Reinforced Concrete Flexural Members Research Articles

Experimental and analytical investigation of T-beams retrofitted through the external post-tensioning using steel and various FRP strands

This study aimed to scrutinize the potential benefits of using fiber-reinforced polymer (FRP) materials as unbonded strands in post-tensioning systems for reinforced concrete (RC) flexural members. The research meticulously evaluated the influence of these FRP materials on global and local response levels. To this end, four-point loading tests were performed on T-beam specimens, and the experimental results were compared with numerical predictions. The study also incorporated insights from previous research on external post-tensioning variables. The results of this study, with their practical implications, demonstrate a strengthening technique that significantly enhances the load-carrying capacity at ultimate and yielding points, as well as the crack initiation load of non-strengthened specimens. The strengthening technique also significantly influences deflection during yielding and maximum loads and plays a crucial role in restoring deflection after unloading. However, it is essential to note that strengthening with external post-tensioning does reduce the member’s ductility. The choice of materials also emerges as a significant factor that leads to noticeable differences in crack widths and distribution among the specimens. These practical findings hold considerable importance and offer valuable insights into the potential use of FRP materials in post-tensioning systems for RC flexural members.

Cyclic Behavior of Reinforced Concrete Flexural Members to Changing Design Parameters

Cyclic Behavior of Reinforced Concrete Flexural Members to Changing Design Parameters

Performance Evaluation of Corrosion Inhibitors in Electrochemical Resistance of Steel Reinforcement in Concrete

Reinforced concrete (RC) structures are often used in the construction sector due to their strength and longevity. Corrosion in steel reinforcement can have detrimental effects like surrounding concrete cracks and spalls as the steel rusts and expands. Furthermore, weakening the structure, corrosion might make the steel’s bond with the surrounding concrete less strong and results in expensive repairs, safety risks, or even collapse. Studying steel reinforcement corrosion in concrete is essential for several reasons. Firstly, it enhances our understanding of the mechanisms underlying corrosion and facilitates the development of effective prevention and mitigation strategies includes designing materials with improved corrosion resistance, applying protective coatings, or implementing cathodic protection systems to inhibit corrosion. The present investigation is aimed to assess the damage in RC flexural members at no corrosion level and 10% corrosion level using non-destructive testing (NDT) techniques to capture various aspects of the specimen condition. The research examined the efficacy of several corrosion inhibitors, including Epoxy, Grout, Geopolymer, Red oxide, and polythene cover. The findings revealed that Geopolymer demonstrated superior performance in mitigating corrosion. This effectiveness is attributed to its ability to form an alkaline protective layer around the reinforcing bars, which significantly impedes corrosion processes compared to the other inhibitors assessed.

Effectiveness of Insulation Layers for Fire Protection of FRP Reinforced Concrete Flexural Members

Effectiveness of Insulation Layers for Fire Protection of FRP Reinforced Concrete Flexural Members

Cyclic bond behavior in reinforced concrete flexural members exposed to elevated temperatures

After enduring fire incidents, many reinforced concrete (RC) structures require assessment of their ability to withstand future seismic loads. A critical factor contributing to seismic behavior of such structures is residual bond between steel and concrete under cyclic loading. This paper examines cyclic bond-slip behavior in RC flexural members that have experienced elevated temperatures. Twenty-six modified beam-end specimens with different diameters of longitudinal reinforcement, concrete strengths, and confinement levels were fabricated. The specimens were subjected to a variety of heating regimes and a natural cool down before loading of their longitudinal rebar in a monotonic or cyclic pattern. The decrease in bond strength after 2-hr exposure to temperatures of 400, 600, and 700 °C were 23, 58, and 65 percent for monotonic loading but 30, 60, and 65 percent for cyclic loading, respectively. Elevated temperatures also caused a decrease in slope and area of hysteretic bond-slip loops. Cyclic bond strength was 25–45% less than monotonic bond strength and better correlated with the residual compressive strength of concrete, unlike monotonic bond strength, which was better correlated with the square root of this property. A model was proposed for backbone bond-slip curves of RC flexural members that are exposed to elevated temperatures.

