Structural response of partially steel-fibre-reinforced concretes

Abstract

Improvements in fatigue strength, improvements in strength under pure torsion and under combined bending and torsion with the addition of fibres to concrete, and the possibility of steel fibres taking over conventional shear reinforcement, have made steel fibre concrete find its place as a convenient structural material. These several benefits achieved by the inclusion of steel fibres into concrete and their promising structural potentials justify carrying out further investigations. The investigations herein reported are aimed at gaining a better understanding of the behaviour of steel fibre reinforced concrete and at enhancing the scope of its potential structural applications. The various aspects investigated and reported in this thesis are now briefly outlined. Chapter 1 deals with the need for a wider spectrum of construction materials, keeping in view the expectations of construction activity regarding improvements in conventional concrete. A brief review of the past work and the scope of the present investigation have been presented in Chapter 2. The review concentrates mainly on the theories that explain the strengthening mechanism of fibres in cement mortars and concretes, and on the structurally oriented properties of steel fibre reinforced concretes. Potential applications have been mentioned and the need for further study has thereby been established. As a sequel to this need, the scope of the present work can be cited as:

to explore the possibility of utilizing steel fibre reinforced concrete more efficiently in the field of its potential application, and to continue the investigation regarding the effect of steel fibres when used in conjunction with conventional reinforcement.

This scope has been brought out in the form of topics listed below: i. partially steel fibre reinforced concrete small size beams ii. conventionally reinforced fibrous concrete large size beams iii. conventionally reinforced partially fibrous concrete large size beams. In Chapter 3, analytical investigation and pertinent formulations have been dealt with at length. In the analysis of partially fibrous concrete beams, the possibility of using fibres for a certain thickness only on the tension side has been examined in detail. This mode of use is regarded as partially fibre reinforcing the concrete member. An engineering parameter, defined as the tensile strain enhancement factor, has been identified for designing partially fibrous concrete beams. In the analysis of conventionally reinforced fully fibrous and partially fibrous concrete beams, the effect of the presence of fibres on structural characteristics has been explored. The deformational characteristics include curvature, deflection, and steel and concrete strains. The analyses are based on: i. stress-strain relationships of plain concrete and fibrous concretes in direct compression and tension ii. modulus of rupture of plain concrete iii. stress-strain relationships of reinforcing steels in direct tension. In Chapter 4, complete details and findings of the experimental investigation have been presented. The experimental investigation is confined to determining: i. compressive and tensile strains and deflections in small size beams of plain concrete, fully fibrous concrete, and partially fibrous concrete ii. compressive strains in concrete, tensile strains at steel level, and deflections in large size beams of conventionally reinforced plain concrete and conventionally reinforced fully fibrous and partially fibrous concretes iii. stress-strain relationships of plain and fibrous concretes in direct tension and compression iv. stress-strain relationships of reinforcing steels v. properties required of other constituent materials. The main variable in the experimental study is the volume fraction of the fibres. Three different volume fractions were used for small size beams, and two different volume fractions for large size beams. Chapter 5 includes a comparative study of analytical and experimental findings, accompanied by detailed discussion. Partially fibrous and fully fibrous concrete beams are found to exhibit reasonably comparable moment capacities and load deflection characteristics. In conventionally reinforced beams, the increase in ultimate moments depends on both the volume fraction of fibres and the disposition of fibres. For a given fibre content, the increase in ultimate moment becomes less pronounced as the quantity of conventional steel increases. The deformational characteristics that require modification depend on whether the member is under reinforced or over reinforced. In under reinforced beams, the improvement in serviceability is nearly the same whether fibres are provided over full depth or half depth. In over reinforced beams, ductility is improved only when fibres are provided over the full depth; half depth addition does not bring favourable improvement. In general, any addition of steel fibres into concrete amounts to increasing the quantity of conventional steel in a uniformly distributed form throughout the concrete mass, contributing to modified deformational characteristics in a controlled manner. A brief summary and specific conclusions are presented in Chapter 6. It is economical to use steel fibres in concrete pavements over a partial thickness on the tension side. The required partial thickness can be designed using the tensile strain enhancement factor obtained from direct tension tests. Partially fibrous sections exhibit practically identical moment capacities and load deflection characteristics compared to fully fibrous sections. In conventionally reinforced beams, moment enhancement depends on the percentage of steel, volume fraction of fibres, and thickness over which fibres are distributed. In under reinforced beams, half depth fibrous concrete provides serviceability improvements similar to those from full depth fibrous concrete. In over reinforced beams, ductility improves only with full depth fibrous concrete. Suitable parameters can thus be evolved, based on direct tension tests, to reflect analytically the role of fibres in conventionally reinforced beams. The following papers from this investigation have been prepared: (i) Structural Response of Partially Fibrous Concrete Beams, Journal of Structural Engineering, ASCE, Vol. 110, No. 11, 1982, pp. 2798-2812. (ii) Flexural Strength Prediction of Fibrous Concretes - communicated to the Journal of the American Concrete Institute. (iii) Structural Response of Reinforced Fibrous Concrete Beams - communicated to the Journal of Structural Engineering, ASCE.