Effect Of Hybrid Fibre Reinforcement In Concrete Deep Beams

Transcription

1 Effect Of Hybrid Fibre Reinforcement In Concrete Deep Beams Mr. Suhail Shaikh 1, Prof. S. K. Kulkarni 2, Dr. S. S. Patil 3, Dr. Halkude S.A. 4 1 PG student, Walchand Institute of Technology, Solapur. 2 Asst. Professor and Research scholar, Walchand Institute of Technology, Solapur. 3 Professor in Civil Engg, Walchand Institute of Technology, Solapur. 4 Professor in Civil Engg, and Principal, Walchand Institute of Technology, Solapur Abstract: To enhance resistance of concrete against crack formation, fiber reinforcement has been the focus of studies by various researchers over past few decades. Combining two or more types of fibers for getting high quality hybrid fiber reinforced concrete (HFRC) has been a recent trend. Use of small, discrete, randomly oriented steel fibers improves the strength and resistance against deformation characteristics of concrete members. Polypropylene (PP) fibers contribute to enhanced ductility, crack resistance, energy absorption and impact strength of concrete. This paper presents data obtained from experiments carried by using combination of these fibers in predefined proportions in the concrete and its effect on strength characteristics of HFRC deep beams. The effect on strength of beams due to variation in shear span is also studied. The beams cast & cured are tested in heavy structure laboratory using 1000 kn capacity loading frame. The two-point loading is applied. The shear span to depth ratios considered is 0.43 and During experiment, first crack load, ultimate load and central deflections of deep beams are recorded. It is observed that, in comparison with conventional deep beams with shear span to depth ratio 0.43, there is improvement of 9% in first crack load and 25% in load at permissible deflection of HFRC deep beams cast with optimum mix containing 0.9% steel fiber and 0.3% PP fiber by volume of concrete. Keywords: Steel fiber, polypropylene fibre, Deep beam, shear span, two point loading, hybrid fibre reinforcement. I. INTRODUCTION A beam shall be deemed to be a deep beam when the ratio of effective span to overall depth is less than two 1. The common examples of deep beam are shear walls, transfer girders, pile caps, walls of water tanks, bunkers and silos, etc. Force transfer mechanism in deep beams is quite different from that in slender beams as major part of the external load on deep beam is carried by diagonal strut joining loading point to support point. This leads to non-linear variation of stresses and strains. Strength of deep beams is controlled by shear rather than flexure. When principal tensile stress exceeds the permissible tensile stress of concrete, diagonal cracks occur in the shear span region and results in low first crack load and low shear strength. The uses of discrete, randomly oriented fibres provide post-cracking tensile resistance to concrete. Their use as shear reinforcement in reinforced concrete (RC) beams has been the focus of several investigations in past, few of them are as follows. T.M.Roberts, N.L.Ho 2 (1982) have presented experimental results of several tests on deep fiber reinforced concrete beams. The authors found that randomly oriented small discrete fibres prohibit crack formation in concrete and hence improve its durability. Steel fibers can make failure of deep beams more ductile and improve its strength. The Steel fibers prevent shear failure in deep beams. Shanmugam, Swaddiwudhipong 3 (1984) carried out an experimental investigation to study the ultimate load behaviour of steel fibre reinforced concrete deep beams and they found that the addition of steel fibres results in increased failure loads. Tan K. H 4 et al (1998) carried out experiments on High strength Concrete Deep Beams and they observed that for beams with shear span-to-depth ratios less than one, failure mode is largely influenced by the shear span-to-depth ratio. V. M. Sounthararajan 5 (2013) performed experiments on compressive and flexure strength of concrete for various mix proportions containing different percentage of polypropylene fibers from 0% to 0.3%. Imperial Journal of Interdisciplinary Research (IJIR) Page 238

