BEHAVIOR OF REINFORCED CONCRETE ONE-WAY SLABS STRENGTHENED BY CFRP SHEETS IN FLEXURAL ZONE

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 10, October 2018, pp , Article ID: IJCIET_09_10_097 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed BEHAVIOR OF REINFORCED CONCRETE ONE-WAY SLABS STRENGTHENED BY CFRP SHEETS IN FLEXURAL ZONE Wissam D. Salman, Ahmed Abdullah Mansor and Mohammed Mahmood Civil Engineering Department, the University of Diyala, Diyala, Iraq ABSTRACT This paper presents experimental data and observations to the behavior of six reinforced concrete one-way slabs with opening strengthened by CFRP sheets. The specimens were mm length, width and thickness respectively. 200mm square opening was created at the centroid of some specimens. The main parameters that were investigated are the length and width of CFRP sheets also the presences of the opening. The behavior of each specimen was evaluated by the beginning of cracking, crack pattern, deflection, ultimate load and failure mode. The results showed that the presence of opening in one-way slabs could decrease the load capacity by 50%. The effectiveness of using CFRP sheets for strengthening RC slabs with opening had been clearly recorded. The ultimate strength increased by 24% to 92%, the mid-span deflection was reduced by 40% to 49% at service load stage and by 47% to 62% at ultimate load stage, the cracking loads were increased by 74% to 88%, the crack width was reduced by 44% to 76% at service load stage and 86% to 95% at ultimate load stage. Keywords: One way slabs, Strengthening, CFRP sheet, Slabs with opening, Flexural zone Cite this Article: Wissam D. Salman, Ahmed Abdullah Mansor and Mohammed Mahmood, Behavior of Reinforced Concrete One-Way Slabs Strengthened By Cfrp Sheets in Flexural Zone, International Journal of Civil Engineering and Technology, 9(10), 2018, pp INTRODUCTION The strengthening of reinforced concrete buildings is required in some unexpected cases such as increasing in the applied load, changing the functionality of the building and to overcome design or construction mistakes [1-3].Openings in reinforced concrete slabs are essential for stairs, elevators or any other services such as air heating, wiring, conditioning ducts. The presence of the opening in reinforced concrete slabs could result in clear reduction in the slab strength and rigidity

2 Wissam D. Salman, Ahmed Abdullah Mansor and Mohammed Mahmood Many of studies were conducted to understand the behavior of reinforced concrete slabs with openings and propose some design recommendations [4-6]. The use of Carbon Fibers Reinforced Polymers (CFRP) can be considered as a replacement for steel plate. CFRP overweighs the steel plates by many aspects. For instance, CFRP has higher strength to weight ratio, easier to place at irregular places and the corrosion is not a problem in CFRP [7-11]. Improvement in flexural capacity of RC slabs through the horizontal restraint in the borders has been sufficiently verified by both theoretical and experimental applications. It was stated that increasing in the capacity due to the compressive membrane forecast the restraints can be considered [12]. The research described in this paper is investigating the use of CFRP sheet as a high-performance technique for repairing and strengthening reinforced concrete one-way slabs with openings. The main aim of the research is to study the effect of length and width of CFRP sheet. 2. EXPERIMENTAL PROGRAM The experimental program involved casting, strengthening and testing of six reinforced concrete one-way slabs (five out of six with opening). The specimens' dimensions are ( ) mm. ACI [13] specificationswereadopted to design the specimens. The main steel reinforcement was 4ϕ10, representing a 0.843% steel ratio. Secondary reinforcement was 9 ϕ10 in the transverse direction. S1is a solid slab without opening and without strengthening. It was considered as a reference specimen and the remaining five specimens contain square openings (200mm side) at their centroids. S2was with opening but without strengthening. S3, S4, S5 and S6 were with openings and they were strengthened using CFRP sheets. The strengthening of specimens was divided into two main groups, each group involves two specimens have length of CFRP sheet (600mm and 800mm), but in the first group involve using two segments of CFRP sheet with 100mm width and in the second group involve using one segment of CFRP sheet with 500mm width. The properties of the six specimens are specified in Table 1. Figures 1and 2 present the geometrical dimensions and reinforcement details of the specimens respectively. The molding, casting and curing of specimens are shown in Figure 3. Specimen Opening size (mm) Table 1Test matrix CFRP length (mm) CFRP width (mm) Number of CFRP sheets f c ' MPa S S2 200mm S3 200mm S4 200mm S5 200mm S6 200mm editor@iaeme.com

3 Behavior of Reinforced Concrete One-Way Slabs Strengthened By Cfrp Sheets in Flexural Zone Figure 1 Dimensions of specimens and reinforcement details Figure 2 Details of tested specimens editor@iaeme.com

