Minnesota Department of Transportation

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NRRA Flexible and Intelligent Construction Technologies Teams

Asphalt Pavement Milling Best Practices through Enhanced Understanding of Milling Process

Status: Active
Contract #: 1036343WO2
Project Start Date: February 1, 2021
Project End Date: October 31, 2023

Project objectives

A majority of pavement construction projects that are currently underway in the NRRA members states (as well as the United States in general) are related to pavement rehabilitation and preservation, a large share of which are mill and overlay (mill-and-fill) projects. There is a growing concern among many state and local agencies that many mill-and-fill asphalt pavements do not last for their intended design life. This has been recognized by NRRA; several NRRA efforts are underway to enhance performance of asphalt mixtures as well as to develop tools to better predict and quantify performance properties of asphalt overlays. However, at present there is a lack of understanding with respect to the extent of damage that current construction processes may induce to existing pavement during rehabilitation stages.

Milling is an unavoidable process to enable asphalt pavement construction without altering roadside structure locations, geometric features, and the retrieval of reclaimed asphalt pavement (RAP) for recycling. A key question that has never been addressed is: how does milling affect the pavement layers that remain after milling? Milling is a very high energy activity in which the asphalt mix is removed by very high impact forces within a very short period. This type of rapid and high stress loading has never received any attention from researchers. A review of milling specifications shows that the only concerns that have been addressed so far are that of safety, environmental factors, delamination, and accuracy. The depth to milling is a rule of thumb decision, primarily based on project goals and economics, with minimal consideration of existing pavement properties.

The objectives of the proposed innovative research are to:

  1. quantitatively determine changes to physical and mechanical properties of asphalt pavement from milling operations;
  2. identify factors that have the most significant impacts on milling induced changes to the existing pavement; and,
  3. develop best practices guidance for use in milling, design and construction specifications.

Project tasks

Task 1: State of the practice survey and review

In this task, the research team will conduct a thorough review of the current milling specifications with an emphasis on the current state of practice with respect to selection of appropriate milling depths as well as parameters associated with equipment and construction operations. Review will also include a summary and list of the most popular milling equipment and their features related to the expected damage on the milled pavements (such as, weight, dimensions, wheel/track types, operational speed). Specifications and construction guidance documents for State transportation agencies within United States and industry guidelines will be reviewed and findings will be summarized. Detailed information on each NRRA member agency’s practices will be reviewed through surveys and if necessary, by follow-up interviews with agency personnel. A state-of-the-art review will be conducted to determine significant factors that have been identified by agencies, contractors, researchers, and equipment manufacturers that are linked to the condition of post-milled pavement, especially material in the immediate vicinity of the milling line. A survey will also be developed and sent to various contractors, consultants, and equipment manufacturers (especially those that are associate members of NRRA) to document current industry approaches for determining milling depth and operational parameters (such as milling speed, teeth pattern and pavement temperature.).

The list of parameters that should be considered in the selection of field projects, material sampling, and field and lab testing will be determined in this task. This list will be finalized with input from the project Technical Advisory Panel (TAP) and used to guide the activities in Task 2. The outcome of this task will help researchers establish the current state of the practice and the state of the art with respect to asphalt pavement milling operations and its impacts on existing pavement. Synthesis of various agency practices as well as industry preferences will aid research team in the later tasks on development of guidelines and recommendations.

Task 2: Project identification and testing and sampling plan development

In this task, suitable projects for inclusion in this study will be identified and, testing and sampling plans for both in-situ and laboratory testing will be developed. An initial list of significant parameters that will be considered in selection of sufficiently varied projects include:

  • Age of pavement;
  • Structure of existing pavement (lifts and their thicknesses);
  • Ability to obtain ride-quality and texture information on the milled pavement;
  • Timing between milling and post-mill overlay construction;
  • Type and condition of pavement base/subbase/subgrade;
  • Interface strength between various asphalt lifts of pavement (light, moderate, heavy tack coat and/or tack coats of emulsion and asphalt).
  • Depth of milling (especially proximity of existing pavement layer interface to milling line);
  • Operational/Equipment Parameters: Drum diameters, speed (rotational speed of drum and longitudinal speed), teeth wrap patterns, teeth density, teeth condition etc.; and,
  • Time of milling with respect to pavement temperature.