2 Their experimental results showed that PP fibers possess increased extensibility and tensile strength, both at first crack and at ultimate load, particularly under flexural loading; and the fibers were able to hold the matrix together even after extensive cracking. Also, they found that, inclusion of PP fibers resulted in loss of workability. II. OBJECTIVES OF THE STUDY The present research is devoted to study the variation in shear strength of HFRC deep beams with variation in fiber content. The shear strength of HFRC deep beams is compared with that of conventional deep beams. Hybrid fiber reinforcement consists of hooked end steel fibers and fibrillated polypropylene (PP) fibers, used as shear reinforcement in place of conventional shear reinforcement in deep beams. Study of literature reveals that generally steel fiber content in the range of 0.5% to 1% by volume of concrete improves strength and deformation characteristics of concrete with minimum effect on its workability 3, 7, 12. Polypropylene fiber in fibrillated form occupies more volume with respect to weight and 0.3% polypropylene fiber is efficient in minimization of cracks in concrete 5. Therefore, to study the effect of combination of both fibers in concrete, the steel fiber content considered is 0.5, 0.7, 0.9, 1.1 and 1.3% and for each percentage of steel fiber the PP fiber content taken is 0.1, 0.2 and 0.3% by volume of concrete. The details of variation of fiber content are presented in Table 1. The objective of present research isto study the effect of hybrid fiber content on deep beam and to arrive at suitable proportions of steel and PP fibre content to attain higher shear strength. III. EXPERIMENTAL PROGRAM A. Materials used for casting of deep beams along with their properties: i. Casting of conventional deep beams: The materials used for making the concrete for conventional deep beams are 43 grade ordinary Portland cement conforming to IS 269:2015, natural sand conforming to zone II of IS 383:1997, and coarse aggregate of maximum size of 20 mm. The material used for shear reinforcement as well as tensile main reinforcement in conventional deep beams is steel of grade Fe 500. ii. Casting of HFRC deep beams: For casting of HFRC deep beams, all the materials used are same as in case of conventional deep beams except shear reinforcement, which is replaced completely by hooked end steel fibers and polypropylene fibers. Steel fibres used (as shown in Image 1-a) are of aspect ratio 80, diameter 0.75 mm and length 60 mm. Modulus of elasticity of steel fiberis 200 GPa and tensile strength is 1100 MPa. Fibrillated polypropylene fibres used (as shown in Image1-b) are of modulus of elasticity 3.5 GPa and average tensile strength of 453 MPa. a) Hooked end steel fibre b) Fibrillated polypropylene fibre Image 1: Fibres used B. mix proportions: i) For conventional deep beams: Design mix of M20 grade concrete is carried out in accordance with IS 10262:2009. Mix proportion obtained is 1:1.98:3.09 (by weight) with water cement ratio of ii) For HFRC deep beams: Same mix design, which was designed for conventional deep beams, is used for HFRC deep Imperial Journal of Interdisciplinary Research (IJIR) Page 239

3 beams, only difference being addition of fibers in HFRC mix. C. Mixing of conventional and HFRC concrete and their workability and compressive strength: Dry mixing of ingredients of concrete was done with uniform addition of fibers to avoid balling effect. Wet mixing was carried out by adding exact quantity of water and uniform mix was obtained. Slump of each mix was measured using slump cone apparatus. Standard cubes of size 150 x 150 x 150 mm were cast from each mix. After casting the specimens were kept under 90% humidity for 24 hours, then molds were removed and specimens were immersed under water in a curing basin. After 28 days of curing, cubes were tested in Compression testing machine. The various proportions of fibers taken, their workability and compressive strength of cubes are as shown in Table No.1 below. Table 1: Variation in slump of mix and compressive strength of concrete cubes with addition of fibres: & 0.3% PP fibre content which are as presented in Type of % Steel % Slump % drop Permissible Compressi % increase in Concrete fibre PPfibre of slump slump for ve strength comp. ( M20 Grade) medium of cubes Strength workabilty (MPa) compared to mm conventional (as per IS 456- concrete 2000) Conventional Hybrid Fibre reinforced Table 1 as well as in Fig.1. Workability: In comparison with conventional concrete, there is a drop in slump of 3.13% for minimum fibre content i.e. combined 0.5% steel and 0.1% PP fibre content. For the same steel fibre content of 0.5%, when PP fibre content was increased to 0.2 % & 0.3 %, slump drop was observed to the of 2.08 % in comparison with conventional concrete. This is due to good cohesion & mixability of PP fibre with concrete 5. A mix with steel fiber content of 0.9% and PP fiber content of 0.3%, shows slump drop of 19.8%. in comparison with conventional concrete. Fig. 1 shows Variation in slump value of M20 concrete mix with different fiber percentages. It makes clear that increase in the percentage of steel fiber, causes decrease in workability of the mix. Also it can be observed that increase in percentage of PP fiber improves workability of mix slightly. Same trend is observed in case of 0.7, & 1.3% steel fibre content combined with 0.1, 0.2 Imperial Journal of Interdisciplinary Research (IJIR) Page 240

