Asphalt

Modern asphalt mixtures are usually a combination of various materials from different sources, including reclaimed asphalt pavement (RAP) and recycling agents (RAs), and are used to attain sustainable growth. However, the lack of a well-established method for determining compatibility between various sources and types of virgin binder, aged binder within RAP, and RAs has been a major impediment in current asphalt material selection and specification. Therefore, the objective of this study was to evaluate various binder and mixture testing methods to characterize the compatibility between complex components of asphalt mixtures, specifically from the perspective of assessing their cracking performance. The primary evaluation consisted of laboratory-prepared materials that used three RAP sources, three asphalt binders (one PG 58–28, two PG 64–22), and two RAs (petroleum-based and bio-oil-based) for both binder and mixture characterization. The binder tests consisted of rheological characterization using the dynamic shear rheometer (DSR) and thermal analysis using the differential scanning calorimeter (DSC), whereas the mixture tests included complex modulus (E*), semi-circular bend (SCB), and disk-shaped compact tension (DCT) tests. The results indicated that the rheological characterization of asphalt binder and mixture may not adequately capture the incompatibility between virgin binder, RAP, and Ras. However, binder DSC analysis and mixture fracture tests have shown promising results for evaluating the compatibility of various mixture components. Therefore, the findings of this study provide agencies with a framework to select the most compatible component materials from various sources for their projects.

Asphalt milling is an essential construction activity. It requires concentrated high-intensity applications of force to the existing pavement to remove the asphalt material. The impact that the induced stresses have on the pavement below the mill line is unknown. Consequently, selected milling parameters rarely consider the impact the milling may have on the remaining layers. This study evaluates milling parameters to provide an enhanced understanding of their impacts on the layer directly below the mill line. Five parameters were evaluated and include the time between milling and post-mill overlay construction, existing pavement structure, temperature while milling, depth of milling relative to layer interface, and rotor speed. Pre- and post-milling cores were collected adjacent to each other and evaluated for physical and mechanical properties. The measured properties of the pre- and post-milling cores were statistically compared to determine the impact of milling operations on the integrity of the asphalt concrete immediately below the mill line. Based on the results from this study, it was determined that leaving milled pavement exposed for longer periods of time or milling at cooler temperatures can cause a decrease in the strength of the layer below the mill line and a decrease in the expected pavement life of the new pavement structure. The depth of milling or changing the rotor speed while milling did not have significant impacts on the layer directly below the mill line. In consideration of the results of this study, research with a wider variety of pavements and milling conditions is warranted.

The purpose of this three month study has been to determine whether it is feasible to simulate an asphalt pavement section using the elastic theory. This has been done by first obtaining samples from Investigation 183, Test Section 102 and determining the stress-strain properties of the various layers using the repeated load triaxial test under various conditions. Appropriate moduli have then been put. into an elastic layered system which is used to calculate stresses and strains within the system. The Benkelman beam deflections measured in the field have been simulated in this manner and the comparison between computed and measured deflections is used to show whether the elastic theory simulates a flexible pavement. The possibility of determining equivalencies between stabilized base courses and granular base courses are explored as are the use of other parameters for design purposes.

This study evaluates methods used to reduce the amount of reflective cracking in new asphalt concrete overlays on existing PCC pavements. Methods being studied include asphalt-rubber interlayers as stress absorbing membrane interlayers (SAMI), cracking of PCC pavements before overlay and variable thicknesses of asphalt concrete overlays. To date only increased thickness of asphalt concrete overlays has significantly retarded reflective cracks. This report is an interim report and additional information is expected with time.

Cold-recycling processes such as Cold in Place Recycling (CIR) and Cold-Central Plant Recycling (CCPR) offer opportunities for innovation through the use of recycling additives (RAs). The objective of this project was to evaluate the efficacy of rejuvenating asphalt emulsions in the CIR and/or CCPR process in terms of potential performance benefits relative to existing stabilization options. An experimental matrix was designed to include several of the mix design factors known or thought to control mix performance. Rejuvenating asphalt emulsions containing both Bio-based and petroleum-based RAs were produced and compared to a control engineered emulsion with a proven field history of performance. Inclusion of RAs did not negatively impact mixture stability or the mechanism of strength, while generally improving the CT Index of the tested cold recycled mixes compared to the use of a similarly graded control emulsion. The concept of utilizing a “Balanced Mix Design” approach was explored to quantify the performance attributes of these materials. Mixture stability at 40 °C and mixture IDEAL CT Index at 25 °C were ultimately selected as the performance tests used in the balanced mix design framework. To aid rapid implementation of the results, the recommendations were also written in the form of a specification amendment document, included in the appendix of this report.

