Urban mining for low-noise urban roads towards more sustainability in the urban environment

The urban environment faces two challenges: waste and noise. The work presented here shows that the use of certain waste and marginal materials for semi-dense asphalt low-noise pavements is a sustainable option. To this end, mechanical, acoustic and life-cycle assessment evaluations were performed, comparing different scenarios. The results show that these alternative materials can, for the most part, reach the mechanical and acoustic performance required for low-noise pavements both at the binder scale and the mixture scale. Depending on the waste material, mix design adjustments might be necessary such as increasing binder content and the selection of substitution fraction. The results showed that the mechanical performance reached 80% of that of all virgin materials in most cases and the acoustic performance was similar. However, the Technical Readiness Level (TRL) for their use varies and depends strongly on the geographic region. To determine if using alternative materials makes sense in addition to mechanical performance, environmental and economic performance should be considered.


Introduction
The World Bank reports that worldwide 2.01 billion tons of municipal solid waste (MSW) are generated every year with 30% not managed in an environment-friendly manner.The trend is increasing and they expect this to rise to 3.40 billion tons per year by 2050.The type of waste generation is dependent on the income level of countries.For example, in high-income countries 51% of waste is categorised as dry waste that could be recycled, including plastic, paper, cardboard, metal and glass (World Bank, n.d.).Similarly, the global Construction & demolition (C&D) waste market is estimated to be USD 26.6 billion in 2021 and is projected to reach USD 34.4 billion by 2026.The rising demand for more sustainability in the construction sector provides opportunities and challenges.Opportunities are listed as  Market Report, 2021).The alarming waste generation rates and the opportunities that present themselves indicate that it is imperative for all industrial sectors to do what is possible to contribute to a zero-waste society.
The recycling of waste and marginal materials can reduce resource use, reduce greenhouse gas emissions and make a significant contribution to a sustainable built environment (Poulikakos et al., 2017).Marginal materials refer to waste materials that are not considered suitable as primary materials.This can be due to the lower quality of recycling into the same materials and as a result, they can end up in landfills.Therefore, it is critical that the construction industry shifts towards a greener approach to construction.
The urban environment faces many challenges; excessive exposure to noise and the production of waste are two important aspects.According to the Swiss federal office for the environment, one in seven people in Switzerland (1.1 million people) is exposed to noise levels that exceed the exposure limit specified in the Noise Abatement Ordinance (NAO) (Bafu, NAO; Swiss Noise Abatement Ordinance, n.d.).The NOA that was passed in 1986 aimed to reduce the exposure of residents to noise.Three methods are used to implement this policy (1) reduction of noise at the source using low-noise pavements: (2) use of noise-reducing barriers; (3) low-noise windows at the receiver's end.To reduce the traffic noise at the source semi-dense asphalt (SDA) pavements are used.These are gap-graded pavements with 12-16% air voids.
The results presented herewith are part of a larger Swiss national project titled 'Urban Mining for Low Noise Urban Roads and Optimized Design of Street Canyons'.The overall goal of this project was to investigate the suitability of using urban waste materials for low-noise pavements by using mechanical tests and life-cycle assessment.The research by the authors has shown that there are many urban waste materials that could be suitable for use on roads.At the same time, the Technical Readiness Level (TRL) of these materials as used on roads is not uniform (Piao et al., 2021;Poulikakos et al., 2017), indicating that there are barriers to their use that are not necessarily technical.These could be due to a lack of incentives or legislation or trust in their use.
This research project addresses the gap in knowledge pertaining to the use of urban waste materials for low-noise pavements, thereby addressing both urban challenges of waste and noise.

Materials and methods
The materials and methods used in this analysis are described in this section.

