Rectangular Reinforced Concrete Research Articles

Shear strength components of adjustable hybrid bonded CFRP shear-strengthened RC beams

This paper presents an experimental study on shear behavior of full-scale rectangular reinforced concrete (RC) beams under shear-strengthening of carbon fiber-reinforced polymer (CFRP) U-strips. The test involved five RC beams with transverse reinforcement, including one control beam, one shear-strengthened by externally bonded (EB) CFRP U-strips only, and the remaining three strengthened in shear by a hybrid bonded (HB) CFRP system under different normal pressures applied on the CFRP U-strips. The transverse reinforcing steel bars and CFRP U-strips were extensively instrumented with strain gauges, which enabled the accurate quantitative assessment of shear strength contributions from CFRP (Vf), stirrups (Vs) and concrete (Vc) during the whole loading process of a shear test. Further, by comparing Vs and Vc for beams with and without CFRP strengthening, the interactions can be evaluated and clarified. In addition, key features of shear behaviors, e.g. failure mode, load-deflection curves, shear capacity and ductility, shear crack angle, and strain development and distribution of stirrups as well as CFRP strips, are presented, compared and analyzed.

A constitutive model for confined concrete in slender rectangular RC sections incorporating compressive energy

Reinforced concrete (RC) structural walls can provide superior stiffness in tall buildings while maintaining notable ductility when subjected to seismic demands. In flexure-dominated walls, this ductility is based on the ability of the end regions (also known as boundary elements) to maintain load carrying capacity through increasing tension and compression cycles. In order to achieve this behavior, modern design practice aims to force substantial yielding in tension prior to loss of compressive strength. As such, a sound understanding of the compressive behavior of wall end regions is paramount to understanding RC wall ductility.A database of both rectangular RC prism (axial) and RC wall tests was assembled, and the results were compared with a popular analytical constitutive model for confined concrete. The existing model assumes that compressive behavior is based on fracture of the transverse reinforcement, however the data indicates that bar buckling and/or crushing of the core can occur well before transverse bar fracture. Therefore, the existing model significantly overestimates both compressive strength and strain capacity in many conditions consistent with modern design.Formulations for compressive strength and strain capacity, as well as a monotonic constitutive model, are proposed to capture the compressive behavior observed in the data. Both the number of longitudinal bars and the use of crossties in place of closed hoops were found to have a significant impact on the compressive behavior of confined concrete, and are therefore integral to the proposed model. The model was developed based on tests performed by the authors, and then validated using the aforementioned database.

Finite element analysis of rectangular reinforced concrete columns wrapped with FRP composites

Fibre reinforced polymer (FRP) wrapping of reinforced concrete (RC) columns has increasingly become the most suitable method used to strengthen and rehabilitate RC columns. It is clear that limited studies have investigated the behaviour of eccentrically loaded RC columns wrapped with FRP composites. In the present study, a three-dimensional finite element (FE) model was developed to simulate the behaviour of rectangular RC columns wrapped with glass fibre-reinforced polymer (GFRP) sheets under concentric and eccentric loading. The FE model was developed in the finite element analysis software ANSYS. The variables within the FE model are the number of GFRP layers and the magnitude of load eccentricity. The FE analysis results showed that GFRP wrapping significantly improved the performance of the strengthened columns by delaying concrete rupture. The presence of load eccentricity reduced the load carrying capacity and performance of the strengthened RC columns. The FE model correlated well with the stress distribution trends observed in the literature.

