Coordination Action FP Seventh Framework Programme Theme 7: Transport

Transcription

1 Tyre and Road Surface Optimisation for Skid Resistance and Further Effects Coordination Action FP Seventh Framework Programme Theme 7: Transport D14 Interdependencies of parameters influencing skid resistance, rolling resistance and noise emission 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) Karen Scharnigg, BASt, Germany Phone: , scharnigg@bast.de Gernot Schwalbe, BASt, Germany Phone: , schwalbe@bast.de Due Date 28 th February 2010 Delivery Date 16 th March 2010 Work Package Dissemination Level WP3 Road surface properties skid resistance / rolling resistance / noise emissions Public (PU) Project Coordinator Mr. Manfred HAIDER, Austrian Institute of Technology, Austria phone: , manfred.haider@ait.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) Karen Scharnigg, BASt, Germany Phone: , Gernot Schwalbe, BASt, Germany Phone: , Marco Conter, Austrian Institute of Technology, Austria phone: , Malal Kane, LCPC, France Phone: , malal.kane@lcpc.fr Darko Kokot, ZAG Ljubljana, Slovenia Phone: , darko.kokot@zag.si Peter Roe, TRL, UK Phone: , proe@trl.co.uk Review Reviewer(s) Philippe Nitsche, Austrian Institute of Technology, Austria Helen Viner, TRL, UK Date: 17/03/2010, Version: (67)

3 Control Sheet Version History Version Date Editor Summary of Modifications July 2009 Karen Scharnigg Initial structure outline, chapter November 2008 Karen Scharnigg chapter December 2009 Gernot Schwalbe, Malal Kane, Darko Kokot Chapter 3.1, December 2009 Karen Scharnigg Chapter 3.2, January 2010 Karen Scharnigg Chapter January 2010 Marco Conter chapter January 2010 Karen Scharnigg editing chapter January 2010 Karen Scharnigg, Gernot Schwalbe Chapter 3.4, 5; editing chapter January 2010 Marco Conter input for chapter February 2010 Karen Scharnigg, Gernot Schwalbe Chapter 4, 5 and 6, executive summary February 2010 Karen Scharnigg incorporate partner comments February Karen Scharnigg Version for Peer Review March 2010 Karen Scharnigg incorporate comments from Peer Review March 2010 Peter Roe March 2010 Manfred Haider Final editing as part of English Review, including revised Conclusions and Executive summary. Release version after final review by the project coordinator March 2010 Peter Roe Manfred Haider Updated version with correction of minor typographical errors Final Version released by Circulated to Name Date Recipient Date Manfred Haider, Project Coordinator 16 March 2010 Coordinator 16 March 2010 Consortium 16 March 2010 European Commission 16 March 2010 Date: 17/03/2010, Version: (67)

4 Table of Contents 1 Introduction Background General comments Road surfacing-related parameters that affect the three surface properties Tyre-related parameters that affect the three surface properties Other parameters that affect the three surface properties Interdependencies of road surfacing parameters and their influence on the three surface properties Introductory comments Interdependency matrix of surface parameters influencing the road surface properties Surfacing parameter interactions related to skid resistance Surfacing parameter interactions related to rolling resistance Surfacing parameter interactions related to noise emissions Interdependencies of tyre parameters and their influence on the three surface properties Introductory comments Interdependency matrix of tyre parameters influencing the road surface properties Tyre parameter interactions related to tyre/road friction Passenger car tyres Truck (HGV) tyres Tyre parameter interactions related to rolling resistance Passenger car tyres Truck (HGV) tyres Tyre parameter interactions related to noise emissions Passenger car tyres Truck (HGV) tyres Optimising the designs of road surfaces and tyres to increase safety General comments Optimising surfaces and tyres for safety from a theoretical point of view Implementing optimised designs in practice Implications for rolling resistance and noise when optimising for safety Conclusion References...65 Date: 17/03/2010, Version: (67)

5 Abbreviations Abbreviation ABS BFC EFI FI HGV IFI IRFI LFC Meaning Antilock Braking System Braking (force) Friction Coefficient (=LFC) European Friction Index Flakiness index Heavy goods vehicle 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 ) PC PSV PWS SFC SI SRI VFB VMA Passenger car Polished Stone Value Polierwert Wehner/Schulze (= polishing value of the Wehner/Schulze device) Sideway (force) Friction Coefficient Shape index Skid Resistance Index (=EFI) Voids filled with bitumen Voids in mineral aggregate 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) Date: 17/03/2010, Version: (67)

6 ASTM BASt BRITE CEDR CEN COST DRI FAA FEHRL ISO LCPC NASA PIARC RWS TRL 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 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 Seitenkraftmessverfahren Stuttgarter Reibungsmesser (DE) Date: 17/03/2010, Version: (67)

7 Definitions Term Adhesion Airfield operational testing Bound surface Braking force coefficient Calibration Contact area Fixed slip Fixed-slip friction Friction Horizontal force (drag) Horizontal force (side force) Longitudinal friction coefficient (LFC) 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 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. 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 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. 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. Date: 17/03/2010, Version: (67)

8 Term Macrotexture Mean profile depth Megatexture Microtexture Nearside wheel path Operating speed Pedestrian slip resistance Push mode Repeatability r Reproducibility R Routine testing Sampling length/interval Side force coefficient (SFC) Definition 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 0.63 mm to 50 mm centre wavelengths. Descriptor of macro texture, obtained from a texture profile measurement as defined in EN ISO and EN ISO 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. 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. Speed at which the device traverses the test surface. 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 ) 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. Date: 17/03/2010, Version: (67)

9 Term Skid resistance Slip angle Slip ratio Slip speed Subsection Test section Theoretical water film thickness Tow mode Vertical force Water delivery system Water flow rate Wet road skid resistance Wheel paths Definition 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. 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. 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. 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. Date: 17/03/2010, Version: (67)

10 List of Figures Figure 2.1: Texture wavelength influence on tyre/road interactions (based on [16])...19 Figure 3.1: Parameters influencing the three surface properties (skid resistance, rolling resistance and noise emission) from the perspective of the road surface...24 Figure 3.3: Different surface texture types and their impact on friction values of wet surfaces, based on [21]...35 Figure 3.4: Degree of sound absorption depending on the use of different types of aggregates (left: granite, right: diabase), based on [22]...36 Figure 3.5: Texture spectrum of a dense homogeneous surface (arrows point in the direction to modify the spectrum to archive a low noise surface) [15]...40 Figure 4.1: Parameters influencing the three surface properties (skid resistance, rolling resistance and noise emission) from the perspective of the tyre...43 Figure 5.1: Texture wavelength influence on tyre/road interactions (based on [15])...60 List of Tables Table 1.1: Overview of the major outcomes of the individual Tasks of WP Table 3.1: Interdependency matrix of surface parameters...26 Table 3.2: Aggregate properties and their impact on skid resistance...29 Table 4.1: Interdependency matrix of tyre parameters...45 Table 5.1: Parameters most relevant to optimising skid resistance of asphalt surfacings...57 Table 5.2: Parameters most relevant to optimising skid resistance of concrete surfacings...57 Table 5.3: Parameters most relevant to optimising tyre/road-friction of tyres...58 Table 6.1: Different areas in which recommendations are made for road surfacings and tyres...62 Date: 17/03/2010, Version: (67)

11 Executive Summary 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. TYROSAFE is focussed on three main road surface properties, as they are referred to in this report: skid resistance, rolling resistance or noise emission. These properties are governed by a number of individual factors (or values that describe them), referred to as parameters that relate to the construction or manufacture of road surfaces and tyres, or their component parts, which potentially act or interact to influence the three main properties. The project is being carried out in a number of Work Packages (WP) and the objective of WP3, to which this report relates, was to describe the different parameters of road surfaces and tyres that are relevant to skid resistance, rolling resistance and noise emission, leading to a matrix that clarifies the interdependencies of the different factors. At the outset of the TYROSAFE project, it was clear that road surfacings and tyres are currently developed independently of one another. In relation to the three particular surface properties on which the project concentrates this becomes especially obvious as roads are developed without real thought to tyres and tyres are tested without reference to road surfacings in every-day use. The main purpose of this report is to draw together what is known of the various parameters from many research projects, practical experience and discussions in expert workshops, to examine how the parameters interact with one another in different ways and at different levels, with a view to optimising them in relation to the three surface properties. Acting together, roads and tyres make a vital contribution to road safety but, as they do so, they have an impact on the environment. Interaction between the tyre and the road surface provides grip to allow vehicles to manoeuvre; the same process can also give rise to rolling resistance, with a potential increase in fuel consumption and CO 2 emissions, while the interactions generate noise both in vehicles and in areas close to the road. Many parameters of road surfaces and tyres are involved in these interactions and they can affect one another adversely. For instance, in a road surfacing, something that improves skid resistance may result in higher rolling resistance or increased noise, and vice-versa. Other demands are placed on road surfacings (such as requiring a long working life in a wide range of environmental and traffic conditions) that must also be taken into account in their design. A similar situation applies to tyres. The European Parliament and Council have published a directive regarding tyre labelling which focuses on fuel efficiency (CO 2 -emissions), safety and noise emission. These translate directly to the tyre s equivalent of the three surface properties (wet grip, rolling resistance and noise) but there are also recommendations for tyres regarding mileage (i.e. durability), weight, and wet and dry handling as well as stability at high speed. Date: 17/03/2010, Version: (67)

12 Assessing these parameters is complicated by the fact that, for safety (skid resistance or friction) wet conditions are used, whereas for noise or rolling resistance, measurements are made when the road surface is dry. From the perspective of the road surfacing, by far the most important factor is its texture which, through its different scales (micro-, macro- and megatexture), affects all three surface properties. All texture scales can have an effect on tyre/road friction but the dominant parameters in this context are microtexture and macrotexture. Macrotexture has some influence on friction at low speeds, albeit to a much lesser extent than microtexture, but it is the dominant factor affecting friction at higher speeds on wet roads. Although of vital importance for skid resistance, microtexture has little or no effect on rolling resistance and noise emission: macrotexure and megatexture are the more-important texture scales influencing those properties. Of the many different properties of tyres that affect their interaction with the road surface, the most important are the tread depth and pattern, the tyre structure and the rubber compound. For skid resistance, the tread depth and pattern, together with the rubber compound have a major effect. The main influence of the tread pattern is in the ability of the tyre to expel water from the contact and allow the rubber to make intimate contact with the road surface, when the compound becomes particularly important. If the road surface macrotexture is good, then the tread depth is relatively unimportant but as texture deteriorates then tread depth becomes increasingly significant as a contributor to safety in wet conditions. Part of the issue of optimisation of roads and tyres regarding the skid resistance property of road surfaces, therefore, is to find the best balance of the contributions of macrotexture and tread depth. In doing so, allowance must be made for the fact that roads have widely varying texture and are used by a wide range of vehicles that have wide variations in tread depths on their tyres. Regarding rolling resistance, the major tyre influence on this property comes from the rubber compound. The same interaction forces that generate adhesion for grip, together with flexing of the rubber, cause heat generation with consequent energy loss. Another influence on the rolling resistance can be found in the flexing of the belt, the carcass and the sidewalls of the tyre. This mechanical bending of different materials (mainly rubber and steel) generates heat with a resulting energy loss. Tyre/road noise is produced by the interaction of the tyre tread with the road surface through different mechanisms and the tread pattern has a major effect on the loudness and the frequency of the emitted noise. The noise effects, although generated by the rolling tyre, are also influenced by many parameters not related to it, such as road surface texture and environmental factors, as well as driving behaviour, speed and acceleration. To illustrate these interactions, interdependency matrices have been developed and discussed to help identify the main parameters that can be used to optimise performance. It has been found that interactions occur on two levels. Some parameters have a direct impact on a property whereas others are important secondary parameters that influence the main parameters. Factors affecting the surface properties can also be grouped depending upon Date: 17/03/2010, Version: (67)

