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Influence of aggregates on permanent deformation of asphalt

Time: Thu 2020-12-10 10.00

Location: Via videolänk, Du som saknar dator/datorvana kan kontakta Tom Thöyrä / If you lack a computer or computer skills, please contact Tom Thöyrä, Stockholm (English)

Subject area: Soil and Rock Mechanics

Doctoral student: Bernardita Francisca Lira Miranda , Jord- och bergmekanik

Opponent: Professor Alan Carter, École de technologie supérieure, Université du Québec

Supervisor: Docent Robert Lundström, Byggvetenskap, NCC Industry AB; Professor Stefan Larsson, Jord- och bergmekanik

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One of the most common failure modes for asphalt pavements is the occurrence of permanent deformation on the wheel path. This leads to not only other distresses, such as potholes and stripping, but also decreases substantially road safety and increases maintenance costs. Resistance to permanent deformation of asphalt mixtures depends mostly on the aggregates, bitumen and air void content of the mixture. Mineral aggregates are the major component of asphalt mixtures, representing more than 90 % of the weight. The structure formed by the aggregates within the mixture depend highly on the size distribution, shape and texture of the aggregates. To manipulate the characteristics of the mineral aggregates is usually considered the most economical and convenient way to improve asphalt mixture performance.

The study presented in this thesis has investigated the influence of aggregate particles on the resistance to permanent deformation of asphalt mixtures. In the theoretical part, some pre-existing models have been considered and then further developed to obtain a framework able to characterise asphalt mixtures based on their aggregate gradation and relate these properties to performance. In the gradation-based framework two structures are identified: the primary structure, which provides the load-bearing capacity, and the secondary structure, which contributes with stability and durability. The experimental part has focused mainly on testing the assumptions included in the developed framework with regards to aggregates size distribution, gradation, and shape. Two series have been tested, one with varying aggregate gradation and one with varying flakiness index. All bitumen parameters and aggregate source have been kept constant for each series.

Results from the first series show that the aggregate gradation, especially the coarse fraction, has a significant influence on the resistance to permanent deformation as measured with the wheel tracking test. Mixtures where even tested with the cyclic compression test, where results were mostly able to identify good and bad, but no clear relationship to aggregate gradation. In the second series, the influence of particle shape was taken into account by varying the flakiness index of five, otherwise, identical mixtures. Results from the wheel tracking test showed no influence of flakiness index on the resistance to permanent deformation. Additionally, there was no higher breakage observed for the flaky particles than for the non-flaky particles.

The gradation-based framework has been applied using spheres representing aggregate particles. Even though the primary structure showed a certain correlation with permanent deformation, this was not statistically significant. Instead, the empirical findings in this study show a significant linear relationship between resistance to permanent deformation and the total coarse fraction of the mixtures, independent of its flakiness index.