Transverse Steel Reinforcement Research Articles

Investigation of shear response due to variation of fiber volume fraction, transverse steel, and placement method on short span R/UHPC beams

This study investigates the response of small-scale, reinforced ultra high performance concrete beams. Seven specimens were cast and experimentally tested. Steel fiber volume fraction, amount of longitudinal and transverse steel reinforcement, and casting direction varied between specimens. High longitudinal reinforcement ratios of 4.1% and 9.5% were expected to drive relatively large shear demands and low steel fiber volume fractions of 0.5% and 1% were expected to decrease the UHPC’s ability to resist tension, shear, and compression. Specimens were either cast at one end or mid-span. All specimens failed in shear, as expected. R/UHPC beams with only 0.5% fibers and a stirrup spacing equal to half the beam’s depth carried more ultimate load than beams with 1% fibers and twice the stirrup spacing. At a 0.5% fiber volume fraction, UHPC performed well, exhibiting fiber bridging in tension and resisting spalling in compression. As longitudinal reinforcement ratio increased from 4.1% to 9.5%, load carrying capacity generally increased, but not proportional to the increase in steel. Placement method did not influence fiber orientation in the R/UHPC beams when measured in 2D planes transverse or longitudinal to the specimens’ major axis. The longitudinal and transverse reinforcement interrupted fiber flow and prevented significant fiber alignment; combined with the short span of the beams, these factors mitigated the influence of placement method.

Prediction of Flexural Strength of RC Circular Columns Considering Lateral Confinement Effects of FRCM and Transverse Steel Reinforcement

There has recently been growing interest in making a sustainable and durable fiber reinforced cementitious matrix (FRCM) to improve the seismic performance of RC column members. However, most studies evaluating the lateral confinement effect on FRCM jacketing concrete have excluded the confinement effect of transverse steel reinforcement and focused solely on mechanical properties. This paper, based on existing studies, proposes a peak axial stress formula that considers the lateral confinement effects of FRCM composite and transverse steel reinforcement. Additionally, the study assesses the structural performance of FRCM composites with flexural strengthening in actual size RC circular columns and predicts the maximum flexural strength by applying the proposed formula. The proposed peak axial stress formula, considering the influence of transverse reinforcement, was compared with the experimental value (databases) and the predicted value, and the suitability was confirmed with a standard deviation of 27.6% and a coefficient of variation of 0.856. In addition, the maximum flexural strength of actual size RC circular columns predicted by applying this formula ranged from 0.97 to 1.02, which effectively predicted the experimental values.

Experimental and Analytical Study on the Axial Behavior of Circular High-Strength Concrete Columns with Hybrid Carbon Fibers and Steel Confinement System

This paper describes a work that examines a new solution to the problem that arises from the relatively high amount of transverse reinforcement required in HSC columns. It presents an alternative to common transverse steel reinforcement, a dual system comprising steel ties and a carbon-fiber mesh (CFM) applied internally together with steel ties. The behavior of the proposed system was examined in this work in a series of twelve laboratory tests of circular stub column specimens. The experiments performed in this work focused on the columns’ load and displacement capacities. The tests were planned with the aid of an analytical model that was originally developed for a hybrid system of external fiber-reinforced polymer (FRP) sheets and internal steel, and was adapted for the current system. An analysis of the results shows that for a given amount of conventional transverse steel, the application of the carbon-fiber meshes adds efficiency to the rebar confinement system, in terms of both the load bearing capacity and the ductility, and for specimens with the hybrid confinement system, the higher the carbon fiber amount the larger the ductility improvement. Furthermore, fair to good agreement was observed between the model and the measured stress–strain curves, especially those of the peak stresses. Based on the above findings and the added benefit of fire resistance, the hybrid method appears to be promising for confining HSC columns.