Unified Flexural Resistance Design Method and Evaluation Frame for the B-Regions of RC Flexural Members—Theory and Application

The load and resistance factor design (LRFD) method is normally used to design B-regions of reinforced concrete (RC) flexural members. The design includes many checks corresponding to different limit states. The LRFD method requires many loop calculation steps in the design, demonstrating its relative inefficiency. It cannot be applied to compare limit states directly and quantitatively. Different design limit states are separated and isolated. How to improve the analytical calculation efficiency of the LRFD method and to realize direct and quantitative comparisons between limit states are very important problems in structural engineering. This paper presents an innovative unified flexural resistance design (UFRD) method and a unified flexural resistance evaluation (UFRE) frame to solve these problems to some extent. The main contents include the unified flexural resistance (UFR) principles, formulas for the unified flexural resistance design (UFRD) method, the operation procedure to facilitate its usage, the UFRE framework to compare limit states, and three examples. The results show that the UFRD method can provide the same design outcomes as the LRFD one. However, UFRD calculations are simpler, requiring at most 20% of the calculation steps of the LRFD method. The UFRE frame can make different limit states compare with each other directly and quantitatively, which cannot be realized by the LRFD method. It helps expose some potential and insufficient flexural resistance hazards for some limit states, such as the only 10% relative strength reservation of one example. Thus, the UFRD method and the UFRE frame supplement and develop the LRFD method to some degree. The simplicity and practicality of the approach and the frame make them appropriate for many applications.

Structural performance assessment of RC flexural members through crack-based fibre beam-column model updating

For reinforced concrete (RC) structures in service, original finite element models (FEMs) based on design information usually deviate from actual structures due to structural damage such as cracks, displacement and material degradation. Although conventional FEMs can be updated to give results that match the response data used during the updating process, they can hardly predict the subsequent structural performance. In this study, an innovative fibre beam-column model updating method based on static deflection and crack width is proposed. First, the self-developed fibre beam-column finite element program COMPONA-MARC is modified to efficiently simulate the nonlinear cracking behaviour of RC flexural members. Second, sensitivity studies using the discrete central difference method based on the Lagrange interpolation function are conducted to select updating parameters. Third, with the combination of modified COMPONA-MARC and an automation program written in Python, plenty of sample data are obtained, and artificial neural networks (ANNs) are trained by Bayesian Regularization Backpropagation (BRB). Finally, the updated parameters are acquired by inputting the measured static deflections and crack widths. Typical RC beams and RC slabs from the literature are utilized to validate the proposed methodology and explore potential implementation scenarios. The results show that updated FEMs can satisfactorily predict mechanical behaviour and assess structural performance. The proposed method offers enlightening insights about combining structural health monitoring and inspection (SHMI) results with model updating, which can promote the applications of digital twins in performance assessments of RC structures.

Flexural Performance of Composite RC Beams Having an ECC Layer at the Tension Face

This paper presents an experimental study on the flexural behavior of composite Reinforced Concrete (RC) beams having a monolithic Engineered Cementitious Composites (ECC) layer at the tension face. Due to the brittle nature of normal concrete, clear cover on the tension side of beam cracks results in spalling and corrosion of reinforcement. The proposed technique overcomes the inherent brittle behavior of normal concrete with the incorporation of ECC on the tension face. This also helps in reducing bond-splitting, cover-spalling, and buckling of reinforcement in RC flexural members. For testing purposes, six full-scale beam specimens (225 mm x 300 mm x 2400 mm) with the same reinforcement were cast and tested. Out of six, two specimens were made of conventional concrete, whereas the remaining four (two each) had an ECC layer of 75mm and 100mm thick at the tension face respectively. Each specimen was installed with three strain gauges (one each at the midspan top & bottom surface of concrete and one midspan rebar on the tension face) and one LVDT at midspan. The samples were then subjected to simple monotonic loading under a third-point bending test as per ASTM C78. The load-displacement, stress-strain and moment-curvature curves were obtained for all the tested specimens. It was found that ECC-strengthened beam samples displayed an increased flexural performance at first crack, yield, and ultimate load-carrying capacity as compared to conventional RC specimens. Whereas a better crack arrest with even distribution of cracks and improvement in ductility was observed for the ECC-strengthened composite beams.

