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The behavior of concrete structures subjected to shear forces involves the contribution of several shear force transfer actions, such as aggregate interlock, dowel action, concrete residual tensile along the fracture process zone, and transfer through the uncracked concrete. While the contribution and interaction between these actions dependt on the shape and kinematics of the critical shear crack, the prediction models available either do not consider all these actions or are based on linear or bilinear shapes for the critical crack. In this context, this paper presents a multiaction model in which the shear actions are considered in the equilibrium of forces and moments based on the shape and kinematics of the critical shear crack. The crack shape, represented by a quadratic Bézier curve, was obtained from an optimization process using a genetic algorithm. The results obtained from the model include the location, shape, and kinematics of the critical crack and the final shear strength and the contributions of each mechanism. The outcomes of the proposed approach were compared with experimental results reported in the literature for beams without stirrups and made with fiber-reinforced concrete (FRC) and fiber-reinforced polymer (FRP) or conventional steel bars. The experimental behavior was accurately simulated, especially for the critical shape and shear strength, which presented relative errors of around 1% and 7%, indicating good agreement. In addition, the model revealed that the residual tensile stress resulting from a discrete fiber action had the highest impact on the load-bearing capacity, following the same observations as the reference studies. The proposed model represents a promising methodology to predict the shear behavior of reinforced concrete beams with various combinations of materials. [ABSTRACT FROM AUTHOR]

A steel-fiber reinforced polymer (FRP) composite bar (SFCB) consists of an inner steel bar and an outer continuous FRP layer, which exhibits excellent corrosion resistance, a high elastic modulus, and customizable postyield stiffness in tension. However, its response and buckling characteristics under compression remain unclear. This paper presents an experimental study on the axial compressive properties of bare SFCB with varying equivalent slenderness ratios (λeq) and compares them with those of BFRP bars and stainless steel bars. To enhance the buckling resistance of the rebars and to solve the rebar congestion in precast members, corrugated-pipe confined (CPC) specimens with bundled reinforcement were designed and tested under compression with two pipe diameters (40 mm and 60 mm). The compressive damage of SFCB included FRP fracture, debonding between fibers, or debonding between the FRP and the inner steel bar. SFCBs with λeq less than 8 demonstrated a slight postyield stiffness, and the load capacity exhibited a slight decrease with an increase in λeq. The compressive load capacity of SFCBs was significantly lower than that under tension and could hardly exceed the yield load, with a maximum of only 34% of the tensile load. The CPC specimens could result in a higher overall load capacity than the sum of the compressive loads of the individual components as well as better axial deformation ability of the inside rebars. The maximum load capacity of SFCB inside the CPC specimens increased by approximately 100%, exceeding the yield load and demonstrating postyield stiffness characteristics, which is effective in enhancing the seismic performance of the structure. BFRP bars are sensitive to the pipe diameter of CPC specimens, and the compressive load capacity and deformation capacity in D60-CPC specimens can reach more than twice that of bare BFRP bars, while the performance of D40-CPC was significantly degraded. [ABSTRACT FROM AUTHOR]

Carbon fiber–reinforced polymer–steel composite bars (C-FSCBs) and coral sea-sand aggregate seawater concrete (CSSC) are attractive choices as construction materials for island and atoll engineering construction. Understanding the bond behavior between the C-FSCB and CSSC is crucial for evaluating the mechanical properties of C-FSCB-reinforced CSSC structures. In this study, the bond–slip behavior between the C-FSCB and CSSC was experimentally assessed via pullout tests. The influence of various factors on the bond behavior was discussed, and the bond mechanism between the C-FSCB and CSSC was analyzed. The results indicate that, unlike steel bars, the low rigidity of fiber-reinforced polymer caused surface fiber stripping (i.e., shear damage) of the C-FSCB after interface slip, consequently reducing the squeezing fragmentation of concrete between the ribs. As the diameter and bond length of the C-FSCB increased, the bond strength decreased. Compared to specimens with C-FSCBs, the bond strength of specimens with steel bars of the same diameter increased by 17.9%. The degree of coarse aggregate fracture at the CSSC interface in the splitting failure was much higher than that of normal concrete. Based on existing research data, a formula for calculating the bond strength of C-FSCBs in CSSC has been established to determine the anchorage length of C-FSCBs, and the calculated values accurately predicted the test values. [ABSTRACT FROM AUTHOR]

In 2019, a major code change made by the American Concrete Institute incorporated the extensive usage and applications of high-strength steel reinforcing bars (HSRBs). The adoption of HSRBs was motivated by several factors, including the reduction in steel congestion and construction cost, gain in member strength, and reduction in the carbon footprint of reinforced concrete buildings. However, code adoption of HSRBs has lagged in masonry design, which is due to the absence of relevant analytical and experimental data. Currently, masonry design standards define the maximum allowable design stress for reinforcing bars at 420 MPa (60,000 psi). This paper presents the first research study on HSRBs for structural masonry with a focus on the lap-splice demand for Grade 80 (550 MPa) deformed bars. The research study included 22 contact lap-splice experiments in concrete and clay brick masonry, with test variables being the bar size, the lap-splice length, and the standard specification of the bars. A high-fidelity finite-element model was developed to simulate the bond-slip behavior of HSRBs in structural masonry and complement the experimental findings. Experimental and numerical results corroborated the technical feasibility of embedding Grade 80 bars in structural masonry with reasonable lap-splice lengths, following revisions to the code design formulas. Furthermore, it was found that different standard specifications of bars present different lap-splice length requirements, a distinction that is not currently made by existing code provisions. [ABSTRACT FROM AUTHOR]

The exposure of reinforced concrete bridges to chloride induces a corrosion phenomenon, leading to pitting corrosion on the steel bars and subsequent stress concentration development within them. This process accelerates corrosion, resulting in structural degradation. The stress corrosion coupling effect of pitting steel bars was investigated in this paper. First, based on electrochemical theory, the stress corrosion correction factor (SCCF) was proposed for carbon steel bars to chloride attack, which was applicable throughout the elastic-plastic stage. Then, an electrochemical experiment of steel bars in concrete was conducted to obtain the parameters for the SCCF equation. Electrochemical impedance spectroscopy (EIS) and linear polarization resistance (LPR) measurements were performed at various strain levels to investigate the stress corrosion behavior of steel bars under simulated chloride ion intrusion conditions. Next, based on the experiment results, a time-varying stress corrosion model was established to examine the corrosion pits of the steel bars under load. Finally, a numerical model of stress corrosion in concrete-exposed steel bars subject to chloride erosion was developed. The impact of loading on the stress corrosion coupling effect and the relationship between the SCCF and the stress concentration factor (K) were discussed. The analyses confirmed an accelerated corrosion rate in steel bars under stress, particularly magnifying as the steel bar transitions into the plasticity phase. Furthermore, the stress corrosion coupling effect was amplified with increasing loads. Within the elastic range, the SCCF and K were basically linear. [ABSTRACT FROM AUTHOR]

In this paper, some recent analytical expressions including those of codes for shear strength calculation of reinforced concrete (RC) beams without and with stirrups are discussed and verified against experimental data available in the literature. An analytical expression proposed by the author is also introduced and discussed. Two different sets of experimental data were analyzed: one for beams without stirrups composed of 719 data and the other one for beams with stirrups composed of 152 data. Comparison between experimental data and analytical results showed that all of the analytical expressions of international codes are conservative in prediction. The proposed equation gives the same grade of accuracy of the other existing analytical expressions for the prediction of experimental data, and it is derived by a mechanical approach. Comparison shows that although most of the analytical expressions give safe and conservative results, more effort is still necessary to reduce the high scatter of prediction of all the expressions used here, especially for beams without stirrups. [ABSTRACT FROM AUTHOR]

