David Newcomb

Development of Mechanistic-Empirical Pavement Design in Minnesota

The next AASHTO guide on pavement design will encourage a broader use of mechanistic-empirical (M-E) approaches. While M-E design is conceptually straightforward, the development and implementation of such a procedure are somewhat more complicated. The development of an M-E design procedure at the University of Minnesota, in conjunction with the Minnesota Department of Transportation, is described. Specifically, issues concerning mechanistic computer models, material characterization, load configuration, pavement life equations, accumulating damage, and seasonal variations in material properties are discussed. Each of these components fits into the proposed M-E design procedure for Minnesota but is entirely compartmentalized. For example, as better computer models are developed, they may simply be inserted into the design method to yield more accurate pavement response predictions. Material characterization, in terms of modulus, will rely on falling-weight deflectometer and laboratory data. Additionally, backcalculated values from the Minnesota Road Research Project will aid in determining the seasonal variation of moduli. The abundance of weigh-in-motion data will allow for more accurate load characterization in terms of load spectra rather than load equivalency. Pavement life equations to predict fatigue and rutting in conjunction with Miner’s hypothesis of accumulating damage are continually being refined to match observed performance in Minnesota. Ultimately, a computer program that incorporates the proposed M-E design method into a user-friendly Windows environment will be developed.

Perpetual pavement design for flexible pavements in the US

Long lasting asphalt pavements have been designed and constructed in the United States for many decades. Pavements that have avoided structural failure in the presence of heavy traffic and which require only periodic resurfacing are advantageous in terms of minimizing life cycle cost and user delays. Such pavements are known as perpetual pavements and the keys to their performance have become evident through a number of studies. These pavements have features that allow them to minimize critical responses to loading. Furthermore, recent studies have confirmed the likelihood of strain thresholds below which damage does not seem to accumulate in pavement structures. A design procedure (PerRoad 2.4) has been developed that encompasses these principles, and performs a Monte Carlo simulation for a probabilistic analysis of critical pavement responses. The designer can then select a pavement section that minimizes the risk of structural failure.

INCORPORATION OF RELIABILITY INTO MECHANISTIC-EMPIRICAL PAVEMENT DESIGN

Pavement thickness design traditionally has been based on empiricism. However, mechanistic-empirical (M-E) design procedures are becoming more prevalent, and there is a current effort by AASHTO to establish a nationwide M-E standard design practice. Concurrently, an M-E design procedure for flexible pavements tailored to conditions within Minnesota has been developed and is being implemented. Regardless of the design procedure type, inherent variability associated with the design input parameters will produce variable pavement performance predictions. Consequently, for a complete design procedure, the input variability must be addressed. To account for input variability, reliability analysis was incorporated into the M-E design procedure for Minnesota. Monte Carlo simulation was chosen for reliability analysis and was incorporated into the computer pavement design tool, ROADENT. A sensitivity analysis was conducted by using ROADENT in conjunction with data collected from the Minnesota Road Research Project and the literature. The analysis demonstrated the interactions between the input parameters and showed that traffic weight variability exerts the largest influence on predicted performance variability. The sensitivity analysis also established a minimum number of Monte Carlo cycles for design (5,000) and characterized the predicted pavement performance distribution by an extreme value Type I function. Finally, design comparisons made between ROADENT, the 1993 AASHTO pavement design guide, and the existing Minnesota design methods showed that ROADENT produced comparable designs for rutting performance but was somewhat conservative for fatigue cracking.

Calibration of Flexible Pavement Performance Equations for Minnesota Road Research Project

As mechanistic-empirical (M-E) pavement design gains wider acceptance as a viable design methodology, there is a critical need for a well-calibrated design system. Calibration of the pavement performance equations is essential to link pavement responses under load to observed field performance. A field calibration procedure for asphalt pavements that incorporates live traffic, environmental effects, observed performance, and in situ material characterization was developed. The procedure follows the M-E design process, iterating the transfer function coefficients until the performance equation accurately predicts pavement distress. Test sections from the Minnesota Road Research Project were used to demonstrate the calibration process, and fatigue and rutting performance equations were developed. It is recommended that further calibration studies be undertaken with this methodology, possibly by using sections from the Long-Term Pavement Performance project.