13 whether they can be influenced in the process of optimising a surfacing or whether they are outer factors, that play a part but outside the control of the engineer such as the weather. A good example of this is that of road surface microtexture, which is of considerable importance for development of skid resistance (and wet grip). However, microtexture is a parameter that cannot be directly measured at present and, over the life of a surfacing, it is heavily influenced by the polishing resistance of the aggregate. This aggregate parameter, in turn, interacts with traffic, rainfall, frost and other factors to result in the actual level of skid resistance available at a particular location and time. This example alone demonstrates that the interactions identified are complex; it has therefore not proved straightforward to represent them in a combined manner. Consequently, separate interdependency matrices were developed for surfacings and tyres to provide an overview of the general impact of the various parameters on each of the three surface properties. In analysing these issues further, it was found that a smaller set of parameters could be used as the basis of optimising road surfacing performance in relation to the three main properties and these have been used to suggest what properties an optimised road surfacing might have, namely: Small aggregate size (5 or max. 8 mm). Polishing resistance appropriate to the expected traffic and skid resistance level required over the life of the surfacing. High angularity of aggregates Cubic shape of aggregates Binder viscosity optimised for the application (preferred polymer modified bitumen). A concave surface texture (without separately applied surface chippings for asphalt or an exposed aggregate form for concrete). This approach offers possibilities to improve all three properties without deterioration of one of the others; none of them will necessarily achieve the best possible values but seldom is that necessary. Existing knowledge and practical experience suggest that it should be possible to optimise surface texture in the process of constructing a road surfacing without making a great many other changes to the surfacing mixture, although achieving this in practice may not be easy. However, with tyres, although they have properties (tread compound, tread pattern and depth, carcass construction) that are analogous to the different surface texture scales in some of their effects on the three properties, changing one can affect the behaviour of the whole tyre, not always advantageously. In addition, the different characteristics of truck and car tyres mean that different approaches to optimisation may be necessary. Nevertheless, some optimisation in relation to the three properties of wet grip, rolling resistance and noise is theoretically possible for tyres but because their response depends heavily on the road surfacings on which they run, and these are currently so varied, it is Date: 17/03/2010, Version: (67)

14 difficult to establish at present where the best optimisation can be achieved. Fortunately, relatively few parameters work against one another. The whole process of optimisation would be made easier if there were common international standards relating to tyre properties that were based on a response on real roads and if there were a harmonised approach to setting requirements for road surfacings in relation to the three properties, an aspect discussed in TYROSAFE WP1. This exercise of examining the interactions between the various parameters has highlighted a number of areas where current knowledge is lacking and further research will be of value. These issues will be considered and reported separately, in D15, the final deliverable from TYROSAFE WP3. Date: 17/03/2010, Version: (67)

15 1 Introduction 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 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 WP3, to which this report relates, is to describe the different parameters of road surfaces and tyres that are relevant to skid resistance, rolling resistance and noise emission, leading to a matrix that clarifies the interdependencies of the different factors. Through this process, it will be possible to identify knowledge gaps, make recommendations regarding the design of road surfaces to increase road safety, and propose suggestions for further research into the optimisation of road surfaces. To reach its objective, WP3 was divided into four Tasks: Task 3.1 collecting knowledge of parameters influencing skid resistance, covering both road surfaces and tyres. Task 3.2 collecting knowledge of parameters influencing rolling resistance. Task 3.3 collecting the knowledge of parameters influencing noise emissions Task 3.4 developing matrices which show the interdependencies of the different parameters, identifying knowledge gaps and requirements for further research, organising two workshops (Spring 2009/Spring 2010) to obtain input from experts from different fields and countries. Date: 17/03/2010, Version: (67)

16 Table 1.1 gives an overview of the major outcomes planned for the individual Tasks of WP3. The 19-month programme of work began in December It has involved an initial literature review and two expert workshops (the first held in Brussels on the 13th May 2009 and the second workshop held in Cologne on the 10th February 2010, during the Tire Technology Expo), in addition to collating the TYROSAFE team s existing knowledge. Date: 17/03/2010, Version: (67)

17 Table 1.1: Overview of the major outcomes of the individual Tasks of WP3 Task Deliverable Name Month 3.1,3.2,3.3 D10 Report on different parameters influencing M14 skid resistance, rolling resistance and noise emissions 3.4 D14 Report on matrices on interdependencies of M20 parameters influencing skid resistance, 3.4 D15 Report on knowledge gaps and proposals M23 for further research concerning optimisation of road surfaces and tyres for skid resistance, rolling resistance an noise emissions Two dedicated workshops M10 and M21 This report, Deliverable D14, is the main output from Task 3.4. For the avoidance of confusion, the term property in relation to road surfaces is usually used throughout this report to refer to skid resistance, rolling resistance or noise emission; these are the three main aspects on which TYROSAFE is focussed (important for safety, health and energy saving on the European road network). The term parameter is used to refer to individual factors (or values that describe them) that relate to the construction or manufacture of road surfaces and tyres, or their component parts, which potentially act or interact to influence the three main properties. The main purpose of this report is to draw together what is known of the parameters that were described in D10 [1], and discuss their interactions with a view to optimising them in relation to the three surface properties. The analysis draws on the results of historic and current research on these topics and incorporates the outcomes of discussions at the expert workshops. Chapter 2 provides some background regarding the impacts that different characteristics of road surfacings and tyres have, identifying the parameters that are likely to be the most relevant or important for optimising the surface properties. Chapters 3 and 4 examine in more detail the various parameters of road surfacings and tyres and the influences that they have in the process of optimising them together in order to provide the best outcomes for the three surface properties. Chapter 3 looks at the optimisation of the design of road surfaces while Chapter 4 considers optimisation of tyre parameters in relation to road surface properties. Each of these chapters contains an interdependency matrix showing the various parameters and their impacts on the three surface properties. Chapter 5 focuses on the most relevant parameters for optimising surfaces and tyres with regard to safety (skid resistance), firstly from a theoretical perspective and secondly in relation to practical implementation. The final part Chapter 5 then incorporates the implications for the other two surface properties. Chapter 6 concludes the report by summarising the findings of the report as a whole. Date: 17/03/2010, Version: (67)

18 2 Background 2.1 General comments Acting together, roads and tyres make a vital contribution to road safety but, as they do so, they have an impact on the environment. Interaction between the tyre and the road surface provides grip to allow vehicles to manoeuvre; the same process can also give rise to rolling resistance, with a potential increase in fuel consumption and CO 2 emissions, while the interactions generate noise both in vehicles and in areas close to the road. Many parameters of road surfaces and tyres are involved in these interactions and they can affect one another adversely. For instance, in a road surfacing, something that improves skid resistance may result in higher rolling resistance or increased noise, and vice-versa. As well as the three properties that are the focus of TYROSAFE, other demands placed on road surfacings (such as requiring a long working life in a wide range of environmental and traffic conditions) must also be taken into account in their design. This can mean that parameters that are not related specifically to the interface where tyre and road meet but are used to improve the wider performance of the surfacing (such as providing high resistance to deformation or cracking) may also have an indirect impact on the three surface properties. A similar situation applies to tyres. The European Parliament and Council have published a directive regarding tyre labelling which focuses on fuel efficiency (CO 2 -emissions), safety and noise emission. These translate directly to the tyre s equivalent of the three surface properties (wet grip, rolling resistance and noise) but there are also recommendations for tyres regarding mileage (i.e. durability), weight, and wet and dry handling as well as stability at high speed; optimising tyres for their contribution to the key surface properties should not worsen these parameters. In this report, therefore, an attempt is made to review the properties of road surfacings and tyres together in an attempt to find a solution that optimises both road surfaces and tyres. Skid resistance is at its lowest when the road is wet (apart, of course, from icy conditions) and throughout this report, unless otherwise stated, all discussions of surfacing or tyre parameters relating to this safety aspect refer by default to wet conditions. Many other factors can affect the key surface properties or vehicle responses to them because of the way in which they influence local conditions and the behaviour of the traffic using a road. These include: Environmental effects, such as rainfall (which influences aggregate polishing processes and, instantaneously, the thickness of the water film on the road); Individual vehicle loads and speed Traffic levels Road geometry (such as cross-fall, bendiness or curve radius). These types of factor are not considered directly within this report because neither road constructors nor the automobile industry can influence them: they are essentially external to the surfaces or tyres themselves and are referred to later in the report as outer factors (see also Chapters 3 and 4). Date: 17/03/2010, Version: (67)

19 The unevenness of a surface is a factor that may have an adverse impact on road safety. Unevenness can be controlled during construction by appropriate use of paving machinery but, over a road s service life, the influence of the paving and compaction process is very small in comparison with the many parameters relating to the composition of the surfacing material, such as the properties of the aggregates and bitumen used. For this reason, unevenness is not considered specifically in this report. The impact of environmental effects on the surface properties (and vice-versa), as well as the likely influences of climate change in this context, are being dealt with in WP4 of TYROSAFE and have not been taken into account explicitly here in developing the interdependency matrices. WP4 will take account the findings of WP3 relating to the optimisation of surfaces and tyres in its considerations and include these in its reports, particularly D16. The level of friction between the tyre and road surface at any time is the result of a process in which many factors are involved rather than an inherent property of the road surface itself. The same is true of rolling resistance and noise. The factors involved can be grouped into four categories: Road surface characteristics Tyre properties Vehicle operational parameters Environmental factors. The many parameters that could be involved in the processes influencing the key surface properties have already been discussed in detail in TYROSAFE Deliverable D10 [1]. The remaining sub-sections of this Chapter provide some background relating to the main factors that have been identified as most important prior to discussing the issues for optimisation; for convenience, these have been divided into parameters related to the road surface (2.2), those related to tyres (2.3) and other parameters. 2.2 Road surfacing-related parameters that affect the three surface properties As discussed in D10 [1], there are a great many parameters that could have an influence to a greater or lesser extent on the road surface properties. From the perspective of the road surfacing, by far the most important factor is its texture which, through its different scales (micro-, macro- and megatexture), affects all three aspects. All texture scales can have an effect on tyre/road friction but the dominant parameters in this context are microtexture and macrotexture. Both should be high in order to increase adhesion, hysteresis and water drainage. Microtexture affects the level of friction achieved over the range from almost zero up to the maximum possible friction and is important at all speeds. Macrotexture has some influence on friction at low speeds, albeit to a much lesser extent than microtexture, but it is the dominant factor affecting friction at higher speeds on wet roads. Although of vital importance for skid resistance, microtexture has little or no effect Date: 17/03/2010, Version: (67)

20 on rolling resistance and noise emission: macrotexure and megatexture are the moreimportant texture scales influencing those properties. Figure 2.1 provides an overview of the impact that different surface textures (based upon their wavelength) have on the different (surface) properties. This allows the relevant wavelengths for the textures relevant to the various properties to be identified. Tyre footprint Wavelength (mm) Range of irregularities Microtexture Macrotexture Megatexture Roughness Rolling resistance Influenced performances Adhesion Skid resistance Drainage Optical properties Riding comfort Roadhold Splash & spray Dynamic loads Tyre wear Vehicle wear Tyre/road noise Figure 2.1: Texture wavelength influence on tyre/road interactions (based on [16]) Because texture is so important overall, parameters that play a significant part in its generation must be considered. Both microtexture and macrotexture are influenced by the nature of the road surfacing materials and the constituents used in their manufacture. Microtexture is generally provided in asphalt pavements by the relative roughness of the aggregate particles and in concrete surfaces by the fine aggregate or sand in the mortar on the finished surface. Macrotexture is generally provided in asphalt pavements through the appropriate choice of aggregate grading (in dense materials) or by aggregate chippings spread on the surface (in surface dressings or chip-seals and micro-surfacings as well as chipping newly laid surfacings to improve microtexture in the very early life of the surfacing). Porous surfacings can enhance the drainage characteristic of macrotexture by providing interconnected voids that run through the surfacing layer, again achieved through careful control of aggregate grading. On concrete surfaces macrotexture is provided by an additional treatment such a brushing the still-wet laitence, exposing the coarse aggregate or by means of sawn grooving. Currently, there is no simple solution to produce an optimal texture through the mixture composition alone. Paving and compaction methods also have considerable influence on the properties of the finished surface. Thus, two areas give rise to parameters which impact on surface texture: Properties of aggregates. Composition, paving (including compaction and surface texturing techniques). Date: 17/03/2010, Version: (67)

21 Aggregate properties are the dominant area for microtexture. Macrotexture is influenced by the surfacing material composition in combination with paving and compaction, while megatexture is mainly influenced by paving and compaction. These groups of characteristics are not only significant when a road is new. Their favourable qualities should be maintained through the life of the road because they will have an impact on the long-term durability of texture under the cumulative effects of traffic and environmental influences. Both the ability of the aggregate at the surface to resist polishing and abrasion or wear and the stability of the surfacing layer maintaining the desired nature of the running surface are important. The aggregate used in a road surfacing material has great impact on the three surface properties, both in terms of the material exposed at the surface that makes contact with tyres (i.e. its microtexture) and the structure of the surfacing that provides the coarser scales of texture and durability. Of particular importance are particle size, polishing resistance (typically expressed as Polished Stone Value, PSV), and particle shape. Higher levels of polishing resistance can be an advantage in retaining microtexture, and hence the skid resistance, under the influence of heavy traffic. However, with some aggregates high polishing resistance is associated with poor abrasion or wear resistance; this can lead to reduced macrotexture in service, so both aspects must be considered together for optimum skid-resistance performance. As a further example of the kinds of issues that might be addressed for optimising the surface properties, there is increasing evidence that, when used appropriately, particle sizes smaller than are typically used at present, about 5 or 6 mm, can show positive effects in relation to all three properties: better skid resistance, less rolling resistance and reduced noise emission. However, particle shape, in combination with size, can have an important influence on how macrotexture is provided or maintained, so controlling size alone is insufficient. 2.3 Tyre-related parameters that affect the three surface properties Of the many different properties of tyres that affect their interaction with the road surface, the most important are the tread depth and pattern, the tyre structure and the rubber compound. For skid resistance, the tread depth and pattern, together with the rubber compound have a major effect. The main effect that causes adhesion of the tyre to the road, and therefore the ability of the vehicle to accelerate, brake and to corner, is micro-slippage of the tyre rubber on the road surface. In the contact, area the tread blocks are flattened and pressed onto the road surface and lifted up again as the vehicle moves. The flattening results in micro-slippage; the rubber compound reacts with the minerals at the road surface and molecular adhesion forces, especially Van-der-Waals forces are created allowing the tyre to grip the road. The main influence of the tread pattern is in the ability of the tyre to expel water from the contact and allow the rubber to make intimate contact with the road surface. In order to Date: 17/03/2010, Version: (67)