A new type of buckling-restrained brace (BRB), called induction-hardened buckling-restrained brace (IHBRB), is proposed in this study. The IHBRB uses a round bar as the core steel and a hollow round section as the restrainer. The core steel undergoes partial induction hardening, which makes its strength lower in the middle but higher at the two ends. This strength difference along the length is expected to trigger energy dissipation in the middle but keep the ends undamaged. The restrainer has an inner diameter similar to that of the core steel, which minimizes the gap between them and avoids time-wasting grouting. This study introduces experimental and analytical research on four types of IHBRB. Tensile coupon tests revealed that the core steel of the IHBRB-600 series satisfied the target yield strength of 600 MPa in the middle and 1,200 MPa at both ends. The core steel of the IHBRB-800 series also satisfied the target yield strength of 800 MPa in the middle and 1,200 MPa at both ends. The ascending cyclic loading test showed that the IHBRB exerted a load capacity of over 1,000 kN as designed and exhibited tensile-compressive symmetric hysteresis behavior. IHBRB-600L, the specimen that failed by tensile fracture, showed that the high-strength brace end succeeded in remaining elastic as planned, while the low-strength brace middle showed a higher-order mode buckling. However, unexpected overall buckling at the joint occurred on other specimens. The low cycle fatigue specimens showed large cumulative inelastic deformation ratios of more than 200. A three-dimensional finite-element model of IHBRB-800L built by ABAQUS/Explicit (Dassault Systèmes Simulia Corp., Paris) captured the experimental results well. A parametric study revealed that the longer restrainer fixed at one side of the core steel end was one of the solutions to guarantee the great seismic performance of the IHBRB. [ABSTRACT FROM AUTHOR]

Corrosion of steel bars embedded in reinforced concrete structures considerably reduces the service life and even causes early failure of the structure. This paper investigates the early corrosion behavior of an alloy corrosion-resistant steel bar in seawater–sea sand concrete (SWSSC) using various electrochemical measurements [electrochemical impedance spectroscopy (EIS) linear polarization resistance (LPR)] and surface characterization techniques [scanning electron microscope-energy dispersive spectroscopy (SEM-EDS), atomic force microscope (AFM), and X-ray photoelectron spectroscopy (XPS)], in comparison to low carbon steel bar (LC) and stainless steel bar (SS). The results reveal that the Cr10Mo1 alloy corrosion-resistant steel bar (CR) in SWSSC shows significant increase in the radius of the capacitive arc in later stages, particularly in the low frequency region. This increase in CR can be attributed to the enhanced corrosion resistance of the passivation film, as well as the rise of impedance at the interface between the passivation film and the concrete matrix. These improvements have effectively inhibited the penetration of free chloride ions into the concrete, thereby impeding the initiation and propagation of corrosion in CR In addition, the mechanical properties, pore structure and the hydration production of SWSSC were investigated. Based on the interactions between steel substrate and concrete matrix, the possible resistance mechanism of CR was proposed. This study innovatively provides a comprehensive evaluation of the effectiveness of CR in SWSSC. [ABSTRACT FROM AUTHOR]

To obtain a precise finite-element model (FEM) for analyzing the dynamic response of corroded beams at high temperatures, a stepwise FEM modification strategy is proposed based on the improved extreme learning machine. Three concrete beams were designed and cast, and the dynamic response characteristics of corroded concrete beams at room temperature and high temperature are discussed. Firstly, electrical accelerated corrosion tests and vibration tests were conducted on simply supported beams at room temperature. The fundamental frequencies of concrete beams under different corrosion ratios were measured. The attenuation law of fundamental frequency with corrosion ratio also was studied. Subsequently, the FEM under different corrosion ratios was modified. The bond-slip between steel bars and concrete under different degrees of corrosion was considered during the correction process. Finally, a vibration test at high temperature was performed. The modal attenuation law of corroded beams at high temperatures was analyzed. Based on the modified FEM, numerical analysis at high temperature was performed. The proposed FEM modification strategy and the study of the attenuation regularities of modal information under fire exposure provide a foundation for further research on the damage development of corroded reinforced concrete (RC) beams under fire exposure. [ABSTRACT FROM AUTHOR]

This study proposed a novel type of self-centering (SC) timber beam–column connection utilizing shape memory alloy (SMA) bars and investigated its cyclic behavior through experimental and numerical studies. In this SC connection, anchor bars consisting of SMA bars, steel bars, and couplers were utilized to connect the beam and column and achieve SC and energy dissipation capabilities. The configuration of the SC timber beam–column connection and material properties of the SMA bars were first introduced. The cyclic behavior of the SC timber beam–column connection was systematically investigated through experimental and numerical studies. A series of cyclic loading tests were conducted on the SC timber beam–column connection to evaluate its stiffness, SC, and energy dissipation capabilities, and the effect of multiearthquake loading. A detailed finite element model of the timber connection was also built and validated using the experimental results. Results indicated that the timber connection could exhibit a desirable flag-shaped hysteretic behavior, indicating favorable SC and moderate energy dissipation capabilities. The connection remained functional without any repair work after experiencing two consecutive cyclic loads up to 4% drift ratios, demonstrating its potential to withstand multiple seismic events. [ABSTRACT FROM AUTHOR]

The combination of glass fiber–reinforced polymer (GFRP) and ultrahigh-performance concrete (UHPC) to form structural members has generated significant interest due to their excellent durability and mechanical properties. This paper presents the flexural behavior and design methodology of GFRP-reinforced UHPC beams. Eight reinforced UHPC beams were tested to failure, varying in longitudinal reinforcement type (steel and GFRP), flexural reinforcement ratio, and steel fiber volume fraction (1% and 2%). Two flexural failure modes, including crack localization followed by rupture of GFRP (tension failure) and progressive crushing of UHPC followed by rupture of GFRP (compression failure), were observed in the tested GFRP-reinforced beams. Substitution of steel bars with GFRP bars resulted in delayed crack localization and a significant improvement in flexural strength by 54.9% and ultimate displacement by 55.7%. Increasing the GFRP reinforcement ratio showed a trend of increased flexural capacity, ultimate deformation, and energy dissipation capacity. Increasing the steel fiber volume in UHPC improved the flexural capacity of the tension failure–controlled beam, but had a slight effect on the flexural capacity of the compression failure–controlled beam. In addition, two different models were used to calculate beam deflection, and were compared with experimental results at the service load levels. Considering the fiber-bridging mechanism, a flexural strength model for GFRP-reinforced UHPC beams was developed. Finally, a minimum reinforcement ratio was proposed to ensure progressive failure of GFRP-reinforced UHPC beams. [ABSTRACT FROM AUTHOR]

The flexural behavior of hybrid timber–steel encased beams comprising coniferous radiata pine (Pinus radiata) (MGP10) and Douglas fir (Pseudotsuga menziesii) (F8) timber lamellae with bonded-in steel bars is studied. The effect of cross-section depth, steel bar size, timber species/grade, and steel bar arrangements (only bottom and top-and-bottom) on the hybrid beams' stiffness, failure mode, ductility, and load-carrying capacity were investigated. The flexural capacity and stiffness of the doubly (top-and-bottom) reinforced beams are increased by 127% and 71%, respectively. However, in the singly (bottom) reinforced beams, the flexural capacity and stiffness are increased only by 41% and 25%, respectively, highlighting the important role of the compressive bars. The failure of all beams was associated with tensile flexural failure of timber, but the steel bars improved the ductility of the beams. The maximum coefficient of variation of the peak load in hybrid beams (CoV=14.3%) is lower than that of the bare timber beams (CoV=21.7%). Two analytical models were developed based on a linear and a bilinear stress–strain relationship for timber. The analytically predicted peak load and stiffness agree well (less than 13% and 12% difference) with the experimental results. [ABSTRACT FROM AUTHOR]