Experimental and finite element analysis of reinforced concrete multi-cell box girders retrofitted with carbon fiber reinforced polymer strips under torsion

This study expands the state of the art in studies that assess torsional retrofit of reinforced concrete (RC) multi-cell box girders with carbon fiber reinforced polymer (CFRP) strips. The torsional behavior of non-damaged and pre-damaged RC multi-cell box girder specimens externally retrofitted by CFRP strips was investigated through a series of laboratory experiments. It was found that retrofitting the pre-damaged specimens with CFRP strips increased the ultimate torsional capacity by more than 50% as compared to the un-damaged specimens subjected to equivalent retrofitting. This indicated that the retrofit has been less effective for the girder specimen that did not develop distortion beforehand as a result of pre-loading. From experimental observations, when the girder specimen was cracked and the transverse steel reinforcement bars started straining, the CFRP strips started to work more effectively and contributed together with the transverse steel reinforcement to resist torsion load. Additionally, Finite Element (FE) simulations were developed using Abaqus to model the torsional behavior of the retrofitted girders, the results of which were validated with the experimental data. With the numerical model, the effect of concrete compressive strength, transverse reinforcement spacing, and CFRP strip spacing were examined. The effectiveness of the CFRP strips in enhancing the torsional strength of the girder was found to increase with increasing the spacing between the transverse reinforcement.

Torsional behavior of FRP-strengthened reinforced concrete members considering various wrapping configurations: Theoretical analysis and modeling

Reinforced concrete (RC) structural members such as spandrel, flanged and curved beams, slab beams, and bridge girders may be subjected to significant torsional loading. Despite extensive research on the strengthening of RC members, very little is known about the torsional behavior of RC members strengthened with fiber-reinforced polymer (FRP) sheets. Therefore, very limited guidelines apply to the design of externally bonded FRP systems for torsional strengthening, and no design provisions exist for the U-wrapped and 2-sided strengthening systems in torsion. This study aims to contribute to this growing area of research by developing a new analytical model to predict the full torsional response of strengthened RC members with many conventional configurations such as spiral, transverse and longitudinal FRP, continuous and intermittent strips along the beam length, side bonded, and full-wrapped systems. Moreover, this study provided an analytical model accurately predicting the contribution, efficiency, and failure mode of side-bonded strengthening systems. Additionally, the influence of various parameters such as FRP stiffness and transverse steel reinforcement ratio is taken into account in the constitutive law of concrete and internal reinforcement for the first time. The good agreement between the test results of 32 collected FRP-strengthened RC beams from the literature and the theoretical ones, which were predicted through programming proves the validity and accuracy of the presented theory.

Centrifugal concrete poles for transmission lines reinforced with prestressed FRP rebar

Reinforced with steel rebar concrete centrifuged conical poles for power transmission poles receive corrosion damage to steel reinforcement during operation. These damages can lead to the failure of structures. Complete replacement of both longitudinal and transverse steel reinforcement by composite one is a way to increase the reliability and durability of such structures. This method also allows the use of cost-effective concretes with a low cement content. The research results have shown the viability of such solutions, but the use of composite reinforcement requires its prestressing. This article discusses the Russian practice of testing concrete centrifuged conical poles for strength, crack resistance and rigidity. A brief description of the prototypes and methods of their testing is given. The main results of bending tests of full-scale test samples are presented in the form of diagrams of their deformation. Analysis of the results showed that the test samples of poles are correspond to requirements of Russian norms for deformability, crack resistance and strength. According to the results the studied centrifugal concrete poles with prestressed longitudinal GFRP rebars and transverse spiral GFRP rebars are like traditional ones with steel reinforcement (in standardized strength, crack resistance and deflection values) and, therefore, can be used in building construction.

