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This study investigates the shear behavior of reinforced concrete (RC) beams that have been strengthened using carbon fiber reinforced polymer (CFRP) grids with engineered cementitious composite (ECC) through finite element (FE) analysis. The analysis includes twelve simply supported and continuous beams strengthened with different parameters such as CFRP sheets, CFRP grid cross-sectional area, and CFRP grid size. To conduct the analysis, FE models of the RC beams were created and analyzed using ABAQUS software. Research results show that the strengthened RC beams with CFRP grids and ECC had approx. 30–50% higher shear capacity than reference RC beams. The composite action of CFRP grids with the ECCs also showed a significant ability to limit diagonal cracks and prevent the degradation of the bending stiffness of the RC beams. Furthermore, this study calculated the shear capacity of the strengthened beams using an analytical model and compared it with the numerical analysis results. The analytical equations showed only a 4% difference from the numerical results, indicating that the analytical model can be used in practice.

To develop feasible carbon fiber reinforced polymer (CFRP) retrofit schemes for the shear strengthening of real three-dimensional reinforced concrete (RC) beam-column joints, a series of parameters in relation to the contributions of the CFRP sheets externally bonded to joint panels was numerically investigated in this study. The parameters include CFRP reinforcement ratio, CFRP layout, transverse beam-to-joint panel width ratio, transverse beam-to-joint panel height ratio, location of transverse beam, and number of transverse beams. Strengthening efficiency, a new dimensionless index, was introduced to evaluate the residual effect of a CFRP-strengthening system weakened by the presence of transverse beams in comparison with the increase in joint shear capacity in relation to the one-way counterpart. The results obtained from 44 nonlinear finite element models, which were calibrated against experimental observations, confirmed the effectiveness of the CFRP strengthening technique with regard to the relatively wide ranges of the parameters. The significant differences among the roles of the parameters were revealed, and the reasons behind the differences were analyzed. Furthermore, the shear mechanism of the CFRP-retrofitted joint panels was discussed with the proposed strut-and-tie model.

International Transaction Journal of Engineering, Management, & Applied Sciences & Technologies, 13, 2, 13A2P: 1-7

Existing reinforced concrete (RC) structures suffer from degradation in their structural capacity. These structures require strengthening and retrofitting to integrate sustainability and improve their serviceability and durability. RC members strengthened with fiber-reinforced polymer (FRP) composites usually suffer from FRP debonding; therefore, researchers proposed several types of sustainable materials to overcome the shortcomings of FRP composites. Limited experimental studies have been conducted for shear strengthening of RC beams using sustainable fabric-reinforced cementitious matrix (FRCM) composites; moreover, the application of 3D-printed strips in strengthening RC beams has never been established. The current research experimentally investigates the efficiency of FRCM composites, 3D-printed sheets (CD), and CFRP plates (CP) in strengthening RC beams that are weak in shear. Various strengthening configurations were adopted, including vertical, oblique, zigzag, and several-slanted layouts. Eight simply supported beams were prepared to find the most efficient shear-strengthening configuration and material for RC beams. Test results showed that FRCM and CP are both efficient for shear strengthening in terms of maximum load capacity, initial stiffness, and ductility. However, CD showed a limited effect on enhancing the performance of shear-strengthened beams. The best shear enhancement was found in the beam strengthened with vertical CP, with improvements in load-carrying capacity, stiffness, and ductility of 43%, 23%, and 23%, respectively. The vertical and oblique strengthening configurations were more efficient than the zigzag and several-slanted layouts. The ACI 440.2R-17 model yielded accurate predictions with an average (Vc, test/Vc, ACI 440) of 1.11.

The favorable contribution of externally bonded fiber-reinforced polymer (EB-FRP) sheets to the shear strengthening of reinforced concrete (RC) beams is widely acknowledged. Nonetheless, the premature debonding of EB-FRP materials remains a limitation for widespread on-site application. Once debonding appears, it is highly likely that brittle failure will occur in the strengthened RC structural member; therefore, it is essential to be alerted of the debonding incident immediately and to intervene. This may not be always possible, particularly if the EB-FRP strengthened RC member is located in an inaccessible area for fast inspection, such as bridge piers. The ability to identify debonding immediately via remote control would contribute to the safer application of the technique by eliminating the negative outcomes of debonding. The current investigation involves the detection of EB-FRP sheet debonding using a remotely controlled electromechanical admittance (EMA)-based structural health monitoring (SHM) system that utilizes piezoelectric lead zirconate titanate (PZT) sensors. An experimental investigation on RC T-beams strengthened for shear with EB-FRP sheets has been performed. The PZT sensors are installed at various locations on the surface of the EB-FRP sheets to evaluate the SHM system’s ability to detect debonding. Additionally, strain gauges were attached on the surface of the EB-FRP sheets near the PZT sensors to monitor the deformation of the FRP and draw useful conclusions through comparison of the results to the wave-based data provided by the PZT sensors. The experimental results indicate that although EB-FRP sheets increase the shear resistance of the RC T-beams, premature failure occurs due to sheet debonding. The applied SHM system can sufficiently identify the debonding in real-time and appears to be feasible for on-site applications.

In this paper, the influence of the strengthening configuration on the structural response of U-wrapped SRG (Steel-Reinforced Grout) shear-strengthened reinforced concrete beams was analyzed both experimentally and theoretically. The parameters varied were the ratio sf/wf, sf being the distance between two consecutive U-shaped strips and wf the width of the strips, and the number of steel fabric layers (one and two). Two series of real-scale beams, eleven beams in total with one un-strengthened and ten strengthened, were tested up to failure (the results obtained in the first series of tests were reported in a previously published paper). The obtained results highlighted that the debonding phenomena, which occurred for all the examined strengthening configurations, strongly affected the shear capacity of strengthened beams. An analytical model found based on the “effective strain” of the SRG strips was developed to predict the shear capacity of SRG shear-strengthened reinforced concrete beams. Two analytical relationships were proposed to evaluate the “effective strain” of the SRG strips: The first one was based on a statistical analysis of results obtained by tests on FRCM (Fabric-Reinforced Cementitious Mortar) and SRG shear-strengthened reinforced concrete beams, while the second was developed through the results of single lap direct shear tests performed on SRG-to-concrete joints. The effectiveness of the proposed analytical methods was evaluated through the comparison of their predictions and available experimental results.

