SOME LITERATURE STUDIES ON ENGINEERING PROPERTIES OF FIBRE-REINFORCED CONCRETE

Transcription

1 SOME LITERATURE STUDIES ON ENGINEERING PROPERTIES OF FIBRE-REINFORCED CONCRETE 1 Ankit sharma, 1Nikesh Kalani, 1Monil Mehta, 2Gaurav gohil 1 Student Civil Department, Sardar Patel College of Engineering, Anand, India 2 Asst.Professor Civil Department, Sardar Patel College of Engineering, Anand, India Corresponding Author Address: Ankit sharma Student Department of Civil Engineering, Sardar Patel College of Engineering, Anand, India. ABSTRACT In conventional concrete, micro-cracks develop before structure is loaded because of drying shrinkage and other causes of volume change. When the structure is loaded, the micro cracks open up and propagate because of development of such microcracks, results in inelastic deformation in concrete. Fibre reinforced concrete (FRC) is cementing concrete reinforced mixture with more or less randomly distributed small fibres. In the FRC, a numbers of small fibres are dispersed and distributed randomly in the concrete at the time of mixing, and thus improve concrete properties in all directions. The fibers help to transfer load to the internal micro cracks. FRC is cement based composite material that has been developed in recent years. It has been successfully used in construction with its excellent flexural-tensile strength, resistance to spitting, impact resistance and excellent permeability and frost resistance. It is an effective way to increase toughness, shock resistance and resistance to plastic shrinkage cracking of the mortar. These fibers have many benefits. Steel fibers can improve the structural strength to reduce in the heavy steel reinforcement requirement. Freeze thaw resistance of the concrete is improved. Durability of the concrete is improved to reduce in the crack widths. Polypropylene and Nylon fibers are used to improve the impact resistance. Many developments have been made in the fiber reinforced concrete. KEYWORDS: Fiber Reinforced Concrete; Types of fibre; Mechanical and Structural Properties. INTRODUCTION Concrete is weak in tension and has a brittle character. The concept of using fibers to improve the characteristics of construction materials is very old. Early applications include addition of straw to mud bricks, horse hair to reinforce plaster and asbestos to reinforce pottery. Use of continuous reinforcement in concrete (reinforced concrete) increases strength and ductility, but requires careful placement and labour skill. Alternatively, introduction of fibers in discrete form in plain or reinforced concrete may provide a better solution. The modern development of FRC started in the early sixties [1]. Addition of fibers Page 84

2 to concrete makes it a homogeneous and isotropic material. When concrete cracks, the randomly oriented fibers start functioning, arrest crack formation and propagation, and thus improve strength and ductility. The failure modes of FRC are either bond failure between fiber and matrix or material failure. In this paper, the state-of-the-art of fiber reinforced concrete is discussed and results of intensive tests made by the author on the properties of fiber reinforced concrete using local materials are reported. The advantage of reinforcing and prestressing technology utilizing steel reinforcement as high tensile steel wires have helped in overcoming the incapacity of concrete in tension but the durability and resistance to cracking is not improved. These properties can be improved by the use of fibres in the concrete. It has been revealed that concrete reinforced with a permissible amount of fibre acquires better performance in compression, flexure, toughness and energy absorption, in which the degree of improvement relies on the types of fibres used. Experiments have been carried out by several investigators using fibres of glass, carbon, asbestos, polypropylene etc[2].moreover fibres also helps in restricting the growth of micro-cracks at the mortaraggregate interface thus transforming an inherently brittle matrix i.e. cement concrete with its low tensile and impact resistances, into a strong composite with superior crack resistance, improved ductility and distinctive post cracking behavior prior to failure[3]. It must be well noted however that the benefits of adding fibres to concrete in construction, which is principally to improve on the residual load-bearing capacity, is influenced by the content, orientation and type of fibres in use [4]. The world has a witnessed rapid increase in the use of fibre reinforced polymer FRP materials as a substitute for conventional steel bars in some concrete structures, due to the numerous benefits: high strength, improved toughness, resistance to post-crack propagation and light weight amongst others [5]. There have been extra efforts by researchers with respect to the various fibre concrete types. Different experimental and theoretical studies have reported on varied mechanical properties of steel, synthetic, natural and glass fibre reinforced concrete, in view of structural applications [6]. Therefore, the use of FRC, derived by the combination of steel or synthetic fibres and plain-concrete, is gradually gaining ground in civil engineering and structural applications due to its beneficial mechanical properties [7]. There is considerable improvement in the post-cracking behavior of concretes containing fibers. Although in the FRC the ultimate tensile strengths do not increase appreciably, the tensile strains at rupture do. Compared to plain concrete, FRC is much tougher and more resistant to impact. Plain concrete fails suddenly once the deflection corresponding to the ultimate flexural strength is exceeded; on the other hand, FRC continue to sustain considerable loads even at deflections considerably in excess of the fracture deflection of the plain concrete. Examination of fractured specimens of FRC shows that failure takes place primarily due to fiber pull-out or de-bonding. Thus unlike plain concrete, a fiber-reinforced concrete specimen does not break immediately after initiation of the first crack. This has the effect of increasing the work of fracture, which is referred to as toughness and is represented by the area under the load deflection curve [9]. In FRC crack density is increased, but the crack size is decreased. The failure mechanism is by pullout. You never exceed the tensile strength of the fiber. Bond is much weaker. Steel fiber in terms of durability is the best. The addition of any type of fibers to plain concrete reduces the workability [10]. Concrete mixtures containing fibers Page 85

