Journal of Composites for Construction

Carbon fiber-reinforced polymer (CFRP) materials have been widely used in prestressed and non-prestressed systems for the flexural strengthening of concrete structures in many construction projects worldwide. Strengthening with non-prestressed CFRP materials, which is known as passive repair, could lead to an increase in the ultimate capacity of the concrete member. However, it has little effect on its serviceability performance (e.g., cracking, yielding, and deflection properties). Flexural strengthening with prestressed CFRPs provides an active repair technique that positively effects the strength and serviceability of structures. In addition, strengthening with prestressed fiber-reinforced composites efficiently uses the material’s high tensile strength. Although different fiber-reinforced polymer (FRP) materials and forms have been used in prestressing applications, this paper focused on the use of CFRP laminates (e.g., sheets, strips, or plates). These laminates have been widely used due to their excellent mechanical properties, ease of application in various geometrical shapes, good conforming surface area of contact with structures, and low installation costs. A comprehensive literature review was presented on strengthening using prestressed CFRP laminates in conventionally and internally prestressed concrete structures (PCSs). The externally bonded and unbonded prestressed CFRP laminate applications that used different anchorage systems were presented. The flexural behavior, failure modes, and serviceability performance of the strengthened concrete members were discussed. The effects of the prestressing level on ductility and energy absorption and a summary of the recommended optimum prestressing level discussions were presented in this paper. Furthermore, a review of the theoretical studies that were conducted for prestressed CFRP applications was provided. The knowledge gaps in the research area and future research recommendations were presented.

This paper examines the punching shear behavior of a new concrete flat slab design, incorporating a glass fiber-reinforced polymer (GFRP) stay-in-place (SIP) structural form system and orthogonal GFRP top rebar mesh. The SIP form system comprises I-beams supported on the four sides of a column and flat plates with T-up ribs supported by, and adhesively bonded to, the bottom flanges of the I-beams. The web and top flange of the I-beams are embedded in the slab, thereby providing flexural and shear reinforcement, while the SIP ribbed plates provide the bottom reinforcement. Four full-scale interior slab–column specimens, 2000 × 2000 × 200 mm³, were tested under axial compression applied to the column. The slabs have a central 300 × 300-mm square column extending 300 mm on either side of the slab. The study assessed the contributions to punching shear strength of different components of the GFRP system. The new design experienced a 29% higher punching shear strength than the control slab with GFRP rebar only, and was much more ductile. The load dropped gradually over a large range of deflection, increasing the ductility index from 1.6 in the control slab to 3.1 in the slab incorporating the new design. An analytical model is developed for punching shear strength, accounting for concrete contribution and flexural and web contributions of I-beams. Results agreed with experimental strength, within −6% to +13%. A parametric study examined a range of rebar reinforcement ratios, different GFRP I-beam sizes and a comparable steel I-beam.

This work focused on material and Global Warming Potential (GWP) savings for the construction of textile-reinforced concrete (TRC) beams made of carbon fabrics (without steel rebars), which were characterized by their flexural behavior. Two beam design approaches were explored for material and GWP savings: (1) regular TRC beam designs, in which the steel rebars were replaced by a carbon textile, which eliminated the need for a thick concrete protective layer that is required in steel reinforcement; and (2) optimized truss-like TRC beams that were based on topology optimization, which distributed the concrete and fabric at their optimal locations. The aim was to minimize concrete consumption through the development of lightweight structural concrete elements with irregular shapes. This exploited the ability of the fabrics to conform to complex shapes and their corrosion resistance. Examples of concrete beams with optimal configurations that were demonstrated in this work showed the potential for significant savings in concrete and reinforcement and the related GWP. In addition, weight was significantly reduced when the textile was used as reinforcement instead of steel.

An appropriate hybrid form of steel and fiber-reinforced polymer (FRP) bars in reinforced concrete members can prevent steel corrosion and ensure capacity and ductility. To investigate the impact resistance of concrete beams reinforced with hybrid steel and basalt FRP (BFRP) bars, a three-dimensional numerical model was developed that takes into account the strain-rate strengthening effect. The validity of the numerical model was first verified. Subsequently, drop hammer impacts were simulated on 10 concrete beams reinforced with hybrid steel–BFRP bars to determine the influence of the BFRP bar proportion and position on their impact resistance. The results indicated that the level of concrete damage in the hybrid bar–RC beams is between those of the concrete beams with steel bars and concrete beams with BFRP bars. As the proportion of BFRP bars increased from 0 to 1, the peak impact force and internal force of the beams linearly decreased by 20%, the reaction force linearly declined by 50%, while the peak midspan displacement linearly increased by 20%. Except for the midspan deflection, the comparable degree of damage among beams with four different bar configurations indicates the equivalent impact resistance of the beams. The change in the frequency of the hybrid bar–RC beams before and after impact loading can reflect their impact resistance.

