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Estimating the stress of reinforcing bars and its variations in service conditions can be useful to determine the reserve capacity of structures or to assess the risk of fatigue in the reinforcement. This paper investigates the use crack width measurements to estimate the stress in the bars. In existing structures, crack width formulations can be used to estimate the stress in the reinforcement from crack width measurements, profiting from additional information that can be measured in‐situ, such as the crack spacing. Recent experimental results show that the values of the mean bond stress typically considered in code formulations overestimate the actual bond stresses activated in cracked concrete specimens. This paper presents the results of an experimental program consisting of reinforced concrete ties and beams instrumented with Digital Image Correlation and fiber optical measurements. The results confirm the differences with typically assumed bond stresses. A formulation to estimate the bond stresses in service conditions is derived from the results of the numerical integration of a previously developed local bond–slip relationship. Their pertinence for the estimation of the stress in the reinforcement from the measured crack width is evaluated with satisfactory results for monotonic loading and for the maximum force in cyclic tests. [ABSTRACT FROM AUTHOR]

Models used to study the fracture process of concrete are often considered 2D, ignoring the influence of specimen width. However, during the fracture process in pre-cracked concrete beams, the crack length varies along the thickness direction, especially in reinforced concrete. To study the influence of specimen width on reinforced concrete fracture behavior, a 3D numerical method was used to simulate the crack propagation processes of lightly reinforced concrete beams based on Fracture Mechanics. Nonlinear spring elements with different stress-displacement constitutive laws were employed to characterize the softening behavior of concrete and the bond-slip behavior between the steel bars and concrete, respectively. It is assumed that the crack begins to propagate when the maximum stress intensity factor at the crack tip along the beam width reaches the initial fracture toughness of concrete. To verify the validity of the proposed method, the completed crack propagation processes of lightly reinforced concrete three-point bending notched beams were simulated, and the calculated load-crack mouth opening displacement curves showed a reasonable agreement with the experimental data. Moreover, the impact of the 2D reinforced concrete beam model on the crack propagation process was analyzed. The results indicate that at the initial loading stage, the external load P obtained from the 2D model is significantly larger than the result from the presented 3D model. [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]

Steel-fiber reinforced polymer (FRP) composite bars (SFCBs) can enhance the controllability of damage in concrete structures; thus, studying the interfacial bonding between them is fundamental and a prerequisite for achieving deformation coordination and collaboration. However, research on the interfacial bonding performance between SFCBs and concrete remains inadequate. This study conducted central pullout tests on SFCB-concrete specimens with different concrete strengths (C30, C50, and C70), bar diameters (12, 16 and 20 mm), and hoop reinforcement constraints, analyzing variations in failure modes, bond-slip curves, bond strength, etc. Additionally, finite element simulations were performed using ABAQUS software to further validate the bonding mechanism of SFCB-concrete. The results showed that the failure mode of the specimens was related to the confinement effect on the bars. Insufficient concrete cover and lack of hoop restraint led to splitting failure, whereas pullout failure occurred otherwise. For the specimens with pullout failure, the interfacial damage between the SFCB and concrete was mainly caused by the surface fibers wear of the bar and the shear of the concrete lugs, which indicated that the bond of the SFCB-concrete interface consisted mainly of mechanical interlocking forces. In addition, the variation of concrete strength as well as bar diameter did not affect the bond-slip relationship of SFCB-concrete. However, the bond strength of SFCB-concrete increased with the increase of concrete strength. For example, compared with C30 concrete, when the concrete strength was increased to C70, the bond strength of the specimens under the same conditions was increased to 50–101.6%. In contrast, the bond strength of the specimens decreased by 13.29–28.71% when the bar diameter was increased from 12 to 14 mm. These discoveries serve as valuable references for the implementation of sustainable SFCB-reinforced concrete structures. [ABSTRACT FROM AUTHOR]

Traditional cast-in-place beam–column joints have the defects of high complexity and high construction difficulty, which seriously affect the efficiency and safety of the building construction line, and precast beam–column joints (PBCJs) can greatly improve the construction efficiency and quality. At present, the investigations on the seismic behavior of precast reinforced concrete structures are still mainly focused on experiments, while the numerical simulations for their own characteristics are still relatively lacking. In the present study, the seismic behavior of novel precast beam–column joints with mechanical connections (PBCJs-MCs) is investigated numerically. Based on the available experimental data, fiber models for four PBCJs-MCs are developed. Then, the simulated and experimental seismic behaviors of the prefabricated BCJs are compared and discussed. Finally, the factors influencing the seismic behavior of the PBCJs-MCs are further investigated numerically. The numerical results indicate that the fiber models can consider the effect of the bond–slip relationship of concrete and reinforcement under reciprocating loads. The relative errors of the simulated seismic behavior indexes are about 15%. The bearing capacity and displacement ductility coefficients of the PBCJs-MCs decrease rapidly as the shear-to-span ratio (λ) increases. It is recommended that the optimum λ for PBCJs-MCs is 2.0–2.5. The effect of the axial load ratio on the seismic behavior of PBCJs-MCs can be negligible in the case of the PBCJs-MCs with a moderate value of λ. [ABSTRACT FROM AUTHOR]

Pull-out tests were conducted to investigate the effects of corrosion of both the longitudinal bars and stirrups on the bond slip behaviour of reinforced concrete specimens. The main experimental variables include concrete strength (26.7 MPa, 37.7 MPa and 45.2 MPa) and expected corrosion loss (0%, 4%, 8% and 12%), with a total of 63 specimens fabricated. The results show that the relative bonding strength of specimens under different concrete strengths gradually decreases with increasing corrosion loss, but the higher the concrete strength is, the faster its degradation rate. The influence of stirrup corrosion on the peak slip can be ignored, but it will further aggravate the degradation of the bonding strength of the specimens. This reduction in bonding strength is linearly related to the stirrup corrosion loss. Based on the experimental results of this work and the achievements of other scholars, a modified relative bonding strength degradation model and a bond–slipbond–slip constitutive model of corroded reinforced concrete are presented by accounting for the influence coefficient of concrete strength. The results show that the constitutive model is in good agreement with the relevant experimental results. [ABSTRACT FROM AUTHOR]