22 achieve this, the tread provides several channels that enable the water to escape from the contact area to the area beneath, in front of or behind the tyre. Clearly, the ability of the tread pattern to assist water removal becomes more important as the drainage capacity of the road surface worsens. If the road surface macrotexture is good, then the tread depth is relatively unimportant but as texture deteriorates then tread depth becomes increasingly significant as a contributor to safety in wet conditions. Part of the issue of optimisation of roads and tyres regarding the skid resistance property of road surfaces, therefore, is to find the best balance of the contributions of macrotexture and tread depth, allowing for the fact that roads are used by a wide range of vehicles with widely varying tread depths on their tyres. Regarding rolling resistance, the major tyre influence on this property comes from the rubber compound. The same interaction forces that generate adhesion for grip, together with flexing of the rubber, cause heat generation with consequent energy loss. Another influence on the rolling resistance can be found in the flexing of the belt, the carcass and the sidewalls of the tyre. This mechanical bending of different materials (mainly rubber and steel) generates heat with a resulting energy loss. Tyre/road noise is produced by the interaction of the tyre tread with the road surface and the tread pattern has a major effect on the loudness and the frequency of the emitted noise. The noise effects, although generated by the rolling tyre, are also influenced by many parameters not related to it, such as road surface texture and environmental factors, as well as driving behaviour, speed and acceleration. Two main groups of mechanisms generate tyre/road noise: aerodynamically related mechanisms. vibration related mechanisms. The aerodynamically related mechanisms can be divided into the effect known as airpumping (in which air is compressed by the tyre into the texture of the road and released again) and resonant effects due to the shape of the area in front of and behind the tyre/road contact patch. Other resonances also occur due to the shape of the tread pattern that forms pipes in the footprint of the tyre where air can be pumped out. All these effects are described in detail in D10 [1]. The vibration-related mechanisms occur because rubber is an elastic material and happen when the tyre rubber interacts with the road surface roughness. This causes compression and decompression of the rubber blocks. Vibration occurs in both directions of the tyre, radial as well as transverse. This vibration initiates movements of the rubber blocks and emits noise. The vibration also initiates movement of the belt and the sidewalls. The adhesion forces of the rubber cause the rubber blocks to stick to surface. As the vehicle continues to move, the rubber blocks are drawn from the surface. This movement causes radial vibration and vibration of the belt. Date: 17/03/2010, Version: (67)

23 2.4 Other parameters that affect the three surface properties As well as parameters of the road surfacing and tyres, the three surface properties are affected by environmental parameters and aspects of road geometry, together with traffic, speed, load and evenness. The majority of these factors are largely outside the control of the road surfacing designer or contractor. They may influence decisions in relation to other parameters (traffic level could influence the choice of aggregate PSV, traffic speed the required level of macrotexture, for example) but only the evenness of the finished surface can actually be influenced at the construction stage. The environmental factors include rainfall (leading to a wet surface, affecting water film thickness), temperature (resulting in changes to surface characteristics such as deformation, flushing of binder to the surface or cracking) and the presence of ice or snow; these are independent of the surfacing or the tyre. For this reason, factors in this group are collectively referred to as OUTER FACTORS in this report. Water on the road surface has a significant adverse effect on skid resistance and tyre/road friction. It has also an impact on noise emissions but the full nature and scale of the effects are still the subject of research. The impact of wet surfaces on rolling resistance is not really known, because the environmental aspects are usually not included in research about this surface property. As already discussed, the influence of environmental effects in general is being dealt with in TYROSAFE within Work Package 4. The evenness of surfaces has only a small impact on skid resistance and noise emission (in the latter case through inducing deformation of the tyre carcase and resulting vibrations), but rolling resistance is influenced by the amplitude of deformation, so unevenness has a major influence on energy consumption. Excitations with a low frequency (unevenness means texture wavelengths greater than 500 mm) result in deformation of the tyres as well as movements in the vehicle s dampers. Energy dissipation in the vehicle s dampers is, by definition, not a part of the tyres rolling resistance, but it can be regarded as a part of rolling resistance in the broadest sense, since the energy loss is induced in response to an influence of the road surface. Unevenness can also have an indirect effect on safety. It influences vehicle movements, which may affect the way in which tyre/road contact is maintained. It can also cause extra stress in the pavement and thus increase the possibility of permanent deformation as well as cracks, which together can have a knock-on effect on road safety. Road design characteristics such as typical cross section, curve radius, the bendiness or even the cross-fall and the longitudinal gradient could influence the three road surface properties of interest to this report but the effects are largely indirect with surfacing and tyre parameters having greater influence in most cases. At present, the impact of road design in relation to the surface properties cannot be quantified because there is no research known on this topic. Date: 17/03/2010, Version: (67)

24 3 Interdependencies of road surfacing parameters and their influence on the three surface properties 3.1 Introductory comments As the background chapter (2.2) has indicated, it is not easy to optimise the road surfacing parameters affecting the three surface properties in such a way that providing the best for one property has no negative impact on the other two. It is therefore worth considering which factors can be effectively influenced in a way that permits optimisation while taking some account of the others. To help in this, in the following discussion the various parameters have been grouped as follows: SURFACE PARAMETERS. These parameters relate specifically to the nature of the road surface and have the main influence in the tyre/road interface. INNER FACTORS. These parameters are secondary to the surface parameters but they can be influenced in the optimisation process to have a direct impact on the surface parameters and the three surface properties. These factors can be divided into two categories: mix design and construction. OUTER FACTORS. These factors may have an impact on the three surface properties but they cannot be influenced in the optimisation process. They fall into two categories: traffic interaction and environment. This concept of the grouping of parameters and their potential interactions with one another from the perspective of the road surface is illustrated in Figure 3.1. Date: 17/03/2010, Version: (67)

25 Figure 3.1: Parameters influencing the three surface properties (skid resistance, rolling resistance and noise emission) from the perspective of the road surface Optimisation, almost by definition, involves some kind of compromise in that it may not be possible to achieve the best of everything at the same time. Thus, when attempting to optimise the three surface properties by adjusting the various factors it is unrealistic to expect to achieve the optimum condition for all of them. Rather, the only practical approach is to try to improve each property compared with the current norm where this is possible. In other words, whereas at present surfaces are typically optimised with regard to only one property, if they are optimised to all three properties none of them should worsen compared with what is currently achieved. This approach will not necessarily lead to very-high skid resistance surfaces, to very-low rolling resistance or to very-low noise emission surfaces: instead, it should result in surfacings that simultaneously help to improve road safety, to lower fuel consumption (and CO 2 -emissions) and to lower tyre/road noise, because they are optimised to all three properties. It should be noted that within the TYROSAFE project, all three properties are Date: 17/03/2010, Version: (67)

26 given equal emphasis because all three are important for European roads. Only a balance between the individual properties allows optimisation of tyre/road interaction. 3.2 Interdependency matrix of surface parameters influencing the road surface properties An interdependency matrix is a way of arranging the various parameters that affect the road surface properties so that the interactions between them can be shown. Information gathered from many research projects (such as those in the reference list) and from practical experience has been used to derive a matrix. An early version of the matrix was discussed at the expert workshop in Cologne during the Tire Technology Expo. It was recognised number of parameters and the interactions between them relating to different types of road surfaces and different types of tyres are so many and so complex that would not be practical to represent them all together in a simple way (three dimensions would be needed). For this reason, in the interests providing an overview and for clarity, two separate matrices have been prepared showing the interactions for the most relevant surface and tyre parameters separately, with only a limited breakdown by surfacing type. The matrix for the road surface parameters, which draws on the results of research covering mineralogy, asphalt and concrete optimisation as well as the texture parameters related to the three main surface properties, has been set out in Table 3.1. Before presenting the Table, however, some explanation is necessary. As has been discussed elsewhere in the TYROSAFE project, there are many ways of measuring skid resistance, in a wide range of test conditions; it is therefore not possible to make direct comparisons of numerical values. Rolling resistance is often determined through an indirect measurement of fuel consumption rather than direct measurements of the property and there are a number of different ways of measuring noise. Therefore, in the interdependency matrix for road surface parameters the following approach has been taken. Within Table 3.1, the parameters included are those that were described as Surface parameters and Inner Factors in Figure 3.1. For each parameter, an indicator has been provided to show the impact on each of the three road surface properties when the parameter value changes. An arrow symbol indicates the change in magnitude of the parameter that is being considered (an upwards arrow implies an increasing value, a downward arrow a decrease). In the columns to the right, a sign indicates the general impact that such a change might be expected to bring for each property: A plus sign (combined with green cell shading) indicates a positive impact, i.e. an increase in skid resistance or a decrease in rolling resistance or noise. A minus sign (with red cell shading) shows a negative impact, i.e. decreasing skid resistance, increased rolling resistance or noise. A zero (with grey shading) indicates that research has shown there is no impact for this surface property. Date: 17/03/2010, Version: (67)

27 A question mark in an un-shaded cell indicates that the response is unknown and is used for interactions where knowledge gaps still exist. (These knowledge gaps will be discussed in more detail in the final deliverable from WP3, D15). The matrix has been laid out so that the parameters and their responses relating to aggregates, mix design and laying are separated in relation to the type of surfacing (asphalt or concrete), with other parameters considered together. It will be noted that there is more existing knowledge relating to asphalt surfaces, reflecting the fact that asphalt is much more widely used than concrete surfaces and research is more often directed at asphalt. As previously explained, all parameters relating to skid resistance refer to wet conditions. However, the parameters for optimising rolling resistance or noise refer to dry conditions; measurements relating to these properties are usually made when the road is dry and little is known about the effects in the wet. When working through Table 3.1, the reader should take note that in the studies establishing the behaviours indicated, most measurements of the road surface properties used new or special tyres, whereas the tyres in everyday use on cars or trucks are not new. This could lead to behaviours in practice that differ from those predicted by the measurements. Further explanation of the interactions of the individual road surface parameters are discussed in more detail in relation to each main property in sub-sections 3.3, 3.4 and 3.5. Table 3.1: Interdependency matrix of surface parameters Skid Resistance Rolling Resistance Noise Emission asphalt shape of aggregates (SI/FI) + o 1? + [2] aggregate properties angularity of aggregates +? o polishing resistance (Polished Stone Value (PSV)/coarse aggregates) polishing resistance (PWS /fine aggregates) + 2? o [2] + 3? o [2] hardness +?? aggregate composition and Structure (percent of hard fraction by visual examination and petrographic analysis) abrasion/wear resistance (Micro Deval) +?? +?? maximum aggregate size mixture parameters binder content + -?? binder type (viscosity) void content (mix design) Depends on the composition used, according to EN , -5, -6, -7 (AC, MA: o, SMA, PA: +) 2 On mastic asphalt, according to EN , it can take a long time for the aggregates to become exposed to the contact area of the tyre; consequently, these properties do not have a positive effect on skid resistance until later in the service life. 3 As with coarse aggregate, on mastic asphalt according to EN it can take a long time for the aggregates to become exposed to the contact area of the tyre; consequently, these properties do not have a positive effect on skid resistance until later in the service life. 4 On mastic asphalt according to EN , there is no known impact of the aggregate size used. 5 Rubber modified binder may have a positive effect on noise reduction 6 Only in dense asphalt mixtures; not in porous asphalt mixtures Date: 17/03/2010, Version: (67)

