Strengthened Reinforced Concrete Beams Research Articles

Effect of fatigue loading on flexural performance of NSM CFRP-strengthened RC beams under different service temperatures

This paper presents an experimental work to investigate the effect of fatigue loading and high service temperature on the flexural performance of near-surface mounted (NSM) carbon fiber reinforced polymer (CFRP)-strengthened reinforced concrete (RC) beams. The experimental program included 11 beams, where the effect of the strengthening (CFRP) ratio (0.06 % and 0.18 %), fatigue load level (R1 = 0.57 and R2 = 0.38) and applied temperature (20 °C and 70 °C) have been considered. Experimental results on fatigue tests showed that high service temperature resulted in an increase in deflections and reduction in the stiffness of the specimens along the test. Moreover, the application of fatigue cycles with larger amplitude resulted in the failure of the specimens (by steel rupture), whilst specimens submitted to lower fatigue amplitude survived to the fatigue cycles. The NSM CFRP-strengthening helped to postpone the final failure of strengthened RC beams after steel reinforcement rupture during the fatigue test. Besides, damage due to fatigue derived in lower residual yielding and ultimate load in the post-fatigue test.

Strengthening Reinforced Beams Subjected to Pure Torsion by Near Surface Mounted Rebars

This paper investigates the possibility of strengthening Reinforced Concrete (RC) beams under pure torsion loadings. The torsional behaviour of strengthened RC beams with near-surface mounted steel and CFRP bars was investigated. The verification with the experimental work was performed to ensure the validity and accuracy which revealed a good agreement through the torque-rotation relationship, ultimate torque, and rotation, and crack pattern. This numerical study included testing of thirteen specimens (one of them was control beams while the remaining 12 were strengthened beams) with several parameters such as mounting spacing and configuration. The analytical results revealed that the addition of NSM rebar redistributed the internal stresses and enhanced the ultimate torsional strength, torque-rotation capacity, ductility, and energy absorption of the concrete beams. Most of the strengthened beams revealed the appearance of the cracks at a phase less than the reference beam by an average of (9%). Concerning the NSM strengthening, the CFRP bars provided a higher enhancement ratio when compared with the beams that strengthened with NSM steel rebar especially for the strengthening space equal to 130 mm and more. The ultimate torsional strength increased by (3.5%) and rotation decreased by (4%) approximately when the steel rebar was replaced by the carbon bar. The ductility and energy absorption of the analysed beams showed that the strengthening enhanced the ductility of the twisted beams. The ductility values varied according to the method of strengthening used, as it showed the highest values of the beam that was strengthened small spacing.

Flexural Strengthening of Reinforced Concrete Beams with Variable Compressive Strength Using Near-Surface Mounted Carbon-Fiber-Reinforced Polymer Strips [NSM-CFRP

An experimental and analytical investigation was conducted on reinforced concrete (RC) beams strengthened in flexure using the near-surface mounted carbon-fiber-reinforced polymers (NSM-CFRPs) technique. A total of 11 full-scale RC rectangular beams were cast and tested under a monotonic three-point bending test, up to failure. The main test variables adopted in this study were the concrete compressive strength (high, medium, and low), the number of CFRP strips, and the strip length. The results indicated that the use of NSM-CFRPs strips in different configurations efficiently increased the load-carrying capacity of the strengthened RC beams, in which all these beams exhibited a higher moment resistance than the corresponding un-strengthened beam. Results also showed that all strengthening schemes were successful in increasing the flexural capacity of the specimens tested. Such increases ranged between 10.36% and 52.28%. Notably, a significant improvement in the ultimate load ratio was observed with beams having a low compressive strength of 17-MPa, then followed by the beams with medium strength (32-MPa), and finally beams with high compressive strength (47-MPa). The NSM technique reduced the occurrence possibility of the CFRP de-bonding failure mode. Furthermore, the test results were compared with theoretical predictions using the ACI 440.2R17 guidelines and showed a good agreement between these results.

