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Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04

Main author: Ulf Sandberg. Swedish Road and Transport Research Institute (VTI) I

MIRIAM

_____________________________________________ Models for rolling resistance In Road Infrastructure Asset Management systems

Road surface influence on tyre/road rolling resistance

Authors: Ulf Sandberg, Swedish National Road and Transport Research Institute (VTI)

Anneleen Bergiers, Belgian Road Research Centre (BRRC) Jerzy A. Ejsmont, Technical University of Gdansk (TUG)

Luc Goubert, Belgian Road Research Centre (BRRC) Rune Karlsson, Swedish National Road and Transport Research Institute (VTI)

Marek Zöller, The Federal Highway Research Institute (BASt) Deliverable # 4 in MIRIAM SP 1

Deliverable Version, 2011-12-31

Document type and No. Report MIRIAM_SP1_04 Sub-project SP 1 Measurement methods and source models Author(s) Ulf Sandberg (a), Jerzy A. Ejsmont (b), Anneleen Bergiers (c), Luc Goubert

(c), Marek Zöller (d), Rune Karlsson (a) Authors' affiliations (acronyms) (a) VTI, (b) TUG, (c) BRRC, d (BASt) Contact data for main author [email protected] Document status and date Deliverable Version 111231 Dissemination level Public File Name MIRIAM_SP1_Road-Surf-Infl_Report

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mailto:[email protected]


Main author: Ulf Sandberg. Swedish Road and Transport Research Institute (VTI) II


Main author: Ulf Sandberg. Swedish Road and Transport Research Institute (VTI) III

Foreword MIRIAM, an acronym for "Models for rolling resistance In Road Infrastructure Asset Manage-ment systems", is a project started by twelve partners from Europe and USA. They have collectively contributed internal and external funding for this project. The managing partner is the Danish Road Institute. The overall purpose of MIRIAM is to provide information useful for achieving a sustainable and environmentally friendly road infrastructure. In this project, the focus is on reducing the energy consumption due to the tyre/road interaction, by selection of pavements with lower rolling resistance – and hence lowering CO2 emissions and increasing energy efficiency. MIRIAM has been divided into five sub-projects (SP). The work reported here has been made within SP 1 "Measurement methods and surface properties model". A first phase of the project has included investigation of pavement characteristics, energy efficiency, modelling, and raising awareness of the project in order to secure economical and political support for a second phase. The second phase will focus on development and implementation of CO2 controlling models into the road infrastructure asset management systems. The website of MIRIAM is http://www.miriam-co2.net/ where extensive project information can be found. The order of authors on the title page, following the main author Ulf Sandberg, is alphabetical and is not related with the extent or importance of the co-authors' contributions. This report is the fourth Deliverable of SP 1. The Deliverables of Phase 1 are the following:

Deliverable 1: “Rolling Resistance – Basic Information and State-of-the-Art on Measurement methods” Deliverable 2: "Rolling Resistance – Measurement Methods for Studies of Road Surface Effects" Deliverable 3: “Comparison of Rolling Resistance Measuring Equipment - Pilot Study" Deliverable 4: “Road surface influence on tyre/road rolling resistance"

See the list of references for where the reports may be downloaded.

http://www.miriam-co2.net/


Main author: Ulf Sandberg. Swedish Road and Transport Research Institute (VTI) IV

Acknowledgements and disclaimer It is gratefully acknowledged that the studies reported here and the production of this report have been funded by the following organizations (in alphabetical order only):

• Belgian Road Research Centre (BRRC) • Pooled funds of project MIRIAM • Swedish National Road and Transport Research Institute (VTI) • Swedish Transport Administration (STA) • Technical University of Gdansk (TUG), Gdansk, Poland • The Federal Highway Research Institute (BASt)

The funding organizations have no responsibility for the contents of this report. Only the authors are responsible for the contents. Any views expressed are views of the authors only.


Main author: Ulf Sandberg. Swedish Road and Transport Research Institute (VTI) V

TABLE OF CONTENTS SUMMARY VII 1 INTRODUCTION 1 2 PURPOSE, LIMITATIONS AND CONCEPT 2 3 TEXTURE AND ROAD UNEVENNESS RANGES AND COMMONLY USED PARAMETERS 3

3.1 Texture and road unevenness ranges 3 3.2 Commonly used measures describing the road surface 5 3.3 Positive and negative textures (skewness) 7 3.4 Tyre tread enveloping of texture 7 3.5 Other road parameters having a potential influence on rolling resistance 8

4 RESULTS OF MEASUREMENTS BEFORE 2000 9 4.1 Swedish measurements of texture effects 9 4.2 Swedish measurements of road condition effects 10 4.3 Belgian measurements of texture and unevenness effects 11 4.4 French measurements 12 4.5 German measurements in the 1990’s 14 4.6 New Zealand measurements in the 1990’s 15 4.7 Other early measurements 16 4.8 Discussion 17

5 RESULTS OF LABORATORY DRUM MEASUREMENTS 18 5.1 Measurements at Dunlop in the UK around 1980 18 5.2 Measurements at BASt in Germany in the 1990's 18 5.3 Measurements at TUG in Poland around year 2000 21

6 RESULTS OF TRAILER MEASUREMENTS IN SWEDEN 2007-2011 22 6.1 Introduction 22 6.2 Test (reference) tyres 23 6.3 Tested road surfaces 24 6.4 Pilot tests with RR trailer 24 6.5 Further tests with RR trailer – Macrotexture influence 24 6.6 Special effects – porosity 27 6.7 Special effects – stiffness 29

7 RESULTS OF COASTDOWN MEASUREMENTS IN SWEDEN 30 7.1 Coastdown measurements 30 7.2 Comparison of results obtained with other methods 31

8 SURVEY OF ROLLING RESISTANCE OF 40 DUTCH TEST TRACK SURFACES IN 2008 32 9 RESULTS FROM THE BELGIAN ARTESIS PROJECT 33

9.1 Background 33


Main author: Ulf Sandberg. Swedish Road and Transport Research Institute (VTI) VI

9.2 Correlation between RRC and texture 33 10 RESULTS FROM THE MIRIAM ROUND ROBIN TEST RELATED TO ROAD SURFACE INFLUENCE

ON ROLLING RESISTANCE 35 10.1 Introduction 35 10.2 Correlation between RRC and texture levels in third-octave bands 35 10.3 Correlation between RRC and macro- and megatexture levels LMa and LMe 36 10.4 Correlation between RRC and Mean Profile Depth (MPD) 38 10.5 Correlation between RRC and texture measures - Overall 40 10.6 Correlation between RRC and unevenness (IRI) 42

11 RESULTS OF MEASUREMENTS IN MINNESOTA 43 12 EFFECTS OF ASYMMETRIC PROFILES 44

12.1 Background 44 12.2 Work at TRL Ltd and Dunlop Tyres Ltd by Parry 44 12.3 Swedish tests in 2011 on polishing off the top of the surface 46 12.4 Results of tests in Minnesota in 2011 47 12.5 Results in the MIRIAM Round Robin Test (RRT) in 2011 47

13 INFLUENCE OF TYRES ON THE ROAD SURFACE EFFECT ON ROLLING RESISTANCE 48 14 OVERVIEW OF RESULTS 49

14.1 General 49 14.2 Macro- and megatexture levels (based on rms of profiles) 49 14.3 MPD 49 14.4 Enveloping 49 14.5 Unevenness and IRI 49 14.6 Texture spectral effects 50 14.7 Other pavement effects 50 14.8 Design of low rolling resistance pavements 50 14.9 The data reported here suggest that the most important texture range for generation of rolling Interactions

with vehicle type 50 15 CONCLUSIONS AND PROPOSED PRELIMINARY MODEL 51 16 RECOMMENDED FURTHER STUDIES 53 17 REFERENCES 54 A. Annex A: Asymmetric profile curves and enveloping procedures 56

A.1 Introduction 56 A.2 Asymmetric profile curves and skewness 56 A.3 MPD as a measure of asymmetry and its relation with skewness 57 A.4 Tyre tread enveloping of texture 57


Main author: Ulf Sandberg. Swedish Road and Transport Research Institute (VTI) VII

SUMMARY MIRIAM has established a sub-project (SP), designated SP 1, to deal with measurement methods for rolling resistance and related issues. This subject forms the most fundamental basis for the MIRIAM ambition to consider rolling resistance in pavement management or other types of infrastructure systems. Without robust measurement methods and equipment that can use them there will be no reliable data as input to such systems and the end result will be most uncertain, if useful at all.

In order to develop and study measurement methods, there must be a basic understanding of the influencing parameters as well as what energy losses that should be included in the concept of rolling resistance. These issues are, therefore, important parts of the work in SP 1.

This report is intended to provide basic knowledge about how and to what degree the rolling resistance is influenced by various fundamental road pavement parameters, such as texture, unevenness and stiffness.

For road management purposes one cannot rely on direct measurements of rolling resis-tance; it is better to develop a model by which rolling resistance can be predicted from collected road pavement data, the latter of which is already made to a large extent in many European countries. This report aims at providing data for such predictions that may be used in sub-project 2 of MIRIAM.

This report summarizes research so far made with regard to this subject, covering the time period from approximately 1980 to and including major studies in 2011. It focuses on the correlation between rolling resistance coefficients or fuel consumption and road surface parameters. The most important work for this purpose is a number of studies in Sweden since 2005 and a Round Robin Test (RRT) made within the first phase of MIRIAM, but there are also many other studies which contribute to the knowledge.

The results presented in this report show the following:

Rolling resistance is not only a property of tyres, but is also a property of the pavement which is of high importance for the energy consumption in the road transport sector and must be systematically considered along with other functional properties in pavement management systems.

As an example, in the MIRIAM RRT, the range of surfaces on the test track (MPD from 0.08 to 2.77 mm) the rolling resistance coefficient for the test tyres increased from the smoothest to the roughest of the surfaces by 21 - 55 %, depending on the tyre type. Such rolling resistance differences correspond to roughly 7 - 18 % in fuel consumption differences, using calculations made in SP 2 of MIRIAM for light vehicles driving on a typical two-lane highway at 90 km/h (to be published in January 2012).

The range in rolling resistance between the best and worst pavements in the MIRIAM partner countries in Europe is at least 50 % (the worst has an RRC 50 % higher than the best), although the more common pavements exposed to high traffic flows show a range of 20-25 % in rolling resistance.

Macrotexture, represented by the parameter MPD, is a major factor influencing rolling resistance. MPD is particularly suited for this purpose as it is sensitive to the vertical direction of the peaks and valleys in the profile curves.

Especially, MPD calculated on an enveloped profile curve seems to give excellent correlation with rolling resistance. It is so well correlated with rolling resistance that it will be difficult to find a better single or major variable for the purpose of quantifying the pavement influence on rolling resistance.


Main author: Ulf Sandberg. Swedish Road and Transport Research Institute (VTI) VIII

Megatexture level might be an alternative parameter, albeit not really as good as MPD, provided it is calculated on an enveloped profile curve. The advantage with this measure is that it is easier to measure by road survey vehicles using profilometers.

The relation between rolling resistance coefficients and MPD is rather consistent measured in different and independent measurement series reported here. The currently best estimate is a coefficient X of 0,0017 to 0,0020 in an equation of RRC = X*MPD + Y, where Y is a constant depending on a large number of factors. The coefficient 0.0020 might be an attractive option as it is easy to remember and to use.

There has been in the past, and to some degree still is, a substantial bias between various series of measurements made by presently available rolling resistance trailers, a "day-to-day" variation; the source of which is not yet known. But it is believed that temperature is part of the solution and that uncertain calibration might be another part of the solution.

It is proposed that a tentative source model for the pavement influence on rolling resistance contains the following significant pavement parameters:

MPD, IRI, pavement stiffness.

Of these three, the MPD and IRI are certainly needed, but the need for stiffness is yet a bit uncertain.

For light vehicles the IRI effect on rolling resistance is probably around 1/3 of that of the effect of MPD. It may be higher for heavy vehicles. Nevertheless, it shall not be neglected.

The best source model for the road surface influence is currently proposed to be:

Rolling resistance coefficient = Constant + 0.0020∙MPD + X∙IRI where MPD is Mean Profile Depth in mm, measured according to ISO 13473-1

and X is a constant yet to be determined

and "Constant" is a value unique to a certain tyre and several other circumstances; usually around 0.008 to 0.012 for light vehicles and approximately 50-60 % of that for heavy vehicles.

This simple model is useful over a speed range of at least 50-110 km/h for the rolling resistance part of the driving resistance. Suspension losses are included only if the IRI term above is specified by assigning a number to its constant "X".

The model is based on light vehicle data. For heavy vehicles, one may use the same model, scaled to representative values of Cr for heavy vehicle tyres, as long as no better model is available, but one must be aware that it is very uncertain for this category.

It is noted that MPD and IRI are collected widely in most countries already, at least for the national and regional road networks. Thus, the use of these variables for rolling resistance prediction will be easy to implement.

Data on pavement stiffness is not commonly collected, but in this case one may find proxy variables, such as a distinction between classes of pavements (cement concrete, asphalt concrete, non-paved surfaces, new and old pavements, temperature, etc).

In the future, it is recommended to develop an enveloping procedure that can be used inter-nationally to calculate more appropriate and relevant MPD values for rolling resistance purposes. The RRT enveloping procedure constitutes a good start.

The work with the rolling resistance property of pavements has only just started. It is a very young discipline and a lot more research is needed in the near future; not the least about measurement methods.

In the special chapter about Recommendations, several suggestions for urgent and important future research are presented.


Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI) 1

1 INTRODUCTION Rolling resistance is a form of energy loss caused by the interaction between a rolling tyre and the road surface. This functional property of road surfaces and also of tyres is one of the most important from both an economical and environmental point of view; something which is acknowledged widely with regard to tyres but which is most often totally neglected with regard to road surfaces. There seems to be the misconception that rolling resistance is a property of tyres alone, rather than an interaction in which both components are equally “guilty”, just like the cases of skid resistance or tyre/road noise. Road surfaces are traditionally selected essentially based on properties such as skid resistance, durability and cost; sometimes also ride comfort and tyre/road noise emission are considered. But rolling resistance is practically never considered. One reason why rolling resistance properties of road surfaces are hardly ever given any importance is the lack of practical measurement methods and thus a lack of data. Rolling resistance of tyres is measured on laboratory drums using ISO and SAE methods but to take these methods out on the road is virtually impossible. The lack of proper measurements has resulted in ignorance about the effect of road surface on rolling resistance. In order to develop and study measurement methods, there must be a basic understanding of the influencing parameters as well as what energy losses that should be included in the concept of rolling resistance. A report which intended to provide basic knowledge about the influence on rolling resistance of various parameters, suggest a definition of rolling resistance and provide some detailed state-of-the-art knowledge about the measurement methods and equipment that are useful for collecting rolling resistance data is already published within project MIRIAM [Sandberg (ed), 2011]. This report presents earlier as well as recent measurements of rolling resistance on a number of road surface types in various countries. Based on these data a relation between rolling resistance, as measured by the special trailers used in this project, and road surface parameters is suggested. This relation may be seen as a kind of source model; explaining the sources of the energy losses and its effects on rolling resistance.



