For reinforced concrete (RC) frame structures, applying more continuous longitudinal bars in the beams is the most effective way to improve the progressive collapse resistance. However, an RC frame has the possibility to experience multiple collapse mechanisms. An RC frame may not only experience progressive collapse caused by critical column removal but may also experience lateral collapse under strong earthquakes. The strong column-weak beam (SCWB) requirement in seismic design does not allow excessive longitudinal reinforcement in the beams. To solve this contradiction, a previous study proposed a novel kinked rebar (KB) configuration to enhance both the seismic and progressive collapse resistances of RC frames. This study was carried out to conduct further experimental validation and investigation of progressive collapse behaviours of RC frames with KB configurations. First, a quasi-static progressive collapse test involving 9 RC frame substructures was carried out. The test results indicated that the application of KBs could increase both the loading and deformation capacities under the catenary mechanism of RC substructures. For RC substructures, the influences on progressive collapse behaviours from various structural parameters were also investigated. Second, finite-element simulations were carried out to model the behaviours of the test specimens. This study experimentally verified that the KB configuration can improve the progressive collapse resistances of RC frame structures and clarified the progressive collapse mechanism of novel RC frame substructures with KB configurations.
A novel joint, inspired by ancient mortise-and-tenon joints in timber structures, was proposed for pultruded FRP beams and concrete-filled FRP columns. FRP structures are widely known to lack appropriate joint solutions. In this regard, the proposed joint effectively provides a feasible solution for FRP frame structures. In this work, the conceptual development of this joint was first addressed. Then, the construction schemes were presented in detail, and a total of four full-scale joints were tested. To assess the structural behavior of the proposed joint, experiments including four specimens were conducted under symmetric and anti-symmetric load, i.e. the loading conditions in sway and non-sway frames. The typical failure modes were identified, including pull-out or tensile fracture of the tensile flanges and end crushing of the compressive flange. Additionally, an analytical study is carried out to develop the design method for this joint. The predicted moment capacity is found to have an excellent accuracy as compared to the experimental results. Moreover, the proposed joint can be classified as the rigid and full-strength joint based on the classifications prescribed by the European design code. It is noted that the proposed joint is completely composed of FRP composites and concrete (i.e., a steel-free joint). Thus, the joint is expected to exhibit excellent corrosion resistance, which is of particular interest for marine structures on/near oceans.
应用壳聚糖－海藻酸盐微囊技术制备了一系列胰岛素微型，并研究了不同反应条件如海藻酸钠浓度，壳聚糖浓度，壳聚糖分子量及壳聚糖溶液ｐＨ值对生囊的胰岛素包封率及其释放性能的影响。结果表明，海藻酸钠浓度越高，微囊对胰岛素的包封率越高，在模拟小肠液中释放速度越低；壳聚糖浓度越大，微囊的胰岛素包封率及其在模拟胃液中释放率越高，在模拟肠液中释放达最大值所需时间越长；而随壳聚糖分子量减小，微囊在胃液中释放率增高；壳
A load-strain model for a steel-concrete-FRP-concrete (SCFC) hybrid column section in compression is proposed. The section layout has a square steel tube as the outer layer and a circular fiber-reinforced polymer (FRP) tube as the inner layer, and concrete is filled between these two layers and inside the FRP tube. Thus the section can be regarded as a concrete-filled steel tube (CFST) with a FRP-confined concrete core (FCCC), in which the FCCC is essentially a concrete-filled FRP tube (CFFT) in sectional configuration. However, the mechanical behavior of a SCFC is superior to the simple superposition of CFST and CFFT without consideration of the interaction mechanisms among the different materials. The load-strain behavior of a SCFC differs from that of a CFST or CFFT in that it includes an initial parabola portion, a second linear portion, and a postpeak portion. The model is established by superposing four load-strain models of the constituent layers and attempting to reveal the mechanical responses of the SCFC sections under axial compression. In the modeling, several mechanical characteristics, namely yielding point, peak strain, peak load, and postpeak residual bearing portion, are investigated and the effects of three parameters, FRP thickness, steel thickness, and concrete strength, are examined. Comparisons between the modeling results and experimental results show good agreement in terms of yielding strain, yielding load, peak strain, and peak load. Furthermore, a set of predictions for peak load covering a greater range of parameters than the experiments is developed according to this load-strain model.