Axial compressive behaviour of corroded steel reinforced concrete columns retrofitted with a basalt fibre reinforced polymer-ultrahigh performance concrete jacket

• An accelerated retrofit approach is developed. • The axial compressive behaviour of retrofitted columns are analyzed in detail. • A simple model was proposed with relatively conservative predictions. Steel-reinforced concrete (SRC) members suffer from severe corrosion in some scenarios. Therefore, new techniques and materials are required for the timely rehabilitation of the deficient structures. To address this challenge, the authors have developed an accelerated retrofit approach using a basalt fibre-reinforced polymer (BFRP) sheet combined with an ultrahigh-performance concrete (UHPC) jacket. The BFRP sheet was internally embedded within the UHPC matrix to enhance its durability under marine environmental conditions. The treatment of corroded steel stirrups in SRC columns replaced with fiber-reinforced polymer (FRP) composites is also considered in the retrofit approach. The authors then investigated the axial compressive behaviour of a series of corroded RC columns retrofitted with a BFRP–UHPC jacket or with a UHPC jacket. Specifically, 16 RC columns (six control specimens and 10 retrofitted specimens) were prepared and tested under monotonic axial compression. The test variables were the sectional geometry (i.e. circular or square cross -section), transverse steel reinforcement ratio (i.e. two different stirrup spacings), and retrofitting method (i.e. BFRP–UHPC jacket or UHPC jacket). The test results showed that the maximum increase in the peak load after retrofitting was as large as 145.89%. The authors also developed a simple model for retrofitted RC columns that can provide reasonably accurate but relatively conservative predictions.

Numerical modeling of iron-based SMA confined concrete columns under axial compressive loading

This paper presents the results of a Finite Element (FE) model developed to simulate the behavior of confined concrete columns with iron-based shape memory alloys (Fe-SMA) under compressive loading. The performed simulations aimed to accurately demonstrate the combined effect of external shape memory alloy (SMA) reinforcement and internal transverse steel reinforcement under different loading conditions and presented a simplified approach to model external confinement as discrete elements near the surface of the concrete element.The primary objective of this study was to develop a FE model that can be used to predict and replicate the response of actively confined columns using Fe-SMA. The FE model was developed using the results of an experimental study conducted on circular Fe-SMA confined columns under concentric loading. The experimental study evaluated the influence of: 1) the presence of internal reinforcement, and 2) the ratio of external SMA confinement used. The simplifications and approximations considered in the numerical approach to predict the capacity and deformation responses showed to be adequate, and the results obtained were in good agreement with the compressive behavior of the actively SMA confined columns obtained from the experimental work.In addition to the concentric case, eccentric loading simulations are presented in this study to evaluate the combined confinement mechanism in terms of the structural element response to a different loading configuration. The predictions obtained in terms of load-carrying capacity and deformations can be used to expand the research work done using the SMA as a form of active confinement.

Artificial neural networks approach for prediction of axial loading capacity of circular normal strength concrete columns confined by both transverse steel reinforcement and carbon fiber reinforced polymer wraps

A few empirical models for the axial loading capacity (ALC) of circular normal strength concrete (NSC) columns wrapped by carbon fiber reinforced polymer (CFRP) sheets and interior transverse steel reinforcement (TSR) (CSC columns) are available in the literature. The deficiency of those models is that they were proposed based on a small number of tests by considering limited parameters of CSC columns. Therefore, the main aim of the current investigation is to propose the improved empirical models for the ALC of CSC columns by including the interaction mechanism between TSR and FRP confining behavior. To secure this aim, a general regression analysis technique and artificial neural networks (NNs) on the experimental outcomes of 76 CSC columns collected from the previous investigations were employed. The proposed NN model was adjusted for the different number of hidden layers and neurons to achieve an optimized model. The suggested NN and empirical models portrayed a close agreement with the testing database with R2 = 0.998 and R2 = 0.892, respectively. The NN model reported a higher accuracy than the theoretical model. The comparative investigation solidly authenticated the superiority and accuracy of the anticipated strength models for CSC columns.

