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This report presents the results of a study on asphalt pavement (RAP) mixtures. The report includes a literature review on RAP and a summary of past research. The research focuses on the use of resilient modulus and complex modulus testing to compare mixtures compacted with only virgin materials to those compacted with varying amounts of RAP. The addition of RAP makes the mixture stiffer, as evidenced by an increase in resilient modulus and complex modulus measurements, according to the research. The addition of RAP also decreases the mixture phase angle, which corresponds to an increase in the elastic properties and a decrease in the viscous mixture properties. The report includes recommendations regarding the proper temperatures and loading frequencies for use in the complex modulus test. It also gives the percentage of RAP and the respective asphalt binder grade necessary to yield the stiffness similar to a virgin mixture.

This report presents the results of an investigation into the use of the Superpave asphalt mix design methodology at the local government level in Minnesota. In the project, researchers combined low-cost natural sand with locally available aggregates from four sources: limestone, quartzite, and partially crushed river gravel, and granite. They evaluated coarse and fine aggregate gradations, along with the use of two asphalt grades. It was difficult to achieve the Superpave volumetric requirements of voids in mineral aggregate (VMA) and voids filled with asphalt (VFA) at 4% air voids, regardless of the gradation. A target air void content of 3% satisfied the VFA requirement, even though the VMA requirement could not be fulfilled. The fine aggregate gradations produced densities indicating that the mixtures might be tender during construction, but not necessarily be susceptible to rutting. The coarse-graded mixtures did not show the tenderness problem, but did show that they might be susceptible to rutting. Resilient modulus testing showed little or no difference in the mixtures, regardless of aggregate source or gradation. The difference in resilient modulus due to asphalt grade was apparent only at the intermediate temperatures, and not at the highest or lowest test temperatures. Moisture sensitivity testing showed that all the mixtures studied had adequate durability. Indirect tensile creep and APA rut testing indicated that resistance to low temperature cracking and rutting may be improved by decreasing the lower PG binder grade and increasing the upper PG binder grade.

This research project investigates the level of Voids in the Mineral Aggregate (VMA) in Minnesota paving projects. Researchers analyzed 10 paving projects from 1996 to determine if a VMA decrease occurred, the magnitude of the decrease, and the potential causes of the decrease. Potential causes include the generation of fines, high-production temperatures, and long storage or cure times. Three of the 10 projects had a VMA decrease of 1.9 or more. These three projects also had the highest plant temperatures and fairly long storage times, which makes increased asphalt absorption a likely cause of the VMA decrease. Five projects showed a moderate drop in VMA. Most had some increases in fines, and some had moderately high plant temperatures and storage times. The two projects with little or no change in VMA had very little change in gradation, and moderate to low plant temperatures and storage times.

This research was conducted on the behavior of materials at the Minnesota Road Research Project (Mn/ROAD), the construction of which was completed in 1994. Falling-Weight Deflectometer (FWD) tests were conducted on the test sections at all stages of pavement construction (pre-base, post-base, and post pavement). Deflection values were highly variable due to variability in surface condition, soil moisture content, density, and stress-dependent effects. In general, backcalculated subgrade moduli tended to increase with increasing FWD sensor offset (decreasing stress). Post-base and post-pavement testing also indicated an apparent increase in subgrade modulus relative to pre-base values. Moduli values were also determined for granular base results. These results were compared to Dynamic Cone Penetrometer (DCP) penetration index values as well as to resilient moduli values from tests conducted on subgrade and granular base materials in the lab. The backcalculated subgrade soil modulus values compared well with the laboratory resilient modulus values, but no correlation was seen between backcalculated moduli and penetration index values. The results of this testing will provide a baseline for future analysis of the test sections at Mn/ROAD.

Poor compaction can lead to early deterioration of an asphalt pavement. It often happens when paving occurs during adverse weather conditions. Yet, in Minnesota, paving must often occur under adverse conditions. A new tool now simulates the cooling of an asphalt mat behind the paver under a variety of environmental conditions. The software, PaveCool Version 2.0, offers users insights into how adverse climate conditions will affect their ability to produce a durable, quality road surface. Users input the type of existing surface, type of asphalt mix, and weather conditions. The output shows a cooling curve with recommended compaction starting and stopping times. Field tests confirm the value of this program as an aid to cold weather paving. A Windows program, PaveCool 2.0 runs on laptop computers (Windows 95, 98, or NT required). This report documents the study of thermal properties and compactibility of hot-mix asphalt, related laboratory tests on the thermal diffusivity and thermal conductivity of hot-mix asphalt at typical compaction temperatures, a literature review, and testing results. It also includes a copy of the PaveCool Version 2.0 software.

In Mechanistic-empirical (M-E) pavement design, the Monte Carlo method has proven to be an effective means of determining reliability. One drawback is the amount of computing time required to carry out a simulation. Performing a Monte Carlo simulation on a complex load spectrum can require hours on a typical desktop computer. The Minnesota Department of Transportation is developing a shortcut method for estimating reliability in its J90 MnPAVE flexible pavement design program. In this program, variability in the layer thicknesses and moduli are expressed as coefficients of variation (CV). Development of the shortcut method involves running a large number of pavement design simulations to generate damage factors (using Miner's Hypothesis) and reliability values (using the Monte Carlo method). For a given set of CV values, there is a strong correlation between the damage factor and reliability. For the two asphalt fatigue models tested, this correlation exists regardless of the number and thickness of pavement layers, layer moduli, number of seasons, and number of axle configurations. For the rutting model the addition of more seasons and loading configurations increases scatter in the data. A family of curves can be generated to include the range of expected CV combinations, and the resulting equations can be used to estimate the reliability of a pavement design based on a single damage calculation.

Mn/DOT is in the process of upgrading its Mechanistic-Empirical asphalt pavement design software (MnPAVE). In the process of evaluating the existing software, some problems with the reliability method (allowed repetitions method) were encountered. This paper presents a description of these problems and proposes the implementation of a new reliability method (damage factor method). In order to compare the reliability methods, eight pavement designs corresponding to a previous study were analyzed. In this study, two low-volume and two high volume pavements designed using FLEXPAVE were evaluated with the existing program. Simulations were conducted to determine the damage factors and reliabilities of the FLEXPAVE designs. A memo to the M/E Design Implementation Group at Mn/DOT describes the original study (see Appendix A).

This paper is a review of development of the Automated Laser Profile System (ALPS) designed for use at the Minnesota Road Research Project (MnROAD). The initial development was designed to replace the six-foot straight edge rutting measurements conducted on the hot mix asphalt test cells for both the mainline and low volume road (LVR). This paper discusses not only the development of the equipment but also covers other possible uses of this equipment along with an initial comparison of the ALPS data compared to the six foot straight edge measurements at MnROAD.