Both the corrosion resistance and bond strength of silica particle-modified enamel (SPME)-coated steel bars are experimentally investigated, and uncoated and pure enamel (PE)-coated bars are also prepared for comparison. The microstructure of SPME coating is analyzed by using scanning electron microscopy (SEM), and the corrosion performance in simulated concrete pore solutions is assessed by using a series of electrochemical techniques. Pull-out tests were conducted to obtain the bond behavior with concrete. SEM images show that silica particles are completely embedded in the enamel matrix. The corrosion resistance of SPME-coated smooth and deformed steel bars is 68 and 28 times lower, respectively, than that of the uncoated bars. The bond strength, bond stiffness, bond ductility, and bond toughness of SPME-coated smooth steel bars are 4.33, 4.61, 35.86, and 20.66 times greater than those of the uncoated bars, while they are 1.29, 4.72, 2.21, and 1.24 times greater than those of the uncoated ones for the deformed steel bars. SPME-coated steel bars can be used to enhance the seismic performance of reinforced concrete structures in earthquake-prone zones, while also improving corrosion resistance. [ABSTRACT FROM AUTHOR]

The objective of this research is to evaluate numerically and experimentally the cyclic performance of corroded columns constructed with accelerated bridge construction (ABC) methods. Three column-to-footing specimens were built and tested under quasi-static cyclic loads; one specimen was subjected to corrosion with a target 10% mass loss (moderate corrosion), one was subjected to a target 25% mass loss corrosion (severe corrosion), and one served as the control. For the moderately corroded specimen, the accelerated corrosion method resulted in an actual mass loss of 10.5% in the longitudinal steel bars and 18.4% in the steel spiral; for the severely corroded specimen there was an actual mass loss of 24.1% in the longitudinal steel bars and 39.9% in the steel spiral. Increased corrosion caused reduction in column lateral displacement capacity in the cyclic load experiments; the control, moderately corroded, and severely corroded specimens reached a drift ratio of 9.0%, 7.0%, and 6.0%, respectively. Computational models were developed for the control and corroded specimens. Corrosion effects were considered by reducing the cross-sectional area, yield strength, and modulus of elasticity of steel bars; reduction in concrete compressive strength was included due to concrete cracking and reduction of bond. The computational models include bond-slip, intentional debonding, low-cycle fatigue, and buckling. Global and local response comparisons of the numerical models with experiments are carried out. The numerical models show good agreement with the experimental results regarding load and displacement capacity and hysteretic energy dissipation; they are able to capture the main effect of corrosion, which was a reduction in lateral displacement capacity. The computational models were used in parametric studies to examine the lateral force and displacement capacity for a range of corrosion levels. [ABSTRACT FROM AUTHOR]

The experimental performance and numerical analysis of a hybrid two-column bridge bent was examined to evaluate its ability to self-center and dissipate hysteretic energy during earthquakes. The experiment was conducted using quasi-static cyclic loads on a scaled specimen with posttensioning bars for self-centering and stretch length anchors for hysteretic energy dissipation. Posttensioning bars were used to connect the cap beam, columns, and footings without any mild steel reinforcing bars crossing the cap beam-to-column and column-to-footing interfaces. The number of posttensioning bars and initial posttensioning forces and the number of stretch length anchors were determined using mechanics and equivalent design of reinforced concrete columns. No yielding of posttensioning bars, mild steel reinforcing bars, or steel spirals was observed up to a 3.5% drift ratio; yielding occurred only in the stretch length anchors that are replaceable after an earthquake. Several stretch length anchors experienced tensile elongation, which exceeded their diameter. The residual drift of the hybrid bridge bent remained below 0.5% at the maximum applied 3.5% drift ratio without any visible damage to the concrete, steel collars, or steel chairs. A nonlinear numerical model was developed to represent the hysteretic response of the rocking bridge bent and was compared with the experimental results showing acceptable global and local response. The experiment and the nonlinear numerical model demonstrate that the bridge bent remains serviceable after significant drift cycles without any damage requiring repairs, which improves seismic resilience. [ABSTRACT FROM AUTHOR]

Steel reinforcement is widely used in reinforced concrete (RC) constructions. However, the durability of the steel RC elements is often compromised due to corrosion under adverse environmental conditions. Glass fiber‒reinforced polymer (GFRP) bars have become an attractive alternative for replacing steel bars in several RC applications due to their excellent durability properties. However, the GFRP RC columns exhibit lesser stiffness and fail in a brittle manner compared to steel RC columns. Also, the second-order moments and deformations can increase in the GFRP RC columns due to the low elastic modulus of GFRP bars. The pseudoductility and postcracking behavior of GFRP RC columns can be improved by adding discrete fibers. In this study, 12 steel and GFRP RC slender columns are tested under eccentric compression (e/h ratio of 0.5), and the results are presented. The test matrix consists of (1) steel RC columns without fibers; (2) GFRP RC columns without fibers; and (3) GFRP RC columns with four different fiber dosages of (1) 0.50% and 1.0% of macrosynthetic polyolefin (PO); and (2) 0.5% and 1.0% of hybrid (HB) combination of PO and steel fibers. The test results are compared with analytical predictions considering the geometrical and material nonlinearities. The test results show that adding HB fibers resulted in higher strength and pseudoductile failure than columns reinforced with only PO fibers. HB fibers significantly increased flexural rigidity and the ultimate strength of slender GFRP RC columns. [ABSTRACT FROM AUTHOR]

To study the corrosion behavior of coupled steel bars under applied stresses, experiments on the influence of applied tensile and compressive stresses on the corrosion of coupled steel bars in simulated concrete pore solutions were conducted. The corrosion processes of the individual steel bars and coupled steel bars with different stress levels, including three tensile stress levels (1/3fy , 2/3fy , and 3/3fy) and three compressive stress levels (−1/3fy , −2/3fy , and −3/3fy), were monitored regularly, by several electrochemical measurements, in chloride contained simulated concrete pore solution for 45 days. The experimental results showed that applied stresses had significant effects on the corrosion of steel bars. In a chloride-free environment, the applied tensile stress was found to protect the passive film. However, once corrosion initiated, the applied tensile stress would aggravate the corrosion, while the applied compressive stress might inhibit the corrosion. Microcell corrosion and macrocell corrosion were significantly influenced by the applied stresses. The applied stresses might alter the corrosion mode. Tensile-stressed steel bars and compressive-stressed steel bars showed the susceptibility to the pitting corrosion. By comparison, steel bars with applied tensile stresses tended to corrode more uniformly; however, those with applied compressive stresses were more likely to undergo localized corrosion. Reinforced concrete structures usually endure various loads such as traffic and wind loads. The external loads have been proven to influence the corrosion process of steels in concrete. However, the specific principles and results of the effect of external loads on the corrosion of steels have yet to be thoroughly studied. This study aims to investigate the corrosion process of steel under different stress levels in a simulated concrete environment. Compared with an experiment in practical concrete structures, an experiment in simulated concrete pore solutions has a shorter test cycle and the experimental phenomena are more obvious. The results show that the applied tensile stress will aggravate the corrosion of steel, while the applied compressive stress will restrain the corrosion process. Based on the research results, engineers should pay attention to the effect of external loads on the durability of structures in the design stage. Moreover, this study provides a reference for further experimental studies in practical reinforced concrete structures. [ABSTRACT FROM AUTHOR]

