PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Use of waste and marginal materials for silent roads
Lily POULIKAKOS1, Sahand ATHARI1,2,Peter MIKHAILENKO1, Zhengyin PIAO1,2, Muhammad
Rafiq Khan KAKAR1, Moises BUENO1, Reto PIEREN1, Kurt HEUTSCHI1
1Empa, Swiss Federal Laboratories for Materials Science and Technology, Switzerland
2ETHZ, Swiss Federal Institute of Technology, Switzerland

ABSTRACTInhabitants of the urban environment are exposed to high levels of traffic noise and increase in amounts ofwaste. One way to mitigate their exposure to noise is to take advantage of the existing public spaces between
buildings and transform the available surfaces of pavements into elements that foster comfort and health forurban residents.
This work aims to develop low noise pavements by using certain waste materials. Semi-dense asphalt (SDA)
mixtures (Air Voids = 12-16%) is used in this study due to their acoustic properties. Part of the aggregate
matrix incorporates crumb rubber (CR). The various mixtures produced are compacted into 100mm dimeter
specimens and subjected to texture measurements by 3D laser profilometry, porosity measurements andsound absorption measurements by the impedance tube method (EN 10534-2). The SDA mixtures showed ahigher surface macrotexture amplitude, MPD. They also showed a more negative surface profile Rsk,
indicating better noise properties based on the shape. This study gives us an understanding on how wastematerials impact the texture and acoustic properties of roads and ultimately, to what degree they can beincorporated in low-noise asphalt mixtures.
Keywords: Semi Dense Asphalt; Low Noise Pavements, Crumb Rubber

1. INTRODUCTION
Greater demands on the road transport infrastructure as a result of economic growth havemanifested themselves in an increase in the number of vehicles worldwide. This increase is inherently
accompanied by increase in congestion, noise, energy use and gaseous and particle emissions as wellas an increase in the infrastructure overuse. A recent report by the World Health Organization (WHO)indicates that in the EU and Norway, traffic noise is the second biggest environmental problem
affecting health after air pollution (1).
A review by the authors has demonstrated how a considerable amount of waste produced in theurban and peri-urban environment can be recycled in asphalt roads. It was shown that various wastematerials such as glass, asphalt, concrete, wood, tires, plastics etc. have technically a potential for
re-use in asphalt roads. The available quantities of the European target waste materials that would
otherwise be incinerated or disposed of in landfills were consider ed. It was shown that there is highpotential in Europe for recycling in road construction, in particular, under the hypothetical scenario
where 33% of new roads would be made of the target waste materials (excluding reclaimed asphalt
pavement RAP which is already recycled), it is estimated that 16% of the available waste quantities
could be recycled in roads. Four hypothetical roads were analyzed showing a considerable savings incosts, CO2 and energy in comparison to conventional asphalt mixtures using all virgin components (2).
However although technically viable, only those waste materials should be considered that their use in
the roads are also environmentally superior to alternative uses in other applications. A viable example
is the amount of crumb rubber from old tires that is expected to continue to increase and be available
in the near future, and the best use of this material may indeed be road construction. Figure 1 shows
effects related to vehicle and surroundings during driving, such as noise, rolling resistance, and tire
wear, plotted against texture wavelength. It can be seen from this illustration that lower macrotexture
have positive effects on road noise and the opposite effect on rolling resistance that is a safety
consideration. Introduction of rubber in mixtures, including ground tires, can also contribute towards
reduction of traffic noise by 2.5 to 4 dB (3). Development of low noise pavements specifically for the491

urban environment considering the frequencies and loading of such pavements is lacking. InSwitzerland the semi dense mixtures (SDA) have shown promising results (4). These are special typeof gap graded mixtures with high porosity up to 18%.

Figure 1 Illustration of texture wavelengths, anticipated effects showing that noise is affected bymacrotexture (4)One potential urban waste material that has been shown to have low noise properties is crumbrubber (CR) from used tires (3). This project aims to investigate the low noise properties of CR
modified asphalt concrete as potential low noise pavement.

2. MATERIALS
Two semi dense asphalt concrete mixtures with maximum aggregate size of 4 and expected porosityof 14% to 18% (SDA4-16) were produced in the lab. The gradation of both is shown in Figure 2.
According to the standard, conventional polymer modified bitumen was used to prepare the reference
mixture (SDA4-PmB). For the experimental one, crumb rubber produced by mechanical grinding withsize <800μm was used as additive in a mixture with non-modified bitumen. The overall void content(connected and not connected) determined for Marshall cylindrical samples (100mm dimeter; ca60mm height) of the two mixtures was 13.2 and 13.8 for SDA 4-CR and SDA4-Pmb respectively.