2 PURPOSE, LIMITATIONS AND CONCEPT The overall purpose of project MIRIAM is to study the potential for saving energy and CO2 emissions by adding rolling resistance data in road surface management systems. The particular purpose of this report is to provide basic and up-to-date knowledge about the influence on tyre/road rolling resistance of various functional parameters of road surfaces. The concept behind this report is the following:

• Rolling resistance is one of the most important functional properties of road pave-ments, applicable to the entire road network, which means that road authorities need to have information about it and be able to control it

• The direct measurement of rolling resistance is very difficult and requires the use of rather advanced equipment and methodology, operated by very skilled and experienced staff. Consequently, direct measurement of rolling resistance is possible only on a very small part of the road network

• A more practical way of controlling rolling resistance for road management purposes than directly measuring it, is to predict it from road pavement parameters that are already collected for most of the road network, such as texture, unevenness, stiffness and road topography

• Therefore, this report has a focus on modelling the relation between rolling resistance and road pavement parameters, based on the present availability of relevant data.

With regard to limitations, it is important to note the following:

• Rolling resistance is an interaction between tyre and road, although for the purpose of serving MIRIAM, this project has its focus on the road surface contribution

• It is important to understand that the energy losses in other vehicle components than the tyres, mainly the tyre suspension system, may not be well measured by the trailer equipment used so far, although they may be due to road surface properties

• Air resistance of the tyres is not a parameter intended to be included in the relations studied here as it is not a road-related property.



3 TEXTURE AND ROAD UNEVENNESS RANGES AND COMMONLY USED PARAMETERS

3.1 Texture and road unevenness ranges The basis for description of road roughness (texture and unevenness) is the profile of the surface along lines (in this case) representing the rolling paths of vehicle tyres. The profile of the surface is described by two coordinates: one in the surface plane, called distance (the abscissa), and the other in a direction normal to the surface plane, called vertical displacement (the ordinate). The distance may be in the longitudinal or lateral (transverse) directions in relation to the travel direction on a pavement, or any direction between these extremes; although for rolling resistance, the longitudinal profile is the most important one. The transverse profile may have an influence on rolling resistance by the side-forces created when tyres roll on the slopes of a rut. “Texture wavelength" is a descriptor of the wavelength components of the profile and is related to the concept of the Fourier Transform of a time series. The profile may be studied in more or less detail, and the features of these will have different influences on the road/tyre/vehicle interaction. Figure 3.1 attempts to illustrate this.

(The vehicle)

"Single chipping"

Unevenness

Amplification ca. 50 times

Macrotexture

"Tyre/road contact patch"

Microtexture

Reference length:

"Short stretch of road"

Megatexture

"Tyre"



Figure 3.1: Illustration of the various scales of road roughness and their relation to the road/tyre/vehicle interaction.



As appears later, the texture and road unevenness are road surface properties that have major influences on rolling resistance. Therefore it is justified to examine these associated terms a little closer. The following is an adaptation from ISO 13473-5:2009. Texture, or pavement texture, is the deviation of a pavement surface from a true planar surface, with a texture wavelength less than 0.5 m. It is divided into the sub-ranges micro-, macro- and megatexture; see Figure 3.2.

Figure 3.2: Ranges in terms of texture wavelength and spatial frequency of texture and unevenness and their most significant, anticipated effects; from [Sandberg & Ejsmont, 2002]. Note that the figure and especially the range for rolling resistance is an estimation made approx. 10 years ago, well before this report was written. See the Conclusions chapter for a possible update of this range. . Microtexture is the deviation of a pavement surface from a true planar surface with the characteristic dimensions along the surface of less than 0.5 mm, corresponding to texture wavelengths up to 0.5 mm expressed as one-third-octave centre wavelengths. Macrotexture is the deviation of a pavement surface from a true planar surface with the characteristic dimensions along the surface of 0.5 mm to 50 mm, corresponding to texture wavelengths with one-third-octave bands including the range 0.63 mm to 50 mm of centre wavelengths. Megatexture is the corresponding deviations with the characteristic dimensions along the surface of 50 mm to 500 mm, corresponding to texture wavelengths with one-third-octave bands including the range 63 mm to 500 mm of centre wavelengths. Unevenness is the corresponding deviations with the characteristic dimensions along the surface of 0.5 m to 50 m, corresponding to wavelengths with one-third-octave bands including the range 0.63 m to 50 m of centre wavelengths. Texture spectra with texture wavelength scales will appear in later chapters of this report.



3.2 Commonly used measures describing the road surface A common way to quantify texture and unevenness is to filter the profile curve through different bandpass filters having passbands corresponding to the texture wavelengths shown in Figure 3.2 and defined in the text of the previous chapter and then to measure the rms (root-mean-square) output value of the filtered profile curve, using the unit [mm]. The measures in the various ranges may be distinguished by using the symbol aMi for micro-texture, aMa for macrotexture and aMe for megatexture, with values expressed in mm rms. The symbol "a" denotes "amplitude". However, it has been preferred in especially noise-related studies to calculate and use the logarithms of these linear measures, then labelled LMi, LMa and LMe, expressed in dB relative to 1 μm rms. One advantage of this is that in most practical studies, this will result in a statistical distribution of the values which is more normal (Gaussian) than when using the corresponding linear measures. Thus, here we have the following measures:

• Microtexture level, LMi • Microtexture level, LMa • Microtexture level, LMe

For the unevenness range, there is no special symbol commonly used, corresponding to aMa and aMe and the logarithm conversion is seldom used. For the very commonly used ranges macrotexture and unevenness, special measures have been standardised and are commonly used. For macrotexture we have two measures which are commonly used: Mean Texture Depth (MTD) and Mean Profile Depth (MPD). MTD is a measure developed in the middle of the 20th century, where a certain volume of sand (later replaced by glass spheres of 0.17-0.25 mm diameter) is spread out with a tool (a rubber pad, often an ice hockey puck) flush with the peaks in the surface into a circular patch on the road surface, the diameter of which is measured. From the patch diameter and the sand volume, the mean depth of the texture over this patch is calculated. This is called the "volumetric patch method", earlier known as the "sand patch method". MPD is a measured developed in the 1980's and 1990's with the intention to become a replacement of the MTD which could be measured by moving vehicles using lasers and laser sensors to record the profile curve, from which a two-dimensional representative of the three-dimensional patch may be calculated. The corresponding standard, ISO 13473-1, is currently being revised, and the new calculation procedure is illustrated in Figure 3.2. From two halves of a 100 mm long profile (two 50 mm long segments), the so-called Mean Segment Depth (MSD) is calculated. By averaging several such MSD values over a certain road section, the MPD is obtained. The actual calculation is more complex than this description says, so the ISO 13473-1 should be consulted if actual measurements are planned. Sometimes, the term Estimated Texture Depth (ETD) is seen. This is an estimation of the MTD from a measurement of the MPD, with a conversion equation appearing in ISO 13473-1. In the unevenness range, a special measure is the International Roughness Index (IRI). It is calculated using a quarter-car vehicle mathematical model, supposed to be driven at 80 km/h (50 mph), whose response is accumulated to yield a roughness index which is the accumulated slope of the profile curve per km of road, be it negative or positive, expressed in mm/km or m/km. Since its introduction in the 1980's IRI has become the road unevenness index most commonly used worldwide for evaluating and managing road systems. IRI is specified in the international standard ASTM E1926 – 08.



Peak level (2nd)

Mean Segment Depth (MSD) = Peak level (1st) + Peak level (2nd) 2

Segment depth (SD)

Baseline (100 mm)

Average level

( first half of baseline ) ( second half of baseline )

Peak level (1st)Mean Segment Depth (MSD)

- Average level

Figure 3.3: Illustration of the terms Segment, Baseline, Segment Depth (SD), and Mean Segment Depth (MSD) (SD and MSD are expressed in millimetres). In analogy with the macro- and megatexture levels mentioned above, one may filter the profile curve with narrower filters and calculate "spectral levels" in the corresponding pass-bands. The most common bandpass filters are one-third-octave bands. By using such filters one obtains a texture spectrum. A typical texture spectrum (in one-third-octave bands) is shown in Figure 3.4, also including two special octave band levels..

20

30

40

50

60

315 160 80 40 20 10 5 2,5Texture wavelength [mm]

Text

ure

prof

ile le

vel r

el. 1

mic

rom

etre

[dB

]

Fig 4-1 in 13473-5

630

L TX63L TX500

L Me = 44,6 dBL TX63 = 41,5 dBL TX500 = 41,6 dB

Figure 3.4: Example of one-third-octave band texture spectrum with indication also of the texture levels of the octave bands LTX500 and LTX63.



-10

-5

0

5

10

15

1,2 1,3 1,4

Distance (m)

Prof

ile h

eigh

t (m

m)

9

9,5

10

10,5

11

11,5

0,6 0,7 0,8

Distance (m)

Prof

ile H

eigh

t (m

m)

The level of each of the two octave bands is indicated by the level of the top line of each rectangle. Note also that the presented spectrum represents a pavement having a relatively low megatexture; in this case a dense asphalt concrete with maximum 10 mm chippings, in near new condition.

3.3 Positive and negative textures (skewness) A possible asymmetry of the profile, see Figure 3.5, should potentially have significant influence on the rolling resistance. A 'positive' texture (exhibiting protrusions) should show a significantly different behaviour in functional qualities, like skid resistance or noise genera-tion, than a negative texture (exhibiting depressions). To quantify such asymmetry, one may apply an analysis of the skewness, i.e. the third statistical moment of the quantity, to reveal this aspect of the profile. Skewness of the profile, rsk, is defined in ISO 13473-2 as the quotient of the mean cube value of the ordinate values Z(x) and the cube of the rms value, within an evaluation length ℓ, according to the equation:

( )

= ∫

0

d11sk xxZrms

r 33

Skewness is dimensionless. Skewness (or just "skew") is a measure of assymmetry of the amplitude distribution (in this case of the ordinate values). This indicates whether the profile curve exhibits a majority of peaks directed upward (positive skew) or downward (negative skew). For a normal distribution rsk is zero. Much more on this is presented in Annex A of this report.

Figure 3.5: Examples of surface profiles of positive macrotexture (left) and negative macrotexture (right). Skewness of the left profile would be positive (somewhat > 0) while it would be substantially negative for the right profile (<< 0).

3.4 Tyre tread enveloping of texture When a tyre runs on a textured road surface, it does not necessarily make contact with all points on the surface in its wheel path. This is, e.g., the case when the texture shows deep and irregular “valleys” (such as on porous asphalt) or deep and relatively regular “grooves" (such as on transversally grooved concrete). The tyre is said to be "enveloping" the part of the surface with which it is in contact.



It has been known already since the beginning of the 1990's that the fact that a tyre envelops only part of the surface of the pavement plays an important role for the prediction of tyre/road noise. As it is related with the way how the road texture deforms the tyre rolling over it, it should also be important for the aspect of rolling resistance. More or less complex ways of tyre enveloping of road surface texture have been developed and tried in various projects. In the so-called RRT study in MIRIAM, reported in [Bergiers et al, 2011], a simplistic enveloping procedure was tried, with substantial success. The effect of this procedure on a profile curve with a high negative skew is shown in Figure 3.6, as an example.

55

56

57

58

59

60

61

62

63

0,5 0,51 0,52 0,53 0,54 0,55 0,56 0,57 0,58 0,59 0,6

Distance [m]

Verti

cal d

ispl

acem

ent [

mm

]

Original profile curve

Enveloping with d* = 0,054 [1/m]

Figure 3.6: Example of an original profile curve and the resulting profile curve when the enveloping procedure used in the RRT has been applied. The pavement was porous asphalt with max. 6 mm chippings. Much more on the enveloping principle is presented in Annex A of this report.

3.5 Other road parameters having a potential influence on rolling resistance Other road parameters which potentially may influence rolling resistance include pavement stiffness, microtexture, road condition, and rutting. For these, please refer to Chapter 4 of [Sandberg (ed), 2011].

In this report pavement stiffness is not quantified; it is just explained as a difference due to the binder used (bitumen or cement). Road condition and rutting, as well as microtexture, are not addressed.



4 RESULTS OF MEASUREMENTS BEFORE 2000

4.1 Swedish measurements of texture effects In 1983, Sandberg at VTI and his colleagues made measurements of fuel consumption of a Volvo 240 car on 20 road surfaces, cruising at 50, 60 and 70 km/h, with a variation in surface type and texture that covered most of the Swedish range by that time. He considered the fuel consumption differences as approx ¼ of corresponding rolling resistance differences when transforming results to rolling resistance. Texture and "shortwave unevenness" (wavelengths 0.5 – 3.5 m) were measured by means of a mobile laser profilometer mounted in an exceptionally soft-suspended luxury car. The tyres were Pirelli Cinturato C3 175SR14. Results were published no earlier than in 1990 [Sandberg, 1990]. When correlating fuel consumption (FC) with macrotexture level in the wavelength range 2-100 mm, Sandberg obtained the results shown in the right part of Figure 4.1. However, results were better correlated when plotting FC versus shortwave unevenness; see the left part of the figure.

Figure 4.1: Relation between fuel consumption (FC) at 60 km/h and shortwave unevenness in the 0.6-3.5 m roughness wavelength range (at the left) and between FC and macrotexture level LMA in the 2-100 mm texture wavelength range (at the right). Diagrams scanned from [Sandberg, 1990]. The correlation coefficient between FC values averaged for the three speeds 50, 60 and 70 km/h and shortwave unevenness level LSU was 0.91 while it was 0.60 when correlating with macrotexture level LMA. The LMA values can be transformed to MPD by using the equation:

MPD = 5·10-9·LMA4.762

based on later studies of the relation between MPD and LMA, and relative FC differences can be transformed to relative RRC differences by multiplying by 4. In this way one may deduce a relation between RRC and MPD of

RRC = constant + 0.0016·MPD



One shall be aware of that when this study was made, standardised texture and unevenness measures did not exist. MPD was not yet known. Thus, measures chosen were "homemade". As mentioned, the best correlation of FC was obtained with the shortwave unevenness (R = 0.91). Megatexture level came second (R = 0.83) and macrotexture third (R = 0.60). This was further illustrated when correlation between FC and road roughness/texture level as a function of texture wavelength was calculated; see Figure 4.2.

Figure 4.2: Correlation between fuel consumption per km and road roughness/texture level as a function of texture wavelength [Sandberg, 1990].