Damage assessment of reinforced concrete structures under elevated-amplitude cyclic loading using sentry values based on acoustic emission testing

ABSTRACT This article reports the combined application of sentry value and the Gaussian Mixture Modeling (GMM) algorithm to study crack classification in reinforced concrete (RC) beams using acoustic emission (AE) testing. The AE generated from RC beam specimens subjected to elevated-amplitude cyclic loading was used to study crack classification. The specimen approaches yielding, followed by a structural failure, when AE hits linked with shear cracks begin to exceed AE hits associated with tensile cracks. The sentry value for RC beam specimens decreased with subsequent higher amplitude loading phases, indicating the formation of macrocracks. The Kaiser effect phenomenon in RC beams was observed. The yielding of steel rebars and subsequent collapse of the RC beam specimens, followed by unstable crack propagation, were identified using the zero-slope line and a vertical spike in the sentry value versus the time graph. The current study appears to be significant for damage assessment and crack classification of in-situ RC flexural members.

Optimum High-Strength Reinforcing Bar Grade for Reinforced Concrete Flexural Members

Optimum High-Strength Reinforcing Bar Grade for Reinforced Concrete Flexural Members

Effects of shrinkage variation within beam depth and bending creep on flexural behavior of RC members

The effects of shrinkage variation within the beam depth and bending creep on the time-dependent flexural behavior of reinforced concrete (RC) beams were examined via two sets of experiments and numerical analyses. Each test set included four types of experiments: axial creep, beam shrinkage, bending creep, and time-dependent RC beam bending tests. The variations in beam shrinkage and bending creep strain were determined using the American Concrete Institute’s shrinkage and creep prediction models, respectively. Time-dependent finite-element beam equilibrium equations were presented using an incremental form of the age-dependent constitutive equation. The mathematical creep model of the constitutive equation accounted for the aging effects of the elastic modulus and the creep development under a multi-sequential loading condition with a single creep curve. The resulting finite-element formulation captured the mechanical and non-mechanical phenomena of the aging effect of concrete at an early age, shrinkage variations within the beam depth, and bending creep. Comparisons between the age-dependent responses of the measured and predicted results of the RC beam test specimens illustrated the significant effects of shrinkage variation and bending creep on the time-dependent behaviors of RC flexural members.

An Experimental Study on Strengthening of Reinforced Concrete Flexural Members using Steel Wire Mesh

Reinforcing reinforced concrete (RC) beams with galvanized welded steel wire mesh is one of the latest technologies applied in retrofitting. For each sample, the experimental evaluation of 18 small reinforced concrete beam samples with a total length of 1200 mm was carried out to study the bending strength under static load conditions. Experimental testing has been carried out to the activated failure mode, with 11 reinforced samples, 4 integrally cast control beams and three original control beams. Based on the test variables, namely SWM characteristics and connection mechanism, the reinforced beams are divided into two groups A and B. This study also clarified the bending resistance, ductility, stiffness, crack width and deflection. According to the test results obtained, all reinforced beams are designed to fail ductilely. The first group of reinforced beams recovered to an average of 110% of the bearing capacity of the original control beam, while the second group of reinforced beams recovered to an average of 163%. Furthermore, it was found that the reinforcement beam functions in the same way as the general control beam and works as a unit. Therefore, the bottom line is that this reinforcement technology can be used confidently in real-life applications, especially in low- cost buildings.

Hysteretic moment-curvature relations for the analysis of RC flexural members subjected to blast loading

A hysteretic moment-curvature relation for analyzing reinforced concrete (RC) members subjected to blast loading is introduced in this paper. After constructing a monotonic envelope curve for the moment-curvature relation, the hysteretic behaviors of unloading and reloading are defined based on the hysteretic curve of steel. The use of the moment-curvature relation in the blast analysis becomes possible by introducing a dynamic increase factor (DIF), which is defined in terms of the curvature rate. This makes it possible to analyze RC structures composed of many bending structural members. In addition to defining a basic hysteretic moment-curvature relation, additional influencing factors such as the bond-slip effect and direct shear behavior, which are expected to affect the structural responses, are taken into consideration for an exact simulation of the nonlinear dynamic response of RC flexural members. The validity of the introduced hysteretic moment-curvature relation is established by correlation studies between the analytical results and experimental data experiencing repeated unloading and reloading phases. The obtained numerical results also show the importance of the bond-slip effect and the hysteretic behavior on the structural response of RC flexural members subjected to blast loading.

