A state-of-the-art review on reinforced concrete slabs strengthened by frp sheets under monotonic, impact and repeated loading

Composites made of fiber-reinforced polymer (FRP) have gained popularity as new high-performance material for concrete buildings in now days. The advantages of FRP composites include high strength, lightweight, and corrosion resistance. This paper’s primary objective is to assess the affect of FRP strips on the strengthening and repair of plain and reinforced concrete slabs. under the monotonic, impact, and repeated loading. According to studies’ observations, applying FRP strips to RC slabs significantly affects the final load and deflection. The number and positioning of FRP sheets improve the performance of slabs. The failure load magnitude of strengthened slabs would increase as slab thickness, concrete compressive strength, and sheet thickness increase. The ultimate load capacity was increased by (27-52%), depending on the FRP strengthening strategy used. GFRP sheets could be employed for improving or enhancing the impact strength of concrete structures. Externally attached GFRP sheets gave good resistance for reinforced concrete slabs against typical explosive blast.


Introduction
Buildings with cracked concrete have become one of the most urgent needs that need to be treated before they become a problem.Some concrete structures developed in recent years are unsuitable for carrying service loads.Poor maintenance, an increase within the allowable load limit, inadequate reinforcing, excessive deflections, structural damages, or steel corrosion, which causes cracks, have all contributed to this inadequate load-carrying capability.In recent years, the amount of money spent on retrofitting existing structures has overtaken the amount spent on building new structures, owing to the use of traditional construction techniques [1].Using FRP for reinforcement and retrofitting in the worst-case loading conditions such as cyclic loads, concrete can be strengthened and retrofitted by rehabilitation/treatment changes to structural parts (such as foundations, columns, beams, and slabs) [2].FRP is more interesting to the civil engineering sector because of its various benefits.They come in a variety of formats, including sheets and laminates.These materials also feature a high strength-to-weight ratio, resistance to corrosion, a quick production time, and flexibility [3].

Theoretical Background Based on Design Codes
2.1.American Code (ACI 440-2R-17) ACI 440.2R-17 represent the producers of fiber-reinforced polymer techniques have established installation techniques for these systems.Installation processes can also differ based on the structure's nature and condition.The behavior of concrete members strengthened or retrofitted with FRP systems depends on a solid concrete substrate, adequate preparation, and surface profile.De-bonding of the FRP system due to improper surface preparation might occur before the intended load transfer is achieved [4].
The material characteristics, such as tension stress, often don't address long-term environmental sensitivity and should be regarded as initial characteristics unless specifically stated by the manufacturer.Materials used in design calculations must be lowered depending on the conditions of environmental exposure since long-term exposure has a negative effect on tensile and creep-rupture and fatigue resistance.Equations 1, 2, and 3 gave the tensile properties that should be used in all design equations [3]. ( ( Environmental reduction factor CE is used to calculate the design's ultimate tensile stress as listed in Table 1.
Table1.Exposure conditions and the environmental reduction factor for FRP systems [4].

Exposure conditions
Fiber 1-Due to the brittle failure mechanism related to de-bonding, the importance of the concrete-FRP bond is essential for strengthening reinforced concrete elements with FRP composites (loss of adhesion).Debonding failure shall not proceed in shear or flexural failure of the improved member according to the capacity design requirements.
2-Reinforced concrete beams' flexural and shear strengthening laminates or sheets may de-bond from the concrete with time.De-bonding can occur at any of the four locations shown in figure 1, It might be found either inside the adhesives, between the adhesive and the concrete, inside the concrete, or inside the FRP reinforcement (e.g. at the interface between two adjacent layers bonded to each other).It is common for failure to occur within the concrete itself, in the form of loss of a layer of material, when the good installation is carried out, the adhesive strength is often significantly greater than the concrete tensile strength (thickness may range from a few millimeters to the whole concrete cover).3-De-bonding failure modes for flexural strengthening are illustrated in graphical form as shown in Figure 2 and can be divided into four modes.

