Structural Strengthening/Repair of Reinforced Concrete (RC) Beams by Different Fiber-Reinforced Cementitious Materials - A State-of-the-Art Review

Sifatullah Bahij^1,2, Safiullah Omary^1*, Francoise Feugeas¹ and Amanullah Faqiri^2
*Correspondence: Safiullah Omary, Department of Civil Engineering, National Institute of Applied Sciences - Strasbourg, 67000, Strasbourg, France, Tel: + 0751522716, Email:

Keywords

Reinforced concrete beams • Strengthening techniques • Fiber-reinforced cementitious materials • Mechanical strengths • Crack pattern • Twisting angle

Introduction

Reinforced concrete (RC) is a combination of concrete and steel reinforcement. Unreinforced concrete has adequate compressive strength but low tensile strength, which results in concrete deterioration under lower traction or flexural applied loads. Therefore, steel reinforcements are needed inside plain concrete for improving the tensile performances [1-3]. It is highly required to update and modernize structures for economic rising and prosperity. For this purpose, improvement is needed in entire infrastructures, particularly RC structures as they will be exposed to severe degradation due to the influence of freeze-thaw, aggressive environments, de-icing salts, and overloading. Hence, it is a decisive issue for civil engineers to protect, retrofit, and maintain these deteriorating structural elements with the execution of new, low-cost repairing techniques to extend the lifetime of deteriorated and new structures [4-8].

Several studies have been conducted to identify various techniques and materials to restore damaged structures and strengthen the new structural elements. Fiber-reinforced polymers (FRPs) are the most commonly exploited materials for strengthening and repairing purposes. Researches were performed to study the behaviors of strengthened structural members with FRP and observed many useful outputs. However, the applications of FRPs contains some deficiencies, which prevent the execution of FRPs under cyclic loading in compression. These performances depend on the parent concrete strength, the bond behaviors between FRPs and concrete, and their durability [9]. Thus, new cementitious materials were generated and applied to repair and strengthen damaged or new RC structural elements, known as fiber reinforced concrete (FRC), high-performance concrete (HPC), high strength concrete (HSC), ultra-high performance fiber reinforced concrete (UHPFRC), steel fiber reinforced high strength lightweight self-compacting concrete (SHLSCC), fabrics reinforced cementitious material (FRCM) and etc. Many researchers underlined the two important features of these concretes (durability and strength) that show promising results [10-12].

These cementitious materials are the newest generation of concrete and are used in many civil engineering applications. Almost two decades ago ultrahigh performance concrete (UHPC) has been invented and characterized by steel fibers, cement, micro silica, sand, superplasticizer, and very low watercement ratio (w/c) [13]. These cementitious materials have high tensile and compressive strengths, high ductility, low permeability, and good durability because of their condensed microstructures. UHPC permits designers to select thinner sections and longer spans for structural elements [14,15]. The incorporation of steel fibers into UHPC improves their mechanical behaviors, reduces their brittleness, and changes the crack propagation performances [16]. Therefore, UHPC was considered for the rehabilitation and strengthening of the structural members. The main purpose was to utilize UHPC to strengthen those parts that are exposed to severe environmental conditions. Furthermore, research investigation has found that UHPC has a perfect bond with ordinary concrete to be used for repair and strengthen techniques, and rough surface preparation contributes to a higher bond [17].

The outcomes of an experiment on RC slabs strengthened with UHPC illustrate that UHPC reduced and postponed cracks growth, demonstrated excellent energy absorption, and increased the ultimate load capacity [18]. An experimental investigation was carried out to examine the structural behaviors of strengthened beams and found that UHPFRC considerably improves the structural performance of RC elements [8]. Researchers studied the efficiency of UHPFRC in strengthening existing RC structures and underlined the excellent performances of RC beams strengthened with UHPFRC 3-sides jacketing [19]. In addition, flexural strengthening of RC beams or slabs with UHPC were studied and found that UHPC could be used to improve such properties of RC elements [20]. Flexural properties of RC beams strengthened by engineered cementitious composites (ECC) were investigated and the results demonstrated that beams strengthened at the tension zone or sides displayed better strength and ductility characteristics compared to the control ones [21]. An experimental and numerical study was conducted to investigate the shear behaviors of RC beams strengthened by steel fiber reinforced concrete (SFRC) precast panels. The findings present that the shear behavior of the beams strengthened by SFRC panels was remarkably enhanced. In addition, nonlinear finite element analysis also found strong agreement with the experiments [22].

