Flexural Reinforced Concrete Beams Research Articles

Theoretical and experimental assessment of the effect of adhesive bond thickness on the flexural capacity of CFRP strengthened beams

Carbon Fiber Reinforced Polymer (CFRP) is a material of choice in the structural strengthening of reinforced concrete (RC) elements. In order to strengthen RC elements, CRFP is bonded externally to RC elements using adhesives. This paper investigates the effect of varying adhesive bond thickness on the moment capacity of CFRP strengthened RC beams in flexure. Thirty-eight (38) Reinforced concrete beams (1.2m length) were cast in the laboratory and their failure loads and corresponding moment capacities obtained when the adhesive bond thickness between the CFRP wraps and RC beams were varied. This paper examines the comparison between flexural moment capacities obtained experimentally and the predicted moment capacities using a theoretical procedure set out in AC440-2R-17 after structural strengthening with CFRP. It was observed that the adhesive thickness had a significant effect on the flexural capacity of strengthened beams. As the adhesive thickness increased beyond certain thresholds, the flexural capacity of strengthened beams reduced. At the optimum adhesive thickness threshold, the ACI 440-2r-17 procedures could predict to high accuracy the moment capacities of strengthened beams.

Performance of corroded RC beams strengthened in flexure using UHPC: Effect of configuration and thickness of the UHPC layers

In the present paper, an experimental investigation was carried out to examine the effect of configurations and thicknesses of ultra high performance fiber reinforced concrete (UHPC) layers on the flexural performance of reinforced concrete (RC) beams strengthened using UHPC layers after undergoing different degrees of accelerated reinforcement corrosion. The beams were tested under flexure to evaluate the effect of UHPC strengthening (with combinations of three UHPC layer thicknesses and two configurations) on failure modes, flexural strengths, and stiffnesses. Statistical analysis of the flexural test results using analysis of variance (ANOVA) showed the significant effects of both configuration and thickness of the UHPC layers on the performance of corrosion-damaged RC beams in flexure, irrespective of the degree of reinforcement corrosion. The RC beams strengthened with UHPC layers applied on three sides exhibited the most significant increase in the ultimate flexural strength in the range of 6 to 63% and an increase in the stiffness in the range of 28 to 112% as compared to that of the uncorroded control beam. Furthermore, analytical models were developed to predict the flexural strength of the corroded beams strengthened using different configurations of UHPC layers. The close agreement of the ultimate flexural strength between the experimentally obtained results and analytically predicted values for the strengthened RC beams indicates the accuracy of the developed analytical models, which can be used to select an optimum strategy for strengthening the corrosion-damaged RC beams.

Effective Strengthening of RC Beams Using Bamboo-Fibre-Reinforced Polymer: A Finite-Element Analysis

This paper presents a finite-element model of the structural behaviour of reinforced concrete (RC) beams with and without openings externally strengthened with bamboo-fibre-reinforced composite (BFRC) plates. The simulation was performed using ABAQUS Unified FEA 2021HF8 software. The stress–strain relationship of the RC was modelled using a model code for concrete structures, whereas the concrete-damaged plasticity model was used to simulate concrete damage. The predicted crack pattern of the beams was comparable to that from experimental observations. The ultimate load-bearing capacity of RC beams in flexure was predicted with an error of up to 1.50%, while the ultimate load-bearing capacity of RC beams with openings in shear was predicted with an error ranging from 1.89 to 13.43%. The most successful arrangement for strengthening a beam with openings in the shear zone was to place BFRC plates perpendicular to the crack on both sides of the beam’s surface, which increased the beam’s original load-bearing capacity by 110.06% compared to that of the control beam (CB). The most effective method for strengthening RC beams in flexure is to attach a BFRC plate to the entire bottom soffit of the RC beam. This maximises the ultimate load-bearing capacity at the expense of the beam’s ductility.

