report on state of the art of test methods seventh framework programme theme 7 transport,

Tyre and Road Surface Optimisation for Skid Resistance and Further Effects

Project Coordinator Mr. Manfred HAIDER, arsenal research, Austria phone: +43 50550 6256, e-mail: [email protected] internet: http://tyrosafe.fehrl.org

This project is part of the FEHRL Strategic Research Programme “SERRP IV” (www.fehrl.org).

Coordination Action FP7-217920 Seventh Framework Programme Theme 7: Transport

D07

Report on state-of-the-art of test surfaces for skid resistance

The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement n°217920

Main Editor(s)

Peter Roe, TRL, UK Phone: +44 1344 770 286, E-Mail: [email protected]

Minh-Tan Do, LCPC, France Phone: +33 2 40 84 57 95, E-Mail: [email protected]

Due Date 1st March 2009

Delivery Date 25th February 2009

Work Package WP2 Harmonisation of skid-resistance methods and choice of reference surfaces

Dissemination Level Public (PU)

TYROSAFE Deliverable 07: The use of reference surfaces for calibration and harmonisation of skid resistance measurements

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Contributor(s)

Main Contributor(s) Peter Roe, TRL, UK Phone: +44 1344 770 286, E-Mail: [email protected]

Contributor(s)

(alphabetical order)

Minh-Tan Do, LCPC, France Phone: +33 2 40 84 57 95, E-Mail: [email protected]

Review

Reviewer(s) Peter Saleh, arsenal research, Austria

Bjarne Schmidt, DRI, Denmark

Manfred Haider, arsenal research, Austria


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Control Sheet Version History

Version Date Editor Summary of Modifications

0 (partial draft)

09 Jan 2009 Peter Roe Initial structure outline and draft Introduction

0.1 26 Jan 2009 Min-Tan Do Addition of draft of text and graphics for Chapter 3

0.2 (first near-complete draft)

30 Jan 2009 Peter Roe

Addition of: first draft text for Chapter 2; revision of text and figures for draft Chapter 3; first draft of text for Chapter 4 (incorporating contribution from BAST); first draft text for Chapter 5; outline points for Chapters 6 and 7.

0.3 02 Feb 2009 Peter Roe

Further minor editorial changes plus incorporation of further suggestions from Minh-Tan Do. Removal of proposed Discussion Chapter 6 since main discussion points covered in earlier chapters.

First Full Draft for Partner comment

0.4 09 Feb 2009 Minh-Tan Do and Peter Roe

Revised in light of partner comments. Chapter 5 restructured and converted to Discussion which now includes previous content, with additional discussion ideas developed from partner comments. Research suggestions form HERMES moved from conclusions to the Discussion and kept in the context of HERMES. Addition of Executive Summary

Full draft for peer review

1.0 16 Feb 2009 Peter Roe Final version implementing Peer Review comments

2.0 25 Feb 2009 Manfred Haider Release version

Final Version released by Circulated to

Name Date Recipient Date

Manfred Haider, Project Coordinator

2009/02/25 Coordinator 2009/02/25

Consortium 2009/02/25

European Commission 2009/02/25


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Table of Contents 1 Introduction ...................................................................................................................12 2 Background ...................................................................................................................16

2.1 What is meant by “calibration”? ..................................................................................16 2.2 What is a “reference surface”? ...................................................................................19

3 Analysis of the state of the art for test surfacings for checking skid resistance measurement equipment..............................................................................................21

3.1 The purpose of the survey and the questionnaire ......................................................21 3.2 Overview of the survey responses..............................................................................21 3.3 General aspects of test surfaces ................................................................................28 3.4 Surfacing properties....................................................................................................30

4 Test surfaces for assessing vehicles and tyres.........................................................34 4.1 Test surfaces used by the motor industry...................................................................34 4.2 The use of drum machines for tyre assessment.........................................................34 4.3 Noise assessment surfaces........................................................................................35

5 Discussion .....................................................................................................................36 5.1 Historic attempts to develop reference surfaces.........................................................36 5.2 Work on reference surfaces in the HERMES project..................................................38 5.3 Some research suggestions based on the ideas proposed by HERMES...................46 5.4 Further discussion ......................................................................................................48

6 Conclusions...................................................................................................................50 7 References .....................................................................................................................51 8 Appendices ....................................................................................................................52

8.1 Questionnaire template...............................................................................................52


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Abbreviations

Abbreviation Meaning

ABS Antilock Braking System

BFC Braking (force) Friction Coefficient (=LFC)

EFI European Friction Index

IFI International Friction Index (developed in the 1992 International PIARC Experiment to Compare and Harmonize Skid Resistance and Texture Measurements)

IRFI International Runway Friction Index (developed in the American Joint Winter Runway Friction Measurement Program, described in ASTM E2100)

LFC Longitudinal (force) Friction Coefficient

MPD Mean Profile Depth (as defined in ISO 13473-1 and ISO 13473-2)

SFC Sideway (force) Friction Coefficient

SRI Skid Resistance Index (=EFI)

HERMES Harmonisation of European Routine and Research Measurement Equipment for Skid Resistance of Roads and Runways (FEHRL project)

JWRFMP Joint Winter Runway Friction Measurement Program (led by Transport Canada and NASA)

SPENS Sustainable Pavements for European New member States (FP6 project)

VERT Vehicle-road-tyre interaction: fully integrated physical model for handling behaviour in potentially dangerous situations (BRITE EURAM project)

ASTM American Society for Testing and Materials

BASt Bundesanstalt für Strassenwesen (DE)

BRITE Basic Research in Industrial Technologies for Europe

CEDR Conference of European Directors of Roads

CEN European Committee for Standardization

COST European Cooperation in Science and Technical research

DRI Danish Road Institute

FAA Federal Aviation Administration (USA)

FEHRL Forum of European National Highway Research Laboratories


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ISO International Standards Organisation

LCPC Laboratoire Central de Ponts et Chaussées (FR)

NASA National Aeronautics and Space Administration (USA)

PIARC Permanent International Association of Road Congresses

RWS Rijkswaterstaat = Department of public works and infrastructure of Ministry of transport (NL)

TRL Transport Research Laboratory (UK)

IMAG Instrument de Mesure Automatique de Glissance (FR)

IRV International IRFI Reference Vehicle

PFT Pavement Friction Tester (UK, TRL)

RoadSTAR Road Surface Tester of Arsenal Research

ROAR Road Analyser and Recorder of Norsemeter

SCRIM Sideway-force Coefficient Routine Investigation Machine

SKM Seitenkraftmessung

SRM Stuttgarter Reibungs Messer (DE)


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Definitions

Term Definition

Adhesion

The transmission of forces by friction against tyre contact surfaces. Resulting from the interaction between tyres and pavement surface, adhesion is influenced by surface roughness, tyre characteristics, the nature and thickness of any intermediate medium such as water or mud, and speed.

Airfield operational testing

Measurement of the skid resistance of a surface on an airfield in response to an operational need and in whatever conditions exist at the time of the test, which may include contamination by ice, snow, slush or water.

Bound surface Top layer or surface course of a road with the aggregates secured permanently in place

Braking force coefficient

Ratio between the longitudinal frictional force and the load on the test tyre, the test tyre mass and the rim mass. This coefficient is without dimension.

Calibration

Periodic adjustment of the offset, the gain and the linearity of the output of a measurement method so that all the calibrated devices of a particular type deliver the same value within a known and accepted range of uncertainty, when measuring under identical conditions within given boundaries or parameters.

Contact area Overall area of the road surface instantaneously in contact with a tyre.

Fixed slip Condition in which a braking system forces the test wheel to roll at a fixed reduction of its operating speed.

Fixed-slip friction Friction between a test tyre and a road surface when the wheel is controlled to move at a fixed proportion of its natural speed.

Friction Resistance to relative motion between two bodies in contact. The frictional force is the force which acts tangentially in the contact area.

Horizontal force (drag)

Horizontal force acting tangentially on the test wheel in line with the direction of travel.

Horizontal force (side force)

Horizontal force acting perpendicular to a freely-rotating, angled test wheel.

Longitudinal friction coefficient (LFC)

Ratio between horizontal force (drag) and vertical force (load) for a braked wheel in controlled conditions. This is normally a decimal number quoted to two significant figures.

Macrotexture Deviation of a pavement from a true planar pavement with characteristic dimensions along the pavement of 0.5 mm to 50 mm, corresponding to texture wavelengths with one-third-octave bands including the range


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0.63 mm to 50 mm centre wavelengths.

Mean profile depth

Descriptor of macrotexture, obtained from a texture profile measurement as defined in EN ISO 13473-1 and EN ISO 13473-2.

Megatexture Roughness elements with a horizontal length of 50 to 500 mm. Roughness of this magnitude can influence accumulations of water on the pavement surface (for instance, in unevenness).

Microtexture

Deviation of a pavement from a true planar pavement with characteristic dimensions along the pavement of less than 0.5 mm, corresponding to texture wavelengths with one-third-octave bands and up to 0.5 mm centre wavelengths.

Nearside wheel path

Wheel path that is closest to the edge of the road in the normal direction of travel. For countries that normally drive on the right, this is the right-hand side and for countries that normally drive on the left, this is the left-hand side.

Operating speed Speed at which the device traverses the test surface.

Pedestrian slip resistance

The property of the trafficked surface to maintain the adhesion of a pedestrian shoe sole.

Push mode When the device is pushed by a pedestrian

Repeatability r

The maximum difference expected between two measurements made by the same machine, with the same tyre, operated by the same crew on the same section of road in a short space of time, with a probability of 95 %. (This equals 2.77 times the repeatability standard deviation: r = 2.77 * σr)

Reproducibility R

The maximum difference expected between two measurements made by different machines with different tyres using different crews on the same section of road in a short space of time, with a probability of 95 %. (This equals 2.77 times the reproducibility standard deviation: R = 2.77 * σR)

Routine testing Measurement of the skid resistance of a surface in standardized test conditions, which normally include a defined water flow rate.

Sampling length/interval

The distance over which responses of the sensors are sampled to determine a single measurement of the recorded variables.

Side force coefficient (SFC)

Ratio between the vertical force (load) and horizontal force (side force) in controlled conditions. This is normally a decimal number quoted to two significant figures.

Skid resistance Characterisation of the friction of a road surface when measured in accordance with a standardised method.

Slip angle The angle between the mid-plane of the test tyre contact surface and the direction of travel.


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Slip ratio Slip speed divided by the operating speed.

Slip speed Relative speed between the test tyre and the travelled surface in the contact area.

Subsection Defined length of surface for which one set of the measured variables is reported by the device.

Test section Length of road between defined points (e.g. location references, specific features, or measured distances) comprising a number of subsections over which a continuous sequence of measurements is made.

Theoretical water film thickness

Theoretical thickness of a water film deposited on the surface in front of the measuring tyre, assuming the surface has zero texture depth.