재료계수를 이용한 철근콘크리트 휨부재의 최대철근비 산정

이 연구는 재료계수와 콘크리트의 비선형 재료모델을 이용하여 철근콘크리트 휨부재의 최대철근비를 제안한 것이다. 이를 위하여 현행 콘크리트구조 학회기준에서 규정하는 철근과 콘크리트의 재료계수와 포물선-사각형 형상의 응력-변형률 관계로 정의되는 콘크리트의 재료모델을 기반으로 하여 휨부재 단면의 변형 적합조건에 의한 최대 중립축 깊이와 힘의 평형관계를 이용하여 최대철근비를 유도하였다. 이와 함께 설계 실무의 편의성 확보를 위하여 인장철근의 항복강도별로 해당 균형철근비에 대한 비율로 최대철근비를 함께 제시하였다. 이 연구에서 제안한 최대철근비는 다른 설계기준의 규정과 비교하여 휨부재의 연성파괴를 충분히 보장할 수 있으며, 시공성 및 경제성을 동시에 확보할 수 있는 것으로 나타났다.

Repair of Fire-Damaged Reinforced Concrete Flexural Members: A Review

The mechanical properties of both concrete and steel reinforcement, and the load-bearing capacity of reinforced concrete (RC) structures are well known to be temperature-sensitive, as demonstrated by the severe damage that major fires cause in buildings, followed—in extreme cases—by their collapse. Since in most cases RC structures survive a fire, retrofitting fire-damaged RC members is a hot subject today. In this paper, after a recall on the performance of RC beams and slabs in fire, different repair techniques are considered, among them externally bonded reinforcement, near surface-mounted fiber-reinforced polymers (FRP), bolted side plating, jacketing with high- and ultra-high performance concretes or mortars, and damaged-concrete replacement. Last but not least, the design equations aimed at evaluating the residual load-bearing capacity after repairing are also presented and discussed.

Experiment on Natural Frequency Change of Reinforced Concrete Members under Low Cycle Loading

The natural frequency change of reinforced concrete (RC) members during damage when subjected to low cycle loading was studied through horizontal cyclic loading experiments. Three groups of RC flexural members were subjected to horizontal, harmonic, low cycle loading to simulate earthquake conditions. The relation of instantaneous load, instantaneous displacement, and instantaneous natural frequency during loading was deduced. Using the resulting equation, the test members’ natural frequencies at any moment during loading could be calculated accurately. Then the natural frequency change curves and their fitting equations were also obtained. The impact of loading period T and loading amplitude A on a test member’s damage rate V was analyzed, which showed that the impact of T on V was quadratic, and the relation between A and V was linear. Finally, by fitting experimental data of number of loading cycles N, loading amplitude A, loading period T, and natural frequency ω, a three‐variable function, ω(N, A, T), was determined, revealing the change process of test members’ frequencies under arbitrary harmonic vibrations.

A partial-interaction approach for extracting FRP-to-concrete bond characteristics from environmentally loaded flexural tests