Experimental and analytical study for shear strengthening of reinforced-concrete beams using a prefabricated geopolymer–mortar panel

This study investigated the performance of geopolymer–mortar panels (GMPs) in improving the shear capacity of reinforced-concrete (RC) beams. A GMP with a thickness of 15 mm was prefabricated and fixed to the shear span of beams using mechanical anchors. Five RC beams were constructed and tested under a four-point bending monotonic load: one unstrengthened, three strengthened with a GMP, and one strengthened with a Portland-cement-mortar panel. The investigated parameters included (a) anchor-bolt spacing, (b) presence of a wire mesh, and (c) binding material types. The results indicated that the GMP is an effective technique for improving the shear capacity of RC beams. The maximum gain in the shear capacity of the strengthened beams ranged from 14.1% to 34.9% with respect to the unstrengthened beam. Moreover, the results suggested that the failure mode of the beam changed from brittle to ductile failure if an anchor bolt spacing of 200 mm and wire mesh were used at the GMP. Based on these findings, the GMP can be used as an alternative method for shear-strengthening RC beams in building applications owing to its high strength and ductility. Finally, an analytical model based on the simplified-modified–compression-field theory model was proposed to predict the shear capacity of RC beams strengthened with the GMP. The analytical model was in reasonable agreement with the test results.

Strategies for strengthening of corroded reinforced concrete beams using CFRP laminates and UHPC jacketing

AbstractThis article aims to investigate the flexural behavior of strengthened corroded reinforced concrete (RC) beams using carbon fiber‐reinforced polymer (CFRP) laminates and using a hybrid system of CFRP laminates and ultrahigh‐performance concrete (UHPC) layers. A total of 15 RC beam specimens were prepared, out of which one specimen was uncorroded–unstrengthened, and 14 specimens were corroded using accelerated corrosion set up to cause a significant reduction in the load‐carrying capacity of the RC beams. The damaged covers of the corroded RC beam specimens were first repaired and then strengthened with different strengthening strategies involving CFRP laminates alone as well as the CFRP laminates and UHPC jacketing together. The experimental results obtained by testing the strengthened RC beam specimens in flexure showed a significant enhancement in the load‐carrying capacity and stiffness of the strengthened corroded RC beams. The number of CFRP laminates, hybridization of the CFRP laminates and UHPC layer, and the thickness of the UHPC layer all significantly improved the load‐carrying capacity and stiffness of the strengthened corroded RC beams indicating the possibility of selecting an optimal strategy out of different options for strengthening the corroded beams to achieve a targeted degree of the efficacy of the strengthening. The analytical model developed in this study to estimate the flexural capacity of the strengthened RC beams was found to predict the values of the load‐bearing capacity of the RC beams strengthened using different strategies very close to their respective experimental values.

Flexural strengthening of RC beams with built-in basalt-fibre-reinforced polymer T-plates

Basalt-fibre-reinforced polymers (BFRPs) are expected to be excellent materials for the repair and rehabilitation of civil infrastructure due to their sound mechanical properties, high corrosion resistance and low cost. However, reported studies investigating reinforced concrete (RC) beams externally strengthened with BFRP are limited. The objective of this work was thus to determine the effect of BFRP strengthening on the flexural performance of RC beams. Four RC beams with a rectangular cross-section were fabricated, strengthened using different BFRP reinforcement techniques and tested under four-point bending until failure. The failure mode, crack widths, number of cracks, load–deflection response curves and ductility of the beam systems were recorded and analysed. The experimental results showed that, compared with a control beam, the stiffness and ultimate capacity of the strengthened beams were increased by 8–23% and 14–30%, respectively. To predict the load-carrying capacity and deflection of the test specimens, three-dimensional finite-element (FE) modelling was performed using Abaqus software to simulate the behaviour of RC beams strengthened with BFRP plates. The predictions of the FE model matched the experimental results well in terms of the mid-span deflection and load-carrying capacity.