This paper will present the shear behavior of the corroded stirrups reinforced concrete (RC) beams strengthened with textile reinforced concrete (TRC). The shear resistance contributions from the corroded RC beams and carbon TRC at various volume ratios were carefully examined. Eighteen beams were tested, including twelve subjected by an electrochemically accelerated aging technique for 60 and 90 days to obtain the theoretical mass loss in their stirrups of 10% and 20%, respectively. The stirrups were locally corroded in the shear span. After the corrosion accelerating treatment, corrosion cracks on concrete surfaces were marked, and their widths were measured to observe their distributions. A three-point bending test was applied to obtain the shear performances of the corroded beams subjected to the monotonic and repeated loading. Eight corroded RC beams were strengthened using 2 and 3 U-wrap layers of bidirectional carbon textile. The shear behavior will be discussed, including the ultimate capacity, failure mode, load-deflection, load–strain relationship, and crack distributions. Compared to the controlled specimens, the averaged shear strength of corroded specimens decreased by 16.08% and 25.34%, corresponding to the degree of corrosion ranging from 12.3% to 23.6%. The experimental results also demonstrate that the shear capacities of the corroded RC beams strengthened with carbon TRC had been improved 60.6% compared to the severely corroded controlled specimens.

For bridges that are experiencing deterioration, action is needed to ensure the structural performance is adequate for the demands imposed. Innovate repair and strengthening techniques can provide a cost-effective means to efficiently and safely extend the service lives of bridges. The use of fiber reinforced polymer (FRP) systems for the repair and strengthening of concrete bridges is increasing in popularity. Recognizing the potential benefits of the widespread use of FRP, a research project was initiated to determine the most appropriate applications of FRP in Indiana and provide recommendations for the use of FRP in the state for the repair and strengthening of bridges. The details of the research are presented in two volumes. Volume 1 provides the details of a study conducted to (i) summarize the state-of-the-art for the application of FRP to concrete bridges, (ii) identify successful examples of FRP implementation for concrete bridges in the literature and examine past applications of FRP in Indiana through case studies, and (iii) better understand FRP usage and installation procedures in the Midwest and Indiana through industry surveys. Volume 2 presents two experimental programs that were conducted to develop and evaluate various repair and strengthening methodologies used to restore the performance of deteriorated concrete bridge beams. The first program investigated FRP flexural strengthening methods, with focus placed on adjacent box beam bridges. The second experimental program examined potential techniques for repairing deteriorated end regions of prestressed concrete bridge girders. Externally bonded FRP and near-surface-mounted (NSM) FRP were considered in both programs.

For bridges that are experiencing deterioration, action is needed to ensure the structural performance is adequate for the demands imposed. Innovate repair and strengthening techniques can provide a cost-effective means to extend the service lives of bridges efficiently and safely. The use of fiber reinforced polymer (FRP) systems for the repair and strengthening of concrete bridges is increasing in popularity. Recognizing the potential benefits of the widespread use of FRP, a research project was initiated to determine the most appropriate applications of FRP in Indiana and provide recommendations for the use of FRP in the state for the repair and strengthening of bridges. The details of the research are presented in two volumes. Volume 1 provides the details of a study conducted to (1) summarize the state-of-the-art methods for the application of FRP to concrete bridges, (2) identify successful examples of FRP implementation for concrete bridges in the literature and examine past applications of FRP in Indiana through case studies, and (3) better understand FRP usage and installation procedures in the Midwest and Indiana through industry surveys. Volume 2 presents two experimental programs that were conducted to develop and evaluate various repair and strengthening methodologies used to restore the performance of deteriorated concrete bridge beams. The first program investigated FRP flexural strengthening methods, with a focus on adjacent box beam bridges. The second experimental program examined potential techniques for repairing deteriorated end regions of prestressed concrete bridge girders. Externally bonded FRP and near-surface-mounted (NSM) FRP were considered in both programs.

Reinforced concrete (RC) beams that are shear-strengthened by externally bonded (EB) fiber reinforced polymer (FRP) often offer limited structural enhancement. This is due to the inherent weaknesses of EB-FRP strengthening, including the premature debonding of the FRP, its brittle rupture, and inadequate deformation capacity of the strengthening system. In this paper, two techniques of shear strengthening using U-wrapped carbon FRP (CFRP) and effective anchoring devices are proposed and tested with the aim of enhancing both the shear strength and the ductility of retrofitted RC beams. One of the techniques involves a hybrid-bonded (HB) CFRP system with adjustable normal pressure applied to the CFRP U-strips, and the other features the CFRP U-strips fastened by an H-type end anchor (EA). The main test variables of the former and latter shear-strengthening systems are the degree of pressure applied to the CFRP U-strips and the width of the deformation segment (or axial stiffness) of the H-type EA, respectively. The results indicate that both strengthening systems significantly enhanced the shear capacity and ductility of the strengthened RC beams. Compared with the control member, the increments in shear capacity were as high as 54.6 and 68.5% for beams retrofitted with the HB-FRP and the EA FRP systems, respectively, and their deflections at peak shear load increased by 43.9 and 84.1%, respectively. The shear failure modes were found to be related to the parameters used in both the HB-FRP and the EA FRP systems. The critical diagonal crack (CDC) inclination of all specimens was less than 45°, ranging from 38° to 44°. Both positive and negative shear interactions were observed between Vf and Vc plus Vs.

