Crimped Steel Fibres Research Articles

Residual Compressive Stress-Strain Relationship for Concrete Subjected to Elevated Temperatures

This study attempts to investigate the post-fire residual stress-strain behaviour of unconfined plain and fibrous concretes under axial compression. The experimental variables of the study were concrete strength levels, volume fractions of flat crimped steel fibres and polypropylene fibres, inclusion of hybrid fibres and temperature of exposure. A total of 147 cylindrical specimens (150 x 450 mm) were cast and tested under this study. The specimens were first exposed to temperatures ranging from room temperature to 800°C and then tested under uni-axial compression after cooling to obtain complete residual stress-strain response. Based on the test data obtained, a simple empirical model is proposed to describe the complete residual stress-strain relationships of plain and fibre reinforced concrete after exposure at elevated temperatures. Important observations have been made in the paper about the influence of temperature on various mechanical properties namely strength, stress-strain curves, compressive toughness and modulus of elasticity of both plain and fibrous concretes.

Effect of the fiber geometry on the pullout response of mechanically deformed steel fibers

A simple model to predict the influence of fiber geometry on the pullout of mechanically deformed steel fibers from cementitious matrix is proposed. During the pullout the mechanically deformed fiber is subjected to repetitive bending and unbending which cause an increase of the tension in the fiber. This increase of the tension depends on the amount of plastic work needed to straighten the fiber during pullout. The model input parameters are mechanical and geometrical properties of mechanically deformed fibers. Model predictions were compared to the experimental results on the hooked-end and crimped steel fiber pullout and good agreement was observed.

Residual Mechanical Properties of Fibre Reinforced Concrete after Exposure to Elevated Temperature

This study aims to find out the compressive strength, split tensile strength, flexural strength, bond strength and permeability of steel fibre reinforced concrete subjected to elevated temperatures ranging from room temperature to 800°C. The specimens were exposed to a heating rate of 10°C/min and the target temperature was maintained for 2 hours to achieve a thermal steady state. A total of 210 specimens of plain and fibre reinforced concrete were tested under the test program. Crimped steel fibres were employed in the study at three volume fractions i.e. 0%, 1% and 1.5%. The results show degradation in strength properties with an increase in maximum heating temperature in both plain and steel fibrous concretes. However, when steel fibres are incorporated in the mix, an improvement of fire resistance and crack resistance at elevated temperature was observed. The results indicate a reduced deterioration in residual compressive, split tensile, flexural and bond strengths of fibre reinforced concrete specimens as compared to controlled plain concrete specimens when the temperature was increased from room temperature to 800°C. Residual permeability characteristics of fibre reinforced concrete show better performance than plain concrete.

Stress-Strain Curves for High-Performance Fiber Reinforced Concrete under Compression

Steel fiber reinforced concrete (SFRC) is increasingly being used day by day as a structural material for various applications. The complete stress-strain curve of this material in compression is needed for the analysis and design of structural elements. An experimental investigation was carried out to generate the complete stress-strain curve of high-performance fiber reinforced concrete (HPFRC) with a strength range of 52–80 MPa. The variation in concrete strength was achieved by varying the water-to- cementitious materials ratio from 0.40 to 0.25, at 10% silica fume replacement. Crimped steel fibers with fiber volume fractions of 0.5%, 1.0% and 1.5%, and aspect ratio of 80 are considered in this study. The effects of these parameters on the shape of stress-strain curves are presented and discussed. Test results indicate that inclusion of steel fibers improves the compressive and flexural strengths, enhances the strain at peak stress, and ductility and toughness of concrete. Equations are proposed to quantify the effect of fibers on compressive strength, strain at peak stress and toughness of concrete in terms of fiber reinforcing index, and elastic modulus as a function of compressive strength. The proposed equations have been found to give good correlation with the experimental values. Keywords: crimped fibers; fiber reinforcing index; high-performance fiber reinforced concrete; compression; stress-strain curve; toughness. Journal of Civil Engineering Research and Practice Vol. 5 (1) 2008: pp. 1-14

Tie-confined fibre-reinforced high-strength concrete short columns

An experimental study was carried out to investigate the behaviour of steel fibre-reinforced high-strength concrete (HSC) short columns confined by square ties under monotonically increasing concentric compression. A total of 72 confined and 24 unconfined specimens were tested in this test programme. The test variables included aspect ratio and volume fraction of crimped steel fibres, volumetric ratio, yield strength and configuration of transverse tie reinforcement and concrete strength. The effects of these variables on the uniaxial behaviour of HSC short columns are presented and discussed. The results indicate that the addition of fibres to the HSC mix prevented the early spalling of the cover and increased the load-carrying capacity and ductility of the specimens over that of comparable non-fibre columns. The effect of mixed aspect ratio of fibres on the stress–strain behaviour of confined HSC was also studied by blending the short and long fibres. It is shown that a higher gain in column strength can be affected by the addition of shorter fibres, and longer fibres can provide better enhancements in the post-peak deformability.

Mechanical Properties of Steel Fiber Reinforced Silica Fume Concrete

This paper presents the investigations towards developing a better understanding on the contribution of steel fibers on the compressive, flexural, and splitting tensile strengths of steel fiber reinforced silica fume concrete. An extensive experimentation was carried out with w/cm ratio ranging from 0.25 to 0.40, and fiber content ranging from zero to1.5 percent with an aspect ratio of 80 and silica fume replacement at 5% and 10%. The influence of fiber content in terms of fiber reinforcing index on the mechanical properties of high performance fiber reinforced concrete (HPFRC) with compressive, flexural, and splitting tensile strengths ranging from 55 to 86 MPa, 5.5 to 11 MPa, and 4 to 8.5 MPa respectively is presented. Based on the test results, equations are proposed using statistical methods to predict 28-day strength of HPFRC in terms of fiber reinforcing index for a wide range of w/cm ratios. Relationship between flexural and splitting tensile strengths has been developed using regression analysis. To examine the validity of the proposed model, the experimental results were compared with the values predicted by the model and by the equations of previous researchers, and the absolute variation obtained was within1.50 percent. Addition of 1.5 % volume fraction of crimped steel fiber resulted in 11 % increase in the compressive strength while modulus of rupture and splitting tensile strength increased by 38 % and 56 % respectively.