Tow mode When the device is towed by a vehicle

Vertical force Force applied by the wheel assembly (the static and dynamic force on the test tyre, the test tyre weight and the rim weight) on the contact area.

Water delivery system

System for depositing a given amount of water in front of the test tyre so that it then passes between the tyre and the surface being measured.

Water flow rate Rate (litres/second) at which water is deposited on the surface to be measured in front of the test tyre.

Wet road skid resistance

Property of a trafficked surface that limits relative movement between the surface and the part of a vehicle tyre in contact with the surface, when lubricated with a film of water.

Wheel paths Parts of the pavement surface where the majority of vehicle wheel passes are concentrated.

List of Figures Figure 1.1 Harmonisation scheme and actions carried out in WP 2 ......................................14 Figure 3.1 Geographical distribution of received questionnaires ...........................................23 Figure 3.2 Number of respondents in the different roles in relation to skid-resistance

measurements ................................................................................................................24 Figure 3.3 Numbers of devices of various types used by the respondents............................25 Figure 3.4 Number of respondents using test surfaces for the three main purposes.............26 Figure 3.5 Number of respondents using test surfaces for accreditation tests at various

frequencies......................................................................................................................26 Figure 3.6 Number of respondents using test surfaces for day-to-day checks at various

frequencies......................................................................................................................27 Figure 3.7 Number of respondents using the different types of reference for comparisons

during accreditation.........................................................................................................27


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Figure 3.8 Number of respondents using the different types of location for test surfaces .....28 Figure 3.9 Length of test sections used for accreditation tests two different countries ..........29 Figure 3.10 Lengths of test sections used for day-to-day checks in two countries ................30 Figure 3.12 Skid resistance levels for test surfaces used by TRL............................................1 Figure 3.12 Skid resistance levels for test surfaces used by CETE Lyon ................................1 Figure 3.13 Comparison of texture depth and skid resistance levels on test surfaces ............1 Figure 5.1 Specially formed surfaces using geometric shapes (l-r): hemispheres, cubes,

tetrahedra, cylinders........................................................................................................42 List of Tables Table 1.1 Overview of the major outcomes of the individual Tasks of WP 2 .........................15 Table 3.1 List of respondents to the questionnaire ................................................................22 Table 3.2 Types of surfacing materials used as test surfaces ...............................................30 Table 5.1 Broad combinations of texture parameters suggested by the HERMES team for a

range of reference surfaces for skid resistance device calibration .................................39 Table 5.2 Artificial aggregates that might be used in a reference surfacing...........................41 Table 5.3 Outline requirements for Calibration Reference surfaces ......................................45


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Executive Summary Many different devices have been developed to measure skid resistance or road surface friction. They all assess skid resistance by measuring friction between rubber and the wet road surface in some way. However, there is no absolute value of skid resistance against which a measuring device can be compared. While it is possible to make test surfaces or identify in-service roads that have levels of skid resistance within a certain range, it is not possible to tell in advance what the actual skid resistance will be – especially as the measured value varies from one device to another anyway. The development of true reference surfaces would be a significant step forward on the road towards a harmonised approach to skid resistance measurement and reporting.

The purpose of this report is to review the topic of test surfaces used for checking and calibrating skid resistance measuring equipment and the potential use of reference surfaces to contribute to the harmonisation purposes. The outcomes of this part of the study, in conjunction with deliverables D04 and D05 will then feed into the next stage of the TYROSAFE project to develop a road map for future harmonisation.

For a survey of current practice, a detailed questionnaire was sent to project partners and, through them, manufacturers, operators of test equipment and those organisations responsible for equipment accreditation were approached. The purpose of this questionnaire was to see how different countries use test surfaces on roads or test tracks to calibrate their measurement devices, in the absence of “true” reference surfaces with known skid-resistance characteristics.

The general conclusion drawn from this part of the work was that many different surfaces are used as test surfaces, most often (but not exclusively) made from conventional road-building materials. However, because they do not have access to test tracks (there are not many of these in Europe), most organisations use in-service roads for their calibration checks. Consequently, the selection of test surfaces is not based just on a specific combination of friction and texture levels but on what is readily available on the road networks concerned. Another consequence of the usage of in-service roads is that the range of friction levels that can be used is limited and low-friction surfaces are missing.

The second aspect covered by this report was the potential use of purpose-made reference surfaces that would have predictable, stable and reproducible skid resistance characteristics. This topic was covered extensively in the FEHRL project HERMES and a literature review for TYROSAFE did not reveal any further published information on the topic. The HERMES report made suggestions as to what the general characteristics of such surfaces might be and how their construction might be approached. However, it was clear then, and remains so now, that research is still necessary to develop such surfaces from a practical point of view.

The choice of suitable materials to achieve predictable and stable performance will be difficult, so work to identify materials that could be reliably specified for use for the reference surfaces will need to be a fundamental aspect for research. The challenge is to find suitable combinations of a regular and repeatable form and level of macrotexture with appropriate treatments or additives to provide predictable, controlled and durable microtexture. This, in turn, will need to be combined with consideration of the contribution of different test tyre compounds and their potential interaction with any proposed surfacing material.


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1 Introduction The safe passage of road traffic needs a certain amount of grip (friction) between the tyres of the vehicles and the road surface. The frictional forces are necessary for the vehicle to accelerate, decelerate or safely change direction. The level of frictional forces that can be built up depends on the properties of both the road surface and the tyres. Much research has shown that the limiting frictional forces for a given road surface and tyre combination depend on many factors, including tyre load, tyre tread compound and depth, road surface characteristics, the presence of water, ice or other contaminants in the tyre/road interface and vehicle speed. In order to characterise road surfaces with respect to friction, for decades many countries have derived their own test methods. These are, of necessity, very much simplified in order to assess specifically the condition of the road surface. They all measure in some way the frictional force developed between a moving tyre or slider and the road surface (which is usually wetted) and record the quotient of the measured force with the applied vertical load (a friction coefficient). For each test method the effects of many of the potential influencing factors are controlled by standardising the measuring conditions. The standard conditions chosen reflect the practicalities of carrying out the particular test and are assumed to be relevant for characterizing the complex reality of friction in the tyre/road interface. Usually the measurement is called the “skid resistance” and is represented by a single value. Because the test methods and the chosen conditions vary, the actual numbers recorded can differ widely for the same road surface. Several European countries have investigated the link between skid resistance level and accident rates. The result of this research is that with a sufficiently high value of skid resistance the safety of roads can be improved by reducing the risk of skidding and hence the number or severity of accidents. Many European countries have developed their own skid policies for the road networks for which they are responsible. The approaches vary between countries but they often contain elements such as periodic routine monitoring of skid resistance of in service road and comparing the results with pre-determined values. In some countries the measurements are also used for comparison with acceptance levels for new works. As has been explained, the available standardized test methods all simplify the reality of the complex friction process in the tyre/road interface during vehicle manoeuvres and they do that in different ways. It therefore should be no surprise that a direct comparison of skid values from country to country is not an easy task. Also the relevance of the different test methods with respect to safety will be different since the techniques and standardised test conditions reflect different aspects of the tyre/road friction mechanism. For example, at one extreme, some methods simulate conditions close to those experienced by a tyre braking under the control of an anti-lock braking system while, at the other, some devices use a skidding locked wheel.


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Some individual countries set standards for skid resistance on their road networks (or parts of them) based on measurements with devices local (and often unique) to them. However, the absence of an accepted common scale for characterizing road surfaces with respect to skid resistance properties is a serious hindrance for developing consistent policies for skid resistance that would make the European road network safer. The TYROSAFE Project is a Coordination and Support Action (CSA) in the Seventh EU Framework Programme and aims at coordinating and preparing for European harmonisation and optimisation of the assessment and management of essential tyre/road interaction parameters to increase safety and support the greening of European road transport. This work is being carried out in the following six work packages (WP):

WP1: Policies of EU countries for skid resistance / rolling resistance / noise emissions;

WP2: Harmonisation of skid-resistance test methods and choice of reference; surfaces

WP3: Road surfaces properties – skid resistance / rolling resistance / noise emissions;

WP4: Environmental effects and impact of climatic change – skid resistance / rolling resistance / noise emissions;

WP5: Dissemination and raising awareness; WP6: Management.

The objective of Work Package 2 of TYROSAFE is to arrive at a widely-supported road map towards future skid-resistance harmonisation policy by 2020, including aspects such as testing equipment, quality assurance and implementation strategy. The major field of application in mind is for monitoring the skid resistance quality of the European road network and for new work acceptance control. Basically, the lines being followed are those formulated in 2005 by the CEN working group on Surface Characteristics (CEN/TC227 WG5), to prepare in the longer term (over 10 years) “a harmonised standard based on the measurement of a friction index with a common and single European friction measuring equipment”. The harmonisation process is illustrated in Figure 1.1, along with actions to be carried out in WP2 of TYROSAFE.


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Local (national) test methods

Local friction devices Local ref. surfaces

EU test method

SESRD Reference surfaces

2008

2010

2015

2020

Partners

+ Experts

+ Road authorities

Correlationlocal/reference- research needs- QA procedure

State of the art(current practices, previous projects, standards)

Development

Specifications Specifications

Alternatives

Pilot tests

State of the art- existing standards

2.1 2.2

2.3

2.4

Local (national) test methods

Local friction devices Local ref. surfaces

EU test method

SESRD Reference surfacesSESRD Reference surfaces

2008

2010

2015

2010

2015

2020

Partners

+ Experts

+ Road authorities

Partners

+ Experts

+ Road authorities





Development

Specifications

Development

Specifications Specifications

Alternatives

Specifications

Alternatives

Pilot testsPilot tests



2.1 2.2

2.3

2.4

Figure 1.1 Harmonisation scheme and actions carried out in WP 2

To reach its objective, WP2 is split into four Tasks:

In Task 2.1 knowledge of current national skid resistance test methods will be collated, together with findings of previous harmonisation research projects, which will be collected and analysed. Based on the outcomes of these exercises, proposals will be formulated for possible options for the specification of a Standard European Skid Resistance Device (SESRD).

In Task 2.2 the focus will be on the use and harmonisation of reference surfaces in the Quality Assurance part of the harmonisation policy as was suggested by the HERMES project.

In Task 2.3, based on the results of Task 2.1 and 2.2, a road map or implementation plan will be developed to point the way towards a harmonised approach to wet skid resistance test methods by 2020. Special attention will be paid to intermediate stages (2010, 2015) to allow for the need for individual countries to make a smooth transition to the new approach. The focus in this transition period will be to maintain consistency with existing historical data and to maximize the possible use of the present fleet of testing devices until the end of their technical working lives. This Task will also initiate promoting activities for finding a number of pilot countries for early implementation in their national monitoring programmes.

To obtain constructive input from stakeholders and experts and to mobilize support for the road map/implementation plan, several workshops will be organised in Task 2.4.