The effectiveness of the Near Surface Mounted (NSM) technique with Carbon Fiber Reinforced Polymer (CFRP) laminates for the shear strengthening of relatively high T cross section (h = 600 mm) Reinforced Concrete (RC) beams was assessed by experimental research. Two inclinations of the laminates were tested (52° and 90°) and, for each one, two percentages of CFRP were adopted. The RC beams shear strengthened with NSM laminates had a percentage of steel stirrups of 0.09%. The experimental results showed that NSM shear strengthening technique with CFRP laminates is very effective in RC beams of relatively high cross section height, not only in terms of increasing the load carrying capacity of the beams, but also in assuring higher mobilization of the tensile capacity of the CFRP. Furthermore, inclined laminates were more effective than vertical laminates and the shear capacity of the beams increased with the percentage of laminates. Taking into account available experimental results obtained with the same test setup, but using RC beams of lower depth of T cross section (h = 400 mm), it can be concluded that the effectiveness of the NSM technique with CFRP laminates increase with the increase of the depth of the cross section. The formulations provided by Nanni et al. (2004) and Dias and Barros (2013) for the NSM shear strengthening technique predicted a CFRP contribution around 51% and 67%, respectively, of the experimentally registered values., The authors acknowledge the financial support provided by QREN (through the Operational Program COMPETE) in the scope of the CutInov Project (n. 38780) involving the Clever Reinforcement Company and the Structural Composites Research group of ISISE-Minho University. The authors also acknowledges the support provided by the “Empreiteiros Casais” and Secil (Unibetão, Braga)., info:eu-repo/semantics/publishedVersion

This paper presents a study of the shear capacity of reinforced concrete (RC) beams strengthened in shear with externally bonded (EB) fiber reinforced polymer (FRP) composites, based on the performance of existing published tests. The main objective of this study is to create a comprehensive overview of the performance of different shear strengthening configurations and the failure modes observed in experimental tests. In addition, the influence of the following parameters on the strengthening efficiency has been analyzed: shear span to depth ratio (a/d), existing steel transverse reinforcement ratio (ρsw), concrete strength (fck), size effect (d), longitudinal reinforcement ratio (ρsL), strengthening configurations, type of FRP: sheets or laminates (CFRP, GFRP), and FRP axial stiffness (AfEf). This paper has been developed under the framework of the Research Projects “BIA2015‐64672‐C4‐1‐R” and “RTI2018‐097314‐BC21” funded by the Spanish Ministry of Economy and Competitiveness (MINECO) and co‐funded by the European Regional Development Funds (ERDF). The authors would also like to thank the Cost Action TU1207 which funded a short term scientific mission of the second author in Lodz University of Technology.

One of the most frequent and catastrophic problems encountered in reinforced concrete (RC) structures is failure due to shear deficiency in RC structures, especially in RC beams. Thus, in the scope of this experimental study, the effectiveness of using prefabricated ultra-high performance fiber-reinforced concrete (UHPFRC) laminates in the form of plate or strip for shear strengthening was investigated. The change in the plate thicknesses is the first variable of the test program studied herein. Besides, two different application methods were also tested to ensure which can be preferred for shear strengthening with UHPFRC plates; bonding with epoxy, and the use of anchorages. Moreover, as an innovative method, the application of UHPFRC in the form of strip was also investigated. Test results showed that by using UHPFRC laminate as a shear strengthening material; shear strength can be increased by 26% to 47%. Moreover, it is possible to convert behavior from catastrophic shear failure to flexural behavior with the help of UHPFRC laminates. It has been determined that the use of UHPFRC in the strip form helps to increase the load-bearing capacity the most, while the use of anchorage as the bonding method of plates improves the behavior markedly., TUBITAK (The Scientific and Technological Research Council of Turkey)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [114M257], Financial support provided by TUBITAK (The Scientific and Technological Research Council of Turkey) to conduct this research under the grant number 114M257 is greatly acknowledged. The authors also thank the collaboration of Yeni Prefabrike Beton Insaat San. ve Tic. A.S. while producing the test specimens. Finally, the authors would like to acknowledge Reza Jahangiri for his assistance in casting and preparing the laminates.

Steel Reinforced Grout (SRG) materials are generating considerable interest as strengthening system of reinforced concrete (RC) structures. They are finding increasing use in several civil engineering applications mainly due to the advantages they offer over traditional material such as high strength to weight ratio, ease of application, durability and low price. This paper describes the results of an experimental investigation carried out on SRG shear strengthened RC beams and gives evidence of the Digital Image Correlation (DIC) effectiveness as a measurement system. The tests performed had two main objectives: (i) assess the effectiveness of continuous and discontinuous U-wrapped jackets comprising a different number of layers and strips, (ii) assess the shear crack distribution during the tests by means of the DIC measurements. The results confirmed that reinforcing RC beams with SRG jackets can increase the load-bearing capacity, when the beam was reinforced with a continuous two-layered SRG strip, an increase of 84% was observed (compared to the unreinforced beam). The Linear Variable Differential Transformers (LVDT) measurements validated the results obtained by means of the DIC.

Through studying the effect of initial microstructure on the precipitation behavior, microstructure evolution and mechanical properties of Mg-6Gd-4Y-0.5Zn-0.5Zr alloy, an effective way to fabricate high-strength and ductile Mg-RE alloy was proposed by low-temperature extrusion and aging treatment, where the contribution of various strengthening mechanisms was quantitatively analyzed. The results indicate that the difference of initial microstructures had no evident influence on the precipitation behavior of the Mg-RE alloy. Static recrystallization took place in the alloy with low-temperature extrusion of 300 °C during aging treatment at 200 °C due to relatively higher dislocation density in the deformed grains, which offset the reduction of elongation caused by aging treatment and resulted in the good combination of strength and ductility. In the high-strength Mg-RE alloy, grain boundary strengthening, solid solution strengthening, and precipitation strengthening are the main strengthening mechanisms. As shearable β’ particles by basal slip existed in the Mg matrix, precipitate shear strengthening other than Orawan bypassing strengthening should be utilized in predicting the precipitating strengthening behavior of Mg-RE alloy.