When making fuel consumption measurements, in contrast to trailer measurements, the suspension losses are clearly present and they should peak in the area where Sandberg's data peak. It was assumed that the suspension system of the test car was in excellent condition, but it was not tested.

The IRI is sensitive to wavelengths between approximately 1.2 and 30 m, with a maximum response at around 2.4 m wavelength [Sayers & Karamihas, 1998]. This is perfectly located in the "shortwave unevenness" range of the presented study. Therefore, one may say that IRI came out as a very influential parameter in this study.

4.2 Swedish measurements of road condition effects There are a few reports about the effect of snow on rolling resistance [Kihlgren, 1977] [Lidström, 1979] [van Es, 1999], but they were made with aircraft tyres in mind and are a little difficult to assess for road conditions and road tyres. However, there is no doubt that the effects of snow are large. In fact, the model suggested in [Lidström, 1979] is presently imple-mented in VTI's VETO model, although the implementation is not easy since for an articu-lated truck (for example) there are many tyres, some rolling in different lateral positions, where snow conditions differ, some rolling in the same track with different snow compaction.

An effect which is mostly forgotten in studies of texture influence on fuel consumption is the tyre drag effect on surfaces partly covered with water. The water level in ruts and pools (the latter is often an effect of megatexture) will be partly influenced by the macrotexture. Non-



uniform water depth may cause vehicle instability [Hight et al, 1993] but also increased fuel consumption. The water depth influences fuel consumption by at least about 10 % according to [Sävenhed, 1986]. A model developed in an MSc Thesis is presently implemented in the VETO model for calculating the effect of water on the pavement [Olsson, 1984].

4.3 Belgian measurements of texture and unevenness effects Some groundbreaking work on rolling resistance and fuel consumption related to road sur-face properties were made in Belgium, France and Sweden in the 1980's. Using a special RR trailer (see [Sandberg (ed), 2011]), as well as a profilometer for the texture range and another one for the unevenness range, Descornet at BRRC analysed the relation between RRC and unevenness, megatexture and macrotexture [Descornet, 1990]. The test tyre was a pattern-less Michelin SB 14" tyre. Figure 4.3 shows the relations he recorded in terms of RRC (Cr) plotted against (M)TD in the left diagram and against the unevenness amplitude at 2.5 m wavelength in the right diagram. There is in the original article also a corresponding diagram for the unevenness wavelength of 10 m, showing essentially the same results as for 2.5 m.

Figure 4.3: Relation between Cr and (mean) texture depth (at the left) and between Cr and unevenness amplitude at 2.5 m texture wavelength (at the right). Diagrams scanned from [Descornet, 1990]. One should be aware that there might have been a correlation between the (M)TD and the unevenness, and that this may partly be reflected in the result in the right diagram. Note also that the slope in the left diagram is 0.0021 (Descornet is obviously wrong by one decimal in the printed Cr equation), which means that RRC increases by 0.0021 for each mm of texture depth increase. This may be compared with the 0.0016 obtained by Sandberg. As will be shown later, these values are fairly consistent with modern results. Furthermore, Descornet found that the most sensitive spectral range was the megatexture range [Descornet, 1990]. See Fig. 4.4. It appeared that the most sensitive range is mega-texture, but that macrotexture is also very influential, at least when disregarding the 6 sections that were transversely grooved.



Descornet also recorded an interesting relation between Cr and tyre internal temperature, but this is reported in [Sandberg et al, 2011].

Figure 4.4: Correlation between RRC (Cr) and road roughness/texture level as a function of texture wavelength [Descornet, 1990]. The dashed line is the result when the 6 (concrete) pavements that had transverse textures (out of 37 pavements) were neglected. The diagrams of Figures 4.2 and 4.4 look quite differently. However, in a way, the difference is logical, since when making fuel consumption measurements, in contrast to trailer measure-ments, the suspension losses are present and they should peak in the area where Sandberg's data peak. It may be noticed that in the megatexture and macrotexture areas Descornet's and Sandberg's data are not very different.

4.4 French measurements By fuel consumption measurements using a passenger car on roads representative of the French network and by assessment of heat emission related to the conversion of mechanical energy in dampers during tests on a vibration bench, a French study in the 1980's gave some insight into the relation between fuel consumption (FC) and road texture and unevenness, reported as both [Laganier & Lucas, 1990] and [Delanne, 1994]. In fact, these tests also included pure rolling resistance measurements on the wheel of a car driving on 5 surfaces on the Nantes test track; the same test track as now is managed under IFSTTAR.

To begin with the rolling resistance measurements on the test track, it appeared that over the range of MTD from approx. 0 to 5 mm, RRC increased from 0.018 to 0.024. This would correspond to a slope in RRC versus macrotexture (MPD) of 0.00125; which was a bit lower than the Belgian and Swedish results. FC measurements gave approx. the same value if an FC difference is assumed to correspond to 1/3 of an RRC difference.



FC measurements on the roads showed an "overconsumption" of fuel of up to 6 % for a car which had an average fuel consumption of 7 litres/10 km as influenced by unevenness and 5 % as influenced by macrotexture (MTD variation was 0.5-2.5 mm). The results in terms of extra FC versus unevenness are illustrated in Figure 4.5 (note that unevenness is not an IRI scale).

Figure 4.5: Extra fuel consumption according to road unevenness level. See text for explanations. Diagram adapted by the authors from [Laganier & Lucas, 1990].

Results of measurements and calculations of power lost in shock absorbers (dampers) as a function of roughness level are shown in Figure 4.6. This is made for three wavelength ranges, namely small (1 m < λ < 3.3 m), medium (3.3 m < λ < 13 m) and longer than 13 m road roughness wavelengths (λ). Note that the "small" wavelength range, which includes the most sensitive IRI range and is close to the megatexture range, was by far the most important one for suspension losses. Laganier and Lucas considered the effects of unevenness and macrotexture as additional.

Figure 4.6: Extra fuel consumption according to road unevenness level (solid bold curve). The consumption is also presented as contributions within three roughness wavelength ranges. Based on measurements of power loss in suspension when tested in a test bench. See text for explanations. Diagram adapted by the authors from [Laganier & Lucas, 1990].



4.5 German measurements in the 1990’s An early version of the BASt trailer for rolling resistance, see description in [Sandberg (ed); 2011], was used in the 1990's to make measurements on 10 surfaces on the German motorway A555 [Ullrich et al, 1996]. These included the use of 4 different (car) test tyres. Results were presented as "normalized Cr at 25 oC". Probably, "normalized" just referred to temperature correction according to ISO to a reference temperature of 25 oC. Measurement of the textures of the same surfaces was made by means of a laser profilometer. The results were presented as rms values of the profile curves, filtered in three different texture wavelength ranges:

0.3 – 10 mm "fine texture"

10 – 100 mm "coarse texture"

10 – 500 mm "megatexture"

However, the texture rms values were normalized to proportions relative to one surface and that surface was given the value 1.0 mm. Therefore, all texture values are just relative to this surface. They cannot be compared to any "modern" standardized measures.

Figure 4.7 shows the result for the case of texture in the 10-100 mm texture wavelength range, which was the range that gave the best correlation between Cr and texture. Diagrams for the other two texture ranges show similar results (correlation coeff. 0.71 for "fine texture" and 0.67 for "megatexture"). Despite the higher correlation for "coarse texture", 0.75 versus 0.67 for "megatexture", this main author thinks that one should not conclude that mega-texture is less important than "coarse texture" (macrotexture) since the poorer correlation is entirely due to the two smooth surfaces not being so extremely smooth in the megatexture range as they are in the two macrotexture ranges.

Figure 4.7: Relation between rolling resistance coefficient (probably average of 4 car tyres) and rms value of "coarse texture" for the 10 tested road sections. Texture values are given as a proportion of the texture of one of the surfaces (the rightmost data point, which is set as 1.0 mm). From [Ullrich et al, 1996].

These German measurements also showed that the four car tyres had approximately equal correlations to the rms texture in the three texture ranges; see Figure 4.8.



Figure 4.8: Relation between rolling resistance coefficient for the four car tyres A10-A13 and rms value of the three texture ranges for the 10 tested road sections. FT = fine texture, GT = coarse texture and MT = megatexture. From [Ullrich et al, 1996].

See further presentations of this research in the chapter about drum measurements.

4.6 New Zealand measurements in the 1990’s In New Zealand a special measurement method was developed in the late 1980's and early 1990's, called the steady state torque method [Cenek, 1994]. It essentially involves a test vehicle (both a car and a truck have been used) being driven at steady speeds between 20 and 75 km/h. At each speed, the driving torque of one (driven) tyre, together with the relative wind speed and direction are continuously measured. The latter are parts of an on-board anemometry system by which air resistance is controlled. The driving torque is divided by the dynamic tyre radius (1.03 x Static Radius) and corrected for ambient wind to obtain the driving force required to overcome all resistive forces with the exception of driveline losses. It should be noted that this method measures rolling resistance including the contribution by suspension losses. Using this method, comprehensive work was conducted in New Zealand in 1988-1992 to determine the influence of NZ pavements on rolling resistance. The most important document is probably [Cenek, 1994]. The results indicated a rolling resistance range of 55 % between the best and the worst pavement, with MTD values ("sand circle" equal to "sand patch") varying from 0.6 to 2.7 mm and unevenness varying between 37 and 59 NAASRA counts/km (corresponding to IRI of 1.4 to 2.3). The result according to [Cenek, 1994] was expressed as the following equation (in this case C0 can be considered equal to Cr):

C0 = 0.0102 + 8.35·10-4·MTD2 + 1.05·10-3·IRI The authors of this report have made some calculations from the equation. For example, if we assume that IRI is 1.0, an MTD of 0.5 mm will give C0 = 0.01146, while MTD of 2.5 mm will give C0 = 0.0165. This is an increase in rolling resistance of 44 % for 2 mm MTD increase. If we assume that MTD = MPD (they are usually rather close) and neglects the nonlinearity of the equation above, this would correspond to a slope of 0.0022 in the equation of Cr versus MPD, as discussed earlier. In another example, if we assume that MTD is 1.0 mm, an IRI of 0.5 will give C0 = 0.0115, while an IRI of 2.5 will give C0 = 0.0136. This is an increase in rolling resistance of 18 % for an IRI increase from 0.5 to 2.5.



This may suggest that the influence over the range of IRI on a common paved road network would amount to approximately half that of the range of macrotexture. Again, this shows the importance of macrotexture and that unevenness is a parameter that shall not be neglected. Another study in New Zealand; a follow-up regarding the rolling resistance and fuel con-sumption of heavy trucks [Jamieson & Cenek, 2002], concluded that the two most important pavement variables for rolling resistance were:

• Average Rebound Deflection (RD), expressed in mm (this is a measure of the pave-ment stiffness)

• Short Wavelength Roughness (SWR) in the wavelength range 0.5 – 5 m, expressed as band-filtered RMS value in mm

Macrotexture was not very important, and even had a negative relation to FC, despite a rather large range. This indicates what is already known for heavy truck tyres and noise emission: macrotexture has rather small influence, and sometimes even a negative corre-lation with noise [Sandberg & Ejsmont, 2002]. Two medium commercial vehicles, and three tyre sizes commonly found on New Zealand commercial vehicles were selected for the test programme. Each of the trucks was instru-mented to allow determination of component vehicular drag forces using the steady state wheel torque method. The most interesting thing of this study is that for trucks, the shortwave unevenness is the most important range, plus that pavement stiffness is important, while macrotexture is less important. However, one should bear in mind that this probably depends partly on the tyres chosen (drive tyre treads versus steer axle treads).

4.7 Other early measurements A review of early data on the relation between rolling resistance and road surface texture should not miss to mention the work of DeRaad in the 1970's [DeRaad, 1978]. Apart from making indoor drum tests he measured rolling resistance by means of a 5th wheel attached to a light truck. Testing included 10 car tyres run at 30 mph on 6 pavements covering a large range of textures.

The results indicated, as a percentage relative to the Cr of a new cement concrete surface, that Cr varied between 88 and 133 %; i.e. a very large range. However, the polished concrete (88 %) seemed to be an extremely smooth pavement not normally accepted on highways.

Unfortunately, the texture was never quantified and therefore the study by DeRaad has only historical interest to project MIRIAM.

It shall also be mentioned that in the literature one may find several documents reporting rolling resistance or FC measurements on various pavements, where the pavements have been poorly described and surface properties rarely quantified, except that often some kind of unevenness values (only) are reported. These studies are not mentioned here as they would not provide much help in determining a quantified relation between rolling resistance and surface properties.



4.8 Discussion The following conclusions are drawn from the data of these older measurements:

Over the range of road surfaces tested, the surface properties seem to influence car fuel consumption by approx 10 % (the worst – the best), while rolling resistance would be influenced by as much as 40-55 %.

When fuel consumption of a full car has been measured, the highest correlations between Cr and road surface texture and unevenness are obtained for "shortwave unevenness", in a wavelength range which seems to be where IRI values would be most sensitive (although IRI was not a known measure when the measurements were made). It is assumed that for the unevenness range, the energy losses occur in the suspension of the cars and not in the tyres; albeit they may be considered as being related to rolling resistance.

When trailers have been used, the macrotexture and megatexture ranges come out as the most important ones. The same applies to the static wheel torque method involving a full car.

The relation between Cr and macrotexture seems to lie in the range of 0.0016 (and the French study as low as 0.00125) and 0.0022 expressed as the slope coefficient in the regression of macrotexture (in mm texture depth) upon (the dimensionless) Cr.

The main value of these data is that they suggest that one shall measure both the tyre rolling resistance and the suspension losses if one wants to measure the full influence of the road surface properties, and that the megatexture and shortwave unevenness ranges are very important, beside macrotexture.

It will be a challenge to rolling resistance trailers to take suspension losses into consideration in future studies.



5 RESULTS OF LABORATORY DRUM MEASUREMENTS

5.1 Measurements at Dunlop in the UK around 1980 Work at Dunlop tyres in the U.K. to reduce rolling resistance of tyres around 1980 was reported by [Williams, 1981]. The experiments involved the use of a drum facility on which replica road surfaces were fitted (one by one). In total 5 tyres and 5 replica surfaces were tested. The results are summarized in Figure 5.1. It can be seen that the range is almost 100 % increase from smooth steel to a rough-textured surface dressing. If the steel surface is excluded, the range shrinks to 50 %. This approximately corresponded to the texture range of common UK surfaces by that time.

Figure 5.1: Relationship between rolling resistance coefficient and type of tyre and road surface, as measured on a drum facility having various replica road surfaces. From [Williams, 1981].

5.2 Measurements at BASt in Germany in the 1990's Related to the measurements reported in Chapter 4.5, BASt in Germany used their drum facility PFF (PFF = Prüfstand Fahrzeug Fahrbahn, see description in [Sandberg (ed); 2011]), to make measurements on 11 surfaces mounted successively on the drum [Ullrich et al, 1996][Sander, 1996]. Of the surfaces which were examined, three were produced as close replicas of real road surfaces and two surfaces were constructed as “ISO surfaces”; however, becoming much too smooth according to this author. The remaining ones were sandpaper-like surfaces with various grit sizes.

