Journal of Composites for Construction

Reinforced concrete (RC) buildings constructed before introducing ductile design requirements may exhibit undesirable seismic behavior. One such vulnerability is the lack of confinement to end regions that may lead to nonductile compression failures for RC walls. To overcome this vulnerability, strengthening nonductile RC walls is often required. The addition of confinement to end region concrete can be implemented using carbon fiber-reinforced polymer (FRP) laminate and spike anchors. The main issue encountered when applying this strengthening method is the lack of existing design guidance on this confinement method, due to a lack of experimental data to validate the approach. The objective of this study was to provide the necessary experimental data to quantify the behavior of concrete confined with FRP laminate and spike anchors. Static monotonic axial compression tests were conducted on 46 concrete prisms confined with FRP laminate and spike anchors and the other six unconfined concrete prisms. These prisms represented the end regions of RC walls. The variables investigated for the confined test prisms included anchor spacing, anchor cross-sectional area, and cross-sectional aspect ratio. It shows that peak strength increased significantly with the decrease of anchor spacing and the increase of anchor cross-sectional area. An increase in anchor cross-sectional area had a more considerable influence on the failure strain than a decrease in anchor spacing. The cross-sectional aspect ratio of the prisms had an insignificant impact on failure strain. When gross anchor cross-sectional area was constant in the confinement arrangement, larger peak strength, strain at peak, and failure strain were generally observed in prisms confined with smaller and more closely spaced anchors. Design recommendations were given on the test variables based on test results.

To effectively utilize marine resources on reefs or islands and to improve the bearing capacity and serviceability of fiber-reinforced polymer (FRP) reinforced seawater coral aggregate concrete (CAC) structures in marine environments, this paper investigates the applicability of using alkali-activated materials (AAMs) as substitutes for ordinary Portland cement (OPC) in FRP reinforced CAC structures. Three types of FRP bars, i.e., carbon-FRP (CFRP), glass-FRP (GFRP), and basalt-FRP (BFRP) bars, with different bond lengths (L = 50, 70, and 100 mm) were selected to determine the bond characteristics of FRP bars in alkali-activated seawater coral aggregate concrete (AACAC), as well as in cement-based CAC, which was chosen as the reference. Moreover, a scanning electron microscope (SEM) was employed to detect the microstructure characteristic at the interfacial transition zone (ITZ) between the coral aggregates and the paste matrix. The results indicated that the AACAC specimens contained a stronger mechanical bite force at the paste–aggregate interface and exhibited a higher splitting tensile strength (approximately 6.7% improvement) than those of the CAC specimens. Additionally, the ultimate bond strength and the initial slope of the bond–slip curves at the ascending branch (i.e., initial bond stiffness) were significantly improved by utilizing AAMs. Improvements of approximately 26.6%, 26.8%, and 16.9% were achieved in the bond strength for the specimens with CFRP, GFRP, and BFRP bars, respectively. It was concluded that the utilization of AAMs as alternatives for OPC was an effective method in improving the mechanical interaction at the paste–aggregate interface and promoting the anchorage capacity of FRP bars in CAC, which may represent a promising approach for applying AAMs in FRP reinforced CAC structures or members.

Using fiber-reinforced polymer (FRP) grid-reinforced engineered cementitious composites (ECC) for strengthening reinforced concrete columns and beams has been stirring structural research interest in recent years. The objective of this research was to extend the application of this technique, using carbon-fiber-reinforced polymer (CFRP). The authors used compression tests on short concrete columns strengthened with CFRP-ECC to investigate the mechanical behavior of strengthened columns by considering the unconfined concrete strength and reinforcement layers of CFRP grid. Test results show that the failure mode of most strengthened columns lies in the rupture of the embedded CFRP grid. The lateral strains corresponding to peak stresses were close, and their average value was almost equal to the ultimate axial tensile strain of the CFRP-ECC. Meanwhile, with an increased number of reinforcement layers, the peak stress and strain of the strengthening columns increased significantly for low-strength core concrete. However, there was no significant enhancement for the high-strength core concrete. In addition, the cracking stress of CFRP-ECC had obvious effects on the yielding stress, peak stress, peak strain, and the axial–lateral strain relationship. Most importantly, a better understanding of the stress–strain relationship for CFRP-ECC confined columns was established, and the feasibility of this model was verified by the analysis results.