This study investigated the efficacy of near surface mounted (NSM) hybrid carbon/glass fibre reinforced polymer (FRP) strips for strengthening of shear-deficient reinforced-concrete (RC) rectangular deep beams. A specific focus was the interaction of FRP with steel stirrups, which were arranged in both aligned and unaligned configurations for different FRP and stirrup volume fractions. Five unstrengthened reference beams and seven strengthened beams were tested. The NSM-FRP increased the beam shear strength up to 55.8%, alleviated debonding between concrete and strengthening material, and enhanced the deformational characteristics of the strengthened beams. The FRP/stirrups interaction also affected the contribution of NSM-FRP to the shear strength of the beams. Moreover, this study proposes a model based on the modified compression field theory (MCFT) to determine the shear capacity of the beams. The proposed formulation accounts for the FRP/stirrups interaction, which is not considered in the currently available models for the NSM-FRP strengthened RC beams. The average theoretical to experimental shear capacities ratio was 0.96 with a standard deviation of 3.19%. 2020 Elsevier Ltd This paper was made possible by NPRP grant # NPRP 7-1720-2-641 from the Qatar National Research Fund (a member of Qatar Foundation) and by Qatar University grants QUST-CENG-SPR-14/15-15 and QUST-CENG-SPR-14/15-16 . The authors would also like to acknowledge Qatar University for the Graduate Assistantship, students code GTRA-CENG-2019-16 and GTRA-CENG-2019-09. The findings achieved herein are solely the responsibility of the authors. Scopus

Flax fiber sheets provide the advantages of high specific strength, a short growth cycle, environmental friendliness, wide availability, and low cost. Therefore, in this study, the shear capacities of reinforced concrete (RC) beams strengthened with ordinary flax fiber sheets, flax fiber sheets grafted with nano-TiO2, and unidirectional basalt fiber sheets were compared. The bearing characteristics and failure modes of RC beams strengthened with flax fiber sheets were investigated. The results showed that after reinforcement with flax fiber sheets, the bearing capacity and mid-span deflection of the RC beams are considerably improved, and the reinforcing effect of flax fiber sheets grafted with nano-TiO2 is greater than that of unmodified flax fiber sheets. After reinforcement with flax fiber sheets grafted with nano-TiO2, the shear capacity of the RC beams is considerably improved to 653 kN, which is 72.8% higher than that of the unreinforced RC beams. Meanwhile, the mid-range deflection of the beam reached 14.6 mm, which is 75.9% higher than that of the unreinforced RC beams.

WOS: 000367336100003, Reinforced concrete beams with insufficient shear reinforcement were strengthened using glass fiber reinforced polymer (GFRP) plates. In the study, the effect of the number of bolts on the load capacity, energy dissipation, and stiffness of reinforced concrete beams were investigated by using anchor bolt of different numbers. Three strengthened with GFRP specimens, one flexural reference specimen designed in accordance to Regulation on Buildings Constructed in Disaster Areas rules, and one shear reinforcement insufficient reference specimen was tested. Anchorage was made on the surfaces of the beams in strengthened specimens using 2, 3 and 4 bolts respectively. All beams were tested under monotonic loads. Results obtained from the tests of strengthened concrete beams were compared with the result of good flexural reference specimen. The beam in which 4 bolts were used in adhering GFRP plates on beam surfaces carried approximately equal loads with the beam named as a flexural reference. The amount of energy dissipated by strengthened DE5 specimen was 96 % of the amount of energy dissipated by DE1 reference specimen. Strengthened DE5 specimen initial stiffness equal to DE1 reference specimen initial stiffness, but strengthened DE5 specimen yield stiffness about 4 % lower than DE1 reference specimen yield stiffness. Also, DE5 specimen exhibited ductile behavior and was fractured due to bending fracture. Upon the increase of the number of anchorages used in a strengthening collapsing manner of test specimens changed and load capacity and ductility thereof increased.

The changing of the Indonesia National Code provision for accommodating the earthquake responses which previously was the SNI 03-1726-2002 to the new SNI 03-1726-2012 had a significant influence to the design criteria of concrete structures. One of the most underlining necessities of this revision was the tsunami and earthquake disasters affecting Indonesia, leading to the differentiation in the earthquake zones used to designing the 2002 SNI code. The newly introduced code has a predominant influence on the amplification of the designed earthquake load. As a consequence, building designed based on the 2002 SNI code requires re-evaluation and strengthening. One method for strengthening concrete structural elements is by using Fiber Reinforced Polymer (FRP) components. Previous research work on short beams loaded in flexure and reinforced with FRP u-shaped shear strengthening around the perimeter of the beam, beneath the slab demonstrated that this reinforcement provided additional strengthening as mandated in the ACI 440 standard. In this study, the combination of FRP flexural and u-shaped shear strengthening in accordance to this ACI 440 is studied to evaluated the enhancement in the capacity of the beam.

This paper present a design approach to predict the shear capacity of reinforced concrete (RC) beams strengthened with fiber reinforced polymer (FRP) laminates/rods applied according to the near surface mounted (NSM) technique. The new approach is based on the simplified modified compression field theory (SMCFT) and considers the relevant features of the interaction between NSM FRP systems and surrounding concrete, like debond and concrete fracture. In the SMCFT model, the shear strength of a RC element is a function of two parameters: the tensile stress factor in the cracked concrete ( β ), and the inclination of the diagonal compressive stress in the web of the section ( θ ). However, this approach is not a straightforward design methodology due to its iterative nature. A sensitivity analysis is carried out to assess the relative importance of each input parameter that mostly affect the shear capacity of RC beams shear strengthened according to the NSM technique. Taking into account the obtained results, equations to determine β and θ without recurring to an iterative procedure are derived. The experimental results of 112 beams shear strengthened with NSM FRP are used to appraise the predictive performance of the developed approach. By evaluating the ratio between the experimental results and the analytical predictions, an average value of 1.14 is obtained, with a coefficient of variation of 13.1%, being safe estimations 87% of the predictions., The author acknowledges the support provided by FP7-PEOPLE-2013-ITN in the project “endure - European Network for Durable Reinforcement and Rehabilitation”, proposal MC-ITN-2013-607851., info:eu-repo/semantics/publishedVersion

An experimental investigation was undertaken into the effectiveness of unanchored and anchored externally bonded (EB) U-wrapped carbon fibre reinforced polymer (CFRP) shear strengthening for reinforced concrete T-beams at a range of realistic sizes. The T-beam sizes, geometry and reinforcement were chosen to reflect existing slab-on-beam structures with low levels of transverse steel shear reinforcement. Geometrically similar reinforced concrete T-beams were tested across three sizes ranging from 360 to 720 mm in depth and with different amounts of EB CFRP shear reinforcement. The beams were subjected to three-point bending with a span to depth ratio of 3.5. All the beams failed in diagonal shear. The experimental results indicate significant variability in the capacity of unstrengthened control beams, and a number of these control beams showed greater shear capacity than their EB CFRP strengthened counterparts. Greater thicknesses of CFRP reinforcement did not lead to increased shear capacity compared with lesser thicknesses of unanchored or anchored EB CFRP, but anchored EB CFRP did lead to moderate increases in shear capacity compared to both control and unanchored EB CFRP strengthened beams.

