Near Surface Mounted Research Articles

Full-Range fatigue behavior of NSM CFRP-to-Concrete bonded joints

The interfacial bond characteristics and failure mechanism of near-surface mounted (NSM) fiber reinforced polymer (FRP) composites in concrete under fatigue loading are less understood compared to their bond behavior under static loading. In this paper, degradation of the local bond strength of FRP-to-concrete bonded joints subjected to direct pull-out fatigue loading is presented for both carbon FRP (CFRP) rods and strips under different fatigue load ranges, i.e., 10–50%, 10–60%, and 10–70%, of the corresponding static load-carrying capacity. The interfacial debonding between FRP and adhesive was observed during fatigue cycles for CFRP rod specimens to different extents. Specimens using CFRP rods with sand-coated and spirally wound surface treatment presented a higher local bond strength and a slower degradation rate, as compared to the ones using CFRP rods with roughened or sand-coated treatment. Using epoxy adhesives with higher strengths benefitted the fatigue bond performance of the tested specimens in terms of local bond strength and stable behavior. Higher fatigue load ranges substantially accelerated the local bond degradation resulting in a much shorter fatigue life of the bonded joints.

Torsional behaviour of RC beams strengthened by NSM GFRP bars

Near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement is an innovative, serviceable, and beneficial method for strengthening reinforced concrete (RC) structures. Previous studies have proven that the epoxy-resin bonded NSM FRP method effectively achieved shear and flexural strengthening. However, only a few researches have employed this technique for improving torsion but, no research has been conducted on the use of NSM with GFRP bars and epoxy adhesives. This study investigates the behavior of RC beams subjected to combined torsion when they have been enhanced with NSM GFRP U-shaped and closed stirrups with different overlap sites. Four RC beams with dimensions of (150×250×2000) mm (width X depth X length) were considered in the experimental tests. One RC beam is not strengthened and used as a control beam (CB), while three beams are enhanced with NSM GFRP bars in different shapes and overlap configurations. The findings of the tests in terms of twist angle_applied torque curves, the torque of initial cracking, the ultimate torsional carrying capacity, and the ultimate angle of twist were all recorded and evaluated. It has also been shown that a considerable improvement in the performance of RC beams in twisting can be achieved by using NSM GFRP stirrups. From the different NSM strengthening configurations used in the tests, results have demonstrated that beams strengthened by closed GFRP stirrups at the opposite overlap with 100 cm spacing outperformed other strengthened beams of about 181.5% compared to the CB. In contrast, the beam, strengthened by U-shaped GFRP stirrups with spacing at 200 cm, had the lowest torsional strength of about 48.1% relative to the CB.

Behaviour of reinforced concrete corbels strengthened by FRP composites and steel bars: a critical review

In reinforced concrete (RC) structures, corbels that extend from the faces of reinforced concrete columns are commonly employed to support primary beams and girders. Frequently, the use of corbels is limited to cantilevers with a shear span-depth ratio that is often less than 2. Because of this relatively low ratio, corbel strength is normally dominated by shear, which is comparable to deep beams. However, owing to some reasons (e.g., design and/or construction faults, modification of facility use, deterioration, and aging), strengthening of existing corbels is becoming of prime importance. The aim of the present paper is to conduct a critical review of commonly proposed techniques that have been implemented to strengthen RC corbels. These techniques comprised near surface mounted (NSM) steel bars, NSM fibre reinforced polymer (FRP) bars, and FRP sheets. The latter strategy included carbon (CFRP) and glass (GFRP) sheets. Hybrid strengthening (combining FRP sheets and FRP/steel bars) was also highlighted in this paper. The strengthened (upgraded) RC corbels were evaluated considering several parameters, including shear span to depth ratio (a/d), quantity and configuration of reinforcement, FRP type, and levels of damage in the tested specimens.