This work tackles the issue of identifiability of fracture and bond properties in reinforced concrete. The basis for modeling of fracture is a computational model capable of describing damage and failure mechanisms in concrete, as well as bond‐slip which is a result of degradation of the concrete‐steel interface. The discrete approximation combines ED‐FEM for concrete crack representation in each element and X‐FEM representation of bond‐slip along a particular reinforcement bar. The uncertain model parameters are modeled as random variables and identified via Bayesian inference with the help of observations from tensile tests on concrete tie beams with a single embedded reinforcement bar. We discuss how the choice of observation type affects the parameter identifiability and propose combinations which improve the estimation capabilities and reduce the discrepancy between the computed and observed quantities of interest. [ABSTRACT FROM AUTHOR]

The study of interfacial behaviour between ribbed steel rebars and concrete is a subject that has been widely studied. However, the definition of the bond stress distribution throughout the embedded length of the steel rebar is still controversial due to the difficulty of experimentally obtaining such distribution for a fixed load magnitude. It is also undeniable its relevancy for the better understanding and model reinforced concrete (RC) structures. So, the definition of the local behaviour between the ribbed steel rebar and concrete is critical to correctly simulate the adherence between both materials. In this matter, the local bond-slip models recommended in codes seem to satisfy some researchers while others suggest prudence in using them. Therefore, only choosing the correct bond-slip relationship may lead to exact interpretations and conclusions of the structural behaviour of a concrete structure but with the existing different bond-slip types, researchers can be misled inadvertently. This work aims to clarify some of these aspects by numerically simulating several pull-out tests under different conditions and checking their influence (or not) on real-scale specimens. After the validation of the numerical model through a proposed new bond-slip relationship, other parameters were studied also. Although the type of the bond-slip relationship influences the detachment of the steel rebar from the concrete, the yielding of the former material was found to be the main parameter that masks the differences in the behaviour of real-scale RC structures when different types of bond-slip relationships were considered in the numerical simulations. [ABSTRACT FROM AUTHOR]

This paper conducts research based on the hollow slab members in the reconstruction and expansion project of expressways, two types of numerical finite element models with and without considering bond-slip relationship of reinforcement and concrete are established, and verified by tests. The distribution characteristics of crack spacing in reinforced concrete beams are studied. The results show that the bond-slip characteristics of reinforced concrete have little effect on the load-deflection characteristics of 8m hollow slab beam. Due to the influence of the bond-slip relationship of reinforced concrete, the load-deflection curve is partially serrated, while without considering the bond-slip relationship of reinforced concrete, the load-deflection curve is smooth. In the numerical model without considering the bond-slip characteristics, almost all damage occurs in the longitudinal direction, and the distribution characteristics of cracks can't be accurately determined. Regardless of whether the bond-slip is considered or not, the macroscopic characteristics of the stress distribution is: smaller near the support and larger at the mid-span. As secondary flexural cracks expand, models with and without consideration of bond-slip characteristics can't calculate crack spacing based on the stress distribution characteristics of the reinforcement. [ABSTRACT FROM AUTHOR]

The numerical evaluation of the shear strength of Steel-Fiber Reinforced Concrete (SFRC) beams without coarse aggregate using data from previous research has been performed in MIDAS FEA. The SFRC beams are modeled by using the total strain crack model with Thorenfeldt for compression behavior and brittle tension behavior. Modified bond stress-slip functions proposed by fib have been used to model the bond between concrete and reinforcement. From the finite element analysis, the cracking pattern and the maximum load at failure of SFRC beams without coarse aggregate have been similar to the test results. As the longitudinal reinforcement ratio increases above 3%, the finite element analysis starts to give overestimated maximum load. The highest ratio of maximum load from analysis and test results equal to 136% has been obtained from 7.82% reinforcement. It is also observed that the influence of longitudinal reinforcement ratio on the shear strength of SFRC without coarse aggregate is lower compared to normal-strength concrete predicted by ACI 318-19. Further studies on bond stress-slip relationship of SFRC without coarse aggregate are needed. [ABSTRACT FROM AUTHOR]

The bonding characteristics between steel bars and concrete in reinforced concrete (RC) structures are crucial for the prediction of load-bearing capacity for seismic design. Nevertheless, most previous studies on bond-slip performance focus on the bond strength based on the pull-out experiments, it is often overlooked that the effect on the failure modes of RC members and the deformation performance due to the bond characteristics. In this research, the effect of the diameter and its arrangement of the reinforcement of the RC column on the bond failure mode and load-bearing capacity based on the cyclic loading tests and the FE analysis are carried out. In the cyclic loading test, it was found that two RC columns with different diameters and reinforcement arrangements showed distinct load-bearing capacity, deformation performance, and failure mode. Despite those columns having the same longitudinal reinforcement ratios. In addition, by applying an advanced finite element analysis using a bond-slip model that induces splitting failure, we succeeded in reproducing the cyclic deformation behavior and local damage obtained in experiments with high accuracy. The proposed model brings in the advanced prediction of the seismic behavior of RC structures and the enhancement of seismic resistance of social infrastructure facilities to earthquake disasters. [ABSTRACT FROM AUTHOR]

This study seeks to identify bond strength and bond-slip behavior of deformed reinforcing bars under high sustained loads. ASTM A944 beam-end test specimens were subjected to sustained load levels ranging from 71 to 100% of their ultimate capacity. The sustained loading tests were conducted until bond failure or at least 20 days. Three of the specimens failed under sustained load at load levels as low as 80% of the control specimens. Subsequent loading of the specimens that survived the high sustained load showed that the residual ultimate capacity was not reduced by the application of sustained load. The slip creep under sustained load was on average 124% of the initial slip, and over 45% of the slip creep occurred in the first day. The effects of concrete cover depth and bond length were examined. With the available data, a simple time-dependent model for determining the bond-slip under high sustained loading was suggested. [ABSTRACT FROM AUTHOR]

