There is no current consensus among state highway and transportation agencies as to the appropriate binder specification test methods required for adequate quality control and acceptance of modified binders. Supplemental tests have been adopted in addition to the conventional performance grade (PG) tests and are often referred to as “PG+” procedures. Many agencies have implemented these additional testing protocols; however, differences exist between the PG+ test methods, test conditions, and performance limits being specified. As a result, it is difficult to satisfy variable criteria when producing and supplying modified binders that consistently and uniformly meet agency expectations. The intent of this project is to provide essential information to state and local agencies to support standardization of PG+ specifications by identifying those PG+ test methods that are reproducible and show promise in simulating actual field performance.
From a practical perspective, formal ruggedness testing and statements of bias and repeatability do not exist for many procedure variants and PG+ specifications. A study of test method variability and reliability for promising PG+ tests identified in this project will help reduced binder testing disputes and associated costs during construction. Furthermore, it will help identify purchase specification test procedures that are unacceptable due to high variability and result in greater confidence in quality assurance testing and material compliance.
The Multiple Stress Creep and Recovery (MSCR) test is used to characterize the rutting resistance of both unmodified and modified binders. Recently specification limits were adopted by AASHTO in provisional method MP-19 as an eventual replacement for current SuperPave guidelines. This development represents a transition in binder grading protocols to test methods that are capable of evaluating the products currently in the market using properties better related to field performance. Conceptually, as a result of implementing the MSCR state agencies will be able to select binders based on properties better related to rutting resistance and with the desired level of elastic response. Rutting distress may not represent the critical mode of failure as many pavements fail due to cracking at intermediate and low temperatures. This concern is well recognized in current the current SuperPave grading system “PG+” specifications as numerous tests in both temperature regimes are required.
Recently, a number of test methods to improve evaluation of materials at intermediate and low temperatures were submitted to the AASHTO Subcommittee on Materials for consideration. Using these tests there is an opportunity to extend the motivating concepts of the MSCR, namely 1) a performance related test 2) capable of evaluating all binder types, to the intermediate and low temperature regimes. As a result, state agencies would have the ability to select materials based on parameters related to field performance across all service temperatures and levels of aging. However, to realize this objective significant efforts are needed in regards to deployment of the MSCR grading criteria and to assess the suitability of newly proposed test methods as agency specification tests for intermediate and low temperature binder properties.
Participation in this pooled fund presents an opportunity for specifying agencies and users to support research on development of specifications and acceptance criteria for modified binders with a focus on test methods that are most related to performance and meet specific needs. The product of this research is a unified modified binder specification developed in cooperation with partner states and User-Producer Groups delivered in a format that will allow for rapid implementation and deployment.
Based on the stated needs and goals, the main objectives of the proposed pooled fund research include:
• Perform detailed assessment of current PG+ and modified binder quality control procedures in partnering states in terms of reliability, applicability, and relevance to performance and quality of modified asphalt binders.
• Use a range of modified binders, representative of the products currently specified by partner states, to develop unified test procedures and specification criteria based on products placed in the field.
• Improve product quality and reliability through conduct of ruggedness studies and development of precision and bias statements for selected tests.
• Introduce consistency to current products supplied by elimination or reduction of differences in modified binder acceptance tests and criteria throughout member states.
• Validate and establish relevance of suggested PG+ and quality control procedures in terms of mixture performance.
Relevant Experience of Research Team
The pavement material research group at the University of Wisconsin Madison and the Modified Asphalt Research Center (MARC) under the directorship of Professor Hussain Bahia has had extensive experience evaluating, improving, and developing asphalt binder characterization methods and test procedures. Such efforts notably include the NCHRP project 9-10 on design of asphalt mixtures using modified asphalt binders which led to the development of the repeated creep test which later became the basis for development of the Multiple Stress Creep and Recovery (MCSR) test.
Through the Asphalt Research consortium project (ARC) the team has been involved in evaluating and validating existing modified binder testing procedures and development of standards /specification criteria. The project involved collaboration with the Western Cooperative Test Group (WCTG), the Rocky Mountain Asphalt User-Produce Group (RMAUPG), state highway agencies and industry to improve PG+ specifications currently in use through documentation of benefits and costs associated with these tests, development of protocols for new binder tests, and development of specification criteria for new tests based on field evaluation of construction and performance. Additionally, interviews and surveys for soliciting feedback on binder tests and specifications were conducted among a large number of state highway agencies as part of this effort. As part of ARC, MARC has also been involved in validating newly developed asphalt binder procedures using comparison with field performance of Long-Term Pavement Performance (LTPP) and MnROAD test sections.
As part of MARC’s involvement in regional modified binder specifications, the group completed Phase 1 of the WisDOT project for development of “Modified Asphalt Binder Guidelines” and is currently the principal on a follow up study intended to identify test methods and specification limits based on field performance measurements from modified asphalt pavement sections constructed in the early 2000s across Wisconsin.
Scope of Work
A. Analysis of Current Modified Asphalt Characterization Procedures (Year 1)
• Conduct detailed analyses on the most promising PG+tests. Identify PG+ tests that correlate to performance in laboratory mixtures and project sites.
• Recommend PG+ tests that best correlate to performance.
• Recommend specification limits that could be used on a regional or unified basis.
• Where necessary and appropriate suggest replacement methods and procedures for consideration.
B. Perform Statistical Analysis and Reliability Assessment on Selected PG+ Tests (Year 1)
• Conduct statistical analyses of binder test data for each binder sample as well as an annual analysis of variability.
• Final written report and recommendations for improving current PG+ testing protocols and introduce alternative test procedures where required. The report will also recommend specification limits that could be used on a unified basis.
C. Determination and Validation of Specification with Laboratory Mix Performance Data (Year 2)
• Laboratory mixture testing will be performed on loose mix samples to verify the predictive value of PG+ test in terms of primary distresses including, but not limited to rutting, fatigue, and low temperature cracking.
D. Validation using Field Performance Data (Year 2)
• Work with participating states to collect existing field data.
• Use uniform analysis procedure of collected field performance data for validation of proposed PG+ test methods.
• Create and populate a mixture and field performance database. The database will summarize results collected in Tasks A and B.
E. Develop Unified Modified Binder Characterization and Quality Control Specification (Year 2)
Partners are asked to commit a minimum of $25,000 per year. This pooled fund will last no longer than 36 months.
Future obligations will occur as needed to fund further activities set forth by the Technical Committee.