28 Skid Resistance Rolling Resistance Noise Emission laying and compacting chippings aggregate size chippings PSV/PWS +? o degree of compaction?? - concrete aggregate properties mixture parameters laying shape of aggregates (SI/FI)?? + angularity of aggregates +? o polishing resistance (Polished Stone Value (PSV)/coarse aggregates) polishing resistance (PWS /fine aggregates) +? o +? o hardness??? aggregate composition and structure (percent of hard fraction by visual examination and petrographic analysis) abrasion/wear resistance (Micro Deval)?????? maximum aggregate size water/cement-ratio??? stability of the concrete -?? consistency + -? + - additive??? miscellaneous finished surface thickness of the surface mortar??? surface textures exposed aggregate concrete - +? + surface textures burlap - -? + surface texture brushed concrete surface texture depth - mean texture depth (MTD) - +? surface type (asphalt or concrete) asphalt o? + 7 damper function of the base layer -? - + shape factor (g) > 70%?? + characteristic shape length (g ) mm maximum of the spectral roughness depth (R max )? o µm?? + absorption o o + layer thickness o o + o 8 flow resistance o o + further texture parameters (micro- / macrotexture) see Figure 2.1 and also Figure o unevenness - -? 7 In [56] it was found that a dense concrete surface with chippings (e.g. 5 mm maximum stone size) created a surface which was about 2 db(a) louder than the same chippings used on an asphalt surface. This effect derives from the higher internal damping of asphalt compared with concrete. 8 On porous surfaces; on dense surfaces the layer thickness has no impact on noise reduction 9 At high frequencies: the increase of the texture amplitudes at wavelengths in the range 0.5 to 10 mm (microtexture) may reduce noise generation, particularly at frequencies generally above 1 khz. At low frequencies: the increase of the texture amplitudes at wavelengths in the range 10 to 500 mm (macro texture) causes an increase of noise, particularly at frequencies below 1 khz. Date: 17/03/2010, Version: (67)

29 increasing/rising value or category (e.g. PSV) decreasing value or category (e.g. shape factor) + positive impact (e.g. higher skid resistance, lower rolling resistance, lower noise emission) o no impact (e.g. no changes on surface property due to changing the parameter) Skid Resistance Rolling Resistance Noise Emission - negative impact (e.g. lower skid resistance, rolling resistance, higher noise emission)? impact unknown 3.3 Surfacing parameter interactions related to skid resistance As explained in Section 2.2, the main pavement characteristic on which road designers must focus in relation to skid resistance is the pavement texture, in particular micro- and macro texture and PIARC [38], has defined ranges of wavelengths and peak-to-peak amplitudes to identify the various texture scales (see Figure 2.1). Macrotexture is of a similar order of size to tyre tread elements, with a wavelength range of 0.5 mm to 50 mm and peak amplitudes between and 0.1 and 20 mm. Microtexture is represented by all wavelengths below 0.5 mm and peak amplitudes from to 0.5 mm. It is generally too small to be seen easily with the naked eye but, when touched with a finger, it makes the surface feel more or less harsh. Both of these texture scales influence both mechanisms (adhesion and hysteresis) involved in developing friction between the tyre and the road surface. Generally, adhesion is related to microtexture and dominates both wet and dry friction level at low speeds. Hysteresis is mainly related to macrotexture, which also facilitates the drainage of water at higher speeds so that microtexture can penetrate the water film and re-establish the adhesive component of friction. Aggregate properties The most important properties of aggregates in terms of skid resistance design and performance are: Mineralogical and petrographic properties: These properties are related to the composition and the mineral hardness of aggregates. Aggregates that are typically composed of hard, strongly bonded, interlocking mineral crystals embedded in a matrix of softer minerals exhibit the highest levels of polish resistance and resistance to wear. Because of differential wear rates, due to differences in grain hardness and the breaking off harder grains from a softer matrix of softer minerals, the abrasive surface is constantly renewed. High hardness and lower cleavability of minerals in the aggregates raises an effective resistance to levelling of the former fracture surface. A directionless and bulky texture of the steric configuration of the mineral crystals has advantages for polishing resistance compared with a parallel texture. A high content of microcrystalline crystals (at a size of mm) shows benefits in relation to polishing resistance [35]. Physical and geometrical properties: The angularity, shape and texture of aggregates are important for defining the microtexture and macrotexture of the road surface. Finer aggregates present angular edges and cubical or irregular shapes so the level of microtexture will be higher. For coarse aggregates, sharp and angular particles can Date: 17/03/2010, Version: (67)

30 more readily interlock and produce a deep macrotexture compared with rounded, smooth particles. Mechanical properties: These properties are related to the capacity of aggregates to resist polishing, abrasion and wear. Aggregates that are resistant to abrasion are important for avoiding breakdown during mixing and compaction, and also under traffic action once in service. The breakdown of aggregates can significantly modify the grading and hence the macrotexture. Resistance to polishing of aggregates under traffic load is a major factor in long-term skid resistance performance; polish-resistant aggregates will retain their microtexture under the grinding and shearing effects of the repeated passage of tyres, especially those of heavy vehicles. Durability properties: This property is principally related to aggregate soundness. Sound aggregates, which are aggregates that can resist to degradation due to climatic and environmental effects, are important for avoiding breakdown in harsh climates. Table 3.2 provides a list of aggregate properties, and the corresponding tests for characterising them, related to their effect on skid resistance performance and durability. The last column of the table shows how increasing values of each property influence skid resistance. Table 3.2: Aggregate properties and their impact on skid resistance aggregate property test type comments [2]-[13] effect of increased values on skid resistance mineralogical and petrographic hardness aggregate composition and Structure Mohs scratch hardness petrographic Analysis hard minerals: greater than 6 soft minerals: from 3 to 5 difference between soft and hard: from 2 to 3 percent of hard fraction of natural aggregates between 50 to 70 average Size of hard grain equal 200 µm shape of hard grain has to be angular? physical and geometrical Angularity Shape content of crushed aggregates Shape index / Flakiness index aggregate particles shape should be conical aggregate particles size from 3 to 13 mm Abrasion/Wear resistance Micro Deval percentage loss less than 17 to 20 mechanical tight-abrasion resistance Los Angeles percentage loss less than 35 to 45 Polishing Resistance Polished Stone Value typically > 50 is required but see footnote 10. durability Soundness Magnesium Sulphate Soundness percentage loss less than 10 to 20 Date: 17/03/2010, Version: (67)

31 To optimise surfaces in relation to skid resistance, polishing resistance (whether measured by PSV or the Wehner-Schulze test (PWS)), should be as high as possible 10 ; the percentage of the angularity should also be high; the shape index or flakiness index of the coarse aggregates should be low, as should the Micro Deval values. However, it should be borne in mind that polishing resistance has little or no impact on initial skid resistance, but it is very important in providing and maintaining skid resistance at appropriate levels over the long term. The fine aggregates contribute to skid resistance through the sharpness of their edges; aggregates with quartzitic origin show advantages regarding to skid resistance compared with aggregates with a siliceous or carbonatic origin [35]. Regarding the petrography of the aggregate, there should be minerals with differences in hardness and cleavability (high hardness and lower cleavability). In concrete surfaces, however, an opposite behaviour is shown in relation to the fine aggregate. The stability of the cement, stone and fine aggregates should be nearly equal; therefore, fine aggregates with a calcitic origin contribute higher skid resistance than fine aggregates with quartzitic origin [19], [20]. In addition to these important properties, the aggregates have to meet all the other requirements for use in surfacing materials, e.g. impact crushing strength, adhesion between aggregates and bitumen as well as frost and heat resistance. Mixture properties Composition (mixture type) - asphalt: Asphalt surfacings can be divided in four different mixture types (according to the various parts of EN 13108): asphalt concrete (AC), stone mastic asphalt (SMA), mastic asphalt (MA) and porous asphalt (PA). Each type makes a different contribution to the skid resistance of the surface: In AC, both the coarse and fine aggregates contribute to skid resistance because both sizes of aggregates are used in approximately similar proportions and so the properties of both fractions (especially PSV) are important. For SMA, which has a much higher proportion of coarse aggregate (about 70% or more), the coarse aggregate has the greatest influence on the skid resistance of the finished surface and good polishing resistance and shape is needed. Although they cannot be ignored, the properties of the fine aggregates are less important in this type of material. For MA, the properties of the chipping material (especially polishing resistance) are much more important than the aggregates used in the mixture. The polishing resistance of the coarse aggregate used in the mixture only becomes important for 10 Typically, PSV should be 50; increased values may be necessary for locations carrying high levels of heavy traffic or subjected to significant braking or cornering stresses. Practice varies in Europe depending on local availability of suitable aggregates and policies relating to skid resistance levels. Date: 17/03/2010, Version: (67)

32 skid resistance in the longer term as it becomes exposed at the surface because of wear or chipping loss. For PA, in which hardly any fine aggregates are used, only the coarse aggregate contributes to the skid resistance. It should be noted that on all asphalt surfaces for which the finished surface is initially coated in bitumen, there might be reductions in skid resistance early in the life of a surfacing that are unrelated to the properties of the aggregate. The percentage of the filler used within the mixture only impacts on skid resistance in rolled asphalt surfacings where areas of the asphalt matrix may be exposed in un-chipped areas. A high content of filler (especially limestone) can reduce the skid resistance of the surface. Composition - concrete: Practical experience has shown that a water/cement-ratio of about 0.40 to 0.45, a cement content of 350 kg/m³ and a stiff consistency provide a mortar layer with an optimal thickness of about mm. Research results show that the water/cement-ratio and the content of the surface mortar have no impact on the durability of the surface texture [19]. Furthermore, the difference in stability of the hardened mortar and the fine aggregates used should not be too great. If fine aggregate stability is higher than the mortar, the aggregates could quarry out and no longer contribute to the skid resistance of the surface. The use of high performance concrete does not necessarily lead to better results because the high stability mortar covers the aggregates so they cannot contribute to the surface skid resistance [19]. Maximum aggregate size: The + shown in Table 3.1 for this mixture parameter refers to aggregate sizes currently used: using a smaller aggregate would lead to higher skid resistance. Asphalt mixtures with a maximum aggregate size of 5 mm or 8 mm (and with appropriate macrotexture) could contribute to higher skid resistance compared with the current general practice [2]. Binder content: The amount of bitumen used in the asphalt can also have impact on skid resistance. The absolute binder content used in the mixture will depend on the mixture type. High binder contents will tend to produce a low void content, which could lead to bleeding. This may result in negative effects on skid resistance due to a loss of microtexture and macrotexture, as the bitumen covers the aggregate, and fills surface voids. Insufficient binder content can lead to loss of aggregate particles from the surface, affecting skid resistance in unpredictable ways (usually adverse). It is therefore necessary to optimise the binder content during the mix design stage to take account of the type of mixture (the required grading), the nature of the aggregates used, maximum aggregate size and binder viscosity, as well as various boundary conditions (such as traffic load and environmental parameters) for the length of road to be surfaced. Date: 17/03/2010, Version: (67)

33 Binder type (viscosity): There is no evidence in the literature to suggest that the type of bitumen used in the asphalt mix has a noticeable effect on skid resistance. However, for carriageways with heavy traffic it is generally better to use bitumen with higher viscosity to provide a durable surface (e.g. to prevent rutting) and therefore, indirectly, its skid resistance. Bitumen with lower viscosity tends to lead to fatting-up at high temperatures and heavy traffic with adverse impact on skid resistance. Practical experience shows that the use of polymer-modified bitumen (PMB) in SMA mixtures tends to produce higher levels of skid resistance compared to standard bitumen, especially at high traffic volumes [36]. This could be due to the higher viscosity of PMB and therefore a higher softening point, resulting in a pavement that has a higher resistance to permanent deformation at high temperatures. However, bitumen without polymer modifiers with a higher softening point tends to embrittle earlier and this could lead to loss of aggregate particles from the surface. The type of bitumen or bitumen viscosity should be chosen with regard to the traffic load and environmental impacts like temperature as well as other asphalt properties, e.g. deformation, low temperature or fatigue cracking. Void content (mix design): The optimal void content (V) for an asphalt mixture depends on its type. For example, higher void content is a key feature of PA but lower void contents apply to dense asphalt mixtures. However, void content is not the only parameter relevant to mixture properties; other parameters, such as voids filled with bitumen (VFB) and voids in mineral aggregate (VMA), must also be taken into account. Different studies and practical experience show that with lower void content, and therefore rising voids filled with bitumen (VFB) at the surface, skid resistance tends to decrease because of loss of micro- and macrotexture. The void content should be at least between 3 and 4 percent by volume. A critical range of V is about 2 percent by volume and VFB about 90%. At values of VFB greater than 90% the kind of aggregate used (and thus its polishing resistance) has little influence on skid resistance [17]. Laying and compacting Chippings aggregate size: In some countries, small chippings are applied to new asphalt surfaces in order to improve skid resistance during the early life period. In this context, the smaller the size of the chippings the higher is the skid resistance of the surface. The size of the chippings of about 3 mm to 5 mm could lead to higher early life skid resistance compared with surfaces without chippings [2]. The size of the chippings in this context should not be larger than the size of used aggregate in the mixture. On coarser-textured surfaces, such chippings may have an adverse effect on skid resistance by filling the surface voids and reducing macrotexture. Date: 17/03/2010, Version: (67)

