M.M. Villani

Contribution of Hysteresis Component of Tire Rubber Friction on Stone Surfaces

This research examines the hysteresis friction of a sliding elastomer on various types of stone surfaces. The hysteresis friction is calculated with an analytical model that considers the energy spent by the local deformation of the rubber due to surface asperities of the stone surface. By establishing the fractal character of the stone surfaces, one can account for the contribution to rubber friction of stone roughness at different length scales. A high-resolution surface profilometer is used to calculate the three main surface descriptors and the minimal length scale that can contribute to hysteresis friction. The rubber is treated as a Zener visco-elastic material model. Modeling of the contact between the elastomer and the stone surface is based on the analytical model of Klüppel and Heinrich, which is a generalization of the Greenwood and Williamson theory of contact between spheres that are statistically distributed about a mean plane. The results show that this method can be used in order to characterize in an elegant manner the surface morphology of various stone surfaces and to quantify the friction coefficient of sliding rubber as a function of surface roughness, load, and speed.

The development and use of the Skid Resistance and Smart Ravelling Interface Testing Device

Since June 2012, tyre manufacturers have been required to provide data in relation to the performance of their tyres through testing. The tyre label covers rolling resistance, wet grip or skid resistance, noise emission. These are important criteria to consider. In the future this kind of labelling system will be applied on roads. In such a case it is not desirable to build costly test sites to evaluate these criteria. There is a need of accurate laboratory testing device to perform such tests. Currently available lab and field friction testing devices do not allow for the evaluation of the interaction between relative speed, pressure and temperature on friction. A Skid Resistance & Smart Ravelling Interface Testing Device (SR-ITD®) has been designed and built for the study of the influence and the interaction of the various phenomena occurring at the rubber pavement interface. The device enables various combinations of slip velocity and pressure to be applied with concurrent measurement of temperature in the interface regions. With this device it is also possible to study the ravelling resistance of a surfacing material (asphalt, concrete or a surface dressing). The device was developed to study and evaluate materials in the laboratory on a smaller scale and much faster than in real practice. The materials to test can be the rubber on the tyres or the surface of the road, asphalt, concrete or other surface materials. This paper will describe the development and use of the machine till current date. An optimal tyre can only be developed with a representative road surface also an optimal road surface can only be developed with a representative tyre. With accurate testing devices the interaction between tyre and road can be solved.

Importance of Rubber Characteristics in the Frictional Response of Asphalt Concrete Surfaces

During rubber–asphalt concrete (AC) interaction, the indentation of the elastomer by the AC asperities causes its deformation and hence dissipation of internal energy. The amount of expended energy is related primarily to surface roughness, thermomechanical response of the rubber, speed, temperature, and applied pressure. Friction can be evaluated on the basis of the amount of expended energy. Because of the many factors involved and their interrelation, laboratory or in situ measured friction is only an indicative value heavily dependent on the specific set of testing conditions. Some conditions, such as rubber characteristics or temperature, are difficult to control, and their influence on friction is difficult to quantify. This research examined the interaction between two rubber types (with characteristics similar to those used for in situ testing) and three typical AC mixes (AC10, stone mix asphalt, and porous asphalt). The AC surface characteristics were studied with a laser scanner and the viscoelastic properties of the rubber with dynamic shear rheometer tests, and the interaction between the two materials was investigated with a purpose-developed and purpose-built skid-resistance interface testing device. The laboratory results were further elaborated with a newly developed computational tool, M2D. Through a detailed laboratory study followed by computational analyses, this research demonstrates the importance of accounting for rubber characteristics during friction evaluation and demonstrates how the characteristics of the rubber can be taken into account in friction prediction tools.

American and European Mix Design Approaches Combined: Use of NCHRP Performance Indicators to Analyze Comité Européen de Normalisation Test Results

This paper describes a project that is part of NL-LAB, a larger, long-term program at Delft University of Technology, Netherlands, which aims to establish the predictive capacity of the current European functional tests for Dutch asphalt concrete (AC) mixtures. In this NL-LAB program, the functional characteristics of resistance to rutting (EN 12697–25), fatigue (EN 12679-24), stiffness (EN 12697–26), and moisture sensitivity (EN 12697-12 and EN 12697–23) are determined for specimens that are (a) mixed and compacted in the lab, (b) mixed in the plant and compacted in the lab, and (c) mixed in the plant and compacted in the road. Eventually, these tests will provide insight into the effect of mixing and compaction on the functional characteristics. The project described in this paper focused on the indirect tensile strength (ITS) and the triaxial cyclic compression test for two AC mixes. The properties found for all three stages of preparation were analyzed with the use of formalistic expressions from the NCHRP Design Guide 1–37A, Level 2, for the estimation of performance indicators. This project aimed to see if these relations remained valid for the Comité Européen de Normalisation tests, especially for mixes with high recycled asphalt pavement content. In the Netherlands, 50% reclaimed asphalt pavement is standard. It was found that the NCHRP Design Guide 1–37A expressions fit the Comité Européen de Normalisation test data quite well for the reclaimed asphalt pavement that contained mixes, especially for the ITS. Currently, another two construction projects are being sampled, and the results will be used to validate and improve the relations.