Highlights

• Fiber-reinforced cementitious materials have the perfect bond with the host concrete.

• Shear, flexural, and torsional load capacities of strengthened RC beams have improved.

• Load capacity of sandblasting method was lower than the epoxy adhesive technique.

• UHPFRC is the most recommended materials for strengthening and repair of RC beams.

Literature Review

Besides the experimental studies, several researchers have conducted the finite element method (FEM) to simulate the structural elements in recent FEM software. In this context, the researchers found a good agreement for UHPC beams studied by experimental and FEM methods [23]. In order to better understand the structural behaviors of the strengthened/repaired RC beams, a wide-ranging literature review was performed to evaluate the current state of the art for flexural, shear, and torsional-strengthening of RC beams using various fiber-reinforced cementitious materials. Moreover, the main aims of this review article are to emphasize on the effective strengthening schemes for flexural, shear, and torsional strengthening, and to investigate the deflection and failure mechanism of RC beams using these strengthening materials.

Fiber-reinforced cementitious materials

Scientists have tried to find a proper solution for the brittleness behaviors of materials from the very beginning of civil engineering applications. Previously, organic fibers have been incorporated into their mixtures to modify the brittleness of clay bricks but recently steel fibers are satisfactorily used to improve the behaviors of cementitious materials [24]. It is reported that more than 30 companies produce steel fibers and more than 100 types of steel fibers are produced worldwide. Mostly straight fibers were manufactured during their first productions, but more than 90% of steel fibers have been recently produced as a shaped fiber to increase their anchorage in concrete mixtures. Moreover, the fibers were produced over the last 40 years in twisted, crimped, flattened, spaded, coned, hooked, surface-textured, and melt-cast shapes of various diameter and lengths [25]. Ultra-high performance concrete (UHPC) is a recent type of fiber-reinforced concrete and has been characterized by fine steel fibers (2-10)%, no coarse aggregate, a high volume of fine aggregate, a high range of water reducing agent (superplasticizer), micro silica and low water-cement (w/c) ratio. UHPC possesses high compressive, flexural and tensile strengths, high toughness, high elastic modulus, low permeability, adequate freezing and thawing resistance, low carbonation depth, selfcompacting behavior, high durability, dense microstructure and etc. [26,27].

Experimental work was performed to examine the mechanical properties of UHPC and found that compressive strength was 150MPa, modulus elasticity was 47GPa and flexural strength improved to 35MPa [28]. Fiber-reinforced concrete is used in many civil engineering applications such as industrial floors, roads, airports, shell structures, railways, tunnels, reservoirs, bridges and etc. For example, several bridges in Canada, Korea, Japan; roof structures in France and Netherland; the cooling tower of a power station in France and etc. Coming to the point, many researchers found through their research investigations that fiber-reinforced cementitious materials are known as the most effective materials for strengthening and repairing activities, therefore, they reported these materials [29,30].

Bond between normal concrete and fiber-reinforced cementitious materials

Bonding properties between the host concrete (ordinary concrete) and the strengthening materials is one of the most important issues in rehabilitation processes. Several researchers have conducted experimental work and found a perfect bond between these two materials, and recommended fiber reinforced cementitious materials to strengthen and repair the structural elements. Experimental work was performed to study the bonding properties between normal concrete and UHPFRC. In this context, various tests were carried out such as; slant-shear test with the inclined bond interface at 55°, 60°, and 70°, pull off, and splitting tensile tests for two different bonds methods, epoxy-bonded (EP), and sandblasted (SB). The outputs present that normal concrete specimen with rough surfaces made by sandblasting present higher slant shear strength compared to epoxy-bonded ones. Furthermore, the findings of splitting tensile strength reported a perfect bond between normal concrete and UHPFRC [31]. Similarly, split tensile strength and slant-shear tests were conducted to measure the bond strength between the host concrete and ultra-high performance fiber concrete (UHPFC). The results indicated that UHPFC provides perfect bonding at the early repairing age, and works strongly together with the surface of the normal concrete [32]. Moreover, experimental work was carried out and found excellent bonding between the host concrete and UHPC. The outputs of the tensile splitting test highlight that the failure commonly happened in normal concrete samples. This means that UHPC bonded very powerfully and efficiently with the normal concrete, where the wire brush and scabbing techniques behave almost monolithic [17].