The rolling resistance measurements included the use of four different (car) tyres. Results were presented as "normalized Cr at 25 oC". Probably, "normalized" just referred to



temperature correction according to ISO to a reference temperature of 25 oC. Measurement of the textures of the drum surfaces was made by means of a laser profilometer. The results were presented as RMS values of the profile curves, filtered in three different texture wavelength ranges:

0.3 – 10 mm "fine texture"

10 – 100 mm "coarse texture"

10 – 500 mm "megatexture"

Figure 5.2 shows the average Cr for the tyres, distinguishing between the four dimensions used, over 10 of the 11 drum surfaces. In the figure the three rightmost surfaces were asphalt concrete with gradations indicated and the four in the middle were asphalt surfaces with chippings spread on the surface of various indicated gradations. The two rougher of these should be possible to use on real roads. The three surfaces at the left were various sandpaper-type surfaces.

30

40

50

60

70

0,6-1,0P24

0,7-1,2P20

1,0-1,7P16

1,0-1,7Kunstharz

0,7-1,4 2,0-2,8 4,0-5,6 8,0-11,0 0/8ISO unbeh.

0/8ISO beh.

0/11S beh.

Mitt

l. R

ollw

ider

stan

d R

25 /

[N]

195/65 R15 H 205/60 R15 V 175/70 R13 T 155/70 R13 T

Korn/mm:Art/ Träger: Korund/ Schmirgelleinen Splitt/ Kunstharz Asphaltbeton

Figure 5.2: Average rolling resistance (Cr) values from drum tests - two runs for each of four tyres at three different speeds (50/90/120 km/h) on 10 of the 11 drum surfaces.

Figure 5.2 shows that the four tyres ranked the surfaces in a very similar way; there is just a certain bias between the four curves.

Figure 5.3 shows the relation between the average Cr values and the macrotexture, the latter expressed as RMS value of the profile within the texture wavelength band 0.3 – 10 mm. Similar diagrams were reported also for the other two texture ranges, but as they gave lower correlations they are not reproduced here.

It is interesting to note that the highest correlations here were obtained for the “fine macro-texture”. This is opposite to all other studies. Probably, it has to do with the selection of the surfaces in the test program, since 7 of the 10 surfaces were too smooth or had too small chippings to be realistic on real roads. Nevertheless, it is notable that the relation appears to be rather linear even down to the very smooth textures in this test program.

A compilation of the correlation coefficients between rolling resistance and texture in the three bands, for both the drum and the trailer measurements (see Chapter 4.5) is shown in Table 5.1.



Figure 5.3: Relation of rolling resistance to fine macrotexture (0.3 - 10 mm) for drum tests (using tyre 175/70 R13T).

Table 5.1: The correlation coefficients between rolling resistance and texture in the three bands, for both the drum and the trailer measurements.

Texture parameter

Drum Trailer

Fine texture 0.3 - 10 mm

0.97 0.71

Coarse texture 10 - 100 mm

0.89 0.75

Megatexture 100 - 500 mm

0.74 0.67

A compilation of the rolling resistance coefficients measured on four tyres, both on the drum and on the road (a very smooth surface) is shown in Table 5.2.

Table 5.2: Average rolling resistance coefficient values (temperature corrected) in %, measured on the drum and compared with trailer measurements on a road surface.

Tyre type Drum (PFF) Trailer (A555/H)

A10 0.802 0.794

A11 1.051 1.178

A12 0.826 0.962

A13 1.046 1.264



5.3 Measurements at TUG in Poland around year 2000 Results of tests made on the rolling resistance drum facility at TUG in Gdansk in a VTI-TUG project including measurements of Cr (RRC) on approx 100 car tyres, are shown in Figure 5.4. The tests were made on the TUG drum facility for car tyres (drum diameter 1.7 m), having two very different drum surfaces, one smooth sandpaper (brand name "Safety Walk") and one surface dressing with max. 11 mm chippings. The latter is a commercial product labelled APS-4 and produced by a French company as a carpet with embedded chippings.

Fig. 4.5: RRC versus speed, tested for approx. 100 car tyres of various brands and dimen-sions. Unpublished data from TUG/VTI.

The sandpaper surface had an estimated MPD of 0.12 mm and the surface dressing an estimated MPD of 2.4 mm. Assuming a linear relationship, from the MPD values and the difference in RRC, one may calculate the slope of the RRC versus MPD as 0.0021. Note that this is for approx. 100 car tyres. An important issue is whether the effect of MPD on RR depends on the speed. The measurements were actually made at the three speeds 80, 100 and 120 km/h. A multiple regression analysis indicated the following relation between RRC, the road surface parameter MPD and speed:

RRC = 0,01065 + 0,002012·MPD + 0,0000064·MPD·(V-20)

where MPD is in mm and V (speed) in m/s.

The combined term MPD(V-20) has a very small coefficient, which is not significantly different from zero for the data available. RRC versus speed data indicates that RRC slightly depends on the speed. However, this dependence does not necessarily have to be coupled to MPD. A conclusion from this is that the MPD effect is either independent of the speed or depends very weakly on it, at least in the speed range 80-120 km/h

Another set of car tyres was measured a few years later in the EU project SILENCE [Sandberg et al, 2008]. The surfaces were the same, but the tyres were 6 tyres chosen to be representative of popular market tyres. They were tested in new condition and at various state of wear (8, 6, 4 and 2 mm tread depth); i.e. for 24 tyre/tread depth combinations. The result was a slope coefficient of 0.0020 as an average for all tread depths, and 0.0021 for full tread depth and 0.0018 for 2 mm tread depth. It suggests that worn-out tyres are a little less sensitive to texture than new ones.

10

11

12

13

14

15

16

80 100 120

Tyre rolling speed [km/h]

RR

C [k

g/to

n]

Smooth drum surface(sandpaper)

Rough drum surface(surface dressing)



6 RESULTS OF TRAILER MEASUREMENTS IN SWEDEN 2007-2011

6.1 Introduction The Technical University of Gdansk (TUG) in Poland, under the leadership of Prof Jerzy A. Ejsmont, 5-8 years ago produced an improved version of the Belgian trailer used by Descornet; the results of which are summarized in Chapter 4.3. VTI ordered the first measurements by this trailer in 2005 while it still was not validated and used the results (see below) to increase the interest in the subject at the Swedish Road Administration. After it became evident that Swedish roads had very unfavourable textures for rolling resistance, and thus creating extra fuel consumption and CO2 emissions from traffic, the interest in the subject has been steadily increasing in Sweden and the intention is that models for selection of pavements and their maintenance shall include a rolling resistance parameter in the near future. The measurements in 2005 have been followed by more and more extensive measurements, made by improved versions of the TUG trailer until the present time. In most cases also noise and texture measurements have been made on the same surfaces. The latter have been made by the VTI mobile laser profilometer. The following sub-chapters will present the essential results of these measurements; however, excluding the noise properties. The measurement instruments are presented in [Sandberg (ed), 2011]. However, a picture of the TUG trailer appears in Figure 6.1.

Figure 6.1: The tyre/road rolling resistance measurement trailer from TUG in the shape and condition of 2010 (before 2009 the test tyre was not enclosed).



6.2 Test (reference) tyres As it is very impractical to measure rolling resistance using a great number of tyres representing the tyre market, when classifying or ranking pavement properties for rolling resistance, it is practical if not necessary to use reference tyres. The purpose of these is to be representative of the category of tyres that they are intended to represent and to provide stable and repeatable conditions. A common reference tyre concept is that one tyre shall represent the fleet of automobile tyres on the roads (tyre category C1), and another tyre shall represent the fleet of heavy truck tyres (C3). One might also want to have a tyre representing the middle range; van tyres (C2). Reference tyres must be available for a long time.

This concept is already implemented in the drafts ISO/DIS 11819-2 and ISO/TS 11819-3 which are two documents specifying the so-called CPX method for classification of noise properties of pavements. A draft for an ASTM method for a "Standard Test Method for Measurement of Tire-Pavement Noise Using the On-Board Sound Intensity (OBSI) Method" specifies the use of one reference tyre (the SRTT). The tyres used in the CPX method by ISO are shown on the left (SRTT) and in the middle (AAV4) in Figure 6.2. The tyre on the right is an extra tyre used by TUG from the time when they started to make RR measure-ments, and has been kept since then for the purpose of providing a link to old measure-ments.

Figure 6.2: Reference tyres used in the tests reported in this article. Refer to the text for more information.

The SRTT ("Standard Reference Test Tire") is a tyre specified in ASTM F2393 as a reference tyre for various purposes. The Avon AV4 tyre (designated "AAV4") is a tyre tested and found to classify pavements (for noise) in roughly the same way as a selection of regular heavy truck tyres do. It is in fact a light truck tyre, but as the smallest dimension for this series of tyres is used, the AAV4 fits on large passenger cars, as does the SRTT.

The SRTT and the AAV4 are tyres considered in the MIRIAM project to become reference tyres also for rolling resistance, and will be tested for this purpose in MIRIAM.

As it is a reference tyre specified by ASTM, the SRTT is likely to be available for several decades in the future. The AAV4 tyre will not be manufactured in the future unless the users of CPX tyres orders a full batch of 100 or more tyres simultaneously, which is indeed the plan.



6.3 Tested road surfaces On behalf of VTI, TUG has measured RRC on a number of road surfaces in Sweden and Denmark in the past 5 years using the TUG "R2 trailer". The tested pavements in this time period, 2005-2010, include the following numbers and types:

11 Dense asphalt concrete, max. aggr. sizes 6, 8, 11, 16 mm 9 SMA (stone matrix asphalt), max. aggr. sizes 6, 8, 11, 16 mm 1 Hot rolled asphalt (HRA), UK type, max. aggr. size 16 mm 3 Dense-graded asphalt rubber (Arizona type adapted to Sweden), max. aggr. sizes 11,

16 mm 1 Open-graded asphalt rubber (Arizona type adapted to Sweden), max. aggr. size 11 mm 3 Porous asphalt concrete, single-layer, max. aggr. sizes in top layer 8, 11 mm 3 Porous asphalt concrete, double-layer, max. aggr. sizes in top layer 8, 11 mm 2 Chip seals (surface dressings), single layer, max. aggr. size 11 mm 6 Thin asphalt layers (dense), max. aggr. sizes 6, 8, 16 mm 1 Exposed aggregate cement concrete, max aggr. size 16 mm 1 SMA, max. aggr. size 16 mm, medium texture but very uneven

These have been in various conditions; including both new pavements and a few near the end of life. Some of the pavements have been tested in Denmark (on the other side of the Oresund, close to southern Sweden) but most of them are Swedish pavements.

6.4 Pilot tests with RR trailer In this chapter, the results will not be listed in tables for each test condition, tyre and pavement. It would be a huge table difficult to evaluate. Instead, the RRC data will be plotted against the MPD data in regression diagrams.

The first diagram shows results measured in 2005 and 2007, at a time when the test tyre was not yet protected from the air flow by an enclosure. Figure 6.3 shows results corrected for the air flow effect (by testing at various speeds and deducting the speed effect), where RRC has been plotted against MPD values for the same pavements, at 80 km/h. These measurements were made by using four automobile tyres, including one SRTT. The round (grey) symbols are for dense asphalt and SMA pavements having max. aggr. sizes 8, 11 and 16 mm, and an extremely rough-textured chip seal at the top; while the square (red) symbol is for an exposed aggregate cement concrete (EACC) with max. aggr. size 16 mm.

Apart from a correlation so high that it is probably just by chance, it may be noted from this diagram that the EACC pavement follows the same RRC-MPD trend as the asphaltic pavements. It is claimed by the cement concrete industry in North America that such rigid pavements have lower RRC than flexible pavements but our measurements could not verify this. Note also the very large range of RRC: over the measured MPD range, the highest RRC on the regression line is 45 % higher than the lowest RRC on the same regression line. One may say that this MPD range covers the macrotexture range of Swedish paved roads; if one excludes pavements with faults such as bleeding asphalt and pavements with lots of ravelling and pot-holes.

6.5 Further tests with RR trailer – Macrotexture influence When measurements were made after 2008, TUG had fitted an enclosure over the test tire; more or less eliminating the air flow resistance around the tire adding to the rolling resistance. In the years 2009-2010 a number of series of rolling resistance measurements were made in Sweden (and one in Denmark). A compilation of such RRC data, plotted as a function of Mean Profile Depth (MPD), the latter measured according to ISO 13473-1, appears in Figure 6.4.



y = 0,0024x + 0,0082R2 = 0,9853

0,008

0,009

0,010

0,011

0,012

0,013

0,014

0 0,5 1 1,5 2 2,5

MPD in mm (ISO 13473-1)

RRC

aver

. for

4 ty

res

Figure 6.3: Rolling resistance coefficient plotted against macrotexture (MPD) for measure-ments 2005 and 2007. The round (grey) symbols are for dense asphalt, and SMA pavements and an extremely rough-textured chip seal (the highest point); while the square (red) symbol is for an EACC (cement concrete). Note that, opposed to what is written in 6.2, these measurements were made with four tyres (RRC averaged), of which one was the SRTT; the others were regular market automobile tyres.

The figure includes only the results at 80 km/h. The reason is that it appeared that the RRC data measured at 50 km/h correlated almost perfectly to those at 80 km/h. Thus; adding more speeds does not add more information. This also shows that the measurements were repeatable (within the day) and subject to only small disturbances. But due to this, in this report, for the Swedish 2009-2010 data, only the results for 80 km/h are displayed.

In Figure 6.4, all measurements in 2009-2010 are put into the same diagram without distinction. R2 which shows the variance explained by the RRC-MPD regression as part of the total variance in this set of data, is only 0.26. This is statistically significant on the 95 % level, but it is very disappointing compared to the earlier results plotted in Figure 6.3. One may therefore suspect that there is one variable (or more) out of control in this scenario which ruins the correlation.



All series merged into one

y = 0,0017x + 0,0088R2 = 0,2626

0,007

0,008

0,009

0,010

0,011

0,012

0,013

0,014

0,015

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1,2 1,3 1,4 1,5 1,6 1,7 1,8 1,9 2,0

MPD [mm]

RR

C fo

r ave

rage

of t

he th

ree

test

tire

s

Figure 6.4: Rolling resistance coefficient RRC plotted against macrotexture (MPD) for measurements in 2009 and 2010. All data for 80 km/h and averaged for all three test tires (Figure 6.2) have been put together without distinction.

The next figure (Figure 6.5) reveals at least part of the problem. In this diagram, data have been analyzed and plotted separately for each measurement series. By a measurement series is meant a set of measurements at similar temperatures covering one or two consecutive days. In such cases calibrations are the same for all measurements within the series. Note that the two points in the upper right corner are for a chip seal (in two different tracks) which had some potholes and were measured at temperatures near freezing point.