Geopolymer concrete reinforced with Glass fiber-reinforced polymer (GFRP) bar presents a novel, durable, eco-friendly strengthening system with significant promise for practical engineering. To employ this strengthening system, a clear understanding of the bond performance between GFRP bars and geopolymer concrete under bending conditions is essential. Interface behaviors of GFRP bars-geopolymer concrete was investigated using hinged beam tests, considering the following parameters: concrete type(geopolymer and ordinary), concrete strength(C30, C50, C70), surface treatment of bars(shallow thread, spiral wrap ribs, and sandblasted), bar diameters(6 mm, 10 mm, and 16 mm), and bond lengths(2, 4, and 8 times the bar diameter). Results indicate a consistent bond mechanism between geopolymer and ordinary concrete. The bond strength increases with higher concrete strength, and the initial bond stiffness is improved by spiral wrap ribs. Moreover, the mBPE model was used to describe the local bond-slip relationship, and the effects of different parameters on interface shear stress distribution were discussed. The development length of GFRP bars in concrete was evaluated and compared with existing codes. These findings provide valuable references for the performance assessment and optimization of GFRP bar-reinforced geopolymer concrete structures in engineering applications. • GFRP bars-geopolymer concrete bond behavior was investigated using hinged beam tests. • Geopolymer concrete exhibits similar bonding mechanisms to ordinary concrete. • The mBPE model was used to describe the local bond-slip relationship. • Developed stress versus bonded length is compared with existing codes formula. [ABSTRACT FROM AUTHOR]

Abstract Corrosion-induced bond degradation of deformed steel rebar in concrete is experimentally investigated with acoustic emission (AE) and 3D laser scan technique. Concrete specimens were fabricated and subjected to direct pullout test after being corroded to different levels. The number and width of cracks present during the corrosion tests and the pull-out tests were recorded. The energy released during the pullout tests were captured with AE probes, and the frequency characteristics was analyzed. After pullout tests, the surface morphology of corroded steel rebars was determined with a 3D laser scanner. A modified bond deterioration model was proposed and the parameters associated with the model were analyzed. Results indicated that two types of AE signals were acquired during pullout tests: concrete cracking in high frequency range of 35 ~ 41 kHz and steel-concrete friction in low frequency range of 3 ~ 15 kHz. The bond strength and the bond-slip characteristics depend upon the level of corrosion as well as the number and width of cracks. The reduction factor of the bond-slip model exponentially decreases as a function of the average cross-sectional area loss and linearly decreases with an increase of the rib area loss.

The bond and slip between concrete and the reinforcement bars, cover a key role in the inter-material force transfer of Reinforced Concrete (RC) structures. In light of the lack of tools able to thoroughly inspect the inner workings of RC structures and to extract reliable bond stress values, modern bond stress–slip (Bond–slip) models are often inaccurate and in contradiction with each other. Considering the recent surge of novel hyper-performant strain sampling tools (Distributed Sensing for example), their application for the creation of novel and physically accurate Bond–slip models is just a matter of time. This being said, one of the main reasons behind the modern coexistence of multiple inaccurate and at times contradictory Bond–slip models is the absence of a tool that has allowed researchers to rapidly corroborate and calibrate their newly created models. To this end, the present article proposes such a Bond–slip validation tool for RC elements. This one is designed to extract reinforcement strain profiles at any given load level on the grounds of a specific bond–slip law and geometrical inputs. Said profile is then compared against an experimentally extracted one based on specimens with identical geometrical features. The performance of the validation tool is demonstrated through an application to six existing bond–slip models. Granted the proposal of validation tools is paramount for the future of the discussion on bond–slip modelling, stress-transfer analyses and serviceability of RC structures, the here proposed validation tool is a first significant step in that direction. [ABSTRACT FROM AUTHOR]

The detection of bond-slip between the reinforcing bar (RB) and concrete is of great importance to ensure the safety of reinforced concrete (RC) structures. The techniques to monitor the connection between the RB and concrete are in constant development, with special focus on the ones with straightforward operation and simple non-intrusive implementation. In this work, a simple configuration is developed using 10 optical fiber sensors, allowing different sections of the same RC structure to be monitored. Since the RB may suffer different strains along its length, the location of the sensors is critical to provide an early warning about any displacement. Bragg gratings were inscribed in both silica and polymer optical fibers and these devices worked as displacement sensors by monitoring the strain variations on the fibers. The results showed that these sensors can be easily implemented in a civil construction environment, and due to the small dimensions, they can be a non-intrusive technique when multiple sensors are implemented in the same RC structure. [ABSTRACT FROM AUTHOR]

Bond–slip is an important characteristic that determines the stiffness, displacement, and load-bearing capacity of a reinforced concrete (RC) beam. It is essential for performing a precise numerical analysis of the beam. In most cases, bond–slip models can define the bond–slip curve only when there are experimental data. However, many bond test data have been obtained from pull-out tests, and the dominant view is that the bond–slip behavior observed in the pull-out test is quite different from that in an actual RC beam. Therefore, a mapping function that makes it possible to estimate the bond–slip behaviors of beam specimens using those of pull-out specimens was developed in this study. A total of 255 pull-out specimen data and 75 beam specimen data were collected from previous studies, and the importance and influence of each feature of the two groups were analyzed using random forest and K-means clustering. The mapping function was derived using genetic programming, and its accuracy was verified through a comparison with existing models. The proposed model exhibits a high degree of accuracy in estimating bond–slip and bond strength in beam specimens and can provide useful information for understanding the difference in bond–slip behaviors between the two groups. [ABSTRACT FROM AUTHOR]

To perform accurate seismic analyses of corroded reinforced concrete (RC) buildings, it is necessary to predict the bond-slip relationships to modify the moment-curvature relationships. In this study, empirical models were developed to predict the bond strength of corroded RC columns. Twenty-five RC columns were tested under combined cyclic lateral displacement excursions for three concrete strength levels—specifically, 9, 27, and 37 MPa (1305, 3915, and 5365 psi, respectively)—at two axial load ratios of 0.20 and 0.40 and at four corrosion levels. An accelerated corrosion method was applied to corrode the reinforcing bars embedded in the RC columns. The actual corrosion levels were determined by breaking the concrete and extracting all the longitudinal and transverse reinforcing bars from the RC columns. Three empirical models were developed. The first model was used to predict the development length of the corroded reinforcing bars. The second and third models were used to predict the bond strength and slip in the reinforcing bars as a function of the corrosion levels. The results indicated that previous models that predict the bond strength of corroded RC members were limited because of the theoretically considered development length of the noncorroded reinforcing bars. [ABSTRACT FROM AUTHOR]

