Shear-critical Reinforced Concrete Research Articles

A machine learning-based structural load estimation model for shear-critical RC beams and slabs using multifractal analysis

This paper presents a machine learning model for load-level estimation for shear-critical reinforced concrete (RC) beams and slabs using multifractal features of their characteristic crack patterns to automate and provide well-informed decisions for RC damage assessment. Multifractal analysis was conducted on a database of 508 images, of which critical features were extracted from the singularity and generalized dimension spectra. These features are used as predictors for the load-level estimation model. The extreme gradient boosting algorithm yielded the best performance among the four machine learning models considered. The mean of the predicted-to-true ratio for the developed model was 1.04 with a coefficient of variation of 0.27. Upon applying Shapley additive explanations, the fractal dimension, information dimension, correlation dimension and the area under the left branch of the singularity spectrum were the critical features influencing load-level estimation. The proposed model can be useful to RC building inspectors.

Verification of Truss-Arch model unified for RC beams with stirrups using shear database

A shear strength analysis model called “Truss-Arch Model Unified (TAMU)” was proposed for shear-critical reinforced concrete (RC) beams with transverse reinforcement. The TAMU has its roots in the Compatibility Strut-and-Tie Model (C-STM) which is a computational modeling approach for the overall force–deformation analysis of deep beams. The TAMU simplified the C-STM so that a limited analysis can be executed by hand methods of analysis to predict the maximum load-carrying capacity of RC deep beams. While the tensile stress of concrete is generally taken into account to evaluate the shear strength of RC beams, the TAMU limit analysis focuses on the strength reduction of the principal diagonal concrete arch due to the transverse principal tensile strain. In the present study, a shear database is assembled from previous works reported in the literature to further substantiate the validity of the TAMU approach. The shear database consists of tested RC beams with shear reinforcement whose shear span-depth ratios (a/d) are between 0.5 and 4.5. Comparative TAMU analysis results show satisfactory agreement with the ultimate strengths observed in 460 RC beam tests regardless of D- and B-regions, demonstrating the method’s ability to predict the maximum load-carrying capacity for shear-critical beams. Additionally, the TAMU approach shows better accuracy and consistency against the database compared to AASHTO sectional shear design methods. The probability of reasonable prediction of the TAMU method is 92% in d-regions and 78% in B-regions, respectively, which is significantly higher than those of AASHTO sectional shear design methods, especially in d-regions. Thus, the TAMU method may be a viable improvement as an alternative strength analysis method for shear-critical RC beams with transverse reinforcement.

Shear strengthening of RC beams using FRCM system made with industrial wastes

In the present study, industrial wastes were used as a partial replacement for cement to form cement-based repair mortars, and the feasibility of using these mortars as the matrix in fiber reinforced cementitious matrix (FRCM) system for shear strengthening of reinforced concrete (RC) beam specimens was investigated. Accordingly, an experimental program comprising two parts was carried out. Firstly, the properties of the mortars were examined through compressive strength, tensile strength, sulfuric acid resistivity, and electrical resistivity tests. Finally, 6 shear-critical RC beam specimens were manufactured, two of which remained un-strengthened acting as control specimens. Two specimens were strengthened in shear by carbon fiber reinforced polymer (CFRP) strips and two specimens were strengthened by fiber reinforced cementitious matrix (FRCM) layers made with prepared mortars. The shear capacity of the specimens was measured through 3-point bending test. The parameters investigated in the beam specimens included the effect of shear strengthening system (FRP/FRCM) and shear span/depth ratio. It was shown that the specimens strengthened by the FRCM system made with waste materials had similar or slightly better performance compared to the FRP strengthened specimens.

