Rectangular Reinforced Concrete Research Articles

Carbon Fiber Reinforced Polymer (CFRP) for Structural Capacity Enhancement of RC Beams Incorporating Innovative Side Hybrid (SH) Technique

Reinforced concrete (RC) infrastructure is an essential part of modern civilization. However, the serviceability of RC infrastructure in extreme weather has become challenging due to the susceptibility of the initiation of cracks. Hence, the demand for strengthening and retrofitting RC infrastructure is rapidly increasing. The RC specimens strengthened with existing externally bonded reinforcement (EBR) and near-surface mounted (NSM) techniques; however, they suffered a prematurely brittle or debonding failure. Hence, the merging of side near surface mounting (SNSM) and side externally bonded reinforcement (S-EBR) methods ended up resulting in the development of an innovative side hybrid (SH) strengthening approach that is designed to overcome these drawbacks. In this investigation, six rectangular RC beam specimens were flexurally strengthened utilizing carbon fiber-reinforced polymer (CFRP) with the SH technique, and then four-point bending experiments were performed to failure. The beam specimens were categorized into two types: (I) control specimens and (II) specimens strengthened with the SH technique applying CFRP varying bonded length from 1600 mm to 1900 mm. The initial cracking, yield, and ultimate load-bearing capabilities, deflection, failure modes, cracking characteristics, stiffness, energy absorption capacity, and strain on the utmost fiber of concrete, the tensile strain of major steel rebars, SNSM bars, and S-EB plates were assessed from the experimental investigation. The SH technique substantially improved the flexural performance of the beam specimens. The initial cracking load, yield, and ultimate load-bearing capabilities were enhanced remarkably by 387%, 108%, and 163%, respectively, over the reference specimen. The flexural stiffness and energy absorption capacity substantially improved by 120% and 103%, respectively, compared with the reference specimen.

Design and economic implications of using steel fibers in elevated slabs of multi‐story buildings

AbstractThis paper presents a comprehensive comparison between conventional reinforced concrete (RC) and steel fiber‐reinforced concrete (SFRC) slabs. Twenty‐two‐way rectangular RC and SFRC slabs are designed with varying dimensions, aspect ratios, and boundary conditions, following verified international standards to compare the steel quantities in a unique study. SFRC slabs with larger aspect ratios and discontinuous edges required significantly higher steel percentages than square slabs, as the random three‐dimensional distribution of steel fibers suited the load distribution pattern of square slabs. To further conduct a detailed design and economic feasibility case study on a realistic building model, two 12‐story buildings are computationally designed with RC and SFRC alternatives. The results indicate that SFRC slabs required approximately 25% more steel than RC slabs due to the uniform distribution of steel fibers throughout the section. Project cost and time estimates were conducted based on standard productivity manuals and cost databases in two distinct economies, India and the USA. Despite the higher steel requirements, the 12‐story SFRC building showed time savings of 34.1 days (5.44%) and cost savings of 4.88% and 1.27% when constructed in the USA and India, respectively. The results are compared with limited available data in literature. In conclusion, the study suggests that SFRC slabs in multi‐story buildings could be more economically favorable in countries with higher labor costs.

Plastic hinge behavior of rectangular CFRP-confined RC columns: Meso-scale modelling and formulation

The ultimate rotation capacity, depending on the plastic hinge length, is a vital factor in the seismic design of flexural members. This paper focuses on the investigation of the plastic hinge length and size effect of rectangular carbon fiber-reinforced polymer (CFRP)-confined reinforced concrete (RC) columns. Firstly, considerable scatter and evident bias of the calculation results were found after evaluating existing models for calculating the plastic hinge length of FRP-confined RC columns. Accordingly, a meso-scale numerical approach was developed, with concrete heterogeneity considered, to investigate the influences of the confinement ratio and corner radius on the plastic hinge length and size effect of rectangular CFRP-confined RC columns. Results show that columns having different cross-sectional sizes exhibit similar failure patterns, and the region of lateral crack propagation for columns with lower confinement ratios is larger than that for columns with higher confinement ratios. The actual plastic hinge length is determined based on the yielding zone of reinforcement, the crushing zone of concrete, and the localization zone of curvature, while the localization zone of curvature is employed to determine the real plastic hinge zone. The size effect is found to exist in the plastic hinge length ratio, and it is weakened with the increase of the corner radius, which can be attributed to the fact that columns with higher corner ratios have more effective confinement. Taking into account the size effect, a new model for calculating the ultimate drift ratio of CFRP-confined RC columns was proposed based on an assumption of curvature distribution and presented accurate predictions.