4 slump % PP 0.2% PP 0.3% PP conventional 0 O.O % of steel fiber Fig.1.Variation in slump value of M20 concrete mixwith different fiber percentages. It is observed that workability of mix with steel fiber percentage beyond 0.9%, the slump value recorded do not satisfy the minimum requirement of 75 mm as per I.S Hence, mixes with steel fiber content 1.1 and 1.3% for all PP fibre contents are found to be not workable mixes. Optimum mix from workability considerations: A mix with 0.9% steel fiber and 0.3% PP fiber shows slump of 79 mm which is within acceptable range as per IS 456. Hence from workability point of view this mix seems to be an optimum mix with max HFRC percentage satisfying wortkability criterion. Compressive strength of concrete cubes: The compressive strength of HFRC increased by 3.83% with addition of minimum fibre content i.e. combined 0.5% steel and 0.1% PP fiber as compared to conventional concrete. For the same steel fibre content, when PP fibre content was increased to 0.2% & 0.3%, the increase in compressive strength observed was by 6.13% and 6.87% respectively. This indicates that the compressive strength of concrete for a given steel fibre % increases marginally with increase in PP fiber content however its workability is getting increased with increasing PP fibre content. When steel fiber content is 0.9% and PP fiber content 0.3%, compressive strength of concrete is observed to be 13.3% more in comparison with conventional concrete. Further it is observed that there is alittle increase in compressive strength of concrete for 1.1% and 1.3% steel fiber contentwith considerable loss in workability, i.e. the slump observed is less than the minimum requirement. Optimum mix from Compressive strength considerations: A mix with 0.9% steel fiber and 0.3% PP fiber shows 13.3% improvement in compressive strength as compared to conventional concrete. This seems to be optimum mix from compressive strength point of view. Design of deep beams: Deep beams are designed as per IS 456:2000. The loading considered for design is two point loads of 75kN each. Two different shear spans to depth ratios (a v /D) viz and 0.56 are taken into consideration. Design reinforcementused for conventional and HFRC deep beamis as shown in Figure 2(a) and Figure 2(b) respectively. Conventional shear reinforcement is completely eliminated (Figure2 b) for HFRC deep beams, and is replaced by combination of steel fibers (0.5% to 1.3% by volume of concrete) and PP fibres (0.1% to 0.3% by volume of concrete) as presented in Table No.1. Imperial Journal of Interdisciplinary Research (IJIR) Page 241

5 Figure 2: Reinforcement details of deep beam. F. Test Specimen, Casting and Curing Procedures: A total of ninety HFRC deep beams having length 700 mm, height 400 mm and thickness 150 mm were cast. For comparison purpose, 06 nos. of conventional deep beams of same dimensions were also cast simultaneously. After casting all specimens werekept under 90% humidity for 24 hours, then formwork was removed and specimens were cured in a curing basin for 28 days. G. Flexural Testing of beams: The experimental setup is as shown in Image 2. Beams were tested using loading frame of capacity 1000kN. Initial arrangements of supports and loading points were checked to confirm design requirements. Bearing plates were placed on roller supports over which the beam is positioned. In order to avoid failure at supports and loading points the bearing plates were provided to transfer the point load into uniform pressure on specimen. For a given thickness, the area of bearing plate required is determined by considering the permissible bearing stress in concrete. Initial reading of the dial gauge was recorded. The rate of loading was 400kg/min as per IS 516:1959. Deflection of beam was recorded using dial gauge for each load interval of 50 kn. The firstcrack load and ultimate load observed are as presented in Table 2 and Table3. Image 2: Deep beam subjected to two points loading. Imperial Journal of Interdisciplinary Research (IJIR) Page 242

