Empirical Relationship between the Impact Energy and Compressive Strength for Fiber Reinforced Concrete

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1 Journal of Scientific & Industrial Research Vol. 73, July 2014, pp Empirical Relationship between the Impact Energy and Compressive Strength for Fiber Reinforced Concrete G Murali, A S Santhi * and G Mohan Ganesh * School of Mechanical and Building Sciences, VIT University, Vellore, Tamil Nadu, India Received 19 February 2013; revised 27 September 2013; accepted 17 April 2014 In this study, an empirical relationship between the impact energy and compressive strength is developed using regression analysis. For this, simple, practical, and economical drop weight test was performed on fiber reinforced concrete which was based on the testing procedure recommended by ACI committee 544. Crimped steel fiber of 50 mm length and an aspect ratio of 50 was used as the reinforcing material in four different volume fractions such as 0%, 0.5%, 1.0% and 1.5% with water cement ratio of The test results indicated that increasing volume fraction of fiber increased the impact resistance of concrete specimen. It is also found that the empirical relationship obtained from the regression analysis is accurate and preferable to evaluate the impact energy by using compressive strength of fiber reinforced concrete thus eliminating the drop weight test. Keywords: Steel fibers, drop-weight test, impact energy, compressive strength Introduction Different guidelines have been suggested by various impact test methods 1-3 such as projectile impact test, drop-weight test, and explosive test and they may be used for the investigation of impact resistance of concrete. Among these test methods, the drop-weight test proposed by the ACI (American Concrete Institution) committee 544 is the simplest method for evaluating the impact strength of fiber reinforced concrete 4-5. Nataraja et al 6 investigated the impact strength of steel fiber-reinforced concrete with a compressive strength of 30 MPa and 50 MPa and an aspect ratio of 40. The results showed that the impact strength of reference and fibre reinforced concrete for the first crack and final fracture increased as the volume fraction of fibers increased. Mahmoud et al 7 examined the combined effect of silica fume and steel fiber on the impact resistance and mechanical properties of concrete with the water cement ratio of 0.36 and Hooked end steel fiber of 60mm length and aspect ratio of 80, with three volume fraction of 0.5, 1.0 and 1.5% were used. In predetermined mixtures, silica fume was used as a cement replacement material at 8% weight of cement. The results showed that when the 8% of cement replaced by silica fume in steel fiber concrete, the *Author for correspondence as_santhi@vit.ac.in ductility and the impact resistance of the resulting concrete are considerably increased. Alavi et al 8 made a comparison between the impact load results obtained from numerical simulation and that of the experimental testing data. In the present study, an attempt has been made to develop an expression, to predict the impact energy from the compressive strength of fiber reinforced concrete. Experiments Material properties Ordinary Portland cement corresponding to ASTM Type I cement with an initial and final setting time of 150 and 260 minutes respectively and a specific gravity of 3.25 was used in concrete mixtures. Crushed granite gravel with a size of 12.5 and 20 mm were used as the coarse aggregate. The natural siliceous river sand having a specific gravity of 2.64 was used as a fine aggregate. A commercial highperformance superplasticizer (polycarboxylic ether) was used as high-range water-reducing agent to produce a workable fibre reinforced concrete. The dosage of superplasticizer on mass basis varies from 0.3 to 1.0 % of the cement content. The crimped fiber with an aspect ratio 50, length of 50 mm and equivalent diameter of 1 mm were used. The density of the fiber was 7.8 g/cm 3 and tensile strength of fiber was 1000 MPa.