Corrosion greatly affects the bond behaviour of reinforced concrete. In the case of reinforcing low-corrosion reinforced concrete, the bond strength of steel bars and concrete is at an enhanced stage. Accurate prediction of bond behaviour at this stage is crucial for the rational reinforcement. This study designed ten groups, totalling 30 pull-out specimens, were accelerated corroded with various current densities and electrification times. Through 3D scanning and image analysis technology, we observed that increasing the current density yielded less homogeneous corrosion, creating pit-like rust marks. Meanwhile, we developed a model for predicting the corrosion level. The model is applicable to the corrosion level design of accelerated corrosion. Pull-out tests showed that as corrosion increased, the specimen failure mode changed from pull-out to splitting and back to pull-out, with the bond strength first increasing and then decreasing. A bond-slip prediction model for corroded reinforced concrete was also established, considering the effects of corrosion on bond strength in different stages, especially the enhancement stage. The model performs well in predicting the bond behaviour of low-corrosion reinforced concrete and can be used in conjunction with corrosion prediction models and provides guidance for reinforcement. • 3D scanning visualizes steel bar corrosion. • Predictive model for corrosion level in concrete developed. • Low-corrosion bond-slip model aids reinforcement strategies. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive investigation of the bond characteristics of steel bar reinforced geopolymer concrete (GPC). The ASTM A944 beam end tests were conducted on GPC beams reinforced with plain or ribbed bars. The bond–slip curves and the bond strength of GPC beams were obtained. The relationship between the bond stress and relative slip in plain and ribbed bar reinforced GPC has been represented by empirical formulae. The bond testing results were compared with those of corresponding ordinary Portland cement concrete (OPC) using statistical hypothesis tests. The results of hypothesis testing showed that GPC was significantly superior to OPC in terms of bond capability with plain bars and bond stiffness with ribbed bars. The statistical analysis indicated that the bond–slip relations derived for OPC are inapplicable to GPC; thus, new bond–slip relations are suggested to estimate the development of bond stress and relative slip between GPC and steel bars. [ABSTRACT FROM AUTHOR]

Reliable and robust numerical modeling of RC columns under lateral load is commonly performed using one-dimensional elements. Although the strain is assumed to vary linearly through the section, bond-slip and buckling of reinforcing bars at the joints/connections contribute substantially to the column response. This paper presents a three-dimensional (3-D) fiber-based model with explicit representation of the bond-slip that occurs at the column-footing interface for cast-in-place columns and at the interface between bars and mechanical couplers for precast columns with grouted sleeve (GS) connections. The model contains beam elements with fiber sections for concrete, beam elements for longitudinal steel bars and sleeves, and spring elements for interfacial bond-slip behavior. The novelty is that the model can be achieved using existing software implementations, retains the efficiency of fiber-based elements under cyclic loading, and parameters are calibrated based on component-level testing, unlike other resultant or hybrid models that require arbitrary backbone and cyclic property specification. An extensive experimental program was conducted to investigate the behavior of GS couplers and calibrate the bond-slip models. Four large-scale RC columns were then used to verify the proposed model by comparing the measured load-displacement, energy dissipation, curvature, and failure mode with the numerical responses. [ABSTRACT FROM AUTHOR]

This paper investigated seismic performance of reinforced concrete (RC) single-column bridge bent with flexural failure under near-fault ground motion. Innovative nonlinear fiber-based finite element models (FEM) with combined damage mechanisms were proposed. Cracking and spalling of cover concrete, buckling of longitudinal reinforcing steel bars, and bond-slip effect were considered. To study bond-slip effect, two FEM were developed: model 1 (without bond-slip) and model 2 (with bond-slip). Nonlinear static cyclic pushover analyses and nonlinear response history analyses under scaled near-fault ground motion were conducted. The simulation results were compared with available pseudo-dynamic test results. Various ductility coefficients were evaluated to assess the seismic performance of RC bridge column. The attributes of near-fault ground motion on the seismic responses of RC bridge column were discussed. Model 1 overestimated the ultimate lateral load resistance, longitudinal reinforcing steel bar strain, and cover concrete strain. Model 1 also underestimated the lateral deflection of RC bridge column. The results of model 2 agree well with experimental observations including hysteretic responses and damage mechanisms. In general, the predictions of both models are in good agreement with the experimental observations. However, model 2 provides improved predictions on the seismic performance of RC single-column bridge bent under near-fault ground motion. It was also observed that maximum responses of RC bridge columns under near-fault motion were characterized by one or few large hysteretic cycles. The proposed models could also help practicing engineers and researchers simulate seismic performance of RC bridge columns under near-fault ground motions in a computationally efficient manner. [ABSTRACT FROM AUTHOR]

A simplified reinforcement model is proposed to consider the effects of both corrosion and reinforcement slip-on seismic performance of reinforced concrete columns. The mechanical properties deterioration of corroded reinforcement are obtained by regression analysis of the available experimental data. Since corrosion may also have a significant influence on bond-slip, a local bond-slip curve with corrosion effect is constructed, by shifting the curve for uncorroded rebar in the slip direction. To fully consider the non-uniformity of bond stress distribution along the development length, a stepped equivalent bond stress is used to simplify the integral calculation of theoretical slip. Subsequently, the cumulative slip and the equivalent strain with slip effect are formulated to modify the parameters of monotonic and hysteretic models of perfectly bonded corroded reinforcement. The response of corroded reinforced concrete columns under cyclic loading is simulated by the fiber section beam element model. Finally, the numerical simulation results and the experimental data are compared. The results show that the ignorance of slip will overestimate the stiffness and bearing capacity of the column by 28.3 and 13.5% respectively, and the yield load, peak load and peak lateral displacement of column decrease with corrosion in longitudinal reinforcement gradually, by 33.8, 16.3 and 56.6% for mass loss percentage 8.0%, respectively. This confirms the ability of the proposed model to simulate the nonlinear seismic response of structures in a corrosive environment. [ABSTRACT FROM AUTHOR]

