Please use this identifier to cite or link to this item: http://localhost:8081/xmlui/handle/123456789/1226
Authors: P. J., Sasturkar
Issue Date: 1990
Abstract: Fibre reinforced concrete is an ordinary concrete containing discontinuous, discrete fibres of short length and small diameter. Among the various types of fibres currently available, steel fibres are most widely used. It has been recognised that the addition of small amount of closely spaced and uniformly dispersed discrete steel fibres to concrete substantially improves its static properties like tensile, shear, flexural strength and ductility. In conventional reinforced concrete, steel acts as a substitute material for the concrete under tensile stresses while in fibrous concrete, the fibres act as crack arrestors and restrict the growth of crack in the matrix under stress. Conventional reinforced concrete is not a two phase material in the true sense of the word. In other words, the existence of one phase (i.e. steel or concrete) does not improve the basic strength properties of the other phase and consequently the overall performance of composite material is dictated by the individual performance of the concrete phase and the steel phase. This has led to search for new materials in which the weak matrix is reinforced with strong stiff fibres to produce a composite of superior properties. Steel fibre reinforced concrete is one of the materials which overcomes a number of deficiencies of the plain concrete. Considerable research has been carried out to evaluate the mechanical properties of steel fibre reinforced cement composites. Compared to the extensive experimental evidence available on the behaviour of concrete sub jected to bending, compression and transverse shear, the attention given to torsional strength is slight. It is well established that the inclusion of fibres in a concrete mix increases its tensile strength and improves the ductility. The failure of concrete in torsion depends upon its tensile strength. An addition (v; CHAPTER 3 - CHARACTERISTICS OF BEAMS UNDER PURE BENDING Page 3.1 Theoretical Analysis 84 3.1.1 General 84 3.1.2 Method of Analysis 84 Assumptions 84 (A) Reinforced Concrete 85 (B) Reinforced Fibre Concrete 85 Formulation of the Method 85 Steps in Analysis 88 Maximum Crack Width 89 3.2 Presentation of Test Results 89 3.3 Analysis and Discussion of Results 90 3.3.1 Reaction at Support 90 3.3.2 Strains in Tension Steel 90 3.3.3 Strains in Concrete 91 3.3.4 Cracking and Ultimate Load 92 3.3.5 Initial Cracking, Yield and Ultimate Moment °4 3.3.6 Curvature 95 3.3.7 Moment-Curvature Relationship 96 3.3.8 Load-Deflection Curves 96 3.3.9 Cracking Characteristics 98 3.3.10 Mode of Failure 101 3.4 Concluding Remarks 1°1 CHAPTER 4 - CHARACTERISTICS OF BEAMS UNDER TORSIONAL EFFECT 4.1 General 132 4.2 Method of Analysis '33 (vi) Page 4.2.1 Beams Subjected to Pure Torsion 134 (A) Method 1 134 (B) Method 2 136 4.2.2 Beams Subjected to Torsion, Bending and Shear 137 4.2.3 Angle of Twist at Ultimate Torque 139 4.3 Presentation of Test Results 140 4.4 Analysis and Discussion of Results 140 4.4.1 Reaction at Support 141 4.4.2 Strains in Steel Reinforcement 141 4.4.3 Strains in Concrete 141 4.4.4 Loads and Torques at First Cracking and Ultimate Failure 142 4.4.5 Load-Deflection Curves 144 4.4.6 Angle of Twist-Torque Curves 1*6 4.4.7 Cracking Characteristics 148 4.4.8 Mode of Failure 150 4.5 Concluding Remarks 152 CHAPTER 5 - CONCLUSIONS 5.1 General 187 5.2 Economics of Fibre Reinforced Concrete 187 5.3 Conclusions ' 189 5.4 Scope for Further Research 193 APPENDIX REFERENCES 200 214 (viii) of short discrete fibres is known to increase the tensile strength and the torsional resistance of concrete. The improvements in torsional and flexural strengths of fibrous concrete has been studied recently. Testing of fibrous beams under different combinations of torsion, bending and shear have been reported in these studies. However, the combined influence of conventional reinforcement and fibre incorporation has been investigated in a very few of the recently published studies. Moreover, majority of the results reported so far are those obtained from small size specimens. Fibre dispersion and orientation which play an important role on the behaviour of fibre reinforced concrete beams can be adversely affected by the size of the member. This was avoided by testing relatively large size specimens with the cardinal aim of exploring a systematic study to examine the potential use of steel fibres under torsional effects. The following test programme was planned to investigate experimentally the various properties of fibrous concrete. (a) To obtain information about the basic properties of the constituents of reinforced cement concrete and fibre reinforced concrete, viz., cement, sand, coarse aggregate, steel fibres and skeletal steel as per relevant Indian Standard Specifications. (b) To obtain information on most of the basic parameters required in structural design, viz., compressive and tensile strength