc. The numerical model can predict the progressive collapse resistance of the subassemblages. In addition, the calculated results were used to better understand the stress distribution of the GFRP in a corner column–removal scenario.

Carbon fiber–reinforced polymer (CFRP) rod panels (CRPs) comprising small-diameter rods at a spacing larger than rod diameter are emerging as an effective adhesively bonded system for retrofit applications. They offer the advantage of a better bond than equivalent CFRP conventional plates due to the increased surface area of the rods. This study aims at characterizing the flexural bond properties of CRPs using the notched concrete beam test specified by the current standard. CRPs with 2-mm rod diameter were used. Rod spacing-to-diameter ratio ($S/D$) was varied from 3 to 6. CRP width-to-beam width ratio $b_f/b_c$ was varied from 0.17 to 0.5. The bond length was varied from 25 to 200 mm on one side of the notch. While debonding failure consistently governed, a development length of 80 mm was found, beyond which insignificant change in ultimate load was noticed. The average bond strength was determined to be 5.1 MPa. It was found that the stress in the individual rods at ultimate load increased from 33% to 53% of their ultimate tensile strength as the $S/D$ ratio increased from 3 to 6. A bilinear bond-slip relation was also established for this system.

One of the emerging applications of fiber-reinforced polymer (FRP) tubes is hollow-core FRP–concrete–steel (HC-FCS) columns. HC-FCS columns demonstrate superior advantages in material savings and mechanical behavior. However, the lack of long-term data for their durability performance hinders their greater acceptance. This study aims to investigate the degradation of HC-FCS cylinders under combined freeze-thaw, heating-cooling, and wet-dry cycles. Sustained axial loads were also applied to the cylinders during environmental conditioning to simulate the service dead load. Compression tests on cylinders were performed and split-disk tensile tests were performed on FRP rings for both conditioned and control specimens. The test results showed that the environmental conditioning slightly degraded the strength and vertical strain capacities of the HC-FCS cylinders, but caused pronounced degradation of the hoop strain capacities of the glass fiber–reinforced polymer (GFRP) tube. Investigating the microstructure of the FRP before and after conditioning revealed that no chemical reaction took place. However, damage to the interphase of fiber and resin led to damage of the fiber bond. This led to strain concentration and early rupture of the FRP. The sustained load had a negative effect on the cylinders, but had a more obvious effect on strain than on strength.

This paper presents an experimental investigation on the use of textile-reinforced mortar (TRM) jackets as a means of improving the out-of-plane performance of masonry infill walls in reinforced concrete (RC) frames during natural or humanmade catastrophic events (e.g., blasts, earthquakes). The experimental program included testing of six half-scale, single-story masonry-infilled RC frames. All specimens were subjected to out-of-plane monotonic loading with the load being distributed at four points on the infill’s body. Two specimens were tested without receiving any retrofitting, thus serving as reference specimens; one with single-wythe and one with double-wythe infill wall. The remaining four specimens were first retrofitted with carbon-fiber TRM jackets and then tested to failure. The examined parameters included: (1) the connection configuration between the masonry infill wall and the surrounding RC frame members and (2) the thickness of the wall. It was found that the out-of-plane performance was dramatically improved in all cases of retrofitted walls, with the examined parameters playing a crucial role in certain aspects of the wall’s behavior. Overall, the risk of collapse was drastically mitigated when the walls were connected to the RC frame, thus enhancing the resilience of masonry-infilled RC buildings subjected to humanmade or natural catastrophic events.