INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 1, No 4, 2011

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1 Experimental study and prediction of tensile strength for steel fiber reinforced concrete Shende.A.M. 1, Pande.A.M 2 1 Assistant Professor and Head. J.L.Chaturvedi College of Engineering, Nagpur 2 Dean and Professor, Y.C.C.E. Nagpur shende_renu@yahoo.com ABSTRACT Results of an investigation conducted to study the tensile strength of steel fibre reinforced concrete (SFRC) containing fibres of 0%, 1%, 2% and 3% volume fraction of Hook tain steel fibres of 50, 60 and 67 aspect ratio are presented. Cylinder specimens of size 150 mm diameter and 300 mm length were tested under compression testing machine as per I.S A result data obtained has been analyzed and compared with control beam (0% fibres). A relationship aspect ratio vs. Tensile strength represented graphically and governing equation of graphs and prepared Mathematical model for Tensile strength can be used to predict Tensile strength of SFRC by using appropriate values of percentage of fibers (V f ) and aspect ratio (A) Keywords: Fibre reinforced concrete, Tensile strength, Predicted Tensile strength 1. Introduction Concrete is one of the most widely used construction material.since the day of its advent, concrete has been undergoing changes as a material and technology.due to the growing needs of performance and durability of concrete there has been a continuous search for upgrading the properties of concrete. High performance concrete, fibre reinforced concrete, self compacting concrete are a few examples of the outcome of the same. It is well known that plain concrete is weak in tension and brittle. A growing tensile crack in plain concrete can very soon lead to failure. In the presence of reinforcement, tensile load is transferred to steel reinforcement. An alternative to increasing the load carrying capacity of concrete in tension is the addition of fibres well dispersed in concrete to bridge the micro cracks that develop in concrete. There are numerous fibre types available for commercial experimental use. The basic fibre categories are steel, synthetic and natural fibre material. Hence this study explores the feasibility of steel fiber reinforcement, aim is to do parametric study on, split tensile strength and prepared Mathematical model for Tensile strength can be used to predict Tensile strength of SFRC by putting appropriate values of percentage of fibers (Vf) and aspect ratio (A) 1.1 Historical background Historically fibres have been used to reinforce brittle materials since ancient times. Straws were used to reinforce sunbaked bricks; horsehair was used to reinforce plaster. More recently asbestos fibres were used to reinforce Portland cement. Eventhough reinforcing a brittle matrix with discrete fibres is an age old concept, modern day use of fibres in concrete started in the early 1960s. In the beginning, only straight steel fibres were used. The major improvement occurred in the areas of ductility and fracture toughness, even though flexural 910

2 strength increases were also reported. The law of mixture was applied to analyze the fibre contributions. It was understood that fibre reinforced concrete can be designed to obtain a specific ductility or energy absorption. Research by Romualdi, Batson, and Mandel in the late 1950 s and early 1960 s represented the first significant steps towards development of steel fibre reinforced concrete (SFRC). Although patents have been granted since the turn of the century for various methods of reinforcing concrete with steel, development of SFRC technology did not progress much until the late 1950 s. Since that time, steel fibres have been optimized to some extent for incorporation into concrete. Also, mixing, placing, consolidating, and finishing techniques employed in SFRC have been improved. The advent of deformed fibres and high range water reducing admixtures provided a big boost to the fibre reinforced concrete use in the field. Ramakrishnan and his colleagues established that fibres with hooked ends can be used at much lower volume fractions than straight steel fibres, producing the same results in the area of ductility and toughness. These fibres were also glued together at the edges with water soluble glue. When added to the concrete, the fibres had a much lower (apparent) aspect ratio. 2. Experimental Programme 2.1 Material used The materials used for this experimental work are cement, sand, water, steel fibres, and superplasticizer. Cement: Ordinary Portland cement of 53 grade was used in this experimentation conforming to I.S Sand: Locally available sand zone II with specific gravity 2.65, water absorption 2% and fineness modulus 2.92, conforming to I.S Water: Potable water was used for the experimentation. Superplasticizer: To impart additional workability a superplasticizer (Rheobuild 1100) 0.6 % to 0.8% by weight of cement was used. It is based on sulphonated naphthalene polymers with following properties as per I.S Fibers: Fibre is a small piece reinforced material possessing certain characteristics properties. They can be circular or flat. It is often describe by a convenient parameter called aspect ratio.the aspect ratio of fibre is the ratio of its length to its diameter. In this experimentation Hook tain Steel fibres were used. The different aspect ratios adopted were 50, 60, and 67 with length of fibers 30 to 35 mm and diameter of fibers 0.40 to 0.70 mm. 3. Experimental Methodology For Split tensile strength test, cylinder specimens of dimension 150 mm diameter and 300 mm length were cast. The specimens were demoulded after 24 hours of casting and were transferred to curing tank where in they were allowed to cure for 28 days. These specimens were tested under compression testing machine. In each category three cylinders were tested and their average value is reported. Split Tensile strength was calculated as follows as split 911