6 IV. TEST RESULTS Shear strength and behavior of deep beams: The observations of first crack load, ultimate crack load and deflection for two different shear span to depth ratios are recorded as shown in Table 2 and Table 3. Table 2:Details of reinforcement type, Fibre percentage, First crack load, Ultimate load and deflections for deep beams with shear span to depth ratio Type of shear reinforce m-ent in beams Conventi onal Fibres Fibre percentage by volume of concrete (%) Steel Polypropylene First crack load Ultimate load Deflection at first crack load Max. deflection Permissibl e deflection (as per IS ) Load at permissibl e Deflection Fig. 3 shows variation in first crack load of of deep beams with different fiber percentages for shear span to depth ratio It can be observed that there is consistent increase in first crack load with increase in percentages of fibers. Imperial Journal of Interdisciplinary Research (IJIR) Page 243

7 load at first crack O.O % of steel fiber 0.1% PP 0.2% PP 0.3% PP conventional Fig. 3: Variation in first crack load of of deep beams with different fiber percentages for shear span to depth ratio Table 3: Details of reinforcement type, Fibre percentage, First crack load, Ultimate load and deflections for deep beams with shear span to depth ratio 0.43 Type of shear reinforcemen t in beams Fibre percentage by volume of concrete (%) Steel Polypropylene First crack load Ultimate load Deflection at first crack load Max. deflection Permissible deflection (as per IS ) Load at permissible Deflection Conventional Fibres Imperial Journal of Interdisciplinary Research (IJIR) Page 244

8 Optimum mix from first crack load considerations: As the fiber percentage increases, first crack load increases for all combinations of steel & PP fibers. The increase in first crack load is marginal beyond 0.9% steel & 0.3% PP fiber content followeed by decrease in workability below acceptable limit (< 75 mm). Therefore, from first crack load point of view also this mix (0.9% steel fiber + 0.3% PP fiber) seems to be optimum one. Fig.4 shows variation in load at permissible deflection of deep beams with different fiber percentages for shear span to depth ratio It is clearely observed that there is consistent increase in load at permissible deflection with increasimg percentages of fibers. Load at permissible deflection O.O % of steel fiber 0.1% PP 0.2% PP 0.3% PP conventional Fig.4: Variation in load at permissible deflection of deep beams with different fiber percentages for shear span to depth ratio Optimum mix from load at permissible deflection considerations: As the fiber percentage increases, load at permissible deflection increases. The increase in load at permissible deflection is marginal beyond 0.9% steel and 0.3% PP fiber content which is followed by decrease in workability below acceptable limit (< 75 mm). Also, load at permissible deflection for the mix containing 0.9% steel fiber and 0.3% PP fiber seems to be heighest one. V. DISCUSSION ON TEST RESULTS: A. Conventional Deep Beams: i) First crack is initiated at a load, which 35 to 38% of ultimate load (as shown in Table No.2 and 3). It originatesin shear zone near the support and with further increase in load the crack propagated towards the loading point. Cracks get continued till failure of beam occurs. The failure of beams observed was in shear mode. B.Hybrid Fiber reinforced deep beams: i) Effect of fibre inclusion: The load at first crack of beams is observed to increase with increasing percentage of fibres. First crack initiated at a load, which is in the range of 50% to 60% of ultimate load for HFRC beams (refer Table No.2 and 3). Initially crack originates in shear zone near support and with further increase in the load, new cracks are developed and propagates towards the loading point. Fibres oriented perpendicular to the crack path, obstruct the continuity of cracks. Therefore generally crack length was observed to be shorter as compared to conventional concrete deep beams. At higher loads, more number of diagonal cracks, shorter in lengths were developed. Most of the beams failed in shear mode. Consistent improvement in first crack load and ultimate load was observed with increase in percentage of fibres. For lower fiber content (0.5% steel + 0.1% PP fiber), the average improvement in load at permissible deflection for HFRC beams is observed to be improved by 1% in comparison with conventional beams. HFRC deep beams of Shear span to depth ratio 0.43 reinforced with 0.9 % steel + 0.3% PP fiber content, showed 25% more shear strength than conventional deep beams at permissible deflection of span/325 (as per I.S. 456 for conventional beams). Deflection is more in case of HFRC beams due to improved Imperial Journal of Interdisciplinary Research (IJIR) Page 245

