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Volume : 44

Part : 2

Proceedings of the Institute of Acoustics

 

 

Low-noise friction courses containing treated and un-treated crumb rubber to mitigate tire/road noise in urban contexts

 

Filippo Giammaria Praticò1, University Mediterranea of Reggio Calabria, Reggio Calabria, Italy

Rosario Fedele2, University Mediterranea of Reggio Calabria, Reggio Calabria, Italy

 

ABSTRACT

 

Tire/road interaction is one of the main causes of traffic noise. This generates health, social, and environmental issues. Bituminous mixtures containing crumb rubber, CR, and with low-nominal maximum aggregate size, NMAS, can be used to mitigate the aforementioned issues in both the short and long period. The main objective of the study presented in this paper is to investigate the variation of volumetric, surface, mechanical, and acoustical properties of friction courses due to the presence of treated and un-treated CR. Low-noise mixtures were designed during the ongoing project “E-VIA” (LIFE18 ENV/IT/000201) and were used to pave a street in Florence. In the laboratory, samples were created using the Superpave Gyratory Compactor (AASHTO T-312, UNI EN 12697-31). Specimens with NMAS=6 mm and bitumen in the range 6-7% were used as a reference. Other two sets of specimens were created adding treated and un-treated CR (dry method). Results show how the different composition affects the properties and performance of the mixtures under investigation. Future studies will include the comparison between the in-lab produced specimens (herein analyzed) and the cores extracted from the aforementioned street.

 

1. INTRODUCTION

 

For both traditional Internal Combustion Engine Vehicles, ICEVs, and Electric Vehicles, EVs, road/tire interaction is one of the main causes of traffic noise [1,2]. This generates health, social, and environmental issues [3–5] especially in urban contexts.

Over the past decades, several studies about process-, technology-, and mixture-related studies were carried out, dealing with Crumb Rubber (CR) and its effect on the performance of construction materials (especially asphalt concretes, cf. [6–11]):

  • From a chemical point of view studied focused on optimal CR percentage, CR-bitumen mixing time, binder viscosity and mixture swelling.
  • From a mechanical and functional point of view, studies focused on the effect of CR on the expected life of ACs (i.e., temperature and water sensitivity, permanent deformation, fatigue resistance, and ageing), on friction, on surface micro- and macro-texture, on elastic modulus, and Frequency Response Functions, FRFs, such as dynamic stiffness and mechanical impedance.
  • From a production point of view, studies focused on the differences between the two main production technologies, i.e. the dry and the wet process, and the improvement of these methods, including storage stability.

 

The on-going project LIFE E-VIA (start: 2019; end: 2023; LIFE18 ENV/IT/000201) [12–15] aims at finding and implementing possible solutions to the problems mentioned above. It aims at using asphalt concrete mixtures containing CR to increase the sustainability of transportation infrastructures (i.e., reducing Life Cycle Cost with respect to actual best practices), and at reducing urban traffic noise produced by both ICEVs and EVs. In more detail, the expected reduction of the traffic noise level was quantified in terms of 1) Noise level measured applying the Close Proximity method at 50kph (i.e., LCPX,50) that should be below 87 dB(A) for the road containing CR (called “optimized track”) and below 90 dB(A) for that without CR (called “reference track”). 2) Noise reduction at the source in comparison with the common old existing pavements (old dense asphalt concrete, DAC, mixtures) of at least 3 to 4 dB(A), which is higher than the gain deriving from having a common new dense-graded pavement (at least 2 dB(A)). 3) Lden and Lnight (cf. 2002/49/CE) reduction of at least 5 dB(A) at receivers living at road side. To this end, more than 150 asphalt concrete mixtures were taken into account (cf. deliverable A.2 of the E-VIA project [15]), and an asphalt concrete mixture with Nominal Maximum Aggregate of 6 mm (AC6) was selected as the best potential low-noise mixture. It is expected an A-weighted noise level (close proximity method, CPX) lower of about 3-4 dB(A) in comparison to other traditional semi-dense and dense mixtures [15,16]).

 

A first in-lab characterization of the AC6-based mixture with and without CR was carried out in a previous study [14]. Based on the results of this latter study, a test track was built in France [13], and the following results were observed: 1) Noise reduction of about 3 dB(A) (LCPX,50 vs. dense asphalt concrete, DAC 0/10 using different tires), and of about 4 dB(A) (controlled pass-by, CPB, vs. DAC 0/10 using EVs). 2) Macro-texture (Mean texture Depth, MTD) of about 0.43 mm for the pavement without CR, and of about 0.52 mm for the one with CR. 3) Reduction of the maximum texture levels of about 3dB (solution without CR) and of about 8 dB (solution with CR), for wavelengths close to 8-16 mm. These results partially confirmed the aforementioned target of the E-VIA project.

 

The following table reports noteworthy examples of projects and studies related to the mitigation of the traffic noise.

 

Table 1: Traffic-noise mitigation projects and studies.

 

 

The remaining part of the paper contains the main objectives and scopes of the study (section 2), the main results of the study and the related discussions (section 3), and the main conclusions (section 4) that are followed by the sections Acknowledgements (section 5) and references (section 6).

 

2. OBJECTIVES

 

The main objectives of the study presented in this paper are to investigate the effect of treated and un-treated CR on volumetric, surface, mechanical, and acoustical properties of low-noise mixtures, which were designed during the ongoing project “E-VIA” (LIFE18 ENV/IT/000201).