The production quality assessment of prefabricated laminated slabs (PLS) is required to ensure the installation of components on the construction site. However, existing inspection methods are difficult to realize PLS production quality inspection during the production process, and the inspection speed cannot match the PLS production speed. To address these issues, a deep learning (DL) method based on You Only Look Once version 4 (YOLOv4) is proposed for the quality detection of PLS in production in this research. Five new measures are introduced to improve the performance of YOLOv4. The ResNet50vd backbone and deformable convolution (DCN) are used to enhance the ability of the model to capture the feature information of small targets with drastically changing morphologies in complex contexts. The k-means++ clustering algorithm is used to recalculate anchor box specifications to overcome the challenge of large differences in scales across targets as well as within the same target. Intersection over Union (IoU)-aware loss is used to optimize the loss function to improve the localization accuracy of small targets. Geometric median filtering pruning (FPGM) is used to simplify the network, and the model parameters are reduced to 44.1%, which significantly improves the detection speed of the components. The results show that the improved YOLOv4 can quickly and accurately detect laminated panels, PVC boxes, and steel bars in PLS when IoU=0.75 , which is more suitable for industrial applications, and the model achieves a mean average precision (mAP), F1 score, and FPS of 91.38%, 94.54%, and 22.91 frame rate (f/s) , respectively, which is superior to the comparison method. The proposed method can be directly applied to PLS in production to preemptively detect errors and discrepancies in the components and improve the qualification rate and quality management efficiency of the components, which has a positive effect on the rapid development of industrialized construction. Combined with the actual situation of prefabricated components production, this paper proposes a PLS production quality inspection method that is more in line with the factory production process. The new method was obtained by introducing five improvements to YOLOv4. The improved YOLOv4 can quickly and accurately detect laminated panels, PVC boxes, and steel bars in PLS when IoU=0.75 and is more suitable for industrial applications, and the model achieves a mAP, F1 score, and FPS of 91.38%, 94.54%, and 22.91 f/s , respectively. The proposed method does not require an additional inspection site and can be directly applied to the PLS production line for quality inspection of the PLS in production without interfering with the normal production process of the components and the outgoing process. By changing the posttest inspection to in-test inspection, the method can preemptively detect errors and discrepancies in PLS production and implement remedial measures in a timely manner, which effectively improves the qualification rate of components and the level of production quality management, and reduces the production cost of components. [ABSTRACT FROM AUTHOR]

One of the obstacles to the recent trend toward taller timber buildings is the limited load-carrying capacity of softwood columns. With the aim of promoting the structural use of European beechwood (Fagus sylvatica L.) in high-performance applications, the buckling behavior of beech glued-laminated timber (GLT) columns reinforced with glued-in steel bars was investigated experimentally and numerically. Axial compression experiments were carried out on full-scale stocky and slender columns, and a finite-element model was developed and validated against the experimental data. The influence of geometric and material parameters on the load-carrying capacity of the steel-reinforced beech GLT columns was studied in parametric analyses. The experimental and numerical data demonstrate the high potential of this new structural product for high-strength columns in demanding residential, office, and industrial applications. The load-carrying capacity mainly depends on the cross section size, the column slenderness, the position and diameter of the reinforcement bars, and the initial deformed shape of the column. An eccentric layout with steel bars in the corners of the cross section is very effective in increasing the load-carrying capacity. Four corner steel bars of 20 mm diameter, 50 mm edge distance, and grade ST900/1100 were found to increase the load-carrying capacity of a 200-mm-wide square GL48h column by almost 40% across the slenderness ratios relevant to structural applications. The glued-in steel reinforcement is also expected to be able to provide an alternative load path for structural robustness, by enabling the columns to carry tensile forces. A design method for corner-reinforced beech GLT columns under axial compression was developed based on the empirical and numerical data. [ABSTRACT FROM AUTHOR]

Heat-treated ribbed bar 630 (HTRB630) is a new type of reinforcing steel produced in China. The tensile properties and low-cycle fatigue properties of HTRB630 high-strength steel bars after exposure to high temperature are crucial for the seismic performance of reinforced concrete structures potentially subjected to fire. With this aim, HTRB630 high-strength steel bar specimens were heated to high temperatures (200°C, 400°C, 600°C, and 800°C), and cooled to the room temperature by water. The appearance and microstructures of the specimens with different exposure temperatures were observed. The tensile tests and different constant strain amplitude low-cycle fatigue tests were carried out. Test results indicated that the specimens exposed to high temperature below 600°C had good tensile properties and low-cycle fatigue properties. Variations of the metallographic structures of the specimens resulted in different tensile properties and different low-cycle fatigue properties. After exposure to the same temperature, as the strain amplitude increased, the shape of the hysteresis curve of the specimen was fuller, its plastic strain amplitude and plastic strain range and elastic strain range ratio increased, its maximum stress per cycle increased slightly, its softening characteristic was more obvious, and its residual dissipated energy density decreased. All specimens performed ductile fracture failures. Therefore, HTRB630 high-strength steel bars have broad application prospects in antifire reinforced concrete structures. [ABSTRACT FROM AUTHOR]

Steel strips as soil reinforcements have two significant advantages over reinforcements with a circular cross section like solid and hollow steel bars. First, the contact area of steel strips is more than that of steel bars when they are compared in terms of their steel volume per unit length. Second, unlike steel bars, most of the contact surfaces of steel strips are perpendicular to the in situ vertical pressure. To further enhance steel strips' efficiencies, denti-steel strips are presented in this study. This form of steel strips further integrates the aforementioned mechanical advantages by welding several dents along a smooth steel strip. The dents are small rectangular pieces of smooth steel placed at regular intervals. A total of 240 pullout tests consisting of 51 experimental tests and 189 numerical simulations were conducted to evaluate the performance of a denti-steel strip as an innovative bearing reinforcement in frictional soil. Three-dimensional numerical simulations were carried out using the finite-difference package FLAC3D and validated with the experimental results. It was found that the height, number, and relative spacing of dents are dominant factors in the pullout capacity, and, unlike ribbed steel strips with an unchangeable configuration, the optimum dent spacing of denti-steel strips can be designed based on the properties of granular soil. This research introduces a sawtooth failure surface for a denti-steel strip when there is no interference between dents. However, it proposes a block failure surface when there is severe interference between them. These failure surfaces are the bases of developing several rational equations for predicting the maximum pullout capacity of denti-steel strips. The results show that the intensity of interference between dents depends on the spacing-to-height ratio of dents. There are two critical ratios beyond which the interference between dents and failure mechanism significantly change. Numerical simulations indicate that the average normal stress on the reinforcement is more than the applied normal pressure. Furthermore, the average-to-applied ratio of normal stress increases with a decrease in the applied normal pressure. [ABSTRACT FROM AUTHOR]

Vertical openings in beams and girders are sometimes used to facilitate the passage of essential services such as HVAC, electricity, and sewage. There is a lack of research on the effects of vertical openings on deep reinforced concrete beams. In this work, 11 RC deep beams were tested to failure, examining key parameters, namely, location of opening, opening shape, and strengthening technique around the opening. The beam ultimate load (Pu) and to a lesser extent stiffness were found to be affected by the presence of openings, wherein those located in the shear span reduced Pu by 12%–19% and those in the constant-moment region contributed to a respective decrease of 8%–13%. The reduction in Pu is also related to the opening shape with rectangular and circular shapes, resulting in the largest and smallest decrease, respectively, and square being in the middle. Two techniques were proposed to mitigate the openings effects, namely, using ultrahigh-performance concrete (UHPC) in the vicinity of the opening, and reinforcing the surrounding opening area with horizontal and vertical steel bars. Both were found effective, with the former resulting in 14% and 10% increase in Pu relative to beams with openings located in the shear and moment regions, respectively, and no strengthening. Additionally, Pu from tests were compared with those calculated using the strut-and-tie model (STM) and an empirical formula from current US standards. Modifications to the STM model, which resulted in better predictions than the expression from current standards, but still needed improvement, were presented, and resulted in an analytical-to-experimental Pu ratio of 0.99 to 1.05. [ABSTRACT FROM AUTHOR]

Reinforced concrete (RC) corbels are commonly utilized in bridges and industrial buildings to support primary beams and girders. Using glass fiber–reinforced polymer (GFRP) reinforcement in corbels can be advantageous due to its corrosion-resistance properties. However, GFRP reinforcement, with a lower modulus of elasticity and shear strength than steel, could affect the capacity of direct shear. This paper presents the experimental results of nine full-scale, double-sided corbels reinforced with either GFRP or steel bent bars. Large-scale double-sided corbels were constructed and tested for failure under monotonic concentric loads. The test parameters included the reinforcement type (GFRP and steel), the main reinforcement ratio (0.5% and 0.7%), the shear span-to-depth ratio (a/d = 0.33 and 0.66), and the amount of crack-control horizontal reinforcement (0.7% and 1.3%). The predictions of corbel capacity using the Canadian standards for FRP-RC structures were conservative, especially for the corbels with crack-control reinforcement. In contrast, the predictions of the American and European codes overestimated the corbel strength, particularly for the higher a/d ratio of 0.66. [ABSTRACT FROM AUTHOR]

