Damaged Reinforced Concrete Columns Research Articles

Axial compressive behavior of predamaged RC columns retrofitted with precast steel-reinforced grout jackets

Steel jacketing and concrete jacketing are popular retrofitting techniques to strengthen the axial compressive behavior of reinforced concrete (RC) columns. However, these retrofitting methods lack efficiency in practical application due to intensive labor and time-consuming installation. In order to improve the retrofit efficiency of damaged RC columns, this study proposed a novel precast steel-reinforced grout jacket (PSRGJ) consisting of early-strength cementitious grout and a steel frame. Ten columns were tested to investigate the axial compressive behavior of predamaged RC columns retrofitted with PSRGJ. The tested parameters were the stirrup reinforcement ratio, the transverse reinforcement ratio of PSRGJ, and the predamage level. The results indicated that the columns’ peak load and ductility index I10 were significantly improved by PSRGJ retrofitting and increased with the increase of transverse reinforcement ratio of PSRGJ. Besides, the peak load and the initial stiffness of PSRGJ-retrofitted RC columns decreased with the predamage level, while the ductility index I10 increased. In addition to the confinement effect, PSRGJ carried 11%−17% of the peak load through the interfacial bonding effect. Based on the test results, the actual confinement pressure model and interfacial bonding strength model were proposed. Further, peak load models considering the size effect were developed for PSRGJ-retrofitted intact and predamaged RC columns. The predictions of the proposed models agreed well with the test results from this study and the literature.

The role of overstrength in welded joints for rebar substitution in damaged RC columns

Rehabilitating corroded or seismically damaged reinforced concrete (RC) bridge columns often involves replacing the longitudinal steel rebar. Welded joints offer a practical and efficient in-situ solution for connecting new rebar segments to existing undamaged rebar, even with irregularities in the steel reinforcement. However, traditional approaches to rebar joint welding have paid little attention to the overstrength of the welding seam, which can result in a brittle response. Properly designing such joints should aim for a ductile rebar failure. However, in-situ rehabilitation solutions often require reducing the seam length to save time and material costs and, not secondarily, avoiding the plastic hinge formation in the rehabilitated column. Therefore, providing the seam with adequate overstrength is crucial to achieving a ductile failure of the welded joint. This study presents an experimental investigation into the most effective methods for connecting existing and new rebar segments through welding, addressing on-site welding challenges such as limited operational space and weld orientation. Over 100 tensile tests have been conducted at Fuzhou University’s laboratory in China to understand the response of nine welded joints. Among indirect butt joints, the study identifies two performance groups based on the ductile failure rate as the welding seams reduce. The symmetry of the joint and the presence of additional direct butt welding also in asymmetric joints can significantly improve the joint performance, determining a ductile failure with minimal welding lengths. In a second step, following a probabilistic approach, the study assesses the overstrength of indirect butt joints as a function of seam length for the two identified performance groups. Empirical failure probabilities of the rebar and the seam are estimated to fit probability density functions representative of the tensile strength of the rebar and the seam. Based on a target reliability class, the overstrength factors are estimated.

Machine vision‐based automated earthquake‐induced drift ratio quantification for reinforced concrete columns

SummaryThis paper presents a novel method for estimating the seismic peak interstory drift ratio (IDR) in reinforced concrete (RC) columns after an earthquake using surface crack image analysis. The quantitative representation of the complexity and irregularity of crack images in damaged RC columns is obtained through the consideration of the generalized fractal dimensions. The authors have compiled a comprehensive database consisting of 445 crack maps obtained from cyclic experiments conducted on 110 rectangular RC column specimens exhibiting double‐curvature deformation mode. This database is utilized by the authors to develop and validate the proposed procedure. The research database contains a wide range of structural and geometric features. Five closed‐form equations are developed with the objective of estimating the peak IDR experienced by the RC columns during a seismic event. The predictive equations are derived through the utilization of symbolic regression technique, with the input parameters varying according to the availability of columns characteristic parameters. Results reveal that generalized fractal dimensions, especially D−1, are strong vision‐based indicator of damage in RC columns having correlation coefficients with IDR ranging from 0.82 to 0.92 across the considered plans. The seismic peak IDR obtained through the empirical equations can serve as the input engineering demand parameter (EDP) in the seismic loss estimation frameworks. This allows for the determination of the probability of exceeding damage states for structural and nonstructural components of concrete buildings. Finally, the practical implementation of the methodology is examined by its application to an actual case of a damaged column during the Kermanshah earthquake of magnitude 7.3 that occurred in 2017.