Previous Studies on Reinforced Concrete Slabs Enhanced by FRP Sheets under Monotonic Loading
Ali and Yehia (2016) [1] improved the ultimate capacity of RC one-way slabs under two-point loads by experimenting with various ways.Using (GFRP Sheets, CFRP Strips, or NSMR) to strengthen RC slabs is an effective method.
The main outcomes were: 1. Stabilizes the original specimens' ductility while increasing their initial stiffness by around half as shown in Figure 3.
2. The improvement can be accomplished by carefully choosing the proportion of the cross-sectional area of the reinforcing steel Af/As and using strengthening materials with elasticity moduli that give substantially close conditions.
3. Cost analysis in this research also helps the end-users select the most appropriate technique for the project's conditions and the governing factors, allowing them to focus on what is most important.2018) [6] evaluated RC cantilever slab behavior enhanced by different techniques.The main cantilever slab's working load (ultimate load) was put onto the cantilever elements to a level of 40%.The cracks were subsequently repaired with epoxy injection, and other strengthening methods were used.The test samples were repeatedly loaded till failure.Seven specimens of the reinforced concrete cantilever, the first one is the control, and the other six were divided into three groups as shown in Table 2.The main outcomes were: 1.All the techniques of strengthening improved the behavior of the cantilever slab as shown in Figure 4.
2. Regardless of how the shear connector bars are installed, the jacket strengthening procedure has a considerable impact on all measured responses.
3. Although GFRP sheets de-bonded, strengthening with 60 percent of the cantilever length resulted in higher ductility and stiffness when compared to 90 percent of the cantilever length.4. Due to early debonding, which may be attributable to the deficiency of the epoxy resin's bonding resistance, the epoxy resin-based technique for reinforcing steel plates does not achieve the desired effect, as determined by test results.The description of tested slabs was listed in Table 3 and the main outcomes were: 1-Strengthening CFRP sheets at the lower face of the slab is an effective way to increase the ultimate load.
2-The final load of slabs enhanced by CFRP strips is up to 132% at failure greater than that of unstrengthened slabs, while the deflection is up to 54% less.
3-In the compression zone, steel reinforcement yields, followed by significant rupture of CFRP strips, and finally concrete is crushed.It is definitely a flexural failure.
4-Multiplying strip thickness results in a large increase in failure load as shown in Figure 5. Table 3. Detials of slabs [7].(i) One slab is the control without repairing (S01).

Group
(ii) Two slabs were repaired and enhanced with GFRP strips bonded to the negative moment zone for (S03) and (S06) as shown in figure 6a.
(iii) The final two slabs were repaired and enhanced with both GFRP strips and anchors for (S04) and (S07) as shown in figure 6b.The main outcomes were: 1.As a top mesh, GFRP is used for strengthening and repairing cantilever slabs and has a substantial impact on enhancing load-carrying capacity.
2. On cantilever slabs, strengthening approaches using GFRP as a top mesh, both without and with anchors, have a greater impact than repairing by GFRP as a top mesh on cantilever slab, both and without anchors.
3. Anchors improved the tension concentration in the area around them.When the anchor fails, the fibers are cut at the anchor position, however when the anchor is not used, the fiber just de-bonds.
4. Anchors have a greater impact on slabs strengthening with GFRP than slabs repaired with GFRP.

