Solid Reinforced Concrete Research Articles

Flexural behaviour of hollow reinforced concrete T-beams strengthened or retrofitted with carbon-fibre-reinforced laminates: Experimental and numerical investigation

This study investigates the effect of longitudinal rectangular openings on the flexural behaviour of reinforced concrete (RC) T-beams strengthened or retrofitted with external carbon-fibre-reinforced polymer (CFRP) laminates. Eight RC T-beams, with one solid and one hollow beam in each of four groups, were analysed using a three-point bending test. The CFRP laminate used in each beam was similar, and the beams had an identical longitudinal reinforcement ratio. The ultimate load capacity of the strengthened hollow RC T-beams showed a 9% decrease compared to that of the strengthened solid RC T-beams, while retrofitted hollow RC T-beams with 100% preloading exhibited a 24% decrease in ultimate load capacity compared to that of retrofitted solid RC T-beams. In addition, the CFRP laminate had a positive effect on reducing strain in hollow RC T-beams as compared to that of solid RC T-beams. The flexural behaviour of the CFRP-strengthened hollow RC T-beams with a 5% cross-sectional hollow core area was similar to that of strengthened solid RC T-beams. However, after preloading, the retrofitted hollow RC T-beams exhibited a decrease in flexural efficiency compared to the retrofitted solid beams. A validated numerical model was developed using ABAQUS to accurately simulate the behaviour of the hollow RC T-beams. Subsequently, a parametric study was conducted using the validated model to investigate the effects of varying the position and size of the hollow section on the response of the RC T-beams.

Investigation of Circular Hollow Concrete Columns Reinforced with GFRP Bars and Spirals

Glass fiber-reinforced polymer (GFRP) reinforcements are useful alternatives to traditional steel bars in concrete structures, particularly in vertical structural elements such as columns, as they are less prone to corrosion, and impart increasing strength and endurance of buildings. There is limited research on the finite element analysis (FEA) of the structural behavior of hollow glass fiber-reinforced polymer reinforced concrete (GFRPRC) columns. The hollow portion can be used for the service duct and for reducing the self-weight of the members. Numerical analysis of the compressive response of circular hollow concrete columns reinforced with GFRP bars and spirals is performed in this study. This article aims to investigate the axial behavior of hollow GFRP concrete columns and compare it with that of solid steel reinforced concrete (RC) columns as well as hollow steel RC columns. The Abaqus software is used to construct finite element models. After calibration of modeling using an experimental test result as a control model, a parametric study is conducted. The columns with the same geometry, loading, and boundary conditions are analyzed in the parametric study. It is resulted that the hollow GFRP concrete columns provide a greater confinement effect than the solid steel RC columns. The average variation in the ultimate axial load-carrying capacities of the experimental results, from that of the FEA values, is noted to be only 3.87%, while the average difference in the corresponding deformations is 7.08%. Moreover, the hollow GFRP concrete columns possess greater axial load and deformation capacities compared with the solid steel RC columns.

Structural Behavior of Large-Scale Hollow Section RC Beams and Strength Enhancement Using Carbon Fiber Reinforced Polymer (CFRP) Composites.

An experimental program was conducted to ascertain the efficiency of Carbon Fiber Reinforced Polymer (CFRP) in enhancing the flexural response of hollow section reinforced concrete (RC) beams. Nine beams were tested under four-point bending in three groups. Beams were categorized to reflect the presence or configuration of the CFRP sheet. Each group consisted of three beams: one with a solid section, one with a square mm × mm opening and 1 with mm × mm opening. Beams in 1st group were tested in as-built conditions. Beams in the 2nd group were strengthened with a single CFRP sheet bonded to their bottom sides. Configuration of CFRP sheet was altered to U-shape applied to the tension side of 3rd group beams. The inclusion of openings, regardless of their size, did not result in degradation of ultimate load and corresponding deflections. However, cracking loads were found to decline as the opening size increased. Regardless of the opening size and CFRP configuration, ultimate loads of beams increased with the application of CFRP. However, this improvement was limited to the debonding and rupture of CFRP in group 2 and 3 beams, respectively. A comparison in the behavior of group 2 and 3 beams revealed that the application of the U-shape CFRP sheet yielded better flexural performance in comparison with the flat-CFRP sheet bonded to the bottom of beams. In the end, In order to further evaluate the economic and performance benefits of these beams, the cost-benefit analysis was also performed. The analysis showed that the feasibility of the hollow section RC beams is more than the solid section RC beams.