Size Effect on Stress-Strain Model of Carbon Fiber- Reinforced Polymer Transverse Steel Reinforcement- Confined Concrete

Size Effect on Stress-Strain Model of Carbon Fiber- Reinforced Polymer Transverse Steel Reinforcement- Confined Concrete

Experimental and numerical investigations on seismic performance of HTRB630 high-strength steel bars reinforced concrete columns

Heat-treated ribbed bar 630 (HTRB630) is a new type of the main 630 MPa reinforcing steel produced in China. To explore the possibility of using HTRB630 high-strength steel bars to replace HRB400 steel bars in reinforced concrete structures, experimental and numerical investigations on seismic performance of HTRB630 high-strength steel bars and HRB400 steel bars reinforced concrete columns are presented in this article. Firstly, the pseudo-static experiment of HTRB630 and HRB400 steel bars reinforced concrete columns is carried out. Secondly, an efficient modeling method is proposed to simulate and analyze the seismic performance of reinforced concrete columns on OpenSees platform based on the measured mechanical properties. ConcreteCM model and Reinforcing Steel model are applied to accurately simulate the uniaxial constitutive relationship of concrete and steel bars respectively. The constitutive parameters of confined concrete of specimens are calculated using the iterative algorithm method to effectively consider the degree of improvement of concrete performance in the core area by transverse steel reinforcements. Considering the bond-slip effect and shear deformation of reinforcements, a rotation spring element and a shear spring element are introduced on the fiber model. Finally, the applicability and accuracy of the numerical simulation results are verified by the experiment results on the hysteresis curves, normalized skeleton curves, bearing capacity, displacement ductility, energy dissipation capacity and degradation of stiffness. The experimental results indicate that the failure modes of the reinforced concrete column specimens are all bending failure. Hysteresis loops of the specimens reinforced HTRB630 and HRB400 steel bars are all bow-shaped. After the equal strength of HTRB630 steel bars, the shape of the hysteresis curve is not changed significantly. HTRB630 high-strength steel bars reinforced concrete columns can achieve good seismic performance under the premise of reasonable reinforcements. The numerical results indicate that the finite element model has a good simulation effect on most seismic performance indicators. For specimens with relatively low axial pressure and insignificant pinching, the error of the simulated hysteresis curve is larger; for specimens with relatively high axial pressure and obvious pinching, the error of the simulated hysteresis curve is smaller. The simulation errors of those seismic performance indicators are about 10%.

Performance Assessment of Reinforced Concrete Frame under Close-In Blast Loading

The need for assessing and retrofitting structures increased with time as terrorism-induced explosion trends rose with time. This paper presents a numerical investigation on performance assessment of a two-story, one-bay seismic resistant reinforced concrete framed building under close-in blast loading. ANSYS AUTODYN, an explicit nonlinear finite element software program, was used for 3D model development and analysis. The experimental results reported in the literature were used to validate proposed FE models. Furthermore, parametric studies on close-in explosive story-to-story locations and charge masses were performed on both conventional and seismically detailed RC framed structures. FEA results showed that a decrease in scaled distance raised effective plastic strain and damage index values. Furthermore, simultaneous use of close-spaced transverse steel reinforcement spacing in mid-height and ends of reinforced concrete columns is found to be effective in reducing both effective plastic strains and damage index values.

Effect of Internal Stirrups on the Eccentric Compression Behavior of FRP-Confined RC Columns Based on Finite-Element Analysis

Fiber-reinforced polymers (FRPs) have been widely used in the retrofitting and rehabilitation of reinforced concrete (RC) columns because they can provide substantial lateral confinement to concrete. Conventionally, the presence of internal stirrups is neglected in the retrofitting design due to a lack of understanding on the effect of internal hoops when both external FRP and internal steel confinement exist, particularly under eccentric loading. In this study, a finite-element (FE) method is developed to investigate the eccentric compression behavior of FRP-confined RC columns. A reliable concrete plastic-damage model that accurately considers the confining stiffness ratio between the FRP and steel is proposed for the first time, which facilitates the study of a difficult problem: the behavior of RC columns with the dual confinement system under eccentric loading. The accuracy of the model is verified by test results. An extensive parametric study of various affecting factors is carried out to investigate the confinement mechanisms of the concrete columns in terms of the axial stress and confining pressure distributions, lateral principal stress ratio, local stress–strain response, and interaction between the FRP/steel hoops and concrete. It is found that the nonuniform confining pressures under eccentric compression are significantly affected by the internal stirrups. With the existence of discrete transverse steel reinforcement, the axial resistance of the concrete core is increased, thereby improving the ultimate load capacity of the columns. An ultimate axial load model for RC columns that considered combined FRP–steel confinement is subsequently proposed. More accurate results are obtained using the proposed model compared with those calculated from existing design codes.