Previous Studies on Reinforced Concrete Slabs Enhanced by FRP Sheets under Repeated or Impact Loading
Guo et al. (2017) [9]used glass fiber reinforced polymer (GFRP) strips attached externally to reinforced concrete (RC) slabs for retrofitting them such that they are better able to withstand conventional explosions.Five RC slabs of (1500*1500*150 mm) including one reference specimen and four slabs with GFRP sheets with two retrofitting schemes were adopted as shown in figure 7 a-b.The main outcomes were: 1.A series of blast tests with increasing intensity were performed on all specimens.As a result, residual damage collected on the slabs before exposed to greater intensity of blast will affect both of the blast wave criteria and the slabs' blast reaction.
2. The slab samples finally broke in the flexural mode under standard explosive blast loading, with GFRP strips entirely rupturing or de-bonding from the bottom face of concrete.The fracture pattern on the rear face is identical to the yield line pattern when the specimens are under static center load.
3. Overall, the strengthened slabs outperformed the control slab and were able to withstand the higher explosive blast.Strengthening scheme 2 [9].Abdel-Kader and Fouda (2017) [10] studied the impact of hard projectiles on 12 plain concrete specimens and also examined the effect of applying GFRP strips with different shapes to strengthen plain concrete panels under this type of loading as shown in figure 8: The main outcomes were: 1. Due to the strengthening effect of GFRP, the peak impact load of specimens with a rear GFRP sheet was approximately 23 % more than that of specimens without a rear GFRP sheet.
2. When GFRP sheets are placed on both faces of the concrete panels, would be more successful (causing less damage to the specimen).
3. As long as the GFRP sheets were not in the axis of the projectile, retrofitting concrete panels with GFRP sheets had no noticeable effect on ballistic performance.
Jamal and Vijayan (2018) [11] investigated slab specimens with elliptical balls only and with elliptical balls and GFRP strips called (bubble-deck slabs) which were tested under uniformly distributed load with appropriate boundary conditions and were analyzed by finite elements using both M25 and M30 grades of concrete.A slab with elliptical balls was modeled of (1730x1350x230) mm and the elliptical void of (180x240) mm and is assumed to be made of high-density polyethylene (HDPE) as shown in figure 9 .The main outcomes were: 1. Slab with elliptical balls and GFRP strips possess a higher ultimate load compared to that of the specimens without sheets 2. Slab with elliptical balls and GFRP strips shows less deflection compared to the specimens without sheets.Soltani, Khaloo, and Sadraie (2020) [12] made a comparison under impact loading between using exterior strengthening by (GFRP) strips to the slab and interior strengthening by using steel fibers with concrete.Performance of fourteen thin two-way (1 × 1 × 0.075 m) concrete slabs including one plain specimen, one steel RC specimen, three steel RC specimens supported by steel fibers with varying volume fractions, and nine steel RC specimens enhanced by GFRP strips as shown in figure 10 under impact loads produced by drop weight is tested experimentally.The main outcomes: 1-Following the experimental findings of slabs with steel fibers, increasing the percentage of S.F improves the impact resistance of specimens up to a specific value.Specimens casted with a high percentage of fibers exhibit a detrimental effect on their strength characteristics and impact resistance.
2-Under impact load, an increase in the number of GFRP strips on the bottom face of the slab has a greater effect on reducing slab displacement than using GFRP sheets on the top face of the slab.
3-The performance of slabs with completely bottom GFRP sheets is superior to those with steel fibers.4-The distributed longitudinal GFRP strips (lower width and more strips) reduced the overall displacement more than the other configurations of GFRP strips among slabs strengthened by GFRP sheets with the same area with various configurations.2020) [13] carried out an experimental investigation on two-way reinforced concrete slabs to study the impact response of glass fiber reinforced polymer (GFRP) sheets as reinforcement (TSRCS).Nine TSRCS of 1000 × 1000 ×60 mm with the following parameters as shown in figure 11: (i) Two GFRP strips of varying widths (50&75 mm) were bonded to the bottom face.
(ii) There were two different techniques to strengthen the structure (vertical and angled directions in two separate directions).The main outcomes were: 1.The stiffness, toughness, consistency, and acceleration parameters of a slab reinforced with GFRP strips were all improved.The greater acceleration means greater the slab's resistance to impact and its ability to absorb collision energy.When GFRP strips were increased from 50 to 75 millimeters wide, the acceleration and impact resistance increased.
2. Two-direction lamination of GFRP strips increased the acceleration values of TSRCS significantly more than strengthening the slab in a single direction.TSRCS's impact resistance was also improved by GFRP strengthening.
3. TSRCS is an excellent composite for buildings under impact loads because of its capacity to absorb collision energy under falling mass impact.TSRCS can be used in a wide range of applications, including concrete on offshore constructions, industrial flooring, transportation infrastructure, airport runways, and protective structures.
Yoo et al. (2020) [14] examined the structural behavior of RC slabs enhanced in four different techniques by FRP sheets and subjected to impact loading transmitted from the upper to the bottom side.
FRP sheet, sprayed FRP, and no-slump high-strength, high-ductility concrete (NSHSDC) were used to strengthen the upper and lower sides of test slabs.Four different methods of reinforcing six two-way slab specimens were used in the testing (FRP, sprayed FRP, NSHSDC, and hybrid) as shown in the figure 12.The main outcomes were: 1.In terms of impact resistance and strength, the NSC-NF was an excellent choice.NSHSDC was found to provide the best combination of strength and stiffness when applied to the upper and lower sides of the test specimens.NSHSDC's steel and PE fibers keep micro-cracking under control and distribute impact energy uniformly across the specimen.10 2. The NC-F specimen had the lowest deflection at the initial blow, approximately 26% less than the reference samples.An increase in early stiffness can be related to CFS.
3. As a hybrid made from PE and steel fibers, the NSC-NF crack not only dispersed the bottom crack evenly but also increased the deflection capability.FRP and sprayed FRP can't be used to confirm cracks because of the FRP adherence, which makes it impossible.

