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Ageing of Asphalt Mixtures: Micro-scale and mixture morphology investigation
KTH, School of Architecture and the Built Environment (ABE), Transport Science, Highway and Railway Engineering.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

There are many variables that affect the viscoelastic properties of asphalt mixtures with time, among which age hardening may be considered one of the important ones. Age hardening of asphalt mixtures is an irreversible process, which contributes to a reduction of the durability of pavements and eventually increases the maintenance cost. Beside the environmental effects, ageing in asphalt mixture depends on the physicochemical properties of bitumen and mixture morphology which is a combined effect of aggregate packing, porosity, air void distribution and their interconnectivity. Thus, a clear understanding on the physicochemical properties of bitumen and mixture morphology may help to predict the performance of asphalt mixtures, which will contribute to longer-lasting and better performing pavements.

When looking at the bitumen at micro-scale, one can see microstructures appearing under certain conditions which can be partially explained by the interaction of the individual phases. Since the thermo-rheological behavior of bitumen depends largely on its chemical structure and intermolecular microstructures, studying these can lead to understanding of the mechanism, speed and conditions under which this phase behavior occurs. Linking this to the changes in properties of bitumen can thus lead to better understanding of the causes of ageing, its dominant parameters and the resulting diminished mechanical response.

To investigate ageing in asphalt pavements, along with physicochemical properties of bitumen one needs to also focus on the influence of mixture morphology.  It is known that asphalt mixtures with similar percentages of air-voids can have different morphologies and thus can age differently. Prediction of ageing behavior without considering the influence of mixture morphology may thus lead to erroneous conclusions and non-optimal mix design. Hence, it is important to understand the interplay between the mixture morphology and ageing susceptibility and relate this to the long term mixture performance.

The aim of this Thesis was to develop fundamental understanding on ageing in asphalt mixtures that can contribute to the asphalt community moving away from the currently used accelerated ageing laboratory tests and empirical models that can lead to erroneous conclusions.

To reach this aim, experimental and numerical micro-scale analyses on bitumen and meso-scale investigations on mixture morphology have been performed which, collectively, allowed for the development of a method for the prediction of asphalt field ageing, incorporating both mixture morphology and micro-scale bitumen mechanisms. For this, first, the mechanisms of surface ageing and diffusion controlled oxidative ageing were identified. Secondly, the influence of mixture morphology on asphalt ageing susceptibility was investigated. Procedures to determine the controlling parameter were then developed and an empirical framework to quantify the long-term field ageing of asphalt mixtures was set-up. For this, a combination of experimental and numerical methods was employed.

An extensive experimental study was carried out to understand the fundamental mechanisms behind the micro-structural phase appearance and the speed or mobility at which they change. Atomic Force Microscopy (AFM) was utilized at different temperatures to investigate the phase separation behavior for four different types of bitumen and co-relate it with the Differential Scanning Calorimetry (DSC) measurements. Based on the experimental findings, it was concluded that the observed phase separation is mainly due to the wax/paraffin fraction presence in bitumen (Paper I). A hypothesis was developed of the appearance of a thin film at the specimen surface due to ageing which is creating a barrier, restricting thus the microstructures to float towards the surface. Furthermore, investigation showed that depending on the bitumen and exposure types this surface thin film is water soluble and thus the moisture damage becomes more severe with the ageing of asphalt pavement (Paper II and IV).

A new empirical relation to obtain the primary structure coating thickness was established utilizing mixture volumetric properties and gradation using a large set of data from different literature sources. It was found that the enhanced morphological framework can be used to optimize the long term performance of asphalt mixtures (Paper III).  Thereafter, the effect of diffusion controlled oxidative ageing on different mixture morphologies based on oxidative ageing mechanism of bitumen and diffusion-reaction process was investigated using the Finite Element Method (FEM). From the FE analyses, the effect of air-void distribution and their interconnectivity combined with the aggregate packing was shown to have a significant effect on age hardening (Paper IV).

It was shown that focusing only on the percentage of air-void as the main predictive ageing parameter may lead to an erroneous conclusion and non-optimal predictions of long-term behavior.  To replace such approaches, a new way to predict the long-term ageing was proposed in this Thesis, utilizing the found influences of mixture morphology and fundamental mechanism. Though additional mechanisms and non-linear coupling between them may be still needed to reach the ‘ultimate’ ageing prediction model, the current model was found to be a significant improvement to the currently used methods and may lead the way towards further enhancing the fundamental knowledge towards asphalt mixture ageing (Paper V).