Review of retrofitting techniques of deteriorated reinforced concrete columns

To ensure sustainability, all types of building structures should be kept in accepted condition throughout their life of service. Rehabilitation has been the subject of extensive research recently because of rising spending on construction and maintenance of constructed infrastructure. The most recent methods to reinforce current reinforced concrete columns, including both conventional and cutting-edge strengthening procedures, are presented in this study. Frequently, general materials used in repairing on-service reinforced concrete columns are concrete, steel, and different kind of Fiber Reinforced Polymer (FRP) composites. Conventional methods and procedures for different mode of failures strengthening, including all types of FRP procedures Near Surface Mounting (NSM) reinforcement, Externally Bonded (EB) FRP, and textile reinforced concrete will all be included in the discussion, and FRP pre-stressing methods for modifying existing structures.

The Effect of Cohesive Debonding Elimination on Enhancing the Flexural Performance of Damaged Unbonded Prestressed Concrete Girders Strengthened Using NSM CFRP

This manuscript studied the effect of U-CFRP wrapped sheet anchorage on the flexural performance of unbonded post-tensioned PC members subjected to partial strand damage and strengthened using CFRP Near-Surface Mounting techniques. The program includes six girders as a control girder, a girder with strand damage of 14.2%, and four girders strengthened by CFRP laminates using the NSM technique with and without U-CFRP wrapped sheet anchorages. The testing results show that the strand damage of 14.2% has reduced the flexural strength of the girder by 5.71%. The NSM-CFRP laminate has a significant effect on flexural strength by 17.4%. On the other hand, the application of end U-CFRP wrapped sheet anchorages improves flexural strength by 27.97% and enhances ductility. The intermediate and successive U-CFRP sheet anchorages increase the flexural strength by 36.56% and 32.61%, enhancing the stiffness at all loading stages and improving the ductility. Semiempirical equations were developed to determine the actual stress of unbonded strands considering the effect of U-FRP-wrapped anchorages.

Shear performance of reinforced concrete beams strengthened in shear using NSM bars and reinforced HSC layers

AbstractThe reinforced concrete (RC) beams could be upgraded in shear using steel and fiber‐reinforced polymer (FRP) having several shapes (bars, strips, or sheets). Using near‐surface mounted (NSM) Glass FRP (GFRP) bars is still limited compared to carbon FRP (CFRP) elements as shear reinforcement. Moreover, using reinforced high‐strength concrete layers (HSCL) as NSM shear strengthening for the RC beams is still limited. Herein, three RC beams internally reinforced in shear using different shear areas (without, 100 mm stirrups apart, and 200 mm stirrups apart) were experimentally tested as reference beams (CB). Other eight beams strengthened in shear using NSM HSCL, and bars were experimentally tested. Moreover, an extended finite element (FE) simulation was implemented through a verified FE model to study the effect of other NSM aspects on shear‐strengthened beam response. The results explained that, regardless of the internal stirrups area and external NSM characteristics, the shear‐strengthened beams showed higher load efficiency than the corresponding CB. The NSM upgraded shear beam had slightly higher load efficiency than the un‐strengthened beam having the same steel area as internally shear stirrups. Moreover, increasing the internal shear stirrups decreased the total efficiency of the NSM shear strengthening and the internal stirrups regardless of the NSM characteristics. Furthermore, the NSM HSCL was more efficient than the corresponding NSM bars in increasing the load capacity and stiffness of the shear‐strengthened beams. The reported analytical model predicted well the shear capacity of the tested beams.

PERFORMANCE EVALUATION OF BEAMS STRENGTHENED WITH MECHANICALLY ANCHORED CARBON FIBER-REINFORCED CEMENTITIOUS MATRIX (FRCM)