3 possess very low consistencies; however, the place ability and compatibility of concrete is much better than reflected by the low consistency.[11] Fibre used in FRC STEEL FIBER-The presence of fibers may alter the failure mode of concrete, but the fibers effect will be minor on the improvement of compressive strength values (0 to 15 percent). The strain of SFRC corresponding to peak compressive strength increases as the volume fraction of fibers increases. As aspect ratio increases, the compressive strength of SFRC also increases marginally. GLASS FIBER Glass fibers mixed thoroughly mixed in the composition and filled in the Steel mould of size 150 x 150 x 150 mm is well tighten and oiled thoroughly. They were allowed for curing in a curing tank for 28 days and they were tested in 200tonnes electro hydraulic closed loop machine. POLYMER FIBER - Compressive strength is essentially matrix dependent.in-plane ( edgewise ) compressive strength will be somewhat lower than cross-plane strength due to the layers of glass fibers affecting the continuity of the matrix. Crossplane compressive strength ( flatwise ) is not influenced by the presence of glass fibers and will be about the same as the compressive strength measured on bulk matrix materials in cube or cylinder tests NATURAL FIBERS - The cubes tests prepared with different fibers, different fibers volumetric ratios and different reductions in coarse aggregate, showed large variations in the test results as compared to the control specimens with no fibers. The variation in the results could be attributed to the relatively small size of the cube which may result in erroneous data compared with 15x30 cm standard cylinders. SYNTHETIC FIBERS - The compressive strength of concrete is one of the most important and useful properties of concrete. In most structural applications concrete is used primarily to resist compressive stress. The compression test was conducted on cube specimens cured for 7, 14 & 28 days. The test cubes were removed from the moist storage 24 hours before testing. The top and bottom bearing plates of the compression testing machine were wiped and cleaned before the placement of the specimen. GLASS FIBER - Flexural stress-strain curves are used to determine values of modulus of elasticity for design purposes. Values of flexural modulus of elasticity are normally in the 1.5 to 2.9 X 106 Psi range, and will vary in accordance with water-cement ratio, sand content, cure, density, and degree of micro cracking. There is a lack of a continuous network of micro cracks at low stress level versus well develop network of micro cracks at or near flexural strength, thus giving lower E-value than normally associated with precast concrete panels. Page 86

