Minnesota Department of Transportation

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Road Research

NRRA Geotechnical Team

Improve Material Inputs into Mechanistic Design Properties for Reclaimed HMA & Recycled Concrete Aggregate (RCA) Roadways

Project Start Date: July 1, 2019
Project End Date: April 30, 2020


In the mechanistic pavement design approach (AASHTO, 2008), pavement performance is evaluated based on mechanistically determined critical stresses, strains, temperatures, and moisture levels that are in turn the inputs to empirical prediction models for specific pavement distresses such as rutting, fatigue cracking, thermal cracking, and roughness for flexible pavements and cracking, faulting, and roughness for rigid pavements. Accurate characterization of the traffic, climate, and material input parameters is therefore important to ensure that the theoretical computation of pavement stresses, strains, temperatures, and moisture levels are accurate at the critical locations within the system. Local calibration is desirable to improve the accuracy of the empirical distress model predictions for a particular state or region. Depending on the desired level of accuracy of input parameter, three levels of input are provided from Level 1 (highest level of accuracy) to level 3 (lowest level of accuracy). Depending on the criticality of the project and the available resources, the designer has the flexibility to choose any one of the input levels for the design as well as use a mix of levels.

The material parameters required for pavement foundation materials including unbound granular materials, subgrade, and bedrock may be classified in one of three major groups: (1) pavement response model material inputs, (2) Enhanced Integrated Climatic Model (EICM) material inputs, and (3) other material inputs. Pavement response model materials input required are resilient modulus, Mr, and Poisson’s ratio, μ used for quantifying the stress dependent stiffness of unbound materials under moving wheel loads. Material parameters associated with EICM are those parameters that are required and used by the EICM models to predict the temperature and moisture conditions within a pavement system. These inputs include Atterberg limits, gradation, and saturated hydraulic conductivity. The “other” category of material properties constitute those associated with special properties required for the design solution. An example of this category is the coefficient of lateral pressure.

While there is a rich database nationwide about Mr and California Bearing Ratio (CBR)/Unconfined Compressive Strength (UCS) of subgrade and conventional unbound granular aggregates, there is no stiffness/strength/gradation/hydraulic conductivity database for reclaimed hot mix asphalt (HMA) materials-reclaimed asphalt pavement (RAP) material and recycled concrete aggregate (RCA) used as a base/subbase layer in pavement systems. The main goal of this project is to collect these data from the National Road Research Alliance (NRRA) member states and the literature. In addition, the list of field and laboratory tests that had been conducted to use in mechanistic pavement design along with the construction specification will be summarized. Moreover, it will be determined whether any data exist that evaluate the impact of any specific characteristics and/or mix design of RAP on the stiffness and strength of pavement systems in short- and long-term. The research team will also conduct some preliminary sensitivity analyses with AASHTOWare Software by using the collected data to determine the most sensitive parameter that may impact the pavement performance predictions in use of RAP as a base/subbase material.


The proposed study will help to establish a database for RAP and RCA material characteristics including resilient modulus (Mr), CBR/UCS, gradations along with construction specifications. Thus, this would lead to more consistent material input and specifications between NRRA agencies. Once a database of material characteristics is established, the cost involved in acquiring and testing field samples for new design is reduced/refined. The outcome of this research in the form of a pavement design specification could be immediately implemented by the department of transportations (DOTs) of states participating in NRRA.

Research methodology

The overall research methodology is proposed by a highly-qualified team with expertise in pavement systems and geotechnical engineering and pavement engineering at Michigan State University (MSU) and the Recycled Materials Research Center (RMRC). The methodology primarily involves collecting and summarizing Mr, CBR/UCS, and gradation data for RAPs tested by NRRA state members. Construction specifications, literature, and design methods for building and design pavement foundation layers with RAP from NRRA member states will also be investigated. In addition, preliminary sensitivity analyses with the pavement AASHTOWare Pavement ME Design software will be conducted via use of RAP data collected. Based on the results of this study it will be determined whether more detailed laboratory and field tests will be necessary to enhance the database.