The application of textile-reinforced mortars (TRM) on masonry walls constructed with natural stones was studied through a set of medium-scale experiments. Fourteen experiments were carried out in total, including in- and out-of-plane cyclic tests as well as two different strengthening configurations. The first consisted of a TRM made of a natural hydraulic lime (NHL) mortar, combined with a natural flax-fiber textile, while for the latter, a novel alkali-activated material (AAM) geopolymer mortar combined with basalt textile was employed. The use of such low-carbon footprint materials instead of conventional ones makes these systems environmentally friendlier, in line with the modern requirements for lowering CO₂ emissions. Both solutions led to a substantial increase of the load-bearing capacity, up to 70% for both in- and out-of-plane experiments. Stiffness and energy dissipation characteristics of the masonry elements were improved as well. If some durability related issues of both configurations and feasibility ones of AAMs are addressed, they could provide good candidates for application in real-life structures.

This paper reports a comprehensive study on the behavior of concavely curved soffit reinforced concrete (RC) beams strengthened in flexure with carbon fiber-reinforced polymer (CFRP) composites under static loading. The main objective of this paper is to explore the effect of surface concavity on the bond performance of externally bonded wet layup CFRP sheets and laminates. An experimental program consisting of flexural strengthening of 24 RC beams with concavely curved soffits was carried out. All specimens were simply supported RC beams tested under three-point bending. Of the 24 beams, 6 beams were flat soffit RC beams, and the remainder were fabricated with concavely curved soffits with a degree of curvature that is ranging from 5 mm/m to 20 mm/m. All tested specimens were 2,700-mm long and had a constant cross section at midspan of 140-mm wide × 260-mm deep. The experiments showed that all strengthened beams failed by intermediate crack-induced (IC) debonding of the CFRP. The experimental results were then used, together with those for other specimens in the literature to set recommendations for strengthening concavely curved RC beams.

An experimental investigation assessed the effectiveness of carbon and polyparaphenylene benzobisoxazole (PBO) fabric-reinforced cementitious matrix (C-FRCM and PBO-FRCM) systems in rehabilitating reinforced concrete beams affected by corrosion of the tensile steel reinforcement. Thirteen reinforced concrete beam specimens were tested using a four-point bending setup under monotonic and cyclic loading. The FRCM strengthening of the corrosion-damaged specimens significantly increased the yield and ultimate capacity. The FRCM composites efficiency was demonstrated under cyclic loading; the restoration of the beams’ strength is maintained over the cyclic life. The FRCM strengthening systems were capable of extending the cyclic life up to five times in comparison with nonstrengthened control beam. The rehabilitated beams maintained their stiffness during the cyclic loading, indicating that FRCM-strengthening systems efficiently resist cyclic loads.

This paper proposes an externally bonded carbon fabric system to mitigate the causes of timber sleeper deterioration. Therefore, three methods of carbon fabric application for timber sleeper reinforcement, namely, carbon fabric-wrapped timber sleepers (CWT), one-layer carbon fabric timber sleepers with anchors (OCT), and two-layer carbon fabric timber sleepers with anchors (TCT), are studied and compared with conventional timber sleepers (CTS). Modal analysis is performed to obtain the sleeper damping ratios, and three bending moment tests are conducted to compare their load capacities. Furthermore, finite-element method (FEM) modeling is developed to compare the stress levels within each carbon fabric-strengthened sleeper. The bending moment test results indicate a high improvement in the load–displacement behavior of timber sleepers in the presence of carbon fabric, especially for the CWT, which shows a 55% improvement compared with the CTS, followed by the TCT and OCT, which show 50% and 33% improvements. Wrapped and two-layer and one-layer anchored systems have damping ratios of 0.21, 0.26, and 0.18, respectively, which are higher than that of conventional timber sleepers, at 0.17. The FEM results show that the stress levels of the timber sleepers with carbon fabric-strengthened systems decrease compared to CTS. Finally, a desirability function is developed to select the optimum carbon fabric system based on the load–displacement behavior, damping ratio, and insertion loss, which indicates a two-layer carbon fabric reinforcing system.