Strengthen of RC beams by fiber-reinforced cementitious materials

Commonly, the RC beams are strengthened/repaired to improve their flexural, shear, torsional, and other structural behaviors. In this context, several research investigations have been conducted to examine the structural behaviors of the RC elements strengthened by various cementitious materials. Although most of the studies were performed only as research but some findings have been put into practice and have shown excellent performances as well. In fact, these properties are directly related to the types of fiberreinforced cementitious materials, retrofitting configurations, and adhesive materials. This section aims to underline and summarize some of the recently performed research investigations regarding the flexural, shear, and torsional strengthening of RC beams using various cementitious materials.

Flexural behavior

Flexural strength, also recognized as rupture modulus and is the highest stress of material just before its yielding in a flexure test. The flexural strength of concrete beams is generally measured using rectangular cross-section or T-shaped samples and with the help of 3-point or 4-point loading setups [33,34].

In this regard, experimental and numerical work was performed to evaluate the flexural behavior of RC beams strengthened by UHPFRC. Overall 6 beams were prepared and tested: 2 of them as control specimens, 2 beams were strengthened by UHPFRC layers, and 2 others were strengthened by combined UHPFRC layers and steel bars. UHPFRC was produced from the mixed proportion of sand, silica fume, ground granulated blast furnace, CEM I 52.5 R, polycarboxylate superplasticizer, 3.0% of steel fibers by volume (13 mm long and 0.16 mm diameter), and w/c ratio of 0.28. The layers of UHPFRC were attached to the RC beams by shotcrete or proper formwork. The outputs indicated that the attachment of UHPFRC layers resulted in increased stiffness and first and ultimate flexural load capacities. While the UHPFRC layer plus steel bars resulted in a significant enhancement of these load capacities over the control ones. Moving to the crack pattern, the control beams started with the first cracks at lower loads, and then crucial cracks for failure of the beams were found in the middle of the span, and finally, beams failed at both compressive and tensile zones. The beams strengthened with the UHPFRC layers started with the first cracks on the UHPFRC layer and propagated toward RC beams, while some local debonding was seen at the interface and eventually the beams failed at the compressive zone. For beams strengthened by combined UHPFRC and steel bars, the first crack began at flexural zones and followed by a single crack propagated through the UHPFRC layer and resulted in beams’ failure. In addition, the bonding at the interface was found to be strong enough and no debonding has happened even during the final failure. The authors reported almost zero slip value at supports and the highest value of slipping was observed near the loading points. Finally, the experimentally tested beams were modeled numerically in ATENA software and the results show a good agreement with the experimental outputs [35].

In the same context, the experimental and numerical study was carried out to investigate the flexural characteristics of RC beams strengthened with the UHPFRC layer by two techniques:

a) Bonding in-situ UHPFRC layers using sandblasting, and

b) Bonding with the prefabricated UHPFRC layers using epoxy adhesive. In total, 8 beams were prepared and considered 3 different configurations:

• Bottom side,

• Two longitudinal faces, and

• Three sides strengthening and the jacketing thickness in each configuration was 30 mm.

The outputs underlined that flexural load capacity improved remarkably for the strengthened beams compared to the control ones. As a comparison, the beams strengthened in 3-sides experienced more improvement than beams strengthened just in the bottom portion. In addition, it was observed that beams strengthened using epoxy present higher load capacity than strengthened with sandblasting technique. Moving to the crack pattern, almost all beams strengthened with the help of sandblasting/epoxy failed in flexure that was started in the mid-span and propagated toward the supports. However, beams strengthened at the bottom showed a combination of flexural and splitting flexural cracks. While beams strengthened in 3-sides had fewer cracks due to the combination of side and bottom jackets, and the flexural cracks during failure were more concentrated to the mid-span. The beams’ load-deflection behavior was almost similar, the load was increased linearly with a slight decrease in stiffness during cracking up to yielding of the steel reinforcement. However, the beams strengthened with UHPFRC experienced higher stiffness compared to the control specimens because with the application of UHPFRC jackets the natural axis comes down. Additionally, the authors simulated the tested beams with the help of a nonlinear finite element method using ABAQUS software. The concrete damage plasticity model (CDPM) was considered to model concrete, while the behaviors of the materials were directly inputted into ABAQUS from the results of previously tested samples. The findings show that the outputs of FEM were best fitted with the experiments [31].