The flexural and durability performance of reinforced ultrahigh-performance fiber-reinforced concrete (UHPFRC) beam structures is expected to improve if the steel bars are replaced with carbon fiber–reinforced polymer (CFRP) bars with high tensile strength and corrosion resistance. Nevertheless, the interfacial bond characteristics between CFRP bars and UHPFRC may deteriorate when subjected to elevated ambient temperatures during service. To investigate the bond–slip behavior of CFRP bars embedded in UHPFRC at elevated ambient temperature, a series of direct pullout tests were conducted under elevated ambient temperatures ranging from 20°C to 80°C. The testing used CFRP bars with two diameters of 10 and 12 mm. Based on the test results, two temperature-dependent bond–slip models (T-MBPE and T-MCMR-SC models) and a temperature-dependent bond strength model were developed. The results indicated that the bond strength decreases dramatically as the ambient temperature increases. The bond strengths of the specimens with 12-mm-diameter CFRP bars are higher than those with 10-mm-diameter CFRP bars, which is mainly due to the greater rib height of the 12-mm-diameter CFRP bars. Compared with the proposed T-MBPE model, the constructed bond–slip relationships of the proposed T-MCMR-SC model were more consistent with the experimental results. The proposed temperature-dependent bond strength model performed well in characterizing the bond degradation under elevated ambient temperatures, with a prediction error of less than 10% compared to the experimental results. The research outcomes can provide an experimental and constitutive basis for using CFRP bars in UHPFRC structures at elevated temperatures. [ABSTRACT FROM AUTHOR]

Ultrahigh-performance concrete (UHPC) is a new cementitious composite material that exhibits a high bond strength with steel reinforcement. To investigate the splice performance levels of steel bars in UHPC, mutual pullout tests on lap-spliced specimens and direct central pullout tests on bond-anchorage specimens were conducted in this work. The main variables included loading mode, concrete cover, and steel bar position. The results showed that the splice strength of lap-spliced steel bars was approximately 0.76 times the bond strength of the anchored steel bar with the same bond length. The splice strengths of specimens under one-way repeated loading were lower than those under monotonic loading; increasing the splice length or concrete cover thickness reduced the weakening effect of one-way repeated loading on lap-spliced specimen performance. The concrete cover thickness showed a strong positive linear correlation with the splice strength in the range of 0.75db to 2.25db and played an important role in stopping crack development in the specimens. The splice strengths decreased when the loading steel bar was moved from the middle of the structure to the side. Based on the experimental results, formulas for the splice strength and critical splice length of lap-spliced steel bars in UHPC under monotonic loading and one-way repeated loading are suggested. Finally, an analytical model for the bond slip of lap-spliced steel bars is proposed, providing a good fit with the test results. [ABSTRACT FROM AUTHOR]

To explore the formation of corrosion products of steel bars in coral aggregate seawater concrete (CASC), this study used X-ray powder diffraction (XRD) to examine the ingredients of corrosion products of CASC steel bar, and scanning electron microscopy (SEM) to scan the surface structure of the steel rust layer to understand the difference between the steel corrosion products under different test environments. In addition, this paper analyzes the steel corrosion in CASC and seawater sea-sand concrete (SSC) at the micro level. The results demonstrate that the CASC in the marine environment undergoes electrochemical reactions to produce insoluble Fe(OH)2. Under the immersion environment, the large amount of α-FeO(OH) contained in the rust layer of the steel bars can accelerate the corrosion process. There are more free chloride ions (Cl−) in the dry and wet cycle environment, which leads to the corrosion products of the steel bar containing more β-FeO(OH). Furthermore, it was noticed that the presence of small openings in the needle-shaped crystals can cause water molecules to become trapped, making the pitting corrosion of the steel bar worse. This situation provides an ideal setting for the occurrence of severe corrosion. The corrosion process in a dry–wet cycle environment is less than the corrosion products of the steel bar under the full immersion environment; the structure of steel corrosion products of CASC are less than that of SSC. [ABSTRACT FROM AUTHOR]

In this work, the influence of the area ratio of the low corrosion probability area (prone to becoming cathode) and high corrosion probability area (prone to becoming anode) on the initiation of macrocell corrosion of a steel bar in a simulated pore solution was investigated, along with the depassivation mechanism under the macrocell effect. Meanwhile, the set cathode-to-anode area ratio (SC/A) and the actual cathode-to-anode (pitted) area ratio (SC/A′) after corrosion initiation were compared. The results reveal that the increase in SC/A not only increases the corrosion current density but also promotes corrosion initiation drastically. A pronounced positive shift was observed at high SC/A in the initiation phase due to intensified chloride adsorption and the double-layer effect on the passive film of the anodic steel bar. The chloride threshold value (CTV) could be reduced to 42% of the original time in the simulated concrete pore solution, and the pit depth was increased by up to 124% due to the macrocell effect. In addition, the actual corrosion area (Sact) is much smaller than the set anode area (SA) because the corrosion area created by chloride ions is tiny, resulting in SC/A′ being higher than SC/A. [ABSTRACT FROM AUTHOR]

The key considerations in the design of a high-pressure tunnel are ensuring stability and limiting water losses. In this paper, a triple-layer, thick-walled cylinder model considering the impact of the loose rock zone was built to investigate the inner water exosmosis and conditional cooperation mechanisms in high-pressure tunnels. An analytical calculation method for the design-related parameters, such as the steel bar stress, maximum crack width, and water losses of the high-pressure tunnel, was proposed. A comparison of the results obtained by the proposed solution with those obtained by other numerical and analytical solutions was presented to validate the proposed model. Then, the research for a hydropower diversion tunnel was conducted, and the impact of the loose rock zone on the design-related parameters was analyzed. The steel bar stress is generally lower than the design expectations due to the conditional cooperation between the concrete lining and surrounding rock during the high-pressure tunnel operation period. The consistency of measured field data in tight rock and calculation outputs further verified the presented analytical model. Finally, the proposed model was applied to investigate various parameters related to the loose rock zone, and the results could be considered general recommendations for developing pervious linings with high internal water pressure (IWP). [ABSTRACT FROM AUTHOR]

This study aims to investigate the flexural performance and design methods of flexural concrete beams reinforced with basalt fiber–reinforced polymer (BFRP) and steel bars. Eight concrete beams, which included six reinforced with a combination of BFRP and steel bars, one reinforced with only steel bars, and one reinforced with only BFRP bars, were constructed and then tested under four-point loading. For hybrid-reinforced concrete (RC) beams, two types of BFRP bars (ribbed and sand-coated) and different reinforcement ratios were considered. The test results are discussed in terms of the flexural capacity, reinforcement strain, deflection responses, and cracking behavior. Experimental results showed that the crack width of the hybrid-RC beam, obtained by adding BFRP bars in the concrete cover of the steel-RC beam, reduced by approximately 40% compared with that of the steel-RC beam, and the strength utilization of BFRP bars in hybrid-RC beams was higher than that in the BFRP-RC beam. Additionally, the cracking moment, nominal flexural strength, and midspan deflection of the hybrid-RC beams were predicted by using existing design models. The bond-dependent coefficients (kb) of the ribbed and sand-coated BFRP bars were assessed by using test data. [ABSTRACT FROM AUTHOR]

