*3.1. Pavements Life Cycle Assessment*

This section presents a brief literature review about traditional pavements life cycle assessment, showing some of the various studies and giving the reader an overview about the subject. AzariJafari et al. [41] highlight the large increase in the number of studies on the life cycle assessment of conventional pavements. Current literature demonstrates a wide range of environmental load implications associated with pavements [42–44]. Chiu et al. [45] demonstrated that actions aimed at sustainable development in pavement construction projects can lead to the reduction of greenhouse gas emissions and their life cycle cost. However, there are still immature concepts, which require more research in the coming years, in different stages of the evaluation of the pavement life cycle. One of the fields still little explored is that of permeable pavements. Few studies regarding the life cycle of these pavements and the environmental benefits that can be achieved through the retention of water and consequent reduction of the problems related to floods and water recharge are found in the literature.

LCA is an appropriate tool that can help designers deal with the environmental aspects of their pavements to achieve the goal of building more sustainable pavements. In fact, LCA helps to quantify, analyse and compare the environmental impacts of different types of pavement, from material extraction to the end of its lifespan [19].

AzariJafari et al. [41] compared publications involving LCA of several types of pavements. The results show a significant heterogeneity of functional units and other components. LCA standards, such as ISO 14040 and 14044, do not have technical details on, for example, phases and processes that should be included in the assessment, the lifespan to be analysed, or what the minimum amount of data is that should be considered in modelling LCA. In addition to inconsistencies between publications, significant differences in calculated life cycle environmental impact outcomes make comparisons of results simply impossible.

Approximately US\$150 billion and 320 million tonnes of building materials are invested annually in the construction, rehabilitation and maintenance of pavements in the United States. However, very little is known about the environmental damages caused by the construction of these pavements [46]. Some studies have shown that the type of pavement can influence vehicle fuel consumption [47,48]. Taylor and Patten [48] have shown that Portland cement-based concrete pavements can decrease

the amount of fuel consumed when compared to pavements constructed with hot-mix asphalt concrete (HMA).

Huang et al. [49] developed a life cycle assessment tool for the construction and maintenance of asphalt pavements. The structure of LCA was composed of process parameters (energy consumed in transport, material production and pavement construction), pavement parameters (size, materials used, lifespan), unit, project inventory and characterization results. The results are divided into different categories, such as depletion of minerals and fossil fuels, depletion of the ozone layer, global warming, acidification, photo-oxidant formation, human toxicity, eco-toxicity, eutrophication, among others. The study proposed a method for grouping and weighting categories, according to the "Eco-points" developed by the Building Research Establishment (UK) for the construction sector, as shown in Figure 3.

**Figure 3.** Grouping and weighting of LCA environmental impact categories. **Source:** Huang et al. [49].

Huang et al. [49] used the proposed LCA methodology to conduct a case study investigating the environmental impacts of the asphalt pavement life cycle on a highway, in which the natural aggregates were partially replaced with glass residues and incineration ash. The results were compared to conventional pavement of the same size and function, but using only virgin aggregates. Asphalt mixing, bitumen and aggregates production consumed, respectively, approximately 62%, 23% and 6% of the total energy and consequently produced more emissions than the other processes. The use of recycled materials reduced the consumption of asphalt binder by about 7%. Another significant benefit of recycling was the saving of 5766 tonnes of aggregates and the recycling of 579 and 989 tonnes of glass waste and incineration ash respectively. Aggregate transport accounted for more than 61% of all diesel use, due to the long transport distance (193 km). Trains with higher fuel efficiency (0.17 MJ/t.km) than trucks (0.46–0.94 MJ/t.km) were used to transport aggregates. Glass and ash were obtained from local sources and the use of diesel to transport asphalt was only 17%, as the highway was located very close to the asphalt plant (6.4 km). The results of this study show the grea<sup>t</sup> dependence of the location of the road and the materials used in the pavement structure, which significantly interfere with the environmental impacts of the life cycle.

Santero and Horvath [50] evaluated the global warming potential of conventional pavements in the United States, analysing several components such as: extraction and production of materials, transportation, equipment used, carbon absorption, heat islands, surface roughness of the pavement, rolling resistance, albedo, among others. Figure 4 shows the emission of carbon dioxide (in Mg CO2e) per kilometre of road over 50 years obtained by Santero and Horvath [50]. Grey bars show variations of global warming potential, while black bars show the extreme values of each component. The results demonstrate the wide range of possible impacts to the components of the pavement life cycle. This impact ranges from insignificantly small to 60,000 Mg CO2e per kilometre of road over 50 years.

**Figure 4.** Impact of the global warming potential for components of the pavement life cycle. **Source:** Santero and Horvath [50].