Machine learning-based collapse prediction for post-earthquake damaged RC columns under subsequent earthquakes

The rapid assessment for numerous post-earthquake damaged reinforced concrete (RC) structures is critical for making informed post-earthquake decisions. However, this often requires significant resources in terms of manpower and calculation costs. This study proposes a machine learning (ML)-based method for predicting the collapse state of post-earthquake damaged RC columns under subsequent earthquakes, which are a critical earthquake-resistant component in RC structures. First, the damage strain field theory is introduced to establish the numerical analysis models for 10000 post-earthquake damaged RC columns. A dataset is constructed using time history analysis results obtained from these numerical analysis models under the action of 1000 ground motion records. The Random Forest (RF) algorithm is then utilized to train an ML model that can predict the collapse state of post-earthquake damaged RC columns under subsequent earthquakes. To evaluate the accuracy of the ML prediction model, a testing dataset consisting of 47 full-size RC columns from the Pacific Earthquake Engineering Research Center (PEER) database is constructed. Results demonstrate that the prediction model has good accuracy, with an Area Under Curve (AUC) value and an F1 measure of 0.87 and 0.76, respectively. The SHapley Additive exPlanation (SHAP) algorithm is then utilized to interpret the ML prediction model, and the influences of design parameters, ground motion intensity parameters, and damage index on the prediction results are discussed. Finally, an interactive and user-friendly graphical user interface (GUI) tool is developed to provide a rapid collapse prediction of post-earthquake damaged RC columns. This study represents a pioneering step towards the application of ML in post-earthquake assessment for damaged RC columns.

Post‐earthquake stiffness loss estimation for reinforced concrete columns using fractal analysis of crack patterns

AbstractThe seismic damage to reinforced concrete (RC) components is conventionally quantified through various damage indices. In this paper, an image‐based procedure is developed for post‐earthquake residual stiffness quantification of RC columns. The proposed scenario‐based methodology is built upon the multifractal indices of the surface crack maps of the seismically damaged RC columns as the complexity measure for the images. An extensive databank from experiments on RC column specimens with a broad range of structural characteristics are collected and employed for generation and validation of the proposed procedure. Based on the availability of the structural information, five closed‐form regression‐aided equations are offered for estimating the stiffness loss in seismically damaged RC columns using predicting parameters among fractal indices, concrete strength, and the aspect ratio of the column. Results reveal that multifractal dimensions of crack maps are largely correlated with the stiffness degradation in RC columns. A sample specimen at various stiffness‐based damage indices is also presented as a case study to evaluate the predicted versus measured damage indices. The updated stiffness values acquired by the proposed equations can be utilized for stability evaluation, system identification, or further analysis of the seismically damaged RC buildings following an earthquake.

Evaluation of the Residual Seismic Capacity of Post-Earthquake Damaged RC Columns Based on the Damage Distribution Model

Evaluation of the residual seismic capacity (RSC) of post-earthquake damaged buildings is instrumental to the formation of reasonable recovery strategies. At present, incremental dynamic analysis (IDA) that considers the mainshock and aftershock is the method most frequently used to evaluate the RSC of damaged structures. However, the mainshock-induced structural damage determined using the IDA method may be inconsistent with the damage observed in actual engineering. This inconsistency could potentially lead to an unreasonable evaluation result. To overcome this drawback, it is necessary to evaluate the RSC of damaged structures according to their observed damage instead of that obtained by the IDA. In this paper, a method of evaluating the RSC of damaged reinforced concrete (RC) columns is proposed. First, the damage degree and distribution of the damaged columns were evaluated via visual inspection after mainshocks. Then, a numerical model was developed to predict the residual behavior of damaged columns subjected to aftershocks. After that, the RSC of damaged columns was estimated based on fragility analysis. The degradation of the collapse capacity of damaged columns was quantified by the collapse fragility index (CFI), and a parameter analysis was conducted to study the effect of structural parameters on the CFI of damaged columns. Lastly, an empirical model for predicting the CFI was proposed, facilitating the application of this study in actual post-earthquake assessments. The parameter analysis indicates that the axial load ratio of the columns and the degree of damage degree accumulated during mainshocks have a significant effect on the CFI. Additionally, the proposed empirical model can effectively predict the degradation of the collapse capacity of RC columns in existing test data, with an accuracy of 0.82.