Properties of Foamed Asphalt for Warm Mix Asphalt Applications

This report presents proposed AASHTO standard test methods for measuring performance-related properties of foamed asphalts and designing foamed asphalt mixes with satisfactory aggregate coating and workability. The objectives of this project were to determine key properties of foamed asphalt binders that significantly influence the performance of asphalt mixtures and develop laboratory protocols for foaming of asphalt binders and laboratory mixing procedures. The production and performance-related properties of foamed asphalt were investigated through a series of laboratory and field experiments. A key finding of the research is that the foaming characteristics of an asphalt binder are primarily affected by its source (i.e., its crude oil slate), the production date for a given refinery and crude oil slate, and polymer modification. A laser-based method was developed to measure parameters associated with the expansion and collapse of foamed asphalt. A digital photographic approach was developed to characterize the size, distribution, and surface area of bubbles formed during production of foamed asphalt. Methods were also identified for determining a coatability index for foamed asphalt and the workability of mixes produced with foamed asphalt. A foamed asphalt mixture design procedure was developed to identify the optimum water content for coating and workability. Finally, the utility and effectiveness of these various methods were verified through their application to foamed asphalt binder and mix produced in full-scale asphalt mix plants. This report fully documents the research. Four appendixes are included: Influence of Binder Properties on Binder Foam Expansion; Draft Commentary on Guidelines Proposed for Revising Appendix to AASHTO R 35; AASHTO Style Standards; and Field Foaming Data Acquisition Form.

Analytical Predictions of Seasonal Variations in Flexible Pavements: Minnesota Road Research Project Site

The mechanistic analysis and design of flexible pavements is very dependent on knowledge of traffic loading, materials, and climatic factors. Seasonal variation of climate factors such as temperature, temperature history, and precipitation affects the subsurface conditions of the pavement layers, including the in situ temperature, moisture content, and state of moisture. In turn, these subsurface conditions have a direct relationship with the pavement strength and stiffness, causing seasonal variations in both strength and pavement layer moduli. Many agencies are now moving toward a mechanistic-empirical pavement design, in which the design inputs include, as a minimum, the seasonal variations in pavement layer moduli. The ability to analytically predict and quantify the climatic effects on pavement strength and stiffness has been investigated by numerous researchers, but few comparisons with measured field data have been completed, because of a lack of pavement sites with extensive arrays of monitoring instrumentation. Detailed is a comparison between field results and predictions obtained from an analytical tool, called the enhanced integrated climate model (ICM). The climatic factors used as inputs into the model include temperature, rainfall, wind speed, and solar radiation. The ICM is used to predict seasonal variations in temperature, moisture content, and layer moduli at two representative flexible pavement test sections at the Minnesota Road Research Project site.

CHARACTERIZING SEASONAL VARIATIONS IN FLEXIBLE PAVEMENT MATERIAL PROPERTIES

As agencies progress toward mechanistic-empirical flexible design procedures, it is critical to quantify the relationships between climate factors and the resulting seasonal variations in pavement layer moduli for a given region. This was investigated for flexible pavement structures and climate conditions specific to Minnesota. The objective of this study was to quantify the relationships between climate factors, subsurface conditions, and pavement material properties for use in a mechanistic-empirical design procedure that reflect conditions specific to Minnesota. The approach used to establish these relationships may suggest possible directions for similar studies in other regions. The data used in this study were obtained from the Minnesota Road Research Project, located on Interstate 94 in central Minnesota. The extensive instrumentation, on-site weather station, and deflection testing performed at this facility provide valuable environmental and pavement response data. The results show that the maximum stiffness for the pavement layers occurs in the winter, and the minimum stiffness occurs at different periods in a typical year for the different layers. The asphalt concrete modulus is at a minimum in the summer when temperatures are high. The base layer modulus is at a minimum in the spring thaw period, and the subgrade layer modulus is at a minimum in the late spring and summer months. The seasonal variations in pavement layer moduli are used to establish seasonal factors for each layer which describe the modulus cycling in a typical year. These factors may be applied to a mechanistic-empirical design procedure specific to Minnesota.

