Malal Kane

Characterisation of ageing processes on the asphalt mixture surface

Ageing of asphalt binders leads to evolution of pavements mechanical performances, due to changes in their rheological behaviour and in the binder composition. Ageing leads to a hardening of asphalt, mainly due to the oxidation of the asphalt binder itself. Oxidation rate is influenced by several parameters, namely outside temperature, ultraviolet (UV) radiation and intrinsic characteristics of the mixture constituents. In order to assess physical and chemical characteristics of the binders in aged pavements, mixture samples of several centimetres in thickness are usually cored from the field. Binders are extracted from these asphalt samples. This process of extraction is typical but it does not allow differentiating bulk and surface characteristics. Indeed, only the surface binder is exposed to UV rays and weathering. In order to assess the ageing of only the surface layer, to verify, for instance the influence of ageing on adhesion characteristics, it is necessary to extract and recover only the surface asphalt binder. A new test protocol, presented in this paper, has been developed to fulfil this purpose. This protocol consists in sampling only small particles at the surface layer, extracting the binder from the particles and performing Fourier transform infrared spectroscopy (FTIR) tests to assess the ageing of the binder. As the quantity of binder is very small, a specific FTIR test method is used. This method consists in keeping the binder diluted in the solvent and placing the solution in the ray of the spectrometer to perform the measurement. This new method has been compared to a more typical one, which consists in applying the binder on a transparent plate after having extracted the binder from the solvent. Once it has been checked that the two methods provide the same results for three kinds of binders, surface ageing tests have been performed. Two different ageing processes were considered: an ageing on site and an accelerated ageing protocol using a climate chamber named Weatherometer (SUNTERTXXL+). In this chamber, temperature, humidity and rain are controlled. The comparison between the ageing in the laboratory and the field allowed correlating both processes to establish an accelerator factor with respect to the increase in carboxyl group.

Laboratory evaluation of aggregate polishing as a function of load and velocity. Application to the prediction of damages on skid resistance of road surfaces due to trucks and passenger cars

Skid resistance of road surfaces depends mostly on pavement texture. This texture is usually divided to two components: microtexture and macrotexture. Microtexture refers to the small-scale texture of the road aggregate component while macrotexture refers to the large-scale texture of the road as a whole due to the aggregate particle arrangement. Both components contribute to the generation of friction between tyre and road. However, due to traffic, the first-cited component is continuously polished over the roads life. Maintaining that microtexture depends greatly on the aggregates used in the wearing course, the traffic level and the vehicle velocities. This work tries to quantify the influence of these three parameters on skid resistance degradation. Different specimens fabricated from different types of aggregates are submitted to different polishing pressure levels and velocities. From experimental results, an updated version of an existing model of skid resistance evolution is thereby proposed and validated. As a possible application of the new model, a prediction of damages induced by truck and passenger car traffic on skid resistance is proposed.

Exploring the Ageing Effect of Binder on Skid Resistance Evolution of Asphalt Pavement

ABSTRACT One of the most important safety requirements of a road is its capability of preventing skidding of vehicles. However, this requirement so called “skid resistance” evolves during the whole pavement life. Phenomena that are identified to be responsible of this evolution are binder removal, aggregate polishing and seasonnal variations. In addition, ageing of bitumen is also identified to play a significant role on skid resistance evolution. One of the main results of previous works about skid resistance evolution was a model that takes into account two of the above listed phenomena such as bitumen scrouring and aggregates polishing. This present work is a continuation of the previous ones. It tries to include the effect of ageing in the above model. An updated model of evolution of skid resistance including this latter effect is proposed. This model is composed by a set of elementery functions in which some of them can be easily identified. The model can predict the evolution of skid resistance by combining effects of ageing, polishing and binder removal.

Development of an accelerated pavement test reproducing the effect of natural ageing on skid resistance

This study deals with the development of a new test method simulating the effect of ageing on skid resistance. This test is applied to bituminous mixes in this study, but can also be applied to concrete. This test relies on two machines, the Wehner–Schulze machine, to measure the friction, and the Weatherometer sunset machine, to simulate weather effects (rain, wind, sunlight, etc.). The relevance of this test is evaluated from comparisons between changes in friction and a chemical function linked to bitumen ageing: the carbonyl index. Validation is performed on bituminous mixture samples exposed to natural and accelerated ageing modes. Three very thin asphalt concrete (VTAC) slabs are manufactured in the laboratory with three different types of bitumens. Two specimens are extracted from each slab: one for natural ageing and the other for accelerated ageing. The evolutions of friction and carbonyl index measured on these samples submitted to the two ageing modes exhibit similar tendencies for all three bitumen types, thus confirming the relevance of the test. A conversion factor describing the correspondence between the durations of accelerated ageing and those of natural ageing is proposed for each type of bitumen. Video abstract Read the transcript Watch the video on Vimeo

