Ignacio Artamendi

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.

Differential Thermal Contraction of Asphalt Components

Large differences between the coefficient of thermal contraction of mineral aggregate and binder has been associated with localised damage at the aggregate-binder interface at low temperatures. In this work, the coefficients of thermal contraction of different binders, aggregates and asphalt mixtures have been determined. Binder specimens were first produced by pouring hot bitumen into 200 x 50 x 50 mm3 moulds. The specimens were conditioned at various temperatures ranging from 10 to -20 0C. The change in length was then measured to determine thermal strains as a result of cooling. It was found that linear coefficients of thermal contraction varied between 115 and 175 x 10− 6 mm/mm/0C depending on grade and type of binder. Coefficient of thermal contraction of different aggregates was also determined. Rock specimens of the same dimension as the binder specimens were cut from large rock cores. The specimens were then conditioned at different temperatures and their change in length was measured. Three types of rocks namely limestone, granite and greywacke typically used in asphalt mixtures were employed. It was found that CTC of the aggregates varied between 7 and 10 x 10− 6 mm/mm/0C, thus, 10 to 25 times lower than those of the binders. The coefficient of thermal contraction of various asphalt mixtures was determined using a volumetric and a composite model. Furthermore, predicted values were compared with those determined experimentally using beam shaped asphalt specimens cut from roller compacted slabs manufactured in the laboratory.

DEVELOPMENT OF UK PROPIETARY ASPHALT SURFACING SKID RESISTANCE AND TEXTURE

This paper considers the development of proprietary asphalt surfacing properties used in the United Kingdom due to trafficking. The project aimed to determine whether asphalt mixes made with lower PSV aggregate in smaller nominal sized asphalt mixes could provide adequate levels of in-situ performance compared to the traditional use of higher PSV aggregate and 14mm nominal sized mixes. It compared SCRIM and GripTester data from full-scale road trials with laboratory Wehner Schulze laboratory data measured using asphalt laid at the road trial. The proprietary mixes were 14mm, 10mm and 6mm maximum aggregate size made with porphyry and quartz granite aggregate. Based on their PSV these aggregate sources would typically not be used on heavily trafficked roads in the UK. Surface texture was monitored using laser sensor during SCRIM assessment. In-situ testing found good levels of skid resistance for all mixes over the last 4 years since installation. The optimum nominal mix size was 10mm. The 14mm mixes had a noticeable drop in texture depth whereas the smaller nominal size mixes remained relatively unchanged. The Wehner Schulze was found to rank the mixes similar to in-situ measurement in contrast to PSV which did not. A model was developed to predict in-situ skid resistance based on Wehner Schulze data.

New performance indicators for polymer modified binders

In Europe, polymer modified bitumens are specified in accordance with EN 14023. These specifications are based on binder penetration and softening point plus a set of properties including resistance to hardening, cohesion, Fraass breaking point and elastic recovery. The European standard also provides a series of classes for each of these properties and enables the selection of the most suitable class for each polymer modified binder. These properties, however, provide limited information about their effect on the performance of an asphalt mixture in the laboratory or during installation, compaction and in-service. In this paper new binder performance indicators are proposed based on the critical workability temperature of the binder and a suite of tests on a standardised sand mixture. Critical workability temperature was determined from binder viscosity data obtained using a rotational viscometer. Tests on a standardised sand mixture, on the other hand, provided information on the effects of binder properties on stiffness, oxidative ageing, adhesion, resistance to deformation and low temperature cracking. Critical workability temperature was then used to classify the polymer modified binders into five classes, from the most workable to the least workable. Standard binder properties, critical binder workability temperature and binder’s properties derived from the sand mixtures were then related to the properties of a dense asphalt concrete mixture. Asphalt mixture properties evaluated in the laboratory included workability, resistance to water damage and, deformation, fatigue and fracture resistance. Finally, performance indicators limits were proposed for each of the binder classes.

