Towards a Better INtegration of the Aggressiveness of Road loads experienced bY wearing course – BINARY

To achieve these objectives, various laboratory tests were conducted, ranging from millimeter-scale tests to full structural laboratory tests, to replicate the in-situ loading conditions experienced by wearing courses over their lifespan:

• Rheological characterization of the mastic in the wearing course and local fracture tests of the mastic between aggregates.

• Rheological tests on wearing course asphalt mixes to characterize their behavior.

• Small-scale laboratory tests, including free-rolling tests and surface wear simulations.

Full-scale tests were conducted on the fatigue carousel facility at IFSTTAR Nantes on a rehabilitated pavement.

A significant modeling effort was undertaken to interpret laboratory tests and model the in-situ behavior of wearing courses:

• A viscoelastic contact law was developed based on mastic tests to simulate the behavior of mastic between particles. This law was implemented in the Discrete Element Method (DEM) code LMGC90.

• A DEM model for asphalt mixtures, based on mechanical and geometric properties measured in the laboratory and using the aforementioned contact law, was developed in LMGC90.

• The behavior of wearing courses under free-rolling (WTT) and braking/acceleration effects (T2R) was simulated using the model implemented in LMGC90.

• LMGC90 was used to simulate the response of pavement structures to moving loads.

• In parallel, the mechanical behavior of the pavement structure was modeled using Viscoroute© 2.0.

Main expected results: - A better definition of an aggressiveness criterion for wearing courses, related to material characteristics (grading, particle shape), binder performance and layer thicknesses, but also to tire/pavement contact conditions and loading conditions: free rolling, breaking/acceleration and their consequences on the design life of the wearing course. - An improved modelling of the interface behavior and its influence on the design of the whole pavement. - Scientific publications of the results obtained including recommendations for the implementation of new design criteria for wearing courses in pavement design methods.

EXPECTED SCIENTIFIC IMPACT This project will contribute to improve scientific knowledge and understanding of the mechanisms of deformation and diffusion of loads in upper pavement layers. It will also lead to develop laboratory tests representative of in situ loading conditions (T2R test). In situ measurements of pressure distributions and strains, with innovative instrumentation (sensors for measuring distribution of contact pressures, optical fiber sensors for the measurement of deformation fields) are essential for understanding the behavior of wearing courses under real heavy vehicle wheel loads and to validate the proposed modeling. From a modeling point of view, approaches based on DEM (LMGC90) appear as the only method able to describe realistically forces/stress distributions in pavement layers with particles in contact, presenting a small thickness relative to the maximum particle size. They have been successfully used for railway structures, to estimate deformations and stability of ballast layers. SOCIO-ECONOMIC IMPACT The objective of better understanding the mechanical behavior of wearing courses, and of modelling their behavior should lead to a better design. This could also help to explain some degradation modes, still not well understood, such as top-down cracking observed on some pavements, or interface debonding. The reduction of the consumption of materials and the increase of design lives, will lead to savings of natural resources. A better understanding of tire pavement interaction could also allow to optimize the design of heavy vehicles and of their tires, to reduce the aggressiveness of their loads.

The dissemination and exploitation of scientific results will be provided throughout the project by communications during conferences (EATA, TRA, BCRRA, TRB, ISAP) and publications in national and international journals (RMPD). The International Conference on Accelerated Pavement Testing (APT2020) will be organized by IFSTTAR in September 2020, and it will give the opportunity to present results of the Binary project to a large international audience.

This project will contribute to the improvement of professional practices of public works companies, and design offices in terms of pavement design. At the end of the project, a summary of the main practical results, intended for the road industry, will be published (addressing methods of measurement of pressure distributions and deflection
levels, evaluation of the impact of loads, calculation models).

Main results dealing with the reduction of tire and axle aggressiveness will be presented to the truck industry and pneumatic industry. Part of the truck industry is already involved in the European project Ensemble (H2020 project starting in 2018) of which IFSTTAR is a member.

IFSTTAR is a member of the standardization commission in charge of the French pavement design standard NFP-98086 and the results could be integrated in this standard.

These results will also be presented during a seminar, organized for the stakeholders (road owners, consultants, contractors) in each of the french regions and during national events like the yearly meeting “Journées Techniques Routes”.

If economic constraints impose nowadays to carry more goods at lower costs, all heavy trucks silhouettes do not have the same effects on infrastructure for the same transported tonnage. In a context of ageing road networks, and where the resources devoted to the maintenance of these networks are shrinking, it is important to better control and understand the road pavement surface degradation mechanisms to optimize their design and maintenance. Currently, specifications exist regarding adhesion, texture, evenness, but no standards or any design method allows defining the mechanical characteristics, guaranteeing lifetime of surface layers which support directly traffic loads. The aim of the project, which combines French and Chinese research laboratories, is to develop more rational and scientific approaches for assessing the effect of traffic loads on wearing courses and for the design of these layers. The aim of the project is to develop more rational and scientific approaches for assessing the effect of traffic loads on wearing courses and for the design of these layers. To achieve these goals, the main mechanisms of deterioration of wearing courses under traffic loads, such as raveling, stripping of asphalt concrete (AC) particles induced by loading of tires and top-down cracking, will be studied. This requires the resolution of three scientific problems, which represent an obstacle to better take into account the aggressiveness of rolling loads: • to better understand the stress and strain fields generated in the wearing course under traffic loads, for different temperature conditions, and also during braking and acceleration phases on AC materials. • to take into account the impact of the behavior of interfaces between asphalt layers on the stress/strain redistribution within the structure, especially close to the pavement surface • to improve the prediction of wearing course lifetime, by studying and modeling the behavior of asphalt under rolling loads and validating the approach by full scale load tests with the IFSTTAR full scale pavement test facility. The critical loads will be used to estimate lifetime of the wearing course. To achieve these goals, various laboratory tests will be performed from micro scale tests till laboratory structure tests in order to reproduce the loadings that the wearing courses encounter in situ during their life time : - rheological characterization of mastic constituting the wearing course and local fracture tests on mastic between two aggregates. - rheological tests on wearing course asphalt mixes - reduced scale laboratory tests, including wheel tracking tests, used to characterize wearing course response under free rolling loads and test simulating surface wear An important modeling work is also planned, for the interpretation of the laboratory tests, and the modeling of the in situ behavior of the wearing courses. DEM will be used and a simplified model will be developped for advanced modelling of pavement with Viscoroute.