9 stiffness and energy absorption of beams because of addition of fibers. ii) Effect of shear span to depth ratio: In case of HFRC deep beams, the shear strength for shear span to depth ratio 0.43 is 15% more than that of shear span to depth ratio It is evident that load carrying capacity of beams will increase with decrease in shear span. Results presented in Table 1, 2 and 3, show that the mix with combination of 0.9% steel fiber and 0.3% PP fiber can be considered as optimum mix which gives good shear strength of beams without affecting workability of concrete. Concrete with inclusion of 1.1% and 1.3% steel fiber is found to have less workability with marginal increase in strength with respect to 0.9% steel and 0.3% PP fiber content. The variation in Maximum load carried by deep beams with variation in fiber percentage for shear span to depth ratio 0.56 is shown in Fig. 4. It shows consistent improvement in load carrying capacity of beams with increase in fiber content. Even though for steel fiber content of 1.3% improvement in load carrying capacity is observed, but still it is not workable concrete, therefore cannot be practiced. VI. CONCLUSION The following conclusions are drawn based on the experimental results: The optimum mixfor HFRC deep beams, based on workability, compressive strength of mix and shear strength of deep beams, isa mix having 0.9% steel and 0.3% PP fibre content. The minimum slump drop of 3.13% is observed for lower fibre content (i.e. 0.5% steel and 0.1% polypropylene) as compared to conventional concrete. A slump drop of 19.8% is observed for optimum mix. This indicates that workability of concrete mix decreases with increase in fibre content. Compressive strength of HFRC cubes increases with increase in fibre content. In comparison with conventional concrete, there is increase of 3.83% in strength for lower fibre content and that of 13.30% for optimum mix. First crack load of HFRC deep beams increases with increase in fibre content. In comparison with conventional deep beams, beams with optimum mix, showed increment of 25% in their first crack load. At permissible deflection, maximum load carried by HFRC deep beams having shear span to depth ratio 0.43, cast with optimum mix was 25% more than conventional deep beams. This shows improved stiffness of HFRC beams due to addition of fibres. VII. REFERENCES: 1. I.S. 456:2000, Indian Standard Code of Practice for Plain and Reinforced Concrete Bureau of IndianStandards, New Delhi. 2. T. M. Roberts, N. L. Ho (1982), Shear failure offiber reinforced concretedeep beams, The International Journal of Cement Composites and Lightweight Concrete, Vol-4, pp Shanmugam N.E., Swaddiwudhipong S. (1984), The ultimate load behavior of fiber reinforced concrete deep beams, Indian Concrete Journal, 58 (8) Tan K. H., Kong F. K., Weng L. W. (1998), High strength Concrete Deep and Short Beams: Shear Design Equations in North American and UK Practice, ACI Structural Journal, V. 95, No V.M. Sounthararajan, et.al. (2013), Evaluation of Composite Polypropylene Fibre Reinforced Concrete, International Journal of Engineering and Technology (IJET), Vol 5, No.2,Apr./May S. K. Madanet.al. (2007), Steel fibres as replacement of web reinforcement for RCC Deep Beams in shear, Asian journal of civil engineering, Vol.8 no.5, pages VinuR.Patel, Pandya.I.I (2010), Evaluation of Shear Strain Distribution in Polypropylene Fiber Reinforced Cement Concrete Moderate Deep Beams, International Journal of Civil and Structural Engineering, ISSN Volume 1, No 3, S.K.Kulkarni, S.S.Patil,Dr.S.A.Halkude(2013),Analysis And Design of R.C. Deep Beams Using Finite Strip Method & I.S A comparative study supported by Experimental Investigation, International Journal of Engineering Research & Imperial Journal of Interdisciplinary Research (IJIR) Page 246

10 Technology (IJERT), Vol. 2,Issue 4, April Rahul Gavade, S.K.Kulkarni (2015), Experimental study and Comparison of Test Results on Fiber Reinforced Concrete, IJERT, Volume 4, Issue 11,Nov IS 10262: 2009, Recommended Guidelines for Concrete Mix Design, Bureau of Indian Standards, New Delhi. Imperial Journal of Interdisciplinary Research (IJIR) Page 247