Fiber Reinforced Polymer Laminates for Strengthening of RC Slabs against Punching Shear: A Review.

Reinforced concrete flat slabs or flat plates continue to be among the most popular floor systems due to speed of construction and inherent flexibility it offers in relation to locations of partitions. However, flat slab/plate floor systems that are deficient in two-way shear strength are susceptible to brittle failure at a slab–column junction that may propagate and lead to progressive collapse of a larger segment of the structural system. Deficiency in two-way shear strength may be due to design/construction errors, material under-strength, or overload. Fiber reinforced polymer (FRP) composite laminates in the form of sheets and/or strips are used in structurally deficient flat slab systems to enhance the two-way shear capacity, flexural strength, stiffness, and ductility. Glass FRP (GFRP) has been used successfully but carbon FRP (CFRP) sheets/strips/laminates are more commonly used as a practical alternative to other expensive and/or challenging methods such column enlargement. This article reviews the literature on the methodology and effectiveness of utilizing FRP sheets/strips and laminates at the column/slab intersection to enhance punching shear strength of flat slabs.


Introduction
Reinforced concrete slabs that are not supported by beams, also known as flat slabs are the most popular floor system in the building construction industry due to architectural flexibility and speed of construction. Speed of constructing flat slab systems is owed primarily to savings in time that would have been needed to build formwork for supporting beams. They also offer designers the flexibility of placing heavy and light partitions anywhere on the floor slab without abiding by location of beams. However, flat slabs are susceptible to brittle two-way shear failure (i.e., punching shear failure) at the slab-column connection that is caused by the transfer of shear and unbalanced moments [1]. Although the mechanics of punching shear is not completely understood, many methods have been developed over the years to prevent this type of failure. Four basic stages in the punching failure of a slab-column connection are generally recognized. Firstly, flexural and shear cracks form in the tension zone of the slab near the face of the loaded area. Secondly, the slab tension steel close to the loaded area yields. Thirdly, flexural and shear cracks extend into what was the compression zone of the concrete. Finally, failure occurs before yielding of reinforcing steel extends beyond the vicinity of the loaded area. A possible reason for punching failure is the rupture of the reduced compression zone in the slab [2]. The failed surface forms a conical shape enclosed by an inclined critical crack pattern which meets a horizontal crack parallel to the reinforcing steel reaching the tension side [3]. When a slab-column connection is loaded, the initial response is linear-elastic, followed by cracking which reduces the  Effect of flexural reinforcement on punching shear capacity Minimum flexural steel must be provided near tension face of the slab in two-perpendicular directions (ρ min = 0.18%, for grade 420 MPa reinforcement) (Section 8.6 [19] and commentary). Integrity reinforcement in column strip is required (8.7.4.2) to provide residual punching shear strength following punching shear failure. A minimum of two bottom bars shall pass in two directions through the region bounded by column vertical reinforcement and be anchored at exterior support.
Incorporated in punching shear strength formula. Clearly EC2 recognizes significant increase in punching shear strength in heavily reinforced slabs.
Effect of slab thickness on punching shear strength It is incorporated in equation "c", indicated punching shear capacity increases with increase in the ratio of effective depth to critical perimeter 0.083 2 + αsd b0 λ f c (MPa) Implicitly addressed in the commentary by indicating that the minimum tension steel development length beyond column face on each side for slabs under predominantly uniform gravity load with clear span to depth ratio (l n /h) at less than 15. For thicker slabs with (l n /h), provide continuous tension steel to intercept punching shear cracks.
However, size effect parameter, limited by upper value of 2.0 is.
Effect of supporting member dimensions on punching shear strength Incorporated in the parameter β, which represents the ratio of the long support side to the short side.
Upper limit on punching shear strength is 0.33λ f c (MPa) around corners of columns and decreases to 0.17λ f c or less along long sides of the support between the two end sections Not explicitly indicated in the capacity expression.
Polymers 2020, 12, 685 4 of 27 As indicated in Table 1, ACI 318 requires integrity longitudinal reinforcing steel to provide residual capacity in the event of loss or damage of vertical load-carrying member. A study by Weng et al. [20] showed that integrity reinforcement plays a significant role in post-punching behavior of flat slabs and recommends a minimum of 0.63% integrity reinforcement ratio to be provided. A typical concern in relation to punching shear failure in flat plates is that its brittle nature may lead to progressive collapse of large segment of the structural system. Mohamed et al. [21] discussed effect of two-way shear on triggering progressive collapse, along with recommended design/detailing measures to enhance the post-failure behavior of the structural system.

Experimental Specimens, FRP Strengthening Patterns, and Methods
The following section includes discussion on details of experimental program, various FRP strengthening patterns and methods and some significant research findings. Binici and Bayrak [25] used 25-mm wide vertical CFRP strips for strengthening the flat slab specimens (2133 mm × 2133 mm × 152 mm) against punching shear. Strengthening of flat slab is achieved by punching 18-mm diameter vertical holes around the loading area. A practical downside to the method is the potential of cutting tension and/or integrity steel around the column which could contribute to failure. Two specimens had four holes in a line extending from each side of the loading area, one specimen had six holes on each side of the loading area, and one specimen had eight holes extending from each side of the loading area. The holes started at distance equals to 1 4 of the effective depth (d/4) from the face of the loaded area and spaced at half of the effective depth (d/2) center-to-center. The authors clearly observed that that the use of diagonal CFRP strips between the holes containing the vertical strips as seen in Figure 1A1-A3 around the loading area is effective in preventing punching shear inside the shear-strengthened area compared to pattern B, with orthogonal strips. Extending CFRP shear-strengthening further along with diagonal strips to join vertical strips (A3) increases load-carrying capacity 1.51 times and the ductility 2.0 times the unstrengthening control specimens. In addition, extending the shear strengthening further from the loading area (A2 and A3), for the specimens with diagonal CFRP strips was effective in changing the failure mode from brittle punching to a more ductile failure mode.
Applying CFRP vertically in the form of shear reinforcement is superior to bonding CFRP strips/laminate on the tension side in terms of enhancing punching shear strength. Sissakis and Sheikh [26] reported 80% increases in punching shear capacity by applying CFRP strips vertically through holes created at constant spacing perpendicular to the loading column in a manner similar to stitching of the slab.
Harajli and Soudki [27] demonstrated that providing reinforcing CFRP strips on the tension face of the slab increased the punching shear strength ranged from 17% to 45% compared to the control flat slab specimens that did not contain CFRP strips for strengthening purpose as seen in Figure 2. The mechanism by which this method of strengthening increases two-way shear strength is by restricting the growth of tension cracks or by increasing flexural strength at the connection. Therefore, strengthening the slab on the tension slide in the column zone may change flexural failure mode to flexure-shear failure or pure two-way shear failure. The authors tested 670 mm square specimens with tension steel reinforcement ranging from 1% to 2%. The loading column is 100 mm × 100 mm which was cast monolithic with the slab. Four CFRP strip widths (50, 100, 150, and 200 mm) were used. Using strips on the tension side only did not affect the punching shear cracking extent and remained at a distance 2.0 × h (h = overall slab depth) from the face of the loading column. Applying CFRP vertically in the form of shear reinforcement is superior to bonding CFRP strips/laminate on the tension side in terms of enhancing punching shear strength. Sissakis and Sheikh [26] reported 80% increases in punching shear capacity by applying CFRP strips vertically through holes created at constant spacing perpendicular to the loading column in a manner similar to stitching of the slab.
