A NEW CFRP STRENGTHENING TECHNIQUE TO ENHANCE PUNCHING SHEAR STRENGTH OF RC SLAB-COLUMN CONNECTIONS

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1 Asia-Pacific Conference on FRP in Structures (APFIS 2007) S.T. Smith (ed) 2007 International Institute for FRP in Construction A NEW STRENGTHENING TECHNIQUE TO ENHANCE PUNCHING SHEAR STRENGTH OF RC SLAB-COLUMN CONNECTIONS H. Erdoğan 1*, G. Özcebe 2 and B. Binici 2 1 Research Assistant, Department. of Civil Engineering, Kocaeli University, Turkey (Currently on leave at the Middle East Technical University for graduate study, Dept. of Civil Engineering, Ankara, Turkey) ehakan@metu.edu.tr 2 Department. of Civil Engineering, Middle East Technical University, Ankara, Turkey ABSTRACT This study presents the results of experiments conducted on a new economical and easy to install strengthening technique for flat plates against punching failures. Four test specimens that were simply supported along four edges with corners free to lift up, representing the interior slab-column connection were tested. The concrete strength and tension longitudinal reinforcement ratio of the specimens were 30 MPa and 1.41%, respectively. Self-manufactured dowels were placed around the column stubs of the flat-plate specimens as vertical shear reinforcement. As a result of experiments, it was observed that the vertical load carrying capacities of the strengthened specimens were increased up to 33.4% relative to reference specimen. In addition, the test results indicated that the post-punching capacities of the specimens were also increased up to 135.5% relative to control specimen. The ultimate load capacities and failure modes of the test specimens were compared with the ACI provisions. The test results show that the proposed FRP retrofit technique can be used successfully to enhance punching shear capacity of slab-column connections. KEYWORDS Flat-plates, carbon fiber reinforced polymer, shear strengthening, and punching shear. INTRODUCTION In recent decades, the use of carbon fiber reinforced polymers () as strengthening material, become very popular in civil engineering industry. Because of advantageous material properties (high tensile strength and low self-weight etc.) structural engineering researchers have focused on investigating the use of as a reinforcing material. Several experimental studies were performed by bonding s to various structural components or members in different ways to rehabilitate the structures. Punching failure of flat-plates is a very serious problem for civil engineers because of the sudden and brittle nature of the phenomena. The inclined shear cracking that occurs in the slab-column connection of the flat-plates due to vertical loads or unbalanced moments can lead to total progressive collapse of the structure without any warning. The first attempt for strengthen punching shear capacity of slab column connections in deficient flat plate structures using was performed by Erki and Heffernan (1995) by bonding fiber reinforced plastic material externally on the flat-plate surfaces. Externally bonding method was also studied by Tan (1996), Chen and Li (2000) and Wang and Tan (2001) by changing the parameters such as type, width and arrangement of the FRP layers. The effect of combination of shear bolts and material on punching shear capacity of the flat plates was investigated by Salakawy et. al (2004) and Harajli et. al (2006). Binici and Bayrak (2003) was first to use s as vertical shear reinforcement to enhance the punching shear capacity of the flat plates. In this study, we propose a new method to install FRPs as shear reinforcement that can reduce the workmanship and save from material which results in economical strengthening designs. 233

2 EXPERIMENTAL PROGRAM Test Specimens Four specimens with plate dimensions of 2300 mm x 2300 mm x 150 mm and column stub dimensions of 250 x 250 x 300 mm extending from both faces were selected as the test structures to mimic near full scale slab column connection regions (Figure 1). Reinforcement on the tension side of the specimens consisted of 16 mm deformed bars with a spacing of 120 mm for both orthogonal directions, whereas compression reinforcement consisted of 12 mm deformed bar with spacing of 150 mm for both orthogonal directions. The column stubs had four 16 mm deformed bars as longitudinal reinforcement with lateral reinforcement of 10 mm deformed bars having spacing of 150 mm. The net effective depth of the specimens measured to the centre of the two orthogonal layers of reinforcement was 114 mm. Specimen named as CS was the control specimen without any strengthening. Specimens SS3, SS4 and SS5 were the strengthened specimens with s. For application of strengthening