An Experimental Study on Hybrid Fibre Reinforced Concrete Filled Steel Tube Columns

Transcription

1 An Experimental Study on Hybrid Fibre Reinforced Concrete Filled Steel Tube s Er. Surya Ravindran 1,Er. Afia S. Hameed 2 1,2 Department Civil Engineering,Saintgits College Engineering,Kottayam Abstract A Concrete-Filled Steel Tube (CFST) column comprises steel hollow section circular or rectangular cross section filled with plain or reinforced concrete.steel confinement helps to reduce columns size and confined columns possess excellent earthquakeresistance and fire resistance properties. M3 design mix concrete amalgamated with.7% steel fibre and.% polypropylene fibre was chosen as hybrid fibre reinforced concrete (HFRC) which is used as an in-fill material for CFST columns. A total 8 column specimens comprising two normal RCC column and seven concrete-filled steel box columns were tested. The paper presents an experimental study on hybrid fibre reinforced Concrete Filled Steel Tube (HFRCFST) short columns axially loaded in compression to failure. Results show that in a CFST column the steel tube acts as longitudinal and lateral reinforcement for the concrete core and play an important role in increasing the compressive strength up to 8%. Failure CFST columns was due to local buckling the steel tube. Index Terms CFST column,hybrid fibre reinforced concrete. I. INTRODUCTION A. Concrete Filled Steel Tube s A Concrete-Filled Steel Tube (CFST) column comprises steel hollow section circular or rectangular shape filled with plain or reinforced concrete. Steel columns have high tensile strength and ductility. Concrete columns have high compressive strength and stiffness. Composite columns combine steel and concrete, resulting in a column that has the beneficial qualities both materials. Steel tubeprovided as a confinement for concrete acts as longitudinal and lateral reinforcement for the concrete core. Also it performs most effectively in tension and in resisting bending moment and also used as permanent formwork. CFSTs are used in buildings to avoid large size columns, supporting platform fshore structures, ros storage tanks, bridge piers, and column in seismic zones. Figure 1 shows a CFST column. Fig1: CFST s B. Hybrid Fibre Reinforced Concrete Concrete made with cement, sand coarse aggregate and water is brittle in nature and has some significant disadvantages such as low tensile and flexural strength, poor deformability and weak crack resistance in the practical usage. Fibre reinforced concrete (FRC) is a composite material consisting cement, sand, coarse aggregate, water and short discrete fibresthat are uniformly distributed and randomly oriented which increases its structural integrity. Fibres are used generally to improve the strength, ductility, post-cracking resistance, toughness etc. Only one fiber cannot improve all the desired properties concrete and hence two or more fibers are rationally combined and the composite is known as Hybrid Fibre Reinforced Concrete (HFRC). Steel fibres (metallic fibre) and polypropylene fibre (non- metallic fibre), when added to concrete improves its properties such as fracture toughness, ductility, impact resistance and reduces the plastic cracking in concrete structures. Hence steel and polypropylene fibres were used in the present study. HFRC has greater flexural strength and tensile strength than plain concrete. IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 18

2 C. Objectives Study the load carrying capacity steel tube columns in-filled with different concretes under compression Compare the ultimate load and strain capacities normal M3 mix and HFRC filled CFST columns with that normal reinforced columns. The effect steel confinement to columns and compare the failure patterns. D. Summary Literature Review In case CFST columns, column capacity was significantly improved due to the concrete strength gained from the confinement provided by the steel tube. Under both cyclic and monotonic tests, specimens with carbon FRP cracked, whereas specimens with glass or hybrid FRP did not show any visible cracks throughout cyclic tests. Among all CFST columns, the hybrid lay-up demonstrated the highest flexural strength and initial stiffness. Additions a small fibre type had a significant influence on the compressive strength, but the splitting tensile strength was only slightly affected. E. Methodology Literature survey Collection materials from different sources Mix design for M3 grade concrete Fabrication steel tubes Experimental investigation Preparation trial mixes Test on fresh and hardened concrete Casting and curing CFST specimens Analysis and Discussion Conclusion II. MATERIALS USED A. Material Properties A1. Cement Ordinary Portland cement (ULTRATECH) grade 3conforming to IS 12269: 1987was used. The physical and chemical properties cement are given in table 1. Sl No. Table 1: Properties Cement Property Result 1 Specific gravity Standard consistency 33% 3 Initial setting time 98 minutes 4 Final setting time 38 minutes Fineness cement 1.82% A2. Fine aggregate and Coarse aggregate M-sand which is free from organic impurities conform to IS 431: 1988and crushed stones 2mm, 12.mm and 6mm sizes conform to IS 2386: 1963 were used. Table 2 gives properties fine and coarse aggregate and figure 2 shows the gradation curve fine aggregate. Table 2: Properties Fine and Coarse Aggregate Sl No. Property 1 Specific gravity fine aggregate 2 Specific gravity coarse aggregate Result Water absorption.% 4 crushing value coarse aggregate 2.67% IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 181

