Concrete is essentially a mixture of cement, aggregate and water. It is widely used in the construction industry because all the raw materials required are widely available and are of low cost. Concrete is very strong in compression, however, it has a very low tensile strength. To improve its tensile strength, steel reinforcing mesh (SRM) and steel bars are used in the concrete. Apart from traditional steel reinforcement, various fibres are also used to improve the properties of concrete, mainly for enhancing the tensile strength. There are four main types of fibres which can be used to reinforce concrete: steel fibre, glass fibre, natural fibre and synthetic fibre (Daniel et al. 2002).
In recent years, macro plastic fibres have widely been used in the construction of concrete footpaths (Alani and Beckett 2013), precast panels (Peyvandi et al. 2013) and shotcrete mine tunnels (Kaufmann et al. 2013). Macro plastic fibres normally have a length of 30-60 mm, a cross section of 0.6-1 mm2 (Yin et al. 2015), a tensile strength of 300-600 MPa and a Young’s modulus of 4-10 GPa (Hasan et al. 2011), depending on the raw materials and manufacturing techniques used. The macro plastic fibres can effectively control drying shrinkage cracks of concrete (Pujadas et al. 2014). The drying shrinkage cracks occur in concrete due to loss of water molecules from hardened concrete (Jafarifar et al. 2014). This type of drying shrinkage cracks can occur in large flat areas like slabs in hot and dry environments, for example, in North Queensland, Australia. A SRM is normally used to prevent the drying shrinkage cracks; but it is now gradually being replaced by the macro plastic fibres due to ease of construction, reduced labour and lower cost. Another significant benefit of using macro plastic fibre is the improvement in post-cracking behaviour of concrete (Buratti et al. 2011). Plain concrete is brittle and has no effective post-cracking ductility, but the macro plastic fibres can considerably improve the post-cracking response of concrete, because the plastic fibres act as a crack arrester, and alter the intrinsically brittle concrete matrix into a tough material
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S. Yin, Development of Recycled Polypropylene Plastic Fibres to Reinforce Concrete, Springer Theses, DOI 10.1007/978-981-10-3719-1_1
with better crack resistance and ductility. Therefore, when concrete cracks, the common large single cracks can be substituted by dense micro-cracks due to the presence of fibre reinforcement (Brandt 2008).
Production of SRM involves significant carbon emissions. For instance, to reinforce 100 m2 of concrete footpath (100 mm thick), typically, seven sheets of SL82 SRM are needed, which produces about 1250 kg of carbon emissions (BPIC 2010; Strezov and Herbertson 2006). However, 40 kg of polypropylene (PP) fibre can achieve the same degree of reinforcement in concrete, producing only 160 kg of carbon emissions (Chilton et al. 2010). Furthermore, the preparation required when using SRM such as laying, cutting and tying requires considerable labour time and cost compared to the use of plastic fibres which can be mixed directly into concrete trucks. Due to the ease of construction, and reduced labour cost, macro plastic fibers, such as PP (Ramezanianpour et al. 2013), high-density polyethylene (HDPE) (Zheng and Feldman 1995) and polyethylene terephthalate (PET) fibres (Fraternali et al. 2014) have become attractive alternatives to SRM for reinforcing concrete (Pelisser et al. 2010).
Presently, thermoplastic waste from disposable consumer packaging and products is increasing, elevating the environmental pollution and wasting useful resources. According to the surveys done by the United States Environmental Protection Agency (U.S.EPA 2014), plastic waste accounted for less than 1%-w/w of municipal solid waste stream in the 1960s, which considerably increased to 12.7%- w/w in 2012. Plastic bottles and plastic bags are becoming increasingly prevalent and have substituted glass bottles, metal containers and paper bags. Global plastic production was 288 and 299 million tonnes in 2012 and 2013, respectively (PlasticsEurope 2015). However, the global recycling rate of plastic waste in this period was less than 5% (Velis 2014). The recycling rate varied for different countries depending upon the maturity of the recycling process. For example, the recycling rates of Europe, Australia and the United State were 26% (PlasticsEurope 2015), 20% (A’Vard and Allan 2014) and 9% (U.S.EPA 2014), respectively.
Consequently, increasing plastic production coupled with low recycling rates has led to a serious increase in pollution, including emissions of powerful greenhouse gases (GHG) such as methane in landfill areas (Zhou et al. 2014), emissions of toxic chemicals (e.g. bisphenol A and polystyrene) (Dodbiba et al. 2008), and poisoning of many marine species (La Vedrine et al. 2015). One of ways to address this problem is to develop various reusing and recycling techniques, such as material recycling (Castro et al. 2014), feedstock recycling (Dormer et al. 2013) and energy recovery (Gallardo et al. 2014), for these materials. Improving the quality of recycled PP products (Eriksson et al. 2005) and extending their applications (Duval and MacLean 2007) is also an effective way to promote the recycling rate. Therefore, preparing high-strength recycled plastic fibres and using them in concrete can be a unique way of reinforcing concrete while also decreasing plastic pollution.
Several techniques and methods have been developed to produce recycled plastic fibres (Fraternali et al. 2011; Gregor-Svetec and Sluga 2005; Kim et al. 2008, 2010; Ochi et al. 2007). A common technique is extruding recycled plastic granulates into fibres, and then slowly stretching the fibres in an oven with a temperature of 130-170 °C (Ochi et al. 2007). Another popular processing technique is extruding recycled plastic granulates through a rectangular die to form film sheets. The resulting film sheets are then slit longitudinally into equal width tapes (Kim et al. 2008, 2010). de Oliveira and Castro-Gomes (2011) and Foti (2011) used a method to produce recycled lamellar PET fibres and ‘O’-shaped PET fibres by simply cutting waste plastic bottles.
A common deficiency in these methods is that they are based mostly on laboratory processes, and they have low production rate resulting in high cost. Hence, these methods are not suitable to produce fibres for large-scale commercial applications. Moreover, the durability of recycled PET fibres in Portland cement matrix is still questionable (Silva et al. 2005). The PET fibres belong to polyester group, and polyester fibres degrade when embedded in Portland cement matrix (Alani and Beckett 2013; Won et al. 2010). According to the degradation tests from EPC company (EPC 2012), the PET fibres only could maintain its performance for 10 years in concrete, after that the strength of fibres decreased significantly. Although recycled PET fibres have recently become a focus of research, literatures on the use of recycled PP fibres in concrete is very limited. Recycled PP fibres have not yet been widely adopted by the construction industry due to limited research focusing on their mechanical properties and performance in concrete. Hence, this research focuses on the development of recycled PP fibres with sufficient mechanical properties like Young’s modulus and tensile strength for the application in concrete. The research also focuses on quantifying the post-cracking performance of recycled PP fibre reinforced concrete and comparing it with the performance of virgin PP fibre reinforced concrete.
In order to help decision makers choose reinforcing materials that cause the lowest environmental impacts, it is very important to carry out a comparative impact analysis. There are a variety of general and industry specific assessment methods, such as GMP-RAM (Jesus et al. 2006), INOVA Systems (Jesus-Hitzschky 2007), and fuzzy logic environmental impact assessment method (Afrinaldi and Zhang 2014). However, life cycle assessment (LCA) is the most comprehensive among the available tools and has been widely used. The LCA methodology is generally considered the best environmental management tool for quantifying and comparing the eco-performance of alternative products. Therefore, this research studies and quantifies the environmental impacts of production of recycled PP fibres by using the LCA methodology, and compares these with the environmental impacts of production of traditionally used virgin PP fibre and SRM.