The use of steel reinforced grout (SRG) has emerged as a promising technique for the strengthening and retrofitting of reinforced concrete (RC) structures, however, the experimental evidence on its application for shear-critical RC beams is rather limited. Accordingly, this study focuses on the structural performance of SRG-strengthened RC beams critical in shear. At this aim, an experimental investigation has been conducted on eleven RC T cross-section beams. Eight beams were strengthened with different types of SRG (different for the density, namely 1.57 and 3.14 cords per centimeter) comprised of ultra-high tensile strength steel fabrics, and three, which were unstrengthened, were used as reference specimens. The test parameters investigated were as follows: (a) steel fabric density, (b) bond scheme, and (c) amount of internal shear reinforcement within the critical shear span, SRG/stirrups interaction. The experimental results revealed the high potential of SRG system for the strengthening of shear-critical RC beams. It has significantly improved the shear capacity and deformation characteristics of the strengthened beams. The percentage increase in the load-carrying capacity over the reference beam of up to 71% was observed. In addition, an analytical model based on the simplified modified compression field theory (SMCFT) has been proposed to predict the shear capacity of SRG-strengthened beams. The SMCFT-based model has resulted in an accurate and safe prediction of the shear capacity of the SRG-strengthened beams with an average ratio of Vth/Vex of 0.92 and a standard deviation of 4.07%. - 2019 Elsevier Ltd This paper was made possible by NPRP grant # NPRP 9-110-2-052 and UREP grant # UREP24-045-2-013 from the Qatar National Research Fund (a member of Qatar Foundation) and Qatar University grant # QUST-CENG-SPR- 14/15-15 . The authors would also gratefully acknowledge Kerakoll SpA for providing the strengthening material. The findings achieved herein are solely the responsibility of the authors. Scopus

This paper presents the experimental data of the behavior of reinforced concrete beams strengthened to shear with carbon fiber composites. The tests were composed of eight T beams, b w=15 cm, h=40 cm, flange width 40 cm, flange height 8 cm, and length 300 cm, divided into two series with the same longitudinal steel reinforcement and a reference beam without strengthening in each series. The beams had two types of arrangement of internal steel stirrups. The test variables were the internal and external geometric ratio of the transverse reinforcement and the mechanical ratio of carbon fiber composites stirrups. All the beams were loaded at two points. The strengthened beams were submitted to a preloading and the strengthening was applied to the cracked beam. All the beams were designed in order to guarantee shear failure, and the ultimate load of the strengthened beams was 36% to 54% greater than the reference beams. The Cracking Sliding Model applied to the strengthened beams was evaluated and showed good agreement with the experimental results. Este artigo apresenta os dados de ensaios de vigas de concreto armado reforçadas à força cortante com compósito de fibras de carbono. Os ensaios de oito vigas T, b w=15 cm, h=40 cm, largura da mesa igual a 40 cm, altura da mesa 8 cm, comprimento de 300 cm, divididos em duas séries com a mesma armadura longitudinal e uma viga de referência sem reforço externo para cada série. As vigas tinham dois tipos de arranjos de estribos internos de aço. As variáveis dos ensaios foram as taxas geométricas das armaduras transversais internas e externas, e a taxa mecânica da armadura em estribos de compósito de fibras de carbono. Todas as vigas foram carregadas em dois pontos. As vigas reforçadas foram submetidas a um carregamento prévio e reforçadas após a fissuração. Todas as vigas foram projetadas para garantir a ruptura por força cortante, e as forças últimas das vigas reforçadas foram de 36% a 54% superiores às das vigas de referência. O Modelo da Fissura Deslizante foi avaliado e mostrou boa concordância com os resultados experimentais.

This article describes the shear strengthening of reinforced concrete structures with one of the latest and most promising method: NSM - Near surface mounted reinforcement. Research in this area started only a few years ago with a combination of epoxy and FRP (fiber reinforced polymer) materials suitable for strengthening concrete elements. In this paper main components of NSM strengthening system are described, design approaches and also numerical models of strengthened concrete beams are also mentioned.

Due to their lightweight, high tensile strength, and ease to install on irregular surfaces, the use of FRP systems for the repair and strengthening of reinforced concrete structures has become an accepted practice within civil engineering community. Extensive research has been conducted on structural members to identify the improvement in their flexural and axial capacity due to FRP strengthening. However, the analytical and experimental studies on shear strengthening with FRP systems are limited. Currently, there is no widely accepted guideline available for the design of concrete structures strengthened in shear using externally bonded FRP systems. Keeping in mind the fact that the use of these strengthening methods has been hindered mostly due to lack of comprehensive design provisions, the available guidelines/specifications are reviewed and the factors that need further investigation are identified. To improve the understanding of the general mechanisms of shear strengthening techniques with externally bonded FRP systems, suggestions for further research effort are itemized and recommendations for an improved guideline are set forth.

The effectiveness of NSM technique with CFRP laminates for the shear strengthening of T-section RC beams of low strength concrete was assessed. Five NSM shear strengthening configurations (three CFRP orientations and two levels of CFRP percentage) were applied in RC beams with a steel stirrups percentage of 0.10% and of 0.17%. The results showed that: NSM technique is still effective in beams of low strength concrete; inclined are more effective than vertical laminates; the beam’s shear resistance increases with the CFRP percentage; the NSM strengthening effectiveness decreases with the increase of the steel stirrups percentage. Using available experimental results obtained with the same test set-up but using beams of higher strength concrete it can be concluded that as minimum is the concrete strength as less effective is the NSM technique. In general, the formulation proposed by Nanni et al. provided safe and acceptable estimates for the contribution of the NSM shear strengthening systems (the predicted values of the CFRP contribution for the shear resistance were 75% of the results registered experimentally)., Fundação para a Ciência e a Tecnologia (FCT) -PTDC/ECM/73099/2006.