3 Percent Passing September 216 IJIRT Volume 3 Issue 4 ISSN: Grading Fine Aggregate IS Code 4 Sample 2 IS Code Mild steel sheets conforming to grade IS 1748 GR2 24 BIS LIC NOCM/L having 2mm thickness was used. A6.Crimped Steel Fibre (SF) Sieve Designation Fig 2: Gradation curve fine aggregate Fig 3: Steel Fibres Table 4: Specifications Steel Fibre Table 3: Physical and chemical properties super plasticizer Sl No. Property Result Property Fibre Type Shape Specification Low Carbon Cold Drawn Wire Type V Steel Type Undulated 1 Appearance Yellow colored liquid 2 ph Minimum 6 3 Volumetric mass at 2 C 1.9 kg/liter 4 Chloride content Nil to IS:46 Dimensions Aspect Ratio 38/ Tensile Strength Equivalent diameter 1. Mm And mm Length 7-9 MPa A7. Polypropylene Fibre (PF) Alkali content 6 Dosage Typically less than 1.g Na 2 O equivalent / liter admixture. to 3. liters/1kg cementetious material A. Mild Steel Sheets Fig 4: Polypropylene Fibres III. EXPERIMENTAL INVESTIGATION IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 182

4 A. Concrete Mix Design Concrete mix design for M3 was done as per IS 46:2 and IS 1262:29. The mix proportion for M3 mix is given in table. Table : Finalized Mix Proportion for M3 Concrete Sl No. Material 1 Cement 36 2 Fine Aggregate 748 Quantity (Kg/m 3 ) 3 Coarse Aggregate Water 146 Super Plasticizer (.% mass cement) Water Cement Ratio.4 Mix (C:FA:CA:W) B. Design Proportion 1: 2.49: 3.342:.4 Design column was done as per IS 46:2. As per the design 4 numbers 1mm diameter bars were provided as main reinforcement and 8mm bars are used as transverse reinforcement. The cover provided is 2mm. The reinforcement detail for the column is shown in figure. Fig : Reinforcement details column C. Test Specimen Details An extensive experimental research has been planned to study the axial compressive behavior square CFST short columns having 2 mm 2 mm 38 mm size in-filled with different types concrete such as normal M3 mix and hybrid fibrereinforced concrete (HFRC). To obtain suitable fibrereinforced concrete mix for columns several trials were done by standard compression tests on cubes and from those test results the optimum dosages different fibres was determined. Details column specimens were given in table 6.Figure 6 shows CFST columns prepared for testing. Specimen Table 6: Details the test specimens Specim en Designa tion Perce nt Steel Fibre RCC C 1 Percent Polypro pylene Fibre RCC Steel Filled Tube C 2 PCC Steel Filled Tube C 3 HFRC Steel Filled Tube C 4.7. IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 183

5 Table 7 gives the compressive strength concrete cubes for different HFRC mix and normal M3.. Fig 6: specimens D. Test Set-Up and Test Procedure The test set-up consists a compression testing machine 3 KN capacity. The test specimens were placed appropriately on the center the end bearing plates Compression testing machine. Axial load was applied to the column, which is fixed supported at both ends, at a constant rate. In order to measure the strains in the column demec gauge were fixed at the mid-section and strain gauges are used for determining strains. Proper care was taken to fix the demec gauge by scraping the paint and using metal adhesive. Strain measured at equal intervals 2 KN load, load at first crack and the ultimate load were recorded. The test setup was shown in figure7. Table 7: Compressive strength HFRC cubes Concret e mix % polypro pylene fibers % steel fiber Average compres sive stress in 7 days(n/ mm 2 ) Avera ge compr essive stress in 14 days (N/m m 2 ) Avera ge compr essive stress in 28 days (N/m m 2 ) Normal HFRC Stress (N/mm 2 ) Compressive Stress for different HFRC mixes 7 Days 14 Days 28 Days. % polypropylene fiber.2 % steel fiber. % polypropylene fiber. % steel fiber. % polypropylene fiber.7 % steel fiber Fig 8: Compressive Stress for Different HFRC Mixes Fig 7: Test Set-up IV. RESULTS AND DISCUSSIONS A. Standard Test Results From the compression test results HFRC cubes.7% steelfibre and.% polypropylene fibre were fixed as the optimum dosage. Table 8 gives the results compressive strength cubes and cylinders, split tensile strength cylinder and IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 184