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Figure 2 Aggregate size distribution of SDA-4 with polymer modified bitumen and with straight run bitumen
50/70 and 1% CR

3. METHODS
3.1 Mixture and Sample Production
All mixtures were produced in the laboratory using polymer modified bitumen and straight runbitumen with 50/70 penetration grade with crumb rubber modification. 1% crumb rubber was addedusing the dry process to the mixture. In the dry process CR is added to the hot aggregates before theaddition of bitumen. After the mixing process, the mixtures were conditioned for 120 minutes at 160°C.
This conditioning time is important for the crumb rubber to interact with the bitumen. The samples
were compacted using the Marshall hammer with 50 blows per side.
3.2 Mechanical Tests
The tensile strength of the specimen was determined using the indirect tensile test according to theEuropean standard (EN 12697-23). In this test, the load is applied to a cylindrical specimen (100mmdimeter, 60mm height) loaded diametrically until failure. The experiments were performed at room
temperature ca 23°C.
3.3 Surface Texture
The surface texture of the asphalt mixture samples was measure by an Ames Engineering 9400HD
3D laser scanner. The tests were conduction on one Marshall sample of each type: dense asphalt with
about 5% pores, semi-dense asphalt (SDA 4 Pmb), and semi-dense asphalt with 1% crumb rubber(SDA 4 CR); both with about 13% pores. The test was conducted by placing the sample horizontally
under the device and conducting two 50x50 mm area scans in two random directions. The resolution s
were 0.005 mm vertically, 0.00635 mm along the length of the scan and 0.02469 mm for the width.
2000 scan lines were conducted for each area scan, for a total of 4000 scan lines for each sample.
3.4 Acoustic Absorption
Three specimens from three types of mixtures were tested in the impedance tube based on ISO EN
10534-2 (7). This procedure measures the normal incidence sound absorption coefficient using thetransfer function method. In these tests one sample of each mixture type was used: SDA4-Pmb
SDA4-CR. Furthermore, a dense asphalt specimen with less than 5% air void content was evaluated ascontrol specimen to analyze the effect of the porosity on the sound absorption. The testing was493

conducted on the same samples as the surface texture. Figure 3 shows these specimens. Each sample
was tested three times with about 120 degrees of rotation. The produced plots are the average of thesethree measurements for each sample. Figure 4 shows the test rig using the B&K two-microphone
impedance measurement tube type 4206.

Figure 3 Specimens used for acoustic tests a)Dense asphalt, b)SDA4 Pmb, c)SDA4 CR

Figure 4 Acoustic test setup showing the impedance tube and data acquisition devices
Initially, the SDA4 Pmb and SDA4 CR specimens were 99mm in diameter, therefore an ai r gap
existed between the specimen and the inner wall of the impedance tube (which is 100mm). The results
obtained in this condition contained an artefact at about 200 Hz to 300 Hz. The tests were repeatedwith the same specimens but this time sealed and fixed using masking tape around the perimeter of thecylindrical specimens. In this condition, the artefacts were not present in the measurement results. The
difference between the results in these two conditions emphasizes the importance of fitting thespecimens correctly in the impedance tube to avoid artefacts.

4. RESULTS
4.1 Mechanical Tests
Figure 5 shows the results of three repetitions for the indirect tensile test on the CR modified
mixture and the conventional mixture with Polymer modified binder. As shown in Figure 5, the CR
modified mixtures could withstand slightly higher loads in comparison to the polymer modified
mixtures.

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Figure 5 Maximum force to mechanical failure as a result of the indirect tensile tests
4.2 Surface Texture
Based on the 3D laser scans, a number of texture parameters were calculated as shown in Table 1.
These parameters were calculated on the Macrotexture as defined in ISO 13473 -1 (8), where only thetexture wavelengths between 0.5-50 mm were considered. The pavement macrotexture has been shown
to be critical to pavement noise (4). Mean Profile Depth (MPD) is a measure of the depth of the profile
deviation from the mean. Rsk is the skewness value of the depths, which is an indication of whether
there is a “positive” and “negative” surface texture. Rku, the kurtosis parameter indicates the presenceof disproportionately high peaks or deep valleys (9). Finally the length and area ratios indicate thelength of the profile versus the horizontal axis, which is a simple indication of roughness.
Table 1 Macrotexture Parameters for Asphalt Mixture Samples
MixtureDense