The proposed team has been working on MnDOT data and NRRA states for years and very familiar with the process. This project contains four tasks: (1) initial memorandum on expected research benefits and potential implementation steps, (2) collection of data from NRRA member states and literature, (3) sensitivity analyses, and (4) final report.


Task 1: Initial memorandum on expected research benefits and potential implementation steps

During the proposal phase and the development of the work plan, key benefits were selected to clearly define the benefits the state will receive from the results and conclusions of this research. This task will provide an initial assessment of research benefits, a proposed methodology, and potential implementation steps.

  • Scheduled date for task final approval: October 30, 2019
  • Deliverable: A memorandum providing initial estimates of expected research benefits, documentation of the methodology, and potential implementation steps.

Task 2: Data collection

Under this task, the research team will collect the Mr, CBR/UCS, and gradation data of RAP from the NRRA member states. In addition, a detailed literature review will also be conducted to investigate the overall average stiffness/strength, gradation range of RAP materials used in pavement foundation layers. Construction specifications, literature, and design methods for building and design pavement foundation layers with RAP from NRRA member states will also be collected and summarized.

Furthermore, Tuncer B. Edil conducted and finalized the previous MnDOT led FHWA pooled-fund study, TPF-5(129): “Recycled Unbound Materials”. This study collected RAP materials from eight different states including Minnesota (MN), Colorado (CO), California (CA), Texas (TX), Ohio (OH), New Jersey (MJ), Wisconsin (WI), and Michigan (MI). This research conducted detailed index tests (sieve & hydrometer), asphalt content, absorption, compaction, Mr tests on all these RAP materials. In addition, it studied the impact of freeze-thaw cycles and wet/dry cycles on the stiffness and strength of three RAPs (TX, NJ, and CO). Moreover, it conducted Mr tests on at different temperatures to evaluate the performance of RAP materials at four different temperatures (7°C, 23°C, 35°C, 50°C).

The degree of saturation level (moisture content) is considered an influential parameter on the stiffness and strength of pavement materials. Therefore, it is very important to collect the Mr data of RAPs prepared at different moisture contents. PIs have worked on similar data analyses (Edil et al. 2012, Rosa et al. 2016, and Rosa et al. 2017). This data will also be collected. Additionally, void ratio, density, and angularity of these materials could be influential thus data will also be collected if available.

  • Scheduled date for task final approval: November 30, 2019
  • Deliverable: An interim report and webinar

Task 3: Sensitivity analyses

Under this task, the project team will perform sensitivity analysis to determine which changes in RAP materials characteristics most affect the prediction of the pavement performance via use of AASHTOWare Pavement ME Design software in NRRA member states.

The values obtained during the sensitivity analysis are normalized by comparing the change in pavement performance with the change in inputs. The normalized sensitivity index (NSI) allows easy comparison as to the magnitude of effect an input has on the pavement performance according to the AASHTOWARE software. A large positive NSI value indicates that increasing the input will greatly increase the output value. A negative NSI value indicates that increasing the input will decrease the output. The equation for this method is displayed below (Li et al. 2013).

  • ∆Y = the change in pavement performance due to change in design input
  • DL = the design limit for the pavement performance parameter
  • ∆X = the change in the design input from the baseline X
  • X = the baseline design input value
  • Scheduled date for task final approval: February 28, 2020
  • Deliverable: Presentation to the TAP and interim report

Task 4: Final report

During this task, the approved report will be processed by MnDOT’s contract editors. InTrans publication staff is very familiar with submitting final reports for MnDOT projects and they will ensure the report is in the correct format and meets all the requirements.

  • Scheduled date for task final approval: April 2020
  • Deliverable: Final published report

Project team

Principal Investigator: Bora Cetin, Assistant Professor, Michigan State University, cetinbor@msu.edu
Technical Liaison: Tim Andersen, Minnesota Department of Transportation, timothy.lee.andersen@state.mn.us
Project Technical Advisory Panel (TAP) – email the TAP
Contact us to join this TAP

  • Terry Beaudry, MnDOT
  • Matt Oman, Mathy
  • Heather Shoup, Illinois DOT
  • Raul Velasquez, MnDOT

Related materials

Final report

Expected April 30, 2020