Experimental study of in-plane flexural behavior of screen-grid insulated concrete form rectangular and T-shaped walls

Screen Grid Insulated Concrete Form (SGICF) walls are gaining popularity in the construction of low and mid-rise buildings due to their unique constructability and insulation properties. In order to validate and ensure the safety of this novel construction system in the face of seismic loads, experimental testing aimed to provide information regarding its nonlinear behavior is of significant relevance. This paper aims to describe analytically and experimentally the in-plane flexural behavior of SGICF panels. An analytical model is presented to estimate the design parameters considering the material properties and the element geometry. Ten test specimens were constructed and tested under monotonic and cyclic in-plane lateral loading to assess the model's effectiveness. Test units were divided into two groups, each constructed using different prefabricated forms made of expanded polystyrene. The first group comprised six rectangular-shaped section walls whereas group two comprised four T-shaped section walls. All specimens were designed and detailed to obtain a flexure-controlled response, considering the presented analytical model. Results showed a ductile and stable response for all test specimens, with yielding of the longitudinal reinforcement and compression crushing of the extreme concrete fibers. This study shows that SGICF walls can reach large inelastic deformations, as well as a stable response, similar to that of conventional reinforced concrete (RC) walls with similar geometric properties. Furthermore, it was verified that flexural resistance models for solid cross-section RC members are applicable to Insulated Concrete Form (ICF) walls, taking into account section discontinuities.

Torsional behavior of solid and hollow concrete beams reinforced with inclined spirals

Enhancing the torsional capacity of reinforced concrete (RC) beams while maintaining reasonably economic and efficient cross-sections poses a practical challenge in the design of such elements. Thus, developing and testing economically viable reinforcing techniques for enhancing the torsional capacity of RC beams is a significant aspect. Among these techniques is the continuous spiral reinforcement system that has shown promising results in enhancing the torsional capacity of rectangular RC elements.In this study, the role of continuous rectangular spiral stirrups, as a transverse reinforcement with a controllable pitch (P) and inclination angle (θ), in enhancing the torsional behavior of rectangular solid and hollow RC beams has been investigated experimentally. The applied torque was directed to lock the spirals for all specimens. The experimental program of this investigation included testing of ten specimens of normal strength RC beams; two of which were solid with closed and spiral rectangular stirrups. While the rest of specimens were hollow specimens included one beam reinforced by closed stirrups and seven beams reinforced by rectangular spiral stirrups with different reinforcing ratios. Different variables were considered to develop a wide range of the beams’ reinforcing scenarios. This includes stirrups type (conventional closed stirrups and rectangular spiral stirrups), inclined stirrups ratio, pitch, inclination angle and finally the longitudinal reinforcement ratio. In addition, a three-dimensional finite element model using ANSYS-15.0 was developed to examine the capability of the model to capture the observed torsional behavior of the experimentally tested beams.The obtained results showed that using inclined spiral rectangular stirrups in reinforcing RC solid and hollow beams greatly enhanced the torsional capacity by about 16% and 18%, respectively compared to using the conventional closed stirrups. Moreover, the results showed that using that technique leads to higher twist- angles, more ductile failure and increased strain energy by about 27% and 16% for solid and hollow beams, respectively. Lastly, the comparison between the results obtained from numerical model and experimental results showed good agreement and thus the numerical model can be used to extend the experimental work for investigating different reinforcing schemes.

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.

An effective simplified model of composite compression struts for partially-restrained steel frame with reinforced concrete infill walls

To resolve the issue regarding inaccurate prediction of the hysteretic behavior by micro-based numerical analysis for partially-restrained (PR) steel frames with solid reinforced concrete (RC) infill walls, an innovative simplified model of composite compression struts is proposed on the basis of experimental observation on the cracking distribution, load transferring mechanism, and failure modes of RC infill walls filled in PR steel frame. The proposed composite compression struts model for the solid RC infill walls is composed of α inclined struts and main diagonal struts. The α inclined struts are used to reflect the part of the lateral force resisted by shear connectors along the frame-wall interface, while the main diagonal struts are introduced to take into account the rest of the lateral force transferred along the diagonal direction due to the complicated interaction between the steel frame and RC infill walls. This study derives appropriate formulas for the effective widths of the α inclined strut and main diagonal strut, respectively. An example of PR steel frame with RC infill walls simulating simulated by the composite inclined compression struts model is illustrated. The maximum lateral strength and the hysteresis curve shape obtained from the proposed composite strut model are in good agreement with those from the test results, and the backbone curve of a PR steel frame with RC infill walls can be predicted precisely when the inter-story drift is within 1%. This simplified model can also predict the structural stiffness and the equivalent viscous damping ratio well when the inter-story drift ratio exceeds 0.5%.