4 NATURAL FIBER - The elastic modulus of composites was determined using tensile tests. Tensile tests were performed according to ASTM D 638 specification. Tensile tests were carried out using an MTS testing machine with load cell capacity of 10kN at a crosshead speed of 5 mm/min. Tensile elastic moduli were determined from the slopes of the stress strain curves. STEEL FIBERS - For flexural strength test beam specimens of dimension 100x100x500 mm were cast. The specimens were demoulded after 24 hours of casting and were transferred to curing tank wherein they were allowed to cure for 28 days. These flexural strength specimens were tested fewer than two point and four point loading as per I.S From Various Literature Studies the following are the mechanical properties showing below : Toughness: For FRC, toughness is about 10 to 40 times that of plain concrete. Fatigue Strength: The addition of fibers increases fatigue strength of about 90 percent and 70 percent of the static strength at 2 x 106 cycles for non-reverse and full reversal of loading, respectively. Flexure: The flexural strength was reported [8] to be increased by 2.5 times using 4 percent fibers. Modulus of Elasticity: Modulus of elasticity of FRC increases slightly with an increase in the fibers content. It was found that for each 1 percent increase in fiber content by volume there is an increase of 3 percent in the modulus of elasticity. Impact Resistance: The impact strength for fibrous concrete is generally 5 to 10 times that of plain concrete depending on the volume of fiber use [8]. The following Literature studies carried by authors shows the engineering properties of FRC: Rajarajeshwari B Vibhuti studied the effect of addition of mono fibers and hybrid fibers on the mechanical properties of concrete for pavements. Steel fibers of 1% and polypropylene fibers 0.036% were added individually to the concrete mixture as mono fibers and then they were added together to form a hybrid fiber reinforced concrete. Mechanical properties such as compressive, split tensile and flexural strength were determined. The results show that hybrid fibers improve the compressive strength marginally as compared to mono fibers. Whereas, hybridization improves split tensile strength and flexural noticeably. She suggested that the improved mechanical properties of HFRC would result in reduction of warping stresses, short and long term cracking and reduction of slab thickness. Page 87

5 Kukreja et al conducted some experiments and reported that, based on the results of three methods such as split tensile test, direct tensile test and flexural test, split tensile strength test was recommended for fibrous concrete. Also increase in tensile strength and post cracking strength, toughness were reported. Goash et al studied tensile strength of SFRC and reported as inclusion of suitable short steel fibres increases the tensile strength of concrete even in low volume fractions. Optimum aspect ratio was found as 80 and the maximum increase in tensile strength was obtained as 33.14% at a fibre content of 0.7% by volume. Also it was reported that cylinder split tensile strength gave more uniform and consistent results than the modulus of rupture test and direct tension test. Kumar et al made a study on statistical prediction of compressive strength of steel fibre reinforced concrete and they reported that the compressive strength of SFRC increased steeply with the increase of fibre content upto 1% (by volume) and beyond which the rate of increase in strength reduced. It was also reported that the compressive strength of SFRC increases with the increase in the aspect ratio upto 60 and beyond this the rate of increase in strength reduces. It was further concluded that Fibre Reinforcing Index (FRI) significantly influences the compressive strength and the strength increased upto FRI = 90 for stright fibres and FRI = 60 for crimpled fibres. Beyond these values, the rate of increase in strength started to decrease. They also proposed some statistical emprical relationships between compressive strength and FRI. Nataraja et al conducted a study on steel fibre reinforced concrete under compression. Here the behavior of steel fibre reinforced concrete under compression for cylinder compressive strength ranged from 30 to 50 N/mm2. Round crimpled fibres with three volume fracions of 0.5 percent, 0.75 percent and 1.0 percent and for two aspect ratios of 55 and 82 are considered. The effect of fibre addition to concrete on compressive strength was studied. It was concluded that the addition of fibres increased the compressive strength and toughness. Some empirical equations were also proposed for compressive strength of concrete in terms of fibre reinforcing index. G. Jyothi Kumari, et al studied behavior of concrete beams reinforced with glass fiber reinforced polymer flats and observed that beams with silica coated GFRP flats shear reinforcement have shown failure at higher loads. Further they observed that GFRP flats as shear reinforcement exhibit fairly good ductility. The strength of the composites, flats or bars depends upon the fiber orientation and fiber to matrix ratio while higher the fiber content higher the higher the tensile strength. Avinash Gornale, et al studied the strength aspect of glass fiber reinforced concrete. The study had revealed that the increase in compressive strength, flexural strength, split tensile strength for M20, M30 and M40 grade of concrete at 3, 7 and 28 days were observed to be Page 88