The research work (experiments, analytical, and FEM) was carried out to analyze the flexural behaviors of RC beams repaired by UHPFRC. Totally, 7 beams were prepared: a control specimen, 3 beams were repaired with different thicknesses of UHPFRC layers on the upper side, and 3 others were strengthened on the bottom side with various thicknesses of UHPFRC jackets. The UHPFRC layers consist of steel fibers that contain 13 mm length and 0.16 mm diameter. The results indicated that beams strengthened with UHPFRC jackets show higher flexural load capacity compared to the reference one. This is attributed to the high strength and strain hardening properties of UHPFRC. In comparison, the enhancement was more for beams containing thicker jacketing because a thicker layer contributes to smaller deformations for a given load and the creation of localized micro-cracks at higher loads. In addition, beams strengthened on the lower side showed better behavior than strengthened on the upper side. Moving to the crack pattern, all beams failed in flexure; the control beams failed in flexure with concrete crushing, while strengthened beams also failed in flexure but with UHPFRC crushing or rebar fracture. Moreover, it was observed that strain at the top of the control and strengthened beams reached the crushing value and resulting in concrete crushing at the fracture load. Except for the beams strengthened at the lower side had the same strain distribution behavior as the reference one, but strain at the top exceeded the crushing value, and bottom steel bars fractured because the strain was reached to the ultimate. The authors also conducted the analytical flexural model and finite element model by using nonlinear FEM software of MSC/Marc. They considered a perfect bond between reinforcements, concrete, and UHPFRC layers. The supports were modeled on plates as a roller with constraining to a single line of nodes at plates. Concrete was considered as a homogenous and initially isotropic material. It was found that analytical and FEM results best match the experiments. However, some differences were reported such as; the analytical model and FEM found that beams were stiffer than experiments. This is attributed to the fact that the experiment contains dry shrinkage, heat evolution during hydration, handling of RC beams that will cause micro-cracks [36].

Moreover, experimental work was carried out to study the flexural behavior of RC beams strengthened with UHPFRC laminates by different bonding techniques and rebar addition. The authors conducted the experimental work in three steps: 1) material characterizations to obtain a proper mix design among four percentages of steel fibers (1.0%, 2.0%, 3.0%, and 4.0%), whereas, 3.0% of steel fibers were selected in terms of both strength and workability, 2) testing of UHPFRC laminates to obtain the bare properties of full-scaled laminates, and 3) testing of RC beams strengthened with UHPFRC laminates in order to examine their flexural properties. Two bonding techniques were applied for strengthening, epoxy resin, and mechanical anchorage. In addition, steel bars were also added into some specimens. The findings highlighted that overall flexural load capacity increased, while beams were strengthened with UHPFRC laminates independent of the bonding method. However, this improvement was more significant for beams strengthened with the help of epoxy resin compared to the mechanical anchorage due to its high tensile strength. In addition, considerable improvement was highlighted, while steel bars were also added into RC beams. On the other hand, all the beams failed in flexural with fracture of laminates, but little difference was observed in cracking initiation, number of cracks, and their locations. In the epoxy resin method, the failure mode was changed from flexural to brittle concrete cover separation without the failure of UHPFRC laminates. In this case, the deflection has reduced because laminates act as rigid plates, and deflection decreased more, while steel bars were added into the beams. Besides, the beams strengthened with the help of mechanical anchorage also failed in flexure, but they were containing concrete crushing and failure of laminates [37].

