Thomas Gabet

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.

Hot Recycling of Bituminous Mixtures

This chapter first presents the results of a survey perfomed on the practices used in Europe. This survey is first report on the subject and could be usefully completed by the states of the art prepared in the framework of the Re-road and Direct-Mat European projecys.

Creep tests on bituminous mixtures and modelling

In order to study the permanent deformations of bituminous mixtures, a temperature-controlled triaxial test has been set up. By means of triaxial creep tests at imposed stress, the influence of different parameters on the behaviour of bituminous materials (confining pressure, deviatoric stress and temperature) has been investigated. A viscoelastic–viscoplastic model has been developed to simulate the creep tests and has shown good ability for simulating the test results.

Mechanical Testing of Bituminous Mixtures

This chapter focuses on permanent deformation, which is at the origin of pavement rutting. First experimental laboratory tools and associated analyses related to permanent deformations are presented. It is either material tests, that can be homogeneous or giving a simple empirical characteristic, or non-homogeneous Structural/Specimen tests.

Cyclic triaxial tests on bituminous mixtures

In the framework of a study on rutting of bituminous layers, thermo-controlled triaxial tests under constant confining pressure on homogeneous asphalt concrete specimen have been performed. These tests are cyclic sinusoidal tests, with the aim to reproduce in some way the loading and unloading phases due to traffic on roads. The influences of confinement and load frequency have been studied. Tests have highlighted that confinement notably decreases the strain rate as well as the level of strain reached at the end of the test. The tests did not allow to clearly observe the influence of frequency on the response of the material, due to problems of repeatability. In order to avoid such problems, another test procedure was adopted. It consisted in changing either confinement or frequency during the test, with the purpose to see an evolution of strain rate, and to establish correlations. These tests have clearly shown the influence of cyclic loading and the relative influence of loading frequency on the response of asphalt concrete. The main objective of this triaxial programme is to develop a numerical model, based on rheological laws, able to predict the behaviour of bituminous layers.

Prediction of the Mechanical Properties of Aged Asphalt Mixes from FTIR Measurements

Setting a method to evaluate the mechanical properties of an aged asphalt mix using a rapid chemical measurement from small samples, would be of high interest at different steps of a road structure life cycle: at the design step, to optimise the estimates of the service lifetime usually based on the properties of unaged materials; during the service life, to update the residual lifetime expectation of an existing structure according to the actual properties of the in situ aged materials; and at the end of the service life or for maintenance management, to evaluate the recyclability of the materials of the structure. This study was carried out aiming at comparing results from Fourier Transform InfraRed and Dynamic Shear Rheometer tests. Experimental data came from an international round robin test, organised by the RILEM TC ATB, designed to set an ageing protocol on loose bituminous mixtures. The evolutions of the carbonyl index (ICO) calculated from the FTIR spectra are compared to the evolutions of the parameters of a modified Huet Rheological Model (1S2P1D) fitted on the experimental linear viscoelastic data measured on the recovered binder. A significant relationship is observed suggesting that chemical and rheological properties could be correlated. Based on this observation, a method is proposed for predicting the G* values of a binder aged at a given ΔICO level. Subsequently, this method could be used to predict the complex modulus E* of a given mix from the shear modulus G* of the binder using existing binder/mix transfer models.

MURE National Project: FTIR spectroscopy study to assess ageing of asphalt mixtures

The French national project MURE, a collaborative research and development programme, started in March 2014. Two main issues are addressed: coupling of recycling and warm-mix asphalt technique and multi-recycling of bituminous mixtures. It gathers the national road federation, academic laboratories, road contractors and project contracting officials. The objective of the first working group (WG1) of MURE is to validate an accelerated ageing process for bituminous mixtures as well as on site than in laboratory. To achieve this goal, it is first necessary to characterize the ageing using a reliable and reproducible method valid for laboratory and plant-made mixtures. Many studies from the literature showed the efficiency of FTIR spectroscopy for ageing identification. In particular, carbonyl and sulfoxide groups found in binders are appropriate ageing markers. Meanwhile, no common FTIR protocol does exist at present. The first step of the experimental work performed by the WG1 was to compare seven FTIR methods, differing from sample preparation and approaches for calculating spectroscopic indexes. One given bitumen was aged eight times successively by RTFOT method (75 min – 163 °C). Two IR spectrum acquisition modes were tested: attenuated total reflectance (ATR) and transmission (spreading of bitumen on an IR transparent plate and bitumen dissolution in a solvent). The second step of the study was to verify the repeatability of FTIR tests on neat and aged bitumen according to sample preparation and calculation methods. The results allow confirming the relevance of FTIR spectroscopy for qualifying bitumen ageing. A single methodology and a common tool for harmonizing the calculation of indexes are proposed. The whole procedure can be used to follow the ageing properties on site at short and long times.

Effects of Ageing on Warm Mix Asphalts with High Rates of Reclaimed Asphalt Pavement

Within the framework of the European Project SUP&R ITN a Ph.D. thesis is carried out to study the durability of asphalt mixtures made with the combination of high rates of reclaimed asphalt pavement (RAP) and warm mix asphalt technologies. For this purpose the complex modulus and fatigue resistance of three different asphalt mixtures, including surfactant modified and foamed warm mix asphalts, combined with RAP has been studied. The extra value is given by the application of an ageing procedure based on the oxidation of compacted materials in laboratory. It follows the recommendations of the RILEM TC-ATB TG5, which distinguishes between short and long term ageing. Fourier Transform InfraRed (FTIR) tests were carried out on the extracted bitumens to quantify the oxidation levels. An increase of the norm and a decrease of the phase angle of \( \left| {{\text{E}}^{*} } \right| \) at 15 °C 10 Hz with ageing and RAP addition are experienced for all the mixtures. Similarly the slopes of the fatigue laws tend to increase with ageing. A consistent correlation is observed between these evolutions and the evolution of the carbonyl index. In general, the tendency is similar for all procedures, so the use of warm technologies combined with high RAP amounts may need to be considered.

Long term properties assessment of HMA and WMA with RAP through the apparent molecular weight distribution variation of the bitumen

The δ-method previously developed in IFSTTAR, is employed to determine the long term properties of asphalt mixes with reclaimed asphalt. This δ-method allows the calculation of apparent molecular weight distribution from rheological measurements. Its potential benefits as a tool for better understanding the key points of bituminous mixes recycling process is assessed. The study is carried out on twelve mixtures including two different types of warm mix asphalt techniques (surfactant and foamed bitumen) combined with the addition of high rates of reclaimed asphalt pavement. Samples are submitted to an oxidative aging process based on the protocol proposed by the RILEM Technical Committee-ATB TG5 consisting in two separate aging phases, a short term stage (corresponding to transport and spreading) and a long term aging equivalent to service life. The changes observed on the apparent molecular weight distribution with ageing and/or when reclaimed asphalt is added show that the δ-method could help to characterize structural ageing according to the different mixtures processes and its evolution. This may allow a better assessment for the long term performances of bituminous mixes depending on the manufacturing process.