Conclusions
The main conclusions that emerged from studying previous literature are as follows: 1-Adding FRP strips or NSMR to strengthen RC slabs is a reliable method that can raise the original specimens' initial stiffness by around 1.5 times while still maintaining their ductile behavior.
2-Increasing FRP sheet area, number, and sheet thickness to double and triple results in a decrease in mid-span displacement and a rise in failure load.
3-Increased flexural resistance capacity against conventional explosive blasts can be achieved through retrofitting RC slabs with GFRP strips that are externally bonded.
4-Superior performance of concrete panels containing GFRP with epoxy resin under the impact load.5-In comparison to other GFRP strip configurations between slabs reinforced by GFRP strips with the identical area with different configurations, the dispersed longitudinal GFRP strips (small width and more strips) led to a superior reduction of the maximum deflection.
6-Strengthening slabs with one or two layers of GFRP sheets at the lower face of concrete slab has a positive effect on reducing deflection by up to 35% and improving ultimate capacity for two layers of GFRP sheets.This is a result of the increased slab rigidity.

Figure 1 .
Figure 1.De-bonding between concrete and FRP [5].3-De-bonding failure modes for flexural strengthening are illustrated in graphical form as shown in Figure2and can be divided into four modes.xMode[1] (sheet/laminate end de-bonding)x Mode[2] (An intermediate de-bonding induced by cracks in flexure)x Mode[3] (De-bonding due to shear cracks)x Mode[4] (Roughness and irregularity of concrete surface due to de-bonding)[5]

Figure 3 .
Figure3.Load-deflection relationship for strengthened slabs[1].Hussein et al. (2018) [6] evaluated RC cantilever slab behavior enhanced by different techniques.The main cantilever slab's working load (ultimate load) was put onto the cantilever elements to a level of 40%.The cracks were subsequently repaired with epoxy injection, and other strengthening methods were used.The test samples were repeatedly loaded till failure.Seven specimens of the reinforced concrete cantilever, the first one is the control, and the other six were divided into three groups as shown in Table2.The main outcomes were:1.All the techniques of strengthening improved the behavior of the cantilever slab as shown in Figure4.2.Regardless of how the shear connector bars are installed, the jacket strengthening procedure has a considerable impact on all measured responses.3.Although GFRP sheets de-bonded, strengthening with 60 percent of the cantilever length resulted in higher ductility and stiffness when compared to 90 percent of the cantilever length.Table2.Cantilever slabs classification[6].

Figure 4 .
Figure 4.Load-deflection curves under applied load[6].Emarah (2019) [7] investigated the response of RC slabs attached to CFRP strips using the ANSYS 15 model then the results were compared to the Egyptian Standing Code (ESC,2005).Additionally, this work demonstrates the impact of the width and location of CFRP on the flexural behavior of RCS.The finite element investigations used for one-way reinforced concrete slabs RCS of (1500*1000*80 mm).The description of tested slabs was listed in Table3and the main outcomes were: 1-Strengthening CFRP sheets at the lower face of the slab is an effective way to increase the ultimate load.2-Thefinal load of slabs enhanced by CFRP strips is up to 132% at failure greater than that of unstrengthened slabs, while the deflection is up to 54% less.3-In the compression zone, steel reinforcement yields, followed by significant rupture of CFRP strips, and finally concrete is crushed.It is definitely a flexural failure.4-Multiplyingstrip thickness results in a large increase in failure load as shown in Figure5.Table3.Detials of slabs[7].