Pretensioned spun high-strength concrete piles usually use the traditional helical grooved steel bars as the main reinforcement, leading to insufficient ductility. Replacing prestressing steel bars with steel strands can effectively overcome the problem. To further enhance the overall seismic performance of the piles, pretensioned centrifugal spun concrete piles with combined use of pretensioned steel strands and nonprestressing deformed steel bars (PSRC piles) have been developed. This paper presents experimental and numerical investigations into the seismic performances of PSRC piles. Three full-scale PSRC pile specimens have been tested under lateral cyclic loading with different axial compressive forces, and the results are analyzed in detail and discussed. The influence of incorporating the deformed steel bars on the cyclic behavior of piles is examined with a comparison to the previous test results of the counterpart piles with only steel strands. A detailed finite element model of the PSRC pile specimens is developed and verified against the test results. Parametric analyses are then carried out using the validated model. The results show that the incorporation of nonprestressing deformed steel bars markedly improves the cracking behavior of the piles with much-diffused crack distributions. The combined use of steel strands and deformed bars also results in better deformation capacity as well as higher load-bearing capacity. By adjusting the concrete wall thickness and selecting a desirable ratio of prestressing steel strands and nonprestressing deformed bars, sufficient deformation and load-bearing capacities can be ensured with the piles. [ABSTRACT FROM AUTHOR]

The bond performance between concrete and steel bars in complex environments forms the basis for the serviceability and safety of RC (reinforced concrete) structures. To investigate the bond behavior between steel bars and concrete under different conditions, a series of pullout tests were conducted. Corrosion and fatigue loading were selected as influencing factors for the degradation of bond behavior, and a series of tests were developed to investigate the corrosion level, surface crack width, failure mode, strain distribution, bond stress, slip under a single effect (fatigue, corrosion), and the coupled effect of corrosion and fatigue loading. The test results indicate that the corrosion level and surface crack width increase with increasing fatigue cycles, fatigue loading has little impact on the bond stress and slip, and chloride penetration exhibits a significant influence on the decrease in peak bond stress and increase in slip. A comparison with the specimens experiencing a single effect indicated that the coupled effect of fatigue loading and corrosion accelerated the degradation process of the bond performance and that the dominant factor of the degradation process transformed from fatigue loading to corrosion with increasing coupled effect time. Considering the nonuniform corrosion morphology of steel bars, a prediction model for bond strength is established based on the single trigonometric series method and minimum potential energy principle. Through evaluation of the experimental data, a model for the bond stress-slip relationship under the coupled effect of fatigue and chloride penetration is proposed. [ABSTRACT FROM AUTHOR]

This work reveals influence of the stress state of steel bars on the critical chloride content for depassivation in a simulated concrete pore solution (SCPS). The passivation behavior of steel in chloride-free SCPS and depassivation behavior in SCPS with incremental addition of chloride were studied by electrochemical techniques including electrochemical impedance spectroscopy, Mott-Shottky test, cyclic voltammetry, and others. Results demonstrated that stress enhanced the redox activity in highly alkaline SCPS, where enhancement by tension was more pronounced than compression. Additionally, stress increased the donor density and dielectric properties of the constant-phase element of the passive film. The critical chloride content of steels can be considerably lowered by 80% under stress. [ABSTRACT FROM AUTHOR]

The maintenance or replacement of critical infrastructure exposed to aggressive environments is a major problem in many countries. To address this issue, alternative noncorrosive materials, such as fiber-reinforced polymers (FRPs), have been proposed to substitute conventional steel bars. However, large-scale tests of beams with small shear-span-to-depth ratios (deep beams) and FRP bars have shown a significant reduction of shear strength compared to similar beams with steel reinforcement. Therefore, the main objective of this paper is to model and explain the mechanisms that govern the reduction in strength. Twelve test specimens from the literature are analyzed based on a crack-based assessment framework for steel-reinforced concrete members, which is extended to account for FRP bars. The crack-based approach shows that large strains in the flexural FRP reinforcement diminish the aggregate interlock resistance and cause premature shear-induced flexural failures in members without shear reinforcement. It is also shown that the model adequately captures local and global deformations, as well as the effects of beam slenderness, longitudinal reinforcement axial stiffness, concrete strength, and crack geometry on the shear response of internally FRP-reinforced concrete deep beams without shear reinforcement. [ABSTRACT FROM AUTHOR]

To explore the application of heat-treated ribbed bar (HTRB) type HTRB630 high-strength steel bars in reinforced concrete bridges, the tensile test and high-cycle fatigue test are carried out. Then, the high-cycle fatigue properties of the specimens are analyzed and discussed from median stress-number of loading cycles (S-N) curve, P-S-N curve, comparison with 500-MPa grade steel bars, and fatigue fracture mechanism. Finally, their high-cycle fatigue properties are investigated numerically, and the influence of stress ratio on their fatigue properties is discussed. The results indicated that the probability-stress-number of loading cycles (P-S-N) curve with a confidence level γ=75% and a reliability guarantee rate P=97.7% is more accurate than the median S-N curves for the fatigue design of reinforced concrete bridges. In the P-S-N curve, the stress ranges at 2 million and 10 million cycles are 237.03 and 203.38 MPa, respectively. In the median S-N curve, the stress ranges at 2 million and 10 million cycles of fatigue life of steel bars are 262.77 and 236.08 MPa. The fatigue properties of the specimens are higher than those for the 500-MPa grade steel bars. The fatigue fracture of the specimens consists of a fatigue source zone, crack propagation zone, and transient fracture zone. Microscopically, it is a cleavage fracture in the crack propagation zone; it is micropore aggregation fractures in the instantaneous fracture zone. At a given fatigue life, the fatigue stress range decreases as the stress ratio increases. The fatigue properties of the specimens are better at a higher stress ratio with the same maximum stress. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The bond strength of steel reinforcement bars in concrete is of prime importance for the strength and serviceability of reinforced concrete structures. The composition of concrete directly affects the bond strength of steel bars in concrete. Research is ongoing to improve the bond properties of concrete with the addition of nanoparticles. In this paper, the effect of adding graphene nanoplatelets (GNP) to geopolymer concrete on the bond strength of steel bars in geopolymer concrete was investigated and compared with the bond strength of steel bars in ordinary portland cement (OPC) concrete. Slag and fly ash were used as aluminosilicate precursors, and ambient curing conditions were adopted for preparing geopolymer concrete. Specimens were cast with geopolymer concrete without and with GNP and OPC concrete without and with GNP. Steel bars of four different diameters (10, 12, 16, and 20 mm) were used for preparing the specimens. The quantity of GNP added in the concrete was 0.5% by weight of aluminosilicate materials in geopolymer concrete and 0.5% by weight of cement in OPC concrete. The addition of GNP to geopolymer concrete enhanced the bond strength by 13%–19% for the steel bars of diameters 10–20 mm. The bond strengths of steel bars in geopolymer concrete were found to be lower than the bond strength of steel bars in OPC concrete. Furthermore, the bond strengths were calculated using equations available in the design codes and available research studies on geopolymer and OPC concrete and compared with the experimental bond strengths. [ABSTRACT FROM AUTHOR]

The seismic performance of structures could be enhanced by the inclusion of supplemental components that dissipate the earthquake-induced energy. This is especially crucial for rocking structures that possess low energy dissipation properties due to their damage avoidance mechanism. Amongst variously developed yielding-type energy dissipaters (EDs), buckling-restrained energy dissipating carbon steel bars have received considerable attention as they make the most use of the inherent energy dissipation of steel and are easy to fabricate. However, maintenance, repair costs, and performance disruptions owing to corrosive environments have been mostly disregarded in past investigations. Employing stainless steel can be a viable solution to overcome such issues. The work described in this two-part study sheds light on various aspects of the buckling-restrained stainless steel EDs through experimental and finite element (FE) investigations. The mechanical properties of type 304L stainless steel including uniaxial monotonic response, strain sensitivity, and cyclic hardening are characterized. It is shown that the material possesses high ductility along with substantial hardening characteristics. In the first phase of testing, energy dissipating bars with different fuse diameters and lengths are designed. Load and strain capacities and bar-tube interactions of the buckling-restrained energy dissipation device are studied through quasi-static tests. In the second phase of testing, various EDs are designed, fabricated with stainless and mild steel, and tested under quasi-static loading to validate the findings of the FE investigations. The test results demonstrate a stable hysteresis response of the buckling-restrained stainless steel EDs with a cyclic average strain capacity of 10%. The FE modelling procedure, calibration, and parametric studies are presented in the companion paper as Part II. [ABSTRACT FROM AUTHOR]

