Enhancing punching shear capacity of flat slab: review of strengthening and repairing techniques

A significant number of concrete flat slabs need to be strengthened against brittle punching shear failure due to conversion of existing buildings, material deterioration, the growth of standards, and detailed defects. Furthermore, there are some traditional approaches to strengthen and repair reinforced concrete slabs which can be either cumbersome or expensive. The repairing of the slabs is very important and not optional because, they will be lost without repairing and reconstruction may not be appropriate due to lack of time or other exceptional circumstances. There were many techniques to strengthening and repair the flat slab depend on suitable way and material availability for strengthening like CFRP sheets, TRM jackets, steel stiffeners, steel bolts, column capital, increasing slab thickness and many other techniques. Typically, the strengthening enhanced punching shear capacity, and the increment may reach 90%. Repair specimens’ punching shear capacity, which may be more than that of the control specimen and the repairing techniques can be used for strengthening. From presented studies, it was found that using the same technique for strengthening and repairing gave better results in strengthening, so strengthening slabs before damages occur is important and enhances resistance better.


Introduction
Flat slab is one of the types of floors that do not contain beams and the loads are transferred directly from the roof to the columns.They are commonly used in construction for easy construct, costeffective, and allow for lower story heights.A flat slab structure, on the other hand, is subjected to enormous transverse stresses focused surround the column, which can lead to non-ductile, sudden, and brittle punching failure [1].
Because of the limited deflection capabilities and hidden cracks on the slab's top side, the punching shear failure happens without any previous warning.A local punching shear failure in flat slab-column connection causes the neighboring flat slab-column connections to undergo amplified shear forces, which may lead to the continuously punching shear failure of the neighboring flat slab-column connections, which can finally lead to the collapse of the entire structure.There are many things that reduce punching shear strength in the slab, one of which is the presence of openings that are usually required for different purposes.The slab-column connection's punching shear capacity is further decreased by the opening because it eliminates some of the concrete volume necessary to resist shear force and unbalanced moment.As a result, the connection is more susceptible to brittle punching shear failure [2].Thus, significant precautions must be taken during the structure's design stage to prevent brittle and abrupt punching shear failure.By utilizing a thicker slab, a larger column, concrete with higher compressive strength, more flexural reinforcement, or additional shear reinforcement, it is possible to increase the punching shear capacity of a flat slab-column connection.For architectural and cost considerations, some techniques might not always be the best choice.The problem of brittle punching shear failure in such conditions can be effectively solved by adding shear reinforcement to the intersection of the flat slab-column system [3].Additionally, several techniques can be applied to enhance and increase the punching shear strength of existing slabs.The existing slab usually needs updating to reach current design specifications or new applications, such as raising live load demands.There are many numbers of the existing slab column need to repair punching failures during the most current process, for example, to recompense for inadequate durability characteristics and details of defects, or to improve loading capacity due to usage variations.
To meet this new need, many systems for punching shear repair have been pre-installed, such as increased slab support, increased resistance of bending, or post-installation of added shear reinforcement.The previous two ways keep the brittle performance of the slabs and yet depend on the strength of tensile of the concrete.In the third system, pre-stressed shear reinforcements must be initially activated by added rotation, so that they can only withstand additional loads [4].

Strengthening of RC flat slab
In spite of their extensive applications in bridges and buildings, RC flat slabs have lately become a worry source for their weakness to failure or damage in many old constructions.Some of these structures, do not comply with shear reinforcement requirements.In addition, insufficient punching shear capacity of modern flat slab structures due to various factors, such as changing their usage and loading, adding new installations or design/construction errors (e.g., relevant updated design codes) as in the updates that take place on ACI [5] provides the necessity to either upgrading existing structures or replacing them with new ones.Nevertheless, replacing deficient structures is often not a practical solution because it includes huge cost.Therefore, the repairing and/or strengthening of inadequate structures denotes a suitable solution, saving money and time [6].The ultimate design punching according to ACI (318- bo = The critical shear perimeter was placed at the distance of (d/2) from the column, Take a lesser of the equations.
There are numerous attempts to improve slabs' punching shear behaviour as shown below.