Table 1.1 gives an overview of the major outcomes planned for the individual Tasks of WP 2.


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Table 1.1 Overview of the major outcomes of the individual Tasks of WP 2

Task Deliverable Name Month 2.1 D04 Report on state-of-the-art test methods M5 2.1 D05 Report on analysis and findings of previous

skid resistance harmonisation research projects

M8

2.2 D07 Report on state-of-the-art of test surfaces for skid resistance

M8

2.3 D09 Road map and implementation plan to future harmonised test methods and reference surfaces

M16

2.4 - Two dedicated workshops M5 and M10 This report is the main output from Task 2.2 and constitutes the deliverable D07. Its main purpose is to a review the topic of test surfaces used for checking and calibrating skid resistance measuring equipment and the potential use of reference surfaces to contribute to the harmonisation purposes. The outcomes of this part of the study, in conjunction with deliverables D04 and D05 will then feed into the next stage of the project to develop a road map for future harmonisation. Chapter 2 provides background to the topic of this report to set the context for the Chapters that follow. This chapter discusses the problem of calibration, what reference surfaces are, why they might be needed and how they might be used. Chapter 3 presents an analysis of a survey made specifically for this project to establish current European practice in the use of test surfaces for checking skid resistance measurement equipment. Chapter 4 briefly discusses the use of test surfaces in relation to assessing commercial road tyres (as opposed to specialised skid resistance test tyres). Chapter 5 is a Discussion covering a number of specific issues relating to requirements for special reference surfaces for use in calibration and harmonisation, in particular considering material properties and how they might be constructed for effective use in a European harmonisation context. This chapter incorporates work from the HERMES project and possible research ideas. The final Chapter 6 summarises the findings of the report.


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2 Background Many different devices have been developed to measure skid resistance or road surface friction. As explained in detail in the companion report TYROSAFE Deliverable D04 “Report on state-of-the-art of test methods”, they all measure skid resistance by measuring friction between rubber and the wet road surface in some way, typically utilising one of three basic principles:

• Measure longitudinal friction (using a measurement wheel that is forced to rotate more slowly than the vehicle speed requires, so it slips or skids over the surface).

• Measure transverse friction (using a measurement wheel that can rotate freely but is set at an angle to the direction of the test vehicle, so that it slips over the surface).

• Use a slider mechanism (in which a pendulum arm with a rubber pad attached, or a rotating head with a number of rubber feet attached, slows down as it passes over the surface).

Task 2.1 of the TYROSAFE project has found that at least 24 different devices are in current use across Europe to measure skid resistance for various purposes. All three of the basic principles are represented but the ways in which these principles are implemented differs from one device to another (some devices can use combinations of them). There is also a wide range of specific test conditions, such as test speed and test tyre properties (for vehicle methods), and different approaches are taken to processing the recorded data for different uses. This range of principles and operating practice gives rise to a range of different numerical values that are reported to represent the skid resistance of the road that is tested. Consequently, if there is to be a wider, harmonised, approach to reporting skid resistance across Europe in order to encourage more consistent standards and improved road safety, it will be essential to establish a harmonised scale through which different measurement techniques can be compared. Over the years there has been considerable research effort attempting to establish such a harmonised approach, a topic discussed in detail in TYROSAFE Deliverable D05 “Report on analysis and findings of previous skid resistance harmonisation research projects”. In the process, significant limitations have been identified, in particular associated with finding ways to ensure that measurement devices are adequately calibrated. It is this aspect of the process of measuring skid resistance that is behind this report.

2.1 What is meant by “calibration”? Calibration has been defined (see Definitions table at the front of the report) as “Periodic adjustment of the offset, the gain and the linearity of the output of a measurement method so that all the calibrated devices of a particular type deliver the same value within a known and accepted range of uncertainty, when measuring under identical conditions within given boundaries or parameters.” In other words, the process of calibration makes sure that a


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measurement device always gives the same result within close, known tolerances. For devices designed to measure skid resistance, there are two components to this: “static” and “dynamic” calibration or calibration checks.

2.1.1 Static calibration Skid resistance measurement devices effectively utilise a “sensor” which is a tyre or rubber slider that moves over the road surface. The systems then typically use transducers to convert the physical forces developed on the sensor into electrical signals that can then be used to compute a value for skid resistance. One of these transducers will measure the horizontal reaction force (longitudinal or transverse) between the tyre (or slider) and the road. Some devices assume that a constant vertical load is applied to the test wheel but many devices are fitted with another transducer, to measure the vertical force acting on the test wheel at the same time as the horizontal reaction is measured. For some devices this transducer may be built into the system that applies and controls the load. Typically, the ratio of the vertical load and reaction force is used to determine an instantaneous value for the friction and these values are then stored temporarily for converting into skid resistance measurements to represent a given length of road. The exception to this approach is the Pendulum Tester, which simply has a pointer to indicate the extent to which the pendulum swing was affected by the friction between the rubber slider and the road. For all the devices it is necessary to calibrate the transducers, to verify that they and their associated electronic systems are operating correctly. Although the physical technique for doing this varies from one device to another, the principle is always to apply a known force to the sensor (for example, by means of calibrated static weights or using a screw mechanism to apply the force which is measured with a separate, independently-calibrated force transducer) and to observe the measured response of the device. Typically, the applied force is varied through the device’s working range to check that the expected response is obtained. If necessary, the gain or offset of the transducers (or their mechanical equivalent) are adjusted to give the required response. Even those devices that assume an applied load must undergo checks to verify the magnitude of static load that is being applied. These processes are known collectively as “static” calibration because they have to be carried out while the test device is stationary.

2.1.2 Dynamic calibration checks While static calibration processes can readily verify that all the measurement transducers in a device are working correctly and that the mechanical components are applying the correct loads in static conditions, in practice the device will be used with the sensor moving along the road (or, in the case of the pendulum or a rotating-head device, sliding over the surface) in dynamic conditions.


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In these circumstances, the forces are developed by the reaction of the tyre or slider to the road surface in the specific test conditions. These in turn may be influenced by dynamic factors such as unevenness in the road surface affecting the applied load, general “noise” resulting from the real effects of a tyre slipping on a road surface, and so on. The effects may vary depending on the speed of the test and a host of other factors relating to the road surface condition (see D05, Chapter 2). Thus, operators of the devices have to make the assumption that if the static calibration of the transducers was within the required tolerances, the skid resistance values recorded must be satisfactory, too. However, there is always a possibility that a device may not respond in dynamic conditions in the expected manner and factors may come in to play that cannot be detected in static tests of the measurement transducers. For example, on SCRIM machines in the UK it was found over the years that the response of the dampers on the test wheels that absorb some of the shock from uneven road surfaces could affect the response of the machines in dynamic conditions. This led to markedly different measurements from different machines even though the static calibrations were all within the required tolerances. This particular problem has been overcome to some extent by careful specification and routine replacement of the dampers and also by measuring the vertical load dynamically (as is the case on UK machines) but, even so, dynamic problems may still not be detected if reliance is placed on static calibration alone. Therefore, operators of skid resistance measuring equipment usually need to run their machines over a road surface somewhere to verify the response in dynamic conditions. This becomes particularly important where several similar devices are being operated and it is expected that they should give the same results on the same road. The major difficulty that is faced by all devices in this context is that of determining what the “correct” result for a measurement on that particular surface at that particular time should be. In reality, road surface skid resistance varies with time as a result of traffic and seasonal influences. Even a surface on a test track that carries little general traffic may still be subject to some variations over time due to seasonal effects, weather conditions or ageing of the surfacing material. Consequently, operators are obliged to rely on the average result from a number of measurements to provide a level of skid resistance for the surface against which devices may be compared. This means making regular observations and comparing any one measurement with the average or range of values which are normally obtained, looking for unusual deviations from what is normally expected to provide an indication that there may be a problem with the equipment in dynamic conditions. For example, in the UK, while static calibration on an individual SCRIM is carried out daily during the operating season, that machine is also required to make weekly dynamic checks.


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2.1.3 Calibration to a common scale Not only are dynamic checks needed to assess whether a device is measuring skid resistance correctly when its transducers are correctly calibrated, but it is also necessary to ensure that where fleets of similar devices are operated, they also report comparable values. In other words, they all measure within known tolerances on a common scale. The common scale at its simplest is that represented by the measurement of that particular type of device when operated under its standard conditions, be that LFC, SFC or some derivative value. In a harmonised environment such as that being considered by the TYROSAFE project, this might be a special scale on which the device is able to report. Such calibration is achieved at present by knowing what the “correct” result for a surface should be and then comparing individual devices with it. They will then be regarded as “calibrated” if their measurements fall within the expected tolerances for the surface. Deciding on the “correct” value is typically achieved in one of two ways:

• Make measurements with a number of similar devices on the test surface(s) and use the average value of all the devices to provide the level at which comparisons will be made. This strategy also requires knowledge of the measurement precision; outliers can then be identified and removed so that they do not unduly influence the result.

• Designate one individual device to provide the reference level by definition and make a large number of measurements with it. This is the approach used in Germany with the SKM where a “golden” machine provides the reference level but measurements are made over several kilometres of road to establish what that level is. However, there is still no absolute check on whether the “golden” device is recording correct values in dynamic conditions other than by comparison with historic records.

Having established the “correct” value, devices may be regarded as “calibrated” if their average measurement falls within the expected tolerance. This is the approach used in the UK at annual accreditation trials for SCRIM, for example. Alternatively, a small calibration coefficient may be used to modify the result to offset identified systematic differences, a practice followed in Germany with the SKM. However, whichever approach is used, the checks still rely on an average measurement from one or all machines to provide a level for comparison. There is no absolute standard and it remains possible that devices or fleets of devices could drift over time.

2.2 What is a “reference surface”? We have seen that there is no absolute value of skid resistance against which a measuring device can be compared. While it is possible to make test surfaces or identify in-service roads that have levels of skid resistance within a certain range, it is not possible to tell in advance what the actual skid resistance will be – especially as the measured value varies from one device to another anyway.


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If skid resistance measuring systems are to be truly calibrated, either to their own scale or to a common, harmonised, scale, then reliable reference surfaces that produce known levels of friction are required. To be an effective reference, the properties should not only be predictable but should also remain stable; they should not change significantly with either age or repeated use. For the purposes of this report, therefore, the term “reference surface” means a surface of this type, with defined, predictable and stable properties. The development of true reference surfaces would be a significant step forward on the road towards a harmonised approach to skid resistance measurement and reporting. The next two chapters of this report review current practice in this area, firstly (Chapter 3) relating to surfaces used to check skid resistance measurement devices in dynamic conditions and secondly (Chapter 4) relating to surfaces used to test commercial tyre (and, by implication, vehicle braking) performance. Chapter 5 is a Discussion focussed specifically on issues relating to the development of reference surfaces for skid resistance device calibration and harmonisation.