Shear behavior of reinforced concrete beams repaired using a hybrid scheme with stainless steel rebars and CFRP sheets

Repairing corroded flexural-deficient reinforced concrete (RC) structures using a hybrid scheme with stainless steel (SS) rebars and carbon-fiber-reinforced polymer (CFRP) sheets revealed excellent structural performance and contributes to lower maintenance costs in the remaining service life of the repaired structure. However, although cutting the bottom arm of stirrups in a hybrid repair method could affect the structural performance of shear-critical RC beams since this effect was not investigated in the past research. In this paper, structural performance of a group of shear-deficient RC beams repaired using the hybrid method is investigated experimentally to examine the effect of cutting the bottom arm of stirrup on the shear capacity of the repaired beams. The test variables included the longitudinal reinforcement types (i.e. carbon and SS), shear reinforcement configurations, and CFRP sheet wrapping schemes. Experimental results demonstrate that cutting the bottom arm of stirrup for replacing the corroded tensile rebar could significantly reduce the shear capacity of the retrofitted beams. Therefore, strengthening using CFRP sheet is needed to restore the shear performance of the retrofitted beams. Test results revealed that complete wrapping of CFRP sheet is effective to overcome the shear deficiency due to cutting the bottom arm of stirrup in the shear-critical RC beams. In addition, the effectiveness of CFRP sheet wrapping schemes on shear performance of SS-RC beams is approximately identical with that of CS-RC beams. Finally, a design methodology to repair the corroded RC bridge using hybrid method is developed and presented based on bending and shear tests.

Machine learning-based shear capacity prediction and reliability analysis of shear-critical RC beams strengthened with inorganic composites

The application of inorganic composites has proven to be an effective strengthening technique for shear-critical reinforced concrete (RC) beams. However, accurate prediction of the shear capacity of RC beams strengthened with inorganic composites has been a challenging problem due to its complex failure mechanism and the interaction between the internal and external shear reinforcements. Besides, the predictive capabilities of the existing models are not satisfactory. Thus, this research proposed machine learning (ML) based models for predicting the shear capacity of RC beams strengthened in shear with inorganic composites, for the first time, considering all important variables. The results of the analyses evidenced that the proposed ML models can be successfully used to predict the shear capacity of shear-critical RC beams strengthened with inorganic composites. Among the ML models examined herein, the extreme gradient boosting (xgBoost) model showed the highest prediction capability. The comparison among the predictions of the proposed xgBoost and existing models evidenced that the efficacy of the xgBoost model is superior to the existing models in terms of accuracy, safety, and economic aspects. Finally, reliability analysis is performed to calibrate the resistance reduction factors in order to attain target reliability indices of 3.5 and 4.0 for the proposed model.

Shear strength of shear-critical rectangular hollow reinforced concrete columns: Experiments and analysis

Rectangular hollow reinforced concrete (RC) columns have been used extensively for bridge structures. Inadequate transverse confinement and thin wall thickness lead to brittle shear failure, whilst limited studies have reported on the shear model. A hollow high-strength concrete and high-strength stirrups (HHSCS) column, which consisted of high-strength materials, were developed in this paper. The prior monotonic loading tests were conducted to investigate the shear performance on eight specimens. The test variables included aspect ratio, axial compression ratio, and transverse reinforcement ratio. The results revealed that all specimens experienced typical shear failure. On the basis of the modified truss-arch model, a theoretical formula was developed for estimating the shear strength of rectangular hollow RC columns. Also, six existing shear strength models were reviewed and compared with the proposed model using the test data including 37 shear-critical rectangular hollow RC columns. The comparison results showed that the proposed model provided high accuracy of shear strength whether shear-critical rectangular hollow RC columns with high-strength or normal-strength materials.

Prediction of Deflection of Shear-Critical RC Beams Using Compatibility-Aided Truss Model

This study proposes a method for predicting the deflection of shear-critical reinforced concrete (RC) beams. Shear deterioration of shear-critical RC beams occurs before flexural yielding. After shear deterioration occurs in the shear-critical RC beams, the deflection caused by shear is greater than the flexural deflection obtained from the elastic bending theory. To reasonably predict the deflection of shear-critical RC beams, it is necessary to evaluate deflections due to shear as well as flexure. In this study, the deflections produced by flexure and shear were calculated and superposed to evaluate the deflection of shear-critical RC beams. The method recommended by ACI 318-19 was employed to calculate the flexural deflection, and a compatibility-aided truss model able to calculate the shear stress and shear deformation at each load stage was used to consider the shear deflection. A comparison of the experimental and analytical results showed that the proposed analytical method can effectively predict the deflection of shear-critical RC beams.