CFRP sheets and TRM jackets
In the University of Babylon at (2018) Majid [6] used CFRP sheet as externally bonded flexural reinforcement, in addition to steel reinforcing bars on the tension face of the slab (1200×1200×100)mm, increased the punching shear capacity and ultimate deflection in comparison to the control slab by around 50 and 31.4%,respectively.Although the use of TRM jackets on the tension face of the slabs improved the punching shear strength, its effectiveness was dependent on the mesh size of the textile (i.e., the use of carbon fiber textile with mesh sizes 20 and 10 mm improved the punching strength by 30.5 and 35.7%, respectively, as compared to control specimen).It can be shown that the punching shear capacity of strengthened slabs increased with the TRM system, but less so than with the CFRP system.
Figure 2. CFRP sheet and TRM layer [6].Reference [7] investigated the influence of CFRP-sheet and TRM-jacket strengthened materials on the punching resistance of slabs.Twelve RC slabs of 600 x 600 x 90 mm dimensions subjected to quasi-static vertical load.The slab specimens were cast with two different concrete grades (grade A = 39.9MPa and grade B = 63.2MPa, tested according to ASTM [8]) six slabs for each of them.The two slabs of each group served as control, while the other four were strengthened by CFRP and TRM.Based on the obtained results, the following conclusions were drawn:  For the slabs strengthened by CFRP, the load carrying capacity increased by 12.4% and 16.6% for concrete grades A and B, respectively.In addition, there was a significant improvement in the energy absorption capacity of slab specimens ranged between 65.5 and 66.1% for two grades of concrete. For the slabs strengthened by TRM-jacket, the punching shear capacity increased by 9.1% and 18.1% for concrete grade A and B, respectively.In addition, the improvement in the energy absorption capacity of slab specimens ranged between 22.0 and 58.7% for the two grades of concrete. It was shown that the CFRP-sheet was more effective than TRM-jacket for enhancing the punching strength of flat slabs.
Reference [9] looked into the effects of applying the CFRP strengthening technique in two-way RC slabs in 2022.Three strengthened RC solid flat slab samples in addition to unstrengthen slab samples (1000×1000×100)mm (as shown in Figure 3) were tested and exposed to static load in order to investigate the punching shear effect on the middle of the slab.According to the test results, the slabs strengthened with CFRP performed better than the control sample in terms of delaying the onset of the first fracture, extending the resistance to the progression of shear cracks, and improving the loaddisplacement and ductility capacities.Also, the punching shear strength and failure mode of the CFRP-enhanced samples were both improved.Additionally, the analytical model to estimate the ultimate punching shear strength had high agreement with the practical code requirements, making it applicable to the design and analysis of CFRP strengthened slabs.

Steel bolts
Haifa et al. [10] present a strengthening technique of reinforced concrete (RC) flat slabs using postinstalled steel bolts.Two substantial RC slabs that represented an internal slab-column connection were put to the test.One slab worked as the control slab and had no shear reinforcement.The second slab had punching shear reinforcement owing to steel bolts with end anchorage steel plates.The findings indicated that increased the slab's deformation capacity by 47.4% and load capacity by 15.4%.The geometry and steel reinforcement details as shown in figure 4 below.

Steel fiber
The effect of using steel fiber with reactive powder concrete on punching shear strength was clear.Two types of slabs were tested square (450×450×50)mm and tringle with 500mm width, 810mm height and 50mm thickness.The results showed that slabs having 0.5% and 1% of steel fibers, respectively, improved punching shear strength by around (37-53) % and (100) %.Steel fiber usage enhanced punching shear resistance and permitted the transfer of greater loads via the slab-column connection [11].

Column capital or steel plate
Widianto [12] utilized the enlarging column part technique to strengthen punching shear.This method utilized the steel collar underneath the slab in order to accomplish the desired result.Figure 6 shows the process by which the steel collars were manufactured from the steel tube.Due to the fact that this strengthening increased the perimeter critical shear, the collar was improving the punching shear capacity as a result.For a flexural reinforcement ratio of 0.5 percent, the punching shear capacity increased by approximately 45 percent, and the deformation capacities increased by approximately 53 percent.For a flexural reinforcement ratio of 1.0 percent, the punching shear capacity increased by approximately 42 percent, and the deformation capacities increased by approximately 15 percent.Elbakry and Allam [13], used the external steel plates to provide experimental and analytical studies to enhance the punching shear of reinforced concrete slabs.Five specimens of slabs (1200×1200×100)mm with concrete reinforced was investigated.The specimens divided into two types one of the specimens represented as a control specimen without strengthening and the other specimens with strengthening with steel plate and provided with shear studs as shown in Figure 7.
Many parameters were taken such as (steel plate thickness, the plate dimensions, shear stud diameter and the arrangement of shear studs).As a result, this technique enhanced the punching shear capacity about (15-39%).The diameter and numbers of shear studs had an important enhancement of the capacity of punching shear and for using a thick steel plate had not resulted in any important influence on the capacity of punching shear, the deflection capacity of the specimens strengthening was reduced about (44 to 60%) compared to the control specimen.And there are many other several techniques to enhancing punching shear capacity.