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3 Analysis of the state of the art for test surfacings for checking skid resistance measurement equipment

As part of the TYROSAFE project, a review was made of current practice in relation to test surfacings across Europe. A detailed questionnaire was sent to project partners and, through them, manufacturers and operators of test equipment and those organisations responsible for equipment accreditation were approached to find out how dynamic checks were made and what surfacings were used. This chapter presents an analysis of the findings of this survey.

3.1 The purpose of the survey and the questionnaire The purpose of the survey was to find out about current practices where different surfaces are used to compare measurement devices. Potentially, this could help to identify approaches that might be developed towards an absolute reference system. The questionnaire (See Appendix, section 8.1) was divided into five parts. Part 1 asked participants about their particular organisation and its role in using skid resistance test surfaces. Parts 2-4 asked about the types of surface and the way they were used in that context, with a further section at the end for any additional comments. Regarding the ways in which surfaces might be used, three possibilities were identified which related mainly to different organisations’ roles and the questionnaire was constructed to reflect this:

• As part of a national accreditation process to verify acceptable operation of individual machines (or a fleet of similar machines) before they are used to gather data on a network;

• By operators of measurement devices for day-to-day checks on the dynamic operation of their machines;

• By manufacturers of measurement devices to verify that new or recently-serviced machines are operating to their satisfaction.

(In the sections that follow, these three roles/uses of surfaces are called “accreditation”, day-to-day checks” and “quality checks”, respectively). Respondents were asked to provide summary information about where the surfaces for each role that they were involved with were located, what they were made of and their general skid resistance properties.

3.2 Overview of the survey responses

Twenty-two organisations responded to the Questionnaire or had responses completed by project partners on their behalf (Table 3.1).


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Table 3.1 List of respondents to the questionnaire

Organisation Country

Technical University of Vienna Austria

Arsenal Research Austria

BRRC Belgium

Central Road and Bridge Laboratory Bulgaria

Mereni PVV Czech Republic

Danish Road Directorate Denmark

CETE Lyon France

BASt Germany

PMS Pavement Management Services Ltd

Ireland (Eire)

Israel National Road Company Israel

Latvia State Roads Latvia

RWS Netherlands

Serbian Roads Directorate Serbia

ZAG Slovenia

Yorkshire County Council United Kingdom

Derbyshire County Council United Kingdom

DRD Road Service (Northern Ireland) United Kingdom

Findlay Irvine United Kingdom

Jacobs Engineering UK Ltd United Kingdom

Surrey County Council United Kingdom

Powy County Council United Kingdom

TRL United Kingdom

Figure 3.1 illustrates the geographical distribution of the 13 countries represented (Israel is outside the area covered by this map). It can be seen that Southern European countries, together with most Scandinavian ones, are missing. These represent countries that regularly experience extremes of weather over long periods (hot periods in summer, and ice and snow in winter). The UK was well represented (eight completed questionnaires, seven from the mainland and one from Northern Ireland). Clearly, the synthesis in the following paragraphs reflects the responses received and may not be fully representative of European practice.


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Figure 3.1 Geographical distribution of received questionnaires

3.2.1 Respondents’ roles in relation to skid resistance measurement From a general understanding of current practices, three main roles were identified for inclusion in the questionnaire: accreditation, measurement provider and device manufacturer. A fourth role, that of research organisation, was also included since such organisations might have developed specialised surfaces. A fifth category “other” provided for other functions to be identified. It should be noted that some respondents could have multiple roles. The roles of the respondents to the questionnaire in relation to skid-resistance measurements are summarized in Figure 3.2.

UK (7) Ireland (1)

Northern Ireland (1)

France (1)

Belgium (1)

Netherlands (1)

Denmark (1)

Germany (1)

Austria (2)

Serbia (1)

Slovenia (1)

Czech Rep (1) UK (7)

Ireland (1)

Northern Ireland (1)

France (1)

Belgium (1)

Netherlands (1)

Denmark (1)

Germany (1)

Austria (2)

Serbia (1)

Bulgaria (1)

Slovenia (1)

Czech Rep (1)

Latvia (1)

Israel (1) not on map


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0

2

4

6

8

10

12

14

16

Accreditation authority

Measurement service provider

Device manufacturer Research Other

Figure 3.2 Number of respondents in the different roles in relation to skid-resistance

measurements

It can be seen that device manufacturers were not well represented among the replies received, explaining why few results are presented later for the use of surfaces dedicated to quality checks. Most respondents were involved in research and measurement activities.

3.2.2 Devices covered The distribution of the measuring devices owned by the respondents is shown in Figure 3.3. For consistency with the classification employed in the companion TYROSAFE report D04 [1], only devices for which Technical Specifications are being drafted by CEN are listed separately. Their descriptions can be found in the D04 report. Other devices, which might also measure texture depth, are grouped in the “other” category.


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0 2 4 6 8 10 12

ADHERA

BV11

SFT

GRIPTESTER

RoadSTAR

ROAR

RWS trailer

SCRIM

Skiddometer BV8

SKM

SRM

TRT

Other than listed by CEN

Figure 3.3 Numbers of devices of various types used by the respondents

It can be seen that SCRIM and GripTester are the dominant individual devices used by respondents to the questionnaire. Part of the explanation is that a third of the responses were provided by organisations from the UK where both SCRIM and GripTester are manufactured and widely used. These two types of device are also widely used elsewhere in Europe, albeit more commonly on airfields than roads in the case of GripTester.

3.2.3 Usage of test surfaces Figure 3.4 shows the distribution of the use that the various responding organisations make of their test surfaces. Not surprisingly, routine verification – the “day-to-day check” – is much more widely practiced than accreditation. Three respondents indicated that, although they fulfilled one of the roles indicated in the questionnaire, they did not make use of test surfaces.


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0

2

4

6

8

10

12

14

16

18

accreditation day-to-day checks quality checks

Figure 3.4 Number of respondents using test surfaces for the three main purposes

The use of test surfaces for accreditation checks is also widely practiced, the frequency of which is shown in Figure 3.5. Two organisations carry out such checks every month and five do not appear to make regular checks. However, five carry out the process of accreditation annually. This frequency tends to be applied to large fleets of measurements devices – such as SCRIM and GripTester in the UK, or ADHERA and GripTester in France – for which the organisation of calibration tests is time consuming. Shorter frequencies are used by organisations dealing with only one or two devices at once.

0

1

2

3

4

5

6

every month every 6 months every year as required other

Figure 3.5 Number of respondents using test surfaces for accreditation tests at various frequencies


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The frequency for day-to-day checks is more variable between organisations (Figure 3.6). The commonest practice is to perform these routine verifications when required (for example before a measurement campaign). Weekly or monthly checks are practiced too, especially where equipment is in regular use. Some organisations carry out day-to-day checks only during the testing season.

0

2

4

6

8

10

every day every week every month as required Figure 3.6 Number of respondents using test surfaces for day-to-day checks at various

frequencies

3.2.4 Definition of reference level An important aspect of the use of test surfaces is the way in which the reference level – that is, the level of skid resistance that is regarded as the “true” result for the surface – is established. Eight organisations responded on this point (Figure 3.7).

0

1

2

3

4

5

single "golden" average all machines average selected machines

Figure 3.7 Number of respondents using the different types of reference for comparisons during accreditation


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For comparisons between devices belonging to the same fleet, the average value of all machines was generally used. Those participants dealing with only one or two machines use the concept of the “golden” machine as the reference with just one organisation taking the mean of a sub-set of machines to provide a reference level for accreditation purposes.

3.3 General aspects of test surfaces

3.3.1 Location

Since test surfaces have to be traversed by the measurement devices being checked, they need to be of a reasonable physical size. As will be discussed in Chapter 5, it is difficult to make surfaces to have specific levels of skid resistance and therefore organisations may use selected sections of in-service roads to serve as test sections. This restricts the range of surfacings that might be used (especially at lower skid resistance levels) but does mean that the surfaces are representative of roads on the network.

Where special surfaces are to be laid (3.4.1), however, these are often placed on test tracks where they are not subjected to extensive trafficking and they can have properties that might not be acceptable on an in-service road. Test tracks also have the advantage that they offer greater control over the testing environment. Figure 3.8 shows the types of location on which the respondents’ test surfaces were placed.

0

5

10

15

test tracks roads both

Accreditation tests

Day-to-day checks

Figure 3.8 Number of respondents using the different types of location for test surfaces

It can be seen that most organisations use in-service roads for their accreditation and day-to-day checks. Very few responding organisations use test tracks - which can be explained by the fact that test tracks of this type are few in Europe: the best known are those at TRL (UK) and LCPC (France). Other tracks or proving grounds exist, usually operated by or on behalf of car and tyre manufacturers, for testing vehicle handling, braking systems or tyre performance (see


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Chapter 4). However, such tracks were not explicitly included in the questionnaire and are not normally used for assessing skid resistance devices.

3.3.2 Numbers and length of test sections used The questionnaires revealed that the number of test sections used by different organisations for the various purposes varied widely. Some organisations use the same set of surfaces for all types of calibration check, others have different sets for different purposes. For example, BRRC in Belgium use nine surfaces for accreditation and just one for day-to-day checks on their Odoliographs, whereas CETE Lyon in France use 19 surfaces day-to-day and three for accreditation. In the UK, TRL uses nine surfaces on a test track for the annual SCRIM accreditation test programme (although only six of these are used to provide the reference values for final accreditation), whereas day-to-day checks for individual SCRIMs require just three surfaces and these are different sets, on in-service roads, chosen by the individual operating organisations. The Pavement Friction Tester (which is a unique device in the UK) uses one surface on a test track for day-to-day checks. Some of the variation in practice may be attributable both to different approaches taken to assessing the devices by different countries or organisations and to the interpretation of the questions by the individuals completing the questionnaire. Nevertheless, it is clear that there is no commonality of approach. The choice of test-section lengths varies between organisations, even for the same type of calibrations. For example, Figure 3.9 shows that, for accreditations, almost all the test sections used by BRRC are the same length whereas in the Czech Republic, the lengths vary markedly. Comparing practice in France and Austria in relation to day-to-day checks (Figure 3.10) shows similar variety.

Figure 3.9 Length of test sections used for accreditation tests two different countries


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Figure 3.10 Lengths of test sections used for day-to-day checks in two countries

3.4 Surfacing properties

3.4.1 Surfacing materials used The questionnaire asked respondents to provide information on the types of surfacing material that were used on the test surfaces. The purpose of this question was to identify whether there were any potential choices for future consideration for reference surfaces. Unsurprisingly, particularly as most surfaces were on existing roads, the information was limited. The results are summarised in Table 3.2.