2 470 J SCI IND RES VOL 73 JULY 2014 Mix proportions In this study, water cement ratio of 0.42 was adopted for M30 grade concrete and crimped steel fibers of 0%, 0.5%, 1.0% and 1.5% volume fraction were used. Mixing procedure and specimens molding The mixing procedure was adopted by trial and error method as follows: in the beginning the fine aggregate and cement were mixed for one minute, and half of the mixing water and superplasticizer (SP) were added to the mix and then it was mixed for two minutes. The remaining water was added to the mix along with coarse aggregate and mixed for five minutes. Finally, fibers were added in various proportions such as 0.5%, 1.0% and 1.5% to the mixture and the mixing was done for five minutes 7-8. Each mix of freshly mixed concrete was then cast into cylinders (100 x 200 mm), cubes (100 mm) and prisms (500 x 100 x 100 mm) which were used in the splitting tensile, compressive and flexural strength tests respectively. Cylindrical (100 x 64 mm) discs which were cut from the cylindrical specimen were used for the impact test. Impact test The impact resistance of the specimens was determined in accordance with the procedure proposed by ACI committee 544.2R-89. For this purpose, from each mix, nine discs of size 100 x 64 mm were cut from 100 x 200 mm cylindrical specimens using a diamond cutter. They were placed on the base plate of impact testing machine and was then struck with repeated blows. The impact load was applied with a 44.5 N hammer dropped repeatedly from a 457 mm height onto a 63.5 mm steel ball, which was located at the center of the top surface of the disc. In each test, the number of blows (N1) required to produce the initiation of crack was recorded as the initial crack strength, and the number of blows (N2) needed to cause complete failure of the specimen was recorded as the fracture strength and this was method used by several researchers The energy absorption capacity of each specimen used in this test was calculated using Equation (1): Results and discussion The average compressive, splitting tensile and flexural strength test results are graphically illustrated in (Fig. 1). Compressive strength The increase in compressive strength at 28 days was 14%, 25% and 34% for a 0.5%, 1.0% and 1.5% volume fraction of fiber respectively. The results suggest that the higher volume fraction of fiber incorporation into the concrete increases the strength of the concrete. Splitting tensile and flexural strength test results The effect of volume fraction of fibers on tensile and flexural strength of concrete is shown in (Fig. 1). It is clear from (Fig. 1) that, in general, as the volume fraction of fiber is increased the splitting tensile strength gets increased. The splitting tensile strength increased by 11.3%, 41.2% and 62.8% when fiber volume fractions were 0.5%, 1.0% and 1.5% respectively. The test results indicate that the high tensile strength, possessed by the steel fiber improved the tensile strength of concrete specimen 13. Also (Fig.1) presents the results of flexural strength versus fiber volume fraction testing carried out on four different mixtures after 28 days. As expected, a higher flexural strength was obtained in 1.5% volume fraction of steel fiber. Introducing steel fiber into the concrete mixtures has led to significant increases in the flexural strength. For instance, the flexural strength increased by 13%, 33.6% and 50.7% when fiber volume fractions were 0.5%, 1.0% and 1.5% respectively. This may be due to the fact that the strength of transition zone in concrete increases when the steel fibers act as crack arresters 7-8. (1) The variables, n, V, m, and U are the number of impacts, impact velocity, drop mass, and absorbed energy, respectively. Fig. 1 Splitting tensile and flexural strength of concrete at the age of 28 days

3 MURALI et al.: EMPIRICAL RELATIONSHIP BETWEEN THE IMPACT ENERGY AND COMPRESSIVE STRENGTH 471 Impact test results The number of blows resulting in the initiation of the first crack (N1) and the number of blow required for final fracture (N2) of plain concrete, as well as the fiber reinforced concrete with different volume fraction of fiber content and the impact resistance performance are shown in (Table 1 & 2). The number of blows (N1 and N2) given in the table are the average values of three disc specimens. From the results, it was noted that by using crimped steel fiber in the mixtures, there was a considerable increase in the number of blows needed for the first visible crack and fracture, when compared to plain concrete. The impact energy delivered by the hammer per blow can be calculated as follows. (2) (3) (4) where, V is the velocity of the hammer at impact, g is acceleration due to gravity, and t is the time required for the hammer to fall from a height of 457 mm. H is the height of the fall, m is mass of the hammer and W is the weight of the hammer. Substituting the relevant values in Eq. (2) yields: Table 1 Test results and predicted value of impact energy Mix name F0 FC0.5 FC1.0 FC1.5 N1 N2 28 Days, (kn mm) Diff 28 Days, (kn mm) Diff N2-N Table 2 Test results and predicted value of impact energy Mix name F0 FC0.5 FC1.0 FC1.5 N1 N2 56 Days, (kn mm) Diff 56 Days, (kn mm) Diff N2-N

4 472 J SCI IND RES VOL 73 JULY 2014 Fig. 2 Number of blows at First visible crack and fracture The impact energy per blow, U, of the hammer can be obtained by substituting the values in Eq. (1) The number of blows needed to cause the initial visible crack and the final fracture of plain concrete, as well as fiber reinforced concrete are shown in (Fig. 2). It is obvious that, by the incorporation of steel fibers into the mixture, a definitive increase in number of blows was observed for the appearance of first crack and final fracture At 28 days, the mix that contains 0.5% steel fiber showed an increase in N1 and N2 by 2.4 and 2.3 times respectively. By increasing the fiber volume fractions to 1%, N1 and N2 increase by 3.8 and 3.5 times respectively. Further increasing the fiber volume fraction to 1.5% showed an increase of 4.8 and 4.4 times in N1 and N2 as compared to the plain concrete. Similarly in 56 days test, the mix containing 0.5% steel fiber increases the N1 and N2 by 2.4 and 2.3 times respectively. Increment of fiber volume fractions to 1% increases the N1 and N2 by 3.3 times. For 1.5% of steel fiber content the N1 and N2 increases by 4 and 3.9 times respectively. The best performance under impact loading has been given by concrete containing 1.5 % volume fraction of fiber followed by 1% and 0.5% respectively 14. These results reveal that the use of steel fiber can exclusively increase impact resistance or ductility performance of the concrete Fig. 3 Relationship between impact energy and compressive strength Comparison between impact energy versus compressive strength The empirical relationship between the impact energy and compressive strength of fiber reinforced concrete at first visible crack and fracture stage has been evaluated by regression analysis as shown in (Fig. 3), as the volume fraction of fiber increased; both compressive strength and impact energy were increased. It can be seen in (Table 1 & 2) that the predicted value of impact energy is obtained from the regression equation. These predicted values are in good agreement with the experimental observations. Moreover the maximum difference between the experimental and predicted value was 12 % and a higher accuracy is being achieved. In the view of convenience and generality, the empirical relationship is accurate and preferable to evaluate the impact energy by using compressive strength of fiber reinforced concrete without carrying out the drop weight test. Fracture pattern It is observed that, the incorporation of fiber into concrete changed the failure pattern from single large crack to a group of narrow cracks, which displays the beneficial effects of fiber reinforced concrete when subjected to impact loading and this is consistent with previous studies Conclusion Based on the experimental work, the relationship between impact energy and compressive strength is derived. Incorporation of steel fibers in concrete enhanced its mechanical properties such as compressive,