4 Wissam D. Salman, Ahmed Abdullah Mansor and Mohammed Mahmood Figure 3 Specimens moulding, reinforcing, casting and curing The strengthening includes the use of CFRP sheets (SikaWrap Hex-230C), which was bonded to the specimens by structural adhesive paste (Sikadure-330). The dimensions and layouts of the CFRP sheets are shown in Figure 2. The locations of CFRP sheets were roughened by a grinding machine to increase the bonding (Figure 4-a). Then roughened surface was cleaned (Figure 4-b) and about 2mm thickness of structural adhesive paste was applied over the grinded surface (Figure 4-c). CFRP sheets were laid on their predefined locations and hand pressure was applied on CFRP sheets in along direction of fiber to remove air bubbles (Figure 4-d). The temperature of laboratory was around 20 o ± 2 o C during strengthening. Figure 4 Strengthening procedure A hydraulic jack with 600kN capacity load cell was used to perform four points bending tests to the specimens as shown in Figure 5-a. The shear span to effective depth ratio (a/d) for all specimens was equal to 6. The deflection has been measured at three points (E1, E2 and E3) (Figure 5-b). To measure the strain in concrete, a strain gauge was attached to each specimen at the top surface of all specimens as shown in Figure 5-b. Optical micrometer with an accuracy of (0.02mm) was used to measure the crack width editor@iaeme.com

5 Behavior of Reinforced Concrete One-Way Slabs Strengthened By Cfrp Sheets in Flexural Zone 3. RESULTS AND DISCUSSION Figure 5 Test setup 3.1 Load deflection behavior The flexural behavior of specimens is presented in terms of load deflection relationship (Figure 6). A load-deflection curve identifies three stages: a linear partuntilthe cracking load (initial stage), a transition stage of continuous yielding (yield stage) and full plastic deformation (ultimate stage). Therefore, the behavior can be presented by tri linear model. At the pre-cracking part, the trend was approximately straight line with constant stiffness. After the crack initiation, the slope of the curve shows significant decreasing in stiffness. In the post-cracking part, the strengthening presents clear impact on the stiffness. For all strengthened specimens; there was an obvious improvement in the flexural strength and stiffness. However, the failure turned to brittle manner. Table 6presents the experimental results of all specimens (crack and ultimate loads, deflection, strain and crack width in service and ultimate loads) editor@iaeme.com

6 Wissam D. Salman, Ahmed Abdullah Mansor and Mohammed Mahmood Specimen Crack load P cr (kn) Ultimate load P u (kn) Figure 6 Load deflection relationship Table 2Experimental results of specimens mid-span deflection s (mm) at service mid-span deflection u (mm) at ult. Strain εs *10-3 at service load Strain εu *10-3 at ultimate load crack width at service W s (mm) crack width at ultimate W u (mm) S S S S S S Load carrying capacity Referring to Table 2, the presence of opening in specimens S2 decreased by 50% comparing with the slab without opening S2.However, there was noticeable increase in the ultimate strength by using the CFRP sheet. The enhancement in the ultimate load was ranged from 24% to 92% comparing to S2. The maximum improvement in the ultimate strength was obtained by using one segment of CFRP sheet with 500mm width and 800mm length. For specimens strengthened by CFRP sheets with length of 600mm and 800mmthe percentage of improvement was between 42% and 70%. However, the increasing in the ultimate strength was in the range of 24% to70% for 100mm sheets width (two segments) and (42% and 92%) for 500mm sheets width (one segments). Figure 7 presents the percentage of improvement in the ultimate load capacity due to the use of CFRP sheet comparing with S editor@iaeme.com

7 Behavior of Reinforced Concrete One-Way Slabs Strengthened By Cfrp Sheets in Flexural Zone Figure 7 Effect of strengthening on ultimate strength 3.3. Mid-span deflection The mid-span deflection presented in Table 2 shows that the presence of the opening results in decreasing the mid-span deflection at the ultimate strength by 28%.The strengthening has clear impact on the deflection, and it turned the failure mode to a brittle manner. Further reduction in the mid-span deflection was recorded with the strengthening. At the service stage (before the first crack) mid-span deflection reduced by 49% and 62% at the ultimate strength. This behaviour could be related to the properties of the material that was used in strengthening, where the CFRP sheet deemed brittle materials which have 1.5% strain at failure. The decreasing in the mid-span service and ultimate loads compared to S2 can be seen in Figures 8- a and b respectively. Figure 8 Decreasing of mid-span deflection at service and ultimate loads due to strengthening 3.4. Compressive Strain of Concrete The strain was monitored during the tests using strain gauges, which were placed at the upper side of the specimens at the mid-span (Figure 5-b). It can be seen from Figure 9 that the compressive strain in concrete was reduced significantly with the use of CFRP sheet.the reductions were 8% to 23% at service load stages and 30%to 56% at ultimate load stages compared with unstrengthened one-way slab with opening. This reduction may be as a result of large stiffness of CFRP sheet. The decreasing of concrete strain in service (ε s ) and ultimate (ε u ) compared withs2 is presented in Figure editor@iaeme.com