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.

Closed-form approximation of the axial force-bending moment interaction diagram for hollow circular reinforced concrete cross-sections

This paper proposes a simple closed-form approximation of the axial force-bending moment interaction diagram for solid and hollow circular reinforced concrete (RC) cross-sections with an arbitrary number of layers of longitudinal reinforcement. The implemented nonlinear material models are those of the European building code. Specifically, the parabola-rectangular strain-stress relation and the stress-block model are considered for the concrete in compression. On the other hand, the elastic-perfectly plastic behavior is assumed for the reinforcing steel bars. The proposed semi-exact closed-form approximation involves one parameter only, which is calibrated by means of a numerical optimization procedure taking into account a large database of cross-sections. Moreover, an efficient procedure is developed for the design of the outer layer of longitudinal reinforcement in solid or hollow circular RC cross-sections under combined axial force and bending moment (the mechanical ratio of any other layers of longitudinal reinforcement is considered as known function of the mechanical ratio of the outer layer of longitudinal reinforcement). Finally, several numerical examples are included to demonstrate the accuracy of the approximated axial force-bending moment interaction diagram and the effectiveness of the derived design procedure.

Performance assessment of novel hollow and solid RC columns using simulation of shaking table test

The purpose of this study was to investigate the seismic performance of novel hollow and solid reinforced-concrete (RC) columns with triangular reinforcement details when subjected to impulsive near-fault ground motions. The results of numerical simulations of shaking table tests on six hollow columns and six solid columns are presented. The developed reinforcement details are shown to be economically feasible and rational, and they facilitate shorter construction periods. The use of a sophisticated non-linear finite-element analysis program enhanced the accuracy and objectivity of the assessment process. The computer program RCAHEST (Reinforced Concrete Analysis in Higher Evaluation System Technology) was used to analyse the RC structures. The equations of motion were solved by numerical integration using the Hilber–Hughes–Taylor algorithm. The numerical method used gave a realistic prediction of seismic performance throughout the input ground motions for several column specimens investigated. As a result, the developed triangular reinforcement details were considered to be superior to existing reinforcement details in terms of the required seismic performance. Such an analysis to study the seismic response of novel hollow and solid RC columns should lead to realistic and safe design.

Torsional behavior of RC beams strengthened with PBO-FRCM composite – An experimental study

The use of fiber reinforced cementitious matrix (FRCM) composites has been studied for flexural and shear strengthening of reinforced concrete (RC) members, but currently there are no studies on its use for torsional strengthening. This paper presents the results of an experimental study in which solid rectangular RC beams were externally strengthened with PBO-FRCM composite material in different wrapping configurations to investigate the torsional behavior in terms of strength, rotational ductility, and failure mode. Increases in the cracking torque, torsional strength, and corresponding values of twist were achieved by beams strengthened with a 4-sided wrapping configuration relative to the control (unstrengthened) beam. On the other hand, the 3-sided wrapping configuration was found to be largely ineffective in improving the torsional performance due to excessive fiber slippage. The contribution of the strengthening system to the torsional strength was reasonably predicted (±20%) by the strain measured in the composite fibers. Provisions used to estimate the torsional strength of RC beams with fully-wrapped, externally-bonded fiber reinforced polymer (FRP) composites were found to be applicable to beams strengthened with PBO-FRCM composite.

Effect of openings on in-plane structural behavior of reinforced concrete floor slabs

To determine the inelastic seismic response of reinforced concrete (RC) buildings, typically a tri-linear in-plane load-displacement idealization is used for modeling the behavior of RC floor diaphragms, to account for cracking and yielding prior to failure. In the 1980s, solid (without openings) beam-supported RC two-way slab panels were experimentally studied at Lehigh University under in-plane monotonic and cyclic loads, with and without service gravity loads, to determine their in-plane load-displacement and hysteretic characteristics. Subsequently, these results could be implemented in nonlinear damage analysis computational tools developed for analyzing RC buildings with flexible floor diaphragms, ignoring the effect of openings. Due to the lack of experimental data, in the present study a finite element (FE) approach is used to investigate the inelastic behavior of RC floor diaphragms with openings. A general purpose FE software was initially used to create a nonlinear 3D model of the solid panels tested at Lehigh University, and the obtained results from the actual experiment were used to verify the validity of the FE model. This model uses eight-node concrete brick elements (SOLID65) combined with embedded steel reinforcement elements (REIN264). After the accuracy of the solid (with no opening) FE model was verified, openings were placed in the model, and then a sensitivity study has been conducted where the effects of varying opening sizes (0, 6.25%, 14%, and 25% of the floor panel area) and out-of-plane loading (zero and full service load) on the in-plane load-displacement characteristics of the floor panels are investigated. Results indicate that the drop in ultimate in-plane load capacity of the floor diaphragm due to the presence of out-of-plane service loading becomes less significant as the opening size increases (4% for 25% opening vs. 15% for the solid slab). Also, the first significant variation from the initial linear portion of the in-plane load-displacement curve moves up from 30% to about 50% of the ultimate load capacity for the slab with the larger size opening. The failure mechanisms changed due to the presence of the openings, where yielding of the bars around the opening corners appeared to significantly affect the behavior of the slabs. The positive contribution of inclined reinforcing bars, placed at opening corners, in strengthening the in-plane capacity of the slab panels with opening, is effectively demonstrated by use of the FE model.