Hybrid fiber–reinforced polymer (HFRP) bars possessing high tensile ductility and excellent corrosion resistance are an attractive alternative to steel bars. However, the effect of long-term embedment of HFRP bars in concrete on the tensile ductility of bars must be investigated. This study exposes a series of ductile carbon/glass-HFRP bars to a simulated ordinary concrete environment for 3 and 6 months. The accelerated aging method was conducted at 60°C to accelerate the diffusion of moisture and hydroxyl ions into the carbon/glass-HFRP bars, which could rapidly aggravate the degradation of the HFRP bars. Moisture absorption tests, scanning electron microscopy, and Fourier-transform infrared spectroscopy were performed to reveal the degradation mechanisms of the fiber, matrix, and fiber/matrix interfaces in the exposed samples. The tensile ductility of the samples after exposure was lower than that of the samples before exposure. The tensile strength retention of the exposed samples depended on the volume fraction of carbon fibers and the total volume fraction of the fibers, which could be considered in the prediction model of tensile strength retention of the exposed carbon/glass-HFRP bars.

Fiber-reinforced polymer (FRP) grid-reinforced ultra-high performance concrete (UHPC) slabs are new structural solutions that take advantage of the mechanical properties of FRP and UHPC. However, the punching shear behavior of this new slab type has not been characterized. Therefore, punching shear tests were conducted to eight carbon fiber-reinforced polymer (CFRP) bars or grid-reinforced UHPC square slabs (600 mm side width × 40 mm thick). Several influential factors [e.g., use of CFRP bars or grids as flexural reinforcements, type of strengthening short fiber, steel fiber (SF) content, and presence of shear reinforcements] were investigated. The test results showed that FRP grids and short fibers helped to distribute the applied loads and dissipate the input energy; therefore, more cracks were observed, and higher punching shear capacities were achieved. Furthermore, increasing the reinforcement ratio in the FRP grids led to a more significant postcrack ductility response, which increased the punching shear capacity by 17%. In addition, SF addition could enhance the initial cracking load of the slab (V_cr), and polyethylene (PE) fiber addition could intensify the postcrack ductility response, both enhanced the punching shear capacity. The installation of shear reinforcements (eight pieces of 80 mm long CFRP grid strips) appeared to be more cost-effective than increasing SF content. Finally, compared with the current design provisions for conventional reinforced concrete slabs, a more accurate (average error of 8%) punching shear model was proposed for FRP grid-reinforced UHPC slabs with or without SF additions. However, the robustness of the proposed model will be assessed with more test data in the future.

The design of shear strengthening with fabric-reinforced cementitious matrix (FRCM) is currently based on an additive formula where the contribution of the substrate is added to that of the FRCM. This latter, in current regulations, only depends on the strength and equivalent thickness of the fabric, while the contribution of the mortar matrix is neglected. Aiming to evaluate the effective shear capacity of the FRCM material, an experimental setup consisting of an articulated steel frame was designed for testing small-scale squared-shaped samples under ideally pure shear stress loading conditions. The behavior of four FRCM materials obtained with two basalt fabrics embedded in two different mortar matrices was then investigated to highlight the influence of fabrics architecture, matrix properties, and relative interaction. Digital image correlation (DIC) was used for measuring displacements and strains and for detecting the crack pattern of the small-scale specimens, while a micromechanic numerical approach was used to study the fabric-to-matrix shear stress transfer mechanism. The whole experimental and numerical research provides an in-depth comprehension of the shear mechanical response of the FRCM material.

Today’s construction industry has been more inclined to apply innovative rehabilitation techniques, including externally bonded (EB) fiber-reinforced polymer (FRP) composites on existing RC structures, rather than traditional strengthening methods. Indeed, the higher fatigue resistance of carbon FRPs (CFRPs) has made them superior strengthening tools for structures subjected to fatigue loading, such as RC bridge girders against daily traffic loads. However, the performance of RC structures retrofitted with EB–CFRP composites can be highly influenced by the bonding mechanism at the CFRP-to-concrete interface. The occurrence of debonding failure at the CFRP-to-concrete interface can result in premature failure of the EB–CFRP retrofitting system, thereby leading to rupture of the deficient structure. Therefore, an in-depth understanding of the bond behavior, especially under fatigue loading, would contribute to improving the efficiency of EB–CFRP strengthening techniques. To this end, in this work, we investigated the bond fatigue behavior through an experimental study. The parameters of CFRP composite type (laminate vs. fabric sheet), bond length, and CFRP-to-concrete width ratio were considered herein in order to examine their effectiveness on fatigue bond performance at the CFRP-to-concrete interface. The research results revealed that the CFRP composite system composed of the fabric sheet CFRP and the corresponding epoxy adhesive performed better than the CFRP-bonded joint with laminate CFRP in terms of fatigue life and residual load-carrying capacity. Furthermore, a modified bond fatigue-life model (S–N model), validated with existing research data, was proposed that successfully takes into consideration the effects of fatigue loading, concrete compressive strength, and CFRP-to-concrete width ratio.