Table 3.2 Types of surfacing materials used as test surfaces

Number of sections reported Surfacing type

Accreditation tests Day-to-day checks

Asphalt 33 54

Concrete 6 8

Synthetic 3 2

generic asphalt concrete 10 16

thin surfacing 3 4

porous asphalt 3 2

SMA 5 7

surface dressing 4 10

HRA 7 14

Breakdown of types of asphalt surfaces

Other 1 1 Asphalt was the dominant general material type: conventional asphalt concrete surfaces dominated in mainland Europe while hot-rolled asphalt (HRA) was a significant material for organisations operating in the UK and Ireland. (HRA is a dense asphalt surface unique to the British Isles into which 20 mm chippings that have been lightly coated in bitumen are


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rolled, while the mat is hot, to provide micro- and macrotexture). It is possible that some of the surfaces reported as being “asphalt concrete”, especially those that represent longer sections of road, actually include examples of other asphalt material types. The three synthetic surfaces are special low-friction materials (two are known to be epoxy resin) laid on test tracks.

3.4.2 Skid resistance levels

The questionnaire provided information on the typical skid resistance levels recorded on the various test sections. Because these were given in terms of the measure from the individual devices at their particular test speed(s) – incidentally illustrating the problem of making comparisons without a harmonised scale – it is not possible to make detailed comparisons.

However, it is possible to make some very general observations: • A wide range of skid resistance levels is used for test surfaces across Europe, but

there is no consistent pattern. • Most countries tend to use surfaces with skid resistance levels in the middle to upper

ranges of the measurement values. • Few countries include very low friction levels and in those that do, the surfaces are on

test tracks using special materials such as epoxy resins. • It would appear that choices are influenced by the availability of different friction

levels, especially where in-service roads are used.

The two countries that reported the greatest number of test surfaces were France and the UK and it is illustrative to compare some of the results for which the same basic measurement, SFC with SCRIM (albeit at slightly different test speeds), is assessed. Figure 3.12 compares the general skid resistance levels (SFC measured at 50 km/h) of the surfaces used by TRL for accreditation trials (where the whole UK SCRIM fleet is compared) with those used for day-to-day checks in which one SCRIM is checked routinely. The accreditation check sections are on a test track whereas the day-to-day sections are on in-service roads. Figure 3.12 shows similar data for CETE Lyon (in this case, SFC measured at 60 km/h). In this case, the accreditation test sections are also on a test track but there are only three such sections, while the day-to-day checks use a mixture of test track and road surfaces. In the UK, the skid resistance range is relatively large both for accreditations and for day-to-day checks. (As referred to earlier, for day to day checks in the UK, there are specific requirements for individual operating organisations to identify three sites that broadly represent the range of levels covered by the UK skid resistance standards for in-service trunk roads and to use these). In France, the skid resistance levels on the surfaces used for accreditations are close to each other, whereas they vary more for day-to-day checks.


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In both cases, the ranges are greatly “helped” by the inclusion of very-low-friction epoxy resin test track surfaces. Apart from these, there are no skid resistance levels below 0.4. In fact, this was true throughout the responses with the exception of a 0.2 surface on a test track in the Czech Republic and in a reply from Israel where test sections with skid resistance levels of 0.35 and 0.25 on roads were reported. In that case this may reflect the general levels of skid resistance on a network where limestone aggregate predominates.

3.4.3 Macrotexture

An important aspect of the development of skid resistance overall is, of course, the macrotexture of the surface. Although this may not be a significant factor where measurements that are routinely made at the same speed, it might have an effect on the general levels measured. A comparison was made between the reported texture depth and skid resistance levels for the surfaces for which both factors were reported, summarised in Figure 3.13. It should be borne in mind that the skid resistance levels are on the scales for the individual devices and the texture depth may also be on different scales depending on the measurement technique. Nevertheless, the results suggest that:

• For surfaces dedicated to accreditations, texture depth tends to be greater for higher friction values;

Figure 3.12 Skid resistance levels for test surfaces used by CETE Lyon

Figure 3.11 Skid resistance levels for test surfaces used by TRL


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• For day-to-day checks, there is no clear tendency although there is a greater proportion of lower texture levels (<1 mm), probably reflecting the dominance of asphalt concrete.

Figure 3.13 Comparison of texture depth and skid resistance levels on test surfaces


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4 Test surfaces for assessing vehicles and tyres

4.1 Test surfaces used by the motor industry The HERMES project report included a short review of the way in which the motor industry utilises test surfaces and this is summarised here. Vehicle and tyre manufacturers often need to assess the performance of their vehicles and tyres and for this purpose may use proving grounds that include special surfaces designed to deliver different levels of road/tyre friction. These allow comparisons to be made between different braking or control systems or to compare the performances of different tyre designs. Proving grounds may have surfaces designed to give different friction levels and some special materials are commonly used, such as polished basalt tiles or ceramic tiles used to provide low friction for wet straight-line braking tests. However the surfaces are more often of asphalt concrete or Portland cement concrete that are considered to be “typical” of the host country’s road network. The surfaces do not have standardised friction characteristics; rather, they provide a means for comparative testing and friction is deduced from the braking performance of the vehicle or tyre. In Spain, for example, IDIADA (Instituto De Investigación Aplicada Del Automóvil – Applied Automotive Research Institute) built a test pavement on their research track. The pavement friction was tested at the end of the construction work, using the Pendulum tester. However, no dynamic skid resistance measurements with standardised devices were carried out. Instead, friction values are estimated from the braking distance of the different commercial vehicles visiting the site. This process of calculation was considered sufficient to characterise the surfaces since the vehicles had already been officially approved by some standard. In this context, the friction can be even more variable than with standard friction test devices given the range of systems, tyre compounds and tread patterns likely to be used. Some organisations carry out regular friction tests on their test surfaces using standard methods such as the ASTM skid resistance trailer or even the pendulum tester but, because the sites are out-doors, the surfaces are still subject to the variations that are associated with changing seasonal conditions.

4.2 The use of drum machines for tyre assessment Some tyre manufacturers and research organisations (including universities) use drum facilities that use manufactured panels to simulate road surfaces (some artificial, some actual road materials) for tyre assessment tests. In Germany, for example, the Federal Highway Research Institute (BASt) uses a large scale interior drum testing facility to investigate tyre/road interaction. The main part of this facility is a vertically-mounted rotating drum with a diameter of 3.8 m and a maximum rotational speed of 230 km/h. Cassettes one metre long and 0.55 m wide are built into the drum to contain the pavement surface material. Tyres up to 20-inch rim diameter are then set against the track system.


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The test wheel and drum are in a fully enclosed housing that is equipped with a climate control system to simulate various weather conditions. Temperatures ranging from –5°C to 40°C and controlled water films depths up to 5 mm can be simulated. This interior drum test facility can reproduce and vary the main factors that influence a braking tyre, including wheel load, speed of road and tyre, wheel alignment, temperature and water depth. The test pavement surface, however, like all such surfaces in current use, changes as it is used but its characteristics can be adjusted and sharpened by means rotary tools like carbide-tipped saws and studded tires. While the effect of the tools can be controlled by the velocity of the tools and/or the drum and the load on the tool, ultimately the character of the surface is checked by means of measurements low-speed skid resistance with the Pendulum tester and texture depth. For type approval of a test tyre a standard test is used in which the friction versus slip ratio curve is determined (a concept explained in more detail in TYROSAFE Deliverable D04). In the UK, drum machines (with the pavement surface mounted both internally and externally) have been used for research purposes, including manufacture of artificial surfaces to act as references for tyre assessment (see Section 5.1).

4.3 Noise assessment surfaces Another use that is made of so-called reference surfaces by the motor industry is to assess road/tyre noise. There have been various attempts to provide surfaces for this purpose and these are usually of a modern negative-textured design such as variations of Stone Mastic Asphalt. Because they are manufactured from conventional asphalt materials and their properties are designed specifically for drive-over tests rather than friction tests, they offer little help to dealing with the problem of reference surfaces for skid resistance measurement calibration.


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5 Discussion In this chapter, the report discusses a number of issues relating to the development of reference surfaces for use in harmonisation and calibration of skid resistance measurement devices. The FEHRL project HERMES [2] included a significant component dealing with the idea of reference surfaces. As part of the TYROSAFE project, a further literature review has been carried out in order to find out whether any new information had been published since the work included in the HERMES project report was written. This review did not find anything new and consequently this Chapter draws heavily on the HERMES final report (some of the contributing authors to this report were also members of the HERMES project team and wrote the relevant sections of that report). The chapter begins with a short review of previous attempts to develop such surfaces, followed by a summary of the suggestions made by the HERMES project team relating to the properties needed for reference surfaces. Later sections of this chapter discuss further some other aspects and also suggest some ideas that might be considered for further research based on the HERMES suggestions .

5.1 Historic attempts to develop reference surfaces As earlier chapters have shown, there have been various attempts to prepare surfaces that can be used for checking measuring equipment. There have also been attempts to develop surfaces that could be regarded as a “primary reference”, which might meet, at least in part, the concept of a reference surface that is the focus of this report. This section has been adapted from the HERMES project report (the relevant sections were written by some of the authors of this report) to summarise that work. In a laboratory exercise in the early 1970s, Britton et al [7] investigated the criteria needed for the design of primary standard reference pavement surfaces. Model surfaces were created using particles of known and easily-controlled geometry on a flat substrate, such as spheres set in epoxy resin. Adhering artificial or natural fines controlled the microtexture. The different particles had the same shape factor but represented different chemical structures. Over 600 samples were made for the experiment, covering a wide range of macrotexture and several materials including synthetic aggregates. Measurements of skid resistance were made with a Pendulum Tester, reported as what was then often referred to as BPN (British Pendulum Number). No evident difference was observed between the samples made of different materials and of the same particles size, within the limit of the sensitivity of the experiments, but the effects of macrotexture, size and shape were found to be more significant. Their work indicated that skid resistance was influenced by both the macro texture (size of aggregate, spacing and shape) and the microtexture (size of the fines, spacing and shape) which, of course, had already been observed in practical work on roads. .


