Design issues for concrete reinforced with steel fibers, including fibers recovered from used tires

The writers are investigating the use of steel fibers, recovered from used tires (RSF), as concrete reinforcement, aiming at the development of design recommendations. This paper presents part of this research and examines initially an existing design guideline, developed by RILEM for steel fiber-reinforced concrete (SFRC), in order to assess the suitability of the guideline for the flexural design of concrete reinforced with RSF (RSFRC). This examination indicates that, although the RILEM guideline is in general suitable for the flexural design of RSFRC, there are some fundamental issues related to the evaluation of the tensile stress-strain behavior of SFRC that affect the accuracy of the guideline. Thus, based on this conclusion, a new approach is outlined for the evaluation of the tensile stress - strain behavior of SFRC and models are derived for different types of RSF and industrially produced fibers. These models are applied to the flexural design of concrete reinforced with RSF (RSFRC) and results are compared with those obtained by using the RILEM tensile stress-strain models. It is concluded that the model proposed in this study is more conservative and accurate than the RILEM models. Recommendations are also made on values of tensile strain to be used as the ultimate limit state, when predicting the resistance capacity of SFRC and RSFRC. © 2006 ASCE

Modelling of SFRC using inverse finite element analysis

A method of inverse finite element analysis is used to determine the constitutive relationship of SFRC in tension, using primary experimental data. Based on beam bending test results and results from pull-out tests, an attempt is made to explain the physical processes taking place during the cracking stage. Basic models predicting the behaviour of SFRC in tension are proposed. © RILEM 2006

Reuse of tyre steel fibres as concrete reinforcement

To attain economically viable and environmentally friendly tyre recycling, it is necessary to develop new applications and products, which will use tyre by-products (especially the steel cord) as raw materials. The authors demonstrate that the steel fibres recovered from used tyres can be used to reinforce concrete elements. This application has a great potential, as it is estimated that more than 500000 t of high-quality steel fibres could be recovered annually from used tyres in the EU alone. This paper presents the work carried out as part of various ongoing projects on the use of steel fibres in concrete construction. The first part of the paper deals with waste management issues, the methods used to recover steel fibres from tyres, and existing applications of used tyres. The second part presents the mechanical behaviour of concrete elements reinforced with these steel fibres and discusses the relevant design and economic issues. It is concluded that the use of these steel fibres in concrete construction will benefit not only the construction industry, but also the producers and recyclers of used tyres

Stress-strain characteristic of SFRC using recycled fibres

This paper presents work from a comprehensive study on the development of a flexural design framework for concrete reinforced with steel fibres that are recovered from used tyres. The experimental flexural behaviour of notched concrete prisms reinforced with these fibres is initially presented. For comparison purposes, prisms reinforced with industrially produced fibres are also considered. An attempt to adopt an existing RILEM design framework to derive appropriate tensile stress-strain blocks is made, but problems are identified with key parameters of the framework. The influence of crack propagation and location of neutral axis depth on the tensile stress distribution is examined. Following an analytical study, it is concluded that the uniaxial stress-strain model, proposed by RILEM overestimates the load-carrying capacity and should be modified by utilising more advanced analytical techniques. © RILEM 2006

Steel fibres from waste tyres to concrete : testing, modelling and design.

The disposal of waste tyres and steel fibres from tyres is a serious worldwide environmental problem. This thesis examines the use of steel fibres extracted from waste tyres as reinforcement for concrete. Previous attempts to use such fibres with concrete ended up in balling of the fibres and wasting of the concrete. Four concrete mixes using three different types of fibres (from shredded tyres, from pyrolysis and commercially available fibres) were developed. An optimisation procedure was used to maximise the amount of fibre used whilst maintaining a reasonable degree of workability. Single and double sided pull-out tests were developed to determine the anchoring characteristics of fibres. Double-sided tests with multi-fibres were found to be the most reliable. The anchoring characteristics of tyre fibres were found to be as good as for commercially available fibres. Critical fibres lengths were determined for the different fibres used. A simple fibre pull-out model is proposed. Flexural toughness tests were developed and optimised based on the ASTM and Japanese standards. The crack development, neutral axis depth and characteristic length were examined in detail. An examination of the RILEM 0-£ model has shown that the model can overestimate the flexural capacity of Steel Fibre Reinforced Concrete (FRC). New 0-£ models were derived by adopting inverse analysis techniques on results from flexural tests on notched beams. These models when used with Finite Element Analysis (FEA) can predict the behaviour of the tested prisms accurately. For design purposes the models have been simplified. Parametric studies led to design equations which predict the design moment by using a fibre parameter (relating to length and bond) and the fibre amount by weight. The design equations are applied in several applications including the design of slabs on grade. The results compare favourably with existing design guidelines. Finally, slabs for drainage covers were designed and tested, demonstrating that fibres from tyres can be used for industrial applications

Reuse of tyre steel fibres as concrete reinforcement

To attain economically viable and environmentally friendly tyre recycling, it is necessary to develop new applications and products, which will use tyre by-products (especially the steel cord) as raw materials. The authors demonstrate that the steel fibres recovered from used tyres can be used to reinforce concrete elements. This application has a great potential, as it is estimated that more than 500000 t of high-quality steel fibres could be recovered annually from used tyres in the EU alone. This paper presents the work carried out as part of various ongoing projects on the use of steel fibres in concrete construction. The first part of the paper deals with waste management issues, the methods used to recover steel fibres from tyres, and existing applications of used tyres. The second part presents the mechanical behaviour of concrete elements reinforced with these steel fibres and discusses the relevant design and economic issues. It is concluded that the use of these steel fibres in concrete construction will benefit not only the construction industry, but also the producers and recyclers of used tyres

Stress-strain characteristic of SFRC using recycled fibres

This paper presents work from a comprehensive study on the development of a flexural design framework for concrete reinforced with steel fibres that are recovered from used tyres. The experimental flexural behaviour of notched concrete prisms reinforced with these fibres is initially presented. For comparison purposes, prisms reinforced with industrially produced fibres are also considered. An attempt to adopt an existing RILEM design framework to derive appropriate tensile stress-strain blocks is made, but problems are identified with key parameters of the framework. The influence of crack propagation and location of neutral axis depth on the tensile stress distribution is examined. Following an analytical study, it is concluded that the uniaxial stress-strain model, proposed by RILEM overestimates the load-carrying capacity and should be modified by utilising more advanced analytical techniques. © RILEM 2006