Machine learning-based model for recognizing the failure modes of FRP-strengthened RC beams in flexure

Plate end (PE) debonding and intermediate crack (IC) debonding are the main failure modes of fiber reinforced polymer (FRP) strengthened reinforced concrete (RC) beams in flexure. Different failure modes exhibit different failure characteristics. Therefore, accurately identifying the failure mode is of great significance in selecting methods to prevent the debonding of the strengthened beams. This study first established a primary indicator system through literature research. Then one hundred and eighty-eight FRP-strengthened RC beams containing PE and IC debonding were collected from the published literature of forty-eight researchers. After that, correlation analysis and grey correlation analysis were used to study the data of two failure modes. Finally, the indicator system for the prediction of failure modes was established. After the indicator system was established, six machine learning algorithms, including K-nearest neighbor algorithm (KNN), decision tree (DT), random forest (RF), back propagation neural network (BPNN), logistic regression (LR), and support vector machine (SVM), were used to build the prediction model of failure modes. The evaluation of the models shows that the coefficient of variation of the accuracy of the decision tree is merely 5.4%, which has the best robustness; the average accuracy of the random forest for the two failure modes reaches 93% and 98%, which has the highest precision.

Machine Learning-Based Model in Predicting the Plate-End Debonding of FRP-Strengthened RC Beams in Flexure

Reinforced concrete (RC) beams strengthened with fiber reinforced polymers (FRPs) are structurally complex and prone to plate-end (PE) debonding. In this study, considering the extremely complicated nonlinear relationship between the PE debonding and the parameters, machine learning algorithms, namely, linear regression, ridge regression, decision tree, random forest, and neural network improved by sparrow search algorithm, are established to predict the PE debonding of RC beams strengthened with FRP. The results of reliability evaluation and parameter analysis reveal that ACI, CNR, fib-1, fib-2, and TR55-2 are a little conservative; AS and TR55-1 have the problem of overestimating the shear force; the accuracy and robustness of the SSA-BP model developed in this paper are good; the stirrup reinforcement has the greatest effect on PE debonding; and each parameter shows a complex nonlinear relationship with the shear force when PE debonding occurs.

Explainable machine learning model and reliability analysis for flexural capacity prediction of RC beams strengthened in flexure with FRCM

This paper presents a data-driven approach to determine the load and flexural capacities of reinforced concrete (RC) beams strengthened with fabric reinforced cementitious matrix (FRCM) composites in flexure. A total of seven machine learning (ML) models such as kernel ridge regression, K-nearest neighbors, support vector regression, classification and regression trees, random forest, gradient boosted trees, and extreme gradient boosting (xgBoost) are evaluated to propose the best predictive model for FRCM-strengthened beams. Beam geometry, internal steel reinforcement area, FRCM reinforcement area, and mechanical characteristics of concrete, steel, and FRCM are the main input parameters included in the database. Among the studied ML models, the xgBoost model is the most accurate model with the highest coefficient of determination (R2=99.3%) and least root mean square (RMSE), mean absolute error (MAE), and mean absolute percentage error (MAPE). A comparative study of the performance of the proposed and existing analytical models revealed the superior predictive capability and robustness of the proposed model. The predicted flexural and load capacities of the beams based on the existing analytical models are highly scattered and either over-conservative or unsafe. A unified SHapley Additive exPlanations approach is employed to explain the output of the best ML model and identify the most significant input features and interactions that influence the capacity of FRCM-strengthened RC beams in flexure. Furthermore, a reliability analysis is performed to calibrate the resistance reduction factor (ϕ) to achieve a specified target reliability index (βT=3.5).