Seismic performance of lap-spliced pre-damaged and intact concrete columns strengthened or retrofitted with UHPC and NSM

Reinforced concrete (RC) columns with deficient lap splice details such as short lap splice length and widely spaced stirrups were vulnerable to earthquakes. In the present study, to evaluate the effectiveness of an innovative retrofitting technique using the combination of ultra-high performance concrete (UHPC) and near surface mounted (NSM) technique on the seismic performance of RC columns with deficient lap splices details, cyclic loading tests were performed on four large-sized RC column specimens. Two RC columns were strengthened with UHPC or the combination of UHPC and NSM, and the other two were retrofitted with the combination of UHPC and NSM after the lateral strength decreased to 85% peak strength during the pre-damage cyclic tests. The load–displacement relationship, stiffness degradation, energy dissipation, and residual displacement of RC columns are evaluated. The test results showed that the peak strength and ductility of RC columns with deficient lap splice are significantly improved after strengthening or retrofitting with the combination of UHPC and NSM. The seismic performance of severely damaged RC columns due to deficient lap splice details can still be effectively restored after retrofitting with the combination of UHPC and NSM. Considering the adverse effect of pre-damage, a design method is proposed to predict the flexural capacity of RC columns retrofitting with UHPC or the combination of UHPC and NSM. The proposed method is also applied to predict the flexural capacity of RC beams strengthened with UHPC. The proposed method predicted well the test results of this study and existing tests.

Deep learning‐based evaluation for mechanical property degradation of seismically damaged RC columns

AbstractThe evaluation of mechanical property degradation (i.e., stiffness and strength degradation) for seismically damaged reinforced concrete (RC) components is a critical step in the post‐earthquake assessment of the residual seismic capacity of buildings. In this study, a novel approach based on deep learning (DL) was proposed to evaluate the stiffness and strength degradation of RC columns according to visible seismic damage. A database was constructed by linking the test photos of RC column specimens with the loading points on the hysteretic curves, from which the stiffness and strength reduction factors (λK and λQ, respectively) were analyzed. Two novel convolutional network (CNN) modules were designed to enable feature extraction and integration of seismic damage with a reduced number of parameters, and multitask learning was introduced to enable adaptive feature fusion for stiffness and strength degradation individually. A deep convolutional network (DCNN) was therefore proposed to model the correlation between visible seismic damage and mechanical property degradation of flexural‐dominated RC columns, which can integrate visual characteristics and spatial topologies of visible damage to estimate λK and λQ. The application to two test specimens validated the preferable accuracy and robustness of the proposed DL‐based approach, and demonstrated its high potential for use in post‐earthquake performance assessment of buildings.

Prediction of residual behaviour for post-earthquake damaged reinforced concrete column based on damage distribution model

Predicting the residual behaviour of buildings in the damaged state is essential for quantitative performance assessment of post-earthquake damaged structures, which facilitates post-earthquake reconstruction. Quantification of the damage level and distribution in post-earthquake damaged structures is challenging but necessary for predicting their residual behaviour. The objective of this study was to develop a new damage distribution model for accurately predicting the residual behaviour of an earthquake-damaged reinforced concrete (RC) column. The damage relationship along the column height in the plastic hinge zone between the reference concrete and the RC column was established via fitting to experimental data. According to this relationship and the plane section assumption, the damage distribution in the plastic hinge zone was derived analytically. The damage levels of materials at different locations in the plastic hinge zone can be evaluated. The proposed damage distribution model was used to predict the residual behaviour of an earthquake-damaged RC column, and the revised ASCE-41 model was used for comparison. The verification and analyses indicated that the residual behaviour of the damaged RC column can be predicted more accurately with the proposed damage distribution model than with the ASCE-41 model. The proposed model enables accurate prediction for the residual behaviour of damaged RC columns and provide reference for reconstruction decisions.