Several size-effect laws (SELs) have been proposed and verified for concrete-like quasi-brittle materials. However, there is no SEL that can quantitatively describe the influence of related factors on size effect in carbon fiber–reinforced polymer (CFRP)-strengthened lightweight aggregate concrete (LWC) components without stirrups, for example, the shear failure of beams. The main purpose of the present study is to develop a novel SEL for the evaluation of shear strength of CFRP-strengthened LWC and normal-weight concrete (NWC) beams. A 3D mesoscale simulation method is employed to explore the structural behavior of both LWC and NWC beams wrapped with CFRP sheets, in which both the concrete heterogeneities and the steel/concrete interactions are considered. Based on the modeling approach, the influences of shear span ratio and CFRP ratio on the shear failure behavior and the size effect on shear strength are studied. Also, the shear contribution by CFRP on the LWC beams having different shear span ratios under different structural sizes (i.e., beam-depth) is examined. Finally, a novel size effect law that can quantitatively describe the influence of shear span ratio and CFRP ratio on shear strength of CFRP-strengthened LWC beam is developed, and it is verified with the available test data and simulation results.

Premature debonding is a major obstacle for the utilization of fiber-reinforced polymer (FRP)-reinforced stone structures due to the brittle characteristics of stone materials. The study of the interfacial behavior between FRP sheets and stone materials with mechanical anchoring is critical when relative slip occurs at the joint interface. In this study, 45 single shear tests were performed to investigate the interfacial properties between FRP sheets and granite under static loading. Several variables were considered, including the differences in the FRP sheet stiffnesses and anchor properties (e.g., the number and spacing of fasteners, bolt torque, and bolt diameter). The failure modes, strain distribution, and load–slip curve of the designed specimens were discussed. The results indicate that compared with the external bonding (EB) method, the hybrid bonded FRP (HB-FRP) strengthening technology significantly improves the FRP usage efficiency, and the ultimate bond load and slip can be enhanced with an increase in the number of anchors and the torque. In addition, the stiffness of the FRP sheet and bolt diameter negatively influence the ultimate slip of the FRP sheet–granite interface, and the slip between the two anchors depends on the ratio of the effective bond length (EBL) and anchor space. These test results offer helpful information for designing HB-FRP reinforced stone structures.

This paper presents investigations on the tensile behavior of several textile-reinforced concrete (TRC) material compositions after fire exposure. First, experimental investigations are presented, aiming to provide insights into the thermomechanical performance of fire-exposed TRC, and data that can be used for the development and implementation of analytical or numerical models to design fire-exposed TRC elements. The applicability of the Aveston–Cooper–Kelly (ACK) theory is assessed for ambient and increased temperatures. TRC specimens with various cases of reinforcement (carbon or glass fibers, uncoated or coated textiles, low or high fiber volume fraction) were manufactured and subjected to temperatures reaching 700°C. The residual tensile capacity of the exposed specimens is discussed, focusing on the effect of the varying reinforcement. It is verified that the use of uncoated carbon fibers is the most promising solution for maintaining the residual capacity after fire exposure. Finally, it is concluded that the ACK predictions are reliable for TRC specimens with good bond conditions and when adopting a representative thermal degradation law for the reinforcement.