Experimental and analytical investigation of using externally bonded, hybrid, fiber-reinforced polymers to repair and strengthen heated, damaged RC beams in flexure

PurposeThis study aims to conduct an experimental study and finite element model (FEM) to investigate the flexural behavior of heat-damaged beams strengthened/repaired by hybrid fiber-reinforced polymers (HFRP).Design/methodology/approachTwo groups of beams of (150 × 250 × 1,200) mm were cast, strengthened and repaired using different configurations of HFRP and tested under four-point loadings. The first group was kept at room temperature, while the second group was exposed to a temperature of 400°C.FindingsIt was found that using multiple layers of carbon fiber-reinforced polymer (CFRP) and glass fiber-reinforced polymer (GFRP) enhanced the strength more than a single layer. Also, the order of two layers of FRP showed no effect on flexural behavior of beams. Using a three-layer scheme (attaching the GFRP first and followed by two layers of CFRP) exhibited increase in ultimate load more than the scheme attached by CFRP first. Furthermore, the scheme HGC (heated beam repaired with glass and carbon, in sequence) allowed to achieve residual flexural capacity of specimen exposed to 400°C. Typical flexural failure was observed in control and heat-damaged beams, whereas the strengthened/repaired beams failed by cover separation and FRP debonding, however, specimen repaired with two layers of GFRP failed by FRP rupture. The FEM results showed good agreement with experimental results.Originality/valueFew researchers have studied the effects of HFRP on strengthening and repair of heated, damaged reinforced concrete (RC) beams. This paper investigates, both experimentally and analytically, the performance of externally strengthened and repaired RC beams, in flexure, with different FRP configurations of CFRP and GFRP.

Performance of FRCM composites and FRCM-strengthened RC beams subjected to anodic polarization and cyclic loading

Fabric reinforced cementitious matrix (FRCM) systems can provide structural strengthening (SS) of aging reinforced concrete (RC) structures. Furthermore, impressed current cathodic protection (ICCP) has been integrated with FRCM systems to develop ICCP-SS intervention systems to help improve the long-term performance of FRCM-strengthened RC structures. However, there is a lack of research regarding the fatigue performance of FRCM composites and FRCM-strengthened RC members. This study investigated the effects of fabric layers in carbon-FRCM, anodic polarization in the process of ICCP, and cyclic loading conditions on the fatigue performance of FRCM composites in tension and FRCM-strengthened RC beams in flexure. Cyclic tensile loading tests of FRCM specimens and cyclic four-point bending tests of beam specimens were conducted. Cyclic tensile test results showed that the fatigue life performance of FRCM was superior to that of steel rebar and inferior to that of carbon fiber reinforced polymer (CFRP, a composite that commonly has an epoxy resin matrix). The anodic polarization of ICCP degraded the fatigue life of carbon-FRCM at a given stress level and fatigue strength after two million loading cycles. The cyclic bending test results indicated that the fatigue performance of the beams was largely dependent on the number of fabric layers in the FRCM and slightly influenced by the anodic polarization in the ICCP-SS. A new curve of the steel stress range versus the number of cycles (S-N curve) for FRCM-strengthened beams was obtained in this study and provided satisfactory fatigue life predictions.

External strengthening of reinforced concrete beam with opening by bamboo fiber reinforced composites

This paper presents the structural behaviour of reinforced concrete (RC) beams with and without openings strengthened externally with bamboo fiber reinforced composite (BFRC) plates in shear and flexure, respectively. Mechanical properties include tensile and flexural strength of epoxy, polyester and vinyl-ester based BFRC plates with 0%, 10%, 20%, 30% and 40% fiber volume fractions were evaluated. A total of fourteen beams were cast to evaluate the structural behaviour of RC beams strengthened with BFRC plates. All the beams were tested to failure under four-point bending. The results presented were in terms of load–deflection behaviour, failure mode and crack pattern. A comparison was also made between the performance of epoxy, polyester and vinyl-ester based BFRC plates in shear strengthening of RC beams with openings. Results revealed that the presence of openings in the shear zone reduced the original beam capacity of the control beam to about 52–55%. Shear strengthening of RC beams with openings using epoxy based BFRC plates showed significant improvement in regaining the beam structural capacity to approximately 32–36% higher than the un-strengthened beams. Meanwhile, strengthening of RC beams in flexure with epoxy based BFRC plates managed to regain the beam original capacity up to 98% of the control beam. Bamboo fiber composite reinforced with epoxy, polyester and vinyl-ester resins of 40% fiber volume fraction managed to regain the beam original capacity up to 82%. It was found that BFRC plates could divert and mitigate the formation of cracks away from the strengthened region as well as improved the beam ductility.