34 Chippings applied to rolled asphalt surfaces are typically of larger sizes (19 mm) to provide adequate macrotexture. In surface dressings, smaller chippings tend to increase skid resistance at low speeds but may be associated with reduced macrotexture. Chippings polishing resistance (PSV/PWS): The polishing resistance of the chippings used for early life enhancement is not as important as the polishing resistance of the aggregates used in the mixture. However, where the chippings essentially form the final running surface, such as on MA, rolled asphalt or in surface dressings, an appropriate high polishing resistance should be used [2]. Degree of compaction: The impact of the degree of compaction of the asphalt mixture on skid resistance is unknown, especially if the values achieved are higher than the requirement specified in the contract. In most research projects, the degree of compaction will have fulfilled the requirements. If the degree of compaction is too low, the risk of permanent deformation and surface aggregate loss will increase. The influence that this has on skid resistance is unclear since loss of aggregate could reduce microtexture available in the tyre contact area but at the same time increase macrotexture. Over-compaction might lead to reduced macrotexture or fatting-up with a potentially adverse effect on skid resistance. Paving / compaction: The only effects on skid resistance related to paving and compaction of the asphalt mixture occur through the texture of the finished surface. Poor transfer of the hot asphalt from the delivery truck to the paver can lead to unevenness, in the longer-wavelength profile of the surface. Too many changeovers of vibrating roller compactors can cause an excess of binder to rise to the surface, reducing texture and hence skid resistance. The direction of paving can have an impact on the level of the skid resistance measured. Paving and compaction against the direction of traffic will produce surfaces with higher skid resistance than paving in direction of travel. The reason for this could be the orientation of the aggregates on the surface. Concrete surfaces require an additional process to apply some level of macrotexture. The surface mortar provides microtexture but techniques such as brushing are used to create ridges and grooves in the surface mortar that provide macrotexture. Alternatively, with the use of retarders to slow down its hardening, the mortar can be brushed away to expose the aggregate and provide macrotexture. In this case, the aggregate should have adequate polishing resistance. Brushed surfaces tend to lose their macrotexture over time, especially in the wheel paths on heavily trafficked roads. Grinding or longitudinal grooving can be used to adjust the texture of concrete surfaces. The technique can improve skid resistance but it may not be able to meet all requirements since the properties of the aggregates that become exposed by removal of the surface mortar (polishing resistance especially) become particularly important. This is still an area of research in some countries, especially relating to skid resistance at higher speeds. Date: 17/03/2010, Version: (67)

35 Finished surface There is no research evidence to show differences in skid resistance attributable to the type of finished surface. Whether it is of asphalt or concrete has no direct impact on the level of skid resistance but the surface texture does have an influence. The ways in which micro- and macrotexture are provided does differ between the surface types but these should be chosen with the provision of appropriate skid resistance in mind. The impact of the damper function of the base layer is unknown; this was not a subject investigated in most research projects about skid resistance. The shape factor was developed for assessing the noise emission of surfaces. Steinauer et al. [18] state that a concave surface causes lower rubber penetration and low contact pressure but high contact areas. Convex surfaces show an opposite behaviour; higher rubber penetration and higher contact pressure but low contact areas. This might have implications for skid resistance but there are no research results in which the shape index shows advantages in relation to skid resistance values. There are still knowledge gaps concerning the impact of the parameter shape factor, as well as the characteristic shape length, the maximum of the spectral roughness depth, wavelength and unevenness. The absorption, flow resistance and layer thickness have no impact on the skid resistance of the surface because this property is governed by the surface texture, not conditions within the layer. A surface treatment with reaction resin can be used on asphalt and concrete surfaces enhance the long-term skid resistance and the noise emission. The size of aggregate used can be 1/3 mm or 3/4 mm. To illustrate of some of the issues involved in optimising road surfaces for skid resistance, one of the main interactions is that between the two most important surface parameters (microtexture and macrotexture), where their relative importance changes as vehicle speeds change. The interactions of the different texture scales in this context are illustrated in Figure 3.2 (based on [21]). The diagram illustrates in broad terms how the friction coefficient changes with speed depending on the type and scale of the macrotexture and the drainage mechanisms they encourage. How to categorise the shape and form of macrotexture, with measurement parameters that represent its influences more effectively than is possible using current parameters such as MPD, is still an issue for research. Date: 17/03/2010, Version: (67)

36 Forms of macro texture: 1 drainage systems between the aggregates 2 stamped drainage systems Friction coefficient speed macro texture micro texture Figure 3.2: Different surface texture types and their impact on friction values of wet surfaces, based on [21] 3.4 Surfacing parameter interactions related to rolling resistance The interactions of surface parameters in relation to rolling resistance, particularly those associated with mix design and laying of the surface, have not been researched in detail, hence the large number of? entries in the relevant column of Table 3.1. For this reason, these largely open questions are topics for discussion in TYROSAFE deliverable D15, which will deal explicitly with the knowledge gaps and research needs in this context, so they are not dealt with here. However, a few parameters of the finished surface are known to have an impact on rolling resistance and these are discussed briefly in the following paragraphs. Finished surface Unevenness seems to have a major effect on the energy loss of a vehicle while driving over a road surface. As explained earlier, unevenness results to a major extent in body vibrations of the vehicle and these lead to energy being converted into heat in its dampers. Although not part of the energy loss caused by rolling resistance of the tyres, per se, this is an indirect contribution by the road surface to the overall energy loss of a driven vehicle. VTI [23] found different fuel consumptions when an instrumented car was driven on asphalt and concrete surfaces. As a result, they stated that, in terms of rolling resistance, concrete surfaces might produce lower values compared with asphalt surfaces. This result might be proven by further research but with suitable rolling resistance trailers (rather than instrumented cars) for both passenger-car and truck tyres. The absorption, flow resistance and layer thickness have no impact on rolling resistance. As with skid resistance, this is because rolling resistance will only be affected by the surface Date: 17/03/2010, Version: (67)

37 texture and evenness, not by conditions within the surfacing or pavement layers. It is possible that the pavement or layer stiffness could affect rolling resistance. 3.5 Surfacing parameter interactions related to noise emissions It can be seen from the interdependency matrix (Table 3.1) that many of the surfacing parameters that have an impact on noise emissions are those that affect skid resistance. This is because both properties are dominated by the influence of different scales of surface texture. However, some surface parameters are of greater significance for noise than skid resistance or rolling resistance. It is also important to recognise that passenger car tyres and truck tyres show different behaviours in relation to noise emission. Aggregate properties For favourable noise emission performance, aggregates used should have a high percentage of angularity and the shape index or flakiness index of the coarse aggregates should be as low as possible. However, a research project [14] has shown that using crushed or uncrushed aggregates has no influence on the noise emission of the surfaces produced. The degree of sound absorption can be influenced by the kind of the aggregate used. Figure 3.1 shows the results of sound absorption measurements on cores of porous asphalt with the same gradation curve and binder content but different aggregate types. granite diabase degree of sound absorption α [-] frequency [Hz] frequency [Hz] Figure 3.3: Degree of sound absorption depending on the use of different types of aggregates (left: granite, right: diabase), based on [22] The polishing resistance has no impact on the noise emission of the surface except perhaps in the longer term through the maintenance or loss of microtexture. Mixture properties Composition (asphalt mixture type): For noise emission, asphalt surfacings can be divided into dense and porous types. Porous surfacings show good behaviour regarding noise emission due to the high void content near Date: 17/03/2010, Version: (67)

38 the surface [14]. Dense mixtures can be optimised for noise through their grading and choice of maximum aggregate size. Maximum aggregate size: A maximum aggregate size of about 5 mm appears to be the optimum both for car and for truck tyres [14], [2]. Binder content: The amount of bitumen used in the asphalt mix can have an impact on noise emission. Binder content depends on the type of mixture: high binder contents will tend to produce a low void content, which can have an effect on the surface noise emission. As with skid resistance, noise can be affected by excess bitumen covering microtexture or filling macrotexture. If the binder content is too low, loss of particles from the surface can occur and this could lead to increased noise. The same principles affecting binder content in mix design apply in relation to noise as are used for skid resistance. Binder type (viscosity): Because of the influence of the binder on the surface stiffness, using a binder type that causes a very stiff surface should be avoided. However, the use of binder with high viscosity prevents changes in the surface texture that might result in worsened noise characteristics. The use of rubber in the road surface is currently a topic being covered in the European project PERSUADE. It seems that the use of a large fraction of rubber in the surface mix should result in less noise. However, this kind of surface is still under development and not yet ready for practical use. Void content (mix design): According to the definition of porosity as residual air void content, a porous surface will be acoustically absorbing if the void content is more than 20%. Generally, the porosity should be as high as possible consistent with durability requirements. That means that a maximum of 25-30% of void content can be reached and still have a good mechanical stability. For dense asphalt mixtures, the void content is not as important as the surface texture. The noise optimised dense surfaces have void contents about 10 percent by volume in Germany, which is more than normal dense surfaces have. Laying and compacting Chippings aggregate size (asphalt): The size of the chippings used to improve skid resistance on very new asphalt surfaces should be as small as possible, because surfaces with chippings tend to have lower shape factors (g < 60%) and therefore emit more noise. If possible, the chippings of rolled asphalt should be avoided, in order to obtain a concave texture (g > 70%). The aggregate of the chippings on mastic asphalt should be also as small as possible (about 2-3 mm) [14], [2]. In surface dressings, smaller chipping sizes tend to emit less noise. Date: 17/03/2010, Version: (67)

39 Chippings polishing resistance (PSV/PWS): The polishing resistance of the chippings (if used) has no impact on noise emission except perhaps in relation to the maintenance or loss of microtexture on the aggregate [2]. Degree of compaction: As with skid resistance, the impact of the degree of compaction of the asphalt mixture on noise emissions is unknown, especially if the values achieved are higher than the requirement specified in the contract. In most research projects, the degree of compaction will have fulfilled the requirements. If the degree of compaction is too low, the risk of permanent deformation and surface aggregate loss will increase. The influence that this has on noise is unclear since the size and nature of the voids in the material or near the surface, or changes in macrotexture due to loss of aggregate particles in the tyre contact area could either increase or reduce noise depending on how the difference mechanisms are affected. Paving / compaction: As with skid resistance (Chapter 3.3), the only effects on noise emission related to paving and compaction of the asphalt mixture occur through the texture of the finished surface. Poor transfer of the hot asphalt from the delivery truck to the paver can lead to unevenness, in the longer-wavelength profile of the surface. Too many changeovers of vibrating roller compactors can cause an excess of binder to rise to the surface, altering macrotexture and with it, noise emissions. On porous asphalts and dense surfaces that have been optimised for noise emission, compaction with dynamic vibration should be limited or prevented to avoid crushing of aggregates and therefore changes in the texture and the void content. The direction of paving can have an impact on the level of the tyre/road noise emission. Paving and compaction in the direction of traffic will produce lower noise surfaces than paving against direction of travel. As with skid resistance, this may be due to the orientation of the aggregates at the surface. Concrete surfaces: The processes used to provide macrotexture for skid resistance on concrete surfaces are especially important in relation to noise generation. Transverse brushing techniques can produce good macrotexture but if the ridges and grooves generated are too deep they can generate significant tyre/road noise, which is exacerbated by the repeating nature of the transverse marks. This situation is even more noticeable where transverse sawn grooves are applied to restore macrotexture where brushing has been worn away. As a general principle, to minimise noise generation, transverse brushing or grooving should be avoided. Burlap treatments have inherently low macrotexture and these surfacings can become increasingly noisy as they age. Date: 17/03/2010, Version: (67)

