# A Review of Sustainable pavement Aggregates
## 1 Introduction  
The construction of flexible pavements consumes large amounts of asphalt, water, and stone aggregate. Consequently, their design establishes an interaction between the economic, environmental, and social aspects. This interaction is inherent in pavement materials and requires the search for sustainable alternatives [1]. Pavements cover 16.3 million kilometers of the planet, which represents increasing challenges in terms of the availability of construction materials. Thus, the expectation for 2050 is to reach the figure of 25 million kilometers in the construction of new roads [2]. In this respect, the sustainability of pavement performance improves with the reasonable allocation of traffic flow on a road network [3]. Road engineering tends to use recycled materials as a way to address environmental issues around the world [4]. However, waste must be reduced from the design phase of civil works [5]. A sustainable pavement meets human needs, conserving natural resources, as well as the planet's ecosystems through alternative inputs, obtained from reclaimed asphalt pavement (RAP), recycled concrete (RCA), waste foundry sand (WFS), rubber, polymers, and waste glass (WG) [6]. These solid residues are incorporated as aggregates, coarse and fine, as they occupy approximately 90% of the weight of the asphalt mixtures [7]. RAP is the most commonly used recycled material as an aggregate in pavement layers with or withoutbinder and is frequently used in combination with other recycled materials [8]. Reclaimed asphalt pavement (RAP) for making new hot mix asphalt (HMA) is a recognized technique with economic and environmental benefits that can be maximized by increasing the content of this recycle in the aggregate volume [9]. RAP can be effectively reused, contributing to the reduction of pavement deterioration and the consumption of natural resources, while obtaining results comparable to the conventional aggregate of sand [10]. Another waste material used in construction, recycled concrete aggregate (RCA) is a sustainable alternative to newly mined aggregate in concrete manufacturing that provides the ability to replace up to 25% of the construction, without significantly affecting concrete strength [11]. The use of RCA reduces costs and carbon dioxide footprint, in addition to conserving raw materials [12]. It is important to properly process the RCA and use post-processing measures to strengthen it [13]. In addition, the use of recycled concrete aggregates (RCA) in the construction of road pavements can be applied in all pavement layers, showing the physical and mechanical properties of natural aggregates [14]. In parallel, the residue of waste foundry sand (WFS) has pozzolanic properties that contribute to the improvement of the strength of the concrete [15]. The use of recycled tire rubber in asphalt pavements has proven beneficial in improving overall pavement performance; this material reduces permanent deformation, improves resistance to rutting, reduces construction and maintenance costs, and increases resistance to fatigue damage [16]. However, it is necessary to further investigate micromechanical models in order to better understand the multiphysics interactions between the constituents in the rubber-modified binder material and, thus, be able to more accurately predict its mechanical properties [17]. The use of plastic waste in road construction can reduce the emission of greenhouse gases and decrease the amount of plastic that is dumped into the environment [18]. The incorporation of recycled PET in the treal layer of pavements offers benefits in terms of both physical-mechanical and economic properties, however, it is recommended to determine the optimal percentages of recycled PET content in pavements [19]. The addition of polyethylene terephthalate (PET) in hot modified asphalt mixtures (HMA) improves mechanical properties such as Marshall, indirect tensile strength (ITS), and retained Marshall stability (RMS) [20].