Figure 5 .
Figure 5.Comparison between S20 and S20D [7]Ayash, Abd-Elrahman, and Soliman (2020)[8] studied the experimental and numerical performance of GFRP-repaired or strengthened reinforced concrete cantilever slabs.Five one-way cantilevers simple slabs were subjected to two-point loading in the experimental test program:(i) One slab is the control without repairing (S01).(ii)Two slabs were repaired and enhanced with GFRP strips bonded to the negative moment zone for (S03) and (S06) as shown in figure6a.(iii)The final two slabs were repaired and enhanced with both GFRP strips and anchors for (S04) and (S07) as shown in figure6b.The main outcomes were:1.As a top mesh, GFRP is used for strengthening and repairing cantilever slabs and has a substantial impact on enhancing load-carrying capacity.2.On cantilever slabs, strengthening approaches using GFRP as a top mesh, both without and with anchors, have a greater impact than repairing by GFRP as a top mesh on cantilever slab, both and without anchors.3.Anchors improved the tension concentration in the area around them.When the anchor fails, the fibers are cut at the anchor position, however when the anchor is not used, the fiber just de-bonds.4.Anchors have a greater impact on slabs strengthening with GFRP than slabs repaired with GFRP.

Figure 7 .
Figure 7. Strengthening of specimens: (a) Strengthening Scheme 1 (b)Strengthening scheme 2[9].Abdel-Kader and Fouda (2017)[10] studied the impact of hard projectiles on 12 plain concrete specimens and also examined the effect of applying GFRP strips with different shapes to strengthen plain concrete panels under this type of loading as shown in figure8:The main outcomes were: 1. Due to the strengthening effect of GFRP, the peak impact load of specimens with a rear GFRP sheet was approximately 23 % more than that of specimens without a rear GFRP sheet.2.When GFRP sheets are placed on both faces of the concrete panels, would be more successful (causing less damage to the specimen).3.As long as the GFRP sheets were not in the axis of the projectile, retrofitting concrete panels with GFRP sheets had no noticeable effect on ballistic performance.Jamal and Vijayan (2018)[11] investigated slab specimens with elliptical balls only and with elliptical balls and GFRP strips called (bubble-deck slabs) which were tested under uniformly distributed load with appropriate boundary conditions and were analyzed by finite elements using both M25 and M30 grades of concrete.A slab with elliptical balls was modeled of (1730x1350x230) mm and the elliptical void of (180x240) mm and is assumed to be made of high-density polyethylene (HDPE) as shown in figure9.The main outcomes were:1.Slab with elliptical balls and GFRP strips possess a higher ultimate load compared to that of the specimens without sheets 2. Slab with elliptical balls and GFRP strips shows less deflection compared to the specimens without sheets.

3 .
Slab with elliptical balls and GFRP strips and made with M30 grade concrete possess good performance than M25 grade concrete.

Figure 10 .
Figure10.GFRP strengthened slabs[12].Loganaganandan et al. (2020)  [13] carried out an experimental investigation on two-way reinforced concrete slabs to study the impact response of glass fiber reinforced polymer (GFRP) sheets as reinforcement (TSRCS).Nine TSRCS of 1000 × 1000 ×60 mm with the following parameters as shown in figure11:(i) Two GFRP strips of varying widths (50&75 mm) were bonded to the bottom face.(ii)There were two different techniques to strengthen the structure (vertical and angled directions in two separate directions).The main outcomes were:1.The stiffness, toughness, consistency, and acceleration parameters of a slab reinforced with GFRP strips were all improved.The greater acceleration means greater the slab's resistance to impact and its ability to absorb collision energy.When GFRP strips were increased from 50 to 75 millimeters wide, the acceleration and impact resistance increased.2.Two-direction lamination of GFRP strips increased the acceleration values of TSRCS significantly more than strengthening the slab in a single direction.TSRCS's impact resistance was also improved by GFRP strengthening.3.TSRCS is an excellent composite for buildings under impact loads because of its capacity to absorb collision energy under falling mass impact.TSRCS can be used in a wide range of applications, including concrete on offshore constructions, industrial flooring, transportation infrastructure, airport runways, and protective structures.Yoo et al. (2020)[14] examined the structural behavior of RC slabs enhanced in four different techniques by FRP sheets and subjected to impact loading transmitted from the upper to the bottom side.FRP sheet, sprayed FRP, and no-slump high-strength, high-ductility concrete (NSHSDC) were used to strengthen the upper and lower sides of test slabs.Four different methods of reinforcing six two-way slab specimens were used in the testing (FRP, sprayed FRP, NSHSDC, and hybrid) as shown in the figure12.The main outcomes were:1.In terms of impact resistance and strength, the NSC-NF was an excellent choice.NSHSDC was found to provide the best combination of strength and stiffness when applied to the upper and lower sides of the test specimens.NSHSDC's steel and PE fibers keep micro-cracking under control and distribute impact energy uniformly across the specimen.