Accidental fires in aged concrete structures can lead to combined damage from corrosion and fire. In this study, we aimed to explore the effects of corrosion and high temperature on the residual mechanical properties of hot-rolled plain and ribbed bars. The results indicated that the corroded steel bars still exhibited ductile failure after high-temperature exposure. Moreover, the combined action of high temperature and corrosion has little effect on the elastic modulus. The residual bearing capacity of the hot-rolled steel bar became smaller under the condition of temperatures above 600°C. When the temperatures experienced by the plain and ribbed steel bars were less than 600°C and 500°C, respectively, the high temperature accelerated the decrease in the nominal yield and ultimate strength. However, the yield and ultimate strengths based on the minimum cross-sectional area remained unchanged with an increase in the cross-sectional loss ratio. Similar to that at room temperature, the ultimate strain of steel bars subjected to high-temperature exposure decreased exponentially with an increase in the cross-sectional loss ratio. The new findings of this study can provide a theoretical basis for the safety assessment of aged reinforced concrete structures after exposure to fire. [ABSTRACT FROM AUTHOR]

Sudden failure of reinforced concrete (RC) or prestressed concrete (PC) Gerber saddles of bridges and viaducts has occurred all around in the word in the last few years due to corrosion of steel bars. The danger of sudden and brittle failure is often due to general and pitting corrosion of steel bars, concrete crushing, and loss of bond in steel bars. In this paper, the flexural response of reinforced concrete Gerber supports under their self-weight with or without service loads was investigated through determination of the load-deflection response of beams, with the focus on the consequences of pitting corrosion and loss of bond in steel bars. A simplified strut-and-tie model was developed to predict the flexural response of Gerber supports in terms of load-deflection curves, deduced analytically in a closed form. The model includes the effects of corrosion of steel bars, loss of bond, and concrete crushing due to the biaxial state of stresses. Several experimental laboratory studies regarding the flexural behavior of RC beams with Gerber supports were collected to validate the proposed model. In addition, numerical analyses were performed through a nonlinear finite-element code for comparison with the experimental and analytical results. [ABSTRACT FROM AUTHOR]

The American Concrete Institute has developed a design standard for glass fiber–reinforced polymer (GFRP) RC members, dependent to their standard for steel-reinforced structural concrete. Design examples conforming to code language have been developed and their solutions examined to understand the impacts that code requirements have on the design of GFRP-RC members. The impacts include selecting member dimensions and reinforcement on the basis of serviceability requirements, rather than strength, and the related detailing issues involving bar cutoff points. As the design of GFRP-RC members is typically controlled by serviceability requirements, some reinforcement may be provided to satisfy deflection requirements. Cutoff points for such bars must be located differently than for steel bars, where the cutoff point is determined by strength requirements. A proposed method locates cutoffs by calculating the resulting maximum deflections as the cutoff point is varied. A second, approximate, approach locates cutoffs at points where continuing bars provide the same value for effective moment of inertia as that available from all bars at the critical section. The second method results in longer bar lengths but is more easily applied. Shear and crack control requirements should also be checked at cutoff locations. [ABSTRACT FROM AUTHOR]

Due to its superior mechanical and durability performance, ultrahigh performance concrete (UHPC) has gained increasing applications worldwide. While studies have suggested different strategies to design reinforced UHPC beams, their resulting structural behavior is not yet well understood, and their carbon footprint has not been given considerable attention. Based on nonlinear finite element simulations and cradle-to-gate life-cycle analyses, this study compares the performance of two steel-reinforced concrete beams to steel-reinforced UHPC (R/UHPC) beams with the same flexural load capacity but different design strategies. Results show that an R/UHPC beam of the same cross-section but lower reinforcing ratio of a conventional concrete beam could show 166% higher carbon emission and 64% less deformation capacity than a control steel-reinforced concrete beam. On the other hand, a well-designed R/UHPC beam can (1) maintain the carbon footprint; (2) reduce the section size and steel bar usage by 50% and 30%–53%, respectively; and (3) show equivalent or higher service-level stiffness and ultimate deformation capacity. [ABSTRACT FROM AUTHOR]

Millions of in-service concrete bridges around the world are exceeding their design life. Corrosion may lead to a sudden fracture of prestressing steels, and understanding the fatigue behavior of corroded prestressed concrete (PC) bridges is essential. Accordingly, an effective fatigue analytical model for corroded PC beams is needed. In this study, an experimental investigation of the fatigue behavior of corroded PC beams, was carried out. Thirteen PC beams had experienced acceleration corrosion before high-cycle fatigue testing. All beams failed due to fatigue fracture of the corroded tensile steel bars, and three-dimensional laser scanning results indicated that the fracture occurred at the minimum cross sections with a severe corrosion pit. A segmented linear analytical model to evaluate fatigue response and predict fatigue life was developed which considers fatigue irrecoverable plastic strain of concrete, stress concentration, and longitudinal variation in the cross-sectional areas of corroded steel bars. There is an acceptable agreement between the numerical and experimental results of corroded PC beams. The proposed model can reasonably predict the evolution of strains in concrete, corroded prestressing steel, and rebars in PC beams under high-cycle fatigue loading, thereby allowing fatigue life prediction. Further analysis studied the influence of degree of corrosion in prestressing wires/rebars, stress concentration and longitudinal variation in cross-sectional areas of corroded steel bars, load ranges, and partial prestressing ratios (PPRs). It was found that, to reasonably estimate the fatigue life of corroded PC beams, the influence of stress concentration must be considered with the proposed impact factor. A higher corrosion degree, load range, or PPR shortens the fatigue life of corroded PC beams. [ABSTRACT FROM AUTHOR]

Steel bar corrosion is one of the most common causes of structural performance degradation for reinforced concrete (RC) columns subjected to extreme loads such as earthquakes. Due to the different characteristics of corrosion effects, a comprehensive and efficient finite element (FE) modeling approach is necessary for the performance assessment of corroded RC columns. To this end, this study provides an effective FE modeling approach using a recently developed geometrically nonlinear fiber-based frame element that considers bond-slip, allowing for explicit representation of corrosion-affected bonding. This aspect necessitates the use of an explicit material model for corroded bond-slip and its associated corroded properties. Since the existing models of corroded bonding properties in the literature were developed based on the highly scattered and limited experimental data, this study develops a new probabilistic model for corroded bonding properties based on more experimental data compiled from the literature and quantifies the prevailing uncertainties. To facilitate its use for modeling corroded RC structures, a cyclic corroded bond-slip model in an open-source FE software framework is implemented. In this paper, the FE modeling approach is first used to model two corroded RC columns tested in the literature, with comparison to the conventional modeling approach (i.e., assuming perfect bonding). The influence of corroded bonding on the static and dynamic behaviors of RC columns is further explored deterministically. Furthermore, the impact of uncertainty in corroded bonding properties on the static and dynamic behavior simulations of RC columns is examined. This work is concluded that (1) the FE modeling approach provided for corroded RC columns proves to be effective in capturing corrosion effects; and (2) corroded bonding plays an important role in simulating the behavior of corroded RC columns, and thus cannot be neglected. This is particularly true considering its associated uncertainty and its impact on the performance assessment of RC columns under severe corrosion conditions. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation of the fatigue behavior of ultrahigh-performance concrete (UHPC) beams prestressed with external carbon fiber-reinforced polymer (CFRP) tendons. Seven T-shaped beams were tested, and the effects of the effective prestressing stress, partial prestressing ratio, and fatigue load level on the flexural behavior and stiffness were analyzed and presented. Overall, the CFRP tendon–anchor assembly exhibited excellent fatigue performance. All tested fully prestressed beams survived 2 million cycles in fatigue tests, and the variation in their flexural capacity was limited. The partially prestressed beam failed after approximately 97,200 cycles due to fatigue fracture of the internal reinforcement. After fracture of the tensile steel bars, the partially prestressed beam behaved similarly to the fully prestressed beam and sustained more than half of the flexural capacity after 2 million cycles. A higher effective prestressing stress was favorable for maintaining the stiffness of CFRP prestressed UHPC beams. [ABSTRACT FROM AUTHOR]