of concrete, modulus of elasticity and modulus of rupture of concrete, modulus of elasticity of steel and the stress-strain relationship. (c) To investigate the behaviour of simply supported reinforced steel fibre concrete beams subjected to pure bending, pure torsion, combined torsion and bending, and combined torsion, bending and (ix) shear to study the efficiency of fibres on : i) First cracking load ii) Ultimate load iii) Moment-Curvature relationship iv) Angle of twist-Torque relationship v) Deflection characteristics vi) Crack width, crack spacing and cracking pattern. A total of 48 beams of 125 mm width, 300 mm depth and 2500 mm overall length were tested to failure under four different loading combinations. Phase-1 consisted of 24 beams tested under pure bending. Phase-1 was divided into three groups of 8 beams each. These beams were designed as balanced, under- reinforced and grossly under- reinforced and were categorised as Group-1, Group-2 and Group-3 and were designated as BJB, BU and BGU. The remaining 24 beams were categorised under Phase-2, Phase-3 and Phase-4 of 8 beams each. These beams were tested under pure torsion, combined torsion and bending, and combined torsion, bending and shear. They were designated as T, TB and TBS, respectively. Each group under pure bending consisted of 8 conventionally reinforced concrete beams containing 0, 0.5, 1.0 and 1.5% volume fraction of steel fibres. For each volume fraction, two similar beams were cast. The conventionally reinforced concrete beams under Phases 2, 3 and 4 also consisted of same variation in volume fraction of steel fibres. The mix proportion, type of steel fibre and conventional steel reinfor cement were held constant. The plain-straight round steel fibres of diameter 0.4 6 mm and length 36.8 mm had an aspect ratio of SO. The concrete mix selected had an adequate workability for both plain and fibrous concrete. (x) The mix proportion used was 1:1.84:2.55 (by weight) with a water-cement ratio of 0.4 8. Control specimens of 150 mm cubes, 150 mm diameter cylinders and 100 x 100 x 500 mm prisms were also cast along with the beams to determine the compressive, split tensile and flexural strength of concrete. The cast specimens were stripped after 24 hours and immersed in fresh water for curing for 27 days. All the specimens were kept in an environment room till the time of their testing. The pure bending tests were carried out in an usual manner by using third point loading. The torsion tests were carried out in a specially fabricated test-rig. The effective span of all the test specimens between the two supports was kept at 2000 mm with a bearing of 250 mm on each side. The left hand end of the beam was kept on a roller support along with a slider bearing assembly to ensure free rotation of the specimen at the free end. The right hand side end of the beam rested on a load-cell and was restrained in the lateral direction. A lever arm of 1 m between a parallel force system was used, resulting in pure torsion. The bending effect was achieved by third point loading. Beams under combination of loads were subjected to flexural load first. The torsional loads were brought into play after applying flexural load. Successive load increments were applied by keeping the ratio of torsional moment to flexural moment equal to 0.5, till the beam failed. A Huggenberger deformeter of gauge length 205 mm was used to measure the concrete strains while strains in the reinforcing steel were recorded with electrical resistance strain gauges. The angle of twist over the test length was measured using displacement gauges. At each stage of loading, the readings of all the instruments were recorded. The crack width and crack spacing was noted. The crack propagation was marked. Mode of failure was also noted and the control specimens were tested on the same day. (xi) The observed ultimate strength results have been compared with test data available in literature. The experimental values obtained for ultimate strength under flexural and torsional effects have been verified by existing theoretical models. A statistical analysis of these results has been presented. The experimental results of the present study have been compared with the theoretical expressions suitably developed wherever found necessary. Several interesting results emerged out from the present research. It is seen that the beams tested in pure bending gave an increase of 5 to 63% in the cracking load whereas an increase of 6 to 59°S was observed at ultimate load as the volume fraction of steel fibres increased. The group tested in pure torsion gave an increase of 33% and 44% respectively for the cracking and ultimate torque with an increase in the percentage of fibres. While the beams tested under combined torsion and bending gave an increase of 54% in the cracking torque whereas only 21% increase was noticed at the ultimate stage. A significant increase of 82% and 65% was observed for cracking and ultimate torque respectively, when the beams were