It appears that there is an excellent and reproducible relation between RRC and MPD; i.e. that the rolling resistance pavement effect is largely caused by macrotexture, and/or perhaps something which is very closely correlated with macrotexture. The latter may be mega-texture.

Another observation is that the measurements are influenced by a biasing factor with high influence, related to something which is unique for the series of measurements. One such parameter may be ambient temperature (or pavement or tire temperatures, but they have been shown to be highly correlated with each other). There are some temperature diffe-rences here which can explain some of the bias, but not all of it. These results are not shown here as it is yet not known what appropriate RRC-temperature relations might be and not all data have been analyzed in this way yet. Future analyses and planned measurements will hopefully shed some light on this.



y = 0,0017x + 0,0087R2 = 0,7604y = 0,002x + 0,0098

R2 = 0,9466

y = 0,0017x + 0,0121

y = 0,0016x + 0,0079R2 = 0,7712

y = 0,0022x + 0,0087R2 = 0,8224

y = 0,0021x + 0,0079R2 = 0,976

0,007

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0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1,2 1,3 1,4 1,5 1,6 1,7 1,8 1,9 2,0

MPD [mm]

RR

C fo

r ave

rage

of t

he th

ree

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Figure 6.5: Rolling resistance coefficient RRC plotted against macrotexture (MPD) for measurements in 2009 and 2010. Same data as in the previous figure, but here a distinction is made between different measurement series (i.e. different regressions). The data in grey and black are from measurements in 2010, other colors are from 2009.

Another potentially biasing effect might be calibration; that there is a drift with time in the system which is unknown and not controlled by TUG. If so, this drift seems to have some random or at least inconsistent relation with time, as it is impossible to see a relation between this bias and time when the series was measured. TUG does not currently recognize such a problem, but the main author (Sandberg) thinks that it might exist. This "bias problem" is a matter which must be studied much more in the near future, and will be so within project MIRIAM.

6.6 Special effects – porosity It is interesting to study whether porous road surfaces (single- or double-layer porous asphalt) show different rolling resistance properties than dense surfaces do, assuming that the macrotexture is measured as MPD in both cases. Such effects may be speculated as:

• Less air resistance of the tyres due to (air) drainage in the surface

• Questionable measurements of MPD and questionable representativity of MPD on porous surfaces

For this reason, a few porous surfaces have been included in rolling resistance measurement series. First, Figure 6.6 shows measurements made in Denmark in 2009, where one of the surfaces was porous. Obviously, it seems that the MPD overestimates the Cr of this surface.



Figure 6.6: RRC versus MPD for a measurement series in Denmark, where the red symbol shows the result for a porous asphalt with max 8 mm chippings.

Then, Figure 6.7 shows similar results measured in Skåne, Sweden, in the same month in 2009, where all surfaces with MPD above 1.3 mm are porous. It seems also from this diagram that the porous ones are below an imagined regression line through only the non-porous surface points.

Figure 6.7: RRC versus MPD for a measurement series in Skåne, where the symbols having MPD > 1.3 mm represent porous asphalt surfaces.

The reason for these indications may be one of the two bullet points at the beginning of this sub-chapter, or both combined. The MPD is more sensitive to the positive parts of the profile (peaks) than to the negative parts (valleys), but this may not be enough to represent the profile that is really enveloped by the tyre. Application of an enveloping function (see Annex A) may mean an improvement here. This is discussed more later in this report.

Similar results were obtained already in 2005 when rolling resistance measurements were made with the TUG trailer on a new double-layer porous asphalt with max. 11 mm chippings in the top layer, on motorway E4 in Hallunda southwest of Stockholm; results which were compared with an old SMA 0/16 adjacent to the porous section. The porous asphalt gave an average RRC (two tyres, two speeds, two directions) of 0.0110 compared to 0.0113 for the SMA. Thus RRC was lower despite the porous asphalt for sure would have had a substantially higher MPD than the SMA. MPD was not measured there.

y = 0,0017x + 0,0087R2 = 0,7604

0,0060

0,0070

0,0080

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0,0120

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0,40 0,60 0,80 1,00 1,20 1,40 1,60 1,80 2,00

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RR

C fo

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erag

e ty

re

y = 0,0016x + 0,0079R2 = 0,7712

0,0080

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0,20 0,40 0,60 0,80 1,00 1,20 1,40 1,60 1,80MPD [mm]

RRC

for a

vera

ge ty

re



6.7 Special effects – stiffness As discussed in [Sandberg (ed), 2011], the bow-wave in the pavement substructure in front of a rolling tyre, as well as the deflection of the pavement under a loaded tyre, should result in some energy consumption due to hysteresis losses which should be reflected in a contribution to rolling resistance.

Some results of experiments were presented in [Sandberg (ed), 2011]. The overall conclusion there was that pavement stiffness cannot be excluded as an important factor influencing rolling resistance, and should be included in studies in the MIRIAM project. The still open question is as to what extent and under which conditions (temperature, type of pavement and light versus heavy vehicles) when stiffness is a major factor to consider.

Since then, two new studies have been made. The first one is a comparison of RRC values measured in 2011 with the TUG trailer using the three test tyres on a 1 km section of the cement concrete with exposed aggregate (EACC) on motorway E4 north of Uppsala (Sweden) and another 1 km section paved with SMA 0/16, both approx 4 years old. The results are shown in Table 6.1. The results are averages for the three tyres and for the two test speeds 80 and 110 km/h.

Table 6.1: Comparison between a cement concrete (EACC) and a stone mastic asphalt surface (SMA) having the same max. chipping size (16 mm) and being of similar age.

Tested surface Average RRC for the three tyres MPD [mm] IRI

EACC 0/16 0.00130 0.55 1.2

SMA 0/16 0.00135 0.80 0.7

It appears that the EACC gave a little lower rolling resistance, but it is due to the lower macrotexture rather than stiffer pavement. The air temperature during these measurements was only 9 oC, so even the SMA should have been rather stiff at that temperature.

The second new study is the study made in Minnesota, USA, in September 2011. These measurements were made at much higher temperatures. The results there are interesting in this respect, but permission to publish the results here has not yet been granted by the sponsor (MnDOT).



7 RESULTS OF COASTDOWN MEASUREMENTS IN SWEDEN

7.1 Coastdown measurements During the years 2008-2009, VTI conducted measurements of rolling resistance using the coastdown method. For a description of the method, see [Karlsson et al, 2011] or [Sandberg (ed), 2011]. The measurements were performed on a private car and a heavy truck; both vehicles heavily instrumented to record all possible parameters of interest. For the car, measurements included 22 different road surfaces; for the truck, measurements included six road surfaces; all of them on highways. For example, for the truck measurements the range of MPD was 0.6 – 2.0 mm and the range of IRI was 1.2 – 3.1. For the car, these variations were larger.

The following is based on [Karlsson et al, 2011].

In this study, an attempt was made to obtain more reliable estimates of how measures representing macrotexture (MPD) and unevenness (IRI) affect rolling resistance coefficients. The primary method used here is the coastdown method. It was applied to a private car and to a heavy truck (heavy goods vehicle - HGV).

The studies were made based on the previous experimental work in the ECRPD project [ECRPD, 2010], supplemented by measurements and analyses in a national Swedish project sponsored by the Swedish Transport Administration.

Based on the data collected, different models described by equations of Cr as a function of a number of independent variables, of which MPD and IRI are the two major ones representing road surface properties, were developed and tested.

In the final model selected, the coefficients for the parameters MPD and IRI were found to be as indicated in Table 7.1. Only the most important parameters have been included.

Table 7.1: Best estimations of coefficients for the basic rolling resistance and the most important variables in the model equation, describing the contributions of the parameters to the rolling resistance coefficient. Data from [Karlsson et al, 2011].

Parameter (term in equation) Coefficient (slope) - Car Coefficient (slope) - Truck Cr00 (basic rolling res. constant) 0.00802 (0.00434)* CrMPD (macrotexture influence) 0.00172 (0.00091)* CrIRI (unevenness influence) 0.000466 (0.00030)* CrTemp (temperature influence) 0.000104 -- (temp. did not vary enough)

* To be replaced by more recently obtained coefficients

The standard deviation for CrMPD was 0.0002.

The reason why the truck data are written in parenthesis is that the measurements included only six roads and if only one of them is omitted, the coefficients may change dramatically. Therefore, the data for the truck tests are considered as unstable and should not be used. They are included here for two reasons: (1) to show how they compare to those of the car, for the set of road surfaces chosen, and (2) that further studies of these effects are important, since the present data suggest that the surface effects are substantial. Note that if the car and truck coefficients were normalized to the same Cr00 constant, the relative influences of MPD and IRI of the truck would be rather close to those of the car.



It is also important to note that only one car and only one truck were tested, with only one set of tyres for the truck and two sets for the car (normal tyres and studded winter tyre). However, other measurements presented in this report suggest that different tyres seem to rank surface effects in approximately the same way. Thus, any poor representativity of the vehicles may probably mostly be related to the suspension properties.

Results show that the effect of unevenness is in general significantly smaller than that of macrotexture; albeit not negligible.

The coastdown method provides, besides information about rolling resistance, other useful data for the vehicle, such as air resistance coefficients, temperature coefficients and trans-mission resistance.

As mentioned above, for the heavy truck, the extent of the coastdown measurements was much smaller than for the private car. Results are unstable and it is difficult to draw any definitive conclusions.

A serious disadvantage with the coastdown method, at least when applied to road surface effects, is that it can be implemented in many different ways and that results may differ for different implementations. The difficulty to trace any instabilities in results (regression coefficients) to their sources (measurement errors) is a further weakness of the method. Extreme care must be taken in order to obtain reliable and stable results.

7.2 Comparison of results obtained with other methods Focus has been on the coastdown method in the projects described in 7.1. However, it is interesting to compare those results with results obtained by other methods. What is available are then the measurements using TUG equipment, reported in Chapters 5-6, although these are comparable only for the case of car coastdowns.

Due to different premises for the three methods, results are not fully comparable. For example, the drum results are obtained on a curved surface and despite there is a correction for this, the curvature may distort the results a little. Moreover, neither the drum nor the trailer measure unevenness effects in a representative way.

Still, results from the three methods, with regard to the MPD parameter, clearly point in the same direction. The CrMPD coefficient estimated from coastdown, trailer and drum measurement data, agreed well. The drum and RR trailer values are only somewhat higher than that of the coastdown (0.0017-0.0020 versus 0.0017).



8 SURVEY OF ROLLING RESISTANCE OF 40 DUTCH TEST TRACK SURFACES IN 2008

In 2008 TUG was contracted to conduct measurements of RR on the various test sections of the Kloosterzande test track in southern Netherlands. This had for some years been a test field for noise measurements within the huge Dutch IPG programme. In total 40 test sections were measured, using two test tyres: the SRTT and a Continental CPC2 LI98. The latter is a conventional car tyre. The tests are reported in [Lopez, 2010] and [van Blokland et al, 2009].

There were a few problems associated with the measurements that need to be noted:

• The test sections were very short; most of them were 80 m long, a few were only 40 m long. The actually measured lengths varied between 15 and 76 m, depending on test section. This is much shorter than normal measured distances and thus gives poor uncertainties, even though many extra runs were made to compensate for it.

• The test track surfaces were never exposed to regular traffic; they were in new unworn condition.

Nevertheless, as it is a large data set, the RRC of which was shown in [Sandberg (ed), 2011]. After publication of that report, VTI has received MPD values of 30 of these surfaces. This would be an excellent database, especially to see how the many porous surfaces in this data set behave in relation to dense surfaces. However, the MPD values, the origin of which is currently unknown, appear to be totally unrealistic (at least a factor 2 higher than would be normal).

Therefore, until the problem with the MPD values have been solved, the correlation between RRC and MPD will not be shown here.



9 RESULTS FROM THE BELGIAN ARTESIS PROJECT

9.1 Background In 2009-2011 there had been a cooperation project between BRRC and Artesis Hogeschool in Antwerp, in which Artesis had access to the BRRC trailer for making rolling resistance measurements [De Bie & Hofmans, 2011]. Earlier, they had also made coastdown measure-ments of rolling resistance, using texture equipment and test vehicles from BRRC. Most of the activities dealt with issues important for measurement methods; see [Sandberg et al, 2011]. Activities related to road surface influence on rolling resistance are reported below.

9.2 Correlation between RRC and texture In one part of the Artesis-BRRC project the RRC measurements were compared with texture spectral level measurements. From the results of the measurements the correlations (R²) between the RRC and the texture levels of the test sections were calculated; per each octave band in the texture spectra. The results of the most recent measurements are shown in the pink curve in Figure 9.1 [De Bie & Hofmans, 2011].

The results are compared with other research results of the same kind. The other studies are from [Descornet, 1990] and two groups of Artesis master students of the year 2009-2010 [Aerts & Cools, 2010e] [Aerts & Cools, 2010f] and [Dotsenko & Helsen, 2010]. The latter three used the coastdown method for rolling resistance measurements, while Descornet used the original BRRC trailer.

Figure 9.1: Comparison of RRC - texture spectra correlations obtained in the Artesis-BRRC project with previous research.

Three of the studies shown in Figure 9.1 show high correlations, in the wavelength range from 40 to 320 mm (i.e. mainly megatexture), but very low correlations are found in two of the



projects. The results of Descornet and also the results of the coastdown method from Aerts-Cools [Aerts & Cools, 2010e] give better correlations. The curve also has another shape and peak than found by Descornet and master students Aerts-Cools and Dostenko-Helsen. An explanation for this is probably that the trailer which was used for this research still had a lot of uncertainties (calibration issues, tyre temperature measurement, etc).



10 RESULTS FROM THE MIRIAM ROUND ROBIN TEST RELATED TO ROAD SURFACE INFLUENCE ON ROLLING RESISTANCE

10.1 Introduction The purpose of the Round Robin Test (RRT) was to

• assess the repeatability of the individual devices • evaluate how well the results of the trailers correlate with each other • assess the influence of the texture, expressed in terms of third of octave texture

levels, broad band texture levels and the Mean Profile Depth, on the rolling resistance • measure the influence of the tyres on the rolling resistance and how they classify the

pavements.

For this report it is the third bullet which is of interest. The following sub-chapters deal with this issue.

Texture measurements in the RRT were made by two different mobile laser profilometer systems; one owned by BRRC and the other owned by IFSTTAR. For the range of interest these have approximately equal performance, but the data from the BRRC system has been chosen for use in the data analyses.

These analyses and calculations (MPD, texture spectral levels, macrotexture level, megatexture level, skewness, etc) have been made both on the original profile curve and on a curve which has been modified (enveloped) by means of the procedure invented by von Meier; see further Annex A.