5 MURALI et al.: EMPIRICAL RELATIONSHIP BETWEEN THE IMPACT ENERGY AND COMPRESSIVE STRENGTH 473 tensile and flexural strength. By adding 1.5% steel fiber increased compressive strength by 34%. And also the tensile and flexural strength were increased by 62.8% and 50.7% respectively. Further the impact resistance also increased against the first visible crack and final fracture; which meant the energy absorption capacity in concrete with fibers increased. By introducing steel fiber into the concrete, the failure mode was changed from a brittle to ductile behavior; since the failure crack pattern was turned from a single large crack to a group of narrow cracks, which displays the beneficial effects of fiber reinforced concrete. The empirical relationship between the impact energy and compressive strength obtained from regression analysis showed that the predicted values were in good agreement with the experimental data. The empirical relationship would help to develop preliminary designs of structure where impact resistance is especially important and further reduce the number of experiments. References 1 Song P, Hwang S & Sheu B, Statistical evaluation for impact resistance of steel-fibre reinforced concrete, Mag Concr Res, 56(8) (2004) Song P, Wu J & Hwang S, Assessment of statistical variations in impact resistance of high-strength steel fiberreinforced concrete, Cem Concr Res, 35(2) (2005) Song P, Wu J, Hwang S & Sheu B, Statistical analysis of impact strength and strength reliability of steel polypropylene hybrid fiber-reinforced concrete, Construct & Bldg Mater, 19 (2005) ACI Committee 544, State-of-the-art report on fiber reinforced concrete (544.1R-96), American concrete institute (1996). 5 ACI Committee 544, Measurement of properties of fiber reinforced concrete (544.2R-89), American concrete institute (1989). 6 Nataraja M C, Nagaraj T S & Basavaraja S B, Reproportioning of steel fiber reinforced concrete mixes and their impact resistance, Cem Concr Res, 35 (2005) Mahmoud Nili V & Afroughsabet, Combined effect of silica fume and steel fibers on the impact resistance and mechanical properties of concrete, Int J impact Engg, 37 (2010) Alavi Nia A, Hedayatian M & Mahmoud Nili V, An experimental and numerical study on how steel and polypropylene fibers affect the impact resistance in fiberreinforced concrete, Int J impact Engg, 46 (2012) Taner Yildirim S, Cevdet E & Ekinci, Properties of hybrid fiber reinforced concrete under repeated impact loads, Rus J Ndt Testing, 46(7) (2010) Marar K, Eren O & Celik T, Relationship between impact energy and compression toughness energy of high-strength fiber-reinforced concrete, Mater Lett, 47 (2001) Chen Xiang-yu, Ding Yi-ning & Azevedo C, Combined effect of steel fibres and steel rebars on impact resistance of high performance concrete, J Cntrl South Univ Technol, 18 (2011) Atef B, Ashraf F & Andrew K, Statistical variations in impact resistance of polypropylene fibre-reinforced concrete, Int J impact Engg, 32 (2006) Ilker Bekir Topcu & Mehmet Canbaz, Effect of different fibers on the mechanical properties of concrete containing fly ash, Construct & Bldg Mater, 21 (2007) Mohammadi Y, Carkon-Azad R & Singh S P, Impact resistance of steel fibrous concrete containing fibres of mixed aspect ratio, Construct & Bldg Mater, 23 (2009)

IMPACT RESISTANCE AND STRENGTH RELIABILITY OF FIBER REINFORCED CONCRETE USING TWO PARAMETER WEIBULL DISTRIBUTION

IMPACT RESISTANCE AND STRENGTH RELIABILITY OF FIBER REINFORCED CONCRETE USING TWO PARAMETER WEIBULL DISTRIBUTION G. Murali, A. S. Santhi and G. Mohan Ganesh School of Mechanical and Building Sciences,