 

3. RESULTS AND DISCUSSIONS

 

Several samples were created using the Superpave Gyratory Compactor (UNI EN 12697-31, 130 rotations, dry method, cf. Figures 1-5). The first set of specimens are the reference ones (see Ref in Figure 2), the other two sets were created adding treated (TCR) or un-treated (UCR) crumb rubber (cf. Figure 2). NMAS ranged from 6.4 mm to 7.2 mm. The percentage of bitumen (Pb) used in all the specimens is about 6 %;

 

 

Figure 1: Gradation curves of the aggregates and CRs used in this study.

 

Note that the following figures show the percentage variations of the main parameters with respect to the ones of the reference mix (Ref).

 

 

Figure 2: Volumetric properties.

 

Based on the results reported in Figure 2, it is possible to observe that: 1) The specimens containing CR have quite similar maximum specific gravity (Gmm; [24,25]), while the reference one has a slightly higher Gmm. 2) For bulk specific gravities (Gmb; dimensionless; [23,24) derived using the corelok machine (GmbCOR; dimensionless; ASTM D7063/D7063M) the specimens with CR have Gmbs lower than the ones of TCR-added mixtures. 3) For the effective air void content, note that it is herein termed neff, porosity measured using the corelock machine, sometimes termed ΩCOR, dimensionless, determined according to ASTM D7063/D7063M. neff undergoes minor variations when adding TCR, while it yields an appreciable increase for UCR-added specimens. This affects the consequent value of VMACOR and VFACORB (Voids in the mineral aggregates, and Voids filled with asphalt, cf. [24]), both derived using the specific gravity GmbCOR. Figure3 reports the main results related to surface and acoustic properties. In more detail, surface properties are expressed in terms of Pendulum Test Value (PTV, dimensionless, EN 13036-4, micro-texture) and the Mean Texture Depth (MTD, mm, EN 13036-1, macro-texture). The acoustic properties have been evaluated using the average values of the sound absorption coefficient (a0; dimensionless; Kundt’s tube; ISO 10534-2) derived in the three frequency ranges reported in Figure 3, and the air flow resistivity (r; N⋅s/m4, ISO 9053-2).

 

 

Figure 3: Surface (PTV and MTD) and acoustic (a0_1, a0_1, a0_1, and r) characteristics.

 

The following trends related to the variation of surface and acoustic properties due to the presence of the two types of CR can be observed (cf. Figure 3): 1) Micro-texture (PTV) may slightly decrease (this complies with the literature, cf. [22]). 2) For UCR, macro-texture (MTD) increases (this complies with [22]). 3) While the sound absorption (a0) of the specimens with UCR decreases, the sound absorption of the specimens containing TCR increases, and shows a peak for 670-1132 Hz, which is appreciable for the mitigation of the traffic noise. Further investigations are needed for TCR-added mixtures because of the unexpected behavior. 4) The air flow resistivity (r) decreases and this is a good result for the mitigation of noise.

 

 

Figure 4: Mechanical properties derived using (a) the Marshall test, and (b) the impact hammer test.

 

Figure 4 shows the effects of CRs on the mechanical properties of specimens. In particular, 1) For Marshall stability (MS, kg, cf. [24]), it decreases, which partially complies with the literature (cf. [26,27]). 2) The lowest average MS value refers to UCR-added specimens. 3) For the Marshall Flow (MF, mm, cf. [24]) of the specimens with UCR, it increases. 4) The corresponding Marshall Quotient (MQ, kg/mm, cf. [24]) is affected by the high MF value related to the UCR-added specimens and decreases. 5) Overall, the dynamic stiffness decreases (K1, …, K6, impact hammer test,see frequency ranges in Fig. 4). The increase of MQ and K, together with a0 increase, if linked to the increase of damping properties, could be a good result in terms of mitigation of air- and ground-born noise, and ground-borne vibrations in urban contexts (cf. SNEAK project in Table 1).

 

4. CONCLUSIONS

 

In this study the consequences deriving from the addition of treated and untreated crumb rubber to bituminous mixtures were analyzed. Results show how treated and un-treated crumb rubbers affect the properties and performance of the mixtures under investigation:

  • For volumetric properties, UCR-added mixes seem to undergo an appreciable increase of air voids, while further investigations are needed for TCR-added ones.
  • By referring to surface properties, slight variations can be detected in terms of friction. Here further investigations are needed. At the same time, the appreciable increase of air voids for UCR-added mixes implies higher absorption coefficients, lower resistivity, and higher macro texture. Uncertainties still remain for TCR-added mixtures, where further research is needed.
  • By referring to mechanical properties, the mixtures with treated crumb rubber overrate the ones with untreated crumb rubber. This can be noted based on MS, MQ, and K(200-600Hz). At the same time, minor differences seem to emerge between TCR-added and reference mix tures. Again, due to a number of factors (e.g., low percentage of CR and unsatisfactory sample size), further research is here needed.

 

Further research is needed because of the uncertainties above and the need for sample size increase. These studies will include the comparison between the in-lab and on-site results.

 

5. ACKNOWLEDGEMENTS

 

The authors would like to thank all who sustained them with this research, especially the European Commission for its financial contribution to the LIFE18 ENV/IT/000201 LIFE “E-VIA” Project into the LIFE2018 programme, and the Italian Calabria Region (PAC Calabria 2014–2020).

 

6. REFERENCES

 

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1 filippo.pratico@unirc.it

2 rosario.fedele@unirc.it

Corresponding author: rosario.fedele@unirc.it