method, holes were drilled around the column stub of the strengthened specimens in a double line arrangement from the column face (similar to those used By Binici and Bayrak, 2003). The numbers at the end of strengthened specimen names indicates the number of FRP perimeters away from the column face. For example, specimen SS3 had three FRP dowel perimeters along one side of the column stub as shown in Figure 2. The spacing of the holes was 60 mm, which was equal to about half of effective depth mm bars support points mm 120 mm - tension reinf. (top) mm 120 mm tension reinf. (top) mm 150 mm compression reinf. (bottom) Reinforcement Layout mm 150 mm - compression reinf. (bottom) A 150 mm 2000 mm 150 mm 250 mm 250 mm LVDT and Dial Gage locations Top View 150 mm 2000 mm 150 mm 150 mm Tie-rod A 16 mm bars 3 10 mm bars 300 mm Load Cell RIGID FLOOR A - A Section 12 mm bars Roller Support Hydraulic Ram Figure 1. Details of the test specimens and the test setup Materials The target concrete compressive strength was 30 MPa for the test specimens. The compressive strength of the test specimens on the test day are presented in Table 1. The yield strength of the 10 mm, 12 mm and 16 mm reinforcing bars were obtained from uniaxial compression tests as 430 MPa, 426 MPa, 448 MPa respectively. The thickness of the sheets used for the strengthening of the specimens was mm and the rupture stress and strain values of the sheets were reported as 3450 MPa and respectively by the manufacturer. In order to obtain material properties of composite material (+resin), three sheets having width of 25 mm and effective length of 165 mm were impregnated with epoxy resin and the thickness of the sheets become 0.8 mm after impregnation. These three sheets were tested uniaxially in order to obtain the capacity of the composite metarial. The ultimate stress and strain values of the composite material were obtained as 637 MPa and , respectively. (Table 1) Table 1 Specimen details and material properties Specimen Concrete strength, f c, (MPa ) Yield strength of tension reinf., f yt (MPa) Yield strength of compression reinf.,f yc (MPa) Number of dowels CS 32 - SS SS SS Ultimate stress (MPa) effective depth,d (mm) APFIS

3 application sheets were used as vertical shear reinforcements to strengthen three test specimens. At the first stage of application, 250 mm by 120 mm sheets were cut and impregnated with epoxy resin (Fig. 2a). These sheets were rolled around 6 mm plain bars having a length of 90 mm to have uniform shape as shown in Figure 2b. The contribution of the 6 mm plain bars to the punching shear capacity of the strengthened specimens was neglected due to insufficient anchorage length. Then in-house fabricated dowels in the second stage were placed through the pre-drilled holes around the column stubs of the specimens (Fig. 2c). In the final stage of the implementation, strip patches were bonded on both surfaces of the specimens along the FRP dowel locations. Then the ends of the dowels were fanned out and bonded on the strip patches as shown in Figure 2d. The effect of patches on the flexural capacity of the specimens was neglected because of the insufficient development length of patches. strips for anchorage Column Stub Fans of dowels strips for anchorage sheet dowel Section View Top View Instrumentation and Testing (a) (b) (c) (d) Figure 2. Application of strengthening method All the specimens were simply supported along the four sides with corners free to uplift. Eight linear variable displacement transducers (LVDT) and eight dial gages were located on the tension face of the test specimens to measure the vertical displacement of the specimens (Fig. 1). Nine strain gages were mounted on the reinforcing steel to obtain strain profile of the specimens at the position of column face, 120 mm and 210 mm away from column face, respectively. The concentric vertical load was applied monotonically through the column stub on the compression face of the specimens as shown in Figure 1. At every 50 kn increment of loading, displacement was kept constant and cracks were located and marked on the specimens. TEST RESULTS AND DISCUSSIONS The load values obtained from the test were normalized by a load factor defined as 30/f c ' in order to take into account the slight differences between the concrete compressive strength of the test specimens. The flexural capacities of the test specimens were calculated using expressions derived based on yield line theory, accounting the uplift of the corners (Elstner and Hognestad, 1956): V flex 1 = 8m a 1 r (1) APFIS