6 20% to 30%, 25% to 30% and 25% to 30% respectively after the addition of glass fibers as compared to the plain concrete. Yaghoub Mohammadi and Kaushik (2003) about the effect of mixed aspect ratio of fibres on mechanical strength properties of concrete. 25 mm 50 mm long crimped type flat steel fibres were mixed in different proportions with concrete and tested for split tensile, compressive and static flexural strength. Compressive toughness and flexural toughness were obtained from the test results. It is found that 65% of long fibres and 35% of short fibres gave the optimum composite properties when compared with other mixes. An important note also was given in that literature that use of mixed aspect ratio of fibres does not have a significant effect on the static modulus of elasticity. Piti Sukontasukkul conducted an experimental investigation on toughness of steel and polypropylene fibre reinforced concrete beams under bending using two different methods such as ASTM C1018 and JSCE SF-4. The behaviour of steel fibre reinforced concrete indicated single peak response whereas polypropylene fibre reinforced concrete should double peak response. The deformations under two methods were compared. Faisal F Wafa and Samir A. Ashour. They tested 504 test specimens for different mechanical properties such as compressive strength, split tensile strength, flexural toughness and modulus of rupture. The mix was designed to achieve compressive strength of 94 N/mm2. Three volume fractions of steel fibres such as 0.5%, 1.0% and 1.5% were selected. It was concluded that no real workability problem was encountered upto the addition of 1.5% volume fraction of fibres in concrete. Steel fibres enhanced the ductility and post cracking load carrying capacity of high strength concrete. Some emprical relations were proposed in terms of volume fraction of fibres and compressive strength of conventional concrete CONCLUSION Conclusions drawn from reviewing the published literature are: Workability of the fresh mix is adversely affected by the addition of fibers and further decreases by increasing the fiber volume fraction.. Flexural and tensile strength, ductility, drying shrinkage and toughness of the material is usually benefited by the addition of fibers. Use of fibers in the cement-based material improves its durability. It has been well established by observing improvement in various tests such as freeze-thaw resistance, permeability, carbonation depth and fire resistance. REFRENCES 1. Ramualdi, J.P. and Batson, G.B., The Mechanics of Crack Arrest in Concrete, Journal ofthe Engineering Mechanics Division, ASCE, 89: (June, 1983) Page 89

7 2. R.Gowri, M.AngelineMary (2013). Effect of glass wool fibres on mechanical properties of concrete. International Journal of Engineering Trends and Technology, Volume-4 Issue-7 July Shah, Surendraand Rangan (1994), Effect of Fiber addition on concrete strength, Indian Concrete Journal. [5] Nataraja M.C., Dhang, N. and Gupta, A. P (1999), Stress strain curve for steel fiber reinforced concrete in compression, Cement and Concrete Composites, 4. Zerbin, R., Tobes, J.M., Bossio, M.E. and Giaccio, G., On the orientation of fibres in structural members fabricated with selfcompacting fibre reinforced concrete, Cement and Concrete Composites 5. Yan, J.M Effect of steel and synthetic fibres on flexural behaviour of highstrength concrete beams reinforced with FRP bars. Composites: Part B. [e-journal, Accessed through: Science Direct.] Vol. 43, pp [6] ] 6. Kazemi, S. And Lubell, A.S., 2012, Influence of Specimen Size and Fibre Content on Mechanical Properties of UltraHigh-Performance Fiber-Reinforced Concrete, ACI materials Journal, Vol. 109, No. 6, pp Burati,N., Mazzotti,C. and Savoia, M., Post-crack behaviour of steel and MacroSynthetic fibre-reinforced concretes. Construction and Building Materials, Vol. 25, pp ACE Committee 544, State-of-the-Art Report on Fiber Reinforced Concrete, ACI Concrete International, 4(5): 9-30 (May, 1982) [9] Silva, D. A. D., Betioli, A. M., Gleize, P. J. P., Roman, H. R., Gomez, L. A., & Ribeiro, J. L. D. (2005). Degradation of recycled PET fibers in Portland cement-based materials. Cement and Concrete Research, 35(9), Nia, A. Alavi, M. Hedayatian, M. Nili, and V. AfroughSabet. "An experimental and numerical study on how steel and polypropylene fibers affect the impact resistance in fiber-reinforced concrete." International Journal of Impact Engineering 46 (2012): Mohammadi, Y., Singh, S. P., & Kaushik, S. K. (2008). Properties of steel fibrous concrete containing mixed fibres in fresh and hardened state.construction and Building Materials, 22(5), Wang, H. T., & Wang, L. C. (2013). Experimental study on static and dynamic mechanical properties of steel fiber reinforced lightweight aggregate concrete. Construction and Building Materials, 38, Page 90