Abstract Bonding fibre-reinforced polymers (FRP) to reinforced concrete (RC) members has become a popular means for enhancing load-carrying capacity and extending service life. However, the long-term durability of flexurally strengthened members remains uncertain. In this paper, a numerical solution to a previously developed partial-interaction (PI) moment-rotation approach for intermediate crack (IC) debonding in FRP-strengthened RC flexural members is applied to a set of test results in published literature to extract bond characteristics. This is significant in four respects: (1) the model is based on fundamental PI theory, which directly simulates the formation and widening of cracks associated with the tension-stiffening mechanism, and explicitly allows for any changes at bond interfaces due to environmental loading or corrosion of the steel reinforcement. (2) The model also simulates the mechanism of compressive concrete softening should it occur prior to the complete debonding of FRP laminates. (3) Changes to local bond characteristics at the FRP-to-concrete interface due to environmental loading can be quantified by matching the experimental load-deflection response of deteriorated members. (4) The approach can be applied to a wide range of flexural test data, so as to broaden the bounds of bond-slip models, which are otherwise limited to the bounds of shear bond tests. Using the approach, a set of bond characteristic deterioration factors, derived from full-range load-deflection responses of environmental loaded flexural members, indicate that bond deterioration at the FRP-to-concrete interface generally compromises the ductility and strength of flexural members.

Full-range load-deflection response of FRP-strengthened RC flexural members anchored with FRP anchors

The flexural resistance of reinforced concrete (RC) members such as beams and slabs can be enhanced by bonding fibre-reinforced polymer (FRP) composite plates to their tension face. The effectiveness of the strengthening can be compromised though by premature debonding failure of the FRP which can occur at strains significantly lower than the strain capacity of the FRP. Anchorage of the FRP strengthening can, however, increase its usable strain. A convenient means with which to represent the structural behaviour of such a strengthened and anchored member then is via its load-deflection response. The task of how to model this response therefore arises. The authors, as well as other research groups, have derived closed-form solutions which describe the complete load-deflection response of FRP-strengthened RC flexural members of which pre-defined moment-curvature relationships are observed. This paper reports an extension to such existing theory in order to incorporate the anchorage effect. The theory is finally calibrated with test results of FRP-strengthened RC slabs which have been anchored with FRP anchors. The paper sets a framework for future work centred on quantification of the FRP anchor effect.

An Experimental Study on Strengthening of Reinforced Concrete Flexural Members using Steel Wire Mesh

Abstract One of the major challenges and contemporary research in the field of structural engineering is strengthening of existing structural elements using readily available materials in the market. Several investigations were conducted on strengthening of various structural components using traditional and advanced materials. Many researchers tried to enhance the reinforced concrete (RC) beams strength using steel plate, Glass and Carbon Fibre Reinforced Polymers (GFRP & CFRP). For the reason that high weight to the strength ratio and compatibility in strength between FRP composites and steel bars, steel plates and GFRP and CFRP composites are not used for strengthening works practically. Hence, in this present work the suitability of using wire mesh for the purpose of strengthening the RC flexural members is studied by conducting experimental works. New technique of strengthening system using wire mesh with a view to improve sectional properties and subsequently flexural strength of RC beams is adopted in this work. The results for experimental and theoretical analysis were compared and found that good correlation exists between them. The experimental results indicate that RC beams strengthened with steel wire mesh are easy technique for strengthening of existing flexural members.

An automated computationally efficient two-stage procedure for service load analysis of RC flexural members considering concrete cracking

An automated computationally efficient two-stage procedure has been proposed for service load analysis of reinforced concrete (RC) flexural members considering concrete cracking and tension stiffening. The proposed procedure yields cracked lengths, redistributed bending moments and inelastic deflections. The computation of final state, including cracked lengths and interpolation coefficients for tension stiffening, is automated by the initialization of second stage from the results of first stage. The procedure combines the analytical–numerical procedure developed by authors and the neural networks methodology. The cracked lengths and corresponding interpolation coefficients are rapidly estimated in the first stage using the closed-form expressions which are obtained from the trained neural networks. Eight separate neural networks are trained for the estimation of final cracked lengths and interpolation coefficients at in-span locations and supports. The use of estimated cracked lengths and interpolation coefficients of the first stage, in the beginning of second stage, significantly reduces number of iterations in the second stage. The deflections and bending moments obtained from the two-stage procedure are compared with those from the analytical–numerical procedure for a number of beams. The analytical–numerical procedure requires around six analyses to yield results with sufficient accuracy for design purpose (within 2–3%); whereas, in the two-stage procedure, only two analyses, one in each stage, are required to yield results with similar accuracy. The developed two-stage procedure requires a significantly small computational effort as compared to similarly accurate methods available in literature.