Based on the significant parameters identified in Task 1, the research team will work with NRRA member agencies and associate members and the project TAP to identify six to eight suitable milling projects. Emphasis will be given to mill and overlay projects. Full-depth cored specimens from these projects will be sought to identify the thicknesses of various lifts in the existing pavements as well as to assess the quality of bond between various pavement lifts. Based on these evaluations, four pavement projects will be selected for sampling and testing. Mix design and construction data for these projects will be obtained from the respective agencies.

A 1,000 ft. long study section will be identified for each project. These sections will be selected to represent uniform conditions from the perspective of pavement and construction operations. The following in-situ testing and lab sample coring locations are currently proposed (these will be refined during the current task and on the basis of information gathered from Task 1):

  • In-situ testing: In-situ testing to determine properties of pavement before and after milling is proposed to follow a plan similar to that often used for QA specimen procurement. A grouping of two test locations in roughly a diagonal pattern starting at the middle of the lane and extending into the wheel path will be adopted. Testing will be conducted at four locations along the length of the study section, resulting in 16 tests per project. For some techniques (e.g., density profiling system), continuous measurements will be taken around each group’s location. Testing will be conducted before and after milling. Each test location will be marked using high resolution GPS measurements.
  • Coring: Cores for laboratory evaluation are planned to be taken before and after milling. Cores will be taken as close to in-situ test locations as practically possible. At present eight cores (two from each of the four locations within study section, one near middle of lane and one in wheel path) are planned to be obtained before milling as well as after milling.

Research team will also work with field project owner agency and construction contractor to gather data with respect to the truck types and weights during the construction process. Information regarding the condition of pavement foundation (base, sub-base and subgrade) will be gathered for each field study location.

The in-situ and laboratory testing methods to quantitatively assess the impacts of milling operation on mechanical integrity of the milled asphalt pavement will be undertaken in the next task.

  • Date due: May 31, 2022
  • Deliverable: A summary of the field project list as well as provide detailed testing and sampling plans for in-situ and laboratory testing efforts.

Task 3: Field data measurement and sampling

This task will implement in-situ testing and sampling plans developed in Task 2. The in-situ testing will focus on use of non-destructive techniques that can result in development of innovative assessment methods with respect to milling of asphalt pavements. Project team plans to adopt dielectric measurements and deflection testing as primary in-situ measurement methods (pending Task 1 findings). Dielectric measurement using density profiling system (DPS) is proposed with two objectives: assess density changes to asphalt material immediately below mill line and to evaluate water absorptivity. Water absorptivity will be evaluated by saturating a small portion of the pavement and taking subsequent measurements with time to determine the extent of moisture retention, which is readily apparent in dielectric measurements. Deflection measurements (either using falling weight deflectometer (FWD) or light weight deflectometer (LWD)) to assess stiffness of pre- and post-milled asphalt layers. The purpose of deflection testing is to quantify the degree of material integrity change, if any, due to milling. The portable seismic pavement analyzer (PSPA) will also be considered (pending task 1 and 2 outcomes) to non-destructively measure post-milling stiffness changes in HMA layers as well as to gain insight into the extent of void generation from milling processes. Research team will also coordinate with responsible agency and construction contractor on the sampling of cored specimen from the study sites.

  • Date due: October 31, 2022
  • Deliverables: A comprehensive task report will be developed and delivered that will present in-situ measurement data collected during this task.

Task 4: Laboratory testing

This task will focus on conducting laboratory testing on the cored specimen that will be procured in Task 3. Testing on samples taken before and after milling will include (for cored samples taken before milling, the material above milling line will be removed by sawing): (1) bulk density and air voids measurement; (2) laboratory permeability test; (2) resilient modulus test; and, (4) indirect tensile strength test. The first two physical property measurements will primarily assess the change in the void structure or asphalt mixtures from the impact energies of milling operations. Excessive permeability could pose significant durability challenges due to the potential for higher water absorption and increased freeze-thaw damage. These tests will also provide insight to researchers on quantifying the potential for material/mastic loss as a result of milling. The two mechanical property tests were selected for their simplicity and wide-spread usage as well as applicability to detect material changes in terms of stiffness and strength loss. The results of resilient modulus will be utilized within pavement analysis to quantify the changes in pavement capacity (in terms of allowable traffic levels) due to milling (if any).