6 flexural strength concrete beams for normal M3 mix concrete and finalized HFRC mix. Table 8: Test Results standard specimens Con cret e Nor mal mix HF RC % o f S F % P F comp ressiv e stress cubes (N/m m2) Comp ressiv e Stress cylind ers ((N/m m 2 ) Spli t tens ile stre ss (N/ mm 2 ) Mo dul us rup ture (N/ mm 2 ) B. CFST Test Results Axially loaded columns are the one where load acts at the centroid the column cross-section. Resistance axially loaded column is more against buckling than eccentrically loaded column. Load bearing capacity a column also depends on the end conditions. with a fixed end condition at both ends are stronger than those having both ends free. The test specimens subjected to axial compressive load and the maximum load taken by each specimenwas given in table 9 andfigure 9 is a comparison graph ultimate stress. Table 9: test results Specimen Designa tion Compressi ve stress (MPa) 6 Percentage Increase In Ultimate Load Compared With RC RCC C RCC Filled Steel Tube C % PCC Filled Steel Tube C % HFRC Filled Steel Tube C % Ultimate Stress Comparison s C1 C2 C3 C4 Compressive stress (MPa) Fig 9: Ultimate Stress Comparison s In the initial stages concentric axial loading the CFST columns, both concrete in-fill and structural steel will deform longitudinally. At a certain strain, the expansion the concrete infill laterally increases until it reaches the lateral expansion the steel. Longitudinal stress in the confining tube varies based on the transfer forces between the concrete and steel. The failure CFST specimens occurs as the steel reached its capacity. From the test results column specimens it is clear that RC filled CFST columns have 86.% increases in maximum load carrying capacity than that the normal RC column. In case hybrid fibre reinforced concrete filled CFST column it was observed a 4% increase in load carrying capacity. C. Failure Modes Specimens When reinforced concrete columns are axially loaded, the reinforcement steel and concrete experience stresses. There are different modes failure for CFST specimens based on their material properties and geometric configuration. However the most common mode failure CFST columns was due to local buckling the steel tube. The concrete infill prevents the steel tube from buckling inwards and it forces the steel tube to buckle in an outward direction. From the analysis failure modes CFST columns it was understood that the fibre reinforced concrete in-fill has effects on the buckling steel tube. CFST column in-filled with reinforced concrete has more load carrying capacity than hybrid fibre reinforced concrete column but by comparing the outward buckling both these columns the steel tube RC filled column undergoes bulging at top and bottom ends the column while for HFRC filled column IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 18

7 bottom end steel tube had outward bulging. Failure pattern column specimens were shown in Figure 1 to Figure 13.Most the CFST columns have an elephant foot type (shown in Figure 14) local buckling the steel tube at the top and bottom end the column. Fig1: Crack pattern C 1 Fig 12: Failure pattern C 3 Fig 11: Failure pattern C 2 Fig 13: Failure pattern C 4 Figure 14: Elephant Foot Type Buckling V. CONCLUSION Concrete filled steel tube (CFST) columns having an L/D ratio less than 12 with various infill materials with or without internal bar reinforcements were tested under axial compression. Steel tubes were infilled with different materials included normal concrete and HFRC. The behavior CFST columns was analyzed with respect to ultimate load carrying capacity, and failure modes. From the experimental study the following conclusions were drawn. RC filled CFST columns have 86. % increases in maximum load carrying capacity than that the normal RC column. In case HFRC filled CFST column it was observed a 4% increase in ultimateload carrying capacity. Increase in load carrying capacity is due to the confinement effect provided by steel tube. Fibre reinforced concrete filled CFST columns have minimum buckling steel tube and for most columns the bulging steel tube took place when the load reaches about 8% the ultimate load. This may be due to the effect fibres that they provide structural integrity to concrete and prevents the formation micro cracks. CFST columns were capable carrying large amounts strain than normal RC column. REFERENCES [1] Ao-yuJiang, JuChen, Wei-liangJin Experimentalinvestigationanddesignthinwalledconcrete-filledsteel tubes subject to bending Elsevier-Thin-Walled Structures 63 (213) 44. [2] H. Ravi Kumar, K.U.Muthu, N.S.Kumar Experimental Behavior Circular HSSCFRC Filled Steel Tubular s under Axial Compression IC-RICE Conference Issue Nov [3] Nameer A. Alwash1 and Hayder I. AL-Salih Experimental Investigation on Behavior SCC Filled Steel Tubular Stub s Strengthened with CFRP Construction Engineering (CE) Volume 1 Issue 2, July 213 [4] P. Kiruthika, S. Balasubramanian, M.C. Sundarraja, J. Jegan Strengthening Concrete Filled Steel Tubular s using FRP Composites International Journal Innovative Research in Science, Engineering and Technology Vol. 4, Issue 4, April 21. [] S Bullo, R Di Marco, V Giacomin Behaviour Confined Self-compacting Concrete s 34 th Conference On Our World In Concrete & Structures: August 29, Singapore. IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 186

8 [6] Selina Ruby G., Geethanjali C., Jaison Varghese, P. MuthuPriya Influence Of Hybrid Fiber On Reinforced Concrete International Journal Of Advanced Structures And Geotechnical Engineering, Vol. 3 (January 214). [7] Thanuja H.P, E Ramesh Babu, Dr N S Kumar, A Study on Behavior Circular Stiffened HollowSteel Filled With Self Compacting Concrete Under Monotonic Loading Indian Journal Of Applied Research Volume : 4 Issue : 8 (August 214). [8] Y.F. Yang, L.H.Han Concrete filled steel tube (CFST) columns subjected to concentrically partial compression Elsevier Thin-Walled Structures (212) [9] Yin Chi, A.M.ASCE; LihuaXu; and Yuanyuan Zhang Experimental Study on Hybrid Fiber Reinforced Concrete Subjected to Uniaxial Compression American Society Civil Engineers (214). IJIRT INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 187