A new genetic algorithm-based framework for optimized design of steel-jacketing retrofitting in shear-critical and ductility-critical RC frame structures

Design of seismic retrofitting of reinforced concrete (RC) structures compares structural safety on the one hand, and costs on the other. Steel-jacketing (SJ) is a very common and effective retrofitting technique, used to provide additional ductility and shear capacity to concrete members. However, its application is associated with significant invasiveness and noticeable economical and downtime costs limiting its sustainability. In this paper a new specific optimization framework addressing the minimization of seismic retrofitting-related costs is developed and presented. A new genetic algorithm routine is defined by developing modified genetic operators capable of addressing retrofitting optimization both for RC structures with ductility-critical and shear-critical RC columns, including additional shear demand due to infill-frame interaction. The framework provides the position of the column to retrofit and amount of steel-jacketing reinforcement so that the intervention costs are minimized. Applications of the framework are finally proposed with different case study structures. Results will show that the proposed approach is sufficiently general and robust to handle structural configuration having significantly different structural deficiencies and also that sustainability of retrofitting intervention is achievable by means of artificial intelligence aided optimization.

Shear behaviour of RC beams strengthened using geosynthetic materials by external and internal confinement

The objective of the present article is to study the behaviour of shear deficient reinforced concrete (RC) beams strengthened with geosynthetic materials by the experimental and analytical investigation. Two types of geosynthetic materials viz. geogrid and geotextile were applied to strengthen the RC beam by external and internal confinement. In this experimental investigation, all the RC beam specimens tested under the three-point-bending load. The shear failure is noticed in the shear critical RC beams, which has turned into a flexural-failure after strengthening with geogrid and geotextile materials by external and internal confinement. A remarkable enhancement in load-carrying capacity, energy-dissipation capacity, ductility-factor, and post-yielding behaviour is observed in all the strengthened RC beams compared to the un-strengthened conventional specimens. Further, the performance of geogrid confined specimens on these parameters is better when compared to the geotextile confined RC beams, and it is maximum in the outer confined specimens. The economy index of all the geogrid and geotextile strengthened RC beam specimens is also increased in comparison with the conventional un-strengthened specimens. Hence, it is confirmed that geogrid and geotextile materials can be a very useful strengthening solution in the RC beam by external or internal confinement with very low cost. However, for new construction, these materials can be used as shear reinforcement instead of traditional reinforcing materials, which will be very cost-effective. Further, an analytical study is carried out to illustrate the ultimate load of all the RC beam specimens, where the analytical predictions are very close to the experimental findings and show very well agreement with the test results.

Opportunities of light jacketing with Fibre Reinforced Cementitious Composites for seismic retrofitting of existing RC columns

A new jacketing technique consisting in a thin layer of Fibre Reinforced Cementitious Composites (light FRCC jacketing) in replacement of the original cover has been recently proposed for increasing durability and load bearing capacity of existing reinforced concrete (RC) structures. Although the advantages in terms of durability have been widely investigated, the behaviour of retrofitted columns under cyclic loading still deserves attention. This paper deals with the seismic response of RC columns retrofitted with a light FRCC jacketing. To this scope, flexural and shear critical RC columns were retrofitted and tested under load reversal. The resulting damage pattern was analysed by means of the Digital Image Correlation (DIC) technique. The cyclic response of the FRCC jacketed columns was compared with that of bare columns, to assess the suitability and effectiveness of the FRCC jacketing as a seismic retrofit technique. The experimental results demonstrated that the FRCC jacketing can represent a valid strengthening solution for shear critical members. In the case of flexural columns, the FRCC jacketing was able to provide an increase in ductility but induced a rocking behaviour. Preliminary modelling considerations for assessing the seismic response of FRCC jacketed columns have been finally provided, based on experimental evidences.