4 where m is the moment capacity per unit width, a is the span length of the slab and r is the side length of the square column. The summary of test results and normalized applied load versus net centre deflection curves are presented in Table 2 and Figure 4, respectively. Table 2 Test Results Specimen Failure load, V u (kn) V nu, Δ u, (kn) (mm) V nu V flex V npp, (kn) % Increase % Increase in V nu w.r.t. in Δ u w.r.t CS CS % Increase in V npp w.r.t CS Dissipated Energy (knmm) CS SS SS SS V nu : Normalized ultimate load ; V u : Experimental result; V npp : Normalized post-punching load ; Δ u ; failure mid-deflection Control Specimen, CS In the control specimen, first flexural cracks were initiated at the load level of about 75 kn. Propagation of radial cracks resumed until failure load level. The strain gages located on the flexural reinforcement at the column face were detected yielding at a load level of about 350 kn. However, at the ultimate load level the strain gage, located at 120 mm and 210 mm away from the column face, detected lower values than yielding strain of the steel reinforcement. Therefore, this local yielding of the reinforcing steel at the column face is not sufficient for the specimen to exhibit ductile behavior. The column stub punched through the plate at load level of 484 kn in a sudden and brittle manner with an audible sound and the corresponding displacement value at the centre of the slab was measured to be 17.5 mm. Strengthened Specimens, SS3, SS4, SS5 The first flexural crack occurs at load level of 93 kn, 69 kn and 114 kn, for three strengthened specimens, (SS3, SS4, SS5) respectively. Similar to reference specimen, the first yielding of flexural reinforcing bars at the column face was detected at the load range of kn. Specimens SS3 and SS4 experienced a brittle punching failure at load levels of 573 and 614 kn respectively. This slight difference indicates that increasing the number of dowels around the column stub from 24 to 32 is inadequate to obtain yield mechanism for the specimen and more dowels is needed to resist the shear force to increase the ductility level. Therefore, the number dowels were increased to 40 for the specimen SS5. The failure mode of the strengthened specimen, SS5, was also punching after exhibiting a yield plateau which was not observed in other specimens. The ultimate load value for the specimen SS5 was 646 kn. The punching perimeter was located outside the shear reinforced region for all of the strengthened specimens. This behavior indicates the contribution of dowels to the shear strength of the concrete inside the shear reinforced area. (Figure 5) SS4 SS5 Normalized Load (kn) elastic CS SS Center Displacement (mm) Figure 4. Normalized Load vs Center Deflection APFIS

5 Stiffness, Ductility and Energy Absorption The initial stiffnesses of all four specimens coincide with elastic solution up until the first cracking of the specimens (Fig. 4). After first cracking, all specimens followed a similar load-deformation path until the punching failure occurred except specimen SS5 with a yield plateau. The displacement capacities of the strengthened specimens SS3, SS4 and SS5 were 2.03, 2.05 and 2.81 times greater than the capacity of control specimen CS. Due to increases in load and displacement capacities of the strengthened specimens, the energy absorption capacities (i.e. the areas under load-deflection curves) of the strengthened specimens increased significantly as can be observed in Table 2 (up to three times for SS3 and SS4 and four times for SS5 compared to control specimen CS). Post - Punching Behavior The amount of longitudinal reinforcing steel resistance accounting for dowel action increased due to spread of the punching failure cone for strengthened specimens. This leads to significant increase in post punching capacities of the strengthened specimens up to 135% compared to unstrengthened specimen. ACI CODE PREDICTIONS Figure 5. Views of test specimens after failure ACI defines the punching load capacity of flat plates without any shear reinforcement as the smallest of the three expressions given in Eq 2. On the other hand, ACI proposes two different equations for predicting the punching load capacity of the shear reinforced flat plates considering the failure modes either outside (V c ) or inside (V i ) the shear reinforced region. The smallest capacity obtained from Eqs 2 and 3 governs the behavior of the shear reinforced flat plates. 