Meso-scale analysis of shear performance and size effect of CFRP sheets strengthened RC beams

In order to reflect and reveal the shear failure process and mechanism of carbon fiber reinforced plastic (CFRP) sheets strengthened reinforced concrete (RC) beam, a mechanical analysis model is established by using a 3D meso-scale simulation method. The model considers the heterogeneity of concrete materials, the interaction behaviors between reinforcements and concrete, and the complex interaction behaviors between CFRP sheets and concrete, simultaneously. Different types of meso-scale models of RC beam strengthened with CFRP sheets are designed to explore the influences of stirrup ratio and CFRP fiber ratio on the nominal shear strength and size effect behaviors. Test results indicate that: 1) The action mechanisms of stirrups and CFRP sheets are similar; 2) The nominal shear strength of CFRP sheets strengthened RC beams increases with the increase of stirrup ratio and CFRP fiber ratio, while the degree of the increasing range decreases; 3) The nominal shear strength of RC beams strengthened with CFRP sheets decreases with the increase of section size, nevertheless increasing the stirrup ratio and the CFRP fiber ratio will weaken the size effect; 4) Based on the numerical test results, a novel size effect law simultaneously considering the stirrup ratio, the CFRP fiber ratio and the mutual effects between them is proposed.

Strengthening RC beams with mid-span supporting prestressed CFRP plates: An experimental investigation

This paper proposes a novel strengthening method for reinforced concrete (RC) beams using prestressed carbon fibre-reinforced polymer (CFRP) plates. In the proposed method, the CFRP plate is stressed by adjusting the position of the mid-span deviators. The strengthening process is fast and easy to apply because there is no need for hydraulic jacks or adhesives. To investigate the behaviour of the strengthened RC beams, a series of tests were conducted on both intact and damaged beams with different strengthening configurations, and the full range behaviours of the beams are reported in detail. According to the test results, the maximum stress of the CFRP plate reached 1500 MPa, and the beam strength was enhanced up to 43%. Some of the strengthening schemes achieved good ductility. As a result, the proposed strengthening method is found to be effective and can be further developed into a pseudoplastic CFRP strengthening system.

Examination of parameters impacting flexural and shear strengthening of RC beams

Structural performance of reinforced concrete (RC) structures should be improved due to new design standards, strength reduction, and/or functional changes throughout their service life. To reach a sufficient level of structural performance, the following two options can be considered: reconstruction or strengthening of structures. Although reconstruction has some advantages in terms of using current technological developments, this option can lead to some negative consequences such as interruption of the building's service life, relatively high cost, and sustainability issues. Therefore, strengthening an existing structure mostly stands out as the first choice. The techniques used in strengthening applications can be grouped into two different methods: traditional (addition of new structural members) and non-conventional (seismic base isolation, local retrofitting, and jacketing). Among innovative methods, the fibre-reinforced polymer strengthening method attracts attention due to several advantages including practical applicability, as well as shear and bending capacity increase. Its application to the outer surface of structural members using resin reduces deterioration. Plus, fibres oriented in various directions spread stresses in different directions, therefore, provide effective force distribution. In this study, the effect of carbon fibre reinforced polymer (CFRP) orientation and RC material properties in the strengthening RC beams were investigated using MATLAB software. Accordingly, all stages of the design process are presented for RC beam strengthening considering both flexural and shear effects based on the American Concrete Institute (ACI) 440.2R standard. Through compiling MATLAB code, calculation time reduces, and material characteristics can be obtained more accurately. Plus, using curves obtained by MATLAB coding, shear and bending capacity increases can be observed. According to our findings, the application of one-layer CFRP plate to an RC beam increases the bending capacity by 50.6% and shear capacity by 33.6%. However, as the number of layers increases, the capacity increase rate reduces.

