# Mechanical Performance of Cement Bound Granular Mixtures Using Recycled Aggregate and Coconut Fiber
## 1 Introduction  
The construction of road pavement consumes a considerable amount of natural resources, particularly aggregates. The European Aggregates Association (UEPG) indicates an average annual demand of six tons per capita (in EU+EFTA space, 2018), which amounts to three billion tons per year, placing the aggregates sector as by far the largest consumer of natural resources among the non-energy extractive industries [1]. The UEPG estimates that 20% of the end use of aggregates is in transport infrastructure: roads, runways, railways, and waterways [1]. The majority of the aggregates used is obtained from crushing natural rocks. This permanent consumption of non-renewable resources is not sustainable. In addition, some regions already possess low reserves of quality aggregates [2, 3, 4, 5, 6]. Concomitantly, demolition and renovation actions produce significant waste, commonly designated as construction and demolition waste (CDW). Over the last years, a big portion of these materials was discharged into landfills [2]. As a result, the European Parliament and the European Council issued directives to promote the rational and efficient use of natural resources to tackle this environmental problem [7, 8]. These directives are being transposed into national legislation. For example, in the case of Portugal, DL 102-D/2020 [9] recently increased the cost of CDW disposal into landfills from 11 to 22 EUR/ton and already predicts an increase to 35 EUR/ton in 2025. However, if properly processed, CDW can be used to obtain recycled aggregates (RA). The use of RA, as the total or partial replacement of natural aggregates (NA), has already been validated for cement concrete [4; 10; 11; 12; 13; 14] and unbound granular layers [15; 16; 17; 18; 19; 20; 21; 22; 23]. Compared to NA, in general, RA tend to present lower dry density, higher water absorption, and higher wear by abrasion. When crushed concrete materials mainly constitute the RA, these differences can be explained by the residual mortar adhered to the aggregates. Furthermore, when released and hydrated, this adhered mortar promotes self-cementing reactions that enhance the overall mechanical strength [15; 18; 24; 25]. Regarding cement bound granular mixtures (CBGM), the use of RA has already been evaluated, generally presenting a lower maximum dry density and requiring a higher moisture content [3; 26; 27; 28]. In addition, some authors evaluated the performance of CBGM in full-scale pavement test sections, and when testing with falling weight deflectometers, found the deflections of the sections with RA to be equal to or lower than those of the sections with NA [29; 30; 31]. Thus, the use of RA allowed a higher modulus in the pavement layer, which indicates enhanced mechanical behavior. In bound materials, such as concrete and CBGM, the incorporation of fibers can cause a reinforcement effect. Regarding reinforcement, a material with the potential to be used in CBGM is coconut fiber (CF). CF, a natural fiber, is obtained from coconut husk. Coconut husk is a residue from the food sector, which extracts several products from the coconut. Currently, there is a lack of large-scale applications for this waste material, which is generally discharged into landfills, posing an environmental problem. As natural materials, some variability in properties can occur. However, several authors have already highlighted the good mechanical properties of CF, such as tensile strength (90-286 MPa), tensile modulus (1.1-13 GPa), and a high maximum tensile strain (15-51%) [5]. CF also presents high durability due to its low cellulose and high lignin contents that retard its natural degradation [32]. Several authors have previously explored the use of CF in bound materials (mostly cement concrete) and concluded that such an application was interesting [5].