Textile reinforced mortar (TRM), more popularly known as fiber-reinforced cementitious mortar (FRCM), has unique characteristics such as an excellent bond with the concrete substrate, applicability in a wet environment, and resistance to high temperatures. It is now considered a substitute for the fiber-reinforced polymer (FRP) due to the vast progress in FRCM and the increasing complexity of FRP-concrete or FRP-masonry debonding. Strengthening beams with FRCM can be implemented using direct wet layup, Near-surface mounted (NSM), etc. Observed failure modes in the literature show fiber-fiber slip and fiber-matrix debonding as dominant modes. Some authors also studied mechanical anchorages to improve these interfacial limitations. Anchorage in FRCM includes adding transverse layers at the end and intermediate locations, winding the fabric around the bar, and embedding it into the substrate or fan-type anchorage as used in FRP. The present study, 12 beams were strengthened using an indigenously developed unidirectional carbon FRCM system with anchorage. The study parameters include the number of layers and strengthening strategies such as mechanical anchoring and pressure impregnation. For the study, a novel yarn-based mechanical anchorage is developed suitable for Indian site conditions. It is found that the developed mechanical anchorage strategy improves the ultimate load-carrying capacity of the beam by about 16% compared to conventional cement-matrix impregnation. Fiber-rupture failure was observed for anchored specimens, whereas fiber-fiber slip was observed for impregnated specimens.

COMPARATIVE STUDY ON TUNNEL LINING CONCRETE STRENGTHENED WITH CARBON-FIBER COMPOSITE CABLES AND SHEETS

Most lining concretes of road tunnels in Japan have been gradually deteriorated by aging and various environmental effects. The study focuses on a strengthening system for tunnel lining concrete (TLC) subjected to external forces. Carbon fiber (CF) sheets are often used in TLC for preventing concrete exfoliation. The strengthening effect of CF sheet for TLC was not sufficiently known. The experimental study conducted a mock-up loading test to examine failure mode and load-capacity of the TLC model bonding CF sheet. In addition, the study prepared TLC models strengthened with near-surface mounted (NSM) technique using carbon-fiber composite cables (CFCC). The strengthening effects of CF sheet and NSM-CFCC were quantified and compared via the mock-up loading test. The experimental study revealed that the TLC model bonding CF sheet failed by intermediate-crack-induced debonding at around 100 kN, and the load carrying capacity was almost equivalent to the capacity of non-strengthened TLC model. On the other hand, the NSM-CFCC strengthened TLC model indicated higher ductility after concrete cracking and superior load capacity. The experimental study confirmed the NSM-CFCC as an effective strengthening system for TLC members.

Performance of NSM and EB methods on the flexural capacity of the RC beams strengthened with reinforced HSC layers

The near-surface mounted (NSM), in which strips or rods are fixed by an adhesive in slits formed inside the concrete cover, and the externally bonding (EB), in which the reinforcement is glued to the concrete surface, are well-known strengthening methods. In the present work, an experimental and numerical study was carried out on reinforced concrete (RC) beams strengthened with NSM and EB-reinforced high-strength concrete (RHSC) layers. The RHSC layers were reinforced by steel or glass fiber reinforced polymer (GFRP) rods and bonded to the RC beams without and with a concrete cover. Seven specimens were cast and loaded in flexure up to failure to investigate the effect of RHSC layer position and reinforcement on the RC beams' efficiency. The three-dimensional finite element program (ABAQUS) verified the experimental results numerically. Also, a parametric study was numerically performed to examine the factors affecting the flexural performance of the strengthened RC beams using NSM and EB RHSC layers. The strengthened beams by NSM steel RHSC layers increased the load capacity by 147.9% compared to the control beam (CB). Changing the RHSC layer reinforcement from steel to GFRP bars enhanced the load's efficiencies from 147.9% to 172.0% compared to the CB. Decreasing the width and reinforcing area of the RHSC layer decreased the beam load capacity by about 17.5 %. Due to their longer bending arms, the EB RHSC layers showed lower load capacity than the NSM ones, which accelerated the EB RHSC failure. When the carbon fiber-reinforced polymer bars were utilized to reinforce the HSC layers, the strengthened beams showed higher load efficiency than those strengthened with HSC layers reinforced by GFRP rods.

