Le, Jia-Liang

High field density is desired for improving the durability of asphalt pavements. This research aims to develop Superpave 5 mixtures (more compactable than traditional Superpave mixtures) by using locally available materials to improve the field density in Minnesota. First, previous projects in Minnesota were investigated. The mean and standard deviation of field density in Minnesota were about 93.5% Gmm and 1.5% Gmm, respectively. Significant correlations were identified between field density and mix design indices, i.e., Ndesign, NMAS, and fine aggregate angularity (FAA). Four traditional Superpave mixtures were then selected and modified to Superpave 5 mixtures by adjusting their aggregate gradations while maintaining the asphalt binder content. Laboratory performance tests were performed to check the mechanical properties of the modified mixtures. The results showed it was feasible to design Superpave 5 mixtures (more compactable mixtures) by adjusting aggregate gradations, and the improved compactability of the mixtures did not adversely affect the performance of the mixtures for rutting, stiffness, and cracking resistance. Therefore, Superpave 5 mixtures can increase field density as well as other performances of asphalt pavements if implemented.

It is widely acknowledged that early detection of material damage and timely rehabilitation can lead to a significant reduction in the life-cycle cost of asphalt pavements. This research investigates the capabilities of damage detection and healing of graphite nanoplatelet (GNP)-taconite modified asphalt materials. The first part of the research is concerned with the application of GNP-taconite modified asphalt materials for damage detection using electrical conductivity. It is shown that, as compared to conventional asphalt materials, the GNP-taconite modified asphalt materials exhibit an improved electrical conductivity due to the electron hopping mechanism. Based on the mathematical analogy between the elastostatic field and the electrostatic field, a theoretical model is derived to relate the change of electrical conductivity to the damage extent of the material. Although, in principle, the material damage can be accessed using the electrical conductivity, the practical application of this method is complicated by the fact that the conductivity is influenced by the moisture content. The second part of the research investigates the damage healing capability of GNP-taconite modified asphalt materials heated by microwave. GNP-taconite modified asphalt materials can effectively absorb the heat generated by the microwave, and the rising temperature can effectively heal the microcracks in the binder. This damage-healing mechanism is verified by a set of semi-circular beam tests. Finally, microwave heating technology is applied to the tack coat system. It is shown that, with microwave heating, the GNP-taconite modified asphalt material can effectively improve the bond strength of the interface of the tack coat system.

Compaction of asphalt mixtures represents a critical step in the construction process that significantly affects the performance and durability of asphalt pavements. In this research effort; the compaction process of asphalt mixtures was investigated using a combined experimental and computational approach. The primary goal was to understand the main factors responsible for achieving good density and was triggered by the success of a recently proposed Superpave 5 mix design method. First; a two-scale discrete element method (DEM) model was developed to simulate the compaction process of asphalt mixtures. The computational model was anchored by a fluid dynamics-discrete element model; which is capable of capturing the motion of aggregates in the viscous binder. The model was then calibrated and validated by a series of experiments; which included rheological tests of the binder and a compaction test of the mixture. It was concluded that the compaction process was significantly influenced by the rheological properties of the fine aggregate matrix and by the sphericity of the coarse aggregates. Finally; the mechanical properties of two high-density mixtures were determined and compared with mechanical properties of mixtures used for MnROAD 2017 National road Research Alliance (NRRA) test sections. It was found that the properties of high-density mixtures as a group were not significantly different compared to the properties of conventional mixtures.

Pothole repairs continue to be a major maintenance problem for many highway agencies. There is a critical need for finding long-lasting; cost-effective materials and construction technologies for repairing potholes. This research effort investigates critical components associated with pothole formation and pothole repair and proposes solutions to reduce the occurrence of potholes and increase the durability of pothole repairs. The components include investigating and documenting pavement preservation activities; experimental work on traditional repair materials as well as innovative materials and technologies for pothole repairs; stress analysis of pothole repairs to identify whether certain geometric configurations are more beneficial than others; evaluating cost analyses to determine the effectiveness of various repair methods. A number of conclusions and recommendations were made. Potholes are mainly caused by the delayed response to timely fixing common pavement distresses. The state of Minnesota has a number of preservation strategies that are available and have been successfully used. Recommendations are made to improve these strategies using documents made available as part of new Every Day Counts; EDC-4; initiative. Currently; there are no required specifications for patching materials. Mechanical testing can be used to select patching materials based on the estimated durability of the pothole repair; such as short-; medium-; and long-term. A number of new materials and technologies are available for more durable solutions for winter pothole repairs; however; they require additional heat source and are more expensive.

This report explores the application of a discrete computational model for predicting the fracture behavior of asphalt mixtures at low temperatures based on the results of simple laboratory experiments. In this discrete element model; coarse aggregates are explicitly represented by spheres; and these spheres are connected by bonds representing the fine aggregate mixture; a.k.a. FAM; (i.e. asphalt binder with the fine-size aggregates). A literature review examines various methods of computational modeling of asphalt materials; as well as the application of nanomaterials to asphalt materials. Bending beam rheometer (BBR) tests are performed to obtain the mechanical properties of the fine aggregate mixture (FAM) at low temperatures. The computational model is then used to simulate the semi-circular bend (SCB) tests of the mixtures. This study considers both the conventional asphalt materials and graphite nanoplatelet (GNP) reinforced asphalt materials. The comparison between the simulated and experimental results on SCB tests shows that by employing a softening constitutive model of the FAM the discrete element model is capable of predicting the entire load-deflection curve of the SCB specimens. Based on the dimensional analysis; a parametric study is performed to understand the influence of properties of FAM on the predicted behavior of SCB specimens.

This research investigates the relationship between the mechanical properties of SFDR and the final performance of the rehabilitated pavements. The study involves two computational tools (MEPDG and MnPAVE) for the simulation of the long-term rutting behavior of pavements containing SFDR layers. Based on the simulations of three existing MnROAD cells, it is shown that for MEPDG the SFDR layer is best modeled as a bounded asphalt layer. To further investigate the applicability of MEPDG, a series of laboratory experiments are performed on cores taken from several sites constructed with different stabilizers including engineered emulsion, foamed asphalt with cement and CSS-1 with cement. The experiments include IDT creep and tension, semi-circular bending, dynamic modulus and disc compact tension tests. The measured mechanical properties are inputted into MEPDG to predict the rutting performance of these sites and it is shown that the simulated rut depth agrees well with the site measurement. However, it is found that MEPDG may suffer a convergence issue for some ranges of the values of the mechanical properties of SFDR. Due to this limitation, MnPAVE was used as an alternative. It was shown that the results simulated by MnPAVE are consistent with those obtained by MEDPG. A parametric study was performed on the three sites constructed with SFDR to determine the relationship between the long-term reliability of the rut performance and the mechanical properties of the SFDR.