Investigations on Tensile Properties of High Strength Steel Fibre Reinforced Concrete

Transcription

1 Indian Journal of Science and Technology, Vol 8(28), DOI: /ijst/2015/v8i28/84092, October 2015 ISSN (Print) : ISSN (Online) : Investigations on Tensile Properties of High Strength Steel Fibre Reinforced Concrete * Bobby A. Mathew, C. Sudha, P. R. Kannan Rajkumar and P. T. Ravichandran Department of Civil Engineering, SRM University, Kattankulathur , Tamil Nadu, India; sudha.cvl@gmail.com, kannan.struct@gmail.com, ptrsrm@gmail.com Abstract This paper focuses on the analytical and experimental investigations carried out on the tensile properties of High Strength Steel Fibre Reinforced Concrete (HS-SFRC). The steel fibres used here are circular double-end hooked steel fibres and the grade of concrete used is M60. The High Strength concrete is prepared using Silica Fume and Fly Ash as admixtures, along with a Super Plasticizer. The direct tensile tests have been carried out on specimens with different fibre combinations, viz., 0.5% and 1% as well as controlled concrete specimens. The analytical investigations were carried out using the Finite Element Analysis software ABAQUS (Version 6.10). The steel fibres were modelled using the geometry module before distributing and orienting them vertically based on the calculated fibre composition along critical failure zones identified within the concrete specimen. The plain concrete specimen was assigned a linear elastic property prior to analysis. Another model with a homogenous material property has also been generated and analysed for tensile loading. The results obtained from analysis were correlated using the experimental results. Keywords: Fibre Volume Fraction, Heterogeneous Modelling, High Strength Steel Fibre Reinforced Concrete, Hooked Steel Fibres, Ultimate Tensile Strength 1. Introduction The concept of using fibres to improve the mechanical properties of concrete is known for many decades. It has been increasingly used in various fields through structural engineering applications. Adding fibres improves the tensile and shear strengths, compressive strengths, flexural toughness, durability and impact resistance. The mechanical properties of fibre reinforced concrete depend upon the type and the composition of the fibres added to the matrix. Fibres play an important role in reaching a definite load bearing capacity after the matrix fracture, depending on allocation, orientation and embedded length. These parameters are further influenced by concrete composition (aggregate size and shape, concrete content, w/c-ratio, admixtures), fibre type, rheological properties, casting method, consolidation and others. There is a clear dependence of the post-cracking load on the fibre content. Moreover, strength and geometry of fibres have a direct influence on the load bearing capacity of HSSFRC beams without bar reinforcement. Using high-strength fibres would result in a clearly better ductile behaviour and higher load levels in the post-cracking range, compared to normal strength ones 1. Studies conducted to find out the extent to which the mechanical properties like compressive strength, splitting tensile strength and toughness index improved with increase in fibre composition. They have determined that the splitting tensile strengths have increased by 98 % at 2 % fibre while the modulus of rupture increased by 126 % 2. It has been noted that the ductility and tenacity of SFRC are known to increase when the * Author for correspondence