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Viewed from over thirty years later, however, it is clear that a limitation of this exercise was the use of the pendulum test to measure skid resistance. Although at the time it was the only readily-available technique that could be used in the laboratory, it is now appreciated that the test is not able to discriminate reliably between the relative effects of microtexture and macrotexture. (The pendulum tester was originally designed to indicate the level of friction when a patterned tyre (of the 1960s) skids at 50km/h on a medium-textured road surface). In the UK in 1983, Dunlop Limited investigated the manufacture of reference surfaces and proposed a standard reference surface to the relevant International Organisation for Standardisation (ISO) committee (ISO/TC22/SC 9) [8]. This proposal involved replica surfacings to which quartz sand was applied to simulate microtexture. The replicas reproduced microtexture to a high degree of accuracy but it was removed rapidly by the tyre in use, in a similar way to that expected from traffic action had a natural aggregate been used. This could not be considered as a reference surface specification but it was a starting point that was taken into account in a further review when ISO published a technical report detailing the process for creating a standardised test surface for high friction tests [9]. The work carried out to investigate this type of surface indicated that the best results were achieved with a surface dressing of fine silica sand that was spread without rolling on to a bitumen-expanded epoxy binder. With this surface, the high friction depended almost entirely on the microtexture produced by this aggregate, which was selected because it represented the most wear-resistant material known. In the mid-1970s, three field test centres were set up in the USA under the auspices of the Federal Highway Administration (FHWA) in order to improve and standardise the measurement of skid resistance [10]. At these centres, which were in separated geographical locations, various “primary reference surfaces” were constructed. The surfaces were replicated in each location using the same contractor and similar selected naturally-occurring materials, including silica sand and river gravel, all in an epoxy seal coat. Initially, each centre had five test sections 4.6 m wide by 158 m long. ASTM-compliant friction measuring devices from the various state authorities were correlated individually against the test surfaces. A standard vehicle based at each centre was used to provide a reference skid measurement system and provide a correction to take account of variations in the test surfaces over time. Although three centres had been set up initially, it soon became evident that only two were needed to service the population of skid testers and one station was closed after only one year of operation. Although five primary reference surfaces were constructed at each site, there were difficulties in achieving the required target levels of skid resistance. Initially, all the surfaces had higher levels than anticipated. As a result the roughest surface, which was abrading the test tyres of the candidate equipment, was abandoned and further primary surfaces were constructed later to provide low skid resistance. In addition, some of the other primary surfaces at one of


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the centres were affected by surface distress probably brought about by defects in the original binder and by construction joints propagating from the underlying base material. The primary reference surfaces were only trafficked during the testing process but it was found that all exhibited significant variations in skid resistance during the year and there could be significant variation over time. On one surface, the skid resistance Skid Number (the value recorded by ASTM devices) reduced by 27% over the nine year period of the operation of one of the centres (Eldridge et al, 1986). Thus, although so-called durable reference surfaces were made, the experience clearly shows the difficulty in defining and achieving specific levels of skid resistance using natural materials and in making materials that maintain a consistent value over time. In this situation a reference device (but of the same type as the devices being calibrated) had to be used to provide a correction to take account of the variations in the surfaces but, of course, there was no real standard against which that device could be compared. It can be seen that the fundamental requirements of reference surfaces, to be predictable and stable, were not met.

5.2 Work on reference surfaces in the HERMES project

The HERMES project was primarily designed to assess the practical application of a proposed method of harmonisation that had been developed for the CEN Working Group (CEN/TC227 WG5) dealing with test methods for road surface characteristics. The method used a common scale, the Skid Resistance Index, (more commonly called the EFI or European Friction Index). The EFI was derived from the earlier International Friction Index proposed by PIARC but developed specifically to harmonise measurements from skid resistance devices used in Europe. The harmonisation approaches in both of those studies are discussed in more detail in the companion TYROSAFE Deliverable D05 [3].

An important aspect of the EFI was the use of the average of a number of diverse measurement devices to provide a “floating reference” level to which individual devices would be calibrated in a series of comparative measurement exercises. The main objective of HERMES was to test the practical aspects of calibrating a variety of devices to the EFI scale using the proposed methodology, including assessing the stability of the scale over time.

However, the project also considered another aspect of the harmonisation problem, namely that of providing a stable reference level for skid resistance that was independent of the rest of the fleet of devices. This took two forms:

• Proposing a specification for a potential “reference device” to which other devices could be calibrated.

• Examining the possibilities for developing “reference surfaces” that would provide a stable level both for calibrating existing devices for harmonisation purposes and, ultimately, for checking the ongoing calibration of any future “reference device”.

The objective of the work for the second of these aspects was to evaluate the feasibility of designing stable reference surfaces for calibrating friction-testing devices. It was recognised that it was unlikely that a full specification could be produced at that stage and that the task


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would review key aspects of the topic and make proposals that could be developed in the future. The work included:

• A literature review on the general topic. • Contacts with operators of test tracks in the motor industry and with other contacts in

the field in different countries. • General discussion and pooling of expertise within the core group.

5.2.1 Basic requirements for reference surfaces As outlined in Section 2.2, a reference surface should ideally have the following general characteristics:

• It should have a known, preferably predictable, level of skid resistance. • The skid resistance should be stable over time (i.e., it does not change with use or

age). • The surface should have reproducible characteristics (so that more than one can be

made or a replacement can be produced). The HERMES team argue that, if reference surfaces are to be used for calibration purposes, then it must be possible to test any device over its practical range of measurement in operation. Therefore, several reference levels are likely to be required. The two main surface characteristics that contribute to skid resistance are microtexture and macrotexture (characterised by measuring texture depth), governing the underlying friction level and the change in skid resistance with speed. Consequently, any set of reference surfaces should include combinations of these two parameters. Clearly, it would not be realistic to attempt to produce examples of all possible combinations. Neither is it absolutely necessary, for calibration purposes, for surfaces to be specifically representative of any particular type of road surfacing. The HERMES team suggested that four surfaces covering different broad combinations of texture might be adequate (Table 5.1). However, it was not possible to suggest how those levels might be verified.

Table 5.1 Broad combinations of texture parameters suggested by the HERMES team for a range of reference surfaces for skid resistance device calibration

Microtexture Macrotexture

Surface HH High High

Surface HL High Low

Surface LH Low High

Surface LL Low Low


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Also, the practical range of operation would have to include a range of test speeds as well as of friction levels. This places constraints on the areas in which test surfaces are placed, with the need to allow for safe acceleration and deceleration.

5.2.2 Materials properties Although it is possible to suggest in broad terms the levels of characteristics such as micro- and macrotexture that reference surfacings should have (Table 5.1), a major barrier to the practical development of reference surfaces depends upon being able to manufacture surfacings that have the required properties in such a way that they are maintained through time and use. In theory, the HERMES team suggested, a calibration surface itself could be made from three basic types of material:

• Natural aggregate with either a bitumen binder or in a Portland cement concrete. • An artificial aggregate (such as a ceramic) fixed to a substrate with a resin-based

binder. • A completely fabricated surface using man-made materials.

Synthetic surfaces such as epoxy resins, smooth tiles or metal plates can be used to provide very low levels of friction but these are of limited use when devices for road surface assessment are generally required to operate at much higher skid resistance levels.

5.2.2.1 Natural aggregates Although crushed rock aggregates are easy and relatively cheap to obtain, their natural characteristics are likely to be very variable in the context of creating a reference surface. Natural aggregates are already known to change their characteristics with time due to weathering and wear, particularly polishing by traffic. The use of bitumen as a binder is also a marked disadvantage because there is a strong possibility of initial contamination of the aggregate surface. This might be avoided with a surface dressing technique, but this is unlikely to be a successful way of producing a material that will retain its texture depth with the repeated passage of test vehicles. Similarly, it would be difficult to prepare a surface using cement as a binder because of the risk of contamination. Conventional asphalt mixes would be inappropriate for reference surfaces, not only because of the contamination risk but also because where the asphalt matrix forms part of the surface, this can be expected to change over time as the bitumen weathers. Cement concrete mixtures are also likely to gradually wear. In theory it might be possible to make a surface using conventional materials and then to condition it in some way before use as a reference. However, the difficulty with this approach is to know when the correct condition has been reached and what the friction level would be.


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Some gravel aggregates, particularly flint, have naturally smooth, hard surfaces and so it might be possible to use such materials to provide a combination of low microtexture and intermediate or higher macrotexture. This technique – very smooth particles in dense asphalt or exposed aggregate concrete – has been used on research test tracks or motor industry proving grounds to provide intermediate to low friction levels. However, experience on test tracks using natural aggregate surfaces where skid resistance devices are regularly compared has shown that their characteristics can change as a result of repeated testing. For all these reasons, it was recommended that natural aggregates or normal asphalt or concrete mixes should not be used

5.2.2.2 An artificial aggregate and a resin binder There has been a great deal of research over the years into the production of artificial (or synthetic) aggregates. Artificial aggregates can be produced with characteristics that can be controlled and remain relatively stable. Most are derived from naturally-occurring minerals, which are then treated in some way, for example by calcination (heating to high temperatures). Synthetic aggregate particles can be expected to be identical and regular in shape and these therefore, fixed to a substrate with a suitable binder (such as epoxy resin) could be a possibility for the creation of reference surfaces. The different levels of the two components of texture could be achieved by varying the final particle sizes and the asperities in the surface. Examples of artificial aggregates that the HERMES report suggested might be explored are listed in Table 5.2.

Table 5.2 Artificial aggregates that might be used in a reference surfacing

low friction high friction

Ceramics Calcined bauxite

Calcined Flint Burned clay

5.2.2.3 Man-made materials Using man-made materials means that pre-determined shapes and profiles can be manufactured and replicated. The techniques might utilise moulded shapes using, for example, fibreglass and resin, as was tried for special external drum surfaces by Dunlop and TRL during the 1990s. The shapes could be basic geometric shapes such as hemispheres, cuboids tetrahedra or cylinders (Figure 5.1) or castings taken from actual road surfaces.


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Figure 5.1 Specially formed surfaces using geometric shapes (l-r): hemispheres, cubes,

tetrahedra, cylinders

An alternative to moulding surface profiles would be to machine or press them from a metallic plate, although this technique might not allow some of the more complex patterns to be easily reproduced over large areas. The advantage of this type of technique is that it would allow very repeatable surfaces to be made and would permit different forms and scales of macrotexture to be produced. Using casts from real roads, although theoretically possible, might not be appropriate for developing reference surfaces. Apart from deciding what general type of road surface should be used, this approach would make it difficult to ensure homogeneity, both along each modular section and along the length of the assembled test surface. A major limitation of this type of approach, however, is that the materials would be unlikely to have a “natural” microtexture and so this would have to be added somehow, which has proved a problem in the past. On the other hand, this approach could be useful to make a low micro-, high macrotexture surface that might be more consistent than could be achieved with natural aggregates.