Shear strengthening of RC beams with NSM FRP. I: Review of strength models

Near-surface mounted (NSM) fiber-reinforced polymer (FRP) technique is known as a promising alternative to the externally bonded (EB) FRP technique for strengthening reinforced concrete (RC) members, on account of its many advantages such as high bond efficiency with concrete and good durability. Shear strengthening of RC beams by near-surface mounting the FRP bars/strips into the concrete cover on beam sides is one of the most prevalent applications of NSM FRP. Abundant experimental studies have been conducted to investigate the behaviour of NSM FRP shear strengthened beams, and many strength models for predicting the contribution of NSM FRPs to the shear capacity of the beam have been proposed. The present paper presents a comprehensive review of these strength models, in which all the models collected from the existing literature are first classified into three categories based on their used approach and then summarized and discussed. This paper not only aims to bring a deep understanding of the existing strength models for NSM FRP shear strengthened RC beams, but also provides a background and basis for the assessment of these models by using the newly-generated experimental database in the companion paper.

Delamination buckling of GFRP-strips in strengthened RC beams

Fiber Reinforced Polymer (FRP) strips are convenient materials in the strengthening of damaged RC beams of bridges due to corrosion or impact loading and in the rehabilitation of RC beams damaged by seismic actions. The behaviour of Externally Bonded (EB) FRP strips has been extensively studied when the strengthening is subjected to tensile stresses, even if, in many cases, due to cyclic actions, FRP strips glued to the concrete surface may be subjected to compression. Compressive stresses on the FRP strip may cause a buckling mechanism with detachment.In this work, an investigation of reinforced concrete (RC) beam models undamaged and damaged and strengthened with GFRP strips on compressive and tensile sides is shown. Beam models were subjected to four-point bending tests and the delamination buckling mechanism of the GFRP strip has been described. Further, analytical models to study the delamination buckling of strengthening with FRP strips are developed. Finally, experimental and theoretical data are discussed.

Time-dependent behavior of NSM CFRP-strengthened RC beams under different service temperatures

The use of fiber reinforced polymer (FRP) for flexural strengthening of reinforced concrete (RC) beams has become a popular strengthening technique. Significant amount of work is available on the short-term flexural behavior of RC beams strengthened with near-surface mounted (NSM) technique. However, their time-dependent flexural behavior, specifically under high service temperature, has not yet been addressed. This paper presents an experimental work to evaluate the time-dependent behavior of NSM carbon FRP (CFRP)-strengthened RC beams. The experimental program included 23 beams, where the effect of different parameters such as strengthening (CFRP) area, steel reinforcement ratio and applied temperature (20 and 50 °C) have been considered. Experimental results show that the effect of strengthening area is significant on the flexural short-term response of the beams, while minor effects are found on the time-dependent deflections. On the other hand, increasing the service temperature has no significant effect on the short-term tests, but it produces a large increase in the time-dependent deflection of the specimens. Finally, an analytical procedure for the prediction of time-dependent deflections, which is based on the age-adjusted effective modulus method (AEMM), is presented. Good agreement between the experimental results and analytical predictions on time-dependent deflections is shown.

Investigation of the flexural behavior of reinforced concrete beams strengthened with a composite reinforcement layer: Polyvinyl alcohol fiber‐reinforced ferrocement cementitious composite and steel wire mesh

AbstractTo improve the flexural behavior of reinforced concrete (RC) beams, a new ferrocement composite material was proposed. A polyvinyl alcohol fiber‐reinforced ferrocement cementitious composite (PVA‐RFCC) and steel wire mesh (SWM) were attached onto the soffit of the RC beams as a composite reinforcement layer (CRL). One control beam and four strengthened RC beams were investigated. The maximum load carrying capacity, initial stiffness, and displacement ductility parameter of the strengthened beams were up to 99.3%, 48.0%, and 90.9% higher than those of the control beam, respectively. The SWM2 accounted for 91.6% of these improvements. An analytical model was proposed to predict the flexural capacity of strengthened RC beams. The tensile force of the polyvinyl alcohol‐engineered cementitious composite was considered and calculated by rectangular and triangular distributions. The predicted results were in good agreement with the experimental results, which validated the accuracy of the proposed model.