Materials
This investigation used a type of gap-graded mixture used in surface courses in Switzerland placed for its noise reduction properties.The mixtures have typically high-quality aggregates with high void content (14-18%) and high binder content ( > 6%).The gradation is shown in Figure 1.For this project the maximum aggregate size of 4 mm and void content of 14% and a binder content of 6% was targeted.Six types of waste materials were used as shown in Figure 2 and Table 1.Two waste plastics (MP, IP), crumb rubber (CR), electric arc furnace slag (EAFS), Recycled concrete aggregates filler (RCAFL) and polyethylene terephthalate (PET).The Swiss standards require polymer-modified bitumen for SDA mixtures (SN 640 436), as such styrene-butadiene styrene (SBS) polymer-modified binder (PMB), penetration graded at 45/80-65 according to EN 1426 was used for the control mixtures.The mixtures with plastics (MP, IP) and crumb rubber (CR) used straight-run bitumen with penetration grade 70/100.The reason for this was the hypothesis that the plastics and rubber would act similar to the polymer modification and were used as a substitute for polymer modification.Quarried sandstone from Massongex, Switzerland was used as aggregates and filler for all mixtures unless otherwise noted.
The mixtures designated as IP used waste polyethylene plastics from InnoRecycling AG (Switzerland).This is a secondary waste material from the production of plastic packaging.The amount of plastic was 0.3% by mass of mixture with a void content of 16% and binder content of 6.1%.The MP mixtures used commercially available mixed plastic waste.The plastic content was 0.3% with a void content of 16% and a binder content of 6.1%.
The mixtures incorporating CR used the dry process to produce the mixtures.The CR was provided by a local producer using the mechanical grinding process with a maximum size of 0.6 mm and treated with a proprietary chemical modified for use in the dry fabrication process.In this method the CR is added directly to the mixture.The binder content was slightly increased to account for the absorption of the light fractions by CR.The resulting mixture had a binder content of 6.3% and a CR content of 0.7% and a void content of 14.3% (Poulikakos et al., 2021).
The electric arc furnace slag (EAFS) used in this project was provided by the Gerlafingen AG steel recycling plant in Gerlafingen, Switzerland.The slag mixtures were produced with 15% replacement by mass of aggregates of 2/4 mm course and EAFS fractions (SN 640 436;2013).Samples were compacted to 13.6% void content and 6% bitumen content (Mikhailenko et al., submitted).
The PET mixtures used waste from RecyPET (Switzerland) in the form of flakes produced from recycled bottles with a size between 0.5 and 2 mm.PET has a high melting temperature (245.6°C);therefore, it does not melt during the asphalt mixing process as typical mixing temperatures are lower.PET content of 5.1%, binder content of 6.1% and void content of 16% were used for these mixtures (Mikhailenko et al., 2021).
The mixtures that incorporated recycled concrete aggregates (RCA) used waste materials from RCA in the Canton of Zurich, Switzerland.The RCA was sieved to obtain the filler.Further details on the RCA materials are reported in Mikhailenko et al. (2022).
Figure 2 shows the waste materials used in this study and the technology readiness levels (TRL) are shown in Table 1.With '1-4' referring to the progress at laboratory scale or lower; '5-7' indicating that pilot projects have been implemented in the field; '7-9' inferring that the application is partially or completely industrialised.It should be noted that this evaluation and analysis of the TRL was done using the worldwide experience that is publicly available.The local TRL level varies greatly.For example, although the TRL level for steel slag and crumb rubber is very high, this level is very low in Switzerland (Piao et al., 2021).Various elements contribute to this difference.This topic was discussed by the authors and documented (0); some relevant aspects are (1) lack of incentives, (2) lack of know-how and (3) lack of standards (Poulikakos et al., 2017).

Bitumen tests
For the dynamic shear rheometer (DSR) and complex modulus (G * ) measurements, a DSR (Physica MCR 301 DSR, Anton Paar ® GmbH., Austria) was used.Parallel plates' set up with diameters 8 and 25 mm corresponding to thicknesses of 2 and 1 mm, respectively, were used for the low-and high-temperature regimes.A constant strain amplitude of 0.1% was used for the high-temperature range (40-80°C) and 0.05% for the low-temperature range (28°C to −10°C).
For the viscosity measurements, the dynamic shear rheometer DSR (Physica MCR 301 DSR, Anton Paar ® GmbH., Austria) in rotational mode with the C-27 standard spindle at 20 rpm was used.
More details on DSR and viscosity test methods can be found in Kakar et al. (2021).