Toughness enhancement in steel fiber reinforced concrete through fiber hybridization

Crimped steel fibers with large diameters are often used in concrete as reinforcement. Such large diameter fibers are inexpensive, disperse easily and do not unduly reduce the workability of concrete. However, due to their large diameters, such fibers also tend to be inefficient and the toughness of the resulting fiber reinforced concrete (FRC) tends to be low. An experimental program was carried out to investigate if the toughness of FRC with large diameter crimped fibers can be enhanced by hybridization with smaller diameter crimped fibers while maintaining workability, fiber dispersability and low cost. The results show that such hybridization indeed is a promising concept and replacing a portion of the large diameters crimped fibers with smaller diameter crimped fibers can significantly enhance toughness. The results also suggest, however, that such hybrid FRCs fail to reach the toughness levels demonstrated by the smaller diameter fibers alone.

Compressive Stress-Strain Behavior of Small Scale Steel Fibre Reinforced High Strength Concrete Cylinders

An experimental investigation was carried out to generate the complete stress-strain curves of steel fibre reinforced high strength concrete under axial compression. The experimental program consisted of testing 100 x 200 mm concrete cylinders. The experimental variables of the study were concrete strength levels (58.03 MPa and 76.80 MPa), volume fractions (0.5% to 2.0%) and aspect ratios (20 and 40) of flat crimped steel fibres. The effect of the mixed aspect ratio of fibres on the stress-strain behavior of steel fibre high strength concrete was also studied by blending short and long fibres. The effects of these variables on the stress-strain curves are presented and discussed. The results indicate that high strength concrete can be made to behave in a ductile manner by the addition of suitable fibres. It is concluded that short fibres are more effective in controlling early cracking, thereby enhancing the strength of the composite, whereas long fibres are more effective in providing post peak toughness. Concrete strength seemed to have an adverse effect on the deformability of fibre reinforced high strength concrete. Based on the test data obtained, a simple model is proposed to generate the complete stress-strain relationship for steel fibre reinforced high strength concrete. The proposed model has been found to give a good representation of the actual stress-strain response.

High-Velocity Impact Strength of Plain and Fiber- Reinforced Polymer-Modified Concrete

This study deals with establishing high-velocity impact properties of polymer –modified concrete (PMC) including Styrene-Butadiene rubber (SBR), with different weight ratios of polymer to cement: 4%, 8% and 12%. Steel fibers were also included. Sixteen (500mm) diameter, (50mm) thick discs for high-velocity impact tests were used. In addition compressive strength, splitting tensile strength, and flexural strength (modulus of rupture) were companionly recorded. In all the tests, concrete was with and without crimped steel fibers of ratio 1% by volume.In investigating high-velocity impact strength, the decrease in projectile penetration depth was (5-17%) and the scabbing area reduced (15-35%) over reference concrete.In studying PMC including 1% by volume steel fibers, an additional increase was observed in all properties. The increases were quite significant in high-velocity impact strengths. Further reduction was recorded in scabbing area of (64-95%) and penetration depth reduced (28-39%) over control specimens. The fragmentations were reduced also. The range of corresponding compressive was (48-64)MPa ,of splitting tensile strength (4.2-7.8) MPa, and of flexural strength (5-8) MP

Fracture toughness testing by means of the SECRBB test specimen

The single edge crack round bar in bending (SECRBB) test specimen is a practical test specimen for determining the fracture toughness of concrete. The specimens used in this study were prepared from standard 100 mm and 150 mm diameter cylinders with notch depth ratios within the range 0.2 <aD < 0.5. These specimens were more compact than the SECRBB specimens used by earlier researchers (with other materials) with support span/diameter ratios of only 1.5 and 1.67 for the smaller and larger diameter cylinders respectively. Thus an experimental expression was developed which gave K1P values for both cylinder sizes and hence fracture toughness results were determined.The SECRBB test geometry was used to evaluate the fracture toughness of a number of mixes. The effect of maximum coarse aggregate size was studied with inconclusive results — the fracture toughness was reduced for maximum aggregate sizes of 5 mm and 20 mm compared to the results obtained using 10 mm maximum aggregate size. The effect of adding crimped steel fibres in the mix was also investigated. The fracture toughness increased with increasing fibre content but the increase was only half that obtained from other test geometries. This variation was probably due to the method of manufacture of the specimens and it appears that the fracture toughness of steel fibre reinforced concrete may be sensitive to the method of specimen preparation.

Influence of steel fibre reinforcement on the fracture behaviour of concrete in compression

Experimental results are presented to describe the compressive load-deformation characteristics of steel fibre reinforced concrete. The influence of steel fibre reinforcement on the fracture process of concrete is discussed, based on data for various volumes of melt extract, hooked, straight and crimped steel fibres. The results indicate that the primary effect of steel fibre reinforcement on the fracture behaviour of concrete up to ultimate stress is very similar to the characteristics imparted by coarse aggregate inclusions. The initiation and critical stress and strain values decrease with increasing fibre content and the volume of the crack phase in the matrix increases. Steel fibre reinforcement improves the energy absorption capacity of concrete due to the higher lateral strain capacity imparted by steel fibres and due to energy dissipation during pull-out of fibres which bridge tension and shear cracks in the failing matrix.