Similarly, research work was conducted to analyze RC beams strengthened and repaired with high-performance fiber reinforced concrete (HPFRC). The authors considered 4 beams including a control one, beam without steel bars but strengthened by HPFRC, beam containing both steel bars and strengthened jackets, and RC beams repaired by HPFRC. The strengthening materials were obtained from the mix proportion of cement, silica fume, aggregates, superplasticizer, and steel fibers (12 mm length and 0.18 mm diameter). The results indicate a perfect bond and no-slip between host concrete and strengthening materials (HPFRC). The un-strengthened RC beam initiated with flexural bending cracks at 50kN load between two loading points, then cracks were developed to lose the bonding between reinforcement and concrete, and finally, the beam failed in flexure with debonding. On the other hand, the beam strengthened by HPFRC but without steel bars collapsed brittle at 258kN load with a single crack developed close to the mid-span. The RC beam strengthened by HPFRC has presented similar behaviors as the second one due to the presence of jacketing. Since the beam is reinforced a slight reduction in stiffness was observed due to cracks initiation, and finally, the beam collapsed with a single crack near support that contains longitudinal reinforcement rapture as well. The authors also pointed-out that HPFRC jacketing leads to a remarkable improvement in load capacity of the beams and this improvement was 2.15 times for the strengthened beam with steel bars. Additionally, the above-tested beams were numerically analyzed with the help of FEM software DIANA. A 3D model containing iso-parametric 20 nodes brick elements was selected for both concrete and steel reinforcement, and perfect bond was considered between steel and concrete, and between concrete and HPFRC. Generally, it was documented that FEM results were in a perfect agreement with the experimental outputs. However, some differences were recorded between these two findings. For example, variation in the stiffness of the RC beam without jacketing, which could be explained by the presence of splitting cracks in experiments but not existing in FEM due to the perfect bond. In addition, the HPFRC jacketing technique was also considered to repair the pre-damaged beams. The repaired beams had the same properties as the strengthening ones. Where the first cracking loads were similar as was observed for the strengthened beams, but the initial stiffness of the repaired beams was slightly lower than the strengthened ones. The improvement in load capacities of the repaired beams was lower than the strengthened ones [38].

In the same token, experimental work was performed on the RC beams strengthened by steel fiber-reinforced high strength lightweight self-compacting concrete (SHLSCC) to compare its results with the stress model. The mix design of SHLSCC contains: rolled and crushed coarse aggregate, fine aggregate, CEM-I, superplasticizer, fly ash, and steel fibers with the dimensions of 12 mm length and 0.2 mm diameter. Totally 8 beams were cast: one beam as a reference, one was made with half of the normal concrete and half of SHLSCC, and 6 others were strengthened by various thicknesses (40, 50, and 60) mm of the SHLSCC layers. For each strengthening thickness; one beam was considered as a pre-cracked and one as un-cracked. It was highlighted through the results that enough bond and no-slip was detected between the old concrete and SHLSCC, which prove the usage of SHLSCC to strengthen RC members. In addition, the authors noted a significant improvement in both stiffness and flexural load capacity of strengthened beams compared to the reference one and this improvement was more for beams containing a thicker layer of SHLSCC. It was well documented that beams strengthened with U-shaped jackets showed the highest load capacities among all beams. Precracked strengthened beams showed slightly lower flexural load capacities than the un-cracked beams. It was also found that the developed models are more effective to predict the flexural behavior of the beams strengthened with SHLSCC jackets [39].

A research was conducted to investigate the flexural behaviors of the RC beams strengthened by highly ductile fiber-reinforced concrete (HDC) and reactive powder concrete (RPC). A total of 12 beams were prepared and divided into 4 groups; 3 beams as a reference, 3 were strengthened by 30 mm thick HDC at the tension zone, 3 were strengthened with 30 mm thick HDC at the compression zone, and 3 others were strengthened with 30 mm thick RPC at the compression zone. The results highlighted that the ultimate flexural load capacity remarkably increased for the beams strengthened by the HDC layer at the tension zone. The flexural load capacities of the beams strengthened by HDC or RPC in the compression zone have also increased but such improvement was less compared to the beams containing strengthening layers in the tension zone. In addition, this enhancement was more for RPC-based strengthened beams than HDC-based ones. Generally, stiffness decreased for the beams strengthened by HDC or RPC and resulted in greater mid-span deflection compared to the control beams. However, beams strengthened by HDC in the tension zone had more stiffness and resulted in mid-span deflection reduction than retrofitted in the compression zone. Moreover, the control beams had elastic behavior before cracking, whereas the first cracks initiated in the bottom portion of beams, as the load was increased, more cracks were found and then widened between supports. Here, longitudinal reinforcements were yielded, the cracks extended to the compression zone followed by concrete crushing. For the beams strengthened by HDC, the initiation of first cracks were delayed but was found at the bottom, then widened and propagated toward the HDC layer. As the load was increased, firstly, the steel bars inside the HDC layer were yielded, then followed by original bars yielding, and eventually, concrete in the compressive zone was crushed. The delay in cracks occurrence is because of the high ultimate tensile strain of HDC. Overall, beams strengthened with HDC or RPC had fewer horizontal cracks, but beams with the HDC layer experienced debonding between the HDC layer and normal concrete at the end of loading, while the RPC layer had a good bond with the host concrete. The reason behind a good bonding between the RPC layer and host concrete is RPC’s high compressive strength [40].