Shear forces are the most common governing failure mechanism of reinforced concrete bridge deck slabs subjected to loads close to their supports. This paper reveals a comprehensive study of the behaviour of concrete slabs shear-strengthened with a deep embedment fibre reinforced polymer (FRP) technique. In the experimental investigation, a series of eight full scale concrete slabs were created and tested up to failure. Several structural variables were changed to assess the effectiveness of this shear strengthening technique. The behaviour of test slabs is discussed and the influence of this strategy was evaluated by comparing the test results. It was shown that brittle shear failures could be avoided by using this strengthening strategy. The ultimate capacity and deflection at the failure were both enhanced by using the deep embedment strengthening method. Additionally, a nonlinear finite element analysis (NLFEA) was proposed to develop further investigation. This NLFEA model gave excellent predictions for the structural behaviour of the test concrete slabs. Finally, a two-way theoretical model was proposed to predict the loading-carrying capacity of concrete slabs strengthened with deep embedment FRP bars. The ultimate strength predicted by this theoretical method showed good agreement with that from the test results.

The effectiveness of the Near Surface Mounted (NSM) technique with Carbon Fiber Reinforced Polymer (CFRP) laminates for the shear strengthening of T cross section Reinforced Concrete (RC) beams was assessed by experimental research. Three inclinations of the laminates were tested (45 , 60 and 90 ) and, for each one, three percentages of CFRP were adopted. The RC beams withNSMlaminates had a percentage of steel stirrups of 0.10%. The highest percentage of laminates was designed to provide a maximum load similar to that of a reference RC beam with a steel stirrup reinforcement ratio of 0.28%. The results showed that: inclined laminates were more effective than vertical laminates; an increase of the percentage of laminates produced an increase of the shear capacity of the beams; the contribution of the laminates for the beam shear resistance was limited by the concrete tensile strength; the failure modes of the beams were influenced by the percentage of the laminates. For each percentage of laminates, a homologous RC beam strengthened with U-shaped CFRP wet layup sheets (discrete strips) applied according to the Externally Bonded Reinforcement (EBR) technique was also tested, with the purpose of comparing the effectiveness of these two CFRP strengthening techniques. NSM was the most effective, not only in terms of increasing beam shear resistance but also in assuring better utilization of the tensile strength of the CFRP material. Except for the beams with the highest percentage of CFRP, the NSM technique was more effective than the EBR technique in terms of deformation capacity at beam failure. The ACI and fib analytical formulations have predicted a larger contribution of the EBR shear strengthening systems than the values recorded experimentally. The formulation provided by Nanni et al. for the NSM technique predicted a CFRP contribution around 61% of the experimentally registered values., The authors wish to acknowledge the support provided by the "Empreiteiros Casais", Degussa, S&P (R) and Secil (Unibetao, Braga). The study reported in this paper forms a part of the research program supported by FCT, PTDC/ECM/73099/2006.

A recent and promising method for shear strengthening of reinforced concrete (RC) members is the use of near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement. In the NSM method, the reinforcement is embedded in grooves cut onto the surface of the member to be strengthened and filled with an appropriate binding agent such as epoxy paste or cement grout. Only a few studies have been conducted to date on the use of NSM FRP reinforcement for shear strengthening of RC beams. These studies identified some critical failure modes related to debonding between the NSM reinforcement and the concrete substrate. However, more tests need to be conducted to identify all possible failure modes of strengthened beams. Moreover, virtually no test results are available on the behavior of shear-strengthened beams containing steel shear reinforcement, and on the effect of variables such as the type of epoxy used as groove filler. This paper illustrates a research program on shear strengthening of RC beams with NSM reinforcement, aimed at gaining more test results to fill the gaps in knowledge mentioned above. A number of beams were tested to analyze the influence on the structural behavior and failure mode of selected test parameters, i.e. type of NSM reinforcement (round bars and strips), spacing and inclination of the NSM reinforcement, and mechanical properties of the groove-filling epoxy. One beam strengthened in shear with externally bonded FRP laminates was also tested for comparison purposes. All beams had a limited amount of internal steel shear reinforcement to simulate a real strengthening situation. Test results are presented and discussed in the paper.

Near surface mounted (NSM) and externally bonded reinforcement (EBR) strengthening techniques are based on the use of carbon fiber reinforced polymer (CFRP) materials and have been used for the structural rehabilitation of concrete structures. In the present work, the efficacies of the NSM and EBR techniques for the flexural and shear strengthening of reinforced concrete beams are compared carrying out two experimental groups of tests. For the flexural strengthening, the efficacy of applying CFRP laminates according to NSM is compared to those resulting from applying CFRP laminates and wet lay-up CFRP sheets according to EBR technique. The influences of the equivalent reinforcement ratio (steel and laminates) and spacing of the laminates on the efficiency of the NSM technique for the flexural strengthening is also investigated. A numerical strategy is implemented to analyze the applicability of the FRP effective strain concept, proposed by ACI and fib in the design of FRP systems for the flexural strengthening. To assess the efficacy of the NSM technique for the shear strengthening of concrete beams, four beam series of distinct depth and longitudinal tensile steel reinforcement ratio are tested. Each series is composed of one beam without any shear reinforcement and one beam using the following shear reinforcing systems: conventional steel stirrups; strips of wet lay-up CFRP sheet of U configuration applied according to EBR technique; and laminates of CFRP embedded into vertical or inclined (45 ) pre-cut slits on the concrete cover of the beam lateral faces, according to the NSM technique. Using the obtained experimental results, the performance of the analytical formulations proposed by ACI, fib and Italian guidelines is appraised., Fundação para a Ciência e a Tecnologia (FCT) - POCTI/ECM/59033/2004

The possibility of using thin high-performance jackets aimed at improving the structural and shear capacity of existing reinforced concrete members is investigated in this paper. The jackets are made of high-performance fiber-reinforced concrete, with a steel mesh fabric sometimes added (diameter of the wires = 2 mm [0.078 in.]; wire spacing = 25.4 mm [1 in.]; and ultimate strength of steel = 550 N/mm2 [79.8 ksi]). Two different high-performance cementitious composites are investigated: a self-leveling concrete suitable for casting thin members, and a thixotropic concrete requiring no molds during casting operations. To assess the effectiveness of the proposed solutions, different jackets were used to reinforce short, simply supported reinforced concrete beams (2.85 m [9.35 ft] long) lacking stirrups. The beams were tested up to failure in four-point bending, and the numerous test results, including a close comparison with the reference un-strengthened beam, are presented and discussed. The experimental results demonstrate the effectiveness of the technique proposed. All strengthened beams reached their theoretical bending capacity, whereas the unreinforced beam failed in shear.