5.2.3 Surface construction Another aspect of reference surfaces, apart from the types of material from which they are made, is how the surface as a whole is constructed. There are a number of issues in this context that were discussed in some detail in the HERMES report and are summarised here. How the surfaces are to be constructed would depend on where they were to be based. The HERMES team suggested two options:


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• A permanent base with full-length road sections. • A modular system that was either installed in a purpose-build facility or could be

safely stored then transported to an alternative location. Of course, the location of any permanent base or test facility would need careful consideration because of the logistical issues of arranging to calibrate devices from all over Europe. This modular approach would allow the surfaces to be arranged where they could most conveniently be used or allow easier replication for use in a number of separate installations. Clearly, any test surfacing must be built on a structure or foundation that is capable of carrying repeated passes of the weight of the test vehicles. In many cases the test wheel is in line with the vehicle road wheels, either on the same chassis or a trailer. Because they carry large water tanks, some are necessarily large goods vehicles with axle loads of up to eight tonnes. (The Dutch ROAR and some more-recently built SCRIMs, for example, are built on three-axle truck chassis). Other devices have the test wheel on a trailer offset from the wheel path and so the vehicle must pass with its wheels slightly to one side. Whatever form of construction were chosen, it would need to be able to accommodate vehicles of these sizes. As well as being able to carry the weight of the vehicles repeatedly, the test road structure will also need to have adequate drainage to remove the water deposited by the test devices. Ideally, a system that could positively remove excess water between passes would be an advantage. Each test section would also need to be long enough to accommodate the necessary test passes, including an allowance for accelerating to the required test speed and decelerating afterwards. The length of surface on which friction would be measured may only need to be 100 m or so long. (Some organisations that currently use long lengths of road might disagree, but if the reference is predicable and stable there would be no need for testing long lengths. Repeat passes would be adequate to establish precision.) However, some devices, usually those using a variable slip control system, need a certain length in which to stabilise the required slip ratio at the particular speed and friction level. This could require a test surfacing some 300 m long, together with an approach and exit lane to allow for acceleration and braking when higher test speeds are needed. A crucial aspect of reference surfaces is that their characteristics should remain stable over time. This means that ideally, they should be kept clean and should not be exposed for long periods to extremes of weather, particularly frost, rain and strong sunlight. Therefore, a test facility should include some means of protecting the test surfaces from the elements when they are not in use. Consideration should also be given to how any build-up of tyre deposits on the surfacings as a result of repeated testing can be removed without adversely affecting the friction characteristics.


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While it will be important that the characteristics should be stable and durable, the working life of a reference surfacing need not be very long, provided that it can be reproduced and replaced easily, economically and reliably. The HERMES team concluded that a modular form of construction that allows surfaces to be removed and stored when not required could be the preferred approach. An arrangement that enables the sections to be laid adjacent to one another with approach and exit areas common to all sections would reduce the length of road required compared with a linear structure where sections were laid one after the other. This would allow the test surfaces to be relatively narrow (say 1m wide), with neutral areas the width of a normal traffic lane to each side that would potentially allow either left- or right-handed machines to test them without unnecessary wear and tear on the test sections. The team proposed a possible layout that could take advantage of this approach but this level of detail is not included here. It must be re-iterated that the essential feature of a reference surface is that its characteristics should be clearly defined, easily reproduced and consistent through its working life. An individual module need not last for a very long time if it can quickly be replaced by another with the same characteristics.

5.2.4 Outline specification for calibration reference surfaces The HERMES team drew up an outline specification for calibration reference surfaces. This was offered to provide a framework for future investigations; it therefore is particularly relevant to the objectives of TYROSAFE and so is reproduced here. Table 5.3 gives this suggested outline specification, based on the assumption that a modular form of construction would be used, allowing surfaces to be set up at a number of sites or to be moved between suitable locations as required. The surface types (HH, HL, etc.) are those proposed in Table 5.1. Some of the details such as the materials to be used, the layout of the site or facilities needed that have been discussed above have not been included in this table since they would need to be finalised in the light of experience from any future research.


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Table 5.3 Outline requirements for Calibration Reference surfaces

General Property General Requirements

Other Comments

Alignment Straight and level, no cross-fall

These requirements would apply to the track on which vehicles run as well as the test surface. With no cross-fall, suitable drainage or other mechanisims will be needed to rapidly clear excess water from the test surfaces.

Length 100 metres minimum This is the minimum test length. A longer length (up to 300m) may be required to accommodate some devices that use a fixed slip ratio controlled by a servo system that responds to changing surface friction.

Width 1 metre wide test surface There will also be a need for space on either side of the test surface when installed to allow for the passage of the test vehicle, depending upon the relative alignment of the test wheel and the main vehicle’s tyres.

Construction Similar interlocking modules Module size to be chosen to suit ease of construction and handling.

The load bearing capacity for the installed module and associated roadway will need to be able to support normal lorry axle-weights in order to accommodate the larger test vehicles.

General Texture (both micro- and macro-) of the surfacing should be homogeneous along its length and across its width. The texture should not be so aggressive that it causes excessive test tyre wear.

Each module should be similar, with no significant boundary edges in the surface where the modules join. Joints will need to be secure and not collect dirt or allow passage of water (unless suitable sub-surface drainage is provided).

Surface HH BFC20 = 0.75-0.85 MPD = 1.5-2.0 mm

Surface HL BFC20 = 0.75-0.85 MPD = 0.2-0.4 mm

Surface LH BFC20 = 0.25-0.35 MPD = 1.5-2.0 mm

Surface characteristics

Surface LL BFC20 = 0.25-0.35 MPD = 0.2-0.4 mm

These are tentative suggestions of target ranges for the key texture parameters. Eventually, a more precise specification will be needed. BFC (locked-wheel) values at 20km/h have been used here as a suggested indicator of the microtexture level needed: in practice, other low slip-speed measurements could be used.

Surfacing materials To be determined. Should not be bitumen-based or use untreated crushed-rock aggregate.


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Stability Should maintain defined skid resistance and texture depth over a practical temperature range for at least 3 years. Friction should not change over a short period of repeated testing.

To assist this, it may be necessary to keep and use the surfaces in a controlled environment.

Durability Should maintain defined skid resistance and texture depth for up to 1000 repeated test passes with up to 500kg test wheel load.

The suggested number of passes is sufficient to check up to 5 devices (making 5 test passes at 3 speeds plus some additional passes) 3 times a year for 3 years.

5.3 Some research suggestions based on the ideas proposed by HERMES

It is clear that the choice of suitable materials to achieve predictable and stable performance will be difficult, so work to identify materials that could be reliably specified for use for the reference surfaces will need to be a fundamental aspect for research. The challenge is to find suitable combinations of a regular and repeatable form and level of macrotexture with appropriate treatments or additives to provide predictable, controlled and durable microtexture. As a starting point, some or all of the following could be investigated: • Castings of geometric shapes using resin/fibreglass to represent controlled macrotexture

forms (such as in Figure 5.1); • Cut or pressed shapes or patterns in metal; • Proprietary anti-slip or high friction materials/coatings; • Paints with suitable additives to provide microtexture (variations on some road marking

materials might prove suitable); • Conventional materials, either to prove that, as expected, they would not be suitable or to

assess whether in certain conditions they might be used. These suggestions assume an approach in which the required surfacing is applied as some form of overlay to a suitable substrate. An alternative might be to use a material that can be “refreshed” in some way to a standardised level, an approach used in Germany on special surfaces on a large internal drum machine for tyre approval tests. This, of course, begs the question of how the satisfactory outcome of such a “standardised” refreshing process is to be assessed. Another aspect that could be considered for inclusion in ongoing research would be the study of porous or permeable materials, since these are used on the network and a calibration check on the devices with this type of surfacing might be appropriate. However, this is perhaps part of the issue, also a matter for further discussion.


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As well as research that is essential to establish suitable test surfaces, consideration should also be given to how and where they should be placed. In order to overcome potneitla problems with logistics it is likely to be preferable to approach this using a transportable, modular structure rather than permanent road-like structures in just one or two locations. If this view is accepted, then research should address the structure of the modules on which they would be used and how they should be supported at road level on the test site. It is suggested that research on this topic should initially be a desk study to assess how the modules might be engineered, followed by practical tests on the most promising designs. Possible forms of construction for the bearing surface might include a timber bed, concrete slabs or steel/alloy plates. All would need a suitable support framework, together with a carefully designed mechanism to ensure that they would interlock reliably. Some might be ruled out as impractical in order to achieve a workable size and weight for individual modules. Tests using the surface materials applied to fixed substrate might also be considered to allow for the possibility of a permanent installation. A three-stage process involving the following ideas can be envisaged:

1. Investigation on a laboratory or pilot scale of suitable materials, comprising: • Tests to explore suitable material combinations to achieve the required levels

of friction and texture depth. • Accelerated wear testing of the best of the possible materials. • Comparisons of indoor and outdoor tests.

2. Design and testing of a suitable form of construction, comprising:

• A desk study to explore suitable forms of construction for the modules that would carry the test surfaces.

• Practical durability tests on pilot-scale modules.

3. A full-scale trial exercise that would assess both weathering and wear-and-tear under repeated testing over a 3-year period, possibly including:

• Testing of the skid resistance properties of reference surface materials using friction devices covering the three main measurement principles.

• Measurements of physical characteristics using all suitable techniques available to assess gradual changes.

• For accelerated wear tests on the surfaces, consideration should be given to using laboratory scale indoor test machines or larger-scale outdoor machines such as the LCPC carousel at Nantes.

• For tests on the durability of the module construction, facilities such as the Pavement Test Facility at TRL, which allows repeated passing of a lorry-sized wheel under controlled load, might be used.

Clearly, this would be a large programme of work that would require co-operation between several organisations to achieve. It might be worthwhile therefore, to limit the work initially to


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a feasibility study that concentrated on (1), the desk component of (2) and limited tests for (3).

5.4 Further discussion There are clearly significant difficulties to be overcome relating to the design and construction of reference surfaces. The HERMES team were against the idea of using normal asphalt or cement concrete materials for a number of reasons but the use of synthetic resin binders or other materials such as metals may not be an ideal solution either. It can be argued that the resin binders will induce a different response in a test tyre in terms of mechanical impedance compared with either bitumen or a cement concrete surface. This argument, however, could be overcome if the surface were designed such that the resin, bitumen binder films and the asphalt or concrete matrix were excluded from the upper part of the surface with which the test tyre makes contact. A completely artificial surface, such as moulded metal, presents problems of its own because of the different surface properties that such materials have compared with normal road surfacing materials. The grip mechanism of tyre compound elastomers is complex; while it involves mechanical behaviours similar to that in generating friction between rigid bodies, the adhesive component depends on inter-molecular reactions between tyre and road surface. This will be influenced by the electrical characteristics (dielectric) of the interacting surfaces. The process of introducing water into the interface may also have different effects for different types of material. It has also been argued that while the use of an artificial aggregate and a resin binder or the use of man-made materials would allow very repeatable surfaces to be produced, this would not be true to the “real” situation on a road since the materials would be unlikely to have a “natural” microtexture. Current work, in the UK for example, is leading researchers to question the extent to which skid resistance measurements are being influenced by relative areas of contact with surfacing aggregate and the components of the surrounding support matrix as well as the size and shape of the “coarse” aggregate particles. Measurements on newly-laid asphalt with a high proportion of bitumen on the surface are being observed to behave characteristically differently at low speeds from those on well-trafficked roads and this may be due as much to the ability of the surface to be wetted as to its inherent adhesive and other properties. Conversely, behaviour at intermediate and high speeds on bitumen-rich surfaces has been found to be consistent with what might be expected from a very low microtexture surface. This discussion of test surfaces, however, has largely ignored the other fundamental component of the friction measuring device, namely the test tyre or rubber slider. While acknowledging that different types of materials in test surfaces may interact differently with the tyre, it is also necessary to recognise that different devices have different tyres.