Performance of Corroded Reinforced-Concrete Beams in Flexure Strengthened Using Different Basalt Fiber Textile-Reinforced Mortar Schemes

The need for an effective method for the restoration of corrosion-damaged reinforced-concrete elements cannot be overemphasized. In this investigation, the use of textile-reinforced mortar (TRM) for external strengthening of corroded reinforced-concrete beams in flexure is examined. The degrees of corrosion considered were 10% and 20% mass losses of the tension steel at midspan. Sixteen beams were tested in four-point bending; eight beams were cast using normal grade 35 concrete and eight beams were cast with a special high-strength concrete with steel fibers. Three strengthening schemes of a single-ply basalt TRM, covering an equal area on individual beam surfaces, were employed. As expected, the corrosion had a detrimental effect on load-carrying capacity, ductility, energy absorption, and flexural stiffness of RC beams. The strengthening schemes increased the load capacity of the 10% corroded beams within a range of 7.5%–26%, thereby yielding a restoration ranging from 105% to 112% of the peak load capacity. Full recovery of load capacities was not achieved for the 20% corroded beams. The ultimate failure mode observed in all strengthened specimens was fiber rupture, indicating full utilization of composite action of basalt TRM. A U-wrap scheme gave the best load–displacement response and energy absorption recovery.

Effect of U-wrap anchors on the strength and ductility of externally bonded RC beams with mortar bonded GSM sheets

Strengthening of deficient reinforced concrete (RC) structures had been one of the major concerns of researchers over the past few decades. Thus, new materials are being discovered to ease the process of externally strengthening RC structures. One of the various strengthening materials is Galvanized Steel Mesh (GSM) that is available in different cord densities. High and low cord-density GSM sheets can be externally bonded to RC structures using epoxy adhesives, while only low cord-density GSM sheets can be externally bonded with cement Mortar. In this experimental study, low cord-density GSM sheets (3.15 cords/cm) were bonded with cement mortar to the soffit of RC beams surfaces. A total of four RC beams were cast, three of which were externally strengthened in flexure with GSM sheets and bonded with mortar and the remaining beam was kept as a control un-strengthened specimen. In order to avoid the brittle delamination failure mode, one of the strengthened specimens was anchored with end U-Wrap carbon-fiber reinforced polymer (CFRP) sheets, while the other strengthened specimen was anchored with intermediate U-Wrap CFRP sheets, and the remaining third strengthened specimen was not anchored, to serve as a control strengthened specimen. All specimens were subjected to four-point loading test and the load-deflection response curve of each specimen was recorded. It was observed that flexural strengthening using GSM sheets increased the ultimate-load capacity of strengthened specimens by a range of 41.8 to 51.4 %, when compared to control un-strengthened specimen. While, no strength increase was observed when comparing anchored strengthened specimens to non-anchored strengthened specimens. However, the ductility of end anchored specimens increased by 142.1 and 270% when compared to control and non-anchored specimens, respectively. In addition, the ductility of intermediate anchored specimens increased by 134.1 and 257.3 % when compared to control and non-anchored specimens, respectively. Thus, it can be concluded that GSM laminates if properly anchored can enhance the strength and ductility of RC beams in flexure.

Performance of externally strengthened RC beams with side-bonded CFRP sheets

The conventional method of strengthening reinforced concrete (RC) beams in flexure is via bonding carbon-fiber reinforcement polymer (CFRP) laminates to the beam’s soffit. However, the beam’s soffit could be narrow or inaccessible for strengthening. To overcome such obstacles, this paper explores the feasibility of strengthening RC beams in flexure by side-bonded CFRP composite sheets. Accordingly, a total of nine RC beams have been cast, eight of which were strengthened in flexure with different configurations of bottom-bonded and side-bonded CFRP sheets, and tested under four-point bending till failure. The load-deflection response curves, failure modes, and ductility of the tested specimens were recorded and compared. Overall, it is observed that specimens strengthened with similar amount of reinforcement are comparable with percent increase in the flexural strength over the control beam ranged from 62 to 92% for bottom-bonded and 39.7–93.4% for side-bonded strengthened beams. In addition, an analytical model based on ACI 440.2R-08 guidelines is developed and the predicted flexural strength was in good agreement with experimental results with differences ranging between 2.4% and 6.8%. It is concluded that the proposed side-bonded strengthening scheme is a valid alternative to the bottom-bonded scheme of strengthening of RC beams in flexure.