40 Use of exposed aggregate techniques can be an effective way of reducing noise on concrete surfaces. As with asphalt, it is advantageous to use aggregate sizes that are as small as possible, say 5 8 mm. A low megatexture and a fine longitudinal texture are favourable to low noise emission on concrete surface. The best longitudinal texture for this purpose is probably produced by a grinding technique but, as explained in 3.3, the use of this type of treatment may not always be advantageous for skid resistance. For porous surfaces in particular, not only the porosity, but also the texture should be taken into account; high porosity is as relevant as reducing megatexture with regard to noise emission. The texturing of concrete surfaces is one of the areas in which there can be conflicts between the provision of the best forms of texture for skid resistance and for noise. Finished surface Unlike skid resistance, the type of surfacing material asphalt or concrete has an inherent impact on the level of noise emission. For example, it has been found [14] that a dense concrete surface with chippings (e.g. 5 mm maximum stone size) created a surface which was about 2 db (A) louder than the same chippings used on an asphalt surface. This effect derives from the higher internal damping of asphalt compared with concrete. The subsurface or the damper function of the base layer can have different effects on the (rolling) noise emission even if the running surfaces have the same texture. Furthermore, resilient or damping intermediate layers can markedly reduce the noise emission both for car and for truck tyres. It has also been found [14] that achieving surfaces with concave textures ( plateaus with ravines ) with a high shape factor (g > 70%) instead of convex surfaces ( mountains with valleys ) with lower shape factors (g < 60%) should be the objective from the point of view of minimising noise emission. Furthermore, the characteristic shape length (g ) should be between 400 to 700 mm and the maximum of the spectral roughness depth (R max ) should be in the range 60 to 200 µm for low noise. The most important parameter that reduces noise emission is porosity. However, it is also important to note that the road surface ought to be as smooth as possible in order to reduce the texture impact mechanisms. The macrotexture should be maximised at wavelengths around 2 to 6 mm for passenger car tyres and 4 to 8 mm for truck tyres and the megatexture should be minimised, particularly in the wavelength range 50 to 100 mm to decrease the tyre/road noise emissions [15]. In order to optimise the porosity effect, the peak of sound absorption should be placed at 800 Hz to 1000 Hz for high-speed roads (this frequency corresponds to the maximum of car tyres) and 600 Hz for low-speed roads (this frequency is the maximum for truck-tyres). For this reason, the optimum flow resistance should be in the range knsm -4 and between 12 and 30 knsm -4 for low-speed roads. The thickness of the porous layer should be as high as possible, but at least 40 mm. The layer thickness of dense surfaces is not so important, because in this context only the surface texture is relevant for noise emission [15]. Date: 17/03/2010, Version: (67)

41 A porous surface should be optimised not only for porosity, thickness and number of layers but also for texture. The lowest possible mega- and macrotexture should be used at all wavelengths; this means that the aggregate particles should not be sharp. In the case of very low porosity, this recommendation is no longer valid and the same design rules as for a dense surface should be applied. Figure 3.4 shows the texture spectrum of a smooth surface (a Dense AC 11 mm). The arrows represent the desirable direction in which the optimisation should be carried out from the exterior noise point of view. For example, in the low range of the spectrum the texture level should be as low as possible (red range), while in the high part of the spectrum the texture level should be as high as possible (green range). Additionally, the peak in the spectrum should be as pronounced as possible and as far as possible to the right in the diagram. Figure 3.4: Texture spectrum of a dense homogeneous surface (arrows point in the direction to modify the spectrum to achieve a low noise surface) [15] It has been explained that optimising surface parameters for skid resistance is complex and that microtexture is the important parameter in relation to adhesion. To optimise microtexture obtain low noise at high frequencies three suggestions can be made [15]: Adhesion bonds between road and tyre should be low (which is the opposite of what is needed for skid resistance) The road surface and tyre rubber should be hydrophilic Polished surfaces should be avoided (which is also desirable for skid resistance) Macrotexture is a particularly important factor influencing noise emission of the road surface but ways of optimising this parameter lead to some significant conflicting requirements. For instance, the requirements for different vehicle types (reflected in the tyres that they use) are markedly different. For light vehicles, macrotexture should have high amplitudes in the 1 to 8 mm wavelength range (between 0.5 and 12 mm for heavy vehicles) but at the same time Date: 17/03/2010, Version: (67)

42 the macrotexture should have low amplitudes in the mm wavelength range (16-50 mm for heavy vehicles). Low frequency noise generation can be caused by rough texture; consequently, the first suggestion concerning megatexture is to minimise it. Additionally, the aggregate particles at the surface should be arranged homogeneously and be well packed together. The size and the orientation of the chipping should be also homogeneous and as uniform as possible. A cubical shape is a good choice for particle shape because this makes uniform orientation easier to achieve. A surface treatment with reaction resin can be used on asphalt and concrete surfaces to enhance the long-term skid resistance and the noise emission. However, the level of noise reduction depends on the aggregate size used. An aggregate size of 1/3 mm leads to surface textures with the noise emissions that are comparable with burlap textured surfaces. Maintenance: Polishing and abrasion of the road surface by traffic can reduce the sharpness of the aggregate particles at the surface, in some cases rounding them off. For this reason, air drainage between the tyre tread rubber and the road surface will be reduced, affecting the air-pumping noise generation mechanism. For good long-term performance of low noise surfaces, aggregate should be selected that are resistant to polishing and that can maintain their sharpness for as long as possible. Bleeding or flushing of the binder should not occur. Additionally, it could be argued that the most relevant issue related to maintenance of low-noise properties is not the provision of appropriate texture but preventing the texture that is there from becoming clogged with general detritus. Regular cleaning of surfaces using de-clogging machines would help in this respect. Date: 17/03/2010, Version: (67)

43 4 Interdependencies of tyre parameters and their influence on the three surface properties 4.1 Introductory comments The main issues for the optimisation of tyre parameters are similar to those for skid resistance: it is not easy set tyre parameters to optimise one surface property without compromising the other two. In fact, the same groups of parameters that influence skid resistance impinge on tyre performance and hence all contribute directly or indirectly to the three surface properties. The ultimate interface for the tyre is the road surface itself and so the surface parameters, particularly its texture, constitute the principle set of factors with which the tyre parameters interact. The factors that influence the road surface characteristics also contribute to the processes. For tyres, the same set of Outer factors, over which road engineers (nor tyre manufacturers) have little or no control are also Outer factors. However, those factors that are Inner factors from a surfacing perspective (because surfacing engineers can have some influence on them in optimising surfacing characteristics) become another set of Outer factors to the tyre. Thus, from the tyre s perspective there is one set of parameters Tyre parameters that can be influenced during the optimisation process and a secondary set Surface texture parameters that can be partially influenced if optimisation of surfacing design is to consider tyre characteristics. The various groups of factors as seen from the tyre s perspective are illustrated in Figure 4.1. From the tyre industry s point of view, it is possible to optimise only the various tyre parameters, while bearing in mind other outer factors not included in the diagram that also influence tyre performance, such as mileage, handling and wear. As with the road (section 3.1), it would be unrealistic to try to achieve the maximum performance in relation to each surface property. The best practical option for tyre parameter optimisation is also to address the parameters in a way that results in an improvement in all three surface properties compared with the current position. A further difficulty with tyres is that they wear, and consequently their characteristics change, at a much faster rate and over shorter timescales than is normally the case for a road surfacing. Nevertheless, the objective is similar: to set tyre requirements that work in combination with road surfacings that simultaneously help to improve road safety, to lower fuel consumption (and CO 2 -emissions) and to lower tyre/road noise, because they are optimised to all three properties. Date: 17/03/2010, Version: (67)

44 Figure 4.1: Parameters influencing the three surface properties (skid resistance, rolling resistance and noise emission) from the perspective of the tyre Road surfacings can be optimised in terms of specific requirements for particular locations (skid resistance needs, built environment affecting noise requirements, traffic speeds and loadings, and so on). Tyres, however, have to operate on all types of surface and in all conditions; optimisation is of necessity likely to be rather more generalised to typical conditions rather than the specific. In relation to tyre parameters, it must be pointed out that that the rubber compound consists of about 200 different components, from different sources and varying from one Date: 17/03/2010, Version: (67)

45 manufacturer from to another. Therefore, in terms of tyre profile design this report uses as its basis the Road/Tyre Noise Reference Book by Sandberg & Ejsmont (2002) [15] and concerning to the terms for describing basic mechanisms of tyre/road friction the report refers to the Michelin Tyre Handbook [33], [34]. 4.2 Interdependency matrix of tyre parameters influencing the road surface properties A second component of the interdependency matrix, arranging the various tyre parameters that affect the road surface properties in a way that shows the interactions between them, has been prepared following similar principles to those explained in Section 3.2 for surface parameters. As before, the matrix is based both on information gathered from many research projects (such as those in the reference list) and from practical experience. The information from the various projects related to passenger-car tyre and truck tyre optimisation with regard to tyre/road friction, rolling resistance and noise emission. The research results do not necessarily relate directly to typical road surfaces in every-day use, however. In most cases, practical tests will have been on special reference surfaces or steel. Rolling resistance is often assessed indirectly, typically by measuring fuel consumption. In the tyre properties interdependency matrix, (Table 4.1) the same idea of showing the relative effect expected from a change in each property is shown. However, instead of a breakdown by road surface type, the properties are separated into the two types of tyre (passenger car and heavy goods vehicle). When talking about parameters for optimising tyre/road friction in this report, always a wet surface is meant, because only wet surfaces show critical or low friction values. On dry surfaces, the friction values are normally high enough so they do not cause safety problems. The parameters for optimising the other two surface properties (rolling resistance and noise emission) are valid only in dry conditions, because for most of the parameters there are knowledge gaps when talking about wet surfaces. As before, all parameters relating to tyre/road friction refer to wet conditions. However, the parameters for optimising rolling resistance or noise refer to dry conditions; as with the surfacing parameters, measurements relating to these properties are usually made when the road is dry and little is known about the effects in the wet. When working through Table 4.1, the reader should take note that in the studies establishing the behaviours indicated, most measurements of the road surface properties used new or special tyres, whereas the tyres in everyday use on cars or trucks are not new. This could lead to behaviours in practice that differ from those predicted by the measurements. Further explanation of the interactions of the individual tyre parameters are discussed in more detail in relation to each main property in sub-sections 4.3, 4.4 and 4.5. Date: 17/03/2010, Version: (67)

46 Passenger car tyres tyre construction rubber propertie s HGV tyres Table 4.1: Interdependency matrix of tyre parameters Skid Resistance Rolling Resistance Noise Emission tyre dimension o + o tyre width o tyre diameter? + + tread pattern ( according to ventilation) depends on: o + o rather smooth tyre o tyre with high amount of tread grooves inflation pressure stiffness (sidewall /belt)? + / + + / + tread depth rate of tread notching +? - rate of tread blade frequency +?? rubber resilience rubber hardness tyre type (kind of axles used on: power transmission (PTA) or steering axles (STA)) tyre dimension? - o PTA o - - o tyre width o tyre diameter??? tyre construction rubber properti es tread pattern (e.g. according to ventilation) depends on: o + o rather smooth tyre - + +? o tyre with high amount of tread grooves + -? inflation pressure stiffness (sidewall/belt)? /? + / + + / + tread depth rate of tread notching??? rate of tread blade frequency??? rubber resilience rubber hardness? -? -? - miscellaneous age and wear retread tyres?? o + studded tyres? - - load increasing/rising value or category (e.g. PSV) decreasing value or category (e.g. shape factor) + positive impact (e.g. higher skid resistance, lower rolling resistance, lower noise emission) o no impact (e.g. no changes on surface property due to changing the parameter) - negative impact (e.g. lower skid resistance, rolling resistance, higher noise emission)? impact unknown Date: 17/03/2010, Version: (67)

47 Optimisation of tyre properties is very complicated because there are many different requirements, often with legal force, with which tyres have to comply and these can vary from country to country. The basic tyre/road surface properties of wet grip (tyre/road friction), rolling resistance and rolling noise with which this report is concerned are covered, but optimising them must also be considered in the context of many other issues that combine to govern tyre design and manufacture. Several environmental topics are covered, related to fuel consumption (rolling resistance and weight reduction), pollution (avoiding tyre-road noise and toxicological interaction), recycling (the ability to re-tread or re-groove and other recycling concepts) and finally to interior noise. In addition, tyre manufacturers have other factors or targets that their products must address with optimised performance, such as driving comfort, price, weight, rolling resistance coefficient, mileage, stone retention, wet traction, water removal ability, snow traction, highspeed stability. There are no or only limited data (literature, research results) relating to truck tyres in certain areas, but there are some areas in which the truck tyres perform in a similar manner to those on passenger cars. In the following chapters discussing the individual parameters, the full list covered in the matrix is mentioned even if there is little or no existing data. Where truck tyres act in the same way as car tyres, the parameter is covered in the car chapter only. 4.3 Tyre parameter interactions related to tyre/road friction Passenger car tyres Tyre type (summer or winter tyre) In different countries, the different seasons lead to widely varying conditions in which tyres have to perform. Tyre rubber response is affected by road surface temperatures and the amount of rainfall, ice or snow varies markedly over the seasons and between countries. In many countries, these seasonal changes require a change of tyre type from winter to summer tyres and vice versa. Today, winter tyres are built to generate tyre/road friction in cold and wet conditions. To achieve this, they have a siped tread; the lamellas in the tread blocks enable a higher water expulsion rate and therefore better grip in the wet conditions that dominate the winter months in central Europe. The rubber compound incorporates ingredients that are designed to reduce rubber hardness, which increases tyre/road friction during the cold season. During the warmer seasons, rainfall is generally lower than in winter. Therefore, summer tyres have a rather smooth tread pattern compared with winter tyres, with fewer tread blocks and no lamellas within them. The rubber compound is designed to withstand high asphalt temperatures and, for that reason, its shore hardness is rather high. In countries such as the UK, where prolonged periods of very cold temperatures and frequent heavy snowfall are generally rare, summer tyres are used all year round and so performance in terms of tyre/road friction may be slightly reduced in winter when such conditions do occur. Date: 17/03/2010, Version: (67)