Fiber-optic sensing (FOS) provides distributed strain measurement that can enhance both structural health monitoring (SHM) and laboratory testing capabilities. In particular, optical frequency domain reflectometry (OFDR) provides strain or temperature measurements every millimeter over tens of meters of fiber, with a high sampling rate. For RC applications with embedded fiber-optic cables, the sensitivity of the fibers and their ability to measure and survive cracking are both important considerations. In this study, tests were conducted on six RC specimens to investigate the effectiveness of using OFDR strain sensing to evaluate the cracking behavior of concrete and the deformation of steel reinforcing bars. Six types of fiber-optic cables with very different structures, sensitivity, and survivability were tested. Fibers were placed in the concrete and in grooves in the reinforcing bars. A new deconvolution method was developed; using the method, crack widths were accurately measured up to the target load levels using both the more sensitive fiber-optic cables and cables with reduced sensitivity but better survivability. OFDR strain sensing, combined with new methods of data processing, was shown to be capable of detecting distributed microcracking and providing reliable crack widths. The OFDR technique was also used to reveal the location-dependent bond-slip relationships between the concrete and rebar at early stages. [ABSTRACT FROM AUTHOR]

A hybrid strengthening system, comprising a carbon fiber reinforced polymer (CFRP) sheet combined with fiber reinforced concrete (FRC) layer is proposed in this study as a mitigatory solution to brittle debonding problems. Five reinforced concrete (RC) beams, comprising both control cases and those strengthened with the hybrid CFRP-FRC system at various FRP reinforcement ratios, were fabricated and tested to assess the system's effectiveness. A comprehensive finite element (FE) model was also developed to complement the experimental campaign and extend it into a parametric investigation on the effects of FRP reinforcement ratio (ρ f), fiber percentage in FRC (v f), and FRC layer thickness (t FRC). Compared with beams strengthened with FRP alone, using the FRC layer increased the ultimate load (P ult.) of FRP-strengthened beams by 29–59%, depending on ρ f and v f. The ductility of the hybrid FRP-FRC strengthened beams also increased by more than twice compared to the beams strengthened with only FRP. For the FRP reinforcement ratio (ρ f) of 0.18%, use of an FRC layer with v f = 2% yields the highest increase in P ult. and ductility for the hybrid system. Ultimately, the brittle FRP debonding failure was successfully eliminated using the FRC layer, leading to desirable FRP rupture. Built with capabilities to capture material nonlinearity, contact, and debonding phenomena, the FE model predictions excellently matched those of the test results in terms of the beam's capacity, failure modes as well as load-deflection and load-strain responses. • A hybrid strengthening using CFRP sheets combined with FRC layer is proposed. • Performance was evaluated using tests on RC beams at various FRP reinforcement ratios. • A comprehensive finite element model was developed to complement the experiment. • FRC layer serves as a mitigatory solution to brittle FRP debonding and leads to desirable FRP rupture. • Compared with FRP only strengthening, FRC layer increased the capacity by 29–59% along with up to twice the ductility realized by using FRP alone. [ABSTRACT FROM AUTHOR]

Cyclic response of substandard reinforced concrete (RC) columns and structural repair of pre-damaged columns by carbon-fiber-reinforced polymers (CFRPs) were investigated through combination of experimental and advanced numerical modeling methods. The specimens, representing typical RC columns in substandard buildings, were constructed from low-strength concrete, plain round bars, and nonconforming reinforcement details. The key parameters under investigation included the lap-splice, hook detail in the longitudinal reinforcement, and axial load ratio. First, the RC columns were subjected to cyclic loading until the attainment of their heavy damage state. The failure mode was dominated by the flexural response alongside significant reinforcement slip and axial damage, involving concrete crushing and reinforcement buckling. Subsequently, the severe cracks and concrete damage were repaired by the externally wrapped CFRP sheets. The former load-carrying capacities of the repaired RC columns were recovered. The damage formation in the concrete and reinforcement was transferred to CFRP sheets. The final failure mode was characterized by local deformations in the CFRP, with minimal damage observed in the concrete and reinforcement bars. Furthermore, the numerical solution effectively reproduced the nonlinear behavior of the as-built specimens. The crack patterns and capacities observed in the numerical solutions align with the responses observed in the experimental tests. • Cyclic response of substandard RC columns and subsequent structural repair using CFRPs. • Critical parameters such as lap-splice, hook detail, and axial load ratio. • Flexural response alongside significant reinforcement slip, and severe axial damage. • A feasible method for repairing and restoring structural integrity. • Reproduced the nonlinear response matching experimental observations. [ABSTRACT FROM AUTHOR]

Recent earthquakes have shown that the seismic behavior of noncode‐conforming reinforced concrete buildings are in several cases strongly affected by the nonlinear response and possible failure of the beam‐column joints. Beam‐column joint inelasticity is typically due to either shear cracking and failure of the central concrete panel or bond‐slip of the longitudinal steel bars. This paper proposes a new Joint with Hinges macro‐element capable of simulating the most important sources of nonlinearities in the joints. It consists of a quadrilateral central panel whose rigid edges are connected to the beams' and columns' end nodes through nonlinear finite‐length interfaces that represent the plastic hinge zones. The central panel deforms in shear only. The joint macro‐element accounts for the nonlinear shear response of the central panel, for the bond‐slip of the longitudinal rebars crossing the joint and for the nonlinear shear and flexural behavior of the plastic hinge zones of the beams and columns connected to the joint. The resulting model of the reinforced concrete frame is the assembly of nonlinear Joint with Hinges macro‐elements connected by elastic frame elements. The proposed formulation is applied in this paper to plane frames. Validation studies with results from available experimental tests on beam‐column joint subassemblages show the proposed model capability to predict the hysteretic responses and the failure mechanisms of connections with unreinforced joints and different beam reinforcement ratios. [ABSTRACT FROM AUTHOR]