Repair of severely damaged RC columns through FRCM composites

This study presents the results of an experimental campaign aimed at testing the effectiveness of carbon fiber reinforced cementitious matrix (CFRCM) confinement as a repair technique for severely damaged reinforced concrete (RC) columns. To evaluate shape effect and transverse steel reinforcement (TSR) – fiber interaction, two different cross-section shapes, and two TSR rates were investigated. Nine full-scale specimens were analyzed considering three control ones, three undamaged specimens confined with two CFRCM layers, and three severely damaged specimens repaired through confinement by two CFRCM layers. Axial stress-strain curves were analyzed to derive strength and ductility enhancement. Finally, TSR and fiber strains were monitored to better comprehend the contributions of internal and external reinforcement to the overall confinement and possible TSR-FRCM interaction.

Effect of pre-damage on compressive properties of circular RC columns repaired with BFRP composites: Testing and modeling

In reinforced concrete (RC) structures, structural members with damage should be repaired before reuse. Unlike existing studies that focus on strengthening of undamaged RC columns, the present study focused on the compressive properties of pre-damaged RC columns repaired with basalt fiber reinforcement polymer (BFRP) composites. Twenty-two circular columns with a relatively large size were tested. The influence of pre-damage levels, number of BFRP layers, and steel rebars on the compressive properties of the repaired RC columns were investigated. The test results showed that repairing with BFRP increased the strength and ultimate strain of the pre-damaged RC columns by 26–142% and 574–1147%, respectively, compared to the plain concrete counterparts. However, the axial stiffness and compressive strength of the repaired pre-damaged RC columns decreased as the pre-damage level increased. Addressing the adverse effect of pre-damage and the contribution of steel rebars, a strength model, ultimate strain model, and stress–strain relationship model were proposed to predict the compressive responses of the BFRP-repaired RC columns. The proposed models predicted well the test results. This study demonstrates that BFRP repairing is effective in enhancing the compressive strength and deformation capacity of damaged RC columns, even though the pre-damage level is severe.

Effectiveness of hybrid fibre-reinforced polymer retrofitting on behaviour of fire damaged RC columns under axial compression

Reinforced concrete (RC) members when subjected to elevated temperatures due to fire undergo a significant reduction in material strength, concrete cover damage, and loss of bond of reinforcing steel with concrete. In such cases, to ensure the safety of the structure and to allow its future use, it is essential to assess the performance of the fire-affected RC elements and strengthen them appropriately. The objective of this study is to understand the effectiveness of hybrid fibre reinforced polymer (FRP) retrofitting technique on the emergency repair of the severely damaged RC columns under combined fire and axial compression loads. The hybrid FRP retrofitting technique uses the combination of near-surface mounted (NSM) carbon fibre reinforced polymer (CFRP) reinforcement and externally bonded (EB) CFRP wrapping for lateral confinement. In total, eleven short RC columns were damaged under a combination of thermal and axial compression loading conditions. The damaged columns were then retrofitted using an innovative hybrid FRP retrofitting technique. Experimental results reveal that the hybrid FRP retrofitting technique restored 71%–116% of the original capacity of columns before the fire damage.

Significance of non-stationary characteristics of ground-motion on structural damage: shaking table study

This paper reports the results of a set of benchmark medium-scale shaking table tests to investigate the significance of the non-stationary characteristics of ground-motion on nonlinear dynamic responses and the structural damage of reinforced concrete (RC) columns. To examine the influence of ground-motion characteristics, four RC columns are tested under (1) near-field without pulse, (2) near-field pulse-like, and (3) far-field ground-motions. These ground-motion records were spectrally matched by the reweighted Volterra series algorithm without changing non-ergodic characteristics. To explore the confinement effects, two sets of column specimens are designed to represent the modern well-confined and older lightly-confined RC columns. Each column is tested in slight, extensive and complete damage limit states. Then aftershock excitations are conducted to investigate the performance of severely damaged RC columns. Low amplitude white-noise tests are conducted on pristine columns and after each damage limit state experiment to detect natural frequency variant of damaged columns using transfer function estimate. Furthermore, using time–frequency analysis, the real-time variant frequency of test specimens is estimated. The significant duration of ground-motions accounting for the effect of non-stationary characteristics of ground-motion is also estimated by time-cumulative damage analysis of the test results. Finally, the time-variant stiffness degradation of RC columns is estimated.