Flexural behavior of reinforced concrete beams strengthened with externally bonded Aluminum Alloy plates

The objective of this experimental investigation is to study the viability and effectiveness of using Aluminum Alloy (AA) plates as externally bonded flexural reinforcement for reinforced concrete (RC) beams. Ten RC beams were prepared and nine of them were strengthened with externally bonded 2 mm and 3 mm thick AA plates with different mechanical properties. Four strengthened beams had no end wraps or anchorages. Single-layer and double-layer U-wrap CFRP sheets were used in the transverse direction as end anchorages for four strengthened beams and one beam had three double anchorages (two at the ends and one at mid-span). The beams were tested under monotonic load until failure. The goal is to study the effect of using AA plates as externally bonded flexural strengthening material and to explore the effect of end anchorages on the flexural strength and ductility of these beams. The increase in strength over the control unstrengthened specimen ranged from 13% to 40% while the ductility significantly surpassed that of beams strengthened with CFRP sheets. It is observed that the use of end anchorages enhanced the ductility but not the strength of the tested beams. It is also observed that beams without end anchorage failed predominantly in flexure with full de-bonding while beams with end anchorage failed by localized de-bonding and flexure. Furthermore, the performance of the tested beams was compared with numerical predictions by a computer program developed in this study. The results of the numerical models were in close agreement with the measured experimental data. It was concluded that AA plates could be used as an external strengthening material to enhance both the strength and ductility of RC beams in flexure.

Behaviour of Reinforced Concrete Beams Strengthened by CFRP Wraps with and without End Anchorages

Use of Carbon Fibre Reinforced Polymer (CFRP) strips as externally bonded reinforcement, is a technically sound and practically efficient method of strengthening and upgrading reinforced concrete (RC) members. Externally bonded CFRP strips help in improving the structure performance by reducing deflections and/or cracking and increasing ultimate strength. The ultimate capacity of the strengthened beam is controlled by either compression crushing of concrete, rupture of CFRP and flexural shear cracking induced de-bonding atconcrete-CFRP interface. Present study deals with the use of externally bonded CFRP wraps instead of strips to strengthen RC beams in flexure with and without end anchorages. Six beams strengthened with CFRP wraps at bottom with and without anchorages, were tested with different a/d ratios. It was observed that use of CFRP wraps resulted in significantly increasing the stiffness and ductility of the RC beams along with increasing load carrying capacities of the strengthened RC beams with end anchorages.

Experiments of Chloride Ingression in Flexural Reinforced Concrete Beams

This paper focused on the chloride ions' ingression in reinforced concrete beams under flexural loading. Effect of different factors on the chloride ingression in the reinforced concrete beam was investigated. Those factors are water to cement (w/c) ratio, mid-span crack width, chloride ingression method and flexural loading history. In total 20 reinforced concrete beams having different w/c ratios of 0.53, 0.43 and 0.36 were tested. Flexural load was applied to beams by using self-anchoring at both ends. The nominal mid-span crack width designed value range from 0.1 mm to 0.25 mm. Two chloride ingression methods, namely long term immersion and wet-dry cycling are used. Three loading history were also designed for comparison. Chloride ions' contents at different depth in different location of beams were obtained. In addition, chloride diffusion coefficients were also calculated for comparison. It is shown that both the crack and tensile stress have effect on the chloride ions' ingression in reinforced concrete beams.