Shear strengthening of RC shear-deficient beams with highly ductile fiber-reinforced concrete

This study presents an experimental investigation of reinforced concrete (RC) beams strengthened in shear with highly ductile fiber-reinforced concrete (HDC) jacket. Two batches of beam specimens were loaded under the three-point bending test to determine their ultimate shear capacities. The test variables in this experiment included the presence or absence of original stirrups, thickness of the U–shaped jacket (15 and 25 mm), and the presence or absence of strengthening stirrups in jackets. The failure modes, load–deflection curves, and load–strain curves were described and analyzed. The test results reveal that the HDC strengthened RC beams have improved failure modes and cracking patterns than the unstrengthened beams. The maximum gain in the shear capacity of the strengthened beams ranged from 8.4 to 80.67 % with respect to the reference beam. Moreover, peak deflection can be increased by up to 160.6 %. The strengthening stirrups and original stirrups exhibit a similar strain growth trend, which indicates that the HDC jacket and RC beam work well together, enabling the strengthening stirrups to be effectively utilized. Furthermore, in order to calculate the shear capacity of the HDC strengthened beams, three methods were employed and evaluated.

Using NSM pre-stressed rebars for flexural strengthening of RC beams subjected to constant loading

The present study introduces an innovative technique for flexural strengthening of reinforced concrete (RC) beams subjected to constant loading and explores its efficacy through experimental evaluation. The technique involves the application of near-surface mounted (NSM) pre-stressed rebars, which exploits the advantages of both the NSM technique, as an effective one commonly used in strengthening RC beams, and pre-stressing, as an effective technique to eliminate excessive cracking and deformation. The experimental program consists of eight identically reinforced RC beams. Before strengthening, longitudinal grooves are cut out along the tensile face of the specimens in which steel bars are placed and anchored by end plates and an epoxy adhesive. Some of the bars are pre-stressed to a predetermined level using an end bolt and a torque-meter wrench. The test results indicate that the NSM pre-stressed steel rebars lead not only to almost identical amounts of significant increases in the ultimate flexural capacity of RC beams but also to decreased initial cracking and deflection.

Full-scale experimental evaluation of flexural strength and ductility of reinforced concrete beams strengthened with various FRP mechanisms

Carbon fiber reinforced polymer (CFRP) is used for enhancing the structural performance of reinforced concrete (RC) beams. Due to the high resistance of FRP against corrosion and decay, as well as its low specific weight, it has good luck in strengthening the main structural members. Since the FRP system is mainly used for retrofitting than design, its implementation methods are important. In this research, four various implementation methods are evaluated and defined as CFRP sheet, CFRP laminate, Near-surface mounted (NSM method) and combination of NSM and laminate (Hybrid). Full scale tests were done by 1000 kN capacity hydraulic jack. Four points loading is considered and simple support is used. Validation analysis is done and compared with ACI instruction and reasonable results have been achieved. In the past works extensive research has been done on the retrofitting RC beams with FRP and more focused on the internal FRP bars and NSM method. Various external methods are used for strengthening and retrofitting of the RC beams and all tests were done on the real size and full-scale beams. This paper presents an experimental program on five RC beams. One beam is defined as control beam that constructed with typical material and other four beams are retrofitted with various external FPR methods. Findings from the present work shows that maximum flexural capacity achieved by hybrid method. Moreover, the main achievement of this research is that hybrid type led to increase in the ductility and hardening than control beam.

Flexural behavior of recycled aggregate concrete beams strengthened with carbon fiber reinforced polymer

Using recycled aggregate in the construction of reinforced concrete (RC) buildings has become essential to reduce the waste produced from demolished buildings. In this paper, the influence of using carbon fiber reinforced polymer (CFRP) in strengthening of RC beams with recycled coarse aggregate (RCA), as a partial/full replacement for natural coarse aggregate (NCA), was experimentally investigated. Twelve RC beams with various RCA ratios (0%, 30%, 70%, and 100%) were prepared and tested under four-point loading. Four beams without CFRP were tested till failure while the other eight beams were subjected to service loads then strengthened using CFRP and re-tested till failure. The cracking pattern, deflection, as well as strain in longitudinal and transversal reinforcement were recorded. The failure loads of the tested beams were compared to those predicted from various international design codes. Results showed that the flexural cracks appeared in specimens with 100% RCA earlier than those with NCA. Moreover, the increase in RCA% resulted in resisting tensile stresses by stirrups at a lower load value and achieving a more ductile behavior. Also, the experimental results showed that including RCA in beams slightly decreased the failure load for both strengthened and un-strengthened RC beams. Furthermore, beams with higher amount of RCA experienced more deflections than those with lower amount of RCA. Such behavior was found more prominent in the CFRP strengthened beams.

