Asphalt concrete | EPFL Graph Search

Asphalt concrete (commonly called asphalt, blacktop, or pavement in North America, and tarmac or bitumen macadam in the United Kingdom and the Republic of Ireland) is a composite material commonly used to surface roads, parking lots, airports, and the core of embankment dams. Asphalt mixtures have been used in pavement construction since the beginning of the twentieth century. It consists of mineral aggregate bound together with bitumen (also known as asphalt), laid in layers, and compacted. The process was refined and enhanced by Belgian-American inventor Edward De Smedt. The terms asphalt (or asphaltic) concrete, bituminous asphalt concrete, and bituminous mixture are typically used only in engineering and construction documents, which define concrete as any composite material composed of mineral aggregate adhered with a binder. The abbreviation, AC, is sometimes used for asphalt concrete but can also denote asphalt content or asphalt cement, referring to the liquid asphalt port

Advanced methodology for optimization of mixture design of asphalt concrete containing reclaimed asphalt pavement material

Sara Bressi

Despite the massive use of Reclaimed Asphalt Pavement (RAP) in Hot Mix Asphalt (HMA) production, the chemo-physical phenomena that characterise the fabrication of these mixtures have not yet been completely explored. The detection and understanding of these mechanisms as well as the study of the heterogeneity that characterizes high RAP mix production are fundamental to improving the approach to recycling, because they represent the source of the mixture characteristics and performance. This thesis aims to provide a framework for the characterisation of several phenomena and mechanisms occurring during a new mix when RAP material is involved. The variability of binder rheology with different ageing levels was analysed to evaluate the RAP binder heterogeneity and its evolution with time. This stage provided important information for the development of new binder blending charts to understand how the aged and the virgin binder interact and to ascertain whether the blend is homogeneous. The artificially simulated behaviour, assuming the hypothesis of 100% blending, would have provided important information regarding the blend characteristics if the behaviour of the binder had been confirmed in the mixture evaluation. However, in the next stages other phenomena occurred such as RAP particle clustering and the presence of re-activated and non-reactivated binder. Thus, a new methodology was developed using a Clustering Index (ICG*) to detect and quantify the RAP clusters. The same methodology allowed the investigation of the mobilisation of RAP binder and highlighted the presence of different layers in the RAP binder film thickness such as an over-aged crust and softer bitumen under the crust that acts as glue facilitating the clustering. The framework that emerged after these investigations led to the rethinking of the mix design approach, especially when high RAP quantities are used. Therefore, a new methodology was developed for estimating the dosage of virgin bitumen required in asphalt mixtures with or without RAP, investigating the specific contributions of all the components of the mixtures (virgin aggregates, filler and RAP) and taking into account the RAP cluster phenomenon and its consequences during the mixture. Indeed, new revised surface area factors were computed thanks to the 3D images of several stones for the calculation of the specific surface area of the aggregates and the critical filler concentration allowed the volume of bitumen required for coating the filler to be obtained. Finally a RAP clustering prediction model was developed to readjust the grading curve after the RAP particle agglomerations occurring during the mix. The new theoretical formula, thanks to the separation of all the contributions, provides results valid for mixtures with and without RAP (the terms related to the RAP become zero if the RAP percentage introduced in the formula is zero). The verifications carried out in the laboratory gave positive and promising results and the methodology is considered successful in estimating the value of the optimal bitumen quantity for mixtures with and without RAP.

EPFL2016

Multiscale imaging and characterization of the effect of mixing temperature on asphalt concrete containing recycled components