Shear deficient reinforced concrete (RC) structures can be effectively strengthened using external prestressed carbon fibre reinforced polymer (CFRP) straps. Due to the presence of the external elastic straps, a strengthened beam can continue to carry significant load beyond the stages of crack plane slipping and internal shear stirrup yielding, and the concrete is subjected to high tensile strain levels. As a consequence, the concrete material models play a significant role in the context of modeling such behavior. The modified compression field theory (MCFT), which is a widely accepted shear theory for unstrengthened RC structures, incorporates the details of the stress-strain behavior of concrete. The MCFT also considers compatibility as a governing factor, which facilitates the inclusion of the strap system into the MCFT formulation. In the current study, modifications were investigated to model CFRP strap retrofitted RC beams associated with either uniform or non-uniform strap spacings. An experimental investigation on strengthened and unstrengthened rectangular RC beams was carried out to validate the MCFT predictions for various strap layouts. The validation process revealed that, in general, the MCFT was able to model the shear response of the retrofitted RC beams but the representation of the softening of the concrete compressive strain, and stress, was found to be influential in the determination of the ultimate load capacity.

This paper presents the experimental behaviour of two reinforced concrete T-beams strengthened in shear with CFRP. The beams are extracted from an important building of the mid-'30s in Rome. The goal of this research is to evaluate the shear strength of the retrofitted beams with the simultaneous occurrence of a negative bending moment. The tests are performed at the Experimental Laboratory of the Structural Department of the University “Roma Tre”. In the meantime the two beams are modelled with the software ATENA in order to perform numerical simulations of the tests. The numerical results seem to be in good agreement with the experimental data in terms of load– displacement response, strain state, crack pattern and failure mode.

[EN] In light of their excellent mechanical properties and other advantages over conventional systems for repairing and rehabilitating structures, fibre-reinforced polymers (FRPs) are commonly used to enhance the shear strength of RC members. Because of the relatively recent application of the technique in civil engineering, however, it is addressed in very few recommendations or guides. While the design guides in place contain a single, well defined and generally accepted procedure for calculating bending moments, they collectively propose a wide variety of numerical models for shear, none of which is universally acknowledged. The present article focuses on the calculation of ultimate shear strength and describes an experimental study in that context conducted on reinforced concrete beams retrofitted with unidirectional carbon fibre reinforced polymer (CFRP) fabric., [ES] La técnica de refuerzo externo a cortante en elementos de HA con polímeros armados con fibra, conocida como EBR-FRP por sus siglas en inglés, se ha ganado un puesto importante en el área de reparación y rehabilitación de estructuras gracias a las excelentes propiedades mecánicas de estos materiales, y sus enormes ventajas respecto a otras técnicas. Sin embargo, debido a que su uso es relativamente reciente en la ingeniería civil, existen pocas recomendaciones o guías que consideren esta técnica. Las guías de cálculo existentes muestran un procedimiento bien definido y aceptado con respecto al cálculo a flexión, sin embargo, los modelos de cálculo a cortante resultan ser altamente variados y controvertidos. Este documento muestra el enfoque actual para el cálculo de esfuerzos cortantes y sus limitaciones, así como el proceso de una investigación experimental sobre vigas de hormigón armado reforzadas externamente con tejido unidireccional de CFRP.

A three dimensional mechanical model was recently developed to simulate the shear strength contribution provided by a system of Near Surface Mounted (NSM) Fiber Reinforced Polymer (FRP) strips to a Reinforced Concrete (RC) beam throughout its loading process. It was developed fulfilling equilibrium, kinematic compatibility and constitutive laws of both materials, concrete and FRP, and local bond between themselves. In the present paper, that model is first appraised on the basis of some of the most recent experimental results and then is applied to carry out parametric studies. The influence of each of the involved parameters on the contribution provided by a system of NSM FRP strips to the shear strength of a RC beam is investigated. The results of those studies are presented along with the main findings., (undefined), The authors of the present work wish to acknowledge the support provided by the “Empreiteiros Casais”, S&P®, degussa® Portugal, and Secil (Unibetão, Braga). The study reported in this paper forms a part of the research program “CUTINEMO” supported by FCT, PTDC/ECM/73099/2006. Also, this work was carried out under the auspices of the Italian DPC-ReLuis Project (repertory n. 540), Research Line 8, whose financial support is greatly appreciated.

Composite materials are getting more and more common for strengthening existing members and structures; fiber-reinforced polymers (FRP) are widely used, while carbon-fiber-reinforced cement matrix (CFRCM) materials have been more recently proposed especially for strengthening masonry members. In the present paper, the results of an experimental campaign carried out on tuff-masonry walls strengthened in shear by a cement-based composite are reported and commented. The reinforced masonry walls failed after loss of adhesion between the strengthening layer and the masonry substrate. Comparisons between the experimental results and some analytical formulations available in the scientific literature for determining shear resistance of strengthened masonry walls are finally proposed. Huge variability can be observed by applying those alternative formulations which are not able to reproduce the premature nature of the observed failure mode. Consequently, the present study is a thorough experimental report which can be useful for developing and validating more refined theoretical models for describing the ultimate behaviour of masonry walls externally strengthened by FRP.

7 páginas, 7 figuras, 3 tablas.-- El Pdf es la versión post-print de autor., The prediction of the shear capacity of reinforced concrete beams retrofitted in shear by means of externally bonded FRP is very complex as demonstrate the studies carried out up to date. As alternative to the conventional methods two approaches based on artificial intelligence are proposed for the first time. Firstly, the use of neural networks as a means of predicting shear capacity without the need of using complex models and, secondly, the use of genetic algorithms as a means of determining suitably how the shear mechanism works. Predictions obtained with both approaches are compared to experimental values., The writers acknowledge support for the work reported in this paper from the Spanish Ministry of Education and Science (project BIA2007-67790).