Masonry Wall Reinforcement

Unreinforced masonry has been used as a construction material for thousands of years. Whilesome of these structures survive to this day, these tend to have massive walls and would be unpractical formodern use. Unreinforced masonry can be reinforced with proper masonry techniques. As a result,reinforced and confined types of masonry were developed to exploit the strength potential of masonry andto solve its lack of tensile strength, significantly improving not only resistance, but also ductility andenergy dissipation capacity.Fiber reinforced polymer composites have emerged as an effective means of reinforcing masonrystructures due to their high mechanical characteristics, low weight and durability. The role of FRPmaterials is to act as a tensile reinforcement which can be used to improve the ability of a wall against inplane shear and/or out-of-plane bending.This contribution, based on experimentation and numerical modelling, aims at a better understanding ofthe behaviour of structures in masonry. Multiple reinforcement techniques and methods are illustrated inthis summary, such as the reinforcement of masonry walls by composite materials like CFRP (CarbonFiber Reinforced Polymer) and glass fibre reinforced polymer (GFPR), also there are Near surfacemounted (NSM), carbon fibers reinforced polymer (CFRP) and other techniques are cited in this work.

Experimental and Numerical Study on RC Beams Strengthened by NSM Using CFRP Reinforcements

Near Surface Mounted (NSM) Carbon Fiber-Reinforced Polymer (CFRP) reinforcement technique to improve the flexural strength of reinforced concrete members has become increasingly attractive in recent years. In this study, the practical problem of concrete cover depth cutting limitation was investigated. Twelve specimens were tested by four-point bending until failure. Experimental parameters include concrete cover depth, CFRP reinforcement type, CFRP positioning, and stirrups status. Furthermore, a nonlinear FEA model was developed to simulate the tested beams and was able to predict the experimental behavior satisfactorily. A series of parametric studies were then performed using this model to understand the effect of various reinforcement parameters on the flexural performance of the beam. The results showed that Strengthening with CFRP resulted in a significant increase in yield and ultimate strengths, but a significant ductility loss was recorded due to CFRP strip debonding in the strengthened beams, this problem was addressed by using more efficient strengthening techniques utilizing the effective bond length and a proper groove depth and positioning for the NSM bars.

Experimental and FE analysis of NSM FRP bond in low- and normal-strength natural and recycled aggregate concretes

Poor mechanical properties of construction materials constitute one of the major reasons for structural deterioration. Since the extent of the weakening in the material quality could reach an alarming threshold, their suitability for strengthening needs assessment prior to adopting any strengthening strategy. This paper experimentally investigates the bond behavior between near-surface mounted (NSM) fiber-reinforced polymer (FRP) strengthening in low- and normal-strength concrete with compressive strength in the range of 11.6 – 14.8 MPa for the former and 39.4 MPa for the latter. The use of two aggregate types – natural and recycled aggregates – was also investigated in the low-strength concretes. Steel fiber reinforcement was used in some of the mixes to check whether some enhancement is possible in low-quality concrete that can result from inferior recycled material, compromised workmanship, or material deterioration due to durability challenges. Ultimately, the variables of the study include recycled coarse aggregate incorporation ratio of 0%, 30%, 60%, and 100% replacement of the natural aggregate for each of the steel fiber’s percentages of 0%, 0.5%, and 1% of the specimen’s volume. The control NSM FRP-concrete specimen made from normal-strength concrete (39.4 MPa) was prepared and tested as a reference. The low-strength concrete mixtures demonstrated a decrease in the NSM FRP-concrete bond strength with an increase in the RCA incorporation. However, the addition of steel fibers marginally increased the bond failure capacity. Compared to the reference specimen of normal-strength concrete, at least, a 44% reduction in the NSM-concrete bond failure load was observed. Because some design guides (including the ACI 440) suggest a minimum concrete compressive strength for NSM FRP strengthening, a finite element model (FEM) was used to check the full spectrum of the mixture’s quality (both low and high-strength materials) and other key parameters affecting the NSM FRP-concrete bond behavior. The so-developed model is first validated against the experimental observations, followed by using it to run a series of parametric studies using the bar diameter, concrete compressive strength, FRP modulus, and steel fiber content as variables.