Axial compression–bending interaction behavior of severely damaged RC columns rapid repaired and strengthened using hybrid FRP composites

This paper investigates the axial compression and bending interaction behavior of reinforced concrete (RC) columns repaired and strengthened with different FRP (Fiber Reinforced Polymer) techniques after severe damage. The columns were initially damaged under different axial compression to bending load ratios. The loads were applied at different eccentricity (e) to depth (h) ratios to have (i) pure compression (e/h = 0), (ii) eccentric compression (e/h = 0.63) and (iii) pure flexure (e/h = ∞). After initial damage, the specimens underwent severe concrete core degradation with the main steel reinforcement reaching its yield strain. Later, the specimens were repaired using high strength cement grout (HSCG) and further strengthened using (i) near surface mounting (NSM) of carbon FRP (CFRP) laminates and (ii) hybrid FRP technique with a combination of NSM laminates and external bonded (EB) fabrics. After that, the columns were loaded again until failure. An axial compression (P) – bending moment (M) interaction diagram was developed experimentally for the severely damaged columns. Also, the interaction curves were developed analytically using strain compatibility procedure with the modified constitutive models. Experimental results reveal that the specimens repaired and strengthened using hybrid FRP technique can effectively restore the original capacity under different eccentric load levels. The analytical predictions showed a close match when compared to the experimental observations.

Rapid repair of severely damaged reinforced concrete columns under combined axial compression and flexure: An experimental study

Reinforced concrete (RC) columns are in general subjected to combination of axial compression and bending loads. A rapid repair technique for effectively restoring the original strength and stiffness of severely damaged RC columns under combined bending and compression loads is investigated. In total, six RC columns are tested under uniaxial eccentric compression at an eccentricity (e) to depth ratio (d) of 0.63. The columns are initially subjected to severe damage corresponding to a concrete compressive strain (εc) of 0.0035. Damaged columns had buckling of longitudinal reinforcement and concrete crushing. After severe damage, the columns are repaired with three different techniques using (i) quick set cement grout mortar (QS), (ii) near surface mounting (NSM) of CFRP laminates and (iii) hybrid FRP strengthening using a combination of NSM and external bonding (EB) of CFRP fabric. The specimens are tested under high ratio of eccentric compression (e/d = 0.63) in a displacement controlled mode. The performance of different repair techniques under eccentric compression is compared with the similar specimens tested under concentric compression. Test results revealed that hybrid FRP strengthening is very efficient in fully restoring the original strength and failure displacement under combined compression and bending loads. Only NSM strengthening is found to be very efficient under eccentric compression than in concentric compression.

Emergency repair of severely damaged reinforced concrete column elements under axial compression: An experimental study

Reinforced concrete (RC) columns in a framed construction can be severely damaged under earthquake, blast, fatigue and other loads. It is essential to quickly restore the strength and stiffness of columns after its damage for maintaining the serviceability of the building. The objective of this study is to assess the efficiency of different strengthening techniques on emergency repair of the severely damaged RC columns under axial compression. Six short RC columns elements were cast and tested under axial compression until 20% drop in the peak strength. Thereafter, these severely damaged columns were repaired using strengthening schemes, namely (i) repairing the damaged portions using quick setting cement mortar, (ii) Near Surface Mounting (NSM) of CFRP (Carbon Fiber Reinforced Polymer) laminates and (iii) Hybrid strengthening technique. The hybrid strengthening technique uses the combination of CFRP NSM laminates as additional longitudinal reinforcement and external bonding with CFRP fabric for confinement. The hybrid strengthening scheme was found to be the most efficient in restoring the lost strength and initial stiffness when compared to the only NSM strengthening or repair with quick setting cement mortar.

Ductile fibre reinforced cementitious composites (DFRCC) for improved corrosion durability of reinforced concrete columns

