Positive interaction of industrial and recycled steel fibres in fibre reinforced concrete

JOURNAL OF CIVIL ENGINEERING AND MANAGEMENT ISSN 1392-3730 print/ISSN 1822-3605 online 2013 Volume 19(Supplement 1): S50–S60 doi:10.3846/13923730.2013.802710 POSITIVE INTERACTION OF INDUSTRIAL AND RECYCLED STEEL FIBRES IN FIBRE REINFORCED CONCRETE Dubravka BJEGOVICa, Ana BARICEVICa, Stjepan LAKUSICb, Domagoj DAMJANOVICc, Ivan DUVNJAKc aFaculty of Civil Engineering, Department of Materials, University of Zagreb, Fra Andrije Kacic Miosic 26, 10000 Zagreb, Croatia bFaculty of Civil Engineering, Department of Transportation, University of Zagreb, Fra Andrije Kacic Miosic 26, 10000 Zagreb, Croatia cFaculty of Civil Engineering, Department of Engineering Mechanics, University of Zagreb, Fra Andrije Kacic Miosic 26, 10000 Zagreb, Croatia Received 25 Jan 2012; accepted 05 Nov 2012 Abstract. In line with current “green” transport initiatives, Croatia plans to build over the next investment period a high speed railway line which will connect central Croatia and its capital with coastal regions of the country. According to design documents, the track system will be built using ballastless concrete solutions. In the scope of the project “Concrete track system – ECOTRACK”, researchers from the University of Zagreb – Faculty of Civil Engineering analysed a new material, i.e. the rubberized hybrid fibre reinforced concrete (RHFRC), in order to find out whether its properties are adequate for the proposed concrete track system. The RHFRC contains by-products from mechanical recycling of waste tyres (rubber and steel fibres). The study of fibre and rubber interaction and their contribution to mechanical properties of the fibre reinforced concrete is presented, as extensive research on positive interaction between industrial and recycled steel fibres has not as yet been made. The results show that the RHFRC is an innovative, sustainable and cost-effective concrete, which is fully compliant with criteria prescribed in relevant standards. Keywords: fibre reinforced concrete, industrial steel fibres, recycled steel fibres, mechanical properties, recycled rubber, waste tyre. Reference to this paper should be made as follows: Bjegovic, D.; Baricevic, A.; Lakusic, S.; Damjanovic, D.; Duvnjak, I. 2013. Positive interaction of industrial and recycled steel fibres in fibre reinforced concrete, Journal of Civil Engineering and Management 19(Supplement 1): S50–S60. http://dx.doi.org/10.3846/13923730.2013.802710 Introduction Until recently, Croatian investments mostly focused on the construction of one segment of transport infrastructure – motorways. After becoming the EU candidate, Croatia became more oriented towards sustainable development and soon it became clear that “green” transport alternatives must be strengthened. In the oncoming period, Croatia plans to build a high speed railway line, which will connect central Croatia and its capital with coastal regions of the country. According to the design documentation, the track system will be based on ballastless concrete solutions. During realization of the project “Concrete track system – ECOTRACK”, researchers from the University of Zagreb studied a new material, i.e. the rubberized hybrid fibre reinforced concrete (RHFRC), which could prove suitable for the proposed track system (Fig. 1). As the material most commonly used in the construction of balastless concrete track systems, ordinary Fig. 1. ECOTRACK – concrete track system based on innovative, environmentally friendly materials concrete presents many disadvantages such as the low energy absorption capacity, poor post-cracking behaviour, and therefore inadequate durability (Fig. 2). In this paper, fibre reinforced concrete with by-products from mechan- Corresponding author: Ana Baricevic E-mail: S50 Copyright © 2013 Vilnius Gediminas Technical University (VGTU) Press www.tandfonline.com/tcem Journal of Civil Engineering and Management, 2013, (Supplement 1): S50–S60 Fig. 2. Problems encountered during use of the concrete ballastless track System ical recycling of waste tyres (rubber and steel fibres), is studied in order to obtain an adequate alternative to ordinary concrete. Steel fibres are often used as crack arrestors and contributors to strength, deformability and toughness of composites. By reducing crack openings, improving ductility, energy absorption and post-cracking strength of composites, steel fibres are rightly considered as an optimum solution. The aim of this study is to determine a positive synergy between industrial and recycled steel fibres. Additionally, rubber is incorporated to assure enhancement of the concrete energy absorption capacity. The waste management protocol for the oncoming period is clearly defined in numerous EU Directives, and so the research in this field has become attractive from both economic and environmental standpoints. Starting from 2006, any kind of waste tyre disposal is forbidden (Council of the European Union 1999) and, at the same time, 6 200 waste tyres are reused with each kilometre of railway built using the studied material, which has brought about extensive improvements in the sphere of waste tyre management. On the other hand, an extremely high price of industrially processed steel fibres (approximately 1500 €/t) has resulted in the rise of price per each m3 of concrete, which further justifies this research, especially if the price of recycled fibres is taken into account (approximately 150 €/t). 1. Application of recycled steel fibres in concrete technology Several processes can be used to recover recycled steel fibres from waste tyres: the shredding and cryogenic process for mechanical recovery, or the pyrolysis and microwave induced pyrolysis for recovery utilising thermal degradation (Pilakoutas et al. 2004). Previous research in this field mainly consisted in the study of fibre reinforced concrete properties and focused exclusively on recycled steel fibres, and comparison with steel fibre reinforced concrete containing industrial steel fibres (Achilleos et al. 2011; Aiello et al. 2009; Graeff et al. 2009; Ne- S51 ocleous et al. 2006; Tlemat et al. 2004). Recent studies (Graeff et al. 2012) on fatigue resistance of concrete pavements reinforced with recycled steel fibres suggests that for enhanced fatigue performance a combination of recycled and industrially produced fibres would be ideal. Possible synergy of recycled and industrial steel fibres has until now been briefly investigated in few papers only (Angelakopoulos et al. 2011; Bjegovic et al. 2012a, b, c; Krolo et al. 2012). Steel fibres obtained by shredding during the recycling process are irregular in shape and dimensions, and so their ability to provide for an effective stress transfer was considered questionable. Despite irregular undulations it was demonstrated that steel fibres obtained during the shredding process give an effective mechanical contribution to the bond performance (Aiello et al. 2009), and that their strength is best utilized wh (...truncated)