Short-Term Laboratory Conditioning of Asphalt Mixtures

This report develops procedures and associated criteria for laboratory conditioning of asphalt mixtures to simulate short-term aging. The report presents proposed changes to the American Association of State Highway and Transportation Officials (AASHTO) R 30, Mixture Conditioning of Hot-Mix Asphalt (HMA), and a proposed AASHTO practice for conducting plant aging studies. The report will be of immediate interest to materials engineers in state highway agencies and the construction industry with responsibility for design and production of hot and warm mix asphalt.

Mechanistic-empirical methodology for the selection of cost-effective rehabilitation strategy for flexible pavements

Abstract A well-planned rehabilitation approach helps agencies to optimise the allocation of annual investment in pavement rehabilitation programs. Currently, many agencies are struggling with the selection of an optimal time-based and cost-effective rehabilitation solution to address the long-term needs of pavements. This study offers the use of a mechanistic-empirical methodology to develop a series of time-based rehabilitation strategies for high traffic volume flexible pavements located in Oklahoma. Six different pavement family groups are identified in the state, and comprehensive evaluation of existing pavements are conducted through analysis of falling weight deflectometer data and performance measures available in Oklahoma Pavement Management System database. The inadequacy of performance measures to fully characterise the condition of existing pavements are indicated, and damage factor determined from FWD data are suggested as trigger factor to select rehabilitation candidates. Three levels of rehabilitation activities including light, medium and heavy are considered as potential alternatives for rehabilitation candidates. A mechanistic-empirical methodology is employed to obtain an estimate of the performance of rehabilitation and extension in service lives of pavements. Also, an assessment output matrix is developed, which can be served as a supplemental tool to help the decision-makers in the highway agency with the rehabilitation related decision-making process. Cost-effectiveness of rehabilitation alternatives is determined through life cycle cost analysis, and three time-based renewal solutions are developed for pavement family groups that are in need of rehabilitation.

Selection and preliminary evaluation of laboratory cracking tests for routine asphalt mix designs

Cracking has become a primary mode of distress in recent years that frequently drives the need for rehabilitation of asphalt pavements. Meanwhile, asphalt mix designs are becoming more and more complex with the increasing uses of recycled materials, recycling agents, binder additives/modifiers, and multiple warm mix asphalt technologies. Thus, there is an urgent need to identify reliable cracking tests that can be used for routine mix design to eliminate brittle mixes. This paper critically reviewed cracking mechanisms and laboratory tests. A total of 12 cracking tests were discussed at a cracking test workshop held as part of the National Cooperative Highway Research Program Project 9-57. Seven cracking tests were selected for further laboratory evaluation and field validation. Four of the simpler cracking tests from the seven were evaluated in this paper, these being the Texas Overlay Test (OT), Disk-shaped Compact Tension (DCT) test, Semi-Circular Bend test from the Louisiana Transportation Research Center (SCB-LTRC), and SCB test at room temperature from Illinois (SCB-IL). A laboratory sensitivity study was performed, and the results showed that all four cracking tests were generally sensitive to asphalt mix components. However, there were some concerns with the DCT, SCB-LTRC, and SCB-IL. Both the DCT and SCB-IL were found to be not sensitive to asphalt binder content; and both the DCT and SCB-LTRC showed an unexpected increase in cracking resistance when adding RAS to the mix. Additionally, two sets of field test sections were used for preliminary validation of these four cracking tests. It was found that the OT, DCT, and SCB-IL provided rankings which matched the measured field performance for the two sections on US62, Texas; and the OT and SCB-LTRC were valid for six APT test sections. Further validation with different mixes, traffic, and climate is needed.