Modeling of the Road Surface Polishing Based on Contact-mechanics Approach

ABSTRACT In this paper, a new contact-mechanic-based model for road polishing is presented. According to this model, the local polishing rate is controlled by the pressure distribution between the car tire and the road surface. The Boussinesq load-displacement relation is used to calculate this pressure distribution taking into account the surface roughness, the applied load, the mechanical and geometrical properties of the bodies in contact. Coupling the pressure distribution with a wear law, the local evolution of the pavement roughness can be evaluated. The proposed wear law is based on the Preston equation and is proportional to the contact pressure, the relative velocity and the Micro-Deval coefficient of the aggregates used in the mix-designing. To validate this model, polishing tests are performed on road specimens, using the so-called machine Wehner-Schulze. Results show that the predicted road profile evolution is consistent with that given by experiments.

Contribution to pavement friction modelling: an introduction of the wetting effect

ABSTRACT This paper presents a friction model describing the tyre rubber/road interaction that takes into account the viscoelasticity of the tyre rubber, the texture of the road surface and a water layer between the tyre/road interface by introducing explicitly a computation of the water layer effect in the calculation process of the hysteretic friction. The geometry of the wetted portion of the interface model is simplified by transforming it into an equivalent hydrodynamic bearing. Utilising the Reynolds equation, the bearing load capacity is calculated and the resulting forces are subtracted from the contact load when calculating the forces of the hysteretic friction. The mechanical behaviour of the rubber is represented in the model by Kelvin–Voigt model. The frictional forces due to hysteresis are calculated at any given operating conditions (load, slip speed, etc.) from the contact geometry of rough surfaces caused by the viscoelastic behaviour of rubber. To validate the model, a set of surfaces including real pavements and artificially textured slabs were selected covering a wide range of microtexture and macrotexture combinations and the computed and measured friction compared. To describe the contact geometry of rough surfaces using macrotexture and to measure actual friction, the Circular Track Meter and the Dynamic Friction Tester devices were used, respectively. The friction coefficients computed using the model were compared to the measured friction coefficients. The obtained results are presented in the paper and proved to provide high correlation between the measured and modelled friction. The model is capable to predict wet friction at low as well as high speeds on wet surfaces, thus proving to be capable to take adequately the wetting effect on the variation of friction with increasing speed. Recommendations are provided to improve the model and extend it to a tyre friction model.

Kinematic wave approach to model water depth on road surfaces during and after rainfall events

Water on road surfaces significantly degrades the available skid resistance. Indeed, 25% of injury accidents occur on wet roads in France due to lack of skid resistance. Some research results advocate an exponential decrease in skid resistance with increase in water depth. The above conclusions show the importance of knowing the water depth present on road surfaces during and after rainfall events in order to evaluate the available skid resistance for driver safety. In this paper, a prediction model of water depth on road surfaces during and after rainfall events is presented. This model takes into account the road surface characteristics such as macrotexture, slope and porosity, and weather conditions such as temperature, wind speed, relative humidity and rainfall rate. The model is based on the kinematic wave simplification of the Saint Venant equations, which represents the balance of flows from rainfall, run-off, infiltration and evaporation. The model is validated from comparisons to experimental measurements of water depth performed on four different surfaces of the IFSTTAR test track. The experimental device for the validation is composed of sensors that measure water depth, rainfall intensity, wind velocity, air humidity and temperatures. The model predictions on these four surfaces are very satisfactory.

Effect of Aggregate and Asphalt on Pavement Skid Resistance Evolution

When designing pavement, engineers must optimize some requirements such as user safety (skid resistance), environmental impact (noise, rolling resistance...)... However, this skid resistance evolves during the entire pavement life. So it is a common practice to perform laboratory tests to forecast the evolution of skid resistance. Previous works done in the French Laboratory of Bridges and Roads (Laboratoire Central des Ponts et Chaussees, LCPC) have identified phenomena such as binder removal, aggregate polishing and seasonnal variations to be responsible of these variations. This paper focuses on the polished stone values of aggregates and the aging of asphalt on the evolution of pavement skid resistance. Skid resistance of different specimens of nude aggregates and asphalt mixes that are submitted to polishing and aging was studied. On skid resistance point of view, aging of aggregates can be neglected in comparison to those of asphalt. Rocks with high polishing resistance offer less variation of skid resistance. Aging of asphalt tends to increase skid resistance until 12 month and remains this latter constant after.

Evolution of Skid Resistance of Different Materials for Metro Rolling Tracks

This paper investigates the evolution of skid resistance against traffic of different tracks materials for rubber-tired metro. Experimental analysis based on the friction analysis of different track specimens at different polishing states is done. The polishing and friction measurements are performed using the so-called Wehner-Schultz machine. Three materials of which two concrete mixes and steels treated differently at surfaces (before the polishing process) are studied. The results show that concretes offer better skid resistance than steels in the long term, while the initial skid resistance depends exclusively on the treatment applied on the initial surface texture.