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Featured Application: The description of nonlinear damping theory for the dynamic analysis of FRP constrained concrete members considered on the bond slip effect of reinforcing bars provides support for subsequent seismic design. As a composite material, the damping energy consumption mechanism of fiber reinforced polymer-confined reinforced concrete (FRP-C RC) structure is very complex. In previous dynamic calculation models, the bond-slip effect for steel bars was often ignored, which would lead to a considerable error in the response of the FRP-C RC structures. In this paper, a new numerical model of FRP-C RC columns considering the bond-slip for steel bars is established using a zero-length element and nonlinear beam-column elements in the OpenSees software, and the results of the model are verified by experimental results. Based on the complex damping theory, the loss factor expression and nonlinear damping model of FRP-C RC columns with the bond slip effect are proposed. Finally, the dynamic response of FRP-C RC columns with nonlinear damping under harmonic load is calculated and compared with the results available in literature. The results show that the proposed model considering steel bars' bond-slip can provide better prediction for dynamic response of FRP-C RC columns and make the future seismic design of FRP-C RC columns safer. [ABSTRACT FROM AUTHOR]

This paper presents an investigation of the seismic performance of concrete columns reinforced longitudinally with steel bars (referred to as UHS-WB bars) that have an ultrahigh strength and a low bond strength. Nine cantilever columns were tested with variables including the type of longitudinal steel reinforcement, the anchorage method for the UHS-WB bars, the axial load ratio, and the shear-span ratio of the columns. The columns with securely anchored UHS-WB bars and shear-span ratios of 2.5 and 3.0 exhibited very stable hysteretic behavior and a mild hardening postyield behavior up to drift ratio of 4%, whereas the lateral resistance of the column with a shear-span ratio of 4.0 had a slight decline beyond a drift ratio of 1.5% due to significant P-δ effect. However, all the columns with securely anchored UHS-WB bars had a residual drift less than 0.5% after unloading from the drift of 3%. The stable response and low residual drift can be attributed to the low bond strength, which delayed the yielding of the UHS-WB bars. [ABSTRACT FROM AUTHOR]

Externally bonded carbon fiber-reinforced polymer (CFRP) composites are an effective method for flexural and shear strengthening and repair of reinforced concrete (RC) beams. In externally bonded CFRP strengthening, epoxy-based CFRP wet layup systems are predominately used. CFRP composites pre-impregnated with polyurethane (PU) resin and PU primers were demonstrated as an effective alternative in previous studies yet have seen limited applications. Due to the broad range of PU primer and laminate properties, bond between PU-CFRP system and concrete substrate can be potentially promoted. This paper experimentally characterizes the performance of six different PU-CFRP systems for external flexural strengthening of RC beams and investigates the relationship between bond-slip properties and the target flexural responses. Twenty-one lap shear specimens, 30 concrete flexural beams, and seven RC flexural girders were tested and compared with a conventional epoxy-based wet layup system and non-CFRP control specimens. Results indicated that PU adhesive and laminate can be tailored to obtain desired flexural performance, with bond cohesive energy and shear slip being the parameters that best predict the flexural beam strength, deformability, and failure modes. Comparable flexural capacity improvement was also observed between the PU-CFRP and the epoxy-based CFRP wet layup systems. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Zhejiang University: Science A is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper discusses the Truss Model proposed by Panagiotou et al. (ACI Struct J 109(2):205–214, 2012) for modeling reinforced concrete coupled structural walls. The model is validated with the landmark seismic testing of two 1:4 scale seven-story test specimens reported in Santhakumar (The ductility of coupled shear walls. Dissertation, University of Canterbury 1974) and in Paulay and Santhakumar (J Struct Div 102(1):93–108, 1976). The first specimen, Wall A, incorporated conventionally reinforced coupling beams whereas the second specimen, Wall B, incorporated diagonally reinforced coupling beams. These two specimens attained roof drift ratios of at least 1.7% before initiation of lateral strength degradation. Coupling beams in specimen Wall A exhibited significant strength degradation due to sliding shear, whereas coupling beams in specimen Wall B maintained the capacity throughout. We compare key overall and local responses reported for the two specimens with those computed with the Truss Models, as well as responses that could be only computed. In the latter, we show that when the beams effectively coupled the walls, the shear force at the wall base was mainly resisted by the wall being compressed. Moreover, the analysis shows that the first level coupling beams develop greater shear forces than the other beams despite all beams were identically reinforced. This is caused by the restraint provided by the fixed-base walls and by the kinematics of the strongly coupled walls. The Truss Model captures these responses because it explicitly considers the axial-flexure-shear interaction of RC walls and beams. [ABSTRACT FROM AUTHOR]

This study presents the development of a nonlinear finite element (FE) model that is capable of simulating the response and predicting the shear strength of concrete beams reinforced with carbon fiber-reinforced polymer (CFRP) bars. More specifically, this paper investigates the effect of unsymmetrical loading that creates two shear span-to-depth (a/d) ratios within the beam specimen and further complicates shear response. It is observed that the predicted FE results are in close agreement with the measured experimental results. Thus, it can be concluded that the developed models can be used to simulate the behavior of concrete beams reinforced with CFRP bars. [ABSTRACT FROM AUTHOR]

In this paper, an analytical–empirical model, inspired from the damage mechanism observed in tests, is developed to simulate the lateral response of columns reinforced with plain bars and validated with experimental tests accessible in the literature. Using the displacement decomposition approach, and a novel categorization of lateral behavior, the slip part of displacements, as the main source, is calculated through an exact integration solution of the bond-slip degradation for the tensile bar in the column and footing simultaneously. The proposed approach is compared with available test results and using a comprehensive parametric study, a relationship is suggested to predict the rotational capacity of the mentioned columns. [ABSTRACT FROM AUTHOR]