Transverse reinforcement for confinement at plastic hinge of circular composite hollow RC columns

Confined transverse reinforcement was arranged in a plastic hinge region to resist the lateral load that increased the lateral confinement effect in the bridge substructure. Columns increased the seismic performance through securing stiffness and ductility. The calculation method of transverse reinforcements at plastic hinges is reported in the AASHTO-LRFD specification. This specification was only proposed for solid reinforced concrete (RC) columns. Therefore, if this specification is applied for another column as composite column besides the solid RC column, the column cannot be properly evaluated. The application of this specification is particularly limited for composite hollow RC columns. The composite hollow RC column consists of transverse, longitudinal reinforcements, cover concrete, core concrete, and an inner tube inserted in the hollow face. It increases the ductility, strength, and stiffness in composite hollow RC columns. This paper proposes a modified equation for economics and rational design through investigation of displacement ductility when applying the existing specifications at the composite hollow RC column. Moreover, a parametric study was performed to evaluate the detailed behavior. Using these results, a calculation method of economic transverse reinforcements is proposed.

Unified rational formula for pre-cracking torsional stiffness of solid and hollow reinforced concrete members

It is generally assumed that the elastic torsional stiffness of structural reinforced concrete (RC) members that are subjected to a torsional moment less than the cracking torque can be accurately estimated, despite a lack of adequate experimental or theoretical examination. The softened membrane model for torsion (SMMT), which has been validated experimentally, provides accurate estimates of pre-cracking torsional stiffness for solid and hollow RC members. However, the SMMT requires an iterative solution algorithm, and is thus inconvenient for design purposes. This paper presents a simplification of the SMMT and proposes a unified rational formula for the direct calculation of the initial torsional stiffness of solid and hollow RC members. The proposed formula predicts the initial torsional stiffness of nine hollow RC beam specimens almost perfectly. When used to calculate the stiffness of 147 solid and hollow RC beam specimens, the proposed formula is found to be an almost perfect simplification of the SMMT in terms of initial torsional stiffness. However, the elastic stiffness values of the 147 specimens deviate from the formula-calculated values by −40% to +50%, suggesting the values may not be as accurate as commonly presumed. It is also shown that the proposed formula can combine with an existing Tcr-θcr formula to provide a precise bi-linear simplification of the nonlinear pre-cracking torque-twist curves for solid and hollow RC members; this can then be conveniently used in the nonlinear finite element analyses needed in performance-based engineering.

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It is generally assumed that the elastic torsional stiffness of structural reinforced concrete (RC) members that are subjected to a torsional moment less than the cracking torque can be accurately estimated, despite a lack of adequate experimental or theoretical examination. The softened membrane model for torsion (SMMT), which has been validated experimentally, provides accurate estimates of pre-cracking torsional stiffness for solid and hollow RC members. However, the SMMT requires an iterative solution algorithm, and is thus inconvenient for design purposes. This paper presents a simplification of the SMMT and proposes a unified rational formula for the direct calculation of the initial torsional stiffness of solid and hollow RC members. The proposed formula predicts the initial torsional stiffness of nine hollow RC beam specimens almost perfectly. When used to calculate the stiffness of 147 solid and hollow RC beam specimens, the proposed formula is found to be an almost perfect simplification of the SMMT in terms of initial torsional stiffness. However, the elastic stiffness values of the 147 specimens deviate from the formula-calculated values by −40% to +50%, suggesting the values may not be as accurate as commonly presumed. It is also shown that the proposed formula can combine with an existing Tcr-θcr formula to provide a precise bi-linear simplification of the nonlinear pre-cracking torque-twist curves for solid and hollow RC members; this can then be conveniently used in the nonlinear finite element analyses needed in performance-based engineering.