2 Investigations on Tensile Properties of High Strength Steel Fibre Reinforced Concrete fibre content are increased and for the same fibre content, when the fibre s effective length is increased. It is also indicated that due to the bridging effect of fibres, the cube specimens did not crush or explode on failure but held up their integrity until the end of the test 3,4.Experiments conducted on laboratory-scale beam specimens confirmed a higher post-cracking strength when increasing fiber content and/or fiber aspect ratio. A fiber with a higher aspect ratio exhibited higher efficiency than a similar, but more compact type in a higher content. If workability restrictions are respected, economic advantages are obtained when preferring fibers with a higher aspect ratio to reach a specific strength class 5. The fibre composition has been fixed at 0.5 % and 1 % fractions keeping in mind the workability factor. A comparative study has been conducted between the experimental and analytical results to study the behaviour of high strength concrete under direct tension and how the fibres are influential in enhancing the ductility property of the concrete. The purpose of this study was to evaluate the tensile properties of HS-SFRC. The details of the mix used in this study are given in Table 1. Table 1. Details of the mix used Nomenclature Description HS SFRC High Strength Steel fibre Reinforced concrete DB 0 Control concrete specimen DB 1 SFRC specimen with 0.5% fibre volume fraction DB 2 SFRC specimen with 1% fibre volume fraction DB 1H Homogenous specimen with 0.5%fibre DB 2H Homogenous specimen with 1% fibre DB 1A Heterogeneous specimen with 0.5% fibre DB 2A Heterogeneous specimen with 1% fibre 2. Experimental Programme The materials used in producing HS-SFRC were meticulously sorted and tested for quality before commencing the actual experimental study. The details of materials used and preliminary tests were conducted to assess the quality of material are discussed here in brief. 2.1 Materials Used The materials used in producing HS-SFRC were meticulously sorted and tested for quality before commencing the actual experimental study. Cement used for the study was Ordinary Portland Cement conforming to IS: of grade Silica Fume used has a specific gravity It contains over 90% of Silicon Dioxide along with traces of Magnesium, Zinc and other alkali oxides. The Fly Ash used is of Class F having a specific gravity of River sand is used as fine aggregate which has a fineness modulus of 2.82 and a specific gravity of 2.38 and having smooth surface texture. The coarse aggregates used are of nominal size 10 mm and has a fineness modulus of In order to tackle the issue of workability when it comes to casting high strength concrete, a superplasticiser, Structuro 203 is used as admixture to reduce the negative consequences brought about by low water content. It has a specific gravity of The steel fibre used for the study is circular doubleend hooked type having an aspect ratio of 64, i.e, with an effective length of 35 mm and diameter of 0.55 mm. This is shown below in Figure 1. Figure 1. Double end hooked steel fibres. 2.2 Experimental Specimens and Setup The experimental specimens used for the study involved a cubes of 100 mm dimension, cylinders of 200 mm height and 100 mm diameter and dog - bone specimens of 210 mm gauge length, 100 mm width and 75 mm thickness. The latter was used for Direct Tensile testing. A displacement controlled computerized Universal testing machine was used for all the tests. The experimental setup and the geometrical dimensions of the dog bone-shaped specimen used for direct tensile testing are shown in Figure 2. 2 Vol 8 (28) October Indian Journal of Science and Technology

3 Bobby A. Mathew, C. Sudha, P. R. Kannan Rajkumar and P. T. Ravichandran Figure 2. Geometrical dimensions and experimental setup of the dog bone shaped specimen. 2.3 Mix Proportion High Strength Concrete mix proportions for M60 grade concrete were obtained based on the ACI 211 guidelines 7. The details of mix proportions thus obtained for various specimens are given in Table 1. Table 2. Mix proportions for various fibre fractions Description DB 0 DB - 1 DB - 2 Binder Content (kg/m3) Cement (kg/m3) Silica Fume (kg/m3) Fly Ash (kg/m3) Fine Aggregate (kg/m3) Coarse aggregate (kg/m3) Water (l/m3) Super Plasticizer (l/m3) Steel Fibre (kg/m3) Analysis using ABAQUSCae ABAQUS Caeis is one of the versatile Finite Element Analysis software that can be used to model structures both homogenous and heterogeneous, on a macro as well as a micro scale. In this study, a homogenous material model is initially generated in the geometry module using linear elastic property. Young s modulus of the specimen determined through experimental study was used as well as Poisson s ratio. Secondly, a heterogeneous model was created by merging the steel fibres with the concrete specimen. 3.1 Homogenous Model The elements that were used for modelling of the specimen in this report in ABAQUS were Linear 8 noded, reduced integration, brick element. The material properties were chosen as homogenous and isotropic for initial simple analysis using macro-modelling. The value of Young s modulus was derived from the experimental studies that were conducted while the values of Poisson s ratio were obtained from research studies 8. The values of Young s modulus and Poisson s ratio were given as GPa and 0.22 respectively, for 0.5% fibre volume fraction specimens and GPa and 0.22 for 1% fibre specimens. The model has been created using the extrusion option in ABAQUS. The specimen thus modelled for 0.5% fibre content has been given the notation DB H1 while that for 1% fibre content was given the notation DB H2. The aforementioned dimensions of the specimen were allotted. Meshing the specimen through convergence is most crucial as it yields the best results. Hence, the specimen was randomly meshed and displacement/ strain values were noted under constant tensile loading. The point where the slope of the graph transforms to a horizontal profile is marked. The number of elements corresponding to this point is noted. Displacement values are preferred to stress values during convergence as the latter is a derived quantity. As per the results obtained, the number of elements to be meshed came out to be 252. The loading and boundary conditions were given in a way that resembles the actual setup. The bottom portion was clamped with zero displacements at the slanting surfaces. The rotations were not considered for boundary conditions as they do not come into effect in uniaxial tensile loading. The loading was provided as displacements at a strain rate of 0.6 mm/min as this was the actual rate at which the experimental testing was conducted. 3.2 Heterogeneous Model A heterogeneous modeling approach has been done to accurately model and simulate the non-linear material behaviour of the HS-SFRC specimen. For this purpose, the concrete matrix was separately modelled as a linear homogenous material by providing the Young s Modulus Vol 8 (28) October Indian Journal of Science and Technology 3