Microscopy
To investigate the microstructure of the compacted asphalt mixture samples an environmental scanning electron microscope (ESEM) Phillips ESEM Quanta FEG650 in the high vacuum mode was used.The images are produced as a result of the interaction of the atoms of the sample surface with the electron beam.The samples were cut from the centre of the compacted sample impregnated with epoxy resin and polished before they were imaged following the method developed by Poulikakos (Poulikakos & Partl, 2011).

Mechanical tests on mixtures
Various types of tests were performed on these mixtures (Mikhailenko et al., 2022;Mikhailenko et al., submitted;Poulikakos et al., 2021;Mikhailenko et al., 2021).Here for the sake of brevity a representative test is chosen and presented.To this end indirect tensile strength and indirect tensile stiffness modulus were used.The stiffness modulus was obtained according to the European standards EN (EN 12697-26, 2012) using the indirect tensile (IT-CY) set-up with sinusoidal cyclic loading.The sinusoidal loading frequencies presented here were at 10 Hz at 10°C, as prescribed by the type testing European standard (EN 13108-20:2016).Four specimens per mixture were tested.The indirect tensile strength was determined following EN 12697-23 (2003) at the specified test temperature of 20°C, with three samples tested for each mixture.

Noise reduction characterisation
The most important parameters affecting noise reduction properties in asphalt pavements are porosity, connectivity of pores and surface texture (Poulikakos et al., 2022).As the porosity of these mixtures was, for the most part, controlled by using the gyratory compactor, here the surface texture is primarily discussed and presented.The macrotexture has been correlated with the acoustical properties of asphalt pavements.This detailed profiling can be done by laser profilometry.Depending on the particular type of waste material, their use can affect the surface texture (SN 640 436).The surface texture parameter describing the texture in a wide band of wavelengths is used here to compare the results from a different material (SN 640 436).Texture level is presented in decibels as a function of texture wavelengths, λ, by taking the 1/3 octave band power spectral density (PSD) graphs for each scanline and by equations derived from ISO 13473-4 (0).

Environmental analysis method
To evaluate the environmental impacts of using waste materials in SDA pavements, our research applies the methodology of life-cycle assessment (LCA).This method focuses on the whole value chain of a product, aiming at quantifying the environmental performance and the possible burden shifts within the system (Hellweg & Milà i Canals, 2014).The LCA in this research considers the production of virgin materials (natural aggregates and asphalt binder), the processing of waste materials, along with asphalt mixing, construction and demolition.The reclaimed asphalt pavement (RAP) is assumed to be fully recyclable and burden-free.Moreover, the original waste streams and their co-processings are also included in this LCA, to identify the possible burden shifts from pavement production to other industries.

Bitumen tests
The performance index is defined as the ratio of performance of the waste mixture to the control mixture that did not use any waste materials.Figure 3 shows the DSR results in terms of complex modulus G * value of IP modified binder with respect to the PmB-modified binder as a control reference.Figure 3 illustrates that at a higher temperature , for example, from 40°C, with the increase in temperature the effect of IP modification becomes dominant.The performance index value of G * increases in comparison with the reference control (PmB), this infers that at high temperatures, the rutting resistance of IP modification is more effective.Furthermore, at lower temperatures, the effect of G * with the decrease in temperature is the opposite.That is the performance index values of G * for IP-modified binder  becomes lower than PmB and this indicates that with the decrease in temperature, IP modification has an improved tendency of low-temperature performance.Figure 4 represents the performance indicator of a CR-modified binder with respect to the control PmB reference binder.It is worth to mention that the G * values of CR modified binder were measured at a frequency of 1.59 Hz compared to the control value of the PmB binder taken at a frequency of 1.6 Hz.Furthermore, as shown in Figure 4, for some of the G * measurements, CR-modified binders at test temperatures, such as 52°C, 58°C, 28°C and 22°C, were compared to the PmB control values at corresponding temperatures of 50°C, 60°C, 30°C and 20°C, respectively.Overall, the performance index calculated based on the G * values has no clear trend.However, it is interesting to observe that with the increase in temperature from 40°C up to around 60°C, the performance index values of G * for CR modification have improved performance against the rutting and this effect slightly declines as the temperature increases up to nearly 80°C.It is also worth to mention that at a temperature below 30°C, the CR modification has a significant improvement in low-temperature properties and this effect remains consistent at a temperature until 0°C.
Figure 5 shows the results based on the performance index values using viscosity results of IP and CR-modified binder compared to PmB control reference.The results indicate that the performance index values of IP increase with the increase in temperature.However, the effect is the opposite in CR modification.These results similarly reflect the same effect of the materials, as described in terms of performance index values based on G * results.The performance index values of viscosity results infer that the IP modification has consistent dominating effects at higher temperatures similarly evidenced in the measurements of performance index values of G * (cf. Figure 3).Furthermore, the CR modification has shown lower performance index values of viscosity with the increase in temperature and  the same performance has been noticed in the analysis of the G * results (cf. Figure 4) where the CR modification has lower performance index values beyond 60°C.