Performance evaluation of reinforced concrete beams with corroded web reinforcement: Experimental and theoretical study

This study investigated the effect of corroded web reinforcement on the shear behavior of reinforced concrete (RC) beams. In this study, nine shear-critical RC beams were cast and tested. Six of these beams were corroded stirrups, while the remaining three beams were uncorroded stirrups serving as control samples. An accelerated corrosion technique was employed to corrode the stirrups in the beams. After the accelerated corrosion process, the beams were tested in three-point bending over a simply supported span. The shear span-to-depth ratio was kept constant at 3 for all beam specimens. The effects of the corrosion level, the type of steel stirrups (smooth or deformed), and the stirrup diameter were considered in the tests. The corrosion damage could easily be observed in the form of rust stains and corrosion cracks, which developed parallel to the stirrups. The beams with higher corrosion levels showed a reduction in their shear capacities. This reduction was more apparent in the beams with deformed steel stirrups. The shear capacities of the test beams were analyzed using a theoretical model that considers the effects of corrosion of the stirrups. The results of the analysis showed a good correlation between the experimental and theoretical shear strengths of the beam specimens.

Shear strengthening of RC beams with FRP grid-reinforced ECC matrix

Fabric reinforced cementitious matrix (FRCM) composites are increasingly used for flexural and shear strengthening of reinforced concrete (RC) beams. This paper presents the results from an experimental study on the effectiveness and performance of a new FRCM system made of FRP grid-reinforced ECC matrix (referred to as “FGREM” for brevity) for shear strengthening of RC beams. A total of twelve shear-critical RC beams were constructed and tested. The parameters varied between the tests included the type of FRP grids, the type of cementitious matrix, the installation method of the shear strengthening system (prefabricated or cast-in-place), and the shear span-to-depth ratio of the beams. The crack patterns of all the tested beams were monitored using the digital image correlation (DIC) technique. Test results indicated that interfacial debonding of the shear strengthening layer and separation of the concrete side covers were two typical failure modes for the beams strengthened with the cast-in-place method and the prefabricated method, respectively. The gains in the shear capacity of the strengthened beams ranged from 50.9% to 160.6% with respect to the reference beams. Compared to the cast-in-place installation method, the prefabricated FGREM was demonstrated to be more efficient for shear strengthening of the RC beams.

U-Jacketing Applications of Fiber-Reinforced Polymers in Reinforced Concrete T-Beams against Shear—Tests and Design

The application of externally bonded fiber-reinforced polymer (EB-FRP) as shear transverse reinforcement applied in vulnerable reinforced concrete (RC) beams has been proved to be a promising strengthening technique. However, past studies revealed that the effectiveness of this method depends on how well the reinforcement is bonded to the concrete surface. Thus, although the application of EB-FRP wrapping around the perimeter of rectangular cross-sections leads to outstanding results, U-jacketing in shear-critical T-beams seems to undergo premature debonding failures resulting in significant reductions of the predictable strength. In this work, five shear-critical RC beams with T-shaped cross-section were constructed, strengthened and tested in four-point bending. Epoxy bonded carbon FRP (C-FRP) sheets were applied on the three sides and along the entire length of the shear-strengthened T-beams as external transverse reinforcement. Furthermore, the potential enhancement of the C-FRP sheets anchorage using bolted steel laminates has been examined. Test results indicated that although the C-FRP strengthened beams exhibited increased shear capacity, the brittle failure mode was not prevented due to the debonding of the FRP from the concrete surface. Nevertheless, the applied mechanical anchor of the C-FRP sheets delayed the debonding. Moreover, the design provisions of three different code standards (Greek Code of Interventions, Eurocode 8 and ACI Committee 440) concerning the shear capacity of T-shaped RC beams retrofitted with EB-FRP jackets or strips in U-jacketing configuration are investigated. The ability of these code standards to predict safe design estimations is checked against 165 test data from the current experimental project and data available in the literature.