This paper reports the results of an experimental program on the effectiveness of ductile fibre reinforced cementitious composites (DFRCC) in retarding the corrosion of steel, corrosion induced damage and post-corrosion structural behaviour of reinforced concrete (RC) columns. Two series RC columns are included in this program. The first series consists of ordinary RC columns that are subjected to an accelerated corrosion regime. The second series consists of two columns that are similar to first two series in every aspect, except that the ordinary concrete in the column is replaced with a ductile DFRCC material. Two types of DFRCC materials are considered: a hybrid-fibre DFRCC containing 1.5 vol.% PVA fibers and 1 vol.% steel fibers and a mono-fibre DFRCC containing 2.5 vol.% PVA fibers. Specimens of the second series are also subjected to a regime of accelerated corrosion. Corrosion damage, including the possible development of spalling and/or delamination, is monitored in each specimen using a specially-fabricated mechanical expansion collar. Based on the collective findings from theoretically-estimated steel losses, visual recordings of corrosion damage, and measurements of the tendency of cover delaminate, it was concluded that the steel reinforced DFRCC columns had a remarkably higher resistance against reinforcement corrosion compared to the ordinary RC columns. The corrosion damaged DFRCC columns also exhibited about 23–64% higher failure load than that of corrosion damaged RC columns and no sign of spalling of cover of columns in the structural test.

Axial Compression Capacity of Shear-Damaged Reinforced Concrete Columns and Lashing Belt Prestressing

The compressive strength and ductility of concrete can be considerably improved by lateral confinement. In this study, an emergency seismic retrofit technique using lashing belt prestressing is used as to manually retrofit damaged reinforced concrete (RC) columns. The initial prestressing is an important aspect of this technique and is introduced by the ratchet buckle. Thus, this technique offers active and passive confinement as well as shear strengthening. Furthermore, diagonal cracks in the damaged RC columns can be closed by using the active confinement of lashing belts, and the lateral and vertical load-carrying capacity and ductility of the damaged RC columns are recovered. In this study, the recovered axial compression capacity of the retrofitted RC columns and repaired RC columns using epoxy resin was investigated. Finally, the hysteretic behavior of the shear-damaged RC columns after the proposed emergency retrofit was investigated.

A multi-mechanical nonlinear fibre beam-column model for corroded columns

Purpose A new modelling technique is developed to model the nonlinear behaviour of corrosion damaged reinforced concrete (RC) bridge piers subject to cyclic loading. The model employs a nonlinear beam-column element with multi-mechanical fibre sections using OpenSees. The nonlinear uniaxial material models used in the fibre sections account for the effect of corrosion damage on vertical reinforcing, cracked cover concrete due to corrosion of vertical bars and damaged confined concrete due to corrosion of horizontal tie reinforcement. An advance material model is used to simulate the nonlinear behaviour of the vertical reinforcing bars that accounts for combined impact of inelastic buckling and low-cycle fatigue degradation. The basic uncorroded model is verified by comparison of the computation and observed response of RC columns with uncorroded reinforcement. This model is used in an exploration study of recently tested reinforced concrete components to investigate the impact of different corrosion models on the inelastic response of corrosion damaged RC columns. Design/methodology/approach A series of pushover and cyclic analyses on a hypothetical corroded RC columns are conducted. The impact of corrosion on reinforcing steel and concrete is modelled. The influence of cyclic degradation due to low-cycle fatigue is also modelled. Findings (1) Corrosion has a more significant impact on ductility loss of RC columns than the strength loss (plastic moment capacity). (2) It was found that the flexural failure is initiated by buckling of vertical bars and crushing of core concrete which then followed by fracture of bars in tension. (3) The analyses results showed that for seismic performance and evaluation of existing corroded bridges monotonic pushover analysis is insufficient. The cyclic degradation due to low-cycle fatigue has a significant influence on the response of corroded RC columns. Originality/value The finite element developed in this paper is the most comprehensive model to date that is able to capture the onlinear behaviour of corroded RC columns under cyclic loading up to complete collapse.

Evaluation of Damaged RC Columns with GFRP-Strip Device

AbstractDamage of reinforced-concrete (RC) structures commonly occurs during major earthquakes, and becomes worse because of aftershocks. For these reasons, repair technology for seismic damage is needed to prevent possible collapse of damaged RC structures before aftershocks. In this paper, a seismic retrofitting device is proposed to improve the seismic performance of damaged RC columns using glass fiber–reinforced polymer (GFRP), which can be easy to install and quick to repair. The proposed GFRP-strip device consists of a GFRP-composite strip and aluminum clip connectors, used to increase the shear strength of the damaged column. Three RC specimens were constructed with reference to existing RC structures. Two of the specimens were damaged before installing the GFRP-strip devices. After the pretest damage was done, the GFRP-strip devices were installed along the plastic-hinge regions on both ends of the test columns. A comparison of the experimental results indicates that the seismic performance of th...