A research study was conducted to investigate the flexural behaviors of the RC beams strengthened by engineered cementitious cement (ECC) + BFRP grids. Overall, 4 beams were prepared, one beam as a control specimen, and the other 3 were strengthened by 30 mm ECC and 1 mm, 3 mm, and 5 mm thick BFRP grids at the tension zone. The results show that ultimate load capacity remarkably improved for the strengthened beams and this improvement was more for a thicker layer of BFRP sheets. In addition, stiffness of the strengthened beams significantly increased compared to the control sample. Moving to the crack pattern, a flexural crack with concrete crushing was reported for the control beam. While beam strengthened with 1 mm and 3 mm thick BFRP grids, the rupture of BFRP grids at the mid-span was detected and followed by concrete crushing. Finally, the beam strengthened with a 5 mm thick BFRP grid was failed in flexure with debonding and BFRP grids fracture. Furthermore, there was no slip between the strengthening materials and the host concrete which shows the perfect bonding of the interface [41].

Similarly, a research investigation was conducted to study the flexural behaviors of RC beams repaired by various types of concrete. In total 15 beams were prepared and strengthened with four different types of concrete; UHPC, UHPFRC, normal strength concrete (NSC), and cement-based repaired material (CRM). From this, 3 beams were considered as control specimens, and 12 others were previously cracked and then strengthened with the help of the above four types of concrete. The authors found the flexural load capacity increased, while beams were repaired independent to the type of materials. However, this improvement was more for beams strengthened by UHPFRC, then CRM, followed by UHPC, and finally normal strength concrete. In addition, the repaired beams present less mid-span deflection and enhanced stiffness than the control ones. This is attributed to the high modulus of elasticity of repairing materials. As a comparison, the beam repaired by UHPFRC had the least deflection, then beam repaired by CRM, followed by a beam with UHPC, and finally, the beam that contains NSC as a repair material. Moving to the crack pattern, the crack pattern of all the repaired beams is flexure outside the repaired area. However, the beams repaired by UHPFRC and CRM experienced less widen and shorter cracks compared to the control beam and beams strengthened by UHPC and NSC [42].

Furthermore, a research study was performed to explore flexural behaviors of the RC beams retrofitted by HPFRC, designated CARDIFRC. In total, 32 beams were prepared; 4 beams were considered as references and the remaining 28 were strengthened with different configurations using epoxy as adhesive materials. The variable parameters were, retrofitting configurations, mix proportion of HPFRC, and the thickness of retrofitting layer. The results illustrated that retrofitting of RC beams by HFFRC not only enhanced the flexural load capacity, but also increased remarkably the serviceability of the beams in terms of a reduction in the number, width and length of the cracks. As comparison, the beams retrofitted by U-shape strips had the highest load capacity and the least mid-span deflection in both mix proportion compared to the other configurations. Secondly, the beams strengthened at the tension zone and at sides had higher load capacity and stiffness than beams strengthened only at the tension zone. In addition, the flexural load capacity and stiffness increased with the increase of strips thickness in both mix proportion. It was also observed that HPFRC containing long steel fibers were more effective in term of load capacity than with short fibers. Moving to the crack pattern, almost all control beams failed in flexure, but beams strengthened by HPFRC, the cracking mode changed from brittle shear to flexure or flexure-shear. This illustrated the HPFRC can be used effectively to the effectiveness of the strengthening configurations and materials. Finally, an analytical model was developed according to the stress-deformation diagrams of the Model Code CEB-FIP to predict the flexural behaviors of the RC beams that were experimentally tested. It was documented that the outputs for all control and retrofitted beams were well fitted with the experimental outcomes [43].