3 tensile strength: Split Tensile strength = 2P / π DL, Where, P = failure load, D = diameter of cylinder, L = length of cylinder Table 1: Split Tensile strength for control concrete M 20 grade Tensile strength Average Tensile strength Tensile Strength ( Mpa) Tensile Strength (Mpa) Aspect Ratio 1% 2% 3% Figure 1: Variation of Tensile strength of SFRC for different aspect ratios M20 grade Table 3 : Governing equation of graphs Percentage Governing Equation Value of R² of Fibers 1% y = X² X R² = 1 2% y = X² X R² = 1 3% y = 0.070X² X R² = 1 912

4 Table 2: Split Tensile strength of SFRC with 1%, 2% and 3% fibres Different aspect ratios of fibres For SFRC with 1% fibres Tensile Average strength Tensile strength For SFRC with 2% fibres Tensile Average strength Tensile strength For SFRC with 3% fibres Tensile Average strength Tensile strength Mathematical model for Tensile strength: Tensile strength of multiplying factor (F) as follows SFRC (σ St ), using σ St = F x σ mt (1.1) Equation 1.1 can be written for 1%, 2% and 3% fibres, using multiplying factors F 1, F 2 and F 3 as σ St = 1F 1 x σ mt, σ St = 2F 2 x σ mt and σ St = 3F 3 x σ mt from which it can be written F 1 = σ St /1 x σ mt (1.2) F 2 = σ St /2 x σ mt (1.3) F 3 = σ St /3 x σ mt (1.4) Using experimentally observed Tensile strength values, factors F 1, F 2 and F 3 are tabulated as follows. Table 4 : Multiplying factor for Tensile strength M20 grade A.R. F1 (for1% fibres) F2 (for 2% fibres) F2 (for 2% fibres)

5 Using statistical approach, the best fit exponential equations are obtained for multiplying factors F 1, F 2 and F 3, keeping them as dependent variables on aspect ratios (A). F 1 = e A (1.5) _ for 1% F 2 = e A (1.6) _ for 2% F 3 = e 0.003A (1.7) _ for 3% Factors F1, F2 and F3, can be used to predict Tensile strength for SFRC with 1%, 2% and 3% fibres respectively. Now equation 1.1 can be written as σ St = σ mt x P e QA (1.8) Comparing equation 1.5, 1.6, 1.7 and 1.8 it is seen that the variables P and Q depend on fibre percentage. Using equation 1.5, 1.6, 1.7 and statistical approach, the best fit polynomial percentages of fibres (V f ) as follows. P = V f (1.9) Q = V f (2.0) Now rearranging equation 1.8 and putting V f for percentage of fibres it can be written σ St = σ mt x V f xp e QA (2.1) The above model can be used to predict Tensile strength of SFRC by putting appropriate values of percentage of fibers (V f ) and aspect ratio (A). The values of P and Q are given by equation 1.7 and 1.8 Following table gives predicted and observed Tensile strengths of SFRC along with a ratio of predicted Tensile strength to observed Tensile strength, for different aspect ratios of fibers. Table 5 : Predicted and observed Tensile Strengthof SFRC M20 grade Different aspect ratios of fibers For SFRC with 1% fibers For SFRC with 2% fibers For SFRC with 3% fibers Predicted Tensile Observed Tensile Ratio of Predicted to observed Tensile strength Predicted Tensile Observed Tensile Ratio of Predicted to observed Tensile strength Predicted Tensile Observed Tensile Ratio of Predicted to observed Tensile strength