Harajli and Soudki [27] demonstrated that providing reinforcing CFRP strips on the tension face of the slab increased the punching shear strength ranged from 17% to 45% compared to the control flat slab specimens that did not contain CFRP strips for strengthening purpose as seen in Figure 2. The mechanism by which this method of strengthening increases two-way shear strength is by restricting the growth of tension cracks or by increasing flexural strength at the connection. Therefore, strengthening the slab on the tension slide in the column zone may change flexural failure mode to flexure-shear failure or pure two-way shear failure. The authors tested 670 mm square specimens with tension steel reinforcement ranging from 1% to 2%. The loading column is 100 mm × 100 mm which was cast monolithic with the slab. Four CFRP strip widths (50, 100, 150, and 200 mm) were used. Using strips on the tension side only did not affect the punching shear cracking extent and remained at a distance 2.0 × h (h = overall slab depth) from the face of the loading column. strengthening the slab on the tension slide in the column zone may change flexural failure mode to flexure-shear failure or pure two-way shear failure. The authors tested 670 mm square specimens with tension steel reinforcement ranging from 1% to 2%. The loading column is 100 mm × 100 mm which was cast monolithic with the slab. Four CFRP strip widths (50, 100, 150, and 200 mm) were used. Using strips on the tension side only did not affect the punching shear cracking extent and remained at a distance 2.0 × h (h = overall slab depth) from the face of the loading column.  El-Salakawy et al. [28] concluded that applying CFRP or glass FRP (GFRP) strips on the tension surface of flat slab around edge columns increases the flexural stiffness and delays opening of flexural cracks, and hence increases punching shear capacity. Depending on the area of FRP sheets and configuration, the increase of edge column punching shear strength ranges from 2% to 23% compared to unstrengthen specimens. The authors examined effectiveness of strengthening edge columns in flat slab systems. Specimens were 1540 mm × 1020 mm × 120 mm thick and the supporting edge column was 250 mm × 250 mm. The slabs were reinforced with an average of 0.75% tension steel in each direction and an average of 0.45% compression steel in each direction. L-shaped strips were El-Salakawy et al. [28] concluded that applying CFRP or glass FRP (GFRP) strips on the tension surface of flat slab around edge columns increases the flexural stiffness and delays opening of flexural cracks, and hence increases punching shear capacity. Depending on the area of FRP sheets and configuration, the increase of edge column punching shear strength ranges from 2% to 23% compared to unstrengthen specimens. The authors examined effectiveness of strengthening edge columns in flat slab systems. Specimens were 1540 mm × 1020 mm × 120 mm thick and the supporting edge column was 250 mm × 250 mm. The slabs were reinforced with an average of 0.75% tension steel in each direction and an average of 0.45% compression steel in each direction. L-shaped strips were used perpendicular to the free edge while straight strips were applied on the tension side parallel the main reinforcement in each direction. FRP strip width was 50 mm when CFRP was used for strengthening while 75 width strips were used when GFRP was used for strengthening as seen in Figure 3.
Polymers 2020, 12, x 7 of 28 used perpendicular to the free edge while straight strips were applied on the tension side parallel the main reinforcement in each direction. FRP strip width was 50 mm when CFRP was used for strengthening while 75 width strips were used when GFRP was used for strengthening as seen in Figure 3.  A study by Chen and Li [29] confirms that GFRP strengthening sheet increases the punching shear capacity by acting as an external flexural reinforcement. The percentage increase in punching shear capacity is higher in lower grade concrete (16.9 MPa) than in concrete of normal strength (34.4 MPa). However, GFRP sheets change the failure mode from flexure-punching failure to brittle punching failure. This study included 1000 mm × 1000 mm × 150 mm specimens loaded centrally with 150 mm square column stub to simulate interior columns in flat slabs. The flexural tension steel reinforcement ratio was 0.59% in one direction and 1.31% in the perpendicular direction. The GFRP laminate in the form of fabric is applied as one layer (1.31 mm thick) or two layers (1.93 mm thick). Using one layer of GFRP laminate increased the ultimate load from 17% to 45% while using two layers increased the ultimate, depending on the concrete grade and reinforcement ratio. For identical conditions, using two layers of laminates always increased the ultimate load. A study by Chen and Li [29] confirms that GFRP strengthening sheet increases the punching shear capacity by acting as an external flexural reinforcement. The percentage increase in punching shear capacity is higher in lower grade concrete (16.9 MPa) than in concrete of normal strength (34.4 MPa). However, GFRP sheets change the failure mode from flexure-punching failure to brittle punching failure. This study included 1000 mm × 1000 mm × 150 mm specimens loaded centrally with 150 mm square column stub to simulate interior columns in flat slabs. The flexural tension steel reinforcement Polymers 2020, 12, 685 7 of 27 ratio was 0.59% in one direction and 1.31% in the perpendicular direction. The GFRP laminate in the form of fabric is applied as one layer (1.31 mm thick) or two layers (1.93 mm thick). Using one layer of GFRP laminate increased the ultimate load from 17% to 45% while using two layers increased the ultimate, depending on the concrete grade and reinforcement ratio. For identical conditions, using two layers of laminates always increased the ultimate load.
While the failure mode in punching shear created by the GFRP sheets is consistent with published literature reviewed in this paper, the investigators covered the area under the loading column with GFRP fabric, which is not typical for building structures, except for columns supporting the roof. For this study, the authors concluded that the two-way shear prediction formulas proposed by BS 8110 and JSCE produce consistent results with their experimental study while ACI 318 was found to be more conservative. The GFRP strengthening pattern used in the study is shown in Figure 4. Liberati et al. [30] attributed the conservatism in ACI318 prediction of punching shear capacity to the fact that ACI318 does not explicitly consider the positive contribution of flexural reinforcement.
Sharaf et al. [31] observed that strengthening flat slabs in the tension zone around column using externally bonded CFRP strips increases stiffness from 29% to 60% compared the reference unreinforced flat slab depending on amount of reinforcement. Similarly tension-side stiffening around the column increases punching shear strength from 6% to 16% depending on the area and pattern of the strengthening strips. CFRP strips, in general, delayed the initiation of flexural cracks and controlled their propagation. Tested slabs demonstrated failure in punching shear predominantly. Researchers also studied the effect of orthogonal application of CFRP strips (patterns O4 and O8), skewed application of strips (patterns S4 and S8) and combination of skewed and orthogonal strips (OS8) as shown in Figure 5.  All strips had the same width of 100 mm, therefore the effect of CFRP strip area was studied by testing specimens reinforced with either four strips (O4 and S4) or eight strips (O8, S8, and OS8). Specimens with skewed pattern and higher CFRP strip area (S8 and OS8) carried the highest ultimate load compared to orthogonal strips.