The bond of concrete to steel reinforcing bars is the basis for the concept of reinforced concrete as a construction material. One of important bond-related issues is the bond strength-bar slip relation, which has an influence on the failure mode and rotation of structural joints. This relation can be predicted by bond–slip models and it depends on the bond behaviour at the reinforcement–concrete interface. However, the currently-known bond-slip models offer limited applications to well-studied normal concretes only. This article proposes local bond–slip relationships for high performance self-compacting concrete (HPSCC) taking into account the quality of bond conditions. The studies were performed on specimens made of four different high performance self-compacting concrete mixes with varying contents of silica fume. Since the specimens had total heights of 480, 800 and 1600 mm, it was possible to measure the changes of the bond at individual levels. The results were next compared to those obtained for samples made of vibrationally-compacted high-performance concrete (HPC). It was demonstrated that the bond-slip model for normal concretes seems to underestimate the bond stiffness and ultimate bond strength for the HPSCC and HPC mixtures. Moreover, it was proven that a change of the quality of bond conditions has an effect on the local bond-slip relationship. Finally, local bond-slip relationships were presented separately for HPSCC and HPC with respect to the quality of bond conditions. [ABSTRACT FROM AUTHOR]

Here we present a multi-scale model to carry out the computation of brittle composite materials reinforced with fibers, and we show its application to standard reinforced concrete. The computation is carried out within an operator-split framework on the macro-scale, which allows for different failure mechanisms to develop in separate phases, as both the concrete and the bond-slip exhibit non-linear behavior. The computations on the micro-scale are performed for each constituent separately. The reinforcement is taken to be linear elastic, and the bond-slip is handled as a plastic deformation. The standard elastoplastic procedure is used to compute the bond stresses, combined with the X-FEM methodology to give the global representation of slip. The crack development in concrete, on the other hand, is described with a damage model with exponential softening, where ED-FEM is used to represent localized failure. A numerical example is shown to test the developed methodology. [ABSTRACT FROM AUTHOR]

• A novel anchorage system called additional ribs was proposed for FRP bars in NSM strengthening system. • Pull-out tests were conducted to evaluate the effect of different parameter, especially the additional ribs on the bond performance NSM FRP bars in concrete. • The local bond stress distribution and the transfer mechanism of specimens using additional ribs was analysed and expounded. Bond failure due to a lack of anchorage capacity is often observed when using FRP reinforcement as a strengthening method in RC structures, thus restricting the efficiency in upgrading their structural performance. To address this issue, our research group has recently developed a simple and innovative additional ribs anchorage system for FRP bars, which has been verified to have the high efficiency of improving the anchorage performance of FRP bars reinforced in concrete. As a promising strengthening method in RC structures, the near-surface mounted (NSM) fiber-reinforced polymer (FRP) technique has become popular. However, bond failure also exists in the NSM FRP strengthening system. It is also important to initiate a study on the bond performance of NSM FRP bars which are mounted in concrete using the proposed additional ribs. In this study, a total of 36 direct pull-out specimens and 12 beam pull-out specimens were employed. In particular, the influences of additional ribs on the bond behavior of FRP bars mounted in concrete in terms of the failure modes, local bond stress distribution, and load-slip curves were discussed. Several important parameters expected to affect the bond performance were investigated, namely, the type and the bonded length of the FRP bars, groove size and section configuration, and adhesive species. The test results and corresponding analysis in this paper demonstrated the feasibility and significant efficiency of using the proposed additional ribs anchorage system to enhance the bond performance of NSM FRP bars in concrete. [ABSTRACT FROM AUTHOR]

The overall seismic performance of existing pre 1960-70s reinforced concrete (RC) structures is significantly affected by the inadequate length of columns' lap-spliced reinforcement. Due to this crucial structural deficiency, the cyclic response is dominated by premature bond - slip failure, strength and stiffness degradation, poor energy dissipation capacity and low ductility. Recent earthquakes worldwide highlighted the importance of improving the load transfer mechanism between lapspliced bars, while it was clearly demonstrated that the failure of lap splices may result in a devastating effect on structural integrity. Extensive experimental and analytical research was carried out herein, to evaluate the effectiveness and reliability of strengthening techniques applied to RC columns with lap-spliced reinforcement and also accurately predict the columns' response during an earthquake. Ten large scale cantilever column subassemblages, representative of columns found in existing pre 1970s RC structures, were constructed and strengthened by steel or RC jacketing. The enhanced specimens were imposed to earthquake-type loading and their lateral response was evaluated with respect to the hysteresis of two original and two control subassemblages. The main variables examined were the lap splice length, the steel jacket width and the amount of additional confinement offered by the jackets. Moreover, an analytical formulation proposed by Tsonos (2007a, 2019) was modified appropriately and applied to the lap splice region, to calculate shear stress developed in the concrete and predict if yielding of reinforcement is achieved. The accuracy of the analytical method was checked against experimental results from both the literature and the experimental work included herein. [ABSTRACT FROM AUTHOR]

Two simple finite element models denoted as FEM‐B and FEM‐BC are developed in this paper to investigate the structural behavior of fiber‐reinforced polymer (FRP) strengthened reinforced concrete (RC) beams under static and cyclic loading. First, the FEM‐B is developed for analysis of FRP‐strengthened RC beams under static loading and the bond‐slip effect between FRP, adhesive and concrete is included. Then FEM‐BC is developed based on FEM‐B for analysis of structural behavior of FRP‐strengthened RC beams under cyclic loading, and degradations of material properties of concrete, steel reinforcement, and FRP due to cyclic loading are taken into account. The developed FEMs are validated against experimental results and demonstrated to be effective for structural analysis of FRP‐strengthened RC beams. Furthermore, parametric studies are carried out to learn the effects of types, thickness, and length of FRP on the structural behavior of FRP‐strengthened RC beams based on the FEM‐BC model. The research findings are summarized finally. [ABSTRACT FROM AUTHOR]