4 Investigations on Tensile Properties of High Strength Steel Fibre Reinforced Concrete and Poisson s ratio values, while the steel fibre was separately created and modelled with non-linear material behaviour. The Dog-Bone specimen was split up into four main parts, viz., a Trapezoidal part representing the top and bottom portions, cuboidal portion with holes, cuboidal part without holes and steel fibre. Each part was subjected to convergence procedure and the ideal mesh size was determined.the steel fibres were embedded through the holes in the cuboidal part and a Tie interaction without slippage was assigned through the Interaction module in ABAQUS. The specimen was assigned boundary conditions and loading was applied similar to the homogenous model. The loading and boundary conditions were applied at the same rate as that of the experimental specimen as well as that of the homogenous specimen created, before analysing theresults. The stress - strain curves were generated and extracted from the Results module of the software itself. Figure 3 shows the final meshed assembly under loading. graphical illustrations to study the material behaviour which is critical in concluding the appropriateness of the chosen analytical model. 4.1 Compressive Strength Test The compressive strength tests conducted on cube specimens are tabulated here in Table 2 for both fibre s and plain concrete specimens. The compressive strength has shown a marked increase 7.6% in HS-SFRC specimens with fibre volume fraction of 1% rather than those with 0.5% or plain concrete specimen. Hence it can be scrupulously stated that the addition of fibres contribute to the compressive strength of the specimen, which is one of the most important mechanical properties that define concrete. 4.2 Split Tensile Strength Test Results The split tensile test was performed as per IS 5816 specifications 9 and the values were noted accordingly. This is an indirect testing method to evaluate the tensile strength of the specimen. The value normally turns out to be within the range of the values obtained through Direct Tensile tests. The average split tensile strength values for plain concrete and fibre reinforced concrete specimens have been tabulated in Table 3. Table 3. Compressive strength test values Specimen Compressive strength (N/mm 2 ) 7 days 14 days 28 days DB DB DB Figure 3. Meshed assembly of the model. 4. Test Results and Discussion The results for compressive strength tests, splitting tensile strength tests and direct tensile strength tests have been discussed and analysed below, followed by the analysis test results. Comparative studies were also conducted by Here, the test results clearly show that the values of split tensile strength test for fibre reinforced concrete show an increase of 33.3% and 46.5% for 0.5% and 1% fibre s respectively, over the values yielded by plain concrete specimen. This implies that the addition of fibres onto the composite matrix improves the tensile behaviour which is a critical parameter when it comes to failure of the composite. The enhancement of tensile strength can be verified in totality only after conducting direct tensile strength testing, which helps in inducing pure tensile forces on the cross section of the specimen uniaxially. This is discussed in the subsequent sub-section. 4 Vol 8 (28) October Indian Journal of Science and Technology

5 Bobby A. Mathew, C. Sudha, P. R. Kannan Rajkumar and P. T. Ravichandran Table 4. Values of split tensile strength test Specimen Split Tensile Strength (MPa) 7 days 14 days 28 days DB DB DB Direct Tensile Test The direct tensile test was carried out in a computerized displacement-controlled Universal testing Machine, with the setup, as mentioned earlier. Loading was applied at a strain rate of 0.6 mm/min. Values of induced strain were noted down at regular intervals of 2.5 kn or 0.33 MPa. The Tensile strength values obtained for plain concrete and fibre reinforced concrete specimens are given in Table 4. Table 5. Tensile strength values (Experimental) Specimen Tensile Strength (MPa) DB DB DB conditions as that of the experimental setup and the value of Maximum Principal Stress at failure was noted as the tensile strength of the specimen. The tensile stress - strain curves was also extracted from the Results module in ABAQUS. This procedure was followed for both homogenous and heterogeneous models of the specimen. The values of Tensile strength obtained for Homogenous model, symbolised here as DB H and the heterogeneous model, symbolised as DB 1A and DB 2A are tabulated below in Table 5. Table 6. Tensile strength values (Analytical) Specimen Ultimate tensile Strength (MPa) DB 1H 6.4 DB 2H 7.1 DB 1A 8.3 DB 2A 8.7 A comparison between the tensile stress - strain curves of homogenous model and the experimental specimen has been conducted and illustrated below in Figure 5, while Figure 6 shows the combined tensile stress - strain curves of the experimental specimens and the heterogeneous models. Figure 4. Tensile stress - strain values for various fibre fractions (Experimental). Evidently, the values of Tensile strength of the specimens measured through direct tensile tests also show an increase with an increase in the fibre content, thus confirming the results from the split tensile strength test. Besides studying and comparing the values of ultimate tensile strength, the material behaviour under pure uniaxial tension needs to be analysed. For fulfilling this purpose, the tensile stress - strain curves were plotted and have been represented in Figure Analytical Test Results The analysis was run using incremental loading which closely resembled the actual rate of loading and boundary Figure 5. Tensile stress - strain curves for homogeneous models and the experimental specimens. Figure 6. Tensile stress - strain curves for heterogeneous models and the experimental specimens. Vol 8 (28) October Indian Journal of Science and Technology 5