48 Tyre dimension Usually, wider tyres tend to hydroplane at a lower speed than narrow tyres. This effect is a result of less surface pressure because wider tyres have a larger contact patch. A further influence is that, with a larger contact patch, more water has to be displaced from the tyre/road interface [27]. Rubber resilience No existing information or no reliable data. Rubber hardness The wet grip of tyres will decrease with increasing rubber hardness. However, lower levels of rubber hardness result in increased tyre wear and hence reduced mileage. Tread pattern Primarily, the tread pattern is responsible for directing water on road surface away from the contact patch to the areas beside or behind the tyre. Apart from this important role, the type of pattern (summer or winter profile) has less impact on the measured friction coefficient [28]. Tread depth The effects of tread depth are different in wet and dry conditions. On dry surfaces, friction coefficient increases as tread profile depth decreases but in wet conditions the opposite occurs [27], [28]. Thus, while a slick tyre offers good grip performance in dry conditions, the reduction in wet friction (which is, of course, already markedly reduced on most road surfaces compared with dry friction) with decreased tread depth is of greater importance in the context of the TYROSAFE objectives. Inflation pressure Too high an inflation pressure decreases the area of the contact patch hence the ability of the tyre to transmit traction, braking or cornering forces. However, too low an inflation pressure also results in a decreased ability to transmit forces because tyre stability is then decreased. Bachmann [28] states that different inflation pressures (150 kpa to 350 kpa) have no impact on the friction coefficient in wet conditions but, on dry surfaces, friction coefficient increases with a decrease in inflation pressure. Stiffness Normally, the tyre construction has no impact on tyre/road friction. Others Retreaded tyres - No existing information or no reliable data. Studded tyres - No existing information or no reliable data. Date: 17/03/2010, Version: (67)

49 4.3.2 Truck (HGV) tyres Tyre type (tyres for power transmission axle or tyres for steering axles) Because of their high wheel loads compared with braking power, HGVs do not usually have a friction problem. There is a tendency for larger vehicles to roll over or jack-knife rather than skid longitudinally. Apart from winter conditions, the use of traction tread patterns seems unnecessary for driving on paved roads. Therefore, steering axle tyres could also be used on power transmission axles, as is already common with coaches. Tyre dimension The effects are the same as for car tyres. Rubber resilience No existing information or no reliable data. Rubber hardness Truck tyres use different compounds to passenger car tyres in order to achieve the levels of grip required but the effects of changing hardness are similar to those for car tyres. Tread pattern The effects are the same as for car tyres. Tread depth The effects are the same as for car tyres. Inflation pressure The effects are the same as for car tyres. Stiffness No existing information or no reliable data. Others No existing information or no reliable data. 4.4 Tyre parameter interactions related to rolling resistance Passenger car tyres Tyre type (summer or winter tyre) No reliable data known, since winter tyres are tested under the same conditions as summer tyres. Tyre dimension Date: 17/03/2010, Version: (67)

50 Research during the SILENCE project [24] showed that narrow tyres usually produce less energy loss in terms of rolling resistance than wider tyres with the same circumference. These results are not surprising since wider tyres have more material that has to be deformed while rolling over the road surface. The deformation and recovery process generates heat in the tyre and this energy loss contributes to the tyre s rolling resistance. Usually, rolling resistance will decrease with an increase in tyre diameter. Rubber resilience Rubber resilience and rubber hardness are linked to each other to some extent. The effect of tyre ageing causes age hardening of the rubber compound. This effect changes the rolling resistance coefficient of a tyre. Unfortunately, no reliable data relating to this phenomenon are currently known to the authors of this report. Rubber hardness An increase in rubber hardness usually causes a decrease in rolling resistance. Tread pattern There is no reliable information relating to the influence of tread pattern on rolling resistance. A reference was found suggesting that slick tyres sometimes produce a higher rolling resistance than patterned tyres on roads; however, the reasons for that are not clearly understood it could be caused by the relative thick rubber layer at the top of the tyre. Tread depth During the SILENCE project, some rolling resistance measurements with worn tyres were conducted. Without ageing, the tyres were worn on a special tyre wear machine in steps of 2 mm starting from 8 mm tread depth down to 2 mm. Measurements of rolling resistance were made on the smooth steel surface of the interior drum at the tyre/road interaction test facility (PFF) at BASt. The results showed that with decreasing tread depth the rolling resistance of all the tyres tested (six different sizes and types) decreased [24]. Inflation pressure An increase of the inflation pressure may cause decreasing rolling resistance. Stiffness Stiffening the sidewalls is one possible approach to reducing a tyre s rolling resistance coefficient. Others There has been little or no research about rolling resistance of either retreaded tyres or studded tyres but it might be expected that using studded tyres would increase rolling resistance. Date: 17/03/2010, Version: (67)

51 4.4.2 Truck (HGV) tyres Tyre type (tyres for power transmission axle or tyres for steering axles) Usually, steering axle tyres have a lower rolling resistance coefficient than drive axle tyres. In [26], it is shown that, of 15 steering axle tyres tested, apart from one, all produced less rolling resistance when compared with drive-axle tyres with the same size and dimension. The measured rolling resistance coefficient of the steering-axle tyres was between 0.4 kg/t and 1.8 kg/t lower than that of one of the drive-axle tyres. Tyre dimension In [26], the HGV tyres tested were of different dimensions and from various manufacturers. In order to investigate the influence of tyre dimension on rolling resistance, it might be useful to investigate tyres with the same tread pattern but different dimensions under the same conditions but tests of this kind did not take place. No other investigation in which this influence has been investigated is known to the author. Rubber resilience No existing information or no reliable data. Rubber hardness No existing information or no reliable data. Tread pattern No existing information or no reliable data. Tread depth No existing information or no reliable data. Inflation pressure No existing information or no reliable data. Stiffness No existing information or no reliable data. Others Retreaded tyres - no existing information or no reliable data. 4.5 Tyre parameter interactions related to noise emissions Passenger car tyres Tyre type (summer or winter tyre) Most measurements on summer and winter tyres have been made in similar conditions (typically a temperature around 20 C). The rubber compound of winter tyres is rather soft at Date: 17/03/2010, Version: (67)

52 20 C compared to that of summer tyres and this might explain why winter tyres are quieter under the same test conditions. In the SILENCE project, this circumstance was used to create a very quiet tyre by using winter rubber compound with a summer tyre profile. However, opposing interactions occur in this context; as explained in the winter compound leads to a loss of grip and it is not yet well understood whether this approach measure leads to silent and safe summer tyres [37]. Tyre dimension Even if existing truck tyres which are less noisy than some passenger car tyres the width is probably one of the most relevant reasons for the tyre s noise emission. In order to achieve low tyre/road noise emissions the tyre should be as narrow as possible. Leaving all the other parameters unchanged a change from 155 mm width to 195 mm width causes a noise increase of about 2 db [15]. Rubber resilience No existing information or no reliable data. Rubber hardness During the SILENCE project, it was found that when rubber hardness was increased by artificial aging, an increase in tyre/road noise occurred [24]. Regarding the tyre rubber compound, there is clear evidence that softer rubber reduces noise substantially, especially in tyres with an aggressive tread pattern. For winter tyres, this can also be a favourable effect in terms of increased friction but the situation with summer tyres is different. In that case, soft tread compounds, which reduce the noise emission, will lead to larger deformation of the tread block and that may lead to handling problems. According to the Chalmers model, an increase of contact stiffness from 12,500 N/m to 80,000 N/m results in a noise increase of about 4 db in the range Hz, and about 10 db at 800 Hz. This can lead to an increase of 2.5 db [15]. Tread pattern Generally, smooth tyres are less noisy than other tyres but the behaviour also depends on the road surface. When comparing a smooth tyre with a quiet patterned tyre on safety walk sandpaper the decrease in rolling noise is about 9 db, on an ISO surface the difference is about 2 db and on very rough surfaces the smooth tyre emits about 3 db more noise than a quiet patterned tyre [15]. The influence of the tread pattern is one of the most relevant parameters in terms of noise reduction. In a study at the manufacturing company Continental, a standard tyre was compared with a smooth tyre. The standard tyre was around 8.5 db noisier than the smooth tyre. However, a tyre with longitudinal grooves was 3.5 db noisier than a smooth tyre, which means that this kind of grooving also creates noise, but the different pattern showed a decrease of around 5 db compared with the standard tyre [15]. Date: 17/03/2010, Version: (67)

53 Generally, it can be said that the tread impact influences the texture impact and the adhesion mechanism as well as the air pumping mechanism and the resonances. Good ventilation of the tread should avoid the air displacement mechanism and the peak around 1 Hz. Regarding the vibration mechanism, the tread pattern should make contact with the road as gently as possible. If the distance between contact points is kept very small, the tread texture does not create relevant vibrations. If the excitation of the tread blocks is in phase over a substantial part of the tyre width then the vibration effect will be larger. Randomisation of the tread pitch is a further possible approach to reducing the subjective loudness of the noise generated. Some basic requirements for the tread design are summarised in the following list: The tread pattern must not incorporate elements that have shapes coinciding with the outline of the footprint. The tread grooves must be well ventilated in order to reduce pressure changes. Any lateral grooves or grooves in a lateral-diagonal direction should be narrow. Any longitudinal grooves that are not well ventilated should be acoustically choked in order to reduce the pipe resonances. The tread pattern should be well randomised and the ratio between the longest and the shortest pitch should be at least 1:1.4. Nowadays, it should also be possible to have no periodical repetition of the tread elements at all. The shape of the tread blocks should be optimised to reduce the stiffness gradient. The use of sipes in the tread should be obligatory. As an additional optimisation possibility, consideration should be given to testing the tread pattern in a worn condition as well as in a new condition. Tread depth During the SILENCE project, the influence of the state of wear on tyre/road noise emission was investigated. It could be shown that a decrease in tread depth leads to a significant decrease in tyre/road noise [24] (but, as explained elsewhere, this is also accompanied by a reduction in wet grip). Inflation pressure No existing information or no reliable data. Stiffness By fitting absorbing material in the torus cavity inside the tyre, it is possible to achieve a noise reduction of about 1 db. Another possibility is to fill the tyre with some solid and flexible material in order to increase the damping substantially and eliminate the torus cavity Date: 17/03/2010, Version: (67)

54 resonances. This insertion can reduce the overall noise level of 5 db and, for certain frequencies, up to 14 db [15]. Some examples of low noise tyre experiments indicate a noise reduction effect of about 3 db when changing the carcass or the belt to a stiffer one. Another solution could be to increase the damping of the tread [15]. Others Investigations during the LeiStra 2-project have shown that most of different retreaded tyres fulfil the requirements of the directive 2001/43/EG; only few show marginal overstepping, although the requirements of the directive should only be applied on new tyres. Studded tyres are normally winter tyres and tyres with studs are about 2 to 15 db (depending on the height of the studs and on the frequency) louder than winter tyres without studs [15] Truck (HGV) tyres Tyre type (tyres for power transmission axle or tyres for steering axles) Drive-axle tyres usually are up to 3 db(a) louder than steering axle tyres. Therefore, from a noise reduction perspective, using steering axle tyres on the driven axle might be considered. In summer conditions, no loss of vehicle stability from this approach can be foreseen. Usually trucks are more often involved in roll-over or jack-knifing accidents than in skidding accidents. Tyre dimension A literature study of truck tyre noise in 2002 [25] found no connection between tyre/road noise and tyre width/dimension. However, in [15] it is stated that even if there are truck tyres that are less noisy than some passenger car tyres, their width is probably one of the most relevant factors behind the tyres noise emission. Rubber resilience Using different components in the rubber compound might have an influence on the noise emission of truck tyres but no investigations on this topic are known to the authors. Rubber hardness See rubber resilience. Tread pattern See tyre type. Tread depth In [25], some statements on the development of the noise emission as a truck tyre wears are contradictory. Some findings are reported where the noise increases as the tread depth decreases but, on the other hand, for other tyres investigated the opposite is reported. Date: 17/03/2010, Version: (67)

55 Inflation pressure In [25] no significant influence of inflation pressure on the tyre/road noise emission was found for radial rib tyres. The other tyres that were investigated were bias tyres, which are no longer commonly use. Stiffness No existing information or no reliable data. Others Retreaded tyres: the M+P investigation [25] showed that retreaded tyres are quieter than the original new tyres (by approximately 2 db(a)). This result was quite surprising; the report also states that an investigation on the noise emission of retreaded tyres will inevitably be needed. The investigations during the LeiStra 2-project showed almost the same results. All tested truck tyres fulfilled the requirements of Directive 2001/43/EG but some tyres went below the specific values (steering axle or power transmission axle) by up to 3 db(a). Date: 17/03/2010, Version: (67)

