1. Introduction
Hot mix asphalt (HMA) is the most used asphalt mixture for road pavements worldwide. HMAs are produced and laid at high temperatures (150–190 °C) and are durable, weather-resistant, cost-effective, cool down quickly during construction, and let the traffic open in a short period [
1]. Besides the standard hot mixtures composed of aggregates and asphalt binder, several other types of asphalt mixtures have been developed. Asphalt mixtures modified with synthetic or recycled polymers (e.g., styrene-butadiene-styrene, crumb rubber from end-of-life tires) are widely used and have better mechanical performance than standard mixes [
2,
3,
4,
5,
6,
7,
8,
9,
10,
11].
Crumb rubber (CR) from end-of-life tires is used in granular or powder form as a modifying agent for HMAs [
12,
13]. CR can be used to modify asphalt mixtures through wet technology by preliminarily mixing the CR with the asphalt binder to obtain a ductile, elastic, high-viscosity binder, also known as asphalt rubber [
14]. CR is added to asphalt binder in a percentage typically higher than 15% by weight of the total binder, and the mixing process is at temperatures usually higher than 180 °C. The modified asphalt binder can then be combined with aggregates in the hot mix plant to produce asphalt mixtures that are generally of the gap-graded type, with a noncontinuous particle size distribution, and the dense-graded type, with a continuous particle size distribution.
The high temperatures at which HMAs are produced and laid lead to issues such as the gaseous emissions being released by road paving asphalt workers during the construction phase. The exposure of workers to asphalt fumes has been investigated in the literature from the technical and medical points of view. Exposure to dust, particulates, nitrosamines, benzothiazole, polycyclic aromatic hydrocarbons (PAHs), and volatile organic compounds (VOCs) were evaluated by performing medical tests on workers and collecting air samples at the asphalt plant and the paver [
15,
16,
17,
18,
19]. Total particulate, PAHs, VOCs, and benzothiazole were measured by collecting air samples from the asphalt plant during the production of standard and CR HMAs and at the paver during the paving phase. These constituents were higher in gaseous emissions from rubberized asphalt mixtures [
16]. The highest exposure was at the paver vehicle and the asphalt delivery truck [
16]. However, the concentrations of the constituents at the screed and the driver’s paver seat may be different. At the driver’s seat, which is the point closest to the discharge of the HMA from the truck, the fumes can be affected by external factors, such as the wind speed and direction; instead, at the screed, the emissions are more uniformly distributed [
18]. Performing self-reported symptoms, spirometry tests, and blood sampling pre- and post-work on workers employed in paving standard and rubberized asphalt mixtures, dust, PAH, and nitrosamine exposure was not different between standard and asphalt mixtures modified with CR [
15]. However, benzothiazole was higher in workers exposed to fumes from rubberized asphalts, because this chemical is used for the vulcanization process of the rubber for tires [
15,
16]. Sanitary-Environmental Risk Assessment (RA) is an approach used to evaluate impacts due to the use of specific chemicals or physical agents in a specific site and time to protect human health at the local level. In the case of road pavements, RA assesses toxicological and carcinogenic risks to which workers are exposed on site because of the presence of gaseous emissions coming from hot bituminous mixtures laying operations [
17,
20].
Life Cycle Assessment (LCA) is a comprehensive methodology to quantify the environmental performance of goods and services [
21,
22,
23]. In the case of the road pavements, the adoption of the LCA is especially useful in the case of innovative materials containing recycled products, which may be attractive for applications because of their contribution to the reduction of raw materials and nonrenewable resources [
24,
25,
26,
27,
28,
29].
LCA and RA can be used to assess the effects of chemicals and pathogenic agents emitted into the environment on humans and other species. Both tools require similar data, such as emissions, fate, and exposure of receptors [
30]. The LCA impact category on human toxicity and ecotoxicity was developed based on mathematical relationships established by chemical RA [
30]. LCA and RA can be combined to obtain a more comprehensive environmental assessment, inclusive of evaluations performed both at the global and local scales [
30,
31,
32,
33,
34]. There are different ways to link LCA and RA in a life cycle risk assessment (LCRA). For instance, RA could be considered a subset of LCA or the opposite, and they could be used as complementary tools to create a comprehensive picture of a specific case [
34].
The main hypothesis of this work is that HMAs expose paving workers to asphalt fumes containing contaminants, such as PAHs and VOCs, during construction operations. The human toxicity calculated for workers can be included in the life cycle impact assessment of the LCA of road pavements. The use of CR in asphalt mixes may not increase the concentration of the contaminants in the gaseous emissions. Instead, the percentage of the asphalt binder could change this factor. The environmental impact of road pavements built with asphalt mixtures using CR can be lower than that of standard, unmodified pavements.
This paper aims to combine the effects of asphalt fumes on human health during the construction phase of road pavements with the life cycle impacts of CR asphalt mixtures used as wearing course, as compared to standard asphalt mix for reference. Results of the RA were implemented in the LCA, considering the RA ad subset of LCA. We collected air samples at the screed and driver’s seat of a paver during the construction of a full-scale trial section of a wearing course. The air samples were then analyzed to quantify the content of PAHs and VOCs, and the results were used to calculate the human toxicity related to the workers. We calculated the characterization factors (CF) for the human toxicity based on the chemicals detected in the gaseous emissions and then multiplied them by the mass content of PAHs and VOCs measured on site to quantify the human toxicity impact. The workers’ toxicity impact category was added to the other impacts included in the ILCD (International Reference Life Cycle Data System) 2011 midpoint method, assessed using the LCA tool SimaPro [
35]. The quantification of the asphalt fumes from standard and rubberized hot mix asphalts and the calculation of their impact on human health has not been assessed in the literature. This paper represents a contribution to the construction phase of LCA applied to road pavements. LCA practitioners can use these results to consider the health impact of gaseous emissions during the construction phase of asphalt pavements.
4. Conclusions
Results obtained from the LCA analysis described in this paper show that the use of wearing courses containing asphalt rubber produced using wet technology (gap-graded and dense-graded) can lead to significant benefits in terms of energy saving, environmental impact, human health (human toxicity noncancer effects excepted), preservation of ecosystems and minimization of resource depletion. However, these advantages are only guaranteed if mixtures are adequately designed and laid, possibly reducing surface course thickness and maintenance frequency. Risk Assessment is included in ILCD midpoint method results as part of the human toxicity category regarding the health safety of pavement workers in the Turin area. Based on the gathered results, workers’ cancer and noncancer effects were more elevated from asphalt mixtures containing a higher binder content (gap-graded) than the dense-graded and the standard wearing course. Therefore, the integrated method, LCRA, may allow a complete evaluation of global (regional, urban) impacts and local impact (in the examined case, pavement workers) and may be a valuable tool in making decisions considering all the involved aspects.