Cyclic performance of non‐ductile reinforced concrete columns retrofitted by partial steel plate jacketing: Experiment and numerical analysis

AbstractThis paper investigates the strengthening of non‐ductile reinforced concrete (RC) columns using partial steel‐plate jacketing to enhance ductile capacity. Quasi‐static cyclic loading tests were conducted on three rectangular RC columns with varying shear‐span‐to‐depth ratios, evaluating the effectiveness of steel‐plate jacketing. Results show improved ductile flexural response, high drift capacity, and maintained gravity‐loading capacity under high lateral displacements. A numerical investigation, adopting a multi‐spring fiber section model, predicted cyclic lateral strength, drift ratio, and failure mode, accurately simulating reinforcement behavior and capturing the shear‐strength enhancement due to steel‐plate jacketing. Numerical results demonstrated good agreement with experimental findings. Based on these outcomes, a simple design procedure for retrofitted concrete columns, applicable to design engineers, has been developed and reported in this paper. This study contributes valuable insights into the practical applications of steel‐plate jacketing for non‐ductile RC column retrofitting, offering a comprehensive approach to predicting their seismic performance.

Shear capacity model of rectangular RC columns based on Bayesian update

Reinforced concrete (RC) columns are function as structural member resisting vertical loading. The combined effects of vertical loading and transverse loading might induce shear failure of RC columns, posing a significant threat to the structure's integrity. While the accuracy of existing models to determine the shear strength of RC column is crucial for design, there is a need for further refinement. In this context, a study introduces a probabilistic shear capacity model based on the truss-arch theory and incorporating the Bayesian theory Monte Carlo Markov Chain (MCMC) method. This model delineates the shear contribution into four distinct components: concrete, stirrup, axial compression force and longitudinal reinforcement longitudinal reinforcement in the middle of the section parallel to the loading direction. A database, comprising 233 RC column specimen sets, facilitates the model's regression analysis. Calibration is achieved against the density distribution and confidence intervals of the posterior parameters. When set against four established models, the new proposal offers a mean value closely aligned with experimental data and showcases reduced variance. To substantiate the shear mechanism of the proposed model, a thorough contribution of each shear component's contribution is made with those in four existing models: Sezen, ACI 318, Priestley, and GB50010. While the Sezen and ACI 318 models recognize the shear contributions of concrete and stirrup, the Priestley and GB50010 models extend to factor in the beneficial impact of the axial compression force component. Comparative analysis underscores the superior predictive capability of the newly proposed model.

Stress and strain relations of RC circular, square and rectangular columns externally wrapped with fiber ropes

This study explores the potential use of low-cost natural fiber reinforced rope polymers (FRRP) to improve the compressive behavior of circular, square, and rectangular reinforced concrete (RC) specimens. A total of 42 specimens were tested under monotonic axial compression in three groups. Groups were formed to differentiate specimens with different cross-sectional shapes such as circular, square, and rectangular. The findings demonstrate that FRRP can effectively boost the compressive behavior of RC columns. Circular specimens with three-layer hemp FRRP exhibited a 200% increase in compressive strength and a 270% improvement in corresponding strain. Cotton FRRP provided a 117% boost in compressive strength and a 233% enhancement in strain. In square specimens, three-layer hemp FRRP resulted in a 110% rise in compressive strength and a 186% increase in strain, while cotton confinement yielded improvements of 95% and 144%, respectively. For the square and rectangular specimens, the improvement in the compressive behavior was reduced compared to the circular specimens because of stress concentrations near corners. Moreover, the study showed that the hemp FRRP confinement outperformed the cotton confinement. The investigation also revealed that the existing analytical models were inadequate in predicting the mechanical properties of RC confined with natural FRRP. Therefore, the study introduces novel equations to predict the compressive strength and corresponding strain for both hemp and cotton confined concrete in various cross-sectional types. These proposed equations exhibit a good level of accuracy in predicting the compressive strength and corresponding strain.