The present study describes an experimental investigation on shear behavior of GFRP-concrete hybrid beams, consisting of reinforced concrete beams combined with U-shaped glass fiber reinforced polymer pultruded profile constructed as combined tensile reinforcement, permanent formwork and corrosion resistance. A total of six beams were tested, three served as control beams and the remaining were implemented with U-shaped GFRP composites. Test results show that hybrid beam provides significant increase in shear capacity when compared to control RC beam.
Flexural behavior of an adhesive-bolt hybrid connection for pultruded glass fiber reinforced polymer (GFRP) frame is investigated through monotonic and cyclic loading tests. It is shown that the hybrid connection fabricated using resin adhesive and steel bolts is able to exhibit a quasi-plastic behavior. Multistage damage occurred in tests, and the adhesive failed initially, followed by GFRP failure at bolt holes. Effect of shear force was investigated by variable shear-span ratios in tests. Additionally, moment and rotation curves obtained from monotonic and cyclic loading tests are found to be similar, indicating that loading schemes have little impact on structural performance of the hybrid connection. Energy dissipation is analyzed based on cyclic loading tests. Finite element model is also built, and design formulas are proposed to predict the quasi-yield moment and maximum moment. A good agreement is found between experimental results and analytical predictions. In addition, design recommendation on the use of adhesive-bolt hybrid connection is provided.
This review summarizes the latest research progress in regard to the bond behavior of corroded reinforcing steel bar. The literature survey indicates that the previous studies mainly focus on the deterioration of bond strength. The primary factors influencing bond strength, including main bar corrosion, concrete cover, concrete type, stirrups, stirrup corrosion, and corrosion rate, have been well recognized. However, the existing models for bond strength are mostly developed based on specific test results and are not generalized. The corrosion-induced surface crack width is an ideal parameter for the evaluation of bond strength, but models in this respect are still scarce. As for the bond-slip behavior, literature studies reveal that the bond-slip mechanism is hardly affected by corrosion. Currently, discrepancies exist regarding the corrosion influence on the key parameters that shape the bond-slip curves. Preliminary comparisons of the existing bond-slip models show that reliable and generalized models remain to be developed based on larger database. Studies on the bond behavior of corroded steel bar under cyclic loading are quite few. The limited test results indicate that the repeated loading without changing sign shows no apparent influence on bond strength, whereas the cyclic reversed loading exacerbates the corrosion-induced bond deterioration. Knowledge gaps and research areas where further investigations are required are outlined.
The connection between FRP profile and concrete is critical for structural performance. This study introduces a novel fiber-bridging interface to enhance the FRP-concrete interfacial behavior. The interface comprises an epoxy resin adhesive layer, a carbon fabric layer, a mixture of adhesive and sand layer, and U-shaped steel fibers. Central pull-out tests were conducted to investigate the mechanical performance of this novel interface. The investigated variables included bond length and fiber volume fraction. Test results indicate that all specimens failed in a brittle mode at the adhesive layer, with load plateauing after reaching the peak. The number of steel fibers had limited influence on the interfacial behavior. Based on the load-slip curves, a bond stress-slip model for the tangential behavior was developed. An interfacial expansion model was further developed by means of FE analysis and machine learning. The three most widely used machine learning models, i.e., the BP neural network model, the random forest model, and the XGBoost model were selected. Comparisons show that all three models provide reasonable predictions, with the XGBoost model demonstrating the best performance. These models for the tangential and normal behavior of FRP-concrete interface were implemented into FE models for numerical analysis. Comparisons between numerical and experimental results show that the proposed models accurately describe the interfacial behavior of the fiber-bridging interface under brittle failure mode. The innovative interface proposed in this paper can be used for connecting concrete and FRP in various scenarios, and the proposed methodology of calibrating local bond behavior parameters from global response offers a new approach for establishing interfacial bond models.