Flexural Strength of Damaged RC Beams Repaired with Carbon Fiber-Reinforced Polymer (CFRP) Using Different Techniques

In this study, an experimental program was developed to investigate the flexural behavior of pre-damaged reinforced concrete (RC) beams that had been repaired and strengthened using carbon fiber-reinforced polymer (CFRP) laminates under a monotonic load. Two techniques were used: externally bonded reinforcement (EBR) and near-surface-mounted (NSM) reinforcement, to repair and strengthen the tested beams. The experimental program involved casting and testing nine simply supported RC rectangular beams; one beam was considered as the reference beam and did not undergo additional strengthening, and the remaining beams were strengthened using CFRP laminates. These eight beams were divided into two main groups for the purposes of strengthening: beams for which the EBR technique was used, and beams for which the NSM technique was used. The primary variables observed in the EBR and NSM groups included four damage percentages obtained according to the preload (20, 40, 60, and 80%) from the ultimate load carried by the reference beam. The experimental results show that decreasing the damage percentage leads to an increase in ultimate strength from about 3.6% to 17.2% for the beams repaired using the EBR technique and from 27.6% to 57% for the beams repaired using the NSM technique; additionally, the NSM method was more effective than the EBR method in terms of the flexural strength and mode of failure. However, using CFRP laminates enhances the flexure capacity of strengthened RC beams.

Steel-concrete composite beams strengthened with NSM CFRP systems at the hogging-moment regions

Compared with conventional strengthening methods, the near-surface mounted (NSM) technique is more suitable for strengthening steel–concrete composite beams, especially at the hogging-moment regions. This paper investigates the flexural performance of steel–concrete composite beams strengthened with NSM CFRP systems. Four-point bending tests are carried out on four composite beam specimens, including one control beam without strengthening and three strengthened beams with NSM CFRP systems. The failure mode, ultimate load-carrying capacity, deflection distribution, and material strain distribution of the composite beams are presented, based on which the effects of the anchorage method and the number of CFRP bars are studied. The test results demonstrate that the main failure mode of the strengthened beams is the buckling of the steel beam. Compared with the control beam, the load-carrying capacity of the strengthened composite beams with two and four CFRP bars is increased by 10% and 20%, respectively. The use of end anchorage has a slight effect on the flexural capacity of the strengthened beams, while significantly improves the ductility during the later stage of loading. In addition, a nonlinear finite element (FE) model is proposed and calibrated, based on which a parametric study is performed to investigate the effects of the length and section area of CFRP bars, the spacing of grooves, and the thickness of bottom steel flange on the ultimate load-carrying capacity of the strengthened beams.

Effect of the NSM material and area on the flexural response of normal strength concrete beams internally reinforced with GFRP and steel bars

Strengthening the steel-reinforced concrete (RC) beams by fiber-reinforced polymer (FRP) elements have become common several years ago. Recently, the RC beams internally reinforced with FRP reinforcement, especially Glass FRP (GFRP) bars, became popular worldwide. The carbon FRP (CFRP) elements (strips and bars) were recently used to strengthen the internally GFRP RC beams. In contrast, using the GFRP elements to upgrade the RC beams, especially those with GFRP bars as the main tensile reinforcement, was rarely studied. In this paper, the GFRP bars were implemented as near-surface mounted (NSM) elements to upgrade the beams reinforced in their tension side with GFRP or steel bars. All beams were experimentally loaded in flexure using a four-point loading scheme. The experimentally studied parameters were the main tensile reinforcement area, NSM area, and main reinforcement properties. Moreover, a numerical model was established and validated using the available experimental data to investigate the impact of main and NSM materials properties and areas on RC beam response. The experimental results highlighted that increasing the internal and NSM GFRP area by 96% and 100 % increased the beam capacity by 61.6% and 59.7%, respectively, which underlined a slightly higher efficiency of the main reinforcement than NSM bars on the beam load capacity. In addition, substituting the main steel with GFRP bars for the beams upgraded with NSM GFRP reinforcement enhanced their capacities by 11.33–15.1%. This emphasized the great effect of the linear response of the FRP materials and their higher tensile strengths than steel on the interaction between the main and NSM reinforcement, especially after the steel yielding. The FE results showed the highest load capacity using CFRP and GFRP bars as main and NSM reinforcement for the simulated beams. Conversely, using CFRP bars as NSM strengthening was more efficient than GFRP bars for the RC beams with steel or GFRP bars.