The behavior of strengthened RC beams under pure torsion using NSM-CFRP rope

PurposeThe paper aims to introduces an experimental work to investigate the torsional behavior of reinforced concrete (RC) beams strengthened by near-surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) ropes.Design/methodology/approachIn this research, nine rectangular RC beams of 250 mm × 300 mm cross-section and 1,600 mm in length were constructed and tested considering the studied parameters. These parameters include the length of the CFRP rope, the orientation of the CFRP rope, the arrangement of longitudinal and the scheme of NSM-CFRP ropes.FindingsIn comparison to control specimens, the results demonstrate a considerable improvement in the torsional response of RC beams strengthened with the CFRP rope. Additionally, specimens strengthened with 90° vertical ropes increase torsional moment capacity more efficiently than specimens strengthened with 45° inclined ropes since the stress concentration leads to premature debonding of the CFRP rope. Whereas RC beams' ability to withstand torsional moments is reduced as the distance between reinforcing CFRP ropes is increased. According to test results, adding CFRP ropes to RC beams' bottoms had a slightly positive impact on torsional response.Originality/valueThis paper fulfills an identified need to study how the using of the CFRP rope is effective in strengthening RC beam subjected to torsion moment.

Prediction model and sensitivity analysis of ultimate drift ratio for rectangular reinforced concrete columns failed in flexural-shear based on BP-Garson algorithm

Considering that the deformation capacity of reinforced concrete (RC) columns is crucial in performance-based seismic design, the ultimate drift ratio of RC columns failed in flexural-shear are studied using BP neural network and Garson (BP-Garson) algorithm. Firstly, this study employed data onto tests conducted on 35 rectangular RC columns with flexural-shear failure. Nine influencing factors including axial load ratio, shear span ratio, longitudinal reinforcement configuration, transverse reinforcement configuration, and material strength were selected as variables, a BP neural network was used to establish the relationship between the ultimate drift ratio and these variables. Accordingly, a predictive model for the ultimate drift ratio of RC rectangular columns failed in flexural-shear was constructed. Then, the Garson algorithm for local sensitivity analysis was applied to determine the sensitivity of each influence of the ultimate drift ratio of RC rectangular columns failed in flexural-shear. The results showed that the BP neural network could accurately predict the ultimate drift ratio of RC rectangular columns with flexural-shear failure, of which the primary influencing factors are shear span ratio, axial load ratio, volume transverse reinforcement ratio and longitudinal reinforcement ratio. The BP-Garson algorithm integrates model development with influence analysis, which could improve computational efficiency and serves as a reference for evaluating the sensitivity of seismic performance and its influencing factors in other structural components.

Strengthening Reinforced Concrete Rectangular Columns with Various Slenderness Ratios by Fiber Reinforced Polymer (FRP)

Strengthening Reinforced Concrete Rectangular Columns with Various Slenderness Ratios by Fiber Reinforced Polymer (FRP)

Random Forests Machine Learning Applied to PEER Structural Performance Experimental Columns Database

Columns play a very important role in structural performance and, therefore, this paper contributes to the critical need for failure mode prediction of reinforced concrete (RC) columns by exploring the capabilities of random forest machine learning (ML) based on a well-known experimental column database. Known as the PEER structural performance database, it assembles the results of over 400 cyclic, lateral-load tests of reinforced concrete columns. The database describes tests of spiral or circular hoop-confined columns, rectangular tied columns and columns with or without lap splices of longitudinal reinforcement at the critical sections. The efficiency towards the aforementioned goal of supervised ML methods such as random forests using a randomly assigned test set from the Pacific Earthquake Engineering Research Center (PEER) database is examined here. The overall accuracy score for rectangular RC columns is 94% and for circular RC columns is 86%. The latter performances are influenced by the size of the testing and training sets of data and are independent of the number of decision trees in the forest employed in the random forest algorithm. The performances of random forests in postdicting the failure mode of RC columns prove that ML has great promise in revolutionizing the profession of earthquake engineering.