O4 S4 All strips had the same width of 100 mm, therefore the effect of CFRP strip area was studied by testing specimens reinforced with either four strips (O4 and S4) or eight strips (O8, S8, and OS8). Specimens with skewed pattern and higher CFRP strip area (S8 and OS8) carried the highest ultimate load compared to orthogonal strips. All strips had the same width of 100 mm, therefore the effect of CFRP strip area was studied by testing specimens reinforced with either four strips (O4 and S4) or eight strips (O8, S8, and OS8). Specimens with skewed pattern and higher CFRP strip area (S8 and OS8) carried the highest ultimate load compared to orthogonal strips.
Esfahani [32] compared effectiveness of CFRP strengthening applied on the tension side of flat slab when loading is monotonic versus cyclic. It was observed that CFRP strengthening of flat slabs on the tension side increases the punching shear strength under monotonic loading, but cyclic loading decreases the effectiveness of CFRP strips in resisting punching shear. The effectiveness of strips in enhancing punching shear strength under cyclic load was more pronounced with higher tension steel reinforcement ratio. He tested two sets of 1000 mm × 1000 mm × 100 mm slabs with tension steel reinforcement ratio of 0.84 and 1.59, respectively, as shown in Figure 6. Esfahani [32] compared effectiveness of CFRP strengthening applied on the tension side of flat slab when loading is monotonic versus cyclic. It was observed that CFRP strengthening of flat slabs on the tension side increases the punching shear strength under monotonic loading, but cyclic loading decreases the effectiveness of CFRP strips in resisting punching shear. The effectiveness of strips in enhancing punching shear strength under cyclic load was more pronounced with higher tension steel reinforcement ratio. He tested two sets of 1000 mm × 1000 mm × 100 mm slabs with tension steel reinforcement ratio of 0.84 and 1.59, respectively, as shown in Figure 6. Farghaly and Ueda [33] noted that applying 0.167 mm thick CFRP strips to the tension side of flat slab around the loading column increases punching shear strength by 20%-40% compared to the unstrengthen flat slab depending on the area of the CFRP strips. Brittle punching shear failure was the dominant mode for the CFRP strengthened flat slabs. They studied 1600 mm × 1600 mm × 120 mm slab specimens with load applied through 100 mm × 100 mm. One set of slabs was strengthened with CFRP strips that are 50 mm wide (SF5) while a second set of slabs was strengthened with 100 mm wide (SF10) CFRP strips as shown in Figure 7. Steel reinforcement ratio was 1.4% designed to generate punching shear failure mode. Farghaly and Ueda [33] noted that applying 0.167 mm thick CFRP strips to the tension side of flat slab around the loading column increases punching shear strength by 20-40% compared to the unstrengthen flat slab depending on the area of the CFRP strips. Brittle punching shear failure was the dominant mode for the CFRP strengthened flat slabs. They studied 1600 mm × 1600 mm × 120 mm slab specimens with load applied through 100 mm × 100 mm. One set of slabs was strengthened with CFRP strips that are 50 mm wide (SF5) while a second set of slabs was strengthened with 100 mm wide (SF10) CFRP strips as shown in Figure 7. Steel reinforcement ratio was 1.4% designed to generate punching shear failure mode.
Polymers 2020, 12, 685 9 of 27 unstrengthen flat slab depending on the area of the CFRP strips. Brittle punching shear failure was the dominant mode for the CFRP strengthened flat slabs. They studied 1600 mm × 1600 mm × 120 mm slab specimens with load applied through 100 mm × 100 mm. One set of slabs was strengthened with CFRP strips that are 50 mm wide (SF5) while a second set of slabs was strengthened with 100 mm wide (SF10) CFRP strips as shown in Figure 7. Steel reinforcement ratio was 1.4% designed to generate punching shear failure mode. Erdogan [34] observed that vertical CFRP strips arranged in pairs perpendicular to each of the four sides of the column provide better resistance to punching shear compared to the arrangement of the vertical CFRP strips in a circular pattern around the column. In the case of perpendicular reinforcement arrangement, the failed surface was outside of the strengthened zone, while circular arrangement led to failure inside the strengthened zone. Furthermore, CFRP strengthening is more effective when the column is square than when the column is rectangular where a reduction in punching shear capacity was noticed. The investigator studied the effect of CFRP vertical strips (referred to as dowels) arranged around the interior column on punching shear strength as shown in Figure 8. The strips are applied vertically in pairs, perpendicular to each column dimension and through the depth of the slab. The pairs are spaced at d/2 (where d is the effective depth of tension steel) to ensure 45 degree cracks are intercepted. Vertical strips are intended to enhance resistance to punching shear by intercepting inclined punching shear cracks. The vertical CFRP strips were installed after concrete casting into vertical 14 mm diameter cylindrical openings. The openings were Erdogan [34] observed that vertical CFRP strips arranged in pairs perpendicular to each of the four sides of the column provide better resistance to punching shear compared to the arrangement of the vertical CFRP strips in a circular pattern around the column. In the case of perpendicular reinforcement arrangement, the failed surface was outside of the strengthened zone, while circular arrangement led to failure inside the strengthened zone. Furthermore, CFRP strengthening is more effective when the column is square than when the column is rectangular where a reduction in punching shear capacity was noticed. The investigator studied the effect of CFRP vertical strips (referred to as dowels) arranged around the interior column on punching shear strength as shown in Figure 8. The strips are applied vertically in pairs, perpendicular to each column dimension and through the depth of the slab. The pairs are spaced at d/2 (where d is the effective depth of tension steel) to ensure 45 degree cracks are intercepted. Vertical strips are intended to enhance resistance to punching shear by intercepting inclined punching shear cracks. The vertical CFRP strips were installed after concrete casting into vertical 14 mm diameter cylindrical openings. The openings were created by embedding 14 mm × 150 mm polyvinyl chloride (PVC) pipes prior to casting. Pipes were removed after casting. Erdogan [34] also studied the effect of column aspect ratio, as shown in Figure 9 on punching shear capacity. For columns having the same perimeter, changing the aspect ratio from 1 to 3 does not have significant effect on post-punching capacity. However, increasing the column aspect ratio decreases punching shear capacity. This behavior is recognized in ACI318 through the column aspect ratio . Erdogan [34] also studied the effect of column aspect ratio, as shown in Figure 9 on punching shear capacity. For columns having the same perimeter, changing the aspect ratio from 1 to 3 does not have significant effect on post-punching capacity. However, increasing the column aspect ratio decreases punching shear capacity. This behavior is recognized in ACI318 through the column aspect ratio β.