Air VoidsMPDRskRku

LengthArea%mmmmmm

Ratio

Ratio

50.461-0.224

0.188

1.016

1.031

0.132

0.577

1.204

0.008

0.000

0.615

-0.586

0.420

1.036

1.070

0.145

0.483

1.178

0.015

0.000

0.519

-0.311

0.324

1.023

1.047

0.146

0.606

0.883

0.011

0.000

Std Dev
SDA 4 PmB

13

Std Dev
SDA 4 CR
Std Dev

13

The texture result show higher amplitude of macrotexture for the SDA mixtures in terms of MPD,
as found with previous studies with more pervious asphalts (9,10). This can primarily be attributed tothe higher content of voids for SDA causing an increased quantity of cavities at the surface. This
increase in macrotexture or generally higher roughness is also visible from the higher length and area
ratios for the SDA mixtures. The lower amplitude in the SDA 4 sample with CR is not clear, and could
be attributed to the variation sample to sample, so more robust testing is needed, although some studies
have also found a lower MPD with CR (11).
The Rsk results showed negative skewness for all of the samples, with the SDA samples morenegatively skewed. This negative skewness indicates the presence of deep valleys in the samples (12),and this type of surface profile has been shown to cause less vibration and therefore less noise duringservice (13). The R ku kurtosis values were less than 3 for all of the samples, indicating a more bum py
as opposed to spikey surface. This is an indication of better resistance to wear (12). In this parameter,
the dense asphalt has a lower value, which indicates better wearing properties.
The standard deviations for the MPD, R sk and Rku values are quite high due to having been derived
from a series of small segments from 0.5-50mm. The length and area ratios have very little variability
due to having been derived from the whole length segments or areas.

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4.3 Acoustic absorption
Figure 6 shows the values of absorption coefficient for the sealed and non-sealed conditions on a
vertical axis from 0 to 0.2. The horizontal axis shows the frequency spectrum from 63 Hz to 2000 Hz.
As mentioned before, the artefacts at 200 Hz to 300 Hz seem to have been occurred due to the air gap
around the sample and passage of acoustic energy to the back of the specimen.
The SDA4_CR sample has slightly higher absorption coefficients than SDA4_Pmb. Thisdifference reaches 0.015 at its maximum which considering the uncertainty in the measurements arenearly identical.

Figure 6 Absorption coefficient of two SDA specimens in two conditions
Figure 7 shows the results of the impedance tube test for the two specimens for both sealed andnon-sealed conditions together with the results for the dense asphalt. The absorption coefficient for a
75mm-thick foam sample is also included to give a reference for an effective porous absorber material.
As it can be observed, regardless of the medium air void contents, the range of absorption
coefficients of these asphalt samples is considerably small along the frequency spectrum, i.e. always
below 0.1.

Figure 7 Absorption coefficient of the three asphalt samples in comparison to a highly absorbent foam sample
Figure 7 shows the difference between the absorption coefficient of the three specimens SDA4_CR,
SDA4_Pmb and dense asphalt for the correctly sealed condition.

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Figure 8 Absorption coefficient of SDA4_CR; SDA4_Pmb and dense asphalt
As illustrated on Figure 8, the absorption coefficient of both semi-dense asphalts is only slightly
higher than dense asphalt. As a quantitative example, the ratio of the absorption coefficient ofSDA_CR to that of dense asphalt changes from a minimum of 1.7 to a maximum of 8.1. This meansthat SDA_CR is at least 1.7 time more absorbing than dense asphalt. However these preliminary
measurements show that SDA4 is not an effective absorber and its low noise properties are due to othercharacteristics.

5. SUMMARY AND CONCLUSIONS
The mechanical and acoustic performance of semi dense asphalt with and without crumb rubber
waste from tires were tested and compared. The results show that crumb rubber modification does notcompromise the mechanical performance of the mixture in terms of strength.
The surface texture properties indicate higher macrotexture and overall texture of SDA mixtures
relative to the dense asphalt. The SDA CR sample had a lower profile than the SDA PmB sample, butmore testing is needed to find if this is a trend. The samples also showed a negative skewness,indicating better shape properties for noise. All of the samples tested showed a bumpy (as opposed tospikey) profile, indicating better resistance to wear, although the dense asphalt had the best result withthis parameter.
The acoustic absorption tests show that the absorption coefficient of SDA is considerably higher
than the dense asphalt, however both materials are not effective absorbers.
The semi-dense asphalt with crumb rubber has similar absorption coefficients compared to the one
without crumb rubber. On that basis, sound absorption does not seem to be the dominant noise
reduction mechanism when incorporating crumb rubber in the asphalt mixture.
It is probable that other mechanisms of tyre-pavement noise generation such as air pumping andtyre vibrations can contribute more to the acoustic effectiveness of asphalt mixtures with crumb rubber.
Measuring other properties of asphalt mixtures such as mechanical impedance, surface texture a nd
porosity can help obtaining better expressions for the difference in the acoustic performance of themixtures. The future work will be focused on measuring these properties and predicting noisereduction using more parameters than the absorption coefficient.

ACKNOWLEDGEMENTS
This research project is funded by a grant from the Swiss National Science Foundation grant number
205121_178991/1.

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