Repairing of RC flat slab
Since all structural members deteriorate over time, repair is not optional.If slabs are not repaired, they will eventually be lost.Sometimes, increased loads transferred to the slab may result in high shear stresses that cause structure collapse or dangerous damages because of the change in the slab's function.There are many common reasons for repairing of slab-column against punching shear [4]:  The details and the design of the punching shear strength are insufficient. Before the concrete reaches its required strength, the strength of concrete is low because of the quality of construction was bad or rapid construction and the initial loading of the structure. Wind loads, dangerous earthquakes, changes in building use, damaging effects of fires or insufficient additional loads on building extensions increase the number of floors. Partial loss of shear strength of concrete because of cracking and forming the cover of concrete due to damage through intense ground motion or corrosion of steel. A new duct or service pipe that requires an opening need to be installed on the slab near the column.Almost, strengthening techniques can be used for repairing.Many researchers used different techniques for repairing damaged slabs.
Antonio et al. [14] pointed out that repairing the concrete damaged with the new concrete can restore the original strength without using a bonding agent.The original control specimen with the dimension of (1800 mm × 1800 mm × 100 mm) were loaded centrally until they were destroyed and then the concrete damaged was repaired.In this study two repairing ways used.Frist, one done by (separated the damaged of concrete, cleaned the surface from the past concrete, made the surface of specimens in (a saturated-dry) condition, and casted the fresh concrete).This technique showed good results.With this method a punching resistance similar to that of the undamaged slab may be obtained if the surface of the old concrete is well prepared, a suitable repair concrete is used and a good curing is performed.The repairing specimens were failed in the punching shear through the old concrete and not into the bonding at new concrete.The second technique applied to a slab, which it damaged by punching.The damaged concrete replaced by a repair concrete and the model was strengthening with a column head of steel beams connected to the column and slab by epoxy resin and steel anchors.This repairing method showed to be able to develop a good punching resistance, increasing also the bending resistance.
Al-Wetaefi [15] studied the effects of increasing column size on the punching shear strength by applying a precast high strength concrete of (column capital or hollow column).After the loading through repeated load up to (70%) from the ultimate punching shear for reference specimen, the specimens improved by applying the precast column capital.At first, used an electric grinder to grind the column capital and column surface to obtain a suitable clean surface and expose the aggregate without any defects to install satisfactory bonding from the epoxy to the (CFRP) laminates and also the epoxy to the concrete of column capital as shown in Figure8.
The edge from the column capital by (10 mm) is rounded to limit stress concentrations in the (CFRP) laminates.A hole was drilled with a diameter (12 mm) then used with a (rotary hammer drill) that was rotated to the slab bottom plate to a bending reinforcement level (hole length 60 mm) to enter a dowel of steel from the column capital, after that a vacuum cleaner was utilized in order to get rid of the dust and other fine materials.The results showed that the column envelope of the using the (hallow column or column capitals) useful in extending a critical shear perimeter compared to the control specimen, thus improving the capacity of the punching shear by (87.5% and 83.5%) respectively.Hamed [16] presented a solution that is mostly dependent on patching up the deteriorating concrete and adding vertical studs in different configurations through holes perforated in the plates.The experimental program consisted of eight different specimens had the same dimensions (1200×1200×190)mm and flexural reinforcements.However, the concrete strengths and shear reinforcement ratios varied between each specimen.According to the findings of the tests, the interior cracks in the tested slabs within the stud region initially developed when the applied load reached, on average, almost 81% of the ultimate load for NSC and 90.37% of the ultimate load for HSC.The ductility of the repaired slabs increased by the suggested repair method using shear studs.With NSC slabs, adding more shear studs greatly increases the ductility and rotational resistance, whereas this impact is less apparent in HSC slabs.The punched shear capacity of NSC slabs can be higher than that of control specimen by increasing the number of shear studs.HSC slabs had this impact, but at a reduced rate.[16].