Performance of NSM-FRP RC beams in flexure and shear using locally developed CFRP rebars

In the present study, an experimental investigation was carried out to evaluate the flexural and shear capacity of near surface mounted (NSM) reinforced concrete (RC) beams using carbon fibre reinforced polymer (CFRP) rebars. A total of 16 half scale beams were tested to study the flexural and shear responses of strengthened RC beams. The CFRP rebars were manufactured at structural testing centre using locally developed twisting equipment to strengthen the RC beams. Also, new anchor system was locally developed to predict the tensile characteristics of CFRP bars using universal testing machine. To examine the flexural and shear behaviour, all beams were tested under two-point loading system. It was observed that the NSM strengthened RC beams have higher load carrying capacity than the corresponding control beam in flexure and shear by 68% and 35.6%, respectively. Typical failure modes of the beams during the experimental investigation were CFRP rupture, CFRP debonding, crushing of concrete and shear failure. Finally, a design example has been presented for better understanding in the analysis and design of NSM FRP strengthened RC beams. Further, the values obtained from the analytical study are in good agreement with the experimental values.

Strengthening RC Beams in Flexure Using New Hybrid FRP Sheet/Ductile Anchor System

This paper explores a new hybrid fiber-reinforced polymer (FRP) sheet/ductile anchor system for rehabilitation of reinforced concrete (RC) beams. The advantages of the proposed strengthening method is that it overcomes the problem of low ductility that is associated with brittle failure mode in conventional methods of strengthening beams using epoxy-bonded FRP sheets. The proposed system leads to a ductile failure mode by triggering yielding to occur in a steel anchor system (steel links) rather than by rupture or debonding of FRP sheets, which is sudden in nature. Four half-scale RC T-beams were tested under four-point bending. Three retrofitted beams were strengthened using one layer of carbon FRP sheet. The results of the two beams that were strengthened with the new hybrid FRP sheet/ductile anchor system were compared with the results from the beam strengthened with conventional FRP bonding method and the control beam. The results show the effectiveness of the proposed strengthening system in increasing flexural capacity and ductility of RC beams.

Pitting corrosion and structural reliability of corroding RC structures: Experimental data and probabilistic analysis

A stochastic analysis is developed to assess the temporal and spatial variability of pitting corrosion on the reliability of corroding reinforced concrete (RC) structures. The structure considered herein is a singly reinforced RC beam with Y16 or Y27 reinforcing bars. Experimental data obtained from corrosion tests are used to characterise the probability distribution of pit depth. The RC beam is discretised into a series of small elements and maximum pit depths are generated for each reinforcing steel bar in each element. The loss of cross-sectional area, reduction in yield strength and reduction in flexural resistance are then inferred. The analysis considers various member spans, loading ratios, bar diameters and numbers of bars in a given cross-section, and moment diagrams. It was found that the maximum corrosion loss in a reinforcing bar conditional on beam collapse was no more than 16%. The probabilities of failure considering spatial variability of pitting corrosion were up to 200% higher than probabilities of failure obtained from a non-spatial analysis after 50 years of corrosion. This shows the importance of considering spatial variability in a structural reliability analysis for deteriorating structures, particularly for corroding RC beams in flexure.

Spatial variability of pitting corrosion and its influence on structural fragility and reliability of RC beams in flexure

A stochastic process for assessing the effect of spatial variability of pitting corrosion on structural reliability and fragility is developed for singly reinforced simply supported reinforced concrete (RC) beams in flexure. A distribution of pitting corrosion is inferred from existing literature. The RC beam is discretised into a series of elements and maximum pit depths are generated for each reinforcing steel bar in each element. The analysis considered various member spans, loading ratios, bar diameters and number of bars in a given cross-section. Comparisons made with general corrosion showed that pitting corrosion is more critical for smaller diameter reinforcing steel bars. It was also found that up to a three-fold increase in 50-year probabilities of failure were observed if spatial variability of pitting corrosion are included in the analysis when compared to probabilities of failure based on the mid-span limit states only. This strongly suggests that the inclusion of spatial variability of pitting corrosion can lead to significant decreases in structural reliability for flexural RC members.