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In a skid resistance measuring device the tyre is the sensor. Thus, the properties of the rubber will be a critical component in the measurement process and variations between individual (albeit nominally the same) tyres can be expected which would need to be taken into account in any calibration process. Further, different devices use different rubber compounds in their tyres and these are typically not chosen to be similar to production vehicle tyres. Rather, they are designed to be sensitive to the state of polish of the road surface (which is what they are most often used to measure). Also, different sizes of test wheel result in different contact patches areas and aspect ratios and operate under different loading and slip ratio conditions, all of which will influence their response to any particular surface. This raises the question as to whether a surface should be specified in terms of its micro- and macrotexture as has been suggested or whether it should also be “tuned” to a particular type of rubber compound. If it could be shown, for example, that the effect of a different test tyre compounds is to give rise to systematic differences between different devices (which may be different on different surfaces, of course) then the process of calibration should be able to resolve the problem. Overall, there is clearly a fundamental conceptual point to be considered and resolved: if a reference surface is to be used simply to provide an absolute level for the purposes of verifying the calibration of a measurement device, home much does it need to reflect reality? Arguably, the important issue is not how realistic the surface is but whether a correctly set-up and operated measurement device will always give the same predictable response on it. As well as the broad approaches for research suggested in section 5.3, if reliable artificial reference surfaces are to be created, the friction mechanisms of elastomers on potential surfaces might need to be investigated more closely, including both the generation of microscopic bonds between the tyre and the surface and the way in which the surface might be wetted. These are all aspects which will need to be discussed as part of the process of developing the “road map” later in the TYROSAFE project.


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6 Conclusions The purpose of this report was to review the current use of test surfaces for checking and calibrating skid-resistance measuring equipments and the potential use of reference surfaces to contribute to the harmonisation process. For a survey of current practice, a detailed questionnaire was sent to project partners and, through them, manufacturers, operators of test equipment and those organisations responsible for equipment accreditation were approached. The purpose of this questionnaire was to see how different countries use test surfaces on roads or test tracks to calibrate their measurement devices, in the absence of “true” reference surfaces with known skid-resistance characteristics. The general conclusion to be drawn from this part of the work is that many different surfaces are used as test surfaces, most often (but not exclusively) made from conventional road-building materials. However, because they do not have access to test tracks (there are not many of these in Europe), most organisations use in-service roads for their calibration checks. Consequently, the selection of test surfaces is not based just on a specific combination of friction and texture levels but on what is readily available on the road networks concerned. Another consequence of the usage of in-service roads is that the range of friction levels that can be used is limited and low-friction surfaces are missing. Since roads are subjected to the action of both traffic and climate actions, with their characteristics probably altering over time, it is not clear how such variations could be taken into account in a calibration procedure for harmonisation work. The second aspect covered by this report was the potential use of purpose-made reference surfaces that would have predictable, stable and reproducible skid resistance characteristics. This topic was covered extensively in the final report from the FEHRL project HERMES published in 2006 and a literature review for TYROSAFE did not reveal any further published information on the topic. The HERMES report made suggestions as to what the general characteristics of such surfaces might be and how their construction might be approached. However, it was clear then, and remains so now, that research is still necessary to develop such surfaces from a practical point of view. It is clear that the choice of suitable materials to achieve predictable and stable performance will be difficult, so work to identify materials that could be reliably specified for use for the reference surfaces will need to be a fundamental aspect for research. The challenge is to find suitable combinations of a regular and repeatable form and level of macrotexture with appropriate treatments or additives to provide predictable, controlled and durable microtexture. This, in turn, will need to be combined with consideration of the contribution of different test tyre compounds and their potential interaction with any proposed surfacing material.


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7 References [1] M.-T. Do, P. Roe, E. Vos, J. Groenendijk, State of the Art of Skid-Resistance Test

Methods, TYROSAFE Deliverable D04, December 2008. [2] Descornet G et al., 2006, Harmonisation of European Routine and Research

Measurement Equipment for Skid Resistance, FEHRL Report 2006/01, Brussels BE. [3] J. Groenendijk, E. Vos, P. Roe, M.-T. Do, Analysis and Findings of Previous Skid

Resistance Harmonisation Research Projects, TYROSAFE Deliverable D05, March 2009. [4] ISO 5725-2:1994, Accuracy (trueness and precision) of measurement methods and

results – Part 2: Basic method for the determination of repeatability and reproducibility of a standard measurement method.

[5] ISO 13473-1:2004, Characterization of pavement texture using surface profiles - Determination of Mean Profile Depth.

[6] ISO 13473-5 CD, Characterization of pavement texture using surface profiles - Measurement of megatexture.

[7] Britton SC, Ledbetter WB, and Gallaway BM, 1974. "Estimation of Skid Numbers from Surface Texture Parameters in the Rational Design of Standard Reference Pavements for Test Equipment Calibration". Journal of testing and Evaluation, JTEVA, Vol 2 , Nº2 p73-83.

[8] ISO/TC22/SC 9. "Vehicle dynamics and road-holding ability", ISO. [9] ISO/TR 8350-1986 (E). "Road vehicles - High Friction test track surface- Specification",

ISO. [10] Huckins, H.C., 1977 “FHWA Skid Measurement Test Centres”. Public Roads. Federal

Highway Administration. Washington D.C. September, 1977. [11] Eldridge A, Whitehurst EA and Neuhardt JB, 1986. “Time-History Performance of

Reference Surfaces. Page 61-71. The Tire Pavement Interface”. Pottinger/Yager edition. ASTM. Baltimore,.


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8 Appendices

8.1 Questionnaire template

EXPLANATION

One aspect of the TYROSAFE project is to assess possibilities for harmonising skid resistance policies and measurement practices in Europe. The calibration of different measurement devices, their relationship to one another and routine quality control are an important part of this.

Many organisations in different countries make or use skid resistance measurements. Different processes are used to verify that consistent data are being produced.

As well as calibration of their sensors, measurement devices are often tested to check that they work correctly in dynamic conditions.

Typically, specially chosen test surfaces or sections of road are used for this.

If special surfaces could be manufactured with known, constant levels of skid resistance, they would provide an absolute reference to compare and calibrate individual measurement devices. They could also be used to compare and calibrate different types of device.

However, such “Reference Surfaces”, would not be conventional road surfacing materials and probably do not exist at present.

The purpose of this questionnaire is to find out about current practices where different surfaces are used to compare measurement devices. This could help to identify any approaches that might be developed towards an absolute reference system.

The questionnaire is divided into five Parts. Part 1 asks about your particular organisation and its role in using skid resistance test surfaces. Parts 2-4 ask about the types of surface and the way that you use them. Part 5 provides space for any additional comments you might like to add.

We have identified three specific ways in which surfaces might be used:

? As part of a national accreditation process to verify acceptable operation of individual machines (or a fleet of similar machines) before they are used to gather data on a network. Part 2 asks about surfaces that are used for this purpose.

? By operators of measurement devices for day-to-day checks on the dynamic operation of their machines. Part 3 asks about surfaces that are used in this way.

? By manufacturers of measurement devices to verify that new or recently-serviced machines are operating to their satisfaction. Part 4 asks about surfaces used like this.

Please complete Part 1 and whichever of the remaining Parts are appropriate to your organisation.

PART 1 – your organisation

Name : Country :

Accreditation authority:

Measurement service provider:

Device manufacturer:

Research:

What is your organisation’s role in relation to skid resistance measurement? (please tick the appropriate boxes)

Other: (please specify)

SCRIM:

GripTester:

Adhera:

SDK:

Odoliograph:

SRM:

ROADSTAR:

ROAR:

Netherlands Trailer:

OSCAR:

What types of measurement device do you use or check?

(Please tick the box for each type of device you work with)

Other: (please specify)

PART 2 – Test surfaces used for device accreditation

How many different surfaces do you use for device accreditation tests?

Every month:

Every six months:

Every year:

Occasionally as required

How often do you normally carry out accreditation tests?

Other: (explain briefly)

If you do the tests once or twice a year, when do they usually happen?

All: Do you use results from all the surfaces for final approval, or a sub-set? Tick the box or enter the number of surfaces in the sub-set.

A selected set:

Use a single “golden” machine:

Use the average of a small number of selected machines:

Use the average of all the machines:

How do you establish the value(s) for comparison?


PART 2 – Test surfaces used for device accreditation (continued)

Please provide some information about the test surfaces that you use for accreditation tests.

Indicate whether the surface is on a test track or an in-service road and include a short description for the type of surfacing on each section – for example: dense asphalt concrete, 0/14mm SMA, surface dressing, Portland cement concrete with brushed texture, epoxy resin.

If you know them, please include geometrical data and typical skid resistance/texture measurements.

Add rows to the table for extra sections if necessary

Typical texture depthSection

T = Test track

R = In-service

road

Surfacing description length (m)

width (m)

Straight or curve?

(give radius if known)

Gradient (%)

Typical skid

resistance MPD Patch SMTD/rms

1

2

3

4

5

6

7

8

9

10


Date: 25/02/2009, Version: 2.0 53 (53)

PART 3 - Test surfaces for day-to-day checks by machine operators

How many different surfaces do you use for day-to-day checks on your machine(s)

Every day:

Every week:

Every month:

Occasionally as required

How often do you normally carry out these checks?


all year

test season

only Do you make these checks all year round or only during the testing season?

How do you use the results?

Please provide some information about the test surfaces that you use for day-to-day checks.

Indicate whether the surface is on a test track or an in-service road and include a short description for the type of surfacing on each section – for example: dense asphalt concrete, 0/14mm SMA, surface dressing, Portland cement concrete with brushed texture, epoxy resin.


If you use have more than one role and use the same sections as in Part 1 of the questionnaire, for this purpose, simply put “as in Part 2” in the description column.

Add rows to the table for extra sections if necessary

Typical texture depth Section

T = Test track

R = In-service

road


width (m)

Straight or curve?


Gradient (%)

Typical skid

resistance MPD Patch SMTD

/rms

1

2

3

4

PART 4 - Test surfaces for quality checks by device manufacturers

How many different surfaces do you use for quality checks on machines that you manufacture or maintain?

When do you normally carry these checks?

How do you use the results?

Please provide some information about the test surfaces that you use to check your machines.

Indicate whether the surface is on a test track (which could be a private area on your premises) or an in-service road and include a short description for the type of surfacing on each one.


If you use the same sections as in Part 2 or 3 of the questionnaire, for this purpose. simply put “as in Part 2 or Part 3 in the description column.

Typical texture depthSection

T = Test track

R = In-service

road


width (m)

Straight or curve?


Gradient (%)

Typical skid

resistance MPD Patch SMTD/rms

1

2

3

4

PART 5 – Any other comments?

report on state of the art of test methods seventh framework programme theme 7 transport,

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