Shear Capacity of RC Beams with Web Openings Strengthened with Multi Layers of CFRP

In some cases a cut-out web opening in a reinforced concrete (RC) beam is required to facilitate the passage of electrical and mechanical pipes and ducting from one side to another. The presence of cut-out web openings will lead to decreasing in the load carrying capacity of these RC beams. Therefore, strengthening the vicinity of these openings by using Carbon Fiber Reinforced Polymer (CFRP) will represent the best practical solution. In this study, thirteen RC beams with different sizes of cut-out web openings and with different layers of CFRP strengthening and also one solid RC beam (control) were prepared and tested to failure. The outcome of this experimental and analytical study showed that the modified Eurocode 2 model can reasonably be used in computing the shear forces of RC beams having the vicinity of their cut-out web openings strengthened with single/multi layers of CFRP. In addition, for safe design purposes, a reduction factor has been suggested as well.

Experimental study and numerical simulation of the damage mode of a square reinforced concrete slab under close-in explosion

Terrorist attacks using improvised explosive devices on reinforced concrete buildings generate a rapid release of energy in the form of shock waves. Therefore, analyzing the damage mode and damage mechanism of structures for different blast loadings is important. The current study investigates the behavior of one-way square reinforced concrete (RC) slabs subjected to a blast load through experiments and numerical simulations. The experiments are conducted using four 1000mm×1000mm×40mm slabs under close-in blast loading. The blast loads are generated by the detonations of 0.2–0.55kg trinitrotoluene explosive located at a 0.4m standoff above the slabs. Different damage levels and modes are observed. Numerical simulation studies of the concrete damage under various blast loadings are also conducted. A three-dimensional solid model, including explosive, air, and RC slab with separated concrete and reinforcing bars, is created to simulate the experiments. The sophisticated concrete and reinforcing bar material models, considering the strain rate effects and the appropriate coupling at the air–solid interface, are applied to simulate the dynamic response of RC slab. The erosion technique is adopted to simulate the damage process. Comparison of the numerical results with experimental data shows a favorable agreement. Based on the experimental and numerical results, the damage criteria are established for different levels of damage. With the increase of the explosive charge, the failure mode of RC slab is shown to gradually change from overall flexure to localized punching failure.

Compressive strength of circular hollow reinforced concrete confined by an internal steel tube

When concrete is confined, its strength is enhanced by confining stress. Therefore many researchers have studied the confinement and its effect in a reinforced concrete (RC) member. However, their research has focused on making confining pressure from the outside of the member. In a hollow RC member, it is impossible to confine its concrete triaxially because it has a hollow section. The concrete in a hollow RC member is usually in the state of biaxially confinement, and it shows lower strength than the concrete in a solid RC member, which is confined triaxially. In this study, the compressive strength of the concrete in a hollow RC column with an internal steel tube was investigated. The internal tube makes the concrete confined triaxially with internal confinement. To show the existence and the effect of the internal confinement, an experiment was performed. Fourteen specimens were tested. The test results showed that the internal confinement existed and made the concrete in the state of triaxial confinement.

Behaviour of a hollow RC column with an internal tube

A non-linear column model of an internally confined hollow (ICH) reinforced concrete (RC) column was suggested and column tests were performed. The suggested column model considered the confining effect and the material non-linearity of the concrete. To verify the suggested column model and to investigate the behaviour of an ICH RC column, quasi-static tests were performed for three different ICH RC columns and a solid RC column. The analysis results by the suggested model were agreeable with the test results, and they showed that it was reasonable to consider the confining effect of concrete. The test results showed that an ICH RC column has high strength, high energy-absorbing capacity, a normal damage index and acceptable ductility, which can be controlled by the thickness and the type of its internal tube.

Damage of Reinforced Concrete Structure due to Severe Soil Expansion

In Irbid City, Jordan, foundation designs made before 1983 were based on bearing capacity criteria with a limited knowledge of high shrink/swell soil problems. The use of wide and shallow foundation systems was generally the practice rather than the exception in this area. Lack of structural rigidity and insufficient dead load pressure of the foundation systems used in the Irbid area where soils of high shrink/swell are present often cause serious problems related to the performance of constructed facilities. This paper presents a case study typical of a severely cracked one-story reinforced concrete (RC) building constructed over the expansive clay of Irbid City. The building is founded on a mat foundation (solid RC structural slab) embedded at a shallow depth and bearing directly on expansive clay. It is unfortunate that the high shrink/swell potential of the foundation soil had not been recognized properly in the design stage. Based on field and laboratory investigations, remedial construction for the damaged building was proposed. The proposed remedial construction was performed, and more than 10 months have elapsed since the remedial work was completed, with the performance of the building in perfect condition.