Compositional factors influencing permeability of Asphalt

Thesis (MEng)--Stellenbosch University, 2022.ENGLISH ABSTRACT: The current challenge that faces South Africa is the development of balanced asphalt mix designs. There is a tradeoff between permanent deformation and fatigue, based primarily on bituminous binder content. These challenges resulted in a significant number of premature failures in asphalt pavement layers due to moisture ingress and cracking. This study focusses on the potential of an asphalt surfacing layer to be both resistant to moisture ingress and cracking at various bituminous binder contents, within specification limits, thus addressing the main challenges that faces South Africa. The potential of an asphalt surfacing layer to be resistant to moisture ingress was evaluated by conducting High Pressure Permeability (HPP) tests, while the resistance to fracture was evaluated by conducting Semi-Circular bending (SCB) tests. This provided the value of permeability, fracture toughness and fracture energy for an asphalt surfacing layer. The results of these tests were used to evaluate the potential of an asphalt surfacing layer to be both resistant to permeability and fracture. This includes the evaluation of the Bailey Ratios and Dominant Aggregate Size Range (DASR) of the gradation of the asphalt mixture, thus investigating the inherent resistance of the gradation to be permeable. he following conclusions were made throughout this research study: I. Results indicate that an increase in the bituminous binder content, while aggregate gradation remains constant, results in a decreased value in permeability. II. Results indicate that an increase in the bituminous binder content, while aggregate gradation remains constant, results in an increase in the fracture resistance of asphalt. This might be due to the increase in bituminous binder film thickness resulting in an increased mastic stiffness to be obtained in the asphalt mixture. III. Results indicate that, unlike the fracture energy of an asphalt mixture, the fracture toughness reach a maximum value thereafter an increase in bituminous binder content results in a decrease in fracture toughness. Therefore, indicating that an optimal condition has been achieved. IV. The overall results indicate that a link exists between the permeability and fracture resistance of an asphalt surfacing layer. V. The overall results indicate that when an asphalt surfacing mixture is manufactured at the lower end of the specification limit, the asphalt becomes unstable and unpredictable. Such that the asphalt mixture, previously classified as impermeable, to now be reclassified as permeable. High variability in test results were found for the asphalt manufactured at the lower specification limit. Therefore, a situation may arise where the asphalt conforms to specification, but is unable to provide sufficient resistance to moisture ingress or fracture. VI. Results indicate that the proposed Bailey Ratio values for permeability and the porosity values of aggregate fraction ranges can be used to determine the inherent resistance to moisture ingress of an asphalt surfacing gradation. For an asphalt surface layer, this research found a connection between permeability and fracture energy. The insights reached during this research project serve as a foundation for future research on various asphalt combinations. In addition, this research proposes a change to the asphalt design technique that incorporates the Bailey Ratio values for permeability and the porosity values of aggregate fraction ranges. The asphalt design technique has been changed to allow for a new computation of the minimum, target, and maximum bituminous binder contents.AFRIKAANSE OPSOMMING: Geen opsomming beskikbaarMaster