Asphalt Laboratory Instruction Transitioning from Marshall Mix to Superpave Mix Design

Transcription

1 Asphalt Laboratory Instruction Transitioning from Marshall Mix to Superpave Mix Design William J. Davis 1 and Thomas R. Dion 2 Abstract Currently three asphalt mix design procedures are used in the United States. These include the Hveem, Marshall and the Superpave mix design methods. The Hveem method is mostly utilized in California and is not taught on a wide scale basis at the university level. The Marshall method, as developed by the Army Corps of Engineers, has been the primary procedure used to design asphalt pavement mixes over the past 50 years. This method has served as the basis for conducting most of the laboratory instruction at the university level. The Superpave mix design procedure and asphalt mix specifications have been under development throughout the 1990 s by a number of researchers on behalf of the Federal Highway Administration. This new method will address problems inherent with the Marshall mix design procedure primarily through the use of test procedures that are more representative of actual asphalt loading and failure conditions in the field. With wide spread adoption of Superpave procedures at the State and Federal level, universities need to upgrade their laboratory equipment, procedures and instructional methods to ensure that course materials and demonstration exercises remain current with applications of professional practice. In today s academic environment of limited funding and constrained funding mechanisms concentrated more on research rather than instructional purposes, upgrading undergraduate laboratory equipment can be problematic endeavor for most academic departments. This paper includes ideas and suggestions for transitioning towards full implementation of Superpave mix design procedures. Recommendations are presented and discussed. Potential transition steps are outlined along with possible sources and strategies for obtaining updated equipment. The transition plan and suggested approaches presented herein encompass the conceptual framework currently being utilized by the Department of Civil & Environmental Engineering at The Citadel to upgrade their undergraduate asphalt laboratory course. Introduction The Marshall Mix design procedure for asphalt pavements was originally developed for airfield design and construction during World War II. After World War II, a huge growth in road construction occurred throughout the US, and knowledge gained from use of the Marshall Mix design procedure was carried over into highways. This design methodology has served the engineering design community, road construction industry and traveling public well over the past 40 years. However, problems inherent to the design and construction of asphalt pavements require more sophisticated approaches. These include design and construction asphalt pavements to resist thermal cracking under extreme cold temperatures and to deter rutting under high temperature extremes. New procedures in the Superpave methodology have been developed to more effectively address these problem areas through the use of physical tests conducted under laboratory conditions that more accurately duplicate pavement loading and failure conditions in the field. Of course, new laboratory equipment is necessary to conduct these 1 Assistant Professor of Civil & Environmental Engineering, The Citadel, Charleston, SC. 2 Professor of Civil & Environmental Engineering, The Citadel, Charleston, SC. 1

2 newly established test procedures. Complicating the obvious funding problem of obtaining new laboratory equipment is the fact that most of the test specific equipment has been developed recently and the cost per unit is still very high. Further confounding the establishment of a Superpave pavement design lab is that more specialized equipment is necessary to conduct the new stability and asphalt binder tests. Another problem in obtaining equipment via traditional civil engineering benefactors such as public agencies and paving contractors is that most equipment being used is still first generation. The result is that public agencies and private companies do not have old Superpave equipment to donate as they have often generously bequeathed in the past from their surplus equipment reserves. Marshall Mix Design Lab Instruction Traditional Marshall mix design procedures conducted in The Citadel s Asphalt Laboratory course have included exercises as summarized in Table 1. Since many public roadway agencies will continue to use Marshall mix design procedures for secondary roads, this procedure remains applicable to college level instruction. Discussion and application of volumetric and density analysis constitute the fundamental principles upon which pavement design criteria is applied. Laboratory procedures related to the determination of optimal values for stability, flow, percent air voids and percent asphalt content remain germane to asphalt pavement design methodology. However, since the most important issues related to pavement design for major highways will be addressed via Superpave criteria, instruction of Marshall mix design procedures alone would be a limiting factor in providing students with a proficient knowledge of the pavement design field as it currently exists. The realization of this fact by academic institutions requires action be taken to incorporate the new procedures into the current curriculum. Table 1 Asphalt Laboratory Exercises Laboratory Exercises Criteria 1 Particle Charge Test ASTM, Part 15, D Coating Test ASTM, Part 15, D Softening Point Test ASTM, Part 15, D Specific Gravity Test ASTM, Part 15, D Flash Point Test ASTM, Part 15, D Penetration Test ASTM, Part 15, D Kinematic Viscosity Test ASTM, Part 15, D Volumetric, Flow & Stability Test MS-2, Sixth Edition Superpave Mix Design Lab Procedures Superpave mix design procedures were developed and established formally in March, 1993 through completion of the research included in the Strategic Highway Research Program (SHRP). The objective of the SHRP research was to develop new mix design procedures and tests that would better replicate actual loading and failure conditions that exist in the field. A number of new asphalt binder tests were developed to address the issue of conducting more representative laboratory tests. In addition, a new device and procedure was developed for the purpose of conducting a more representative compaction test. Related to compaction and wear criteria, other methods are currently being considered as proof tests. Laboratory procedures for the Superpave mix design procedures are summarized in Table 2. 2