A fiber-reinforced polymer (FRP) confined engineered cementitious composite (ECC) column as a new high-performance composite member achieves significant ductility. Understanding its cyclic behavior is necessary to guide seismic design. This paper presents an experimental study on the compressive behavior of large rupture strain (LRS) FRP and traditional glass FRP (GFRP) jacketed ECC cylinders subjected to full and partial cyclic loadings. Effects of FRP confinement level, FRP type, and loading scheme on the failure pattern, stress–strain relationship, ultimate condition, plastic strain, and stress deterioration ratio were examined. Existing equations for plastic strain and stress deterioration ratio of the envelope and internal cycles were evaluated. Based on the experimental results, new calculation formulas were derived for the plastic strain and stress deterioration ratio. A threshold involving partial unloading/reloading factors was determined to define the effective cycles. In addition, a cyclic model composed of a monotonic envelope model and an unloading/reloading model was developed for estimating the stress–strain hysteresis loops of both GFRP- and LRS FRP-confined ECC cylinders.

Abstract

Fiber-reinforced polymer (FRP) composites are becoming increasingly popular as an alternative to conventional civil engineering materials in existing structures as externally applied systems for strengthening purposes, and in new structures as internal reinforcements in the form of bars, meshes, and strands. The next phase of development in the application of FRPs in concrete structures could culminate in elements possessing both internal and external FRP systems simultaneously. With these advancements in the application of FRP in civil engineering, it is conceivable that traditional steel reinforcements could become obsolete in certain aggressive and hostile environments, such as coastal areas. Although higher durability and performance are associated with FRP materials in some respects when compared to steel, concerns remain regarding the damage and defects in this material, many of them related to their unique features. Regardless, similarly to other structural materials, it is necessary to understand the damage and defects associated with the use of FRP composites, as well as to identify their sources. This knowledge will support the positive prospects for FRP technology, promoting its application in civil infrastructure. Accordingly, this study investigated the types, characteristics, and identification of the observed or expected damage and defects associated with using FRP in reinforced and strengthened concrete elements. The damage was classified according to location and time of initiation. In addition, the sources of these defects and damage were determined, and a damage etiology was established for preventing the occurrence of such damage in future use. The results of this study are intended to provide the background for the development of a guide and training material for the inspection of structures that use FRP materials. The ability to inspect and properly maintain structures containing FRP will result in the enhanced durability and service life of concrete structures reinforced or strengthened with FRP materials.

Practical Applications

With advancements in the application of fiber-reinforced polymer (FRP) in civil engineering and the development of practical means for its use in design and construction, it is conceivable that traditional steel reinforcements could become obsolete in certain aggressive and hostile environments, such as coastal areas. Although higher durability and performance is associated with FRP when compared to steel, concerns remain regarding the potential damage and defects that could be associated with its use. To help the owners of concrete structures feel comfortable with this relatively new application, guides and procedures are needed that will support the inspection and detection of potential damage in FRP. For this, it is necessary to understand the damage and defects that can occur in externally and internally applied FRP, and to be able to identify their sources. In this study, we investigated the types, characteristics, and identification of observed or expected damage and defects associated with using FRP in reinforced and strengthened concrete elements. In addition, the sources of these defects and damage were determined and a damage etiology established as a basis for developing a guide and training materials for the inspection of structures that use FRP materials. An accurate understanding of such damage and its sources could be used for maintenance and preventive measures for structures that were constructed using FRP materials.

The full-range debonding process of a fiber-reinforced polymer (FRP)–generic material substrate interface with mechanical end anchorages requires a better understanding. In this study, we developed a nonlinear semianalytical model based on a cohesive crack model to predict the full-range debonding process of such an interface in a single-shear mode, considering the relatively deformed region in the interface as a cohesive zone. Specifically, we produced a double exponential bond–slip constitutive model with residual bonding shear stress and boundary conditions of slip constraints at both anchoring ends. Implicit solutions for the relative slips, bonding shear stresses, and axial forces in the FRP along the bond length, as well as the load–slip responses at the loading end and effective bond length, were obtained using the Chebyshev polynomial approximation strategy. The accuracy of the proposed model was verified using experimental results available in the literature. Then, the evolution of full-range interfacial bonding behavior and load–slip responses for different bond lengths was discussed. Finally, a parametric study highlighting the effect of geometry and material properties on the load–slip response at the loading end was presented.