Additionally, experimental work was carried out to strengthen RC beams with the help of steel fiber-reinforced high strength lightweight self-compacting concrete (SHLSCC). Four various configurations were considered: no layer, 1-layer at the tension zone, 3-sides jacketing, and a half beam from normal concrete and a half from SHLCC. Overall, the peak load and stiffness of the strengthened beams considerably increased at any strengthened configuration. However, this improvement was more for 3-sides jacketed beams and specimens containing a thicker layer of SHLSCC compared to the control beams [44].

In the same context, research works were carried out to study the flexural behaviors of RC beams strengthened by fiber-reinforced cementitious materials with various configurations. The outcomes illustrate that these materials are recommended to improve the structural behaviors of the deteriorated or new structural members. In addition, the improvement level directly depends on the type of cementitious materials, strengthening method, adhesive materials and etc. [45-47]. Table 1 shows the relative percentage of the ultimate flexural load capacity and mid-span deflection, and cracking pattern for the strengthened RC beams.

Conclusions and Recommendations

The following points are summarized from the previously explained literature concerning the structural behaviors of RC beams strengthened by various types of fiber-reinforced cementitious materials and methods:

• Fiber-reinforced cementitious materials with the help of epoxy resin, sandblasting, shotcrete, or other methods have the perfect bonds with the host concrete. In addition, it was documented that the ultra-high performance fiber reinforced concrete (UHPFRC) with a high volume fraction of steel fibers was the most effective material than other types of fiber-reinforced cementitious materials for strengthening/repairing purposes. In addition, epoxy was confirmed as the most suitable adhesive materials than sandblasting, mechanical anchorage or others, while epoxy + bolting was the best connection technique to strengthened RC beams.

• Shear or flexural load capacities have increased remarkably for the beams strengthened by fiber-reinforced cementitious materials and this enhancement was more remarkable for the beams strengthened by 3-sides configuration compared to other retrofitting methods. Moreover, the beams strengthened by continuous strips or retrofitted in the tension zone had higher load capacity than beams having the spaced strips and strengthened in the compression zone, respectively. In addition, flexural and shear load capacities were strongly related and directly proportional to the thickness of the materials, number of the layers, strength of the bonding materials, longitudinal reinforcement ratio, volume fraction of steel fibers, strength of the concrete, beam size, and value of a/d ratio.

• While studying beams in flexure, almost all the strengthened beams had higher stiffness than the control ones because the natural axis is coming down with the introduction of retrofitting jackets. However, this enhancement was weightier for the beams strengthened with a thicker layer and U-jacketing of fiber-reinforced cementitious materials and using epoxy as adhesive materials. Moving to the crack pattern, almost all the beams failed in flexure. The control beams started with the first cracks at the bottom and then widened between supports, followed by the reinforcement yielding and flexural failure in the flexural zones. While, the strengthened beams failed in flexure as well and had similar trends as the control specimens, but followed by separation, debonding, and rapture of the jackets or steel bars. Moreover, fewer cracks were observed for the beams strengthened in 3-sides with a continuous and thicker layer of the strengthened materials, and the flexural cracks during failure were more concentrated to the mid-span.

• The mid-span deflection of the shear-strengthened beams improved remarkably than the control ones. However, the increase in mid-span deflection was more significant and directly related to the numbers and thickness of jacketing, beam size, shear reinforcement ratio, percentage of steel fibers due to its high modulus of elasticity, and etc. Furthermore, attachment of the continuous strips was more effective than spaced jackets because it provides higher strength and continuous confinement along the shear span, use of epoxy and long steel fibers that displayed superlative ductile behavior and were preventing RC beams from brittle failure. On the other hand, the majority of the control beams failed in pure and brittle shear that contains widened diagonal shear cracks and less flexural cracks near to the mid-span. While for the strengthened beams, the failure mode has been changed from the brittle shear to the ductile flexure, which verifies the effectiveness of the strengthening materials. However, the strengthened beams have experienced minor detachment or debonding between RC beams and the strengthening jackets or slippage/rupture occurred in jacketing layers. In addition, it was observed that the debonding could be eliminated with the introduction of steel bars or using epoxy + bolt connection.

