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Evaluation of Soil Cement Bases in the New AASHTO 2002 Pavement Design Guide
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Mixture Design for Recycling Pavements with Cement
Co-Principal Investigators Tom Scullion , Texas Transportation Institute
Curtis Berthelot, Pavement Scientific International,
Saskatoon, Canada
Objectives
Full depth pavement recycling is a rapidly developing flexible pavement rehabilitation technique. Equipment development over the past decade has provided tremendous new capabilities in this area. Many miles of highway have recently been reclaimed with cement and some structural monitoring and initial pavement performance results have been reported. However, many questions remain at the state and local agency level. For example, 1) How can in-place materials be evaluated to judge their suitability for recycling, 2) how should the cement content be determined to ensure optimal long term performance, 3) what construction practices are recommended and 4) what material properties are appropriate for use in pavement design procedures to ensure an adequate pavement structure.
The scope of this project will be to first document the successes that have occurred and the lessons learned from performance studies. A laboratory test program will be undertaken to address several of the critical issues discussed above and to present guidelines on selecting cement contents.
Market Significance of Project
Several State Highway Agencies (SHA's) have reclaimed many miles of pavement whereas others have not used this process. Substantial growth prospects exist in the national and international markets. This approach has been shown to be highly economical: 30 to 50% savings over traditional removal and replacement. With significant portions of the highway networks in the US and Canada showing major levels of deterioration, full depth pavement recycling will continue to expand in the near future. It is critical for PCA to give clear and concise guidelines on how to effectively use cement in this process.
Utilization of Results
The anticipated results from this study will be a document of Case Studies of what is working in the field, and to identify "best practices" with regard to materials design and construction specifications. Field studies will be conducted to determine the levels of pulverization currently obtained in the field. The impact of various factors such as gradation and the amount and viscosity of the asphalt material on the measured engineering properties of the cement stabilized materials will also be determined. These engineering properties will include both strength and durability measurements.
The results from this study will assist SHA's and other agencies in developing appropriate specifications and laboratory testing procedures. The researchers plan to work closely with the SHA's in both Texas and Saskatchewan. These two states will provide PCA with very different climate and pavement conditions. The results of this study will be presented to interested Area Engineers within both SHA's.
Project Description
I. To locate and identify success stories where FDR with cement is working and saving money. To emphasize projects where controlled performance data in terms of visual condition and NDT results are available.
II. Determine the level of field pulverization that can be reasonably achieved for flexible pavements using typical equipment. Determine appropriate gradation limits to meet strength and durability criteria.
III. Determine the effect of asphalt binder on the hydration and/or binding ability of Portland Cement. Establish if the age of the asphalt affects the performance of the recycled material. Set limits on the asphalt ductility in order to achieve appropriate strength and durability. Also determine if there are limits to the amount of Reclaimed Asphalt Pavement (RAP) that should be included in the total recycled mix.
Work Program Tasks (Including Deliverables):
Task 1.0 "State of the Art" Document (TTI and PSI)
In this study a "State of the Art" document will be produced to identify what is working and what lessons can be learn from field studies. Efforts will be made to document existing field, laboratory and design procedures. The experiences from Texas and Canada will be included as well as the latest findings documented in the First International Symposium on Subgrade Stabilization and In-situ Pavement Recycling with Cement held in Spain in October 2001.
As this is a one-year study, this information will be included in the final report. It will include documentation of the existing design procedures and relevant research findings from around the world.
Task 2.0 Field Pulverization studies on SHA projects (TTI and Canada)
Select two upcoming projects in Saskatchewan and Texas in which full depth recycling is planned. Take sample from the pavement at control locations with an auger (or other technique) prior to the first pass of the recycling train. Again sample the pulverized pavement after the first pass. These materials will be returned to the laboratory for gradation studies and the materials obtained will be used in subsequent tasks. The two projects should include different thickness of asphalt, possibly one high volume and one low volume roadway.
Compare the gradations obtained from the field sampling procedure (auger) to that obtained from pulverization. This is a critical first step in conducting laboratory design work on future studies. It is essential that the material gradation from the material sampling technique approximates that obtained during construction.
Task 3.0 Impact of Gradation on Engineering properties (TTI and Canada)
The materials collected in Task 2 will be used in controlled laboratory tests to study the impact of changes in gradation upon the measured engineering properties. The field gradation and the DOT's design cement content will be used to establish the properties of the cement-stabilized materials. The properties of the control mixture to be determined will include the following;
I. Unconfined Compressive Strength (7 day wet cure)
II. Durability (Tube Suction Test)
III. Wet/Dry strength
These properties will be compared with those obtained with three different gradations, one coarser than the control mix and two finer. The effect of percentage of RAP in the mix will be evaluated to determine if limits should be imposed on the amount of RAP allowed.
Task 4.0 Impact of Viscosity of asphalt binder on engineering properties (TTI and Canada)
One concern about the use of cement with asphalt pavements is the presence of large volumes of fresh asphalt (light oils) in the pulverized material. This is a concern but not thought to be common. Normally the deteriorated pavements will contain several thin layers of brittle oxidized asphalt. However the presence of excessive highly ductile materials is a concern as it may inhibit the cementitious process, this will be studied in this task. These will be controlled laboratory tests.
The teams in both Texas and Canada will perform additional control testing on the impact of additional fresh asphalt on the engineering properties. Samples of one of the projects collected in Task 2 will be used in this task. The properties of the control mix will be determined in Task 3. These mixes will be treated with additional amounts of fresh asphalt to determine the impact on the measured properties. In this study 1.5% and 3% asphalt will be added to the pulverized blend, with the control cement content as proposed by the SHA. The properties described in Task 3 will be measured on these materials.
After testing the asphalt will be recovered from the test samples and its viscosity (penetration number) will be measured. Samples which exhibit significant changes in properties will be subjected to additional advanced testing at TTI. Dr Pat Harris of TTI will perform scanning electron-microscope studies on the resulting materials in order to identify the cause of the change in properties.
Task 5.0 Deliverables and Documentation
Final deliverables will include a final report and two publications for presentation at major research forums and/or refereed journals. This documentation will summarize and present all materials testing results, field test results, construction histories and recommended guidelines on selecting cement content on future recycling projects. Recommendations for future research and applications will be identified.
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