A new strengthening technique for increasing the load carrying capacity of rectangular reinforced concrete columns subjected to axial compressive loading

In this work, a new technique for the efficient confinement of reinforced concrete (RC) columns of rectangular cross-section is described and its effectiveness is assessed experimentally. This technique is based on the concept of applying strips of carbon fiber reinforced polymer (CFRP) wet layup sheets with a certain prestress level using a mechanical device. The influence of the cross-section aspect ratio of columns on the axial stress-strain response, strain field in the CFRP and strength increase provided by the different adopted strengthening configurations was investigated. All specimens had a height of 1100 mm, and three cross-sections were considered: 120 × 120 mm2, 240 × 120 mm2 and 480 × 120 mm2, representing cross-section aspect ratios (large/small edge) equal to 1, 2 and 4, respectively. Four types of columns were tested: conventional RC columns (reference columns), fully-wrapped columns, partially-wrapped columns, and columns strengthened according to the new technique. All columns were subjected to axial compression loading until failure. The experimental results show that the cross-section aspect ratio has a significant effect on the confinement effectiveness that CFRP strengthened systems can provide to RC columns of rectangular cross-section. The maximum axial strength and axial strain at the peak load of all columns significantly decrease when the cross-section aspect ratio increases. Based on the obtained experimental results, it is shown that the proposed technique is more efficient in terms of increasing the load carrying capacity of rectangular RC columns than CFRP-based conventional strengthening techniques.

Behavior of large-scale FRP-confined rectangular RC columns under axial compression

Fiber-reinforced polymer (FRP) jacketing has become an attractive technique for strengthening/retrofitting reinforced concrete (RC) columns. Extensive research has been conducted on FRP-confined rectangular columns under axial compression, leading to a significant number of stress-strain models for FRP-confined concrete in these columns. However, most of these models have been developed based on test results of small-scale columns, so their applicability to large FRP-confined rectangular RC columns has yet to be properly validated. To this end, the present paper first presents the test results of an experimental study consisting of nine large-scale rectangular RC columns, including eight FRP-confined RC columns and one RC column without FRP jacketing as the control specimen, tested under axial compression. The experimental program examined the sectional corner radius and the FRP jacket thickness as the key test variables. Five representative design-oriented stress-strain models for FRP-confined concrete in rectangular columns, identified from critical reviews of the existing literature, are then assessed using the test results to examine their validity for these large-scale columns.

AN EFFECTIVE TECHNIQUE UTILIZING FRCM SYSTEM IN STRENGTHENING RC BEAMS IN SHEAR

This paper presents the efficacy of a new technique of fabric reinforced cementitious matrix (FRCM) for the strengthening of shear deficient reinforced concrete (RC) beams. This technique involves embedding the FRCM composites in the concrete cover and is referred to as “near surface embedded” FRCM (NSE-FRCM) technique. Five (5) medium scale rectangular RC beams of width × depth × length of 150 × 330 × 2100 mm were constructed and tested under displacement controlled with three-point loading. One beam was kept unstrengthen as a control specimen whereas the other four beams were strengthened with different types and configurations of the NSE-FRCM system. The test parameters were: (a) geometric configuration (intermittent strips of NSE-FRCM versus full NSE-FRCM plate), and (b) fabric types (carbon versus glass fabrics). The test results indicated that the NSE-FRCM technique can successfully be used to significantly enhance the shear capacity of the strengthened beams. The strengthened specimens exhibited an average enhancement in the shear capacity of 62% over the unstrengthen beam. The full NSE-FRCM plate showed higher enhancement in the shear strength compared to that for the intermittent NSE-FRCM configuration. Moreover, the specimens strengthened with carbon FRCM showed higher shear capacity compared to that with glass FRCM counterparts.

Developing a new procedure for evaluating the ductility capacity of rectangular RC piers subjected to biaxial flexural loadings

The ductility capacity of bridge piers is estimated uniaxially by moment-curvature analysis, regardless of the amount of flexural demand on the perpendicular axis of the section. In this study, by applying a numerical fiber-based model, the ductility capacity of reinforced concrete (RC) piers subjected to biaxial monotonic loadings is investigated. 288 RC cantilever columns with 6 different square/rectangular solid/hollow sections are studied. Variations in shape, aspect ratio, dimensions, axial compression force, concrete characteristic strength and longitudinal reinforcement ratio are considered in the defined samples. Monotonic biaxial displacement demand is applied at the top of the cantilever columns. Different monotonic displacement paths are defined by varying the angle between the loading alignment and the major axes of the section. In order to sum up and interpret the responses obtained from different samples, a framework is developed to derive normalized ductility capacity ratios (RΔ). Based on the findings in this study, the maximum reduction of the mean ductility capacity ratio (RΔ‾) is about 15% and 8% for solid and hollow rectangular RC sections, respectively. According to the obtained results, the maximum reduction of ductility capacity of square and rectangular sections occurs when ϕ angle is between 40°–50° and 50°–70°, respectively. Based on the obtained mean ductility capacity ratios (RΔ‾), a bi-linear equation is proposed for evaluating the ductility capacity of rectangular RC sections under biaxial flexural demands.