Erdogan [34] also studied the effect of column aspect ratio, as shown in Figure 9 on punching shear capacity. For columns having the same perimeter, changing the aspect ratio from 1 to 3 does not have significant effect on post-punching capacity. However, increasing the column aspect ratio decreases punching shear capacity. This behavior is recognized in ACI318 through the column aspect ratio . Urban and Tarka [35] observed an increase of 11% to 36% in punching shear capacity of flat slabs strengthened with CFRP strips around internal support columns. The percentage increase in punching shear capacity depends on the area of CFRP strips and whether or not anchor bolts were used to enhance bonding of the strips to the slab. The investigators studied 2300 mm × 2300 mm × 180 mm flat slab loaded with square 250 mm columns simulating internal flat slab-column intersection as shown in Figure 10a. The tensile steel reinforcement ratio for all specimens was 0.5%. Other than the control specimen, flat slabs were reinforced with 1.4 mm × 90 mm CFRP strips with varying areas (in terms of number of strips) applied around the loading column as shown in Figure  10. In addition to the adhesive, some samples were provided with additional 10 mm diameter anchoring bolts embedded 100 mm deep into the concrete slab in order to enhance bonding of the CFRP strips to concrete. Specimens strengthened using CFRP strips failed in a brittle explosive manner compared to the control specimen that was not strengthened. However, specimens in which CFRP strips were fixed to the slab using anchor bolts failed more gently than identical specimens without anchor bolts. The authors believe this "softer" failure is due to gradual pulling out of the anchor bolts from concrete as illustrated in Figure 10b. Urban and Tarka [35] observed an increase of 11% to 36% in punching shear capacity of flat slabs strengthened with CFRP strips around internal support columns. The percentage increase in punching shear capacity depends on the area of CFRP strips and whether or not anchor bolts were used to enhance bonding of the strips to the slab. The investigators studied 2300 mm × 2300 mm × 180 mm flat slab loaded with square 250 mm columns simulating internal flat slab-column intersection as shown in Figure 10a. The tensile steel reinforcement ratio for all specimens was 0.5%. Other than the control specimen, flat slabs were reinforced with 1.4 mm × 90 mm CFRP strips with varying areas (in terms of number of strips) applied around the loading column as shown in Figure 10. In addition to the adhesive, some samples were provided with additional 10 mm diameter anchoring bolts embedded 100 mm deep into the concrete slab in order to enhance bonding of the CFRP strips to concrete. Specimens strengthened using CFRP strips failed in a brittle explosive manner compared to the control specimen that was not strengthened. However, specimens in which CFRP strips were fixed to the slab using anchor bolts failed more gently than identical specimens without anchor bolts. The authors believe this "softer" failure is due to gradual pulling out of the anchor bolts from concrete as illustrated in Figure 10b. When a smaller area of CFRP strips is applied to the tension side, the effect of using anchor bolts was negligible to crack width development under applied load. However, when a larger area of CFRP strips is used to strengthen the slab, anchor bolts decreased the crack width under the applied load compared to control specimen and identical specimen with CFRP strips without anchor bolts. CFRP strips increase slab stiffness, therefore, deflection of slab strengthened with CFRP strips was much lower than the control unstrengthen flat slab. In addition for the same CFRP area using anchor bolts When a smaller area of CFRP strips is applied to the tension side, the effect of using anchor bolts was negligible to crack width development under applied load. However, when a larger area of CFRP strips is used to strengthen the slab, anchor bolts decreased the crack width under the applied load compared to control specimen and identical specimen with CFRP strips without anchor bolts. CFRP strips increase slab stiffness, therefore, deflection of slab strengthened with CFRP strips was much lower than the control unstrengthen flat slab. In addition for the same CFRP area using anchor bolts may or may not decrease slab deflection, depending on the area of the CFRP strips. For a larger CFRP strip area, anchor bolts decreased deflection more significantly, especially for larger applied loads than control slab or CFRP slab without anchor bolts. Thus, anchor bolts have limited or no effect on deflection of slab strengthened with CFRP strips. However, as shown in Figure 11, CFRP strengthened specimens (WT-CF-8, WT-CF-K-8, and WT-CF-K-16) and were all much stiffer than the control (unstrengthend) specimen (s-2), as indicated by the smaller deflections for the same force. When a smaller area of CFRP strips is applied to the tension side, the effect of using anchor bolts was negligible to crack width development under applied load. However, when a larger area of CFRP strips is used to strengthen the slab, anchor bolts decreased the crack width under the applied load compared to control specimen and identical specimen with CFRP strips without anchor bolts. CFRP strips increase slab stiffness, therefore, deflection of slab strengthened with CFRP strips was much lower than the control unstrengthen flat slab. In addition for the same CFRP area using anchor bolts may or may not decrease slab deflection, depending on the area of the CFRP strips. For a larger CFRP strip area, anchor bolts decreased deflection more significantly, especially for larger applied loads than control slab or CFRP slab without anchor bolts. Thus, anchor bolts have limited or no effect on deflection of slab strengthened with CFRP strips. However, as shown in Figure 11, CFRP strengthened specimens (WT-CF-8, WT-CF-K-8, and WT-CF-K-16) and were all much stiffer than the control (unstrengthend) specimen (s-2), as indicated by the smaller deflections for the same force. The study by Halabi et al. [36] concluded that CFRP strengthening increases the ultimate failure load in one-way spanning flat slabs when the tension-side is strengthened with CFRP strip in the column zone. The investigators concluded that eccentric loading at flat slab-column connection strengthened with CFRP sheet decreases the ductility and ultimate load. Flat slabs designed for flexural failure in negative moment area above supporting column under concentric load, will transform into shear failure occurring at a lower applied load when the failure load is eccentric. The The study by Halabi et al. [36] concluded that CFRP strengthening increases the ultimate failure load in one-way spanning flat slabs when the tension-side is strengthened with CFRP strip in the column zone. The investigators concluded that eccentric loading at flat slab-column connection strengthened with CFRP sheet decreases the ductility and ultimate load. Flat slabs designed for flexural failure in negative moment area above supporting column under concentric load, will transform into shear failure occurring at a lower applied load when the failure load is eccentric. The failure load decreases with eccentricity when the slab is strengthened with CFRP or remains un-strengthened.
