Development of a Performance-Related Mix Design Method for Cold in Place Recycling (CIR)
I. Problem Statement:
Partial-depth cold in-place recycling (CIR) is a method that has been used with varying degrees of success to rehabilitate flexible pavements. For this rehabilitation strategy, the existing pavement is milled to a depth of three to four inches within the asphalt pavement structure, the millings are transferred to a screening unit where the oversize materials are crushed. All materials are combined and mixed in a mixing unit and blended with an asphalt emulsion. This mixture is then placed in a windrow behind a paving “train,” picked up and placed by a paving machine, and compacted. CIR can be used to remove transverse and reflective cracks, restore oxidized pavements, re-establish crowns, maintain clearances, improve aggregate gradations, and minimize the need for new materials, as well as strengthen the pavement.
CIR has been reported to be a cost-effective treatment for deteriorated pavements, and a recent FHWA policy statement recommends recycled materials be considered for all paving projects. A New Mexico study of 45 CIR projects estimated an average saving of $7,074 in construction costs per lane-km ($11,384 per lane-mile), and $7,524 per lane-km in life cycle costs ($12,109 per lane-mile).
A survey conducted of 38 State Departments of Transportation by the Rocky Mountain User Producer Group found that many agencies routinely use the technique. However, the survey identified that there are problems with performance reliability, specifically pointing to the lack of a uniform, well-defined design procedure. Typical problems with this rehabilitation strategy include raveling, thermal cracking, compaction, low early strength and extended curing time. Other agencies are reluctant to try CIR because of those problems.
Much recent hot-mix asphalt research focuses on lessening risks of early pavement failures through the use of performance-related specifications. Some state agencies, such as Minnesota DOT (MnDOT) and Kansas DOT (KDOT), have adopted new CIR mix design methods that include performance-related tests. These tests help identify some of the problems sometimes associated with CIR. Some of the design equipment and procedures use equipment developed for Superpave. Recent research has been performed by the University of Rhode Island (URI) and University of Kansas, but these research projects did not produce comprehensive mix design methods.
The Foundation for Pavement Preservation (FPP) and FHWA identified emulsion mixes with RAP as a research need, as documented in the Research Problem Statement Workshop document. This meeting was held in Sacramento, CA on June 21 – 22, 2001.
Research is needed to assess the design methods used by MnDOT and KDOT (identical), to use applicable findings from the URI and University of Kansas research, and to develop a uniform CIR design method that can be used by any agency. The simple performance tests that are being developed and validated under NCHRP 9-19 should be considered.
II. Research Objective:
The objective of this research is to recommend procedures for a performance-related based CIR mix design method that will improve the predictability and performance of CIR.
III. Proposed Scope of Work
Accomplishment of this objective will require at least the following tasks that are grouped into two separate phases.
Evaluate and recommend procedures for a performance-based mix design procedure for CIR
(1) In addition to the method used by MnDOT and KDOT and the research performed by URI and KSU, collect and review other relevant domestic and foreign literature, research findings, performance data, current practices, and other information relative to the use, testing, and evaluation of partial-depth CIR. This information may be obtained from published and unpublished reports, contacts with transportation agencies and industry organizations, and other sources. The literature review should note distresses that are observed with this rehabilitation strategy.
(2) Identify the performance parameters of the CIR layer that are important to short and long-term performance and develop new procedures or modifications of current test procedures for measuring those performance-related properties. The simple performance tests from NCHRP 9-19 and the material property inputs required by the 2002 Design Guide should be reviewed and evaluated.
(3) Identify the performance parameters of pavements that may be affected by the properties of the CIR layer, including consideration of the layer's structural behavior, constructibility, and related environmental concerns. Identify--with consideration to practicability, accuracy, and other relevant factors--potential new procedures or modifications of current test procedures for measuring those performance-related properties for which no suitable test method has been identified. Recommend procedures for further evaluation in Phase II.
(4) Prepare an updated, detailed work plan for Phase II that includes an experimental plan to evaluate and validate the most promising procedures for measuring CIR properties that relate to pavement performance. A detailed, statistically but practical experimental plan should be developed as a part of this task. The experimental plan should identify those test sections or projects that are to be included in Phase II.
(5) Prepare an interim report that documents the research performed in Phase I and includes the updated work plan for Phase II. Following review of the interim report by the sponsor, the research team will be required to make a presentation to the project panel. Work on Phase II of the project will not begin until the interim report is approved and the Phase II work plan is authorized by the sponsor.
Develop and validate the CIR mix design procedures identified in Phase I
(6)Execute the plan approved in Task 5. Based on the results of this work, recommend sets of tests for evaluating CIR used in pavement layers. Also, recommend criteria for interpreting test results and assessing CIR acceptability for use in pavement layers.
(7) Determine the ability to use the procedures developed for determining CIR-layer structural coefficients that are required inputs for the 1993 AASHTO Design Guide and the mixture properties that are required by the 2002 Design Guide. A library of mixture properties should be developed to be used on a national basis with the 2002 Design Guide, especially if that Guide is adopted by AASHTO for future use in designing flexible pavement structures.
(8) For those tests recommended in Task 6 that are not currently AASHTO or ASTM standards, develop protocols in a format suitable for consideration and adoption by AASHTO.
(9)Submit a final report that documents the entire research effort. The report shall include an implementation plan for moving the results of this research into practice.