Effect of Lithium Based Admixtures on Alkali Silica Reaction in Concrete and Flexural Behavior of Reinforced Concrete Rectangular Beams

In recycling of lithium-ion batteries, lithium is a challenging element, which possesses great threat to the environment. The disposal of electronic waste is a major problem, which our country is currently facing, whereas, the lithium is one of the compound in electronic waste which challenges the recycling of lithium-ion batteries. If lithium is properly extracted, it can be used for structural engineering purpose. Lithium, if used in concrete, it controls the cracks due to alkali silica reaction (ASR), resulting in increased strength and durability of the concrete. Lithium based admixtures are effective in suppression of alkali silica reaction and delayed ettringite formation, if the dosage added in concrete is appropriate. The main objective of this paper is to assess the effectiveness of LiNO3 at various dosage levels in concrete. In this paper, the effect of LiNO3 on the mechanical properties of concrete, ASR gel formation and the flexural behavior of reinforced concrete rectangular beams are studied. The mechanical properties of concrete was investigated for six mix proportions comprising of control mix, lithium nitrate at 0.6M, 0.65M, 0.7M, 0.75M, 0.8M. LiNO3 not only prevents the ASR gel formation, but also it has great influence on mechanical properties of concrete. The results indicate that the concrete with 0.75M LiNO3 has higher strength values. SEM analysis has been performed for 0.75M and control mix to study the micro characteristics of concrete. The ASR gel formed in 0.75M has been suppressed as compared to that of concrete without lithium nitrate as an admixture. The durability studies are performed in concrete for water permeability, water absorption and rapid chloride penetration characteristics. The analytical study of flexural behavior of reinforced concrete rectangular beam is done using the software ANSYS 16.0.

Non-linear flexural behaviour of RC columns including bar buckling and fatigue degradation

An advanced fibre-based modelling technique is developed to characterise the non-linear flexural behaviour of rectangular reinforced-concrete (RC) columns by accounting for the influence of inelastic buckling and low-cycle fatigue degradation of vertical reinforcement. The proposed uniaxial material model of reinforcing steel is calibrated using 22 rectangular RC column tests. The influence of inelastic buckling of vertical reinforcement on the non-linear cyclic response of rectangular RC columns is investigated. The calibrated model is capable of accurately predicting the non-linear response of rectangular RC columns up to complete collapse by taking into account the additional failure modes of the RC columns.

A study of the effect of fiber orientation on the torsional behavior of RC beams strengthened with PBO-FRCM composite

Repair and rehabilitation of reinforced concrete (RC) structures with different types of external reinforcement has been investigated widely. Fiber reinforced cementitious matrix (FRCM) is a new type of composite system that contains continuous fibers embedded in inorganic matrix. This system has been proven to be effective for strengthening RC members under flexure, shear, and axial loadings. However, studies on the use of FRCM composite for torsional strengthening are very limited. This study investigated experimentally the torsional behavior of solid rectangular RC beams strengthened with externally bonded PBO-FRCM composite in different wrapping configurations. The study focused on the effect of fiber orientation as well as other parameters that influence the torsional strength, torsional moment-twist per unit length response, and mode of failure including fiber continuity and number of composite layers. The strains in the internal and external reinforcement and the longitudinal elongation of the strengthened beams were examined, and a comparison with other types of fiber reinforced composite was also discussed. The 90° fiber orientation (perpendicular to the beam longitudinal axis) was more effective in increasing the torsional strength than the 45° fiber orientation since premature debonding of the fibers occurred at the ends of the 45° strips, which contrasted the potential benefits from optimizing the fiber orientation and led to the underutilization of the composite. The 90° fiber orientation was also more effective than the 0° fiber orientation.