Rolling resistance measurements were made by three trailer systems: BASt, BRRC, and TUG. One to four tyres were used depending on the purpose and on the trailer system. See further [Bergiers et al, 2011] for more details. The speeds were 50 and 80 km/h, but except for the BRRC trailer, which is sensitive to tyre air resistance, there should be only marginal differences between the two speeds. Therefore, the choice of speed to show the results for here is often arbitrary.

10.2 Correlation between RRC and texture levels in third-octave bands The first and basic study is what correlation there is between the RRC values and the texture levels, as a function of texture wavelength, since this shows what part of the texture range that one should focus on. Or in other words: what part of the texture range that the measure-ment method and equipment are most sensitive to.

Figure 10.1 shows the correlation between the RRC values for the tyres tested by TUG at 80 km/h and texture spectral level. This is shown as R2 for each one-third-octave texture spectral band as a function of its texture wavelength. There is one curve for each of the four tyres tested by TUG.

It appears that the most important range (R2 > 0.70) is 160 to 500 mm texture wavelength; i.e. the longer megatexture range, although correlation is "fair" (R2 > 0.50) down to approx. 16 mm texture wavelength.

Figure 10.2 shows the same as the previous one, but when first modifying the profile curve to produce an enveloped profile. It appears that the most important range (R2 > 0.70) is now 20 to 500 mm texture wavelength; i.e., the entire megatexture range plus the rougher macro-texture range. Note that one shall not pay attention to the strange shape of the curve for wavelengths lower than 20 mm, since that range is corrupted by the enveloping procedure. The most important range is the part of the megatexture range at 100-500 mm.



Without enveloping - 80 km/h

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R²

AAV4/TUGSRTT/TUGES16/TUGES14/TUG

Figure 10.1: Correlations between RRC and texture spectral level as a function of texture wavelength, and for four tyres. Rolling resistance measurements performed at 80 km/h by TUG – without enveloping the texture; i.e. based on the original profile curve.

With enveloping - 80 km/h

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AAV4/TUGSRTT/TUGES16/TUGES14/TUG

Figure 10.2: As Figure 10.1 but after modifying the profile curve by applying the enveloping procedure to the texture; i.e. based on the original profile curve.

It would have been interesting to see what the curves would look like at longer wavelengths than 0.5 m but these were not measured by the texture measuring equipment. However, the IRI values will give some insight into this range; see below.

10.3 Correlation between RRC and macro- and megatexture levels LMa and LMe In this case the third-octave bands are brought together into much wider ranges; one range covering 0.63-50 mm (macrotexture) and the other covering 63-500 mm (megatexture). In practice no measurement equipment could cover the very shortest wavelengths, so the practical shortest wavelength is approx. 2.5 mm.

Invalid range



The correlation between RRC and macrotexture level LMa is shown in Table 10.1, for the various tyres, speeds and RR trailers used. Table 10.2 shows the same thing but when first applying enveloping to the profile curve. It is clear that enveloping is very favourable.

Table 10.1: Correlations (expressed as R2) between Cr (RRC) and macrotexture level LMa for various tyres, institutes and speeds. No enveloping applied to the profile curve.

BRRC BASt TUG

Speed [km/h] 50 80 50 80 50 80

AAV4 - - 0.33 0.08 0.37 0.41

SRTT - - 0.39 0.45 0.37 0.41

ES16 - - 0.65 0.54 0.48 0.51

ES14 0.62 0.35 - - 0.55 0.49

Table 10.2: Correlations (expressed as R2) between Cr (RRC) and macrotexture level LMa for various tyres, institutes and speeds. Enveloping applied to the profile curve.

BRRC BASt TUG

Speed [km/h] 50 80 50 80 50 80

AAV4 - - 0.60 0.10 0.67 0.71

SRTT - - 0.60 0.73 0.66 0.70

ES16 - - 0.80 0.61 0.77 0.79

ES14 0.57 0.28 - - 0.80 0.77

The next two tables, Table 10.3 and Table 10.4, show the same thing as 10.1 and 10.2, but for megatexture level, LMe, instead of macrotexture level.

It appears that correlations are higher for megatexture than for macrotexture; something which is to be expected according to the results shown in Chapter 10.2. When enveloping is applied, the correlations are even extremely high in some cases, as for TUG they are in the range of R2 = 0.91-0.94.

This is amazingly high degrees of explanation (> 90 % of the variance), considering the difficulties in rolling resistance measurements, and the high correlations together with the consistency in the results (for TUG) suggest that both rolling resistance and texture (at least megatexture) measurements must be of high quality. Also, the enveloping procedure must have worked fine for megatexture; i.e. it must give a relevant enveloping with respect to rolling resistance influences.

Note that both macro- and megatexture levels are calculated based on the rms value of the profile filtered in the macrotexture, respectively megatexture ranges. It means that they are not sensitive to the direction of the profile curve – whether it is a positive or negative profile. But when applying the enveloping, this problem is solved as mainly the part of the profile in contact with the tyre is counted.



Table 10.3: Correlations (expressed as R2) between Cr (RRC) and megatexture level LM e for various tyres, institutes and speeds. No enveloping applied to the profile curve.

BRRC BASt TUG

Speed [km/h] 50 80 50 80 50 80

AAV4 - - 0.53 0.17 0.63 0.67

SRTT - - 0.59 0.59 0.62 0.66

ES16 - - 0.79 0.43 0.69 0.72

ES14 0.56 0.30 - - 0.76 0.71

Table 10.4: Correlations (expressed as R2) between Cr (RRC) and megatexture level LM e for various tyres, institutes and speeds. Enveloping applied to the profile curve.

BRRC BASt TUG

Speed [km/h] 50 80 50 80 50 80

AAV4 - - 0.80 0.22 0.92 0.94

SRTT - - 0.77 0.83 0.91 0.93

ES16 - - 0.84 0.50 0.94 0.93

ES14 0.44 0.20 - - 0.94 0.94

10.4 Correlation between RRC and Mean Profile Depth (MPD) As discussed in Annex A, the MPD measure is inherently sensitive to the vertical direction of the texture. Thus, it will be interesting to see if this gives higher correlations than when using the rms-based levels in the previous sub-chapter.

To enhance comparisons, the same kind of tables as shown in the previous sub-chapter is used here. Table 10.5 shows the correlation between RRC and MPD when no enveloping has been applied, while Table 10.6 shows the correlation when enveloping has been applied before the MPD values were calculated.

It appears that MPD is a good descriptor of RRC as it gives R2 values for the TUG measure-ments of 0.81-0.92 without enveloping. With enveloping it even increases to 0.84-0.98. Therefore, one may conclude that MPD is fine without enveloping, as it is sensitive to the direction of the texture but, even so, it becomes better if the enveloping is first applied.

It is interesting also to look at the slope coefficients in the regression (RRC = constant + slope∙MPD). Tables 10.7-10.8 shows these slope coefficients.

The slope coefficients turn out to lie in the range 0.14-0.22 without the enveloping and 0.15-0.32 with the enveloping. The former values fit well with the range noted in the previous parts of this report.

Finally, Figure 10.3 shows an example of all the regression diagrams that one may produce. It is for the tyre SRTT measured by TUG. It appears that there is one data point that has a really high MPD, while the rest have low or medium values. This is not a perfect situation, but one may note that if the high point is omitted, there will still be a good correlation with almost the same slope.



Table 10.5: Correlations (expressed as R2) between Cr (RRC) and MPD for various tyres, institutes and speeds. No enveloping applied to the profile curve.

BRRC BASt TUG

Speed [km/h] 50 80 50 80 50 80

AAV4 - - 0.79 0.27 0.91 0.92

SRTT - - 0.77 0.82 0.91 0.92

ES16 - - 0.87 0.40 0.88 0.81

ES14 0.43 0.19 - - 0.88 0.90

Table 10.6: Correlations (expressed as R2) between Cr (RRC) and MPD for various tyres, institutes and speeds. Enveloping applied to the profile curve.

BRRC BASt TUG

Speed [km/h] 50 80 50 80 50 80

AAV4 - - 0.91 0.29 0.98 0.97

SRTT - - 0.70 0.93 0.98 0.97

ES16 - - 0.82 0.60 0.92 0.84

ES14 0.33 0.12 - - 0.87 0.93

Table 10.7: Slope coefficients in the regression of RRC versus MPD for various tyres, institutes and speeds. No enveloping applied to the profile curve.

BRRC BASt TUG

Speed [km/h] 50 80 50 80 50 80

AAV4 - - 0.0014 0.0016 0.0014 0.0015

SRTT - - 0.0024 0.0019 0.0020 0.0020

ES16 - - 0.0017 0.0010 0.0017 0.0018

ES14 0.0017 0.0014 - - 0.0016 0.0015

Table 10.8: Slope coefficients in the regression of RRC versus MPD for various tyres, institutes and speeds. Enveloping applied to the profile curve.

BRRC BASt TUG

Speed [km/h] 50 80 50 80 50 80

AAV4 - - 0.0020 0.0032 0.0018 0.0019

SRTT - - 0.0031 0.0027 0.0025 0.0025

ES16 - - 0.0021 0.0023 0.0021 0.0022

ES14 0.0020 0.0015 - - 0.0020 0.0018



TUG - SRTT

y = 0,0020x + 0,0055R2 = 0,9145

y = 0,0020x + 0,0058R2 = 0,9225

0,0000,0020,0040,0060,0080,0100,0120,0140,0160,0180,0200,0220,0240,026

0,0 0,5 1,0 1,5 2,0 2,5 3,0

MPD [mm]

C r

ES16/SRTT 50 km/hES16/SRTT 80 km/h

Figure 10.3: Correlation between MPD and Cr for the SRTT/TUG tyre (based on measure-ments performed by TUG). No enveloping applied.

10.5 Correlation between RRC and texture measures - Overall Figures 10.4 and 10.5 show a summary in bar diagrams of the correlations (expressed as R2) for the various texture measures used and for the tyres, speeds and trailers that were used in the tests. The intention is to give an easy to understand overview.

One may make the following observations:

• The MPD is the best measure, both without and with enveloping applied.

• Megatexture level is second; almost as good as MPD, but only when enveloping is applied.

• Enveloping is consistently successful.

• The TUG measurements give the most consistent results, with generally the highest correlations. Enveloped megatexture and MPD with and without enveloping give so high correlations that the rolling resistance measurements must have been of high quality to produce such results.

• The BRRC results are low at both speeds but especially at 80 km/h. BASt have a serious problem with certain cases (due to some outliers) but in most cases show good correlations.



Without enveloping

00,10,20,30,40,50,60,70,80,9

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G_50

ES14/TU

G_80

Trailer-Tyre-Speed combination

R²

MPDLMaLMe

Figure 10.4: Summarizing graph showing the correlations between RRC (Cr) and one of the texture measures MPD, LMa and LMe for all tyres and speeds and RR trailers. No enveloping applied.

With enveloping

00,10,20,30,40,50,60,70,80,9

1

ES14/BRRC_5

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SRTT/BASt_50

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G_50

ES16/TU

G_80

ES14/TU

G_50

ES14/TU

G_80

Trailer-Tyre-Speed combination

R²

MPDLMaLMe

Figure 10.5: Summarizing graph showing the correlations between RRC (Cr) and one of the texture measures MPD, LMa and LMe for all tyres and speeds and RR trailers. Enveloping applied.



10.6 Correlation between RRC and unevenness (IRI) The test track surfaces have not been produced with the ambition to vary unevenness; rather to keep unevenness as low as possible. However, when IRI was measured it turned out that the range was between 1.07 and 2.24. IRI of 1 is typical of a high-quality highway while IRI of 2.5 is typical of a quite poor highway, so the range is after all of interest.

As shown in [Bergiers et al, 2011], the correlation expressed as R2 between RRC and IRI was very low for all cases: always below 0.1 except for one measurement by BASt (R2 = 0.23). The BRRC trailer had a weak correlation with IRI (R2 = 0.35). It means that one may say that the TUG trailer seems to be insensitive to unevenness, but the BRRC and BASt trailers must be studied more in this respect.



11 RESULTS OF MEASUREMENTS IN MINNESOTA A very interesting and comprehensive experiment was made in September 2011 in Minne-sota, USA. TUG had transported their RR trailer to Minnesota for this study and made rolling resistance measurements on approx 70 different test sections, including both cement and asphalt concrete; some of them with very special textures. The TUG measurements supplemented an excellent database collected by MnDOT, including data on most test sections with regard to MPD, IRI, skewness, skid resistance and noise levels measured by the OBSI method.

The results there have been analyzed by TUG and VTI, and have been found to be very interesting and useful for this report, for many reasons, but permission to publish the results here has not yet been granted by the sponsor (MnDOT).



12 EFFECTS OF ASYMMETRIC PROFILES

12.1 Background As explained in Annex A, the asymmetry of the texture profile and the enveloping of the surface by tyres has essential implications on the suitability of macrotexture measures. The MPD takes asymmetry into reasonable account, but not enough to represent the enveloping of tyres. Annex A includes a detailed discussion about these issues.

In this chapter the most important experimental data collected so far and related to the correlation between rolling resistance and texture parameters are presented.

12.2 Work at TRL Ltd and Dunlop Tyres Ltd by Parry As part of the so-called MARS project, in the 1990's Parry at TRL conducted a study of relations between functional properties of road surfaces and texture [Parry, 1998]. This included some rolling resistance measurements made on the indoor drum facility at the Dynamics Laboratory at Dunlop Tyres Ltd in Birmingham. The drum was equipped with various epoxy replicas of real road surfaces, reproducing the texture of these; apart from the original smooth steel and sandpaper; see Figure 12.1.

Figure 12.1: The drum facility at Dunlop Tyres in Birmingham used for rolling resistance mea-surements, here equipped with three surfaces, from the left: sandpaper ("Safety Walk"), smooth steel, replica of HRA road surface. Photo given to the main author from Dr John Walker, Dunlop Tyres (1999).

In addition, four special surfaces with simple geometric asperities were cast from polymer resin and mounted on the drum. They were made with special geometrical patterns; see Figure 12.2. It is obvious that these surfaces had very special profile asymmetries.

hemispheres cubes tetrahedra discs

Figure 12.2: The special geometrically patterned surfaces produced for the tests. From [Parry, 1998].



For the rolling resistance measurements, three tyre types were used: a smooth-patterned (PIARC), a standard and a wide car tyre.

The texture measurements are interesting. Parry measured the rms value of the texture profile. Then he divided the rms value into two parts:

• The "contact" part (rmsc) which is the rms value of the part of the profile which is in contact with the tyres

• The "non-contact" part (rmsv) which is the rms value of the part of the profile which is not in contact with the tyres (subscript v is for "voids")

• The total rms value (= the arithmetic sum of rmsv and rmsc) is denoted rmst.

