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

Transcription

1 Tyre and Road Surface Optimisation for Skid Resistance and Further Effects Coordination Action FP 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/ ) under grant agreement n Main Editor(s) Peter Roe, TRL, UK Phone: , proe@trl.co.uk Minh-Tan Do, LCPC, France Phone: , minh-tan.do@lcpc.fr Due Date 1 st March 2009 Delivery Date 25 th February 2009 Work Package Dissemination Level WP2 Harmonisation of skid-resistance methods and choice of reference surfaces Public (PU) Project Coordinator Mr. Manfred HAIDER, arsenal research, Austria phone: , manfred.haider@arsenal.ac.at internet: This project is part of the FEHRL Strategic Research Programme SERRP IV (

2 Contributor(s) Main Contributor(s) Contributor(s) (alphabetical order) Peter Roe, TRL, UK Phone: , Minh-Tan Do, LCPC, France Phone: , minh-tan.do@lcpc.fr Review Reviewer(s) Peter Saleh, arsenal research, Austria Bjarne Schmidt, DRI, Denmark Manfred Haider, arsenal research, Austria Date: 25/02/2009, Version: (53)

3 Control Sheet Version History Version Date Editor Summary of Modifications 0 (partial draft) 09 Jan 2009 Peter Roe Initial structure outline and draft Introduction Jan 2009 Min-Tan Do Addition of draft of text and graphics for Chapter (first nearcomplete 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 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 Feb 2009 Minh-Tan Do and Peter Roe First Full Draft for Partner comment 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 Feb 2009 Peter Roe Final version implementing Peer Review comments 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 Date: 25/02/2009, Version: (53)

4 Table of Contents 1 Introduction Background What is meant by calibration? What is a reference surface? Analysis of the state of the art for test surfacings for checking skid resistance measurement equipment The purpose of the survey and the questionnaire Overview of the survey responses General aspects of test surfaces Surfacing properties Test surfaces for assessing vehicles and tyres Test surfaces used by the motor industry The use of drum machines for tyre assessment Noise assessment surfaces Discussion Historic attempts to develop reference surfaces Work on reference surfaces in the HERMES project Some research suggestions based on the ideas proposed by HERMES Further discussion Conclusions References Appendices Questionnaire template...52 Date: 25/02/2009, Version: (53)

5 Abbreviations Abbreviation ABS BFC EFI IFI IRFI LFC Meaning Antilock Braking System Braking (force) Friction Coefficient (=LFC) European Friction Index International Friction Index (developed in the 1992 International PIARC Experiment to Compare and Harmonize Skid Resistance and Texture Measurements) International Runway Friction Index (developed in the American Joint Winter Runway Friction Measurement Program, described in ASTM E2100) Longitudinal (force) Friction Coefficient MPD Mean Profile Depth (as defined in ISO and ISO ) SFC SRI Sideway (force) Friction Coefficient Skid Resistance Index (=EFI) HERMES JWRFMP SPENS VERT Harmonisation of European Routine and Research Measurement Equipment for Skid Resistance of Roads and Runways (FEHRL project) Joint Winter Runway Friction Measurement Program (led by Transport Canada and NASA) Sustainable Pavements for European New member States (FP6 project) Vehicle-road-tyre interaction: fully integrated physical model for handling behaviour in potentially dangerous situations (BRITE EURAM project) ASTM BASt BRITE CEDR CEN COST DRI FAA FEHRL American Society for Testing and Materials Bundesanstalt für Strassenwesen (DE) Basic Research in Industrial Technologies for Europe Conference of European Directors of Roads European Committee for Standardization European Cooperation in Science and Technical research Danish Road Institute Federal Aviation Administration (USA) Forum of European National Highway Research Laboratories Date: 25/02/2009, Version: (53)

6 ISO LCPC NASA PIARC RWS TRL International Standards Organisation Laboratoire Central de Ponts et Chaussées (FR) National Aeronautics and Space Administration (USA) Permanent International Association of Road Congresses Rijkswaterstaat = Department of public works and infrastructure of Ministry of transport (NL) Transport Research Laboratory (UK) IMAG IRV PFT RoadSTAR ROAR SCRIM SKM SRM Instrument de Mesure Automatique de Glissance (FR) International IRFI Reference Vehicle Pavement Friction Tester (UK, TRL) Road Surface Tester of Arsenal Research Road Analyser and Recorder of Norsemeter Sideway-force Coefficient Routine Investigation Machine Seitenkraftmessung Stuttgarter Reibungs Messer (DE) Date: 25/02/2009, Version: (53)

7 Definitions Term Adhesion Airfield operational testing Bound surface Definition 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. 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. Top layer or surface course of a road with the aggregates secured permanently in place Braking coefficient Calibration force 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. 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 Fixed slip Fixed-slip friction Friction Overall area of the road surface instantaneously in contact with a tyre. Condition in which a braking system forces the test wheel to roll at a fixed reduction of its operating speed. Friction between a test tyre and a road surface when the wheel is controlled to move at a fixed proportion of its natural speed. Resistance to relative motion between two bodies in contact. The frictional force is the force which acts tangentially in the contact area. Horizontal (drag) Horizontal (side force) force force Horizontal force acting tangentially on the test wheel in line with the direction of travel. Horizontal force acting perpendicular to a freely-rotating, angled test wheel. Longitudinal friction coefficient (LFC) Macrotexture 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. 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 Date: 25/02/2009, Version: (53)