PHAETHON: Software for Analysis of Shear-Critical Reinforced Concrete Columns

Earthquake collapse of substandard reinforced concrete (RC) buildings, designed and constructed before the development of modern seismic design Codes, has triggered intense efforts by the scientific community for accurate assessment of this building stock. Most of the proposed procedures for the prediction of building strength and deformation indices were validated by assembling databases of RC column specimens tested under axial load and reversed cyclic lateral drift histories. Usually a column structural behavior is assessed by considering all involving mechanisms of behavior, namely flexure with or without the presence of axial load, shear and anchorage. In the present paper a force-based fiber beam/column element was developed accounting for shear and tension stiffening effects in order to provide an analytical test-bed for simulation of experimental cases such as the lightly reinforced columns forced to collapse. Their peculiar characteristics are the outcome of the shear – flexure interaction mechanism modeled here based on the Modified Compression Field Theory (MCFT) and the significant contribution of the tensile reinforcement pullout from its anchorage to the total column’s lateral drift. These features are embedded in this first-proposed stand-alone Windows program named “Phaethon” -with user’s interface written in C++ programming language code- aiming to facilitate engineers in executing analyses both for rectangular and circular substandard RC columns.

Computational Lagrangian Multiplier Method using optimization and sensitivity analysis of rectangular reinforced concrete beams

This study conducts an optimization and sensitivity analysis on rectangular reinforced concrete (RC) beam using Lagrangian Multiplier Method (LMM) as programming optimization computer soft ware. The analysis is conducted to obtain the minimum design cost for both singly and doubly RC beams according to the specifications of three regulations of American concrete institute (ACI), British regulation (BS), and Iranian concrete regulation (ICS). Moreover, a sensitivity analysis on cost is performed with respect to the effective parameters such as length, width, and depth of beam, and area of reinforcement. Accordingly, various curves are developed to be feasibly utilized in design of RC beams. Numerical examples are also represented to better illustrate the design steps. The results indicate that instead of complex optimization relationships, the LMM can be used to minimize the cost of singly and doubly reinforced beams with different boundary conditions. The results of the sensitivity analysis on LMM indicate that each regulation can provide the most optimal values at specific situations. Therefore, using the graphs proposed for different design conditions can effectively help the designer (without necessity of primary optimization knowledge) choose the best regulation and values of design parameters.

An algorithm to simulate the nonlinear behavior of RC 1D structural members under monotonic or cyclic combined loading

Interaction of lateral loading, combined with axial force needs to be determined with care in reinforced concrete (RC) one-dimensional structural members (1D SMs) such as beam-columns (BCs) and columns. RC 1D SMs under heavy axial loading are known to fail by brittle mode and small lateral displacements. In this paper, a macro element-based algorithm is proposed to analyze the RC 1D SMs under monotonic or cyclic combined loading. The 1D SMs are discretized into macro-elements (MEs) located between the critical sections and the inflection points. The critical sections are discretized into fixed rectangular finite elements (FRFE). The nonlinear behavior of confined and unconfined concretes and steel elements are considered in the proposed algorithm. The proposed algorithm has been validated by the results of experimental tests carried out on full-scale RC structural members. The evolution of ultimate strain at extreme compression fiber of a rectangular RC section for different orientations of lateral loading shows that the ultimate strain decreases with increasing the axial force. In the examined cases, this ultimate strain ranges from 0.0024 to 0.0038. Therefore, the 0.003 value given by ACI-318 code for ultimate strain, is not conservative and valid for the combined load cases with significant values of axial force (i.e. for the axial forces heavier than 70% of the ultimate axial force).