Anchored CFRP ropes for flexural capacity recovering of thermally damaged RC one-way slabs

The effectiveness of utilizing near surface mounted (NSM) ropes of carbon fiber reinforced polymers (CFRP) in restoring the mechanical performance of heat-damaged RC slabs were studied using anchored and un-anchored techniques. The CFRP anchorage is supposed to increase the flexural behavior of the RC one-way slabs under investigation. To achieve this purpose, a total of (24) RC 1100 mm (long) × 500 mm (wide) slabs were devised. The experimental slabs were made in three heights; 60 mm, 80 mm, and 100 mm. Using an electric furnace for two hours each time, six of the slab were exposed to 200 °C, another six slabs were exposed to 400 °C, and the third group of six slabs was exposed to 600 °C, while the remaining six slabs were at room temperature. Twelve slabs were reinforced, in tension, with NSM-anchored ropes of CFRP. A test setup for 4-point loading was used to examine the slabs’ mechanical performance, while NSM CFRP ropes’ strain was measured. Also, the cracking patterns and failure modes were observed and documented. The slabs, with/without strengthening and damaged-by-heat/retrofitted, improved their toughness and load capacity up to 189% and 225%, respectively. In contrast, the slabs encountered a degradation in their displacement ductility with respect to the control slabs up to 2%, with the toughness ductility being increased up to 186% based on the slab’s rigidity and the exposed temperature. The present research demonstrated that the anchored NSM CFRP ropes to rehabilitate damaged-by-heat slabs was efficient in regaining the flexural behavior; the monitored behaviors indicated that. Lastly, it was observed that the improvement in the slabs was controlled by the transverse end-anchoring’s effectiveness, which permitted the prime ropes of CFRP to contribute more to the flexural performance without the possibility of the occurrence of un-expected de-bonding caused by cracking, even when the slabs reached their capacity. Based on the research findings, anchoring the utilized CFRP ropes and predicting its capacity using the proposed guidelines is recommended.

Finite Element Analysis of Flexural Strengthening of Reinforced Concrete Beams Using Near-Surface Mounted CFRP Rebars

Most of the standing structures today have already deteriorated or are on the verge of deterioration due to an increase in their age and harsh environmental conditions. For this very reason, the structures are in need of being upgraded or replaced. Retrofitting using near-surface mounted (NSM) fiber-reinforced polymer (FRP) rods has now emerged as a promising technique for strengthening the reinforced concrete members by increasing their flexural strength. This study aims to numerically investigate the efficiency of the NSM technique for flexural strengthening of RC beams by using finite element (FE) software, ABAQUS, and also evaluate the impact of variables such as concrete strength, embedded length of FRP rods, filler material used for the FRP rods, and the number of FRP rods provided. Validation of the finite element model was confirmed by first making a comparison with the experimental study presented in the literature for an un-strengthened beam. Thereafter, the validated model was used to simulate reinforced concrete beams strengthened with the NSM technique. The numerical results of the cracking moment, steel yielding moment, ultimate bending moment, and deflection at failure were reported and the impact of the variables was evaluated. The FE analysis results indicated that for the RC beams strengthened with NSM FRP rods, the flexural capacity significantly increased compared to the control beam, while the mid-span deflections of strengthened beams at failure decreased compared to the control beam.