This study presents a numerical model to evaluate the flexural behavior of concrete beams reinforced with hybrid fiber-reinforced polymer (FRP) and steel bars. The bond–slip action between the reinforcing bars and the surrounding concrete was considered. The cracking load, yielding load, ultimate flexural capacity, strain development, moment–curvature relationship, and midspan deflection were predicted using the proposed numerical model. The model was compared with the experimental results available in the literature and the theoretical results from the design codes. The equations from the design codes underestimated the cracking load by 7%–9%. The averages of numerical yielding moments without and with a bond were 0.96 and 0.97, respectively. The predicted yielding moment considering the bond behavior was approximate to that of neglecting bond. The predicted ultimate flexural capacity without a bond was 3% higher than the test result, whereas the predicted result with the bond was 1% lower than the experimental result. Both design codes underestimated the mid-span deflection under service loads for hybrid-RC beams. The postyielding deflection and deflection behaviors at the service load level were captured well by the proposed model. A parametric study investigated the effects of the reinforcement arrangement. Beams [with a glass FRP (GFRP) reinforcement ratio of 2.06%] with reinforcements placed in one layer presented the highest ultimate capacity, 19.6% and 14.0% higher than those of GFRP bars placed at the outer and inner layers, respectively. Beams with GFRP bars placed in the outer layer showed the highest deformability index when the GFRP reinforcement ratio was less than 1.75%. This study presents a reliable model to simulate the flexural behavior of concrete beams reinforced with hybrid fiber-reinforced polymer (FRP) and steel bars. The bond–slip action was considered in the model. ACI 318-19 and CAN/CSA A23.3-19 underestimated the cracking load by 7% and 9%, respectively. In addition to the tensile strength of the concrete and the geometry of the section, the layout of the reinforcement was considered in the proposed model, thus achieving a more accurate prediction of the cracking load. The model without bond stress overestimated the ultimate flexural capacity of compression-controlled members and underestimated the ultimate capacity of tension-controlled members. Both ACI 318-19 and ACI 440.1R-15 underestimated the midspan deflection under service loads for hybrid-RC beams. The strain in FRP bars was highly related to the effective reinforcement ratio based on the strength transform (Lau's equation) for hybrid-RC beams. For a designed strain limit for the FRP bars, the required effective reinforcement ratio for beams can be obtained from Fig. 9(b). Beams with reinforcements placed in one layer exhibited the highest ultimate capacity. [ABSTRACT FROM AUTHOR]

This paper presents the results of an experimental study carried out to evaluate the structural behavior of precast concrete culverts reinforced with glass fiber–reinforced polymer (GFRP) bars. Five full-scale GFRP reinforced large culverts having a clear span of 1,530 and 960 mm high were tested under different combinations of lateral and vertical loads. The general behavior of the culverts including crack pattern, force-displacement behavior, ultimate capacity, and failure mode were evaluated and analyzed. The behavior of the culverts at the serviceability limit state were also monitored in terms of crack width and deflection. Punching shear failure at the top slab was found to be the typical mode of failure in all of the culverts tested. Failure was characterized by crushing of concrete with no rupturing of the longitudinal GFRP-reinforcement. The results indicated that GFRP-reinforced concrete culverts have an acceptable load-carrying performance with a small residual deflection and crack width. The presence of the lateral load, simulating the effect of soil, resulted in upward deflection (hogging) of the culverts and increased the load-carrying capacity of the culverts. Maximum deflections were observed when two walls of the culverts were unrestrained. The experimental study presented here indicated that serviceability limit state (deflection and crack width) rather than the strength governs the design of GFRP-reinforced concrete culverts. This study showed that GFRP can be a suitable alternative to steel for precast concrete culvert application, especially in aggressive environments where steel bars are vulnerable to corrosion. [ABSTRACT FROM AUTHOR]

The present study aims to develop a two-yoke magnetic sensor system, measuring the mass loss of steel in concrete subjected to nonuniform corrosion. The impressed current method was employed to induce the nonuniform corrosion of steel in concrete specimens. Experiments and theoretical calculations based on the finite-element method were performed to verify the effectiveness of the two-yoke magnetic sensor. Parametric analyses were carried out to investigate the effects of distribution of rust layer, the geometry of steel bar, as well as the depth of concrete cover on the results of magnetic flux density sensed by the sensor. Results showed a good accuracy in estimating the mass loss of nonuniformly corroding steel by the change of magnetic flux density. Little influence was exhibited by varying the amount of corrosion products including the distribution of rust layer and the volume expansion ratio. It revealed the effectiveness and accuracy of the two-yoke magnetic sensor in quantitatively evaluating the corrosion degree of nonuniformly corroding steel irrespective of different distributions of rust layer. With respect to the sensitivity in structures with large concrete cover, the two-yoke magnetic sensor demonstrated a better performance than the external magnetic sensor. [ABSTRACT FROM AUTHOR]

Bimetallic steel bar (BSB) consisting of S30408 stainless steel (cladding layer) and HRB400 carbon steel bar (substrate) has outstanding durability, indicating its suitability for RC structures in corrosive environments. The effects of fire on the mechanical properties of BSB in relation to its metallographic structure (MS) were investigated experimentally by exposing BSB specimens to different elevated temperatures and cooling methods. When the exposure temperature (ET) was lower than the austenite transformation temperature, the changes in the MS were relatively small. When the ET was higher than 700°C, lamellar pearlite and lath martensite were formed in the substrate of BSB specimens with cooling in air (CIA) and cooling in water (CIW), respectively, which led to changes in the mechanical properties. The hardness of the BSB specimens also was affected by the ET and cooling method. There always was a yield plateau in the stress–strain curve of the BSB specimens with CIA. When the ET ranged from 700°C to 900°C, the formation of granular pearlite improved the homogeneity of cementite in ferrite, which enhanced the ductility. For BSB specimens with CIW, when the ETs were 800°C and 900°C, the yield plateau in the stress–strain curve disappeared because of the lath martensite in the substrate. Ductile dimples were observed in the scanning electron microscope images of the BSB specimens with CIA. However, for BSB specimens exposed to 900°C with CIW, the ductile dimples were almost negligible. [ABSTRACT FROM AUTHOR]

A new kind of structural system, ultrahigh-performance concrete (UHPC) shear walls reinforced with carbon fiber–reinforced polymer (CFRP) and steel bars, is proposed in this study. CFRP bars were used in the web region of the shear walls to obtain favorable self-centering capacity, and steel bars in the boundary element were used to achieve high energy-dissipation capacity. Quasi-static tests of four shear-wall specimens were conducted. The self-centering capacity, energy-dissipation capacity, and strength and stiffness degradation of the specimens were analyzed. The tested UHPC shear walls exhibited favorable self-centering and good energy-dissipation capacity. The experimental results showed that the stress in the CFRP bars is the key factor to determine the effectiveness of CFRP bars. The authors proposed a formula for calculating the proper location of CFRP bars and a design axial-load ratio limit for UHPC shear walls with hybrid CFRP/steel reinforcement bars. [ABSTRACT FROM AUTHOR]