Modeling the axial compressive stress-strain behavior of CFRP-confined rectangular RC columns under monotonic and cyclic loading

This paper presents a stress-strain model for rectangular reinforced concrete (RC) columns confined with carbon fiber-reinforced polymer (CFRP) composites and subjected to cyclic axial compression. The model is mainly based on experimental observations and results from a series of tests reported in the companion study, in which 24 large-size CFRP-confined rectangular unreinforced and reinforced concrete columns under monotonic and cyclic axial compression were tested. Relative experimental results compiled from the available literature are also used. The test database covers unconfined concrete compressive strengths ranging from 17.6 MPa to 51.5 MPa and cross-sectional side lengths ranging from 90 mm to 400 mm. The proposed stress-strain model is composed of three main components: (1) a monotonic stress-strain model for the envelope curve of cyclic response; (2) a parabolic expression for the unloading path; and (3) a straight line for the reloading path. The model also considers the effects of key parameters including cross-sectional size, aspect ratio, effective rupture strain of the CFRP, and internal longitudinal and hoop steel reinforcement. The performance of the model is finally verified through comparison of predictions made by this model with selected test results, as well as the predictions of more than 20 models of other researchers.

Shear strengthening of concrete members with TRM jackets: Effect of shear span-to-depth ratio, material and amount of external reinforcement

Abstract An experimental work on reinforced concrete (RC) rectangular beams strengthened in shear with textile reinforced mortar (TRM) jackets is presented in this paper, with focus on the following investigated parameters: (a) the amount of external TRM reinforcement ratio, ρf, by means of using different number of textile layers and different types of textile fibre materials (carbon, glass, basalt); (b) the textile geometry, and (c) the shear span-to-depth ratio, a/d. In total, 22 tests were conducted on simply supported rectangular RC beams under (three-point bending) monotonic loading. The experimental results revealed that: (1) TRM is very effective when the failure is attributed to debonding of the TRM jacket from the concrete substrate; (2) the trend of effective strains for carbon, glass and basalt TRM jackets is descending for increasing values of the TRM reinforcement ratio, ρf, when failure is associated to debonding of the jacket; (3) the effect of textile geometry is significant only for low values of ρf, resulting in variances in the capacity enhancement and the failure modes, and (4) the shear span-to-depth ratio has practically no effect to the failure mode nor to the TRM jacket contribution to the total shear resistance of the RC beams.

Axial strength of FRP-confined rectangular RC columns with different cross-sectional aspect ratios

This paper aims to contribute to a better understanding of the axial behaviour of rectangular reinforced concrete (RC) columns with large cross-sectional aspect ratios (section depth to width ratios) confined by carbon-fibre-reinforced polymer (CFRP). Three series of rectangular RC columns with cross-sectional aspect ratios of 2·25, 2·0 and 1·8 were tested under monotonic uniaxial compressive loads until failure. The effects of aspect ratios and CFRP anchors on the confinement effectiveness of CFRP wraps for rectangular RC columns were evaluated. The test results indicate that CFRP wraps enhanced the compressive strength of CFRP-confined rectangular RC columns even with a large aspect ratio of 2·25; however, the strength enhancement levels decreased with an increase in aspect ratio. CFRP anchors were effective in restraining concrete from bulging out and thus improved the confinement effectiveness of the CFRP wraps. Based on the test results regarding medium- and large-scale CFRP-confined rectangular RC columns available in the literature and this study, some existing confinement models for rectangular RC columns were assessed and a new analytical model was developed through regression analysis of the database. The proposed analytical model was found to be in good agreement with the test results in the database.