The researchers studied a total of six 2000 mm × 1000 mm × 150 mm flat slab specimens shown in Figure 12a,b supported to span in one-way to simulate internal and external slab-beam connections. The loading was applied concentrically in some specimens and eccentrically in others through 250 mm square column stubs extending above and below the flat slab specimens as shown in Figure 13a,b. The tensile steel reinforcement ratio was 0.92% in longitudinal as well as transverse directions; on compression side of the flat slab specimens the steel reinforcement ratio was 0.5%. The reinforcement was chosen for flexural failure by steel yielding prior to concrete crushing.
in Figure 12a,b supported to span in one-way to simulate internal and external slab-beam connections. The loading was applied concentrically in some specimens and eccentrically in others through 250 mm square column stubs extending above and below the flat slab specimens as shown in Figure 13a,b. The tensile steel reinforcement ratio was 0.92% in longitudinal as well as transverse directions; on compression side of the flat slab specimens the steel reinforcement ratio was 0.5%. The reinforcement was chosen for flexural failure by steel yielding prior to concrete crushing. Abbas et al. [37] studied 600 mm × 600 mm × 90 mm flat slabs supported along two parallel sides to span in one-way. The steel reinforcement ratio on the tension face was 0.71%. The CFRP sheet were unidirectional applied to the slab on the tension side parallel to the direction of the span as shown in Figure 14. The loading was applied through a 40-mm diameter steel pipe.
It was observed that CFRP strengthening of one-way spanning flat slab increases the loadcarrying capacity by 12.4% (for 39.9 MPa concrete) and 16.4% (for 63.2 MPa concrete) compared to un-strengthened control slab. However, in this study the investigators noted the presence of two peaks in the load-deflection curve. The first peak is associated with the punching failure of concrete as it loses shear capacity followed by a drop in the curve during which resistance is due to aggregate interlocked and reinforcing steel dowel action as shown in Figure 15. connections. The loading was applied concentrically in some specimens and eccentrically in others through 250 mm square column stubs extending above and below the flat slab specimens as shown in Figure 13a,b. The tensile steel reinforcement ratio was 0.92% in longitudinal as well as transverse directions; on compression side of the flat slab specimens the steel reinforcement ratio was 0.5%. The reinforcement was chosen for flexural failure by steel yielding prior to concrete crushing. Abbas et al. [37] studied 600 mm × 600 mm × 90 mm flat slabs supported along two parallel sides to span in one-way. The steel reinforcement ratio on the tension face was 0.71%. The CFRP sheet were unidirectional applied to the slab on the tension side parallel to the direction of the span as shown in Figure 14. The loading was applied through a 40-mm diameter steel pipe.
It was observed that CFRP strengthening of one-way spanning flat slab increases the loadcarrying capacity by 12.4% (for 39.9 MPa concrete) and 16.4% (for 63.2 MPa concrete) compared to un-strengthened control slab. However, in this study the investigators noted the presence of two peaks in the load-deflection curve. The first peak is associated with the punching failure of concrete as it loses shear capacity followed by a drop in the curve during which resistance is due to aggregate interlocked and reinforcing steel dowel action as shown in Figure 15. Abbas et al. [37] studied 600 mm × 600 mm × 90 mm flat slabs supported along two parallel sides to span in one-way. The steel reinforcement ratio on the tension face was 0.71%. The CFRP sheet were unidirectional applied to the slab on the tension side parallel to the direction of the span as shown in Figure 14. The loading was applied through a 40-mm diameter steel pipe.
Polymers 2020, 12, x 13 of 28 Figure 14. One-way supporting concrete specimen strengthened with CFRP sheet covering the entire specimen except the circular loading area [37].
After cracking the CFRP sheets are engaged in resisting the load leading to second peak in loadcarrying capacity. In comparison to the control slab which would have experience significant drop in load-carrying capacity, the second peak of CFRP offers the slab much higher load-carrying capacity 189.6% (for 39.9 MPa concrete) and 275.5% (for 63.2 MPa concrete). One-way supporting concrete specimen strengthened with CFRP sheet covering the entire specimen except the circular loading area [37].
It was observed that CFRP strengthening of one-way spanning flat slab increases the load-carrying capacity by 12.4% (for 39.9 MPa concrete) and 16.4% (for 63.2 MPa concrete) compared to un-strengthened control slab. However, in this study the investigators noted the presence of two peaks in the load-deflection curve. The first peak is associated with the punching failure of concrete as it loses shear capacity followed by a drop in the curve during which resistance is due to aggregate interlocked and reinforcing steel dowel action as shown in Figure 15.
After cracking the CFRP sheets are engaged in resisting the load leading to second peak in load-carrying capacity. In comparison to the control slab which would have experience significant drop in load-carrying capacity, the second peak of CFRP offers the slab much higher load-carrying capacity 189.6% (for 39.9 MPa concrete) and 275.5% (for 63.2 MPa concrete).

Figure 14.
One-way supporting concrete specimen strengthened with CFRP sheet covering the entire specimen except the circular loading area [37].
After cracking the CFRP sheets are engaged in resisting the load leading to second peak in loadcarrying capacity. In comparison to the control slab which would have experience significant drop in load-carrying capacity, the second peak of CFRP offers the slab much higher load-carrying capacity 189.6% (for 39.9 MPa concrete) and 275.5% (for 63.2 MPa concrete). Radik et al. [38] studied seven 1500 mm × 1500 mm × 152.4 mm flat slabs to evaluate the effect of strengthening on the tension side using 305 mm wide strips applied to the tension side of the slab in two perpendicular directions. Schematic of test apparatus and pattern of GFRP strips bonded to the tension face are shown in Figure 16.  Radik et al. [38] studied seven 1500 mm × 1500 mm × 152.4 mm flat slabs to evaluate the effect of strengthening on the tension side using 305 mm wide strips applied to the tension side of the slab in two perpendicular directions. Schematic of test apparatus and pattern of GFRP strips bonded to the tension face are shown in Figure 16. One-way supporting concrete specimen strengthened with CFRP sheet covering the entire specimen except the circular loading area [37].
After cracking the CFRP sheets are engaged in resisting the load leading to second peak in loadcarrying capacity. In comparison to the control slab which would have experience significant drop in load-carrying capacity, the second peak of CFRP offers the slab much higher load-carrying capacity 189.6% (for 39.9 MPa concrete) and 275.5% (for 63.2 MPa concrete). Radik et al. [38] studied seven 1500 mm × 1500 mm × 152.4 mm flat slabs to evaluate the effect of strengthening on the tension side using 305 mm wide strips applied to the tension side of the slab in two perpendicular directions. Schematic of test apparatus and pattern of GFRP strips bonded to the tension face are shown in Figure 16.  GFRP laminates bonded to the tension side increased both the ultimate load and ductility of the flat slab compared to the control specimen as shown in Figure 17. GFRP laminates bonded to the tension side increased both the ultimate load and ductility of the flat slab compared to the control specimen as shown in Figure 17. Experimental study by Soudki et al. [39] concluded that applied CFRP strips on the tension side around internal columns of flat slab increases the punching shear strength by up to 29% compared to control un-strengthened slab. CFRP strengthened slabs are stiffer, hence, experience less deflections compared to the control slab. Applying strengthening strips near the column increased stiffness of the slab while applying the same strips offset from the column increased punching shear capacity. The most efficient configuration is skewed strips offset further from the column. This investigation was performed on 1220 mm × 1220 mm × 100 mm flat slab specimens made of 25.8 MPa concrete. The effect of CFRP was studied by reinforcing selected specimens using 100 mm wide × 1.2 mm thick strips applied on the tension side around the loading column in various configurations. The orientations and configurations of the CFRP strips studied includes orthogonal to column, skewed, offset from column, or adjacent to columns is shown in Figure 18.  Erdogan et al. [40] examined a flat slab CFRP strengthening technique and concluded that it is capable of restoring the strength of flat slab specimen damaged through punching shear through interior column. It was also noted that rehabilitation of damaged flat slab using CFRP can restore strength to level higher than CFRP strengthened flat slab. They studied five 2130 mm × 2130 mm × Figure 17. Load versus deflection plots for rehabilitated specimens [38].