8 0.63 mm to 50 mm centre wavelengths. Mean depth profile Descriptor of macrotexture, obtained from a texture profile measurement as defined in EN ISO and EN ISO Megatexture Microtexture 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). 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 path wheel 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 righthand side and for countries that normally drive on the left, this is the lefthand side. Operating speed Speed at which the device traverses the test surface. Pedestrian resistance Push mode Repeatability r slip The property of the trafficked surface to maintain the adhesion of a pedestrian shoe sole. When the device is pushed by a pedestrian 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 * σ r ) Reproducibility R Routine testing Sampling length/interval Side force coefficient (SFC) Skid resistance Slip angle 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 * σ R ) Measurement of the skid resistance of a surface in standardized test conditions, which normally include a defined water flow rate. The distance over which responses of the sensors are sampled to determine a single measurement of the recorded variables. 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. Characterisation of the friction of a road surface when measured in accordance with a standardised method. The angle between the mid-plane of the test tyre contact surface and the direction of travel. Date: 25/02/2009, Version: (53)

9 Slip ratio Slip speed Subsection Test section Theoretical water film thickness Tow mode Vertical force Slip speed divided by the operating speed. Relative speed between the test tyre and the travelled surface in the contact area. Defined length of surface for which one set of the measured variables is reported by the device. 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 thickness of a water film deposited on the surface in front of the measuring tyre, assuming the surface has zero texture depth. When the device is towed by a vehicle 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 system delivery 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 Wet road skid resistance Wheel paths Rate (litres/second) at which water is deposited on the surface to be measured in front of the test tyre. 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. 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 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 Date: 25/02/2009, Version: (53)

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

11 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 skidresistance 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. Date: 25/02/2009, Version: (53)

12 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. Date: 25/02/2009, Version: (53)

13 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. Date: 25/02/2009, Version: (53)

14 Local (national) test methods 2008 Local friction devices Local ref. surfaces Correlation State of the art local/reference (current practices, previous projects, standards) - research needs - QA procedure Specifications Development 2.4 Partners Specifications + Experts + Road Alternatives authorities SESRD Reference surfaces Pilot tests State of the art - existing standards EU test method 2020 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 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. Date: 25/02/2009, Version: (53)

15 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 M8 skid resistance harmonisation research projects 2.2 D07 Report on state-of-the-art of test surfaces M8 for skid resistance 2.3 D09 Road map and implementation plan to M16 future harmonised test methods and reference surfaces 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. Date: 25/02/2009, Version: (53)

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

17 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 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 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. Date: 25/02/2009, Version: (53)

18 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. Date: 25/02/2009, Version: (53)

19 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. Date: 25/02/2009, Version: (53)

20 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. Date: 25/02/2009, Version: (53)

21 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, dayto-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). Date: 25/02/2009, Version: (53)

22 Table 3.1 List of respondents to the questionnaire Organisation Technical University of Vienna Arsenal Research BRRC Central Road and Bridge Laboratory Mereni PVV Danish Road Directorate CETE Lyon BASt PMS Pavement Management Services Ltd Israel National Road Company Latvia State Roads RWS Serbian Roads Directorate ZAG Yorkshire County Council Derbyshire County Council DRD Road Service (Northern Ireland) Findlay Irvine Jacobs Engineering UK Ltd Surrey County Council Powy County Council TRL Country Austria Austria Belgium Bulgaria Czech Republic Denmark France Germany Ireland (Eire) Israel Latvia Netherlands Serbia Slovenia United Kingdom United Kingdom United Kingdom United Kingdom United Kingdom United Kingdom United Kingdom 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. Date: 25/02/2009, Version: (53)

23 Denmark (1) Germany (1) Northern Ireland (1) Ireland (1) UK (7) Belgium (1) France (1) Netherlands (1) Latvia (1) Czech Rep (1) Austria (2) Slovenia (1) Serbia (1) Bulgaria (1) Israel (1) not on map Figure 3.1 Geographical distribution of received questionnaires 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. Date: 25/02/2009, Version: (53)

24 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 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. Date: 25/02/2009, Version: (53)

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

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

27 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 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 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) single "golden" average all machines average selected machines Figure 3.7 Number of respondents using the different types of reference for comparisons during accreditation Date: 25/02/2009, Version: (53)

28 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 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. 15 Accreditation tests Day-to-day checks test tracks roads both 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-today 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 Date: 25/02/2009, Version: (53)

29 Chapter 4). However, such tracks were not explicitly included in the questionnaire and are not normally used for assessing skid resistance devices 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-today checks on their Odoliographs, whereas CETE Lyon in France use 19 surfaces day-today 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 Date: 25/02/2009, Version: (53)

30 Figure 3.10 Lengths of test sections used for day-to-day checks in two countries 3.4 Surfacing properties 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 Surfacing type Number of sections reported Accreditation tests Day-to-day checks Asphalt Concrete 6 8 Synthetic 3 2 Breakdown of types of asphalt surfaces generic asphalt concrete thin surfacing 3 4 porous asphalt 3 2 SMA 5 7 surface dressing 4 10 HRA 7 14 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 Date: 25/02/2009, Version: (53)

31 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 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 inservice 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-today 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. Date: 25/02/2009, Version: (53)

32 Figure 3.11 Skid resistance levels for test surfaces used by TRL Figure 3.12 Skid resistance levels for test surfaces used by CETE Lyon 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 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 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; Date: 25/02/2009, Version: (53)

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

34 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. Date: 25/02/2009, Version: (53)

35 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. Date: 25/02/2009, Version: (53)

36 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.. Date: 25/02/2009, Version: (53)

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

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

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

40 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 Materials properties Although it is possible to suggest in broad terms the levels of characteristics such as microand 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 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. Date: 25/02/2009, Version: (53)

41 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 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 Ceramics Calcined Flint high friction Calcined bauxite Burned clay 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. Date: 25/02/2009, Version: (53)

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