Failure Mode Identification and Shear Strength Prediction of Rectangular Hollow RC Columns Using Novel Hybrid Machine Learning Models

Failure mode identification and shear strength prediction are critical issues in designing reinforced concrete (RC) structures. Nevertheless, specific guidelines for identifying the failure modes and for accurate predictions of the shear strength of rectangular hollow RC columns are not provided in design codes. This study develops hybrid machine learning (ML) models to accurately identify the failure modes and precisely predict the shear strength of rectangular hollow RC columns. For this purpose, 121 experimental results of such columns are collected from the literature. Eight widely used ML models are employed to identify the failure modes and predict the shear strength of the column. The moth-flame optimization (MFO) algorithm and five-fold cross-validation are utilized to fine-tune the hyperparameters of the ML models. Additionally, seven empirical formulas are adopted to evaluate the performance of regression ML models in predicting the shear strength. The results reveal that the hybrid MFO-extreme gradient boosting (XGB) model outperforms others in both classifying the failure modes (accuracy of 93%) and predicting the shear strength (R2 = 0.996) of hollow RC columns. Additionally, the results indicate that the MFO-XGB model is more accurate than the empirical models for shear strength prediction. Moreover, the effect of input parameters on the failure modes and shear strength is investigated using the Shapley Additive exPlanations method. Finally, an efficient web application is developed for users who want to use the results of this study or update a new dataset.

Machine vision‐based automated earthquake‐induced drift ratio quantification for reinforced concrete columns

SummaryThis paper presents a novel method for estimating the seismic peak interstory drift ratio (IDR) in reinforced concrete (RC) columns after an earthquake using surface crack image analysis. The quantitative representation of the complexity and irregularity of crack images in damaged RC columns is obtained through the consideration of the generalized fractal dimensions. The authors have compiled a comprehensive database consisting of 445 crack maps obtained from cyclic experiments conducted on 110 rectangular RC column specimens exhibiting double‐curvature deformation mode. This database is utilized by the authors to develop and validate the proposed procedure. The research database contains a wide range of structural and geometric features. Five closed‐form equations are developed with the objective of estimating the peak IDR experienced by the RC columns during a seismic event. The predictive equations are derived through the utilization of symbolic regression technique, with the input parameters varying according to the availability of columns characteristic parameters. Results reveal that generalized fractal dimensions, especially D−1, are strong vision‐based indicator of damage in RC columns having correlation coefficients with IDR ranging from 0.82 to 0.92 across the considered plans. The seismic peak IDR obtained through the empirical equations can serve as the input engineering demand parameter (EDP) in the seismic loss estimation frameworks. This allows for the determination of the probability of exceeding damage states for structural and nonstructural components of concrete buildings. Finally, the practical implementation of the methodology is examined by its application to an actual case of a damaged column during the Kermanshah earthquake of magnitude 7.3 that occurred in 2017.

A unified shear strength equation for slabs and beams longitudinally reinforced with FRP or steel bars based on simplified cracked elastic properties

At service load conditions, deflections and stresses in reinforced concrete (RC) slabs and beams are typically calculated using the properties of the transformed cracked cross section. For more than eighty years, the depth of compression zone factor k in cracked singly-reinforced rectangular RC sections has been calculated using k=2ρn+ρn2-ρn where ρ is the reinforcement ratio and (n) is the modular ratio. It is shown that in slabs and beams with a wide range of concrete compressive strength fc′ that are reinforced longitudinally with steel or FRP bars, ρn ranges from 0.005 to 0.16. Within this range, k can be accurately calculated using k=0.95ρn0.43 or simply k=ρn0.44. The error between the former equation and the exact equation ranges from –2% to + 3 %. This equation is used to show that the shear strength of concrete vc in the ACI–440.11-22 code for GFRP–reinforced members depends effectively on ρ, fc′ and d, just like the strength of steel–reinforced members in the ACI–318-19 code equation. It is proposed to apply to the factor Er/Esteel1/3 where Er and Esteel are the moduli of elasticity of the FRP and the steel bars respectively to the ACI–318 equation to make it suitable also for FRP–reinforced members. This unifies the equation for the two types of reinforcement and provides a better correlation with the experimental results from 233 FRP–reinforced members available in the literature than the current ACI–440.11 code equations. On the other hand, the transformed cracked moment of inertia Icr can be related solely to ρn and approximated using Icr=0.36ρn0.81bd3 where b and d are the sectional width and effective depth respectively. This equation is used to show that the cracked flexural stiffness is effectively related to the modulus of elasticity and the area of the reinforcing bars raised to the power 0.8, to d2.2 and to modulus of elasticity of the concrete raised to the power 0.2.

