*3.2. Permeable Pavements Life Cycle Assessment*

In recent years, the use of permeable concrete as paving material in low volume road applications has gained importance due to its positive environmental aspects. Due to the increased use of permeable concrete in the pavement industry, there is large scope for future research to better understand the material, which will make it a promising material for sustainable future roads [51]. Wang et al. [11] developed a model of LCA that can be applied to permeable pavements of both asphalt and concrete in order to evaluate the environmental impacts caused by these types of pavement. The impacts investigated in the study were related to urban floods, stormwater recycling and water purification. The authors compared the use of a permeable asphalt pavement with a conventional asphalt pavement on a typical four-lane secondary highway. The results showed that in 10 km of the modelled highway, 49 TJ of energy consumption, 6700 tonnes of CO2e emissions, 0.1 tonne of lead emission and 1.0 tonne of zinc emission could be avoided if permeable pavements were used in place of conventional pavement. The study showed that the most significant reduction in energy consumption, greenhouse gas emissions, lead emissions and zinc emissions occurs during the use phase of the pavement. In addition, in an area of 200,000 m<sup>2</sup> (10 km × 20 m), the volume of stormwater recycled to the subgrade annually using the permeable pavement is 154,000 m3.

Spatari et al. [13] examined the reduction of energy consumption and the reduction of greenhouse gas emissions through selected Low Impact Development (LID) strategies using the LCA in an urban watershed model. The LID strategies consisted of a retrofit in the conventional sidewalks (with impervious surface), these being replaced with permeable pavements. An annual energy reduction of 7.3 GJ and a 0.4 tonne reduction in greenhouse gas emissions were estimated for the strategy implemented in a neighbourhood of New York City. Examining the materials for the LID strategy, the rubber mats and concrete sidewalk components contribute most to the embodied energy (31% and 28%, respectively) and greenhouse gas (GHG) emissions (34% and 27%, respectively), while transportation energy accounts for approximately 10% of the construction materials' life cycle energy and 17% of life cycle GHG emissions. The annual savings are small compared to the energy intensity and greenhouse gases of LID materials, resulting in slow environmental return (paybacks ranged from 70 to 180 years). This preliminary analysis suggests that if implemented along an urban watershed, LID strategies can have significant energy cost savings for water pollution control facilities, and may advance in reducing their carbon footprint.

A study by the Brazilian Council for Sustainable Construction [52] carried out the evaluation of the modular life cycle of concrete blocks for interlocking pavements, which can be used as surface of permeable pavements. The study estimated indicators such as material use, water and energy consumption, CO2 emission and waste generation in the production process. The data were collected in 33 block factories, located in different regions of Brazil. The results showed the grea<sup>t</sup> variability in the consumption, depending mainly on the type of production adopted by the factories and also on the dimensions of the blocks. Energy consumption ranged from 50 to 810 MJ/m2. The CO2 emission varied from 10 to 70 kgCO2/m2. Water consumption, in turn, varied from 0.01 to 0.91 litres/piece. The waste generated by the factories is diverse, such as wood, plastic, paper, oil, steel and cementitious material. The percentage of recycling practiced by the factories ranges from 67% to 100%.

Li et al. [53] evaluated the life cycle of different sustainable drainage systems: permeable pavements, green roofs and wetlands. Indicators at all stages of the life cycle (construction, operation, maintenance and final disposal) were evaluated. The results showed that the abiotic depletion potential, the acidification potential and the global warming potential of the three drainage systems obtained the greatest impacts in each category: resource depletion, ecosystems and human health, respectively. The impact on human health is related to the concrete used in construction, directly impacting the exhaustion of resources. Resource depletion has also contributed significantly to ecosystem damage, while high abiotic depletion is mainly due to the transport of materials. The study also showed that permeable pavements contributed significantly to flood reduction, with a runoff control rate of 67.5%. However, permeable pavements obtained the highest abiotic depletion potential, mainly due to the greater use of building materials in their structure.