This study presents a novel composite component comprising of steel-reinforced high-performance fiber reinforced cementitious composites (HPFRCC) circular columns confined by fiber-reinforced polymer (FRP), with the goal of improving the mechanical behavior of conventional reinforced concrete (RC) columns. Quasi-static cyclic loading tests were conducted to investigate various aspects of the column specimens, including failure phenomena, load-displacement response, sectional compression-bending performance, plastic hinge length, and lateral strain distributions in the FRP jacket, etc. A self-designed frictional force measurement device was employed to accurately measure the actual force borne by the specimen. The experiment results revealed that FRP confinement effectively improved the load-bearing capacity and deformation capacity of the specimens and delayed the deterioration of the sectional compression-bending performance. The superior tensile ductility of HPFRCC led to increased yielding displacement and peak load compared to FRP-confined concrete specimen, although this enhancement was limited. Furthermore, the peak load of the FRP-confined HPFRCC specimen increased with axial load, while the sectional compression-bending performance after the peak load remained relatively stable until approaching the collapse prevention (CP) limit state.
No abstract is provided for this article.
Glass Fiber Reinforced Polymer (GFRP) composites have low permeability and high corrosion resistance. In recent years, as the use of GFRP in applications requiring high corrosion resistance has gradually expanded, its long-term performance in highly corrosive environments has caught public attention worldwide. Nevertheless, there is only limited theoretical research and accumulated test data on the corrosion resistance of GFRP. This paper analyses the long-term performance of GFRP in highly corrosive environments. Samples cut from pultruded GFRP profiles were exposed to alkaline, saline, and acidic solutions of different concentrations at 60°C. Additionally, the effects of acidic solutions at high temperature (90°C) were investigated. Flexural properties, changes in the flexural fracture mechanism, hardness, the rate of weight gain, changes in appearance, and scanning electron microscopy (SEM) images of fracture sections were analyzed over periods of 7, 15, 30 and 90days. Results indicate that flexural strength decreases with increased exposure time to both acidic and alkaline solutions. Under conditions of high concentration acidity and high temperature acidity, respectively, flexural strength decreases even more drastically. Additionally, it was found that saturated NaCl solution has little effect on GFRP.
Pultruded fiber reinforced polymer (FRP) composite profiles are usually regarded as the transverse-isotropic material based on the assumption that the fiber and resin are uniformly distributed across the profile sections. However, this is practically not the case due to the limitations of the pultrusion technology. The non-uniform fiber-resin distribution (NUFRD), as a type of inherent initial imperfection for pultruded glass fiber reinforced polymer (GFRP) profiles, was investigated. The resin contents of 49 different pultruded GFRP sections were tested and analyzed. The standard calcination method was used to measure the resin content. The sections were provided by three different manufactures and included the I-sections, box-sections, angle-sections, circular tubes, channel-sections and flat plates. Multiple sampling locations were specified for each type of section. The test results showed that the degree of NUFRD for each type of section is different. In particular, the I-sections showed the most significant material non-uniformity. The largest COV (coefficient of variation) of the measured resin contents is 0.16. Additionally, the influence of NUFRD on the mechanical properties of GFRP members was addressed. The material non-uniformity would increase the initial section eccentricity. Finite element models were built to simulate the compressive GFRP members. It was found that the critical buckling loads of GFRP compressive members will be reduced due to NUFRD.
In this work, the use of pultruded GFRP web stiffeners to enhance the flange local buckling behavior of pultruded GFRP open-section beams was proposed and demonstrated through a numerical study. The buckling strength of the GFRP I-beam was successfully increased by up to 207% via this approach. The influences of the stiffener geometry and spacing were investigated. The buckling strength of the GFRP I-beam increased as the stiffener width increased and the stiffener spacing decreased. Epoxy adhesive was found to provide a sufficient bonding effect between the web stiffeners and the beams. In addition, a design approach was proposed to predict the increased buckling strength of GFRP I-beams strengthened with web stiffeners. A comparison with finite element analysis and experimental results showed that the design equation was able to provide uniformly conservative predictions. Additionally, practical design suggestions were provided to ensure the best performance of the web stiffeners. The strengthening approach addressed in this work could potentially permit a longer span of GFRP profiles and a wider application of GFRP structures.