In principle, Parry in this way by rmsc designed an enveloping function, somewhat similar to those which are described in Annex A. He made the division of rms into two parts by means of a special software which had been developed to predict tyre/road contact areas. It was based on the finding that by mathematically analysing the profile measured by laser, the contact area of a smooth tyre could be accurately predicted.

Parry found that the texture of the contact part of the surface (rmsc) had the greatest influ-ence on the tyre-road contact pressure distributions. Both of these parameters are significantly related to the rolling resistance measurements for these drum shell surfaces; rmsc is the most strongly correlated and the results are shown in Figure 12.3.

0,5

1

1,5

2

2,5

0 0,2 0,4 0,6 0,8 1 1,2

rmsc

rolli

ng re

sist

ance

PIARCAB

Figure 12.3: Rolling resistance coefficient for the three tyres PIARC (smooth), Tyre A and Tyre B, versus texture described by the contact part of the profile rms value; i.e. rmsc. From [Parry, 1998].

These relationships were significant at the 5 % level for the PIARC tyre, 1 % for Tyre B and 0.1 % for Tyre A; where rmsc predicts 83% of the variability in rolling resistance. It can be seen that the PIARC tyre is comparatively insensitive to rmsc whereas the car tyres are more sensitive and have nearly parallel relationships.

It might be expected that the rolling resistance would be related to the texture of the surfaces but, for this special range of surface types, no relationship exists between rolling resistance



and the rms of the texture profile curve. It is only by determining the texture of the actual contact surface that the rolling resistance behaviour can be explained, according to Parry. Overall, it was concluded that rolling resistance:

a) could be predicted from the predicted contact area and roughness of the surface in contact with the tyre, but not from wavelength characteristics (spectra of macro- and megatexture),

b) was related to the shape of the texture; sharper asperities increase rolling resistance, c) was related to the aggregate size; smaller was better.

12.3 Swedish tests in 2011 on polishing off the top of the surface In the summer of 2011 the main author made an experiment to create a road surface with an extreme skewness – an extremely "negative-textured" surface. The basis was a double-layer porous asphalt with 11 mm max chippings in the top layer, laid on motorway E4 through Huskvarna in Sweden. It was one year old at the time of the experiment.

A 60 m long and 0.6 m wide section of this porous asphalt was polished by rotating discs in a machine supplied by HTC Sweden AB in Söderköping, Sweden. Approximately 1-2 mm from the peaks in the texture was polished off, leaving a flat surface of each major chipping facing upwards. After this procedure the surface was cleaned by a very strong vacuum cleaner. Figure 12.4 shows the unpolished and polished textures from a low angle.

Figure 12.4: The unpolished (left) and polished (right) textures on the double-layer porous asphalt on E4 in Huskvarna, seen from a low angle. The coin diameter is 22 mm.

Later, rolling resistance was measured by the TUG trailer, using the same three tyres as described earlier and speeds of 50 and 80 km/h (posted speed on the site is 90 km/h). Since the polished section was only 60 m long, and max 50 m could be utilized for the measurements, as many as 10 runs were made for each tyre/speed combination.

The results are shown in Figure 12.5. They show that creating a flat surface for the tyres to roll on – an extremely negative skewness - is very important in order to reduce rolling resistance in an optimal way.

The polishing is expected to be possible to make at a cost reasonable enough to polish road surfaces at a large scale.



0,000

0,002

0,004

0,006

0,008

0,010

0,012

0,014

0,016

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50 80Speed [km/h]

Rol

ling

resi

stan

ce c

oeff

icie

n

SRTT - Surf polished SRTT - Surf not polished

MCPR - Surf polished MCPR - Surf not polished

AAV4 - Surf polished AAV4 - Not polished

Figure 12.5: Results of the rolling resistance measurements comparing the polished surface with the unpolished surface, for the three tyres and two speeds.

12.4 Results of tests in Minnesota in 2011 Skewness was one of the texture parameters available for comparison with rolling resistance in the study made in Minnesota, USA, in September 2011 by TUG. The results there are interesting in this respect, but permission to publish the results here has not yet been granted by the sponsor (MnDOT).

12.5 Results in the MIRIAM Round Robin Test (RRT) in 2011 Enveloping was applied in the RRT and turned out to be very successful. See Chapter 10 for an extensive presentation of this issue.



13 INFLUENCE OF TYRES ON THE ROAD SURFACE EFFECT ON ROLLING RESISTANCE

The selection of reference tyres for rolling resistance measurements on road surfaces and the effect various tyres have on the relation between RRC and texture, is a very interesting and important issue. However, it is mainly a measurement methodology issue; thus this subject is dealt with in [Sandberg et al, 2011] instead of here.



14 OVERVIEW OF RESULTS

14.1 General Most of the results summarized below are based on measurements of rolling resistance by means of towed light vehicle trailers. These seem to be insensitive to losses in suspension systems. In a few cases coastdown measurements with full vehicles have been made and in such cases one would include also suspension losses in the "rolling resistance" estimations.

14.2 Macro- and megatexture levels (based on rms of profiles) The RRT in MIRIAM in 2011 showed that macrotexture level does not seem to be a good measure, probably because it is neutral with respect to vertical direction of the texture. Megatexture level is much better but not ideal.

This seems to be especially important in case of special textures, as was concluded by [Parry, 1998].

14.3 MPD The MPD has so far consistently appeared to be a measure with very good correlation with rolling resistance. It has the potential to explain approx. 90 % of all variance in an experiment relating RRC with MPD. If one would try to estimate an average for all experiments reported here, with higher weight on results with good correlation RRC-MPD, the slope coefficient should be in the range 0.0017-0.0020. Maybe 0.0020 would be a suitable value to use temporarily, as it is an even number, and easy to remember.

14.4 Enveloping Enveloping has been tried in two experiments reported here: the MIRIAM RRT and the study by Parry. In both cases it has turned out to be very successful; increasing the correlation between texture measures and RRC substantially. MPD after enveloping the profile curve has the potential to explain some 95 % of the total variance in an RRC-texture regression.

14.5 Unevenness and IRI So far, no specially designed experiments exploring the effects of unevenness on rolling resistance with control of texture have been made. The indication in the MIRIAM RRT is that the TUG trailer seems to be rather insensitive to unevenness, at least for low-to-moderate IRI, whereas the BRRC and BASt trailers must explore this issue more. Especially, the BRRC trailer seem to be influenced by IRI (R2 = 0.35). Measurements of relations between rolling resistance and unevenness and texture at various spatial wavelengths have indicated that the unevenness range is important when testing cars full-scale, most probably due to losses occurring in the suspension system.

A problem is how one can take the suspension losses into account in a reproducible and representative way when not using a full-scale car or truck, and "only" a light trailer. To make coastdown measurements with full cars or trucks pose serious problems of many kinds, albeit it is a possibility.



14.6 Texture spectral effects The data reported here suggest that the most important texture range for generation of rolling resistance is the megatexture range, while also the macrotexture range is of some importance. One study suggested that the fine macrotexture range would be the most important range, but this study stands alone in this respect against several others, and might have been caused by the special selection of test surfaces (on a drum). Those studies which have looked at the importance of the unevenness wavelength range have indicated that shortwave unevenness, 0.5-5.0 m, the wavelengths where the IRI is most sensitive, is of great importance, but the longer wavelengths may be neglected.

14.7 Other pavement effects There are no experimental indications so far that microtexture plays a big role in the genera-tion of rolling resistance, but this could be due to the lack of dedicated studies.

The question of whether stiffness plays a role is still unresolved. At low temperatures and for light vehicles it seems that one may forget this effect, unless the surface is softer than a normal asphalt concrete pavement. At high temperatures and for heavy vehicles, there are indications that stiffness is important, but it is not yet known whether this may be only for very soft surfaces or also for normal asphalt concrete.

Until further explored one shall include pavement stiffness in any source model.

Surface condition, such as snow or ice cover, or water on the surface, should be studied more. Such issues absolutely influence rolling resistance but no quantification of such effects is available.

Rutting is not included in the studies in this sub-project, but it is obvious that it has effects on rolling resistance.

14.8 Design of low rolling resistance pavements It is clear that in order to obtain low rolling resistance it is important to have a flat surface (aggregate faces) on which tyres can roll. Spaces between such flat aggregates should not be too wide, as the tyres may deflect down into part of such valleys and thus cause energy losses.

14.9 The data reported here suggest that the most important texture range for generation of rolling Interactions with vehicle type

The data reported here almost exclusively deal with light vehicles, N B cars. This goes also for the tyres used, as they have been of a dimension typical for (mostly large) cars. An exception is the use of the AAV4 tyre, which is in fact a light truck tyre (but having a dimension fitting some large cars) and which is hoped to be able to function as a proxy for heavy vehicle tyres. Results so far, reported in [Sandberg t al, 2011], seem to indicate that it may well work in this way, but much more measurements with heavy trucks and heavy truck tyres are needed. There are expectations that surface textures that are very good for low rolling resistance for light vehicles may not be equally favourable for heavy vehicles, but much more research is needed to clarify this issue.



15 CONCLUSIONS AND PROPOSED PRELIMINARY MODEL The results presented in this report show the following:

Rolling resistance is not only a property of tires, but is also a property of the pavement which is of high importance for the energy consumption in the road transport sector and must be systematically considered along with other functional properties in pavement management systems.

As an example, in the MIRIAM RRT, the range of surfaces on the test track (MPD from 0.08 to 2.77 mm) the rolling resistance coefficient for the test tyres increased from the smoothest to the roughest of the surfaces by 21 - 55 %, depending on the tyre type. Such rolling resistance differences correspond to roughly 7 - 18 % in fuel consumption differences, using calculations made in SP 2 of MIRIAM for light vehicles driving on a typical two-lane highway at 90 km/h (to be published in January 2012).

The range in rolling resistance between the best and worst pavements in the MIRIAM partner countries in Europe is at least 50 % (the worst has an RRC 50 % higher than the best), although the more common pavements exposed to high traffic flows show a range of 20-25 % in rolling resistance.

Macrotexture, represented by the parameter MPD, is a major factor influencing rolling resistance. MPD is particularly suited for this purpose as it is sensitive to the vertical direction of the peaks and valleys in the profile curves.

Especially, MPD calculated on an enveloped profile curve seems to give excellent correlation with rolling resistance. It is so well correlated with rolling resistance that it will be difficult to find a better single or major variable for the purpose of quantifying the pavement influence on rolling resistance.

Megatexture level might be an alternative parameter, albeit not really as good as MPD, provided it is calculated on an enveloped profile curve. The advantage with this measure is that it is easier to measure by road survey vehicles using profilometers.

The relation between rolling resistance coefficients and MPD is rather consistent measured in different and independent measurement series reported here. The currently best estimate is a coefficient X of 0,0017 to 0,0020 in an equation of RRC = X*MPD + Y, where Y is a constant depending on a large number of factors. The coefficient 0.0020 might be an attractive option as it is easy to remember and to use.

There has been in the past, and to some degree still is, a substantial bias between various series of measurements made by presently available rolling resistance trailers, a "day-to-day" variation; the source of which is not yet known. But it is believed that temperature is part of the solution and that uncertain calibration might be another part of the solution.

It is proposed that a tentative source model for the pavement influence on rolling resistance contains the following significant pavement parameters:

MPD, IRI, pavement stiffness.

Of these three, the MPD and IRI are certainly needed, but the need for stiffness is yet a bit uncertain.

For light vehicles the IRI effect on rolling resistance is probably around 1/3 of that of the effect of MPD. It may be higher for heavy vehicles. Nevertheless, it shall not be neglected.



The best source model for the road surface influence is currently proposed to be:

Rolling resistance coefficient = Constant + 0.0020∙MPD + X∙IRI where MPD is Mean Profile Depth in mm, measured according to ISO 13473-1

and X is a constant yet to be determined

and "Constant" is a value unique to a certain tyre and several other circumstances; usually around 0.008 to 0.012.

This simple model is useful over a speed range of at least 50-110 km/h for the rolling resistance part of the driving resistance. Suspension losses are included only if the IRI term above is specified by assigning a number to its constant "X".

It is based on light vehicle data. For heavy vehicles, one may use the same model, scaled to representative values of Cr for heavy vehicle tyres, as long as no better model is available, but one must be aware that it is very uncertain for this category.

It is noted that MPD and IRI are collected widely in most countries already, at least for the national and regional road networks. Thus, the use of these variables will be easy to implement.

Data on pavement stiffness is not commonly collected, but in this case one may find proxy variables, such as a distinction between classes of pavements (cement concrete, asphalt concrete, non-paved surfaces, new and old pavements, temperature, etc).

In the future, it is recommended to develop an enveloping procedure that can be used internationally to calculate more appropriate MPD values for rolling resistance purposes. The RRT procedure constitutes a good start.

The work with the rolling resistance property of pavements has only just started. It is a very young discipline and a lot more research is needed in the near future; not the least about measurement methods.

In the special chapter about Recommendations, several suggestions for urgent and important future research are presented.



16 RECOMMENDED FURTHER STUDIES For Phase 2 of MIRIAM it is recommended to include the following types of studies:

Most of all there is a need to consider rolling resistance of heavy vehicles and its relation to road surface parameters. To this end, more coastdown measurements are needed and also trials with trailers for heavy vehicles should be made.

The relation between rolling resistance and texture parameters should be studied more, using a wider range of road surfaces, since the range in the MIRIAM RRT and other studies so far has been too limited. In particular more cement concrete surfaces should be included and also more special textures.

The influence of IRI on rolling resistance measurements should be studied both for full vehicle systems including suspension systems in operation, and special rolling resistance trailers such as the ones tested in the RRT.

In particular it should be studied what the IRI influence is on the measurements of the BASt and BRRC trailers.

A system should be constructed that is able to representatively measure the suspension losses for uneven roads, perhaps as a supplement to existing trailers, or perhaps as a kind of model that may predict such influences from IRI measurements.

The pavement stiffness effect on rolling resistance must be studied much more, at conditions when it is likely to be important, and to learn for which types of pavements that it need to be considered (if any).

Variations from day-to-day of the trailers, especially the TUG trailer, must be studied.

The BRRC trailer should be modified to include an enclosure protecting the test tyre from the air flow around the tyre.

The enveloping procedure used in the RRT should be studied and tested more widely, and made available to other users than BRRC. One should also look for better versions of it that are easy to apply and give realistic enveloping.

One should consider the performance and possibilities of the present and future rolling resistance trailer systems when measuring in wheel tracks on roads; they shall not measure just in the middle between wheel tracks.

The source model in this report should be further developed, improved and validated; not only for car tyres but also for heavy vehicle tyres.

It is necessary to work out a full and complete international measurement standard for rolling resistance.

A new RRT should be organized, and it should include also coastdown vehicles, and both light and heavy vehicles and their tyres shall be included. Road surface and pavement parameters shall be mapped in more detail than in the present RRT. The selection of surface textures and other properties should be wider than so far.