Experimental study by Soudki et al. [39] concluded that applied CFRP strips on the tension side around internal columns of flat slab increases the punching shear strength by up to 29% compared to control un-strengthened slab. CFRP strengthened slabs are stiffer, hence, experience less deflections compared to the control slab. Applying strengthening strips near the column increased stiffness of the slab while applying the same strips offset from the column increased punching shear capacity. The most efficient configuration is skewed strips offset further from the column. This investigation was performed on 1220 mm × 1220 mm × 100 mm flat slab specimens made of 25.8 MPa concrete. The effect of CFRP was studied by reinforcing selected specimens using 100 mm wide × 1.2 mm thick strips applied on the tension side around the loading column in various configurations. The orientations and configurations of the CFRP strips studied includes orthogonal to column, skewed, offset from column, or adjacent to columns is shown in Figure 18. GFRP laminates bonded to the tension side increased both the ultimate load and ductility of the flat slab compared to the control specimen as shown in Figure 17. Experimental study by Soudki et al. [39] concluded that applied CFRP strips on the tension side around internal columns of flat slab increases the punching shear strength by up to 29% compared to control un-strengthened slab. CFRP strengthened slabs are stiffer, hence, experience less deflections compared to the control slab. Applying strengthening strips near the column increased stiffness of the slab while applying the same strips offset from the column increased punching shear capacity. The most efficient configuration is skewed strips offset further from the column. This investigation was performed on 1220 mm × 1220 mm × 100 mm flat slab specimens made of 25.8 MPa concrete. The effect of CFRP was studied by reinforcing selected specimens using 100 mm wide × 1.2 mm thick strips applied on the tension side around the loading column in various configurations. The orientations and configurations of the CFRP strips studied includes orthogonal to column, skewed, offset from column, or adjacent to columns is shown in Figure 18.  Erdogan et al. [40] examined a flat slab CFRP strengthening technique and concluded that it is capable of restoring the strength of flat slab specimen damaged through punching shear through interior column. It was also noted that rehabilitation of damaged flat slab using CFRP can restore strength to level higher than CFRP strengthened flat slab. They studied five 2130 mm × 2130 mm × Figure 18. Effect of CFRP strip area, orientation with respect to column, and distance from face of column [39].
Erdogan et al. [40] examined a flat slab CFRP strengthening technique and concluded that it is capable of restoring the strength of flat slab specimen damaged through punching shear through interior column. It was also noted that rehabilitation of damaged flat slab using CFRP can restore strength to level higher than CFRP strengthened flat slab. They studied five 2130 mm × 2130 mm × 150 mm flat slabs loaded with 300 mm square plate representing internal column to flat slab connection. The goal of the study was to evaluate effectiveness of CFRP strengthening and repair of pre-loaded slabs that failed due to punching shear. The investigators indicated the dimensions chosen to represent 2/3 scale models of 8-m span flat slab that is 225-mm thick supported by 450-mm square columns. The flat slab specimens were reinforced with 19-mm bars at 140-mm spacing in each direction, without compression steel, which gives a tension reinforcement ratio of 1.86%. Specimen and column dimensions, loading points and eight CFRP rows around loading column are shown in Figure 19.
Polymers 2020, 12, x 15 of 28 150 mm flat slabs loaded with 300 mm square plate representing internal column to flat slab connection. The goal of the study was to evaluate effectiveness of CFRP strengthening and repair of pre-loaded slabs that failed due to punching shear. The investigators indicated the dimensions chosen to represent 2/3 scale models of 8-m span flat slab that is 225-mm thick supported by 450-mm square columns. The flat slab specimens were reinforced with 19-mm bars at 140-mm spacing in each direction, without compression steel, which gives a tension reinforcement ratio of 1.86%. Specimen and column dimensions, loading points and eight CFRP rows around loading column are shown in Figure 19. Hussein and El-Salakawy [41] concluded that increasing the flexural tension GFRP reinforcement ratio of high-strength flat slab (80 MPa) from 1% to 1.5% (50% increase) increases the punching shear strength by 15%, while increasing the reinforcement ratio to 2% (100% increases) increases the punching shear strength by 27%. The longitudinal GFRP tension reinforcement was No. 16 (15.9 mm) having a tensile strength of 1685 MPa.
GFRP shear reinforcement curbed down widening of cracks and controlled its propagation. Control of cracking lead to enhancement of the stiffness and decrease of deflection in normal strength (40 MPa) slab. No. 13 (12.7 mm diameter) GFRP shear studs increased punching shear strength by 51% compared to the control slab without shear reinforcement. No. 10 (9.5 mm) corrugated GFRP shear reinforcement increased punching shear strength by 34% compared to the control slab.
High-strength (80 MPa) flat slabs with tension GFRP reinforcement from 0.5% to 1.5%, but without shear reinforcement, failed in punching shear in a brittle manner. Cracking begins at column corners then propagates radially from the column in all directions. When the load reached 45%-50% of the ultimate load circumferential cracks appeared and connected radial cracks together in all directions. The normal-strength flat slab (40 MPa) failed in the same manner when there is no shear reinforcement. However, the normal-strength flat slab exhibited higher cracking at failure compared to the high-strength flat slab. It was noted that increasing the tension reinforcement ratio decreased the punching failure cone radius by making the inclination angel of the failure crack steeper.
The study was performed on six 2800 mm × 2800 mm × 200 mm flat slab specimens loaded by 300 mm square column. The specimens represent interior column-flat slab connection reinforced with flexural steel on the tension-side. One test series of three specimen was used to evaluate the effect of tension steel reinforcement ratio on punching shear capacity of high-strength concrete, and a second set of three specimens was used to examine the effect of GFRP shear reinforcement type on punching shear capacity of normal-strength concrete. The flat slab specimens (three in total) set made of high-strength concrete were reinforced with 1%, 1.5%, and 2% flexural steel. The normal-strength flat slab set tested for the effect of GFRP shear reinforcement consisted of one control specimen without shear reinforcement, a second specimen reinforced with GFRP shear studs, and a third specimen reinforcement with corrugated GFRP shear reinforcement as seen in Figure 20. The study Hussein and El-Salakawy [41] concluded that increasing the flexural tension GFRP reinforcement ratio of high-strength flat slab (80 MPa) from 1% to 1.5% (50% increase) increases the punching shear strength by 15%, while increasing the reinforcement ratio to 2% (100% increases) increases the punching shear strength by 27%. The longitudinal GFRP tension reinforcement was No. 16 (15.9 mm) having a tensile strength of 1685 MPa.