Data-oriented analysis of axial capacity of externally CFRP-confined concrete columns transversely reinforced with steel hoops or spirals

Limited research is available in the literature for exploring the axial compressive behavior of circular concrete columns confined by both external CFRP wraps and internal transverse steel reinforcement (SCC columns). The main objective of the present work is to develop an empirical model and an analytical model for predicting the axial compressive strength of SCC columns by considering the interaction between the CFRP composite and steel reinforcement confinement action. An extensive experimental database of 76 SCC columns has been developed from the literature work. A general regression analysis method and curve-fitting techniques on the experimental test results of SCC columns are used for developing the new empirical model. The confinement mechanics-based analytical model has also been suggested by considering the influence of CFRP and transverse steel confinement. The comparative study solidly substantiated the superiority of the proposed empirical model over the proposed analytical model and the previous models with =0.892, =12.83, and =9.78. A parametric study is also performed using the proposed empirical strength model to examine the effect of different parameters of SCC columns confined by both CFRP and transverse steel.

Experimental investigation of twin confinement effects of FRP and steel rebar on cyclic compressive behavior of concrete

Concrete confinement is one of the most effective methods to increase the ductility of structural reinforced concrete elements. Externally bonded fiber reinforced polymer (FRP) is a technique for seismic retrofit and rehabilitation of existing structures. Consequently, evaluation of the cyclic behavior of concrete members that are confined simultaneously by transverse steel reinforcement and FRP is of great significance. During recent decades, a significant number of valuable experimental studies have been conducted and various comprehensive analytical models have been proposed to take into account the effect of confinement provided by steel rebar or FRP. However, less attention has been paid to the twin confining effect of FRP and steel reinforcement on the cyclic behavior of concrete. In this study, a total of 32 concrete specimens were tested under monotonic and cyclic axial compression. The specimens were confined using steel bars or/and CFRP with different reinforcement and retrofitting schemes. Afterward, using the obtained experimental results, the accuracy of a previously developed elasto-plastic and fracture-based model is evaluated. Furthermore, minor modifications are proposed to take into account the effect of concrete cover, longitudinal reinforcement, continuous spiral confinement, and weak confining effects. The modified model is used to predict the behavior of the tested specimens with respect to the experimental loading paths. The results are in good agreement with the experimental data, indicating that the model can accurately simulate the cyclic behavior of concrete confined with FRP and steel rebar, simultaneously.

Flexural Strengthening of Substandard Reinforced Concrete Bridge Wall Piers with CFRP Systems under Cyclic Loads

Reinforced concrete bridge wall piers constructed according to older codes experience damage during earthquakes, which can lead to bridge collapse. Deficiencies include inadequate design and detailing of longitudinal and transverse steel reinforcement and deficient lap splices. Three wall piers were constructed using as-built reinforcement details conforming to pre-1973 requirements to evaluate flexural strengthening strategies. The first wall pier was used as the control specimen. The second wall pier was retrofitted using carbon fiber-reinforced polymer (CFRP) jackets and transverse CFRP anchors placed through the section for the confinement of the lap-spliced region and vertical CFRP anchors to improve flexural capacity. The third wall pier was retrofitted similarly to the second pier by using vertical near-surface mounted (NSM) CFRP rods. The piers were tested under cyclic lateral load until failure. Both retrofitted wall piers were able to achieve significantly higher flexural capacity, stiffness, and hysteretic energy dissipation when compared with the control wall pier specimen. The as-built wall specimen was not able to develop the theoretical flexural capacity due to lap splice clamping failure; both retrofitted specimens exceeded the theoretical flexural capacity of the as-built wall specimen.