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. , xi, 94 p.
Series
TRITA-TSC-PHD, 14:002
Keyword [en]
Ageing, Aggregate packing, Atomic force microscopy, Asphalt mixture, Bitumen, Diffusion, Finite element analysis, Gradation, Micro-structure, Mixture morphology, Oxidative ageing, Surface ageing
National Category
Civil Engineering
Identifiers
URN: urn:nbn:se:kth:diva-145051ISBN: 978-91-87353-37-6 (print)OAI: oai:DiVA.org:kth-145051DiVA: diva2:715945
Public defence
2014-05-23, Q2, Osquldas väg 10, KTH, Stockholm, 09:00 (English)
Opponent
Supervisors
Note

QC 20140509

Available from: 2014-05-09 Created: 2014-05-07 Last updated: 2014-05-09Bibliographically approved
List of papers
1. Micromechanical investigation ofphase separation in bitumen bycombining atomic force microscopywith differential scanning calorimetryresults
Open this publication in new window or tab >>Micromechanical investigation ofphase separation in bitumen bycombining atomic force microscopywith differential scanning calorimetryresults
2013 (English)In: International Journal on Road Materials and Pavement Design, ISSN 1468-0629, E-ISSN 2164-7402, Vol. 14, no S1, 25-37 p.Article in journal (Refereed) Published
Abstract [en]

The thermo-rheological behaviour of bitumen depends largely on its chemical structure and intermolecular microstructures. Bitumen is a complex mixture of organic molecules of different sizes and polarities for which the micro-structural knowledge is still rather incomplete. Knowledge at that level can have great implications for behaviour at a larger scale and will help to optimise the bitumen in its production stage. The present study is focused on understanding the fundamental mechanisms behind the micro-structural phase appearance and the speed or mobility at which they change. To do so, atomic force microscopy was utilised at different temperatures to investigate the phase separation behaviour for four different types of bitumen and co-relate it with the differential scanning calorimetry measurements. Based on the experimental evidences, it was found that the observed phase separation is mainly due to the wax/paraffin fraction presence in bitumen and that the investigated bitumen behaves quite differently. Recommendations are made to continue this research into qualitative information to be used on the asphalt mix design level.

Place, publisher, year, edition, pages
Taylor & Francis, 2013
Keyword
atomic force microscopy, microstructure, phase separation, bitumen, wax
National Category
Infrastructure Engineering
Research subject
SRA - Transport; Järnvägsgruppen - Infrastruktur
Identifiers
urn:nbn:se:kth:diva-123701 (URN)10.1080/14680629.2013.774744 (DOI)000319327300004 ()2-s2.0-84878432765 (Scopus ID)
Note

QC 20130625

Available from: 2013-06-14 Created: 2013-06-14 Last updated: 2017-12-06Bibliographically approved
2. Microscale investigation of thin film surface ageing of bitumen
Open this publication in new window or tab >>Microscale investigation of thin film surface ageing of bitumen
2014 (English)In: Journal of Microscopy, ISSN 0022-2720, E-ISSN 1365-2818, Vol. 254, no 2, 95-107 p.Article in journal (Refereed) Published
Abstract [en]

This paper investigates the mechanism of bitumen surface ageing, which was validated utilizing the atomic force microscopy and the differential scanning calorimetry. To validate the surface ageing, three different types of bitumen with different natural wax content were conditioned in four different modes: both ultraviolet and air, only ultraviolet, only air and without any exposure, for 15 and 30 days. From the atomic force microscopy investigation after 15 and 30 days of conditioning period, it was found that regardless the bitumen type, the percentage of microstructure on the surface reduced with the degree of exposure and time. Comparing all the four different exposures, it was observed that ultraviolet radiation caused more surface ageing than the oxidation. It was also found that the combined effect was not simply a summation or multiplication of the individual effects. The differential scanning calorimetry investigation showed that the amount of crystalline fractions in bitumen remain constant even after the systematic conditioning. Interestingly, during the cooling cycle, crystallization of wax molecules started earlier for the exposed specimens than the without exposed one. The analysis of the obtained results indicated that the ageing created a thin film upon the exposed surface, which acts as a barrier and creates difficulty for the wax induced microstructures to float up at the surface. From the differential scanning calorimetry analysis, it can be concluded that the ageing product induced impurities in the bitumen matrix, which acts as a promoter in the crystallization process.

Keyword
oxidation, Atomic force microscopy, surface ageing, DSC, UV radiation, bitumen
National Category
Civil Engineering
Identifiers
urn:nbn:se:kth:diva-145049 (URN)10.1111/jmi.12122 (DOI)000334107000005 ()2-s2.0-84897963787 (Scopus ID)
Note

QC 20140509

Available from: 2014-05-07 Created: 2014-05-07 Last updated: 2017-12-05Bibliographically approved
3. Investigation of the asphalt mixture morphology influence on its ageing susceptibility
Open this publication in new window or tab >>Investigation of the asphalt mixture morphology influence on its ageing susceptibility
2015 (English)In: Materials and Structures, ISSN 1359-5997, E-ISSN 1871-6873, Vol. 48, no 4, 987-1000 p.Article in journal (Refereed) Published
Abstract [en]

The main objective of this study is to investigate the influence that asphalt mixture morphology aspects have on its overall ageing behavior. Since mixture morphology is controllable, having insight into how the various morphological parameters influence the mixture’s long-term behavior can be of great value to optimize its design, regardless of the individual material properties. To do so, this study is utilizing a new framework to characterize the combined effect of aggregate packing, average air void size, porosity and level of compaction on ageing for a large set of data from different sources of field compacted and laboratory produced asphalt mixtures. The paper also hypothesizes about the mechanisms that lay behind the found influences and how thus mixture design improvements can be made. From all the investigated cases, it was found that the framework can be used to optimize the durability performance of asphalt mixtures. It was also observed that prediction of ageing behavior without considering the influence of mixture morphology may lead to erroneous conclusions and non-optimal mix design.