Sara Bressi, Manfred Partl

When producing asphalt concrete mixture with high amounts of reclaimed asphalt pavement (RAP), the mixing temperature plays a significant role in the resulting spatial distribution of the components as well as on the quality of the resulting mixture, in terms of workability during mixing and compaction as well as in service mechanical properties. Asphalt concrete containing 50% RAP was investigated at mixing temperatures of 140, 160 and 180 degrees C, using a multiscale approach. At the microscale, using energy dispersive X-ray spectroscopy the RAP binder film thickness was visualized and measured. It was shown that at higher mixing temperatures this film thickness was reduced. The reduction in film thickness can be attributed to the loss of volatiles as well as the mixing of RAP binder with virgin binder at higher temperatures. X-ray computer tomography was used to characterize statistically the distribution of the RAP and virgin aggregates geometric features: volume, width and shape anisotropy. In addition using X-ray computer tomography, the packing and spatial distribution of the RAP and virgin aggregates was characterized using the nearest neighbour metric. It was shown that mixing temperature may have a positive effect on the spatial distribution of the aggregates but did not affect the packing. The study shows a tendency for the RAP aggregates to be more likely distributed in clusters at lower mixing temperatures. At higher temperatures, they were more homogeneously distributed. This indicates a higher degree of blending both at microscale (binder film) and macroscale (spatial distribution) between RAP and virgin aggregates as a result of increasing mixing temperatures and the ability to quantify this using various imaging techniques.

Wiley-Blackwell2016

Modélisation et prédiction du comportement rhéologique des mélanges bitume caoutchouc

Mehdi Ould Henia

Asphalt rubber is a modified binder widely used in pavement construction. Composition design methods of asphalt rubber blends are exclusively empiric and rely on the selection of the optimal combination of parameters after a long experimental program. The understanding of the phenomena affecting the asphalt rubber behaviour starts with the proposal of a rational theory and its validation with experimental data. To do so, a set of experiments were carried out including three main factors considered as significant for the rheological behaviour of the final binder. These factors are the bitumen chemical composition, the rubber content and its maximum mesh size. The binder complex shear moduli were measured within the viscoelastic linear domain. The measurements allowed a full characterisation of the rheological behaviour of asphalt rubber for the service temperatures and rates of loading. Base bitumen composition leads to final binders with rheological behaviours that are radically different despite the fact that the base bitumen belong to the same commercial grade. The rubber content increase showed significant changes on the properties of the binder, especially an increase of its elasticity and a decrease of its temperature sensitivity. The rubber powder maximum mesh size has no significant effect on the rheological properties of the blends within the service conditions. An analysis of physical and chemical changes was carried out on the phases formed after the curing process. A method for the isolation of the phases without affecting their physical and chemical properties was proposed. A centrifuging process was applied to asphalt rubber thanks to the design of an appropriate equipment adapted to the binder features. An experimental protocol was established to obtain a good separation of the phases with a limited aging. The SARA bitumen group analyses were carried out before and after the curing process. They allowed the determination the bitumen fractions kinetics during the blending. Light fractions of bitumen are the main chemical groups that migrate toward the cross-linked rubber network causing its swelling. The rheological behaviour of the separated bitumen phase has proven that the residual bitumen exhibits a lowered thermal sensitivity which is confirmed by the increase of the colloidal instability index. The rubbery phase has showed an extent of swelling varying between 200 and 400% which modifies considerably the rheological behaviour of the rubber by transferring to it a part of the thermal sensitivity of the bitumen. The influence of the factors was evaluated and their relative importance determined through a statistical model. Micromechanical modelling of the asphalt rubber composite has allowed the validation of observed phenomena that are mainly physical ones. To do this, two main homogenisation procedures were implemented and carried out on the heterogeneous medium: The classical self-consistent approach widely referenced as the equivalent medium approach (EMA) and the generalised self-consistent approach frequently referenced as the equivalent field approach (EFA). A satisfactory modelling of the composite is achieved by taking into account two changes occurring in the phases: their volumetric fractions influenced by the swelling, and their rheological properties due to the diffusion of bitumen into the rubber. An acceptable estimation of the rheological behaviour of the rubbery phase is obtained from the base materials properties with a three-phase self-consistent modelling according to EFA approach. The consideration of an adsorbed fraction of the bitumen on the surface of the rubber particle has allowed a qualitative and a quantitative description of the properties of the rubbery phase. The difficulty of predicting the rheological behaviour of a bitumen from its chemical composition is discussed. Future investigation ways are proposed.