Two methods for calculating the aggregate surface areas, the Surface Area Factor and Index methods, are discussed in this research and the results are further used to compute an average asphalt film thickness in asphalt mixtures. Field performance data from six Minnesota routes and MnROAD, including both coarse and fine gradations, were analyzed to determine significant correlations between asphalt film thickness values and the performance of asphalt mixtures. The analysis showed that the asphalt film thickness is a significant factor affecting the rutting performance for asphalt mixtures. However, the pavement performance is also affected by many other factors such as traffic level and surrounding environment. More research work is needed to investigate the relationship between the asphalt film thickness and the other performance parameters of asphalt mixtures such as fatigue cracking.

This chart is part of report 2019-26 "Cost/Benefit Analysis of the Effectiveness of Crack Sealing Techniques," published in 2019

This paper details the efforts regarding the construction and analysis of three stabilized full-depth reclamation (SFDR) sections (cells 2, 3, and 4) constructed at the Minnesota Road Research Facility (MnROAD) on I-94 in 2008. Three test sections with varying pulverized asphalt concrete/granular base ratios were constructed in order to study the performance of full-depth reclaimed (FDR) pavements stabilized with engineered emulsion. Emulsion content and base structure varied between test sections. Each test section was designed for 3.5 million ESALs over a period of five years. The sections have been subjected to approximately 2.2 million ESALs as of 30 June 2012. Strain gages were embedded at the bottom of the hot-mix asphalt (HMA) and SFDR layers in each test section to measure responses. The strain gages indicate that both the bottom of the HMA and SFDR layers are subject to horizontal tensile strain from falling weight deflectometer (FWD) and heavy vehicle loading. Pavement performance in terms of rutting, cracking, and international roughness index (IRI) has been measured periodically. The results indicate that all three cells are performing well. The only crack in the three cells exists in cell 3, IRI values are well within the acceptable range, and rutting, while progressing, is still acceptable. Finally, the paper concludes with modeled responses and performance predictions from DARWinME and BISAR. Model predictions indicate that a SFDR layer will provide greater structural benefits and increased performance than similar structures with unstabilized FDR or granular base layers. Content Note: This is the author’s version of a work that was accepted for publication in the Transportation Research Record: Journal of the Transportation Research Board, Issue Number: 2368, Publisher: Transportation Research Board ISSN: 0361-1981. The final version can be found at https://doi.org/10.3141/2368-11.

There has been a sustained effort in applying fracture mechanics concepts to crack formation and propagation in bituminous pavement materials. Adequate fracture resistance is an essential requirement for asphalt pavements built in the northern part of the US and Canada for which the prevailing failure mode is cracking due to low-temperature shrinkage stresses. The current Superpave specifications address this issue mainly through the use of strength tests on unnotched (smooth boundary) specimens. However, recent studies have shown the limitations of this approach and have suggested that fracture mechanics concepts, based on tests performed on notched samples, should be employed instead. Research in progress at University of Minnesota investigates the use of fracture mechanics principles to determine the low-temperature fracture properties of asphalt mixtures. This paper presents a testing protocol that allows obtaining multiple measurements of fracture toughness as a function of crack propagation based on the compliance method to measure crack length. An increase in fracture toughness with crack length is observed, which is consistent with the behavior displayed by other brittle materials. The plateau of the curves may be representative of the asphalt concrete resistance to fracture because the initial values can be significantly influenced by the presence of the inelastic zone at the crack tip. Content Note: This is the author’s version of a work that was accepted for publication in the Transportation Research Record: Journal of the Transportation Research Board, Issue Number: 1789, Publisher: Transportation Research Board ISSN: 0361-1981. The final version can be found at https://doi.org/10.3141/1789-21.

This report reviews the asphalt pavements in northern Minnesota on the criteria of density, smoothness, and mixture and gives recommendations for improvements.