• The torsional strength and twisting angle of the RC beams strengthened by different fiber-reinforced cementitious materials have improved significantly compared to the reference ones. This improvement was more remarkable for the fully wrapped beams compared to the 1-layer, 2-sides, and 3-sides configurations. In addition, beams strengthened with a thicker layer of fiber-reinforced materials, the strengthening materials with a high amount of steel fibers, concrete with high strength, and a high ratio of stirrups had enhanced torsional behaviors. Regarding the cracking pattern, it was reported that the control beam had more cracks and a faster rate of cracks development compared to the strengthened beams, while debonding or fibers rupture were noticed between the RC beam interfaces and jackets of the strengthening materials.

• Overall, the FEM results were in good agreement with the experimental findings. However, in some cases analytical and finite element methods represent somewhat stiffer behaviors than the experiments. This is attributed from the fact that the experiment involves dry shrinkage, heat evolution during hydration, and handling of RC beams that causes micro-cracks, while analytical and FEM do not include such micro-cracks.

Perspectives and recommendations for future work

It was observed from the previous research works that a large number of research investigations have been performed to study the structural behaviors of the RC beams retrofitted/repaired by different types of fiber-reinforced cementitious materials and various configurations. However, lots of information still remains unidentified that needs additional investigation and opens a window for future researchers.

• The shear span to effective depth ratio, a/d, fibers percentages, types, shape and orientation, and anchorage conditions have a great effect on both load capacities and displacements/rotation of the RC beams. Therefore, further investigations are needed to consider the effect of such parameters in the strengthened beams.

• The bonding between fiber-reinforced cementitious materials and the host concrete was effective but in some cases a small values of slip at the interface were recorded. Therefore, intensive research works are required to explore the interface characteristics between strengthening materials and the host concrete.

• Comprehensive, updated, and full design guidelines, code of practice, recommendations are required to ensure more rapid and effective applications of fiber-reinforced cementitious materials for the strengthening of structural elements.

• The above literature has summarized that a large variation between test results was reported due to differences in tested specimens, material types, test arrangement, loading configurations, and etc. Therefore, research work is required to develop a standardized testing method and procedure that will cover weather conditions, test duration, specimens’ shapes, and sizes, loading type and configuration, and etc.

Proposed methods and materials

After careful consideration and review of the previous research work, the authors recommend and propose the following retrofitting configurations and materials:

• It was found that the ultra-high performance fiber-reinforced concrete (UHPFRC) and epoxy have greatly enhanced the flexural, shear, and torsional properties, which are strongly proposing to strengthen/repair the RC beams.

• A combination of two configurations is proposed for the flexural strengthening; 1) at shear spans, the inclined strips in both faces and opposite direction of the diagonal cracks initiation, and 2) at flexural zones, full wrapping. Since in flexural loading, generally, the beam will fail in flexural zones, where full wrapping will delay the crack initiation and improve the flexural load capacity, stiffness, and decrease width and depth of the cracks. If the crack pattern will be changed from flexure to shear zones the inclined strips will work effectively.

• For shear strengthening, the switched method as was proposed for the flexural strengthening is suggested; 1) at shear zones, full wrapping, and 2) at flexural parts, the inclined strips in both faces. During shear loading, the beams are commonly designed to fail in shear spans, here, full wrapping will strengthen those portions effectively and will delay the crack initiation and improve shear behaviors of the RC beams. On the other hand, if the failure will happen in flexural zones, the inclined strips will effectively take responsibility.

• In the torsional retrofitting, full wrapping of complete beams is strongly recommended, as there are possibilities for the initiation of torsional cracks on any part of the beams. Such configuration could enhance both torsional load capacity and twisting angle and delay the initiation of the cracks.

Acknowledgements

Thanks to the technical staffs of both laboratories in France and Afghanistan for their help and support.

Declaration of Interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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