  • Date due: December 31, 2022
  • Deliverable: Laboratory testing results on filed cores from study site

Task 5: Analysis of data

The analysis of field measurement and laboratory test results will be undertaken in this task. The task will also include finite element modelling to supplement the data gathered from Task 3 and 4. Analysis of lab and field data will entail two main components. First, statistical hypothesis testing will be conducted using analysis of variance methods (student’s t test and Tukey’s honest significant factor test) and Pearson’s correlation matrix method. The emphasis of the statistical analysis will be to assess the similarity and dissimilarity between various physical and mechanical properties of the pavement layer immediately below the milling line. Based on the outcomes of statistical hypothesis testing, additional evaluations will be conducted to quantify the degree of property changes and extent of spatial domain (depth below milling line) within which these changes happen. Analysis of Variance (ANOVA) will be utilized to identify significant pavement and construction factors impacting the damage from milling operations and linear and non-linear regressions will be adopted to develop decision process with respect to best practices for milling. Prior to this last analysis step, finite element modeling will be conducted to supplement the data obtained from field sections. To maximize the impact of available resources and to complete the project in timely manner only a limited number of relevant field projects can be studied; through use of finite element analysis, several parameters can be varied to explore their effects on amount of damage in post-milled existing pavement. Use of finite element analysis will be to conduct a parametric evaluation with a sufficiently large dataset to support the development of best practices, in the form of tables and/or charts. Variables considered will include speed of milling operation, rotational speed of milling drum, characteristics (geometry and condition) of milling head, stiffness of existing HMA layers, and condition of the pavement foundation. Lastly, recommendations regarding best practices for milling will be developed.

  • Date due: April 30, 2023
  • Deliverable: Data data analysis and finite element analysis and draft version of the milling best practices guidance document

Task 6: Draft final report

In this task a comprehensive project report including guidance for asphalt pavement milling best practices that provide decision processes for selection of appropriate milling depths and construction parameters will be developed. To train agency and construction contractor personnel on use of the project outcomes, a web-based tutorial will be developed by researchers and disseminated through the NRRA webinar series. Any other pertinent tools (such as, decision tool) that might be necessary for agencies to implement project results and for contractors to adapt those requirements will be provided as part of the deliverable tasks.

  • Date due: July 31, 2023
  • Deliverable: Draft final report, web-based training, and decision tool regarding milling

Task 8: Final report

  • Date due: October 31, 2023
  • Deliverable: Final report

Project team

Principal Investigators: Eshan V. Dave, Ph.D., eshan.dave@unh.edu and Jo E. Sias, Ph.D., P.E., jo.sias@unh.edu, Department of Civil and Environmental Engineering, University of New Hampshire
Co-PI: Rajib B. Mallick, Ph.D., P.E.
, Department of Civil and Environmental Engineering Worcester Polytechnic Institute, rajib@wpi.edu
Technical Liaison: John Siekmeier, MnDOT
Project Technical Advisory Panel (TAP) – Email the TAP
Contact us to join the TAP

  • Eric Baker, Astec Industries
  • Emil Bautista, MnDOT
  • Chelsea Bennett, MnDOT
  • Tom Chastain, Wirtgen Group
  • Rebecca Embacher, MnDOT
  • Derek Frederixon, Mathy Construction
  • John Garrity, MnDOT
  • Greg Johnson, MnDOT
  • Dan Kopacz, WisDOT
  • Todd Mansell, Caterpillar
  • Kiran Mohanraj, The Transtec Group
  • Dave Peterson, Caterpillar
  • Dan Schellhammer, Midstate Reclamation
  • Mike Swing, Midstate Reclamation
  • Eyoab Zegeye Teshale, MnDOT
  • Nick Ware, Alpha Milling Company
  • Brett Williams, National Asphalt Pavement Association (NAPA)
  • Tyler Wollmuth, ND DOT

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