Keyword
Ageing, Asphalt mixture morphology, Gradation, Average air void size, Aggregate packing
National Category
Civil Engineering
Identifiers
urn:nbn:se:kth:diva-145050 (URN)10.1617/s11527-013-0209-z (DOI)000350889100014 ()2-s2.0-84924220970 (Scopus ID)
Note

QP 201406

Available from: 2014-05-07 Created: 2014-05-07 Last updated: 2017-12-05Bibliographically approved
4. On the Oxidative Ageing Mechanism and Its Effect on Asphalt Mixtures Morphology
Open this publication in new window or tab >>On the Oxidative Ageing Mechanism and Its Effect on Asphalt Mixtures Morphology
2015 (English)In: Materials and Structures, ISSN 1359-5997, E-ISSN 1871-6873, Vol. 48, no 15, 3113-3127 p.Article in journal (Refereed) Published
Abstract [en]

This paper investigates the influence of mixture morphologies and microstructures on oxidative ageing of asphalt mixtures. For this, an oxidative ageing mechanism based on a diffusion–reaction process was developed. Previously, most asphalt oxidative ageing modeling research focused on unidirectional diffusion of continuous oxygen flow through bitumen films, which is far from the actual boundary conditions in asphalt mixtures. For this reason in the current study, a finite element (FE) analysis has been conducted in which 3D mixture morphology was considered. Mixture morphology is the combination of mineral aggregate packing, porosity, air-void distribution and their interconnectivity. One dense and one open graded field asphalt mixture core were scanned with a computerized tomography X-ray scanner. In the analyses, the developed oxidative ageing model was implemented. The FE analysis showed that the effect of the air-void distribution, their interconnectivity and the mineral aggregate packing has a significant effect on the resulting age hardening of the overall mixture. Furthermore, from the microstructural investigation done in this research, strong indications were found that, depending on the bitumen and its conditioning, water soluble thin films are formed due to ageing. This means that ageing and moisture damage are strongly interlinked and this should thus be considered in the design of the asphaltic materials and the prediction of their long term performance. © 2014, RILEM.

Keyword
Oxidative ageing; Finite element analysis; Computerized tomography; Mixture morphology; Atomic force microscopy (AFM)
National Category
Civil Engineering
Identifiers
urn:nbn:se:kth:diva-145129 (URN)10.1617/s11527-014-0385-5 (DOI)000359942300003 ()2-s2.0-84939464968 (Scopus ID)
Note

QS 2014

Available from: 2014-05-09 Created: 2014-05-09 Last updated: 2017-12-05Bibliographically approved
5. Numerical study on the effect of mixture morphology on long-term asphalt mixture ageing
Open this publication in new window or tab >>Numerical study on the effect of mixture morphology on long-term asphalt mixture ageing
2015 (English)In: The international journal of pavement engineering, ISSN 1029-8436, E-ISSN 1477-268X, Vol. 16, no 8, 710-720 p.Article in journal (Refereed) Published
Abstract [en]

Asphalt mixtures with similar percentages of air voids can have different morphologies and can age differently. Prediction of ageing behaviour without considering the influence of mixture morphology may thus lead to erroneous conclusions and non-optimal mix design. This article investigates the long-term field ageing of asphalt mixtures by incorporating mixture morphology. For this, a computational analysis on diffusion-reaction process has been conducted by implementing fundamental mechanism of ageing and conducting a parametric sweep of the morphology. To investigate the ageing gradient along the depth of asphalt mixture, diffusion controlled oxidative ageing on one dense and one open-graded field core was investigated. The proposed model based on the mixture morphology information was able to predict the aged viscosity better than the existing model. As mixture morphology is controllable, having insight into how the morphology parameter influences the mixture's ageing susceptibility can be of great value to its design.

Keyword
mixture morphology, ageing, diffusion, asphalt mixture
National Category
Civil Engineering
Identifiers
urn:nbn:se:kth:diva-145131 (URN)10.1080/10298436.2014.943222 (DOI)000355127000004 ()2-s2.0-84930091391 (Scopus ID)
Note

QC 20150616. Updated from manuscript to article in journal.

Available from: 2014-05-09 Created: 2014-05-09 Last updated: 2017-12-05Bibliographically approved

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