Figure 9. vertical studs' technique
The use of Fiber Reinforced Polymer (FRP) systems to repair flat slab-edge column connections with (900×900×130)mm exposed to punching shear examined by a research group in 2016.These systems are exterior stirrups made of steel links, carbon fibers, and glass.The findings indicated that repair methods were successful and considerably enhanced these connections' punching shear behaviour.For repaired specimens, repair enhanced the ultimate punching shear strength by 30% to 78% and the ultimate cracking load rose by 19% to 40%.The best repair element was the CFRP intertwined stirrups, which significantly improved stiffness and provided the strongest punching shear strength [17].
In 2019, Haider et al. [4] studied the effectiveness of a repair method for flat slabs that had experienced punched shear stresses as a result of being loaded to half their capacities.The primary method of repair involved retrofitting the harmed concrete and using steel stiffeners of various sizes and numbers.Four flat slabs with similar flexural reinforcement and dimensions of (1.5 × 1.5 × 0.1) m were produced as part of the testing process.Shear strength, deformation characteristics, and crack behaviour were used as experimental measurements to determine the effectiveness of the suggested repairing system.The findings showed that increasing the size and quantity of steel stiffeners, which influenced the crushed area (failure perimeter) around the column, which can significantly extend the area of a slab's potential failure.When compared to the SR (control sample), the punching shear capacity of samples with stiffeners SRS1(100×200), SRS2(200×200), and SRS3(300×200) increased by about 41.7%, 58.8%, and 74.4%, respectively.In terms of load-displacement curves, crack propagation, and the mode of failure, the repaired slabs with steel stiffeners (SRS1, SRS2, and SRS3) were stiffer than those without strengthening.

Conclusion
In this study, the strengthening and repairing of the flat slab investigated by reviewing a some of researches related to this field.The main conclusions are summarized as follows.
 Using steel stiffeners with different size ranged from (100×100, 200×200, and 300×300) mm enhanced the punching shear capacity about (39.84%, 57%, and 99.2%) respectively. Using of CFRP sheet as externally bonded flexural reinforcement, in addition to steel reinforcing bars on the tension face of the slab, increased the punching shear capacity and ultimate deflection by about 50 and 30% respectively. Using of TRM jackets improved punching shear capacity more than 30% depend on the mesh size of TRM jacket. The CFRP-sheet was more effective than TRM-jacket for enhancing the punching strength of flat slabs. The installation of shear steel bolts increased the slab's capacity to deform by 47.4% and to carry loads by 15.4%. Punching shear strength in reactive powder concrete slabs that contain 0.5% and 1% of steel fibres, respectively, increased by around (37-53) % and (100) %.  The enlarging column part by using of the steel collar underneath the slab increased the punching shear capacity more than fifty percent. The punching shear capacity improved by (15-39%) when steel plate with studs used as a strengthening technique. For repairing the slab by the new concrete instead of the concrete damaged can be restored the original strength if the surface of the old concrete is well prepared, a suitable repair concrete is used and a good curing is performed. Increasing column size by applying a precast high strength concrete improved the punching shear strength more than 80%. Using of vertical studs with different arrangements through holes drilled in the repairing slabs increased the applied load to appear inner cracks up to 90%.The punching shear capacity increased as the number of studs rose, possibly exceeding that of the control specimen.
 Repairing slabs by using of Fiber Reinforced Polymers (FRP) systems increased the initial cracking load from 19% to 40% and the ultimate punching shear strength also increased from 30% to 78%. In comparison to samples repaired by stiffeners SRS1(100200), SRS2(200200), and SRS3(300200) with SR (control), the punching shear capacity of the samples improved by about 41.7%, 58.8%, and 74.4%, respectively.
Bc = ratio of long side to short side of column or column capital.s = 40 for interior column.

Figure 5 .
Figure 5. Failure mode for specimens[11].2.5.Column capital or steel plateWidianto[12] utilized the enlarging column part technique to strengthen punching shear.This method utilized the steel collar underneath the slab in order to accomplish the desired result.Figure6shows the process by which the steel collars were manufactured from the steel tube.Due to the fact that this strengthening increased the perimeter critical shear, the collar was improving the punching shear capacity as a result.For a flexural reinforcement ratio of 0.5 percent, the punching shear capacity increased by approximately 45 percent, and the deformation capacities increased by approximately 53 percent.For a flexural reinforcement ratio of 1.0 percent, the punching shear capacity increased by approximately 42 percent, and the deformation capacities increased by approximately 15 percent.