8 Wissam D. Salman, Ahmed Abdullah Mansor and Mohammed Mahmood Figure 9 Effect of Strengthening on concrete compressive strain Figure 10 Concrete compressive strength at service and ultimate loads 3.5. Cracking and failure modes The first flexural crack appears at load of 9.7kN and 6.8kN in S1 and S2 respectively. It was in the interior region of maximum moment (between the two point s loads). Then the cracks number and width started increasing with the application of loading until they reached the compression face on the specimens and both specimens show ductile failure due the yield of reinforcements. The width of first crack decreased at service stage (Ws) (Figure 11 and Figure 12-b). Also, the cracking load (P cr ) increased as a result of strengthening by CFRP sheets (Figure 12). It was found that the first crack was formed at 14%, 20%, 28%, 22%, 26% editor@iaeme.com

9 Behavior of Reinforced Concrete One-Way Slabs Strengthened By Cfrp Sheets in Flexural Zone and 20% of the ultimate load of specimens S1, S2, S3, S4, S5 and S6 respectively. S3, S4, S5 and S6 failed by debonding. Figure 11 Load- Crack width relationships Figure 12 Effect of strengthening on cracking load and crack width 4. CONCLUSIONS The research showed an experimental study to evaluate the behavior of one-way reinforced concrete slabs with opening to shed some light on the effect of CFRP sheet as a strengthening material to act as a replacement to the removed part of reinforced concrete slab at the opening. The availability of opening in one-way slab may decrease the load carrying capacity by 50%, but the use of CFRP sheet could recover this reduction. The effect of using CFRP sheet in strengthening the opening in one-way reinforced concrete slabs can be summarized as follows: editor@iaeme.com

10 Wissam D. Salman, Ahmed Abdullah Mansor and Mohammed Mahmood 1. Increasing in ultimate load capacity of strengthened one-way slab by 24% to 92% compared with unstrengthened one -way slab with opening. 2. Increasing in stiffness of strengthened one-way slabs at all loading stages, so the deflection is reduced at service load by 40% to 49% and by 47% to 62% at ultimate load compared with unstrengthened one -way slab with opening. 3. Increasing in crack loads for strengthened one-way slabs by 74% to 88%, and reducing in crack width at service and ultimate load stages by 44% to 76% and 86% to 95% respectively compared with unstrengthened one-way slab with opening. 4. Decreasing in the concrete compressive strain for strengthened one-way slabs with a range 8% to 23% at service load stages and 30%to 56% at ultimate load stages respectively compared with unstrengthened one-way slab with opening. 5. The use of one segment of CFRP sheets to cover the four sides of the opening provide better performance in terms of crack load, ultimate load, crack width, deflection and the development of concrete compressive strain compared with the use of sing two segments to cover only two sides of the opening of CFRP sheets have equivalent length. REFERENCES [1] Ibrahim, A.M. and M.S. Mahmood, Finite Element Modeling of Reinforced Concrete Beams Strengthened with FRP Laminates. European Journal of Scientific Research, (4): p [2] Yang, J.-Q., et al., Numerical simulation of FRP-strengthened RC slabs anchored with FRP anchors. Construction and Building Materials, : p [3] Mourão, R., et al. Load-Displacement Assessment of One-Way Reinforced Concrete (RC) Slabs Externally Strengthened Using CFRP Strips under Blast Loads. in Multidisciplinary Digital Publishing Institute Proceedings [4] Zaslvasky A., Yield-Line Analysis of Rectangular Slabs with Central Openings, Proceedings ACI Journal, Vol. 64, No. 12, PP , Jan [5] Lash S.D., Banerje A., Strength of Simply Supported Square Plates with Central Square Openings, Trans. Eng. Inst. Can., Vol. 10, No. A-5, PP. 3-11, [6] Islam S. and Park R., Yield-Line Analysis of Two Way RC Slabs with Openings The Structural Engineer Journal, Vol. 49, No. 6, PP , [7] Ichimasu H., RC Slabs Strengthened by Bonded Carbon FRP Plates: Part 1-Laboratory Study, ACI Journal, Vol. 138, Special Publication, PP , Jan [8] Arockiasamy M., Amer A. and Shahawy M., Concrete Beams and Slabs Retrofitted with CFRP Laminates, Proceeding of the 11th Conference on Engineer Mechanics, ASCE, New York, PP , [9] Karbhari V., Seible F., Seim W. and Vasquez A., Post Strengthening of Concrete Slabs, ACI Journal, Vol. 188, Special Publication, PP , Jan [10] Takahashi Y. and Sato Y., Experimental Study on the Strengthening Effect of a CFRP Sheet for RC Slabs, FRPRCS5, Thomas Telford, Cambridge, UK, PP , [11] Teng J., Chen J., Smith S. and Lam L., Flexural Strengthening of Slabs, FRP Strengthened RC Structures, John Wiley and Sons, PP , [12] Lahlouh E. and Waldron P., Membrane Action in One-Way Slab Strips, Proceedings of the Institution of Civil Engineers - Structures and Buildings, Vol. 94, No. 4, PP Nov [13] ACI Committee 318M-14, Building Code Requirements for Structural Concrete and Commentary, ACI 318M-14, Farmington Hills, MI: American Concrete Institute, 473, editor@iaeme.com