6 2.4 Tensile Strength Ten sile Strength (M pa) Predicted Observed Aspect Ratio Figure 2 : Observed and Predicted Tensile Strength of SFRC with 1% fibers Average of ratios from above table is Avedev (Average of absolute deviation from mean) of ratios is To get lower bound prediction of tensile strength multiplying factor (Average Avedev) i.e say 0.95 is adopted. Hence model given by equation 2.1 is modified as where σ SC = Tensile strength of SFRC σ mc = Tensile strength of plain concrete V f = percentage of fibers A = aspect ratio of fiber σ St =0.95σ mt x V f x P e QA (2.2) 3. Conclusions The following conclusions could be drawn from the present investigation. 1. It is observed that Tensile strength are on higher side for 3% fibres as compared to that produced from 0%, 1% and 2% fibres. 2. The Tensile strength properties are observed to be on higher side for aspect ratio of 50 as compared to those for aspect ratio 60 and It is observed that Tensile strength increases from 9 to 29% through utilization of steel fibres. And through utilization of 1% steel fibres Tensile strength increases from 9 to 15%. Through utilization of 2% steel fibres Tensile strength increases from 14 to 19 %. Through utilization of 3% steel fibres Tensile strength increases from 16 to 29 %. 4. It is observed that observed Tensile strength and predicted Tensile strength is nearly equal. 5. Mathematical model develop for prediction of Tensile strength gives fairly accurate results. 915

7 6. When tested for split tensile strength, the control concrete specimen broken into two pieces while the SFRC specimen retain the geometry integrity, indicating improved ductility of SFRC due to the addition of fibers over control concrete. 4. References 1. Dr. Yuwaraj Marotrao Ghugal, (2003), Effects of steel Fibers on Various Strengths of Concrete, ICI journal (Indian Concrete Institute), 4(3), pp N. Ganesan and T. Sekar., (2006), Effect of micro silica and steel fibers on the strength of high performance. Journal of structural engineering, India (SERC), 33(3), pp ACI Committee , State of the Art Report on Fiber Reinforced Concrete. ACI Structural Journal, Vol 92, pp Bhikshma V, Ravande Kishor and Nitturkar Kalidas, (2005), Mechanical properties of fibre reinforced high strength concrete, Recent advances in concrete and construction tech, 6(1), pp Balaguru P and Najm H, (2004), High performance fibre reinforced concrete mixture proportion with high fibre volume fractions, Material Journal, 101(4), pp PitiSukontasukkul, (2003), Tensile Behaviour of High Content Steel and Polypropylene Fiber Reinforced Mortar, Thammasat International Journal Science on Tech, 8(3), pp Tensing D.Jeminah and Jaygopal L S., (2003), Permeability studies on steel fibre reinforced concrete and influence of fly ash, National seminar on advance in construction materials, pp Damgir R.M.and Ishaque M.I.M., (2003), Effect of silica fume and steel fibre composite on strength properties of high performance concrete, Proceeding of the INCONTEST 2003, Coimbatore, pp Raghuprasad.P.S, Ravindranatha, (2003), Experimental investigation on flexural strength of slurry infiltrated fibre concrete, Proceedimg of the INCONTEST 2003, Coimbatore, pp Permalatha J and Govindraj V, (2003), Experimental studies on fibre reinforced concrete, Proceeding of the INCONTEST 2003, Coimbatore, pp Bayasi,Z and Zeng,J., (1993), Properties of polypropylene fibre reinforced concrete, ACI Materials Journal, 90(6), pp Soroushian P. and Bayasi, (1991), Fibre type effect on the performance of steel fibre reinforced concrete, ACI Materials Journal, 88(2), pp Ezeldin A.S and Lowe S.R, Mechanical properties of steel fibre reinforced rapid set materials, ACI Materials Journal, Vol. 88, No 4, pp Ramakrishnan V, Wu G.Y. and Hosalli G, (1989), Flexural behaviour and toughness of fibre reinforced Transportation Research Record, No.1226, pp

8 15. Gopalaratnam, V. S.; Shah, S P.; Batson, G.; Criswell, M.; Ramakrishnan, V.; and Wecharatana, M., (1991), Fracture Toughness of Fiber Reinforced Concrete, ACI Materials Journal, 88(4), pp Zongcai Deng., and Jianhavi li., (2006), Mechanical behaviour of concrete combined with steel and synthetic macro fibers, International Journal of physical science, 2, pp BIS 383:1970 Specification for coarse and fine aggregates from natural sources for concrete (second revision). 18. BIS 10262:2007 Recommended guidelines for concrete mix design. 19. BIS 1727:1967 (Reaffirmed 2004) Methods of test for Pozzolanic materials (first revision) 20. BIS 4031:1988 (Reaffirmed 2000) Methods of physical tests for hydraulic cement 917