Short columns with deficiencies of low-strength concrete and large spacing of transverse reinforcement are quite common in construction practice in many developing countries in seismically active regions. Therefore, it is important to retrofit this type of column to avoid negative consequences in the case of severe damage. External fiber–reinforced polymer (FRP) jacketing is one of the most practical methods to retrofit reinforced concrete (RC) frame members against shear actions. However, knowledge of the seismic behavior of FRP-retrofitted RC short columns is still limited. Thus, the aim of this study was to investigate the seismic response of FRP-retrofitted low-strength RC short columns with and without predamage through the testing of 11 short column specimens representing old and deficient RC buildings. Accordingly, the results of a research program on the seismic behavior of such columns and on their repair and FRP retrofitting, covering experimental and analytical phases, are presented. In the experimental phase, test results obtained for short columns, FRP-retrofitted short columns, and repaired and retrofitted short columns are documented. Test results provide important knowledge about the effects of extremely low-strength concrete, and of different levels of predamage on the behavior of FRP-retrofitted substandard short columns. In the analytical phase, the performance of relevant design documents in predicting the shear strength of such columns retrofitted with FRP jacketing was tested through the comparison of predicted and experimental results.

Fiber-reinforced polymers (FRPs) have been widely used in the retrofitting and rehabilitation of reinforced concrete (RC) columns because they can provide substantial lateral confinement to concrete. Conventionally, the presence of internal stirrups is neglected in the retrofitting design due to a lack of understanding on the effect of internal hoops when both external FRP and internal steel confinement exist, particularly under eccentric loading. In this study, a finite-element (FE) method is developed to investigate the eccentric compression behavior of FRP-confined RC columns. A reliable concrete plastic-damage model that accurately considers the confining stiffness ratio between the FRP and steel is proposed for the first time, which facilitates the study of a difficult problem: the behavior of RC columns with the dual confinement system under eccentric loading. The accuracy of the model is verified by test results. An extensive parametric study of various affecting factors is carried out to investigate the confinement mechanisms of the concrete columns in terms of the axial stress and confining pressure distributions, lateral principal stress ratio, local stress–strain response, and interaction between the FRP/steel hoops and concrete. It is found that the nonuniform confining pressures under eccentric compression are significantly affected by the internal stirrups. With the existence of discrete transverse steel reinforcement, the axial resistance of the concrete core is increased, thereby improving the ultimate load capacity of the columns. An ultimate axial load model for RC columns that considered combined FRP–steel confinement is subsequently proposed. More accurate results are obtained using the proposed model compared with those calculated from existing design codes.

Precast concrete insulated sandwich wall panels have been commonly used as exterior load bearing or cladding wall panels on building structures. These panels are usually designed as composite, partially composite, or noncomposite, depending on the strength of the shear connectors connecting the two exterior concrete wythes. Various proprietary available shear connectors are used to achieve the required degree of composite action. The design information of the shear connectors is provided by the connector supplier based on testing results. Currently, there is no universally accepted method of testing composite shear connectors in practice or in the literature, however, connector suppliers usually use one of two testing methodologies recommended by the International Code Council–Evaluation Service, namely, AC320 or AC422, respectively. It is not known which test provides a better measurement of design values such as strength and stiffness, and there is no such comparison in the literature. In this study, 68 pushoff experiments were presented to evaluate the difference between these two testing methodologies. The testing program consisted of 20 single shear specimens and 48 double shear specimens. Three different commercially available shear connectors were used as well as variable insulation wythe thicknesses. The results were statistically analyzed. Large differences in the measured connectors’ shear strengths and stiffness were observed between the two testing methods, with AC422 providing consistently larger ultimate shear and secant stiffness, but considerably more variability. Large-scale flexural specimens from the literature were modeled using AC320 and AC422 as-measured results, and AC422 tests were found to predict deflections, cracking, and strength more accurately. It was concluded that AC422 produces a more accurate measure of connector properties, but suffers from large variability, resulting in a recommendation that further investigation into changing testing parameters and/or methods is warranted to reduce this variability.