17 REFERENCES Aerts, J.; Cools, M. (2010e); "Rolling resistance of a few Belgian Roads. Rolling resistance determined by texture measurement and coast down method." Non-published paper, Artesis Hogeschool, Antwerp, Belgium.

Aerts, J.; Cools, M. (2010f); “Masterproef: Rolweerstand van Belgische wegdekken - gemeten met textuurmetingen en uitrolproeven”. MSc Thesis 2009-2010, Artesis Hoge-school, Antwerp, Belgium (in Flemish).

Bergiers, Anneleen; Goubert, Luc; Anfosso-Lédée, Fabienne; Dujardin, Niels; Ejsmont, Jerzy A.; Sandberg, Ulf; Zöller, Marek (2011): "Comparison of Rolling Resistance Measuring Equipment - Pilot Study". Deliverable No. 3 of MIRIAM. Downloadable from the MIRIAM website (see Foreword).

Cenek, P.D. (1994): “Rolling Resistance Characteristics of New Zealand Road Surfaces”. Vehicle Interaction, ASTM STP 1225, B.T. Kulakowski, Ed., American Society for Testing and Materials, Philadelphia, 1994, pp. 248-262.

Clapp, T.G. (1984): “Spectral correlation of the surface profile in the development of a tire and pavement interaction model”. Master Thesis, University of North Carolina, Raleigh, NC, USA (1984)

Clapp, T.G.; Eberhardt, A.C.; Kelley, C.T. (1988): "Development and validation of a method for approximating road surface texture-induced contact pressure in tire-pavement inter-action". Tire Science and Technology (TSTCA), Vol. 16, No. 1, January-March 1988, pp 2-17. De Bie, H.; Hofmans, C. (2011): "Rolweerstand van Belgische wegdekken". Unnumbered paper (2010-2011), Artesis Hogeschool, Antwerp, Belgium.

DeRaad, L.W. (1978): ”The Influence of Road Surface Texture on Tire Rolling Resistance”. SAE Technical Paper 780257, Society of Automotive Engineers, Pennsylvania, USA.

Descornet, Guy (1990): ”Road-Surface Influence on Tire Rolling Resistance”. Surface Characteristics of Roadways: International Research and Technologies, ASTM STP 1031, W.E. Meyer and J. Reichert, Eds., American Society for Testing and Materials, Philadelphia, USA.

Dotsenko, V.; Helsen, B. (2010); “Masterproef: Rolweerstand van Belgische wegen - gemeten met de ARW-aanhangwagen”. MSc Thesis 2009-2010, Artesis Hogeschool, Antwerp, Belgium (in Flemish).

ECRPD (2010): "Energy Conservation in Road Pavement Design, Maintenance and Utilisation". ECRPD Final Publishable Report, Grant Agreement: EIE/06/039/SI2.448265, Intelligent Energy Executive Agency (IEEA). Downloadable from http://www.ecrpd.eu/

Fong, Sandy (1998): "Tyre noise predictions from computed road surface texture induced contact pressure". Proc. of Inter-Noise 1998, Christchurch, New Zealand.

Jamieson, N.J.; Cenek, P.D. (2002): "Effects of Pavement Construction on the Fuel Consumption of Trucks". Paper presented at the 5th annual NZ Institute of Highway Technology Ltd conference, 6-8 October 2002, Auckland, New Zealand.

Karlsson, Rune; Hammarström, Ulf; Sörensen, Harry; Eriksson, Olle (2011): "Road surface influence on rolling resistance – Coastdown measurements for a car and an HGV". VTI Notat 24A-2011, Swedish Road and Transport Research Institute (VTI), Linköping, Sweden.

Klein, P. and Hamet, J.-F. (2004): “Road texture and rolling noise - An envelopment procedure for tire road contact”, report LTE 0427 of the Institut National de recherché sur les

http://www.ecrpd.eu/



transports et leur sécurité (INRETS), Lyon (2004). Can be downloaded from the EU project SILVIA's website: www.trl.co.uk/silvia

Lidström, Mats (1979): “Aircraft Rolling Resistance in Loose Dry Snow: A theoretical analysis”. VTI Report 173A, Swedish Road and Transport Research Institute (VTI), Linköping, Sweden.

Olsson, Annika (1984): "Matematisk modell för rullmotstånd i vatten". MSc thesis LIU-MAT-EX-84-44, Linkoping University, Linkoping, Sweden.

Parry, A.R. (1998): "Macrotexture and road safety: Final report". Project Report PR/CE/56/98 CO67L/94 Issue 1, TRL Ltd, Crowthorne, U.K.

Sandberg, Ulf (1990): ”Road Macro- and Megatexture Influence on Fuel Consumption”. Surface Characteristics of Roadways: International Research and Technologies, ASTM STP 1031, W.E. Meyer and J. Reichert, Eds., American Society for Testing and Materials, Philadelphia, USA.

Sandberg, Ulf; Ejsmont, Jerzy A. (2002): "Tyre Road Noise Reference Book". Published by INFORMEX HB, Kisa, Sweden (www.informex.info). Sandberg, Ulf; Glaeser, Klaus-Peter; Ejsmont, Jerzy A.; Schwalbe, Gernot (2008): “The influence of tyre wear and ageing on tyre/road noise emission and rolling resistance”. Deliverable No. C.D6, Project SILENCE. Downloadable from: http://www.transguide.org/Statistik/TRAX_ED/redirect.asp?url=http://www20.vv.se/fud-resultat/Publikationer_000701_000800/Publikation_000755/SILENCE_CD6_080902_VTI_final.pdf Sandberg, Ulf (ed) (2011): “Rolling Resistance – Basic Information and State-of-the-Art on Measurement methods”, Report MIRIAM_SP1_01, project MIRIAM (2011). Downloadable from http://www.transguide.org/VTI%20publ/MIRIAM-SoA-Report-Final-110601.pdf (accessed 2011-11-13).

Sandberg, Ulf; Bergiers, Anneleen; Goubert, Luc; Anfosso-Lédée, Fabienne; Ejsmont, Jerzy A.; Zöller, Marek (2011): "Rolling Resistance – Measurement Methods for Studies of Road Surface Effects". Deliverable No. 2 of MIRIAM. Downloadable from the MIRIAM website (see Foreword).

Sander, K. (1996): „Vergleichsmessungen des Rollwiderstandes auf der Straße und im Prüfstand“, Berichte der BASt, Fahrzeugtechnik, Heft F20, 1996.

Sayers, M.; Karamihas, S. (1998): "The Little Book of Profiling". UMTRI, University of Michigan, Ann Arbor, MI, USA.

Sävenhed, Hans (1986): “Vehicle Fuel Consumption on Different Types of Wearing Courses”. VTI reprint 107, Swedish National Road and Transport Research Institute (VTI), Linköping, Sweden.

Van Es, G.W.H. (1999): ”A method for predicting the rolling resistance of aircraft tires in dry snow” National Aerospace Laboratory NLR, Technical Report NLR-TP-99240, Amsterdam, the Netherlands.

von Meier, A.; van Blokland, G.J.; Descornet, G. (1992): “The influence of texture and sound absorption on the noise of porous road surfaces”, Proceedings of the PIARC Second International Symposium on Road Surface Characteristics, pp. 7-19, Berlin, Germany (1992).

Williams, Roger (1981): "The influence of tyre and road surface design on tyre rolling resistance". Paper IP 81 - 003 at the Institute of Petroleum (IP) conference 1981, London, U.K.

http://www.trl.co.uk/silvia

http://www.informex.info/

http://www.transguide.org/Statistik/TRAX_ED/redirect.asp?url=http://www20.vv.se/fud-resultat/Publikationer_000701_000800/Publikation_000755/SILENCE_CD6_080902_VTI_final.pdf

http://www.transguide.org/Statistik/TRAX_ED/redirect.asp?url=http://www20.vv.se/fud-resultat/Publikationer_000701_000800/Publikation_000755/SILENCE_CD6_080902_VTI_final.pdf

http://www.transguide.org/VTI%20publ/MIRIAM-SoA-Report-Final-110601.pdf



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A. Annex A: Asymmetric profile curves and enveloping procedures

A.1 Introduction This Annex presents the concepts of skewness and enveloping related to pavement textures in more detail than the main text does.

A.2 Asymmetric profile curves and skewness A possible asymmetry of the road surface profile, see Figure A1, should potentially have sig-nificant influence on the rolling resistance. A 'positive' texture (exhibiting protrusions) should show a significantly different behaviour in functional qualities than a negative texture (exhibiting depressions). Despite such profiles may give similar texture spectra, the way they deform the tyre is completely different. The positive texture deforms the tyre much more, while the tyre runs relatively smoothly over a surface with negative texture. It is quite obvious that this should influence properties such as skid resistance or noise generation, but also rolling resistance.

Figure A.1: Examples of surface profiles of positive macrotexture (left) and negative macrotexture (right). Skewness of the left profile would be positive (somewhat > 0) while it would be substantially negative for the right profile (<< 0). A measure quantifying the asymmetry; i.e. how positive or negative a given texture is, is its skewness. Skewness of the profile, rsk, is defined in ISO 13473-2 as the quotient of the mean cube value of the ordinate values Z(x) and the cube of the rms value, within an evaluation length ℓ, according to the equation:

( )

= ∫

0

d11sk xxZrms

r 33

Skewness is dimensionless. Skewness (or just "skew") is a measure of assymmetry of the amplitude distribution (in this case of the ordinate values). This indicates whether the profile curve exhibits a majority of peaks directed upward (positive skew) or downward (negative skew). For a normal distribution rsk is zero.



A.3 MPD as a measure of asymmetry and its relation with skewness The Mean Profile Depth, MPD, defined in ISO 13473-1 (see Chapter 3 of the main text) is a measure where the peak values occurring in the segments have a more important weight than the valleys. In this way, the MPD value is already a measure which is sensitive to the asymmetry of the profile. It may at first be expected that the MPD is well correlated with the skewness as it is sensitive to the asymmetry. However, the experience so far does not verify this expectation. In Figure A.2 the skewness and the MPD values of the IFSTTAR test tracks in Nantes, which were used in the RRT study [Bergiers et al, 2011] are plotted against each other. The two para-meters appear to have no correlation at all for this data set. It may be concluded that MPD is not fully describing the asymmetry of the profile curve.

y = 0,1578x - 0,606R2 = 0,0671

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Figure A.2: Skewness versus MPD for the IFSTTAR test tracks in Nantes, France.

A.4 Tyre tread enveloping of texture When a tyre runs on a textured road surface, it does not necessarily make contact with all points on the surface in its wheel path. This is, e.g., the case when the texture shows deep and irregular “valleys” (such as on porous asphalt) or deep and relatively regular “grooves" (such as on transversally grooved concrete). The tyre is said to be "enveloping" the part of the surface with which it is in contact. It has been known already since the beginning of the 1990's that the fact that a tyre envelops only part of the surface of the pavement plays an important role for the prediction of tyre/road noise. As it is related with the way how the road texture deforms the tyre rolling over it, it should also be important for the aspect of rolling resistance. More or less complex ways of modelling the tyre enveloping of road surface textures have been developed and tried in various projects. Such models may be used to process the texture profile before calculating the texture spectrum or other texture parametres. During this process, called “enveloping”, the points on the profile which are not in contact with the tyre because they lie to deep are discarded and replaced by a point with a higher amplitude, supposed to be in contact with the tyre tread.



The enveloping procedure requires tyre property data, e.g. Young's modulus, and Poisson coefficient, in order to determine how well the tyre tread may envelop the texture, from which a mathematical function can be applied to the profile curve with the effect of cutting away the non-contact points. The ideal is to produce a new curve which follows the tyre tread deflect-tion. There are several ways of doing this, of which these may be the most commonly used ones:

o A mathematical/empirical method proposed by von Meier et al. [von Meier et al, 1992]

o A tyre-physics-based method originally proposed by Clapp [Clapp, 1984], later improved by Clapp et al [Clapp et al, 1988]

o Clapp et al's method improved by Fong [Fong, 1998] o Clapp et al's method improved by Klein and Hamet [Klein and Hamet, 2004].

The method by von Meier et al is not based on a physical model but is an empirical procedure based on the mathematical limitation of the second-order derivative. For a discrete texture profile, one can express this as follows:

(zi – (zi-1 + zi+1)/2) / x² ≤ d* where zi is the amplitude of the i-th point of the profile, x the sampling step and d* the value to which the second derivative of the enveloped profile will be limited. The parameter d* is a measure of the softness of the tyre. A value for d* representing the average stiffness of car tyres is proposed to be 0.054 m-1. A 100 mm long sample of a profile measured on the IFSTTAR test track on a porous asphalt 0/6 is shown in Figure A.3. Three enveloped profiles calculated with the von Meier method are shown, corresponding to a very soft tyre (d* = 0.1 m-1), a stiff tyre (d* = 0,01 m-1) and a "medium tyre".

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Figure A.3: A 100 mm long profile measured on porous asphalt 0/6 on the IFSTTAR test track in Nantes processed (enveloped) by the method by van Meier et al, using three selected constants representing tyre stiffness.



The texture spectra of the corresponding profile curves are shown in Figure A.4.

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Figure A.4: Third-octave-band texture spectrum of the original profile of the porous asphalt 0/6 IFSTTAR test track in Nantes, compared to the spectra representing the three enveloped curves. These spectra are measured and calculated over a length of 210 m. One can see that the main action of the enveloping is suppression of the amplitudes at the shorter texture wavelengths, due to a “smoothening” effect. In contrast to van Meier's method, Clapp’s envelopment procedure is based on a physical model. It consists in evaluating the contact between a rigid body (indentor, in this case the textured pavement) and a semi-infinite elastic body (the tyre). The tyre is characterized by means of its Young's modulus and Poisson's ratio. Clapp solves the problem of finding the displacement of the indented elastic body, but does so in a quite approximate way. In the enveloped profile, straight lines are drawn between consecutive peaks over valleys which are too deep to be reached by the tyre rubber. This seems to be a quite rough approximation and not totally realistic, while von Meier's method instead lacks realism in that the rubber rides only on rather small points created by the profile peaks, which should give very high local contact pressures. The Clapp method was later improved by Fong in New Zealand [Fong, 1998], although still with straight lines for tyre rubber surfaces without texture contact. Further development of Clapp's method was made by Klein and Hamet at INRETS, introducing the mathematical concept of Green’s functions. They proposed an iterative algorithm that rapidly converges and yields realistic enveloping curves, which are similar to what is obtained with the von Meier method. The disadvantage of the INRETS method is the complex mathematical calculations needed and it is far from sure that the results are



significantly “better” than those obtained with the simple and fast von Meier method. Further research is needed on this topic. For the time being, all envelopment calculations on profiles in the MIRIAM project have been carried out with the von Meier method. The authors assume that this is at least a good first approximation of the true enveloped curves.

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