Influence of synthetic fibers on the seismic behavior of reinforced-concrete circular columns

The addition of discrete fibers to the concrete mixture to enhance its toughness and tensile strength is well acknowledged. Studies have also shown that fiber-reinforced concrete (FRC) can be favorable for its enhanced behavior in compression. This behavior, which is attributed to the confinement action of the fibers, is, in general, superior in terms of strength and ductility, compared to plain concrete. Reversed cyclic loading in flexure under constant axial load of large scale synthetic fiber-reinforced concrete columns is scarce as opposed to steel fiber-reinforced concrete columns. This paper presents an experimental study on the behavior of large-scale synthetic fiber-reinforced concrete columns with hoops transverse steel reinforcement under constant axial load and reversed flexure simulating seismic loading. The results from this study show slightly improved cyclic behavior compared to results obtained on similar columns reinforced with spiral transverse reinforcement. Enhanced behavior of synthetic fiber-reinforced concrete is achieved for practical transverse steel reinforcement spacings (⩾75 mm). In this case, similar ductile behavior of a reinforced-concrete column can be reached with a reduced amount of transverse steel reinforcement and the inclusion of synthetic fibers with an aspect ratio of 50–100 to the concrete mixture. Difficulty in placing synthetic fiber-reinforced concrete in very congested columns precludes its use in columns with hoops or spiral spacing less than 75mm until self leveling concrete with large amount of synthetic fibers is developed.

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.

Seismic collapse assessment of new European single-story RC precast buildings with weak connections

Capacity design, according to the modern seismic building codes, requires the application of specific rules and construction details in order to prevent brittle failure modes at material, element and structural level. In particular, with reference to single-story precast reinforced concrete structures with columns joined by pinned beams, the Italian seismic building code, following the Eurocode 8 general principles, requires that beam-to-column connection should be designed in order to avoid the connection failure before the formation of the plastic hinge at the column base. However, no specific details are provided in order to reach such a performance. Recent European earthquakes showed that seismic performance of beam-to-column dowel connections can be very poor. Hence, for European typical dowel beam-to-column connections, this study aims to investigate: (a) possible connection configurations obtained according to seismic design; (b) possible failure modes involving connection elements (steel dowel, transversal steel reinforcement, dowel concrete cover); (c) how available formulations are able to describe such failure modes and the related shear strength; (d) influence of connection failure on the global seismic safety at collapse of new designed single-story RC precast buildings. The reference buildings and beam-to-column connections are designed considering different geometrical layouts of the primary structure and different seismic hazard levels, i.e. four sites and two soil types. Structural capacity is estimated considering global, i.e. related to column plastic hinge degradation measured in terms of top lateral displacements, and local, i.e. related to connection shear strength, failure modes. Nonlinear multi-stripe analyses are performed for the seismic assessment.

Experimental and numerical investigations of reinforced concrete columns confined internally with biaxial geogrids

Geogrids are geosynthetic materials that have been proven effective for stabilization and strengthening applications in infrastructure, such as: reinforcement and stabilization of pavement layers, reinforcement of embankments and soil walls, in addition to soil improvement. The main aim of this study is to investigate the feasibility of using biaxial geogrids as a replacement of ordinary transverse steel reinforcement in columns. Using geogrids as confining material in reinforced concrete columns offers advantages over steel stirrups in that they are less laborious to install and are more durable. The study presents results of experimental and analytical investigations of the behavior of small-scale reinforced concrete columns of 500 mm height subjected to monotonic uniaxial compressive loading. The main test variables include confining material (steel stirrups or geogrids), number of geogrids layers, and the aspect ratio (height to diameter) of the column. Strain gauges were installed on the geogrids to determine the displacement fields of the geogrid elements. Axial displacement and axial load were measured to assess the overall load-displacement behavior, stiffness, ultimate strength, and ductility of the confined column specimens. Analysis of the results revealed that the ductility and energy absorption capacity of concrete column specimens are substantially enhanced without any significant effect on the ultimate load capacity when internally confined by biaxial geogrids. Two distinct analytical models were developed to predict the load-displacement behavior, one for unconfined plain concrete specimens, and the other for columns with transverse reinforcement using biaxial geogrids. The models provided satisfactory predictions of the stress–strain response when the analytical and experimental results were compared.