Microscopy
The ESEM micrographs in Figure 6 show that the crumb rubber particles or plastic particles (in this case IP) do not melt in the mixture and they are present as elastic bodies.This has a direct effect on their performance, as discussed in the next section, as the temperature goes above the softening point temperature of the binder; these particles remain elastic contributing to the performance.

Mechanical tests on the mixture
The data in Table 2 show the designation of mixtures, the type and amount of waste used, the performance method and the value.The performance index was defined as the ratio of performance of the waste mixture to the control mixture that did not use any waste materials.This means that the closer this number is to 1 the more the performance resembles that of the mixture without waste.As apparent from the table the performance of the control mixtures is not the same although SDA4 was used for all.The reason for this is that the control mixtures had different binder contents depending on the type of waste additive to allow for a direct comparison.The CR mixtures, with 0.7% CR surpass the indirect tensile strength of SDA with PmB in dry (shown) and wet state (not shown) (Poulikakos et al., 2021).The RCAFL mixture was characterised by various methods.The results shown here are from the stiffness modulus in comparison to the reference SDA, showing that RCAFL reached 97% of the modulus of the control mixture.A complete set of data can be found in Mikhailenko et al. (2022).The performance index shown in Figure 7 indicates that all mixtures investigated here except for one (PET) were able to reach more than 80% of the performance of the control mixtures.Once this fact is known it is possible to adjust the mix design to accommodate various distress conditions using various test methods, for example, fine-tuning the mix design to accommodate ruttng in hot climates by using a harder binder.

Noise reduction performance
As SDA mixtures are used due to their noise reduction property, some of the mixtures in this investigation were investigated for their texture level which indicates the noise reduction performance of these materials (Mikhailenko et al., 2020).One can distinguish between macrotexture (8-50 mm) and microtexture (0.1-1 mm).The effects of RCA Filler, PET and PE replacement on texture levels are shown  in Figure 8.The figure shows that waste replacement can affect the texture depending on the wavelength.In the lower wavelength levels, RCA filler had the lowest texture level, whereas the PET and PE had similar texture levels to the control.On the higher wavelength regime, the PE mixture had the highest texture level.

Environmental performance
Figure 9 shows the greenhouse gas (GHG) emissions for using different waste materials in SDA as the ratio of the reference scenarios.The results are based on our previous LCA studies regarding waste polyethylene (PE), crumb rubber (CR), recycled concrete aggregates (RCA) and electric arc furnace slag (EAFS) (0,0,0) (Piao et al., 2022a;Piao et al., 2022b;Piao et al., 2022c).It can be seen that the incorporations of waste PE and EAFS in SDA reduce the GHG by 15% and 35%, respectively, compared to the reference scenarios.But the SDAs with CR and RCA have no advantages in GHG emissions.In these studies, the reference scenarios also consider the original waste streams, which are landfilling (for RCA and EAFS), and alternative fuels for clinker production (using CR) and solid waste municipal incineration (using waste PE), if not used in asphalt.This aims to expand the LCA system to include the impacts of waste management so that the potential burden shift from asphalt pavement to other industries can be identified.