GFRP shear reinforcement curbed down widening of cracks and controlled its propagation. Control of cracking lead to enhancement of the stiffness and decrease of deflection in normal strength (40 MPa) slab. No. 13 (12.7 mm diameter) GFRP shear studs increased punching shear strength by 51% compared to the control slab without shear reinforcement. No. 10 (9.5 mm) corrugated GFRP shear reinforcement increased punching shear strength by 34% compared to the control slab.
High-strength (80 MPa) flat slabs with tension GFRP reinforcement from 0.5% to 1.5%, but without shear reinforcement, failed in punching shear in a brittle manner. Cracking begins at column corners then propagates radially from the column in all directions. When the load reached 45-50% of the ultimate load circumferential cracks appeared and connected radial cracks together in all directions. The normal-strength flat slab (40 MPa) failed in the same manner when there is no shear reinforcement. However, the normal-strength flat slab exhibited higher cracking at failure compared to the high-strength flat slab. It was noted that increasing the tension reinforcement ratio decreased the punching failure cone radius by making the inclination angel of the failure crack steeper.
The study was performed on six 2800 mm × 2800 mm × 200 mm flat slab specimens loaded by 300 mm square column. The specimens represent interior column-flat slab connection reinforced with flexural steel on the tension-side. One test series of three specimen was used to evaluate the effect of tension steel reinforcement ratio on punching shear capacity of high-strength concrete, and a second set of three specimens was used to examine the effect of GFRP shear reinforcement type on punching shear capacity of normal-strength concrete. The flat slab specimens (three in total) set made of high-strength concrete were reinforced with 1%, 1.5%, and 2% flexural steel. The normal-strength flat slab set tested for the effect of GFRP shear reinforcement consisted of one control specimen without shear reinforcement, a second specimen reinforced with GFRP shear studs, and a third specimen reinforcement with corrugated GFRP shear reinforcement as seen in Figure 20. The study by Ferreira et al. [42] demonstrated that in the presence of studs, punching shear failure is initiated by loss of anchorage at the base of stud.
Polymers 2020, 12, x 16 of 28 by Ferreira et al. [42] demonstrated that in the presence of studs, punching shear failure is initiated by loss of anchorage at the base of stud. Silva et al. [43] studied eight 1200 mm × 1200 mm × 100 mm flat slab specimens with loading through 100 mm square stub simulating internal column of flat slab. CFRP strengthening strips were 700 mm long × 100 mm wide × 1 mm thick. All strengthening strips, whether orthogonal (O) or skewed (S) were placed starting one effective depth (75 mm) from the face of the column as shown in Figure 21. End anchorage of CFRP strips, when applied, was done through 50 mm × 150 mm transverse steel plates attached to the slab through 10 mm diameter steel bolts. It was observed that the skewed arrangement of CFRP strips can increase punching shear strength flat slabs by nearly 46% compared to the control slabs. Using end anchorage at the end of the strips is particularly effective in enhancing the load-carrying capacity. The critical perimeter in control slabs is located 1.5-to 2.5-times the effective depth from the column face.
Failure in the control slab occurred by formation of radial cracks at distances from 1.5 to 2.5 × d (where d = effective depth) from the face of the column. The study reported that CFRP strengthened slab failed in flexure. All strengthened specimens without end anchorage failed by de-bonding and strips failed in rupture due to their high tensile strength. The study by Durucan and Anil [44] examined nine 2000 mm × 2000 mm × 120 mm flat slab specimens with concrete compressive strength of 20 MPa on average. The control flat slab did not have an opening while the remaining eight specimens contained openings at different locations with respect to the supporting columns, and strengthening with CFRP strips. The study included two (300 mm and 500 mm) square opening sized as shown in Figure 22. Silva et al. [43] studied eight 1200 mm × 1200 mm × 100 mm flat slab specimens with loading through 100 mm square stub simulating internal column of flat slab. CFRP strengthening strips were 700 mm long × 100 mm wide × 1 mm thick. All strengthening strips, whether orthogonal (O) or skewed (S) were placed starting one effective depth (75 mm) from the face of the column as shown in Figure 21. End anchorage of CFRP strips, when applied, was done through 50 mm × 150 mm transverse steel plates attached to the slab through 10 mm diameter steel bolts. It was observed that the skewed arrangement of CFRP strips can increase punching shear strength flat slabs by nearly 46% compared to the control slabs. Using end anchorage at the end of the strips is particularly effective in enhancing the load-carrying capacity. The critical perimeter in control slabs is located 1.5-to 2.5-times the effective depth from the column face.
Failure in the control slab occurred by formation of radial cracks at distances from 1.5 to 2.5 × d (where d = effective depth) from the face of the column. The study reported that CFRP strengthened slab failed in flexure. All strengthened specimens without end anchorage failed by de-bonding and strips failed in rupture due to their high tensile strength.
Polymers 2020, 12, x 16 of 28 by Ferreira et al. [42] demonstrated that in the presence of studs, punching shear failure is initiated by loss of anchorage at the base of stud. Silva et al. [43] studied eight 1200 mm × 1200 mm × 100 mm flat slab specimens with loading through 100 mm square stub simulating internal column of flat slab. CFRP strengthening strips were 700 mm long × 100 mm wide × 1 mm thick. All strengthening strips, whether orthogonal (O) or skewed (S) were placed starting one effective depth (75 mm) from the face of the column as shown in Figure 21. End anchorage of CFRP strips, when applied, was done through 50 mm × 150 mm transverse steel plates attached to the slab through 10 mm diameter steel bolts. It was observed that the skewed arrangement of CFRP strips can increase punching shear strength flat slabs by nearly 46% compared to the control slabs. Using end anchorage at the end of the strips is particularly effective in enhancing the load-carrying capacity. The critical perimeter in control slabs is located 1.5-to 2.5-times the effective depth from the column face.
Failure in the control slab occurred by formation of radial cracks at distances from 1.5 to 2.5 × d (where d = effective depth) from the face of the column. The study reported that CFRP strengthened slab failed in flexure. All strengthened specimens without end anchorage failed by de-bonding and strips failed in rupture due to their high tensile strength. The study by Durucan and Anil [44] examined nine 2000 mm × 2000 mm × 120 mm flat slab specimens with concrete compressive strength of 20 MPa on average. The control flat slab did not have an opening while the remaining eight specimens contained openings at different locations with respect to the supporting columns, and strengthening with CFRP strips. The study included two (300 mm and 500 mm) square opening sized as shown in Figure 22. The study by Durucan and Anil [44] examined nine 2000 mm × 2000 mm × 120 mm flat slab specimens with concrete compressive strength of 20 MPa on average. The control flat slab did not have an opening while the remaining eight specimens contained openings at different locations with respect to the supporting columns, and strengthening with CFRP strips. The study included two (300 mm and 500 mm) square opening sized as shown in Figure 22.