Robustness evaluation of CSMM based finite element for simulation of shear critical hollow RC bridge piers

PurposeThe purpose of this study is to evaluate the effectiveness of two-dimensional (2D) cyclic softened membrane model (CSMM)-based non-linear finite element (NLFE) model in predicting the complete non-linear response of shear critical bridge piers (with walls having aspect ratios greater than 2.5) under combined axial and reversed cyclic uniaxial bending loads. The effectiveness of the 2D CSMM-based NLFE model has been compared with the widely used one-dimensional (1D) fiber-based NLFE models.Design/methodology/approachThree reinforced concrete (RC) hollow rectangular bridge piers tested under reversed cyclic uniaxial bending and sustained axial loads at the National Centre for Research on Earthquake Engineering (NCREE) Taiwan have been simulated using both 1D and 2D models in the present study. The non-linear behavior of the bridge piers has been studied through various parameters such as hysteretic loops, energy dissipation, residual drift, yield load and corresponding drift, peak load and corresponding drift, ultimate loads, ductility, specimen stiffness and critical strains in concrete and steel. The results obtained from CSMM-based NLFE model have been critically compared with the test results and results obtained from the 1D fiber-based NLFE models.FindingsIt has been observed from the analysis results that both 1D and 2D simulation models performed well in predicting the response of flexure critical bridge pier. However, in the case of shear critical bridge piers, predictions from 2D CSMM-based NLFE simulation model are more accurate. It has, thus, been concluded that CSMM-based NLFE model is more accurate and robust to simulate the complete non-linear behavior of shear critical RC hollow rectangular bridge piers.Originality/valueIn this study, a novel attempt has been made to provide a rational and robust FE model for analyzing shear critical hollow RC bridge piers (with walls having aspect ratios greater than 2.5).

Effectiveness of diagonal shear reinforcement on reinforced concrete short beams

In the study, an experimental and numerical study is performed to investigate the efficiency of diagonal shear reinforcement (DSR) on reinforced concrete (RC) short beams. For this purpose, 7 RC short beam specimens were tested under a 4-point loading, and a numerical study is conducted by using finite element method. Additionally, the efficiency of addition of DSR to specimens is observed in the experimental study together with the increase in stirrup spacing. Analysis results are compared in terms of load-displacement behavior and failure modes. As a result of the study, a significant improvement both in shear and displacement capacities of the RC short beams are achieved along with addition of DSR in short beams. Moreover, it is deduced from the numerical results that increasing both the diameter and yield strength of DSR makes a significant contribution to the shear capacity and ductility of shear critical RC members.

Experimental study on shear strengthening of shear critical RC beams using iron-based shape memory alloy strips

Shape memory alloys (SMAs) have been introduced into structural engineering in recent years. This paper presents research results about developing a new methodology to actively strengthen reinforced concrete (RC) beams by low-cost iron-based SMA strips (Fe-SMA). These strips can transversally prestress, or confine, the cross-section of beams thanks to the shape memory effect of Fe-SMA, without having to apply prestressing force. Activation of strips is carried out by heating them up to 160 °C and then cooling them. An experimental campaign was carried out at two levels: characterization of Fe-SMA strips, and the practical application of the strengthening technique on small-scale beams without internal stirrups. The retrofitted beams with activated strips failed by bending, and the appearance of shear cracks was clearly delayed. Meanwhile, the reference beams failed due to shear.

Experimental and analytical study on strengthening of reinforced concrete T-beams in shear using steel reinforced grout (SRG)

The use of steel reinforced grout (SRG) has emerged as a promising technique for the strengthening and retrofitting of reinforced concrete (RC) structures, however, the experimental evidence on its application for shear-critical RC beams is rather limited. Accordingly, this study focuses on the structural performance of SRG-strengthened RC beams critical in shear. At this aim, an experimental investigation has been conducted on eleven RC T cross-section beams. Eight beams were strengthened with different types of SRG (different for the density, namely 1.57 and 3.14 cords per centimeter) comprised of ultra-high tensile strength steel fabrics, and three, which were unstrengthened, were used as reference specimens. The test parameters investigated were as follows: (a) steel fabric density, (b) bond scheme, and (c) amount of internal shear reinforcement within the critical shear span, SRG/stirrups interaction. The experimental results revealed the high potential of SRG system for the strengthening of shear-critical RC beams. It has significantly improved the shear capacity and deformation characteristics of the strengthened beams. The percentage increase in the load-carrying capacity over the reference beam of up to 71% was observed. In addition, an analytical model based on the simplified modified compression field theory (SMCFT) has been proposed to predict the shear capacity of SRG-strengthened beams. The SMCFT-based model has resulted in an accurate and safe prediction of the shear capacity of the SRG-strengthened beams with an average ratio of Vth/Vex of 0.92 and a standard deviation of 4.07%.

