Background: The original Superpave performance model system developed through the Strategic Highway Research Program (SHRP) incorporated an environmental effects model as an integral component. This model provided the capability to predict temperature and moisture conditions in the structure of the pavement throughout its service life and thus account for specific, long-term effects of climate on material properties and pavement performance.

In 1996, a University of Maryland research team reviewed the original SHRP Superpave environmental effects model as a part of the FHWA Superpave Support and Performance Models Management project and identified several critical deficiencies. The research team recommended that Version 2.0 of the Integrated Climatic Model (ICM) developed for FHWA by Dempsey and coworkers be adapted for use as the current Superpave environmental effects model.

At present, the research team for NCHRP Project 1-37A, "Development of the 2002 Guide for the Design of New and Rehabilitated Pavement Structures," is similarly adapting the ICM as the environmental effects model for the 2002 design guide software. The Project 1-37A research team has carried out a limited validation of the ICM with data from ten sections in the Long Term Pavement Performance (LTPP) Seasonal Monitoring Program (SMP). It was found that the ICM was reasonably accurate in predicting pavement temperatures but less so in predicting soil-water content variations and soil-water characteristic curves (SWCC). Subsequent modifications to the ICM have substantially improved the accuracy of the SWCCs and the corresponding soil-water content variations; Version 2.6 of the ICM incorporates these modifications. Further validation of ICM Version 2.6 with the full data set from all hot mix asphalt (HMA) and portland cement concrete (PCC) sections in the LTPP SMP is required to confirm that its temperature and soil-water content predictive capabilities are acceptable for its routine use in both the Superpave performance model system and the 2002 design guide software.

Besides their diurnal and seasonal effects on pavement layer stiffness, temperature and soil-water content also contribute to long-term, irreversible changes in the properties of the asphalt binder and mixtures. Such changes are usually classified under the general term "aging." There are conditioning procedures in the Superpave mix design method to simulate short- and long-term field aging of asphalt binder and HMA. These procedures are also planned for use in conditioning specimens for the materials characterization test under development in Task F of NCHRP Project 9-19, "Superpave Support and Performance Models Management;" these tests and the associated materials characterization model will provide materials input data required by the pavement response and distress prediction models in the Superpave performance model system. A key question is whether these conditioning procedures consistently simulate the effects of aging with sufficient accuracy for performance prediction.


Objective: The objectives of this research are to (1) validate the latest version of Integrated Climatic Model (ICM) developed in NCHRP Project 1-37A with data from the full Long Term Pavement Performance Seasonal Monitoring Program; (2) develop practical guidelines for selecting ICM input data sets; (3) verify the estimated period or rate of aging simulated by the current Superpave binder and hot mix asphalt conditioning procedures -- AASHTO provisional practices PP1 and PP2 -- with data from Long Term Pavement Performance Specific Pavement Studies and other relevant field experiments; and (4) revise the current conditioning procedures as necessary for their use with the materials characterization test and model under development in NCHRP Project 9-19 for the Superpave performance model system.

Tasks: Accomplishing these objectives will require at least the following tasks:

PHASE I (1.) Prepare a detailed, statistically based experiment design for the calibration and validation of Version 2.6 or higher of the ICM developed in NCHRP Project 1-37A with data from (1) the full set of HMA and PCC pavement sections in the LTPP Seasonal Monitoring Program and (2) other appropriate field experiments such as WesTrack and MnRoad. In the experiment design, identify the materials and climatic (moisture, temperature, and frost depth) data required from each section for the validation and explain how they will be obtained or collected. ( 2.) Critically review the literature on the development and validation of the Superpave mix conditioning procedures [AASHTO PP1, Practice for Accelerated Aging of Asphalt Binder Using a Pressurized Aging Vessel (PAV) and AASHTO PP2, Practice for Short and Long-Term Aging of Hot Mix Asphalt (HMA)] that are proposed for preparation of test specimens for the Superpave materials characterization test, in particular SHRP reports A-383, A-384, and A-390; analyze available conditioning results from research and field projects incorporating Superpave-designed mixes; and develop revised estimates of the period or rate of field aging simulated by the procedures. On the basis of these results, design an experiment to verify the revised estimates of the period or rate of aging simulated by the conditioning procedures with laboratory and field data from the LTPP SPS studies and other relevant field experiments. ( 3.) Submit, within 5 months of the effective date of the contract, an interim report summarizing the work accomplished in Tasks 1 and 2 and presenting experiment designs for (1) validation of the ICM and (2) verification of the period or rate of aging simulated by AASHTO PP1 and PP2. The research agency will be required to meet with the NCHRP approximately 1 month later to obtain approval to proceed with Phase II.

PHASE II (4. ) Conduct the experimental plan for validation of the ICM with the full LTPP SMP data set. Using the experimental results, calibrate or further modify the ICM to bring its capability to predict temperature, soil water, and frost depth within an acceptable degree of accuracy. ( 5.) Perform a sensitivity analysis of the fully validated ICM. Distinguish variables that are most important in predicting temperature and soil-water content variations from those that have little or no effect on the predictions. Evaluate how the accuracy of the validated ICM predictions is affected by the use of a range of default input data versus measured, project-specific input data. Investigate whether temperature and soil-water content predictions made with the validated ICM remain reasonable for long design periods (10 years or greater). Using the results of the sensitivity analysis, prepare practical guidelines to assist engineers in selection of appropriate ICM input data sets for such hierarchical levels of design as may be incorporated in the Superpave performance model system and the 2002 design guide. Illustrate by example the sensitivity of the resulting performance prediction, as obtained from the Superpave model system or the 2002 design guide, to the recommended ICM inputs. ( 6.) Submit an interim report presenting the results of Phase II and documenting, in the form of software and supporting documentation, the validated ICM and practical guidelines for selection of its input data sets. The research agency will be required to meet with the NCHRP approximately 1 month later to present the findings and demonstrate the ICM software.

PHASE III ( 7.) Conduct the experiment to verify the revised estimates of the period or rate of aging simulated by AASHTO PP1 and PP2. Prepare (in the form of recommended changes to the practices in AASHTO standard format) any revisions to the procedures needed to improve their ability to simulate the in-situ aging of HMA material properties for use with the materials characterization test and model under development in Task F of NCHRP Project 9-19. ( 8.) Prepare a final report that summarizes findings, draws conclusions, and documents the results of Phases I, II, and III. In the report, present (1) the validated ICM (including necessary source and compiled code), practical guidelines for selection of its input data sets, and specific directions for its incorporation into the Superpave performance model system and the 2002 Guide for the Design of New and Rehabilitated Pavement Structures and (2) recommended revisions in AASHTO standard format to AASHTO PP1 and PP2 to improve their ability to simulate the in-situ aging of HMA material properties.

Status: The project is complete.

Product Availability: The final report for Phase II is available as NCHRP Report 602. The final report for Phase III is available as NCHRP Research Results Digest 324; the full report for this phase is published as NCHRP Web-Only Document 113.