It was demonstrated that CFRP strips were able to restore punching shear capacity of flat slabs affected by the presence of openings near the supporting columns, to nearly the same capacity of the control slabs without openings. Similarly, numerical studies by Mohamed et al. [45] observed that CFRP strengthening around openings located further from the supporting column enhances flexural capacity and overall stiffness of the slab. In fact, Lapi et al. [46] believes that the increase in punching shear strength through strengthening of the flat slab using FRP strips is the indirect effect of the increase in slab stiffness due to the strengthening strips.
A similar reduction in punching shear strength exists in voided flat slabs, where voids are incorporated in the slab to reduce self-weight [47], with applications in seismic areas. In such cases solutions for enhancing punching shear capacity may include use of a steel sheet in the critical area near the supporting column. It was demonstrated that CFRP strips were able to restore punching shear capacity of flat slabs affected by the presence of openings near the supporting columns, to nearly the same capacity of the control slabs without openings. Similarly, numerical studies by Mohamed et al. [45] observed that CFRP strengthening around openings located further from the supporting column enhances flexural capacity and overall stiffness of the slab. In fact, Lapi et al. [46] believes that the increase in punching shear strength through strengthening of the flat slab using FRP strips is the indirect effect of the increase in slab stiffness due to the strengthening strips.
A similar reduction in punching shear strength exists in voided flat slabs, where voids are incorporated in the slab to reduce self-weight [47], with applications in seismic areas. In such cases solutions for enhancing punching shear capacity may include use of a steel sheet in the critical area near the supporting column. Saleh et al. [48] carried out an experimental study on four 2300 mm × 2300 mm × 200 mm flat slabs loaded concentrically through 300 mm square plates. The control flat slab was prepared with 25 MPa concrete and was not reinforced in shear. Three specimens were reinforced in shear using Lshaped CFRP laminates installed through pre-drilled 25-mm holes as shown in Figure 23. The CFRP strips were fixed through the holes with part of it (to form L-shaped) to the top or bottom using epoxy resin. The L-shaped CFRP shear reinforcing strips were applied to the unloaded slab in three Saleh et al. [48] carried out an experimental study on four 2300 mm × 2300 mm × 200 mm flat slabs loaded concentrically through 300 mm square plates. The control flat slab was prepared with 25 MPa concrete and was not reinforced in shear. Three specimens were reinforced in shear using L-shaped CFRP laminates installed through pre-drilled 25-mm holes as shown in Figure 23. The CFRP strips were fixed through the holes with part of it (to form L-shaped) to the top or bottom using epoxy resin. The L-shaped CFRP shear reinforcing strips were applied to the unloaded slab in three perimeters around the loading plate. Perimeter one is located at a distance d/2 (where d is the effective depth) from the loading plate. Perimeters 2 and 3 are located at distances 0.7d from the loading plate.
It was concluded that L-shaped CFRP strips are capable of increasing the ultimate load of flat slab specimens by 97-104% higher than the control flat slab without CFRP strips. Moreover, L-shaped strips increased the deflection at failure to 400% of the control specimen.
Polymers 2020, 12, x 18 of 28 perimeters around the loading plate. Perimeter one is located at a distance d/2 (where d is the effective depth) from the loading plate. Perimeters 2 and 3 are located at distances 0.7d from the loading plate. It was concluded that L-shaped CFRP strips are capable of increasing the ultimate load of flat slab specimens by 97%-104% higher than the control flat slab without CFRP strips. Moreover, Lshaped strips increased the deflection at failure to 400% of the control specimen.

Results of Experimental Studies and Major Findings
Many investigations have been conducted on strengthening the flat slab using both CFRP and GFRP. All have examined methods to delay or prevent punching shear failure. Table 3 shows summary of existing experimental work of FRP strengthening.

Results of Experimental Studies and Major Findings
Many investigations have been conducted on strengthening the flat slab using both CFRP and GFRP. All have examined methods to delay or prevent punching shear failure. Table 3 shows summary of existing experimental work of FRP strengthening.
It is important to note that experimental studies are generally conducted on a scaled-down prototype specimens due to traditional laboratory limitations. Studies by Goh and Hrynyk [49] indicate that flat slab continuity and lateral restraint greatly affect the two-way shear capacity. Studies on multi-bay slabs showed improved two-way shear capacity compared to isolated column-zone specimens.   The study is an experimental evaluation of the effect of CFRP sheets in enhancing punching shear resistance. 1. Applying bonded CFRP strips without end anchor bolts provides relatively small increase in two-way shear strength, not exceeding 10% compared to the un-strengthened specimen. 2. Adding anchor bolts at the end of the CFRP strips increases punching shear strength significantly compared to the same sample without anchor bolts and compared to un-strengthened specimen.
WT  The investigators tested 6 flat slab specimens to determine the effectiveness of externally bonded CFRP strips in enhancing punching shear. 1. All flat slabs failed in punching shear with the CFRP strengthened specimens experiencing ultimate as high as 29% compared to the control flat slab. 2. Higher punching shear capacity was demonstrated by specimens strengthened near the supporting column while higher stiffness is obtained when the CFRP strips are placed near the supporting column. 3. The most effective CFRP strip configuration in enhancing punching shear strength is when the strips are placed further from the supporting column and skewed (with respect to column orientation. 5. Increasing the amount of CFRP strips did not significantly increase the capacity of the slabs. Erdogan et al. [ The investigators tested 9 flat slab specimens with openings at various locations in the vicinity of the supporting column. The control specimen was not strengthened while 8 specimens were strengthened with CFRP. 1. CFRP strengthening increases the punching shear capacity in comparison with the control (unstrengthen) specimen by 55% on average. 2. The adverse effect on punching shear capacity of openings near supporting columns is nearly eliminated by using CFRP strips around openings. 3. Highest ultimate load was demonstrated when opening was diagonal with respect to rectangular column, CFRP strips completely surrounded the opening, opening no adjacent to column, and opening not the largest of the test specimens. 4. CFRP strips increased initial elastic stiffness of flat slab specimens by nearly 486% on average, compared to unstrengthen specimens. CFRP strips also increased the maximum displacement at failure.  The investigators studied 12 flat slabs supported to span in one direction. Control slabs (SA and SB) were not strengthened. CFRP strengthening with done using single sheet with fibers parallel to the span direction. 1. The load-deflection response curve of CFRP Strengthened one-way specimens was characterized by two peaks compared to the control un-strengthened specimen. The investigators believe the second peak is due to the combined effect of dowel action of rebars, aggregate interlock, and unidirectional strengthening sheet. 2. The CFRP strengthened slabs showed nominal increase in the first peak load (9-18%) but there was significant increase in the second peak load.

Conflicts of Interest:
The authors declare no conflict of interest.