Low Temperature Cracking Properties of Asphalt Mixtures Containing Rubber Bitumen Pellets

This paper evaluates the effect of ground tyre rubber (GTR) bitumen pellets on the low temperature cracking properties of asphalt mixtures. Low temperature properties were evaluated at 0 °C using monotonic and cyclic three-point bending tests. Flexural strength was determined using beam specimens and fracture toughness was determined using two specimen geometries: single edge notched beam and semi-circular bending. Cyclic bending tests under controlled load conditions were also carried out using notched beams to determine crack propagation parameters. Dense surface course mixtures were produced with a paving grade binder, two different polymer modified binders and two pellets concentrations. It was found that the flexural strength increased when GTR bitumen pellets and polymer modified binders were used. Differences in fracture toughness between the mixtures were, however, relatively small. Only one of the mixtures with a polymer modified binder showed a significant higher fracture toughness value. Furthermore, good correlation was found between the fracture toughness values determined from the two specimen geometries. Cyclic test, on the other hand, showed that the mixtures produced with the bitumen pellets and with the polymer modified binders were able to sustain more load applications than the unmodified mixture. Furthermore, crack propagation parameters were affected by both the type of binder and the concentration of pellets in the mixture.

Evaluation of Air Voids in Reinstatement Materials for Footways

The specification for reinstatement of openings in highways in the UK prescribes the materials that may be used and sets out the fundamental requirements for compaction. The preferred mixture for the reinstatement of footways is AC 6 complying with EN 13108-1. Meeting in situ air voids requirements for this mixture is not always achievable in practice due to, among others, the variability of plant produced mixtures, compaction conditions, surcharge and test variability. In this work, two standard AC 6 mixtures, a coarse graded and a fine graded, have been evaluated along with an alternative AC 4 mixture designed to give low air voids with minimum compaction effort. The mixtures were compacted in the laboratory at different temperatures and compaction efforts. Air voids were determined using standard methods. X-ray computed tomography (CT) was also used for measuring the internal structure of the asphalt specimens. Results showed large differences in air voids between the two standard AC 6 mixtures. Considerably lower voids were obtained for the AC 4 mixture. CT scanning showed better compactability of the AC 4 mixture as seen by the air voids distribution with specimen thickness. Also, the AC 6 coarse mixture had a relatively small number of large air voids whereas the AC 6 fine mixture had a relative large number of small air voids. The AC 4 material, on the other hand, had the smallest air voids and their number was small compared to the AC 6 fine mixture but similar to that of the AC 6 coarse mixture.

Successful demonstration of lower temperature asphalt on the UK strategic road network

Lower temperature asphalts (LTAs) have been identified as a way of reducing both the energy required to produce and lay asphalt materials and the nuisance of fumes for workers. The use of LTAs has not yet reached its full potential in the United Kingdom. To encourage uptake in appropriate applications and inform the national specifications, a project was undertaken to demonstrate use of a LTA on the strategic road network. The project was realised as a result of successful collaboration between the national road authority, industry and a research organisation. A site with sections of LTA surface and binder course mixtures produced using an ‘injection foaming’ technology was laid and monitored for initial properties. Sections of conventional hot mix materials were also laid and monitored to enable direct comparison. Visual inspection, temperature measurement and thermographic imaging were undertaken at regular intervals throughout construction. Physical properties of the materials used were established through laboratory testing of samples taken at site and the carbon footprint of construction was evaluated. Overall, the demonstration proved that effective application of an LTA technology could be achieved but also yielded some important practical messages for the future application of LTAs.

Strength and fracture properties of aggregates

This paper presents a study of the mechanical and fracture properties of various types of aggregates used in asphalt mixtures. Three types of rocks namely, greywacke, granite and limestone, were evaluated. Compressive and tensile characteristics of the rocks were determined by means of uniaxial compressive and indirect tensile tests, respectively. Resistance to fracture was determined by means of semi-circular bending tests. Results showed that the greywacke had the highest strength both in compression and in tension. The granite, on the other hand, had high compressive strength but the tensile strength was relatively low. Compressive tests also showed that the response of rock specimens under loading was linear elastic. It was found that the compressive elastic modulus of the limestone was the highest followed by the greywacke and the granite. Similarly, indirect tensile tests indicated that the response of the greywacke and the limestone rocks was linear elastic whereas that of the granite was non-linear. Furthermore, tensile elastic modulus values of the greywacke and the limestone were similar and about five times higher than that of the granite. As regards fracture, load-deflection curves for semi-circular bending tests indicated linear elastic behaviour of the three types of rocks. Thus, linear elastic fracture mechanics theory was applied to determine fracture toughness. Results showed that the greywacke had the highest resistance to fracture.