Maiolo et al. [9] developed a methodology based on the sustainability index to evaluate the life cycle of permeable pavements and green roofs implemented in Italy. Figure 5 shows the structure of the permeable pavement used in the study. The application of the LCA highlighted that there are substantial contributions to the layers made up of natural material (sand, gravel), which have an impact due to transportation from the place of origin to the place of execution of the system. In addition, the life cycle of polymeric materials is the same for both drainage systems because of non-renewable sources of energy supply and transport types whose energy class is not particularly competitive. A confirmation of this fact is that the contribution of carbon dioxide has a higher percentage than the emissions of other gases (methane and dinitrogen monoxide), as shown in Figure 6. In conclusion, the authors state that the comparison between the sustainability indices shows that the green infrastructures are technologies that adequately reflect the objective of reducing the environmental impact produced by drainage systems.

A study conducted by the Center for Neighborhood Technology (CNT) and American Rivers [54] showed that air temperature can be reduced by permeable pavements, which absorb less heat than conventional pavements. By reducing the heat island effect in urban areas, such cooling can reduce diseases and fatalities related to excessive heat during extreme events of high temperatures and heat waves.

**Figure 5.** Structure of the permeable pavement used in the study. **Source:** Maiolo et al. [9].

**Figure 6.** Gases emitted during the life cycle of permeable pavement (PP) and green roof (GR). **Source:** Maiolo et al. [9].

De Sousa et al. [15] compared the life cycle assessment of three different drainage systems in the United States. System 1 consisted of green infrastructures including 27.12 ha of permeable pavements, 1.18 ha of bioretention basins, 2.80 ha of infiltration plants, 1.06 ha of rain gardens and 8.54 ha of cisterns in the subgrade. This combination was collectively sized to capture the first 2.5 cm runoff generated from approximately one-third of the total drainage area. The infrastructure of system 1 occupied about 5% of the total area. Systems 2 and 3 were grey infrastructures. System 2 only retains the runoff in a storage tank and launching it into the Bronx River, while system 3 also performed the treatment prior to launching into the river. The installation of system 1 emitted 20,000 t CO2e, compared to 31,500 t CO2e of system 2 and 100,000 t CO2e of system 3. Of the total emissions associated with the construction of green infrastructures, the major contributions came from transport (8500 t CO2e), followed by the production of cement and concrete (8400 t CO2e).

The study also presented a cumulative emission estimate for the phase of operation and maintenance of the systems in a period of 50 years. The net emissions of the green strategy were 19,000 t CO2e, while grey strategies emitted 85,000 t CO2e (detention) and 400,000 t CO2e (detention and treatment). These results were significantly influenced by the emissions associated with the operation and maintenance activities required for systems 2 and 3, and by the sequestration of carbon provided by vegetation in system 1. Thus, it is noted that green infrastructures have a superior environmental performance when compared to grey infrastructure systems.

Yuan et al. [55] compared the environmental and economic impacts of manufacturing permeable paving blocks (with at least 10% porosity) compared to conventional paving blocks in China. The functional unit used in the study was 1 m2. All inputs of raw materials, energy consumption, transport, waste and effluent discharge were calculated using the functional unit as a baseline for the two types of block production processes. Only the phase of production of the blocks and the phase of acquisition of the raw materials were considered. The economic cost to produce blocks of conventional pavement and permeable blocks was 24.26 RMB (in October 2018, 1 Chinese Yuan (RMB) is equal to 0.15 United States Dollar (US\$)) and 29.68 RMB per m2, respectively. The results showed that cement was the material that caused the greatest environmental impact on the permeable blocks. This impact could be optimized by reducing consumption. The result of the calculation showed that if cement consumption were reduced by 5%, the overall environmental impact would be reduced by about 2.21%, and the cost of production would be reduced by 1.02 RMB. The coefficient of permeability of the blocks was 1.8 × 10−<sup>2</sup> m/s. Thus, during a 3-year service period, the blocks would have a stormwater infiltration capacity of 2.01 m<sup>3</sup> per 1 m<sup>2</sup> of area.