Efficient modeling of flexural and shear behaviors in reinforced concrete beams and columns subjected to low-velocity impact loading

Detailed finite element (FE) models with 3-D solid elements are typically used to simulate impact behaviors of reinforced concrete (RC) beams and columns. However, the method usually requires a substantial amount of time and effort to model concrete and reinforcement and conduct nonlinear contact-impact analyses. Also, the accuracy of the method cannot be guaranteed due to the limitations in concrete material models implemented in general-purpose FE codes. In this paper, an efficient modeling method is proposed to capture both flexural and shear behaviors of RC beams and columns under low-velocity impact loading. A macroelement-based contact model was developed in the proposed method to capture interaction behaviors between impacting objects and RC members. In the contact model, a compression-only spring with an initial gap behavior was used to account for the stiffness and the kinematic response of an impacting object, and a combination of an elastic spring and a viscous damper in parallel was employed to simulate the contact stiffness and damping. By properly approximating the strain-rate effects, traditional fiber-section elements were demonstrated to be capable of predicting impact-induced flexural failures for RC members. On this basis, a general approach for both flexural- and shear-critical RC columns under impact loading was presented with the inclusion of additional shear springs in the fiber-section elements. Nearly fifty impact tests on RC beams and columns reported in the literature were employed to validate the proposed modeling method. Comparisons between the experimental and numerical results indicate that failure modes of the impacted members can be identified explicitly by the use of the proposed modeling method. Also, reasonable agreements were achieved for the impact forces and the impact-induced responses obtained from the impact tests and the analyses. The proposed method can be readily implemented without coding in any FE software as long as traditional fiber-section elements, and discrete macroelements are available. This feature would offer an advantage for the applications of the proposed method such as when assessing the structural response and vulnerability of a bridge structure subjected to vessel and vehicle collisions.

Steel-Reinforced Grout (SRG) strengthening of shear-critical RC beams

Abstract This paper investigates the effectiveness of Steel-Reinforced Grout (SRG) jackets to strengthen shear critical reinforced concrete (RC) beams. Eleven RC beams were tested in three-point bending. Key parameters of investigation were the strengthening configuration (U- and fully-wrapped jackets), the density of the fabric (1.57 and 4.72 cords/cm) and the number of the strengthening layers (one and two). The test results demonstrated the efficiency of SRG jacketing in increasing both strength (up to 160%) and deformation capacity (up to 450%) of the shear critical beams. Expressions are proposed for estimating the effective strain of the SRG jacket.

A Simplified Approach to Investigate the Seismic Ductility Demand of Shear-Critical Reinforced Concrete Columns Based on Experimental Calibration

ABSTRACT A simplified approach is proposed to investigate the seismic ductility demand of shear-critical reinforced concrete (RC) columns based on experimental calibration. Incorporating with the Bouc–Wen–Baber–Noori (BWBN) model, the governing equation of the equivalent single-degree-of-freedom (SDOF) system is first presented to describe the typical hysteresis characteristics, where the relationship between the inelastic restoring force and translational displacement is calibrated by comparing with the experimental data of shear-critical RC columns under quasi-static cyclic testing. Based on the developed governing equation, the strength reduction approach is further implemented to estimate the seismic ductility demand of shear-critical RC columns by establishing the constant strength-ductility demand spectral (CSDDS) method. Finally, an empirical expression is regressed to predict the mean and standard deviation of seismic ductility demand of shear-critical RC columns. This study verifies the feasibility of the proposed approach to assess the seismic ductility demand of shear-critical RC columns. The presented idealized SDOF system can simplify the establishment of CSDDS method. The lognormal assumption cannot well describe the probabilistic characteristics of seismic ductility demand if all the uncertainties in model parameters and earthquakes are considered. The developed empirical expression is applicable to investigate the probabilistic seismic risk of shear-critical RC columns under different earthquake excitations.