Near-surface-mounted retrofitting of adobe walls using different materials: Evaluation of seismic performance

This study focuses on the seismic strengthening of adobe walls using different reinforcement materials and different reinforcement schemes through the near-surface-mounted (NSM) technique. Six adobe wall panels (1900 × 1200 × 200 mm) were quasi-statically tested and comprised one unreinforced wall and five specimens reinforced with reinforced mortar strip, bamboo plywood and wood, respectively. The strengthening procedure involved insertion of the reinforcing materials into the grooves cut symmetrically on both sides of the walls. The results were described in terms of energy dissipation, lateral resistance, ultimate displacement, stiffness degradation, ductility, and tolerate maximum peak ground accelerations. The tolerate maximum peak ground accelerations (PGA) of the specimens were analyzed using the N2 method. The test results indicated that the tensile strength, elastic modulus and scheme of reinforcement materials had a significant impact on the seismic performance and failure mode of specimens. The reinforced mortar strips significantly improved the seismic performance of adobe walls, particularly when using horizontal and vertical reinforcement schemes. The reinforcement of adobe walls with bamboo plywood and wood had achieved satisfactory results in terms of ductility, energy dissipation, and PGA. Finally, the masonry calculation formula was modified to establish the formula for estimating lateral resistance of adobe wall.

Performance of Postfire Reinforced Concrete Beams Retrofitted with External Bonded and Near-Surface Mounted CFRP: Experiments and Analyses

Fire imposes negative effects on reinforced concrete (RC) members, shifting postfire structures onto dangerous situations. This study aims at carbon fiber-reinforced polymer (CFRP) retrofit of postfire RC beams to recover their mechanical properties, providing technical information for structural engineers in practice. Toward this aim, experiments were performed for 16 RC beams, which were divided into four groups exposed to 30, 45, 60, and 75 min of ISO 834 fire. After exposure to fire, these beams were strengthened by external bonded (EB) and near-surface mounted (NSM) techniques using a similar amount of CFRP for comparison. These specimens were then loaded until failure. The results are compared with one another and compared with control specimens. It is found that both EB and NSM CFRP retrofitted beams failed in the form of peeling off of concrete, whereas no rupture of CFRP was observed. CFRP retrofit significantly changes the behavior of postfire RC beams, being brittle or exhibiting low ductility. The effectiveness of EB and NSM CFRP techniques is similar and it decreases with the increase in fire duration. CFRP retrofit increases the yield load-carrying capacity by 27.5%−40.9% for 30−60‐min postfire RC beams and recovers the yield load-carrying capacity for 75-min postfire RC beams. CFRP retrofit significantly increases the ultimate load-carrying capacity, whereas its increasing levels depend not only on the CFRP retrofit but also on the fire duration. Both CFRP retrofitting techniques successfully recovered the yield stiffness of postfire RC beams. The moment capacity of CFRP retrofitted postfire RC beams was theoretically analyzed, and a prediction model was proposed. The proposed model is simple, but it accurately predicts the moment capacity of retrofitted postfire beams; thus, it can be useful for structural engineers in practice.

Behavior of reinforced concrete ring beams strengthened with sustainable materials

Reinforced Concrete Ring Beams (RCRBs) have been used in Reinforced Concrete (RC) buildings, tanks, and roof shell structures, such as domes and cones. The curviness feature of RCRBs may result in excessive tensile stresses that may consequently generate critical cracks in concrete, particularly when the beams are constructed with Normal Concrete (NC). This paper presents experimental and numerical investigations on the structural behavior of RCRBs strengthened with sustainable materials. The experimental results on 12 RCRBs constructed with various types of concrete and strengthened with several techniques are presented and discussed. The specimens were made of NC, Engineered Cementitious Composite (ECC), High Strength Concrete (HSC), and High Strength Fiber Reinforced Concrete (HSFRC). Three strengthening techniques were employed for strengthening, including the External Bonded Reinforcement (EBR) with stainless-steel plates (SSPs), the Near Surface Mounted (NSM) with steel bars, and the pre-stressing system. Three-dimensional Finite Element Models (FEMs) were developed by using ABAQUS software to simulate the nonlinear performance of RCRBs. The test results show that the use of sustainable materials in RCRBs can remarkably improve the strength and ductility of RCRBs. In addition, the strengthening techniques are effective in enhancing the responses of RCRBs against the applied loads. Moreover, the thickness and configuration of SSPs significantly affect the ultimate and energy absorption capacities of RCRBs. Finally, a good agreement is observed between numerical predictions and experimental results, suggesting that the finite element model can be employed for further investigations.