Assessment of plastic hinge length in reinforced concrete columns

Plastic hinge properties are crucial parameters in predicting the nonlinear response of structural elements. Because of the intricate material nonlinearity, precise determination of the plastic hinge length (PHL) has encountered several obstacles. Over the last few decades, there have been various definitions and models put forth to forecast this length, nevertheless, the outcomes displayed significant disparities. Therefore, the paper introduces a comprehensive method for determining PHL using certain criteria, including rebar strain profiles, concrete cover and core peak strains, curvature profiles, and damage observations. Furthermore, a set of four full-scale reinforced concrete (RC) columns measuring 400 mm × 400mm × 3000mm and featuring varying transverse reinforcement configurations were constructed and subjected to testing under a high axial load ratio (ALR). The tested results implied that it is necessary to separate the PHLs based on different criteria. The high axial compression load led to enhancing the PHLs, which were based on rebar compressive yield strains, curvature profiles, concrete cover and core peak strains. In contrast, it has a minor effect on PHL based on tensile yield strains. In addition, the amount of transverse reinforcement had an insignificant effect on all PHLs for tested columns. Hence, a revised equation was proposed to estimate the equivalent PHLs of rectangular RC columns based on tested results and a 114-column database. The proposed equation had better accuracy compared with some other model results in the literature.

Flexural repairing of heat damaged reinforced concrete beams using NSM-BFRP bars and NSM-CFRP ropes

The introduction of Basalt Fiber Reinforced Polymer (BFRP) bars marks a significant milestone in the construction industry's pursuit of advanced and sustainable materials. The application of near-surface mounted (NSM) BFRP bars presents a promising, emerging approach for restoring heat-affected reinforced concrete (RC) elements. This study investigated experimentally and theoretically the flexural performance of rectangular RC beams subjected to a temperature of (650 °C) for 3 h, followed by repairing using NSM-BFRP bars and NSM carbon fiber-reinforced polymer (CFRP) ropes. Among the twelve designed RC beams, seven were repaired with NSM-BFRP bars and three with NSM-CFRP ropes to address flexural deficiencies. The primary focus areas included failure modes, ultimate capacity, and the recovery percentage of original flexural capacity. Results showed that recovering percentages ranging from 88.2 % to 127 %, beams repaired using NSM-BFRP bars gave almost the same behavior as those repaired using NSM-CFRP ropes. Theoretical outcomes agreed well with experimental findings, in alignment with the ACI 440.2R-08 guidelines.

Blast resistance of RC columns for varied detonation scenarios: Effect of shear detailing

The growing threat of accidental and malevolent blast loading to building structures has necessitated the consideration of blast-resistant design. Columns are the most vulnerable and critical component of a building, so they must be designed against blast to achieve defined performance goals. The influence of transverse rebar detailing on the ultimate capacity and failure modes of rectangular reinforced concrete (RC) columns against blast loads was investigated under different detonation scenarios. The performance of the columns was quantified using the failure charge mass and post-blast residual axial capacity. Pressure–impulse diagrams were used to study the failure modes. The analyses showed that special transverse detailing measures were the most effective for columns subjected to near-field surface bursts. The confinement of concrete due to ties and axial restraint played a critical role in improving the blast performance of columns. The provision of diagonal rebar improved the blast resistance by preventing direct shear failure, and was most effective when coupled with closely spaced lateral ties. Finally, the performance of code-compliant columns against blast was studied to determine the blast safety margin and the scenarios under which it would be necessary to undertake blast retrofitting of the columns in existing critical buildings.