Seismic behavior of preloaded rectangular RC columns strengthened with precambered steel plates under high axial load ratios

In the past 20years, numerous tests have been conducted to demonstrate the effectiveness of additional fiber reinforced polymers (FRPs) or steel jacketing in the seismic retrofit of circular reinforced concrete (RC) columns. However, very few studies have explored the seismic retrofit of rectangular RC columns, in particular under high axial load ratios (ALRs). To address this issue, a simple and innovative approach is used in this study, in which rectangular RC columns subjected to high ALRs are retrofitted with post-compressed steel plates under severe lateral reversed cyclic loads. An experimental study is conducted to validate the effectiveness of this approach for improving the seismic performance of RC columns under high ALRs. Specimens with identical geometric and reinforcement layouts are fabricated and tested. One of the columns, which is not strengthened, is used as the control whereas the others are strengthened by using bolted steel plates. The effects of plate thickness, initial precamber and ALR on the failure mode, as well as ductility, strength degradation and energy dissipation, are investigated. The results demonstrate that this approach is effective in increasing the lateral displacement ductility capacity while maintaining the high axial load-carrying capacity of the columns.

Effects of axial load on seismic performance of reinforced concrete walls with short shear span

Reinforced concrete (RC) shear walls in tall buildings are found to have a short shear span, particularly in high-degree coupled walls supported on transfer structures in low-to-moderate seismic regions. These non-seismically detailed walls in existing buildings are exposed to a high risk of failing in shear or compression before plastic hinges are formed at their base. Whilst previous research have focused on squat walls used in low-rise structures tested with zero or low axial loads, the structural response of these walls with a short shear span and limited ductility under high axial load is rarely discussed. Therefore, an experimental study that investigates the influence of the axial load ratio (ALR) on RC walls with a short shear span is presented in this paper. The specimens are designed with a low shear span-to-length ratio (SLR) and detailed with a characteristic 2% vertical and longitudinal reinforcement to represent a wall sub-structure above the transfer structure of tall buildings. Four walls are tested under reverse cyclic loading and subjected to target ALRs that range from 0.1 to 0.4 to investigate the seismic performance until gravity collapse. The ALR is found to have significant effects on crack patterns, failure modes and deformability. Two modified empirical prediction models are proposed to estimate the shear strength capacity and ultimate drift ratio of rectangular RC shear walls with a short shear span under the effects of the ALR. A unique model of the drift limit of collapse under axial load asa function of the reinforcement ratio is put forward for performance based design and assessment.

Exploring the impact of chloride-induced corrosion on seismic damage limit states and residual capacity of reinforced concrete structures

A non-linear finite element (FE) framework for time-dependent capacity assessment of corroded rectangular reinforced concrete (RC) columns is developed. The proposed non-linear FE model includes the impact of corrosion on inelastic buckling and low-cycle fatigue degradation of longitudinal reinforcement. The proposed non-linear FE model is validated against a set of experimental data and then extended to evaluate the impact of corrosion on damage limit states to be used in seismic performance and evaluation of corroded structures. This is done through a parametric study on hypothetical RC columns, varied in axial force ratios, mass loss ratios, cover crack widths and confinement levels. Moreover, the application of the proposed model in seismic collapse capacity assessment of corroded structures is shown through non-linear dynamic analyses of prototype columns. Results show that, depending on the axial force ratio, corrosion changes the failure mechanism of the columns. The results of this study suggest that in seismic fragility analysis of corroded structures, the damage limit states should be considered as time-variant parameters.

Numerical Study on the Ultimate Deformation of RC Structural Walls with Confined Boundary Regions

For accurate assessment of performance levels in reinforced concrete (RC) members, it is important to well define deformation limits at particular damage states. For RC walled building, investigation of the deformation limits of RC structural walls is required to define limit states and corresponding limiting values. Numerical investigations were carried out on barbell shape and rectangular RC walls with confined boundaries to evaluate response curves and ultimate deformations. A nonlinear 2D and 3D finite elements (FE) models were built in order to simulate the load-deformation relations under monotonic loading as well as cracking and damage patterns of previously tested walls. The FE models were able to simulate the backbone curves with good accuracy as well as the ability of boundary columns in reducing damage level. The 3D FE model simulated very well the ultimate deformation compared to 2D models. A sectional fibre model combined with plastic hinge length and shear deformation component is proposed in order to simulate the backbone curves and the ultimate deformation with less computational cost compared to 3D FE analysis. The model was able to provide relatively accurate backbone curves with very good estimation of ultimate drift.