Assessment of the Integrity of Reinforced Concrete Beams Strengthened With Carbon Fibre Reinforced Polymer Using the Acoustic Emission Technique

Today, the use of a non-destructive technique to assess the integrity of reinforced concrete structures strengthened with carbon fibre-reinforced polymer (CFRP) is becoming increasingly important. It is highly important to assess the behaviour of the structure under load. This paper presents the evaluation of a reinforced concrete beam laminated with CFRP at the soffit using the acoustic emission technique. Two types of beams were made, a normal reinforced concrete beam and a reinforced concrete beam laminated with CFRP. CFRP was used to reinforce the bottom part of the beam. The beams were subjected to three-point loading and loaded to failure. During the loading tests, the integrity of the beams was monitored using the acoustic emission technique, and the crack patterns were observed visually. The intensity analysis was carried out on two sensors, designated as CH6 and CH7. CH6 and CH7 were located on top of the beam. Based on the intensity analysis of the acoustic emissions, five intensity crack zones were identified, namely zone A-no crack, zone B-minor, zone C-intermediate, zone D-follow up and zone E-major. With increasing load, which tended to progress the crack modes in the beam, the plots in the intensity zones developed for each crack mode from zone A to zone E. The crack progression matched well with the plots in the intensity zones. Using the intensity zones enables the early detection and prediction of damage.

Environmental Impact on the Behavior of CFRP Sheet Attached to Concrete

Carbon fiber-reinforced polymer (CFRP) has many advantages as a construction/structural-strengthening material. However, there are still concerns regarding the long-term performance of these materials when used with reinforced concrete (RC) structures. Environmental conditions have an adverse effect on the behavior of CFRP and the bond between these sheets and concrete. Therefore, the durability of CFRP used for strengthening RC beams was evaluated under different environmental scenarios, including subjection to immersion in deicing agents, tap water, and saltwater, freeze-and-thaw cycles, and outdoor environmental changes. Laboratory tests were performed to examine the influence of these environmental scenarios on the bonding behavior between CFRP sheets and concrete in terms of deformations and modes of failure. Two types of test setups were performed in this study, namely pull-off shearing and three-point bending. Forty-two concrete prisms with CFRP were prepared and tested by using the pull-off shearing setup. It was observed that as the period of exposure increased, noticeable effects on the CFRP sheet as well as the bond stiffness were observed. Exposure to tap water had a greater impact than saltwater on the CFRP–concrete bond strength as well as the CFRP. In addition, eighteen notched concrete beams strengthened with an external CFRP were tested under three-point bending. The tap water exposure showed a 3.6% increase in the bond strength compared to the control specimen. However, the saltwater exposure showed a 10% increase.

Dynamic behavior of CFRP strengthen RC beams based on digital image correlation technology

In this paper, impact behaviors of reinforced concrete (RC) beam and carbon fiber reinforced plastic (CFRP) strengthen RC beam (CFRP-RC beam), including the damage process, failure pattern, impact force peak value and peak deflection were investigated by means of using three-point bending tests with a drop-hammer impact device. A high-speed camera combined with digital image correlation (DIC) method was used to capture the initiation and propagation process of impact multiple-cracks and to record the cracking strain. The results showed that the changes of hammer impact height and bonded CFRP on beams had different effects on dynamic response of beams, and they also caused change of failure pattern of beams. Introduce of CFRP can significantly reduce the crack widths and mid-span deflections of beams and improve the impact resistance of beams. In addition, the crack velocity in RC beam was inhibited effectively by CFRP, however, the inhibitory effect declined as impact velocity increased.