A single-scale model for reinforced concrete, comprising the plain concrete continuum, reinforcement bars and the bond between them, is used as a basis for deriving a two-scale model. The large-scale problem, representing the "effective" reinforced concrete solid, is enriched by an effective reinforcement slip variable. The subscale problem on a Representative Volume Element (RVE) is defined by Dirichlet boundary conditions. The response of the RVEs of different sizes was investigated by means of pull-out tests. The resulting two-scale formulation was used in an FE2 analysis of a deep beam. Load-deflection relations, crack widths, and strain fields were compared to those obtained from a single-scale analysis. Incorporating the independent macroscopic reinforcement slip variable resulted in a more pronounced localisation of the effective strain field. This produced a more accurate estimation of the crack widths than the two-scale formulation neglecting the effective reinforcement slip variable. [ABSTRACT FROM AUTHOR]

• The unbonded ratio is the main factor affecting the performance of an RC beam. • The friction coefficient between the rebar and concrete is related to the load stage. Hybrid aggregates, comprising normal-weight and lightweight aggregates, offer an effective means to enhance the physical and mechanical properties of concrete. However, the comprehensive effect of hybrid aggregate concrete on the flexural performance of reinforced concrete (RC) beams remains largely unexplored. In this study, utilizing LC30 lightweight concrete, LC40 gravel lightweight concrete, hybrid aggregate lightweight concrete, and hybrid aggregate concrete with reduced ceramsite content as matrix materials, RC ordinary beams and RC bond-slip beams with dimensions of 150 mm × 300 mm × 2400 mm were meticulously designed and fabricated. Employing a four-point bending test, the effects of the concrete matrix, the unbonded ratio, and the reinforcement ratio of the bottom longitudinal rebar on the flexural behavior of the two types of beams were scrutinized. Furthermore, the load–deflection and load–slip responses were subjected to numerical simulation using the finite element method. The findings demonstrate that hybrid aggregates exert a substantial influence on adjusting the dry apparent density, strength, and elastic modulus of concrete to align with design specifications. While the physical and mechanical characteristics of both beam types are primarily governed by the reinforcement ratio and unbonded ratio, the concrete matrix, despite its distinct properties, exhibits minimal effect on these attributes. Nevertheless, it significantly affects beam stiffness and the relative slip between the rebar and the concrete matrix. Both beam categories adhere to the plane section assumption and reveal that optimal collaborative performance between the rebar and concrete matrix occurs at a bottom longitudinal reinforcement ratio of 1%, albeit diminishing with escalating unbonded ratios. The failure progression of both beam types can be categorized into three stages: elastic phase, crack propagation phase, and compressive failure phase. The process of crack advancement, the load–deflection profile, and the load–slip behavior of the beams manifest similarities, albeit subject to variations attributable to the concrete matrix, bottom longitudinal reinforcement ratio, and unbonded ratio. The Menetrey–Willam constitutive model effectively replicates the load–deflection and load–slip characteristics of both beam types. However, its simulation precision is influenced by the friction coefficient existing between the rebar and the concrete matrix. Notably, this friction coefficient undergoes alteration in consonance with the concrete failure process, thereby reflecting the gradual degradation of bonding performance between the rebar and the concrete matrix. [ABSTRACT FROM AUTHOR]

A numerical method to estimate the dynamic response of reinforced concrete (RC) columns subjected to axial and blast loads is introduced in this paper. Upon adopting Timoshenko's beam theory, both the flexural and direct shear behaviors are incorporated into the numerical formulation. The moment-curvature relationship of a reinforced concrete (RC) section is based on the construction of the bending stiffness and, in advance, a dynamic increase factor (DIF), usually defined in the stress-strain relations of concrete and steel, is newly designed to be defined in the moment-curvature relation. In addition to the description of the dynamic characteristics in the RC section, additional modification of the moment-curvature relation is also performed to exactly simulate the large plastic deformation concentrated at the mid-span or beam-column joint due to the bond-slip or anchorage slip after yielding of the main reinforcement. Finally, the validity of the proposed method is verified by comparing the analytical results with the experimental data, and then the pressure-impulse (P–I) diagrams are constructed and compared to review the change in the resisting capacity of a RC column according to the variation of the axial force and slenderness ratio. [ABSTRACT FROM AUTHOR]

Cracking in R/C members is a complex phenomenon characterized by high scattering, mainly affected by concrete in tensile zone. The aim of this paper is to evaluate the effectiveness of formulas for crack width prediction developed in last decades. First, an historical review of available literature models is presented and discussed. Second, the progress and missteps, which have characterized the development of international code provisions, are shown. Finally, a comprehensive comparison between predicted crack widths and experimental data is performed. Some formulas more than others provide mean values of crack width in good agreement with experimental data; nevertheless, predictions are always very scattered, as ratios between theoretical and experimental values have a coefficient of variation not lower than 30%. Results show that the prediction capacity does not necessarily increase as the cracking model is refined. [ABSTRACT FROM AUTHOR]

This paper presents an alternative methodology to represent rebars and their bond-slip behavior against concrete based on coupling finite elements. Among the main features and advantages of the proposed technique are: (i) the coupling between the two independent meshes for concrete and reinforcement does not introduce any additional degree of freedom in the global problem; (ii) both rigid and non-rigid coupling can be used to represent the particular cases of perfect adherence and general bond-slip behavior, respectively; (iii) rebars of arbitrary geometry and orientation can be modeled; (iv) the methodology can be applied to 2D and 3D problems and (v) the formulation can be adapted to other type of finite elements and implemented easily in any existing FEM code. Constitutive models based on continuum damage mechanics are used to represent the concrete behavior and concrete-rebar interaction. A number of numerical analyses are performed and the results obtained show the versatility and accuracy of the proposed methodology. [ABSTRACT FROM AUTHOR]

This article introduces a numerical model that simulates the bond-slip behaviour in axially loaded circular concrete-filled tube columns without applying double nodes required in the use of the classical bond-link element. After calculating the nodal displacements of in-filled concrete, the deformation in the steel tube at each node has been found through the back-substitution technique from the first to the final steel element using a governing equation constructed on the basis of the force equilibrium and displacement compatibility at each node. Finally, correlation studies between analytical and experimental results are conducted to verify the efficiency and applicability of the introduced model. [ABSTRACT FROM AUTHOR]