1 ' 1 1 ' Smallest of Vc = fcbo d ; Vc = + fcbo d ; sd 1 ' Vc fcbo d 3 6 3β 12.. bo 6 = α + (2) 1 ' d V = f bd A f (3) i c o + sw y 6 s where f c is the compressive strength of concrete, b o is the critical punching perimeter located d/2 away from the column face or from the outermost shear reinforcement, d is the effective depth, β is the column aspect ratio, α is a coefficient depending on the column location (=40 for interior columns), A sw is the total area of the shear reinforcement ( dowel) in one perimeter, f y is the yield strength of the shear reinforcement, s is the spacing of the vertical shear reinforcement. When considering the contribution of the dowels in Eq 3, yield stress term f y for dowels were assumed to be 290 MPa corresponding to strain value of This strain value is equal to the strain limit recommended by ACI Committee 440 (2002) for FRP material used for shear strengthening purposes. Table 3 Comparison of test results with ACI Code Provisions Test Results ACI Predictions Comparison Specimen Vu V Failure Mode c V i V Failure Mode ACI (kn) (kn) kn V test CS P3 601 Outside Outside P4 571 Outside Outside P5 657 Outside Outside 0.85 V u : Test results V c : Punching cap. outside the shear reinforced zone V i : Punching cap. inside the shear reinforced zone APFIS

6 The ratio of predicted results to experimental results varies between 0.63 and 0.85 with a mean value of 0.79 as shown in Table 3. ACI provisions were conservative predicting the punching load capacity of the test specimens. In addition, the failure modes of the all specimens were predicted correctly. SUMMARY & CONCLUSIONS In the scope of this study, a total of four flat-plate specimens representing slab-column connection were tested under vertical monotonic loading. Three of four test specimens were strengthened to increase punching shear strength with a new economical and easy to install FRP dowel technique. Following important conclusions can be summarized based on the test results. 1. Significant increase in ultimate punching load capacity of the test specimens was obtained by the application of the strengthening method. The maximum increase was about 33% compared to capacity of the control specimen and the failure cone was occurred outside the strengthened region for all the strengthened specimens. 2. The displacement at ultimate load and the residual capacity of the strengthened specimens were also enhanced. The increase in displacement at ultimate load and the residual capacity was about 181% and 136% relative to control specimen, respectively. The brittle nature of the punching failure was gradually converted to a more flexure dominant failure mode when five dowel perimeters were used. 3. Post punching capacity of test specimens increased up to about 2.4 times the post punching capacity of the strengthened test specimens upon retrofit. In other words, it was found that even a punching failure occurs for strengthened slab-column connections, 80% of the unstrengthened punching shear capacity can be maintained. 4. The design provisions of ACI are safe for predicting both punching load capacity and failure modes of the test specimens. REFERENCES ACI Committee 440, (2002). Guide for the design and construction externally bonded FRP systems for strengthening concrete structures (ACI 440.2R-02) American Concrete Institute, Farmington Hills, Mich. American Concrete Institute (ACI), (2005) Building code requirements for structural concrete. ACI , Farmington Hills, Michigan. Binici, B., and Bayrak, O., (2003). Punching shear strengthening of reinforced concrete flat plates using carbon fiber reinforced polymers. Journal of Structural Engineering, ASCE, 129, pp Chen, C.C., and Li, C.Y., (2000). Experimental study on the punching shear behavior of reinforced concrete slabs strengthened by GFRP. International Workshop on Punching Shear Capacity of Reinforced Concrete slabs, Sweden, pp El-Salakawy, E., Soudki, K.A., Polak M.A., (2004). Punching shear behavior of flat slabs strengthened with fiber reinforced polymer laminates. Journal of Composites for Construction, ASCE, 8, pp Elstner, R.C., and Hognestad, E., (1956). Shearing strength of reinforced concrete slabs. ACI Structural Journal, 53, pp Erki, M.A., and Heffernan, P.J., (1995). Reinforced concrete slabs externally strengthened with fiber-reinforced plastic materials. 2nd International Symposium on Nonmetallic (FRP) Reinforcement for Concrete Structures, Belgium, pp European Standard EN 1992, Eurocode 2 (2003). Design of concrete structures, B-1050 Brussels Tan, K. H., (1996). Punching shear strength of reinforced concrete slabs bonded with FRP systems. 2nd International Conference on Advanced Composite Materials in Bridges and Structures, pp Harajli, M.H., and Soudki, K.A., Kudsi, T., (2006). Strengthening of interior slab column connections using a combination of FRP sheets and steel bolts Journal of Composites for Construction, ASCE, 10, pp Wang, J.W., and Tan, K.I., (2001). Punching shear behavior of reinforced concrete flat slabs externally strengthened with systems. 5th International Conference on Fiber-Reinforced Plastic for Reinforced Concrete Structures, London, pp APFIS