Case studies on determining fire resistance of uniaxial bending reinforced concrete columns subjected to 4-side standard fire exposure

This article introduces technical specifications and calculation instructions for fire resistance of uniaxial bending rectangular reinforced concrete (RC) columns under 4-side heating by standard fire. Although a number ofresearch works have been conducted following the Eurocodes to meet the long-term requirement of the country’s development in construction, it is shown that the Russian design standard SP 468.1325800.2019 (SP 468) is also applicable for concrete structures subjected to fire in the Vietnam current context. Tabulated data and simplified calculation methods according to SP 468 are introduced, followed by two practical case studies for demonstration. The calculation results show that by using the simplified calculation method the RC columns’fire-resistant limit can be explicitly determined with a particular safety factor ku, T, and a more economical design solution can be obtained compared to that of the tabulated data method. Additionally, RC columns loaded with small eccentricities exhibit greater fire resistance advantages compared to those with large-eccentricity loading. A number of other useful observations are also discussed in the latter part of this article.

Prediction of flexural resistance of reinforced concrete beam cross-section by practical formula

This paper presents the process of developing a practical formula for predicting the ultimate bending moment of rectangular reinforced concrete (RC) beams through regression analysis. The data used for regression analysis was generated by using the fiber method to analyze a non-linear batch of commonly encountered RC beam cross-sections. The practical formula was obtained by fitting a linear regression model to the training set and then making predictions on the test set. The coefficient of determination, R2, between the bending moment values calculated from the formula and the results of the non-linear analysis was 0.9948, indicating a good predictive capability of the formula.

Suggestions for repairing various defects in the NSM strengthening mechanism with hybrid rebars for RC beams: Experimental and numerical study

NSM bars enhance flexural strength of RC components but degrade ductility and transform the failure mechanism from ductile flexure to brittle shear. Failure due to debonding and other causes still has an impact on the strengthening system's efficiency. The focus of this research will be on how successful hybrid steel-CFRP/GFRP rebar strengthening is in enhancing the overall behaviour of reinforced concrete (RC) beams under flexure. Six rectangular RC beams were cast and strengthened using various semi-NSM procedures (120 mm width by 300 mm depth by 3000 mm length), then tested to failure in four-point bending tests. The beams were classified into four categories: (I) control beam, (II) strengthening with mild steel bar, (III) strengthening with mild steel-CFRP/GFRP hybrid rebars, and (IV) strengthening with high tensile steel-CFRP/GFRP hybrid rebars. To enhance the overall behaviour of RC beams, particularly beam ductility, and to avoid corrosion, FRP confinement was utilized in association with steel bars. End anchoring with strengthening rebars aims to limit unexpected de-bonding of NSM bars at higher loads, resulting in superior flexural performance of the RC members. In compared to the others, strengthening with mild steel-GFRP rebars enhances ductility and beam stiffness. This study also suggests and validates a 3D numerical model to simulate the performance and failure of NSM Strengthening mechanism with hybrid rebars for RC beams using the finite element (FE) software “ANSYS”. Regarding the behaviour of load-deflection, moment-curvature, load-crack width, and mode of failure, the FE results were accurate representations of the experimental results. An additional FEM specimen was examined to corroborate the effect of the experimental parameters.

Mechanical Behaviour Evaluation of Full Iron Tailings Concrete Columns under Large Eccentric Short-Term Loading

In this study, full iron tailings concrete (FITC) was created using iron tailings from a tailings pond in Qian'an, China. Iron tailings account for 86.8% of the total mass of solid raw materials in the FITC. To enable large-scale use of FITC, a comprehensive investigation of the structural behaviour of full-iron tailing-reinforced concrete (FITRC) specimens is warranted. Therefore, eight rectangular reinforced concrete (RC) columns with conventional reinforced concrete (CRC) as a control were tested to investigate the effects of section dimensions, initial eccentricities, and concrete strengths, on the structural behaviour of FITRC columns under large eccentric short-term loading. The experimental and analytical results indicated that the sectional strain of the FITRC columns satisfied the plane-section assumption under short-term loading, and the lateral deflection curve agreed well with the half-sinusoidal curve. In addition, the FITRC columns exhibited a slightly lower cracking load and lower ultimate load capacity than the CRC columns, and the crack widths were larger than those of the CRC columns. The reduction in the load capacity observed in the FITRC was within the permissible range stated in the design code, thereby satisfying the code requirements. The deformation coefficients of the FITRC and CRC columns were identical, and the cracking and ultimate loads calculated according to the current code and theories were in good agreement with the measured results.