Synopsis: The efficacies of the Near Surface Mounted (NSM) and Externally Bonded Reinforcing (EBR) techniques for the shear strengthening of rectangular cross section RC beams are compared. Both techniques are based on the use of carbon fiber reinforced polymer (CFRP) materials. The NSM was the most effective technique, and was also the easiest and fastest to apply, and assured the lowest fragile failure modes. The performance of the ACI and fib analytical formulations for the EBR shear strengthening was appraised. In general, the contribution of the CFRP systems predicted by the analytical formulations was larger than the values registered experimentally. The capability of the De Lorenzis formulation of predicting the contribution of the NSM technique for the shear strengthening of RC beams was appraised using bond stress and CFRP effective strain values obtained in pullout bending tests. This formulation provided values 61% lower than the values obtained experimentally.

Shear strengthening of Reinforced Concrete (RC) beams by means of Near Surface Mounted (NSM) Fiber Reinforced Polymer (FRP) strips is an emerging technique for structural rehabilitation that is gaining increasing interest in the FRP community mainly because of some advantages it provides with respect to the better consolidated technique of the Externally Bonded Reinforcement (EBR). Those advantages encompass, mainly, a better exploitation of material and a higher protection against vandalism along with a relative faster applicability. Yet, the behavior of such NSM FRP strips is extremely complex, as can be gathered by experimental evidence, due to the complex geometry, the non linear mechanical properties of bond, and the scatter affecting the concrete tensile properties along with their non-linearity. In an attempt to provide valuable contribution to a better understanding of their behavior, a three dimensional mechanical model for simulating the shear strength contribution provided by a system of NSM FRPs to a RC beam throughout the loading process was recently developed. Its upgraded version is herein presented along with the main findings. It correctly interprets the experimental evidence taking into account complex phenomena such as the interaction between bond transferred force and concrete fracture along with the interaction between adjacent strips., The authors of the present work wish to acknowledge the support provided by the "Empreiteiros Casais", S&P(R), degussa (R) Portugal, and Secil (Unibetao, Braga). The study reported in this paper forms a part of the research program "SmartReinforcement - Carbon fibre laminates for the strengthening and monitoring of reinforced concrete structures" supported by ADI-IDEIA, Project no. 13-05-04-FDR-00031. Also, this work was carried out under the auspices of the Italian DPC-ReLuis Project (repertory no. 540), Research Line 8, whose financial support is greatly appreciated.

Embedded Through-Section (ETS) technique is a relatively recent shear strengthening strategy for reinforced concrete (RC) beams, and consists on opening holes across the beam thickness, with the desired inclinations, where bars are introduced and are bonded to the concrete substrate with adhesive materials. To assess the effectiveness of this technique, a comprehensive experimental program composed of 14 RC beams was carried out and the obtained results confirm the feasibility of the ETS method and reveal that: (i) inclined ETS strengthening bars were more effective than vertical ETS bars and the shear capacity of the beams has increased with the decrease of the spacing between bars; (ii) brittle shear failure was converted in ductile flexural failure and (iii) the contribution of the ETS strengthening bars for the beam shear resistance was limited by the concrete crushing or due to the yielding of the longitudinal reinforcement., (undefined)

This paper presents a closed-form procedure to evaluate the shear strength contribution provided to a Reinforced Concrete (RC) beam by a system of Near Surface Mounted (NSM) Fiber Reinforced Polymer (FRP) strips. This procedure is based on the evaluation of: a) the constitutive law of the average-available-bond-length NSM FRP strip effectively crossing the shear crack and b) the maximum effective capacity it can attain during the loading process of the strengthened beam. Due to complex phenomena, such as: a) interaction between forces transferred through bond to the surrounding concrete and concrete fracture, and b) interaction among adjacent strips, the NSM FRP strip constitutive law is largely different than the linear elastic one characterizing the FRP behavior in tension. Once the constitutive law of the average-available-bond-length NSM strip is reliably known, its maximum effective capacity can be determined by imposing a coherent kinematic mechanism. The self-contained and ready-to-implement set of analytical equations and logical operations is presented along with the main underlying physical-mechanical principles and assumptions. The formulation proposed is appraised against some of the most recent experimental results and its predictions are also compared with those obtained by a recently developed more sophisticated model., (undefined)

To predict correctly the deformational and the cracking behavior of reinforced concrete elements failing in shear using a smeared crack approach, the strategy adopted to simulate the crack shear stress transfer is crucial. For this purpose, several strategies for modeling the fracture mode II were implemented in a smeared crack model already existing in the FEM-based computer program, FEMIX. Special development was given to a softening shear stress-shear strain diagram adopted for modeling the crack shear stress transfer. The predictive performance of the implemented constitutive model was assessed by simulating up to failure a series of eight beams tested to appraise the effectiveness of a new strengthening technique to increase the shear resistance of reinforced concrete beams. According to this strengthening technique, designated as Embedded Through-Section (ETS), holes are opened through the beam’s section, with the desired inclinations, and bars are introduced into these holes and bonded to the concrete substrate with adhesive materials. The strengthening elements are composed of steel bars bonded to the surrounding concrete with an epoxy adhesive. By using the properties obtained from the experimental programs for the characterization of the relevant properties of the intervening materials, and deriving from inverse analysis the data for the crack shear softening diagram, the simulations carried out have fitted with high accuracy the deformational and cracking behavior of the tested beams, as well as the strain fields in the reinforcements. The constitutive model is briefly described, and the simulations are presented and analyzed., Fundação para a Ciência e a Tecnologia (FCT)

This paper presents a finite element analysis on two reinforced concrete beams, extracted from an important building of the '30s in Rome, strengthened in shear with C-FRP and subjected to the simultaneous occurrence of a negative bending moment and shear stresses. The commercial program ATENA was used to perform numerical simulations of the tests. The numerical results seem to be in good agreement with the experimental data in terms of load-displacement response, strain state, crack pattern and failure mode.

This paper presents the experimental behaviour of two reinforced concrete (RC) beams strengthened in shear with C-FRP. The beams are extracted from an important building of the '30s in Rome. The goal of this research is to evaluate the shear strength of the retrofitted beams with the simultaneous occurrence of a negative bending moment. The tests are performed at the Experimental Laboratory of the Structural Department of the University "Roma Tre".