56 5 Optimising the designs of road surfaces and tyres to increase safety 5.1 General comments The main emphasis of the TYROSAFE project is to identify ways in which policies, measurement techniques, road surface and characteristics can be improved to provide greater road safety while reducing adverse environmental effects of the interaction of tyres and roads across Europe. The emphasis of this report is on the interaction of the various parameters involved. The process of optimising the design of road surfaces and tyres to improve safety is complex. The term safety, in this context, implies better skid resistance (for roads) and wet grip or tyre/road friction (for tyres) to reduce the risk of skidding accidents occurring. However, that does not necessarily mean targeting the highest possible values for skid resistance. TYROSAFE Deliverable D08 [39] explains that, although the driving force for setting standards for skid resistance is to improve road safety, actual target skid resistance levels are dictated by national and local policies. That report has recommended the adoption of a harmonised approach to such policies. This (as is already done in some European countries) would involve basing the policy on equalising the risk of skidding in the many different situations that occur on road networks, rather than setting explicit levels that apply to all parts of all routes. Consequently, the actual level of skid resistance required in any particular situation will vary. Further, because the properties both of road surfacings and of tyres change in use, target levels for skid resistance that apply to new roads may also be set which also make allowance for a reduction in performance over time, but with the intention of adequate provision being maintained over the service life of the road. Therefore, when considering the parameters of greatest interest in relation to the skid resistance or friction properties of roads and tyres, it is important not to focus on these only when the surfacing or tyre is new, but also to consider how the properties change due to use and wear. Tyres, for instance, generally wear out much faster than road surfacings do. In addition, when optimising the parameters for this important surface property, the effects on the other two properties (rolling resistance and noise) should not be worsened. Furthermore, changes made to the surfacing design to improve skid resistance could lead to undesirable changes in other surface characteristics and will need to be avoided (for instance, permanent transverse or longitudinal deformation could have a negative impact on safety). This report has discussed that a road s surface texture is the broad characteristic that has the greatest influence on all three of the surface properties that TYROSAFE is focussed on. The surface properties are influenced in different ways depending upon the amplitude of the texture on different scales (defined by their wavelength and illustrated in Figure 2.1). It has been seen that for skid resistance, both micro- and macrotexture are of great importance. For rolling resistance, however, macro- and megatexture dominate, together with the Date: 17/03/2010, Version: (67)

57 roughness or unevenness of the road, whereas for noise important scales are macrotexure and megatexture. Therefore, it is theoretically possible to optimise microtexture for skid resistance only and roughness/evenness for rolling resistance only, because these texture scales do not affect the other properties. Macrotexture, however, must be optimised with all three properties in mind, especially in certain wavelength ranges within this category. Similar remarks apply to tyres. It can be argued that tread compound constituents, depth, pattern, and construction type each influence wet grip, rolling resistance and noise in ways that are analogous to the road surface texture. Compound is analogous to microtexture (with which it interacts to provide grip); tread pattern and depth are analogous to macrotexture (with which they interact to disperse water for safety and contribute to the generation of rolling resistance and noise) while construction interacts with the larger-scale road surface texture factors to contribute to noise and rolling resistance. Tyres, however, have to run on all road surfacings and in all weather conditions: for them, the fewer the number and range of road surface parameters that they might be expected to encounter, the easier it would be to optimise their properties. 5.2 Optimising surfaces and tyres for safety from a theoretical point of view This subchapter summarises, from a theoretical point of view, those parameters that affect surfacing and tyre properties with regard to skid resistance and tyre/road friction. Tables are used to identify the most important parameters and associated values. To make the summary easier, as in Chapters 3 and 4, surfaces and tyres are covered separately. For many of the parameters it is only possible to suggest levels in broad terms as high or low. Actual values or ranges cannot always be indicated because the appropriate value is either unknown at present or can vary widely and depend on circumstances in particular situations. These most important parameters may themselves be influenced at a secondary level by other factors that are not included in this summary. It should always be borne in mind that measured values related to skid resistance always refer to wet conditions because that generally represents the worst case (excluding ice or snow). Optimised Surfaces The most relevant parameters for optimising road surfacings in relation to road safety (i.e. skid resistance) have been tabulated, divided into the two main types of surfacing, asphalt and concrete, in Date: 17/03/2010, Version: (67)

58 Table 5.1 and Table 5.2 respectively. In each table, they are subdivided into dense and porous surfaces. Date: 17/03/2010, Version: (67)

59 Table 5.1: Parameters most relevant to optimising skid resistance of asphalt surfacings Parameter Dense asphalt Suggested levels Porous asphalt polishing resistance (relevant for long-term behaviour) minimum PSV of 50 but varies with traffic level and skid resistance required in particular situations (not relevant on mastic asphalt) minimum PSV of 55 but varies with traffic levels and skid resistance required in particular situations shape of aggregates SI 20 / FI 20 SI 15 / FI 15 angularity of aggregates C 100/0, C 95/1, C 90/1 C 100/0 max coarse aggregate size 5 to max. 8 mm 5 to max. 8 mm bitumen viscosity polymer modified bitumen (high) polymer modified bitumen void content only relevant with regard to other surface properties as high as possible (25 v-%) chipping material aggregate size max 5 mm - (for use in gritting surfaces in early life) texture polishing resistance Minimum PSV of 50 (only relevant for mastic asphalt) micro texture high macro texture high mega texture low - micro texture high macro texture high mega texture low Table 5.2: Parameters most relevant to optimising skid resistance of concrete surfacings Parameters Dense concrete surface Porous concrete surface polishing resistance minimum PSV of 50 but varies with traffic level and skid resistance required in particular situations Where aggregate is exposed at the surface minimum PSV of 55 shape of aggregates SI 20 / FI 20 SI 15 / FI 15 angularity of aggregates C 100/0, C 90/1 C 100/0 max aggregate size 8 mm 8 mm surface texture type texture exposed aggregate concrete brushed concrete surface (transverse direction) micro texture high macro texture high mega texture low - micro texture high macro texture high mega texture low It can be seen from Date: 17/03/2010, Version: (67)

60 Table 5.1 and Table 5.2 that there are relatively small differences between the optimisation of asphalt and concrete surfaces with regard to skid resistance, especially where the exposed aggregate technique is used to provide surface texture. Interestingly, a suggestion made during the Expert Workshop at the Tire Technology Expo in Cologne, was that it might be possible, in theory at least, to optimise surfacings in different traffic lanes to reflect the traffic that they predominantly carried. For instance, on motorways Lane 1 could be optimised for trucks (or, rather, truck tyres) and lower-speed traffic while the overtaking lanes could be optimised for cars (or passenger-car tyres) and higher-speed traffic in relation to the three the road surface properties (skid resistance, rolling resistance and noise emission). However, as the discussion pointed out, this would assume consistent usage in each lane. In practice, at certain times of day, when there might be relatively few trucks in Lane 1, it is then used mainly by cars travelling at higher speeds; further, the centre lane of three-lane carriageways is often used by a mix of traffic. Optimised Tyres The parameters for tyres that are most relevant to safety (in terms of wet grip or wet tyre/road friction) are summarised in Table 5.3. In this table, tyres are divided into car and truck tyres and only those parameters that can be optimised without worsening one of the other tyre properties are listed. It should be noted that there is greater experience, especially in the published literature relating to passenger car tyres. One possibility for optimising tyre/road friction is the usage of an additive called Nanoprene. This additive has been invented to reduce the wear rate and to optimise the wet traction of the tyre simultaneously. An additional benefit of this compound is a slight reduction of the tyres rolling resistance [31]. Table 5.3: Parameters most relevant to optimising tyre/road-friction of tyres Parameters Car tyres Truck tyres tread pattern optimised to avoid hydro-planing minimum of 4 mm tread depth (depending on actual road surface macrotexture depth and texture type) optimised to avoid hydro-planing minimum of 4 mm tread depth rubber compound optimised (softer rubber leads to better tyre road friction but the other tyre characteristics will get worse) optimised (softer rubber leads to better tyre road friction but the other tyre characteristics will get worse) additive Nanoprene Nanoprene 5.3 Implementing optimised designs in practice While it is possible to make theoretical suggestions for optimising skid resistance or wet grip, in practice it is necessary to know the type of material that should be used, the parameters of the mixture and how the mixture should be laid on the road so that the benefits can be realised. Regarding the texture of the finished surface, it is known that microtexture should be high enough to provide the required skid resistance level and macrotexture should be high enough to limit the loss of friction at higher speeds. Both components of texture should Date: 17/03/2010, Version: (67)

61 stay constant over the life of the surfacing or, given that they may deteriorate with wear, remain at a high enough level to continue to contribute adequate skid resistance. To this end, the polishing resistance of the aggregate used (both coarse and fine) should be high enough to prevent the action of traffic reducing skid resistance unacceptably with time. The size of the coarse aggregate (and, where used, chippings applied to the new surfacing) should be small, typically 8 mm or less, (although 5 mm is better for early-life chipping). The aggregate particles should have irregular surfaces (high content of angularity) and the shape of the stones should be cubic. To prevent fatting up at high temperatures and high traffic it makes sense to use polymer-modified bitumen. Recommendations could also be made related to the laying and the compaction of the mixture, but these are not specific to optimising skid resistance; in this context, it would seem to be adequate for the laying and compacting processes to comply with the normal requirements, e.g. the surface should not be too cold while chipping newly laid dense surfacings. While implementing the optimised design, account must be taken that some surface parameters will change with time (e.g. microtexture, void content), while others will not (e.g. aggregate properties). It is therefore important to know which parameters will change and how they can be expected to change. This is why polishing resistance is such an important parameter for skid resistance: it governs the way in which microtexture will change under the influence of the traffic using the road over its service life. For tyres, the most the relevant boundary condition in practise for optimising safety is that of a wet surface. Hence, the suggestion that tyres should have a minimum tread depth of 4 mm (this could be a little less depending on the macrotexture) and a rubber compound optimised for wet grip but not too soft to prevent increased tyre wear and thus reduced mileage. 5.4 Implications for rolling resistance and noise when optimising for safety At first sight, optimising primarily for safety implies designing road surfaces with parameters that maximise skid resistance. To do this, by maximising macrotexture for example, could lead to noisier surfaces with increased rolling resistance. However, there are practical approaches to producing surfaces that have adequate skid resistance and which limit or reduce the amount of noise and rolling resistance generated. In practice, the optimal solution in a particular situation might mean focussing on just one or two of the main surface properties rather than all three at once. For instance, tyre/road noise emission might be seen as less important than high-speed skid resistance on lightly trafficked roads in remote areas. By contrast, in an urban or suburban environment where traffic is heavier and slower moving, noise reduction may be a greater priority than friction at high speeds. The rolling resistance characteristics of tyres may be more important to consider on long-distance inter-urban highways than on country roads or city streets. The use of an aligned aggregate, with a maximum size in the range 5 to 8 mm for both asphalt and concrete mixtures will contribute to an optimisation for all three surface properties as well as low values of the aggregate shape (SI or FI). If both angularity and polishing resistance are optimised with regard to skid resistance, so far as is known, there isll Date: 17/03/2010, Version: (67)

62 be no adverse impact on rolling resistance or noise emission. This is also true of binder viscosity. Most experiments to date relate to the different wavelength ranges of the texture, but it is a different matter to say how the ideal textures should be produced in regular practice. With regard to noise wavelengths between 10 and 500 mm, the pavement texture should have amplitudes that are as small as possible, to avoid frequencies < 1000 Hz. Figure 5.1, which summarises how different texture scales affect various parameters favourably or adversely, shows that such a texture will also lead to improved tyre/road friction as well as decreasing rolling resistance. A texture depth of mm at wavelength of mm will also lead to improved noise emissions, while avoiding frequencies > 1000 Hz. This texture parameter has also a positive impact on the other two surface properties (increasing skid resistance, decreasing rolling resistance). Figure 5.1: Texture wavelength influence on tyre/road interactions (based on [15]) In addition to these specific texture values in relation to wavelength and amplitude for microtexture, macrotexture and megatexture, some contribution to lower noise emission can be achieved using grooves or textures in the longitudinal direction, if the maximum height is 8 mm. Such textures show no drawbacks for rolling resistance but may have limitations for skid resistance depending on the nature of the surface that has been treated, especially where lower-quality aggregates (from a skid resistance perspective) that were normally covered in the body of the surfacing become exposed to traffic by the grooving process. For optimising tyres with regard to tyre/road friction at the same time as the other two surface properties, it is reassuring that there are relatively few parameters relating to the rubber properties and tyre construction which act in opposing directions regarding safety and noise or rolling resistance. Date: 17/03/2010, Version: (67)