6 Investigations on Tensile Properties of High Strength Steel Fibre Reinforced Concrete It is clear from the above illustrated tensile stress - strain curves that both the homogenous and heterogeneous models used for the analytical investigation closely resemble the material behaviour of the experimental specimen. Moreover, the results obtained from the analysis of the homogenous model is found to be more agreeable with that of the actual experimental results, than the heterogeneous model, although the nature of the curves are almost similar for all, indicating a similar behaviour. The relatively larger extent of agreeability from the homogenous model can be hypothesised using the Rule of mixtures. 5. Conclusion The following are the inferences from the analysis and experimental study performed on HS-SFRC specimens with different fibre compositions, The addition of steel fibres to concrete showed an increase of 7.47% in the compressive strength values when compared to that of controlled concrete specimen. The split tensile strength test results also showed a similar trend in the values yielded. The specimens with 0.5% fibre content showed an increase of 33% while those specimens with 1% fibre content showed an increase of 46.5% when compared to controlled concrete specimen. The direct tensile strength test results imply that the specimens 0.5% fibre content have a tensile strength 12.18% higher than that of concrete specimen, while those with 1% fibre content have a tensile strength 23.64% higher than that of concrete specimen. It has been concluded that orientation of fibres within the concrete matrix in the analytical model generated in ABAQUS resembles the actual specimen in terms of the experimental data available, although the fibres were oriented vertically, penetrating only the failure zones and was not distributed randomly over the entire specimen. It can be presumed from the analytical results that the active participation percentage of fibres during failure or crack formation is very less, i.e., the entire fibre composition within the specimen does not play a role in enhancing the tensile strength. Though as impractical as it may sound, if the fibres can be strategically placed over critical failure zones, once they are appropriately identified, the required tensile strength can be achieved. Thus, construction using Steel fibre reinforced concrete can be made far more economical than it is currently. The fibre reinforced concrete exhibits far better post-cracking load resisting behaviour than plain concrete specimen. 6. References 1. Holschemacher K, Mueller T, Ribakov Y. Effect of steel fibres on mechanical properties of high-strength concrete, Materials and Design. 2010; 51: Song PS, Hwang S. Mechanical properties of high-strength steel fiber-reinforced concrete, Construction and Building Materials. 2004; 18: Olivito RS, Zuccarello FA. An experimental study on the tensile strength of steel fiber reinforced concrete, Composites: Part B.2010;41: Michels J, Christen R,Waldmann D. Experimental and numerical investigation on post cracking behavior of steel fiber reinforced concrete, Engineering Fracture Mechanics.2013; 98: Zhi-Liang W, Yong-Sheng L, ShenRF. Stress strain relationship of steel fiber-reinforced concrete under dynamic compression, Cement and Concrete Composites.2014; 77: IS 12269, Indian Standard for 53 grade OPC, Reaffirmed January 1999, Bureau of Indian Standards, New Delhi; ACI 211.4R-93, Guide for selecting proportions for highstrength concrete with Portland cement and fly ash, American Concrete Institute, Farmington Hills, Michigan, USA. 8. Perenchio WF,Klieger P.Some physical properties of highstrength concrete, Portland Cement Association; IS: 5816, Indian Standard for testing tensile strength of specimen, Reaffirmed January 2012, Bureau of Indian Standards, New Delhi; Vol 8 (28) October Indian Journal of Science and Technology