subjected to the effect of combined torsion, bending and shear. The ultimate capacity of conventionally reinforced concrete beams improved with an increase in the percentage of fibres. But a drop in the capacity was observed in most of the beam cases at the maximum volume fraction of 1.5% used in the investigation. In most of the beams the strain in longitudinal steel exceeded the yield value. The concrete strains were higher for beams under pure bending than beams under torsion. The influence of steel fibre was less pronounced in beams subjected to torsional effects because of the much lower failure loads. Nevertheless, improved strain values were detected due to the inclusion of steel fibres. (xii) The torque-twist relationship and torsional moment capacities were significantly improved by the amount of fibre content due to enhanced tensile strength of the matrix due to addition of fibres. Beams with fibres exhibited a decreased initial deflection. They showed higher cracking and failure loads for higher fibre content. The effect of type of loading on load-deflection behaviour for the same amount of fibre content shows that the beams under pure flexural loads exhibit greater deflec tion than beams subjected to torsional loads. •The average crack-spacing reduced by 6 to 53% in the case of pure bending tests, while a reduction of 13 to 62% was observed when the beams were subjected to torsional moment. As regards the crack-width, in all the 48 beams tested, it was generally observed that the fibrous beams showed reduced crack-widths of 30 to 73% as the percentage of fibres increased. In no case the crack-width exceeded 0.07 mm at the design load. The beams tested under pure bending failed in flexural tension while the beams under torsion failed by bending about a skewed axis due to diagonal tension. Diagonal cracks were observed in beams under pure torsion which spiralled around the three faces of the beam and were joined by a compression zone on the fourth face when failure occurred. The diagonal cracks were accompanied with flexural tension cracks in beams under combined loading. The tension cracks on the opposite faces were inclined at an angle of about 45 . The type of failure of beams were classified as Mode 1 or Mode 2 according to the familiar skew-bending theory. The presence of shear was found to effect the failure mode and Mode 2 changed to Mode 1. This study of conventionally reinforced rectangular concrete beams containing steel fibres leads to the following conclusions : (xiii) 1. The ultimate strength of fibrous beams increased with an increase in the volume fraction of fibres. The maximum increase in strength was obtained at about 1% fibre addition and the strength decreased at 1.5% volume fraction in most of the cases. The test results for the ultimate strength indicate that the effect of fibres is pronounced in grossly under-reinforced beams rather than balanced or just balanced beams. The effect was most significant in beams subjected to combined torsion, bending and shear. 2. The beams tested under pure bending failed in flexural tension. The beams tested under torsional load failed by bending about a skewed axis, by formation of a compression zone either at the top face (Mode 1) or at the side face (Mode 2). 3. The statistical analysis shows a fair agreement between the predicted and experimental values. The theoretical models predict values very close to the experimental results. 4. The statistical analysis confirms the reliability of the test results obtained in the present investigation when compared to the data reported by other investigators. 5. The bilinear moment-curvature relationship assumed for conventionally reinforced concrete beams in flexure can be adopted for reinforced fibrous concrete beams also. 6. There is a pronounced improvement in the load-deflection and torquerotation characteristics of beams containing fibres. 7. The theoretical approach suggested to obtain the theoretical value of the angle of twist at ultimate torque is found quiet satisfactory when compared to experimental results. (xiv) 8. The experimental strains in concrete at first crack for conventionallv reinforced fibrous concrete beams are higher than in reinforced concrete beams without fibres. 9. The degree of cracking with increase in load in the compression zone is much less in fibrous beams as compared to nonfibrous beams. 10. The crack-widths and crack-spacings in fibrous beams were reduced to a considerable extent. In no case the crack width at the design load exceeded 0.07 mm. 11. The maximum crack width formula proposed for fibre reinforced concrete beams gives satisfactory results. 12. The cost analysis indicates that fibre reinforced concrete is economical when the significant increase in the serviceability of the beams is considered. HHHH
Other Identifiers: Ph.D
Research Supervisor/ Guide: Kaushik, S. K.
metadata.dc.type: Doctoral Thesis
Appears in Collections:DOCTORAL THESES (Civil Engg)

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