Conclusions
This article summarised the experience of using various types of waste materials for use on roads.The technical readiness level for use of these materials varies greatly due to a variety of reasons such as lack of incentives, lack of expertise and lack of standards.To lift some of the barriers for use of alternative materials on roads there needs to be an effort to disseminate the knowledge gained.The discussion presented in this paper can pave the path to a contribution from the asphalt community to a zerowaste society.The results can be summarised as follows: 1. Waste and marginal materials can be used to substitute all virgin materials either partially or completely in road construction.The materials investigated here could reach, in most cases, 80% of the performance of semi-dense pavements.2. The type of waste material might warrant mixed design adjustments such as binder content and selection of substitution fraction.For example, it was necessary to increase the binder content of the crumb rubber-modified mixtures to account for the absorption of binder by rubber.3. Depending on the wavelength, low-noise semi-dense asphalt pavements with alternative materials can have similar acoustic performance as ones fabricated using all virgin materials.4. To facilitate their use incentives, guidelines and standards would help. 5. To determine if the use of waste and marginal materials is a viable option a complete life-cycle assessment needs to be done.This should consider the locally relevant waste management practice.6.Any potential harmful effects to the environment need to be investigated such as through standardised leaching experiments or measurements of emissions in asphalt plants and comparison to defined limits.
The discussion presented in this paper can pave the path to a contribution from the asphalt community to a zero-waste society.
(asphalt mixing plant) which is gratefully appreciated.The following companies provided the waste materials for this project and are gratefully acknowledged: FBB (RCA and PmB bitumen), Tyre recycling Solutions (CR), Innoplastics (PE) and RecyPET (PET), Q8 (70/100 bitumen), MacRebur (mixed plastic waste).The authors acknowledge the contribution of Peter Mikhailenko to the contents of the paper including data acquisition and analysis.The authors confirm contribution to the paper as follows: Conceptualization: L.D.P. started the main concept of studying and the use of waste materials.Supervision: L.D.P supervised the study and provided critical feedback.Methodology, validation, formal analysis, investigation, and visualization: Z.P., M.K. fabricated and characterized the materials, Z.P., M.K. and L.D.P. designed and conducted the experiments, Z.P., M.K., L.D.P. analyzed the data.Z.P. designed the LCA experiment and Z.P conducted the LCA experiments and produced the figures.Writing, original draft: L.D.P wrote the initial draft of the paper Z.P., M.K. gave feedback and contributed to the revisions of the paper.All authors have read and approved the final version of the paper.Funding acquisition: L.D.P. acquired financial support for the project leading to this publication.
(1) Innovation in Sorting and Recycling Technologies for Efficient Treatment of Construction & Demolition Waste; (2) Growing Adoption of Construction and Demolition Waste Materials in Urban Areas; (3) Green Building System Promoting Reuse and Recycling of Concrete Elements and Aggregates.The Challenges are listed as (1) Problems Arising Due to Construction Dust; (2) Hindrances in Workflow Due to the Inefficiency of Municipal Bodies and Local Authorities (Construction & Demolition Waste

Figure 1 .
Figure 1.Gradation of the semi-dense asphalt (SDA) and top and bottom limits with 4 mm maximum aggregate size resulting in 16 vol% void content of the Marshall sample as defined by the Swiss standards (SN 640 436; 2013).

Figure 4 .
Figure 4. CR-modified binder performance: G * at high and low temperatures.

Figure 5 .
Figure 5. PES (IP)and CR binder performance: viscosity at high temperature.

Figure 8 .
Figure 8. Texture level as a function of wavelength.

Figure 9 .
Figure 9. Greenhouse gas (GHG) emission index for using different waste materials in SDA as the ratio of ref., based on various system boundaries considering original waste treatments and valorisations.

Table 1 .
Waste materials and their TRL levels.

Table 2 .
Mixture designations and performance.Mixture performance as ratio with respect to control based on different test methods.