# Recycled Polyethylene Fibres for Structural Concrete
## 1 Introduction  
The concrete industry is responsible for 10% of man-made CO\({}_{2}\) emissions [1, 2] and for a large part of the volume of solid waste generated [3]. Being such a polluting industry, several authors have pursued different strategies to reduce the environmental impact of concrete. One of the most widely used methods is the replacement of natural aggregates with recycled aggregates from construction and demolition waste, resulting in what is known as recycled concrete. This is a well-studied topic, which has shown that it is possible to replace aggregates extracted from quarries with waste that would otherwise be sent to landfill, with slightly inferior mechanical and durability properties [4, 5, 6, 7, 8, 9, 10, 11, 12, 13]. Another procedure studied to reduce concrete contamination is the use of self-compacting concrete, as they require a high volume of fines and are capable of absorbing a significant volume of waste in the form of fine particles [14, 15, 16, 17]. Currently, due to the growing importance of fibre-reinforced concrete, the possibility of using recycled plastic in the concrete mix as a more environmentally friendly construction material is beginning to be considered [18, 19, 20]. Since the 20 th century, the use of plastics has expanded in a wide range of products, supported by their good properties, such as low density, high strength-to-weight ratio,high durability, ease of design and manufacture, and low cost [21]. The annual production of manufactured plastics has increased from 2 Mt in 1950 to 380 Mt in 2015, concluding that the total production amounts to 8.3 billion tonnes, of which only 9% has been recycled. As for thermoplastic polymers; historically, the most commonly produced would be: polypropylene (PP) with 21%, low-density polyethylene (LDPE) with 20%, high-density polyethylene (HDPE) with 16%, polyvinyl chloride (PVC) with 12%, and polyethylene terephthalate (PET) with 10% [22]. In Europe, the construction sector accounts for 20% of total consumption, the second largest sector after packaging [23]. One of the biggest problems is plastic that is inappropriately discarded or landfilled, as much of this waste ends up in the sea. Researchers [PERSON] et al. estimated that there is almost 270 kt of plastic in the ocean [24]. A notable case of this problem is the \"rubbish island\" in the middle of the North Pacific Ocean. The characteristics of this island were also quantified, reporting an area almost three times the size of Spain [25]. The traditional linear model of plastic consumption based on: extract, make, use, and dispose is unsustainable. The best alternative for the plastics industry is the so-called circular economy, in which greater emphasis is placed on repairing or reusing products, thus extracting the maximum value from them during their useful life. At the end of their useful life, they are recovered and recycled [26]. The exponential growth of waste makes it essential to look for recycling alternatives, such as valorisation. In addition, there is growing concern and studies associated with the valorisation of different materials [27; 28; 29; 30]. The use of natural fibres (e.g., straw or horsehair fibres) as reinforcement in construction has been widespread since historical times [31]. From the 1960s onwards, there was a quick development of fibre reinforcement in the construction sector, mainly motivated by the search for new structural materials with better mechanical properties. Fibres are composed of a wide variety of materials, including steel, glass and PP [32]. Nowadays, fibres have been shown to improve some properties of concrete, such as durability, elastic modulus, shrinkage control, and flexural strength, among others [33]. In particular, polymeric fibres are a suitable solution for structures exposed to moisture, as they are less prone to corrosion processes. In addition, they can reduce structural weight and control concrete cracking [34].