3 Table 2 Superpave Laboratory Procedures Laboratory Procedure Purpose 1 Dynamic Shear Rheometer Evaluates elastic & viscous properties 2 Rotational Viscometer Evaluates handling & pumping properties 3 Bending Beam Rheometer Evaluates low temp. stiffness properties 4 Direct Tension Tester Evaluates low temp. ability to stretch 5 Binder Aging Methods Simulates aging characteristics 6 Gyratory Compactor Test Realistic compaction to max. density Transition to Superpave Initially, the Citadel s asphalt laboratory course will be modified to address and incorporate Superpave design criteria without the benefit of any new equipment. The lectures and laboratory procedures, currently conducted based on the Marshall mix design procedure, will be presented within the context of how the procedure is either similar or differs from the new Superpave criteria. On the surface, this approach is less than desirable, however, will hopefully serve to buy time as available funding and equipment sources are identified. Over time a significant amount of the course discussion currently focusing on the Marshall mix design will be replaced with material emphasizing Superpave procedures. Specific components of the transition plan are summarized in Table 3. The long range plan will be to acquire a gyratory compactor and then compliment that piece of equipment with other asphalt cement binder testing equipment as second generation models replace first generation models that are in use today. Table 3 Transition to Superpave Laboratory Procedures Actions Priority 1 Continue to conduct Marshall mix design exercises in lab Ongoing 2 Discuss advantages & disadvantages of Marshall procedure Immediate 3 Compare particle size distribution for Marshall, Superpave, etc. Immediate 4 Contrast Superpave vs. Marshall binder test methods Immediate 5 Discuss differences in material selection for Superpave Immediate 6 Provide evaluation of gyratory compactor criteria Immediate 7 Develop Superpave handout materials for laboratory course Immediate 8 Visit contractor s Superpave laboratory High 9 Obtain surplus 1 st generation gyratory compactor Medium 10 Obtain surplus 1 st generation a.c. binder test equipment Low 3

4 Obtaining New Laboratory Equipment Various approaches and priorities for obtaining the necessary laboratory equipment to demonstrate Superpave mix design procedures are recommended by the National Center for Asphalt Technology (NCAT) located at Auburn University. These approaches are summarized as options/steps in Table 4. This is obviously a critical aspect for most institutions in efficiently updating laboratory courses so instructional material remains current with professional practice. Fortunately, it is to the advantage of public agencies, material suppliers and road contractors for engineers entering the job market to be proficient in current design and construction practices. This motivation from interested parties affords academic institutions some leverage when approaching interested organizations for support and advice. NCAT recommends the following ideas for obtaining laboratory equipment needed to provide academic instruction related to Superpave mix design criteria: Contact State Asphalt Pavement Association. Identify alumni that may work in the highway contracting industry. Contact your State Department of Transportation, Materials Engineering Division. Contact local or regional aggregate material suppliers. Contact local refineries that produce asphalt cement for road construction. Table 4 Approaches for Setting Superpave Laboratory Option Items Needed Option 1 Minimum equipment needed to test Superpave mixes Gyratory Compactor (with large metal molds) Aggregate Sieves & vibratory shaker Aggregate oven Mixing apparatus Option 2 Minimum equipment needed to test Superpave a.c. binder Dynamic Shear Rheometer Rotational Viscometer Direct Tension Tester Rolling Thin Film Oven or Pressure Aging Vessel Bending Beam Rheometer Option 3 Equipment needed for full Superpave Laboratory All equipment listed for Option 1 and 2 4

5 Conclusion Change in technology requires academic institutions to continually monitor changes that are occurring in industry. Curriculum, course changes and upgraded facilities are all required to keep pace with an ever-changing professional environment. These type changes must be periodically addressed to ensure that technical degrees remain current with the demands of the job market. Towards accomplishing this important goal, all universities conducting undergraduate laboratory instruction in asphalt pavement design, need to develop a plan to transition towards use of Superpave mix procedures. There are numerous potential sources for support and assistance in making this transition. Creation and diligent pursuit of an effective and realistic implementation plan is a critical element in instituting the changes needed to address technical and equipment related deficiencies. References 1. Hot Mix Asphalt Materials, Mixture Design and Construction, 2 nd Edition, National Center for Asphalt Technology, Napa Education Foundation, Lanham, Maryland, Mix Design Methods for Asphalt Concrete and Other Hot-Mix Types, Sixth Edition, Asphalt Institute Manual Series No. 2 (MS-2), Lexington, Kentucky, Background of Superpave Asphalt Mixture Design and Analysis (FHWA-SA ), US Department of Transportation, Federal Highway Administration, Washington, DC, Background of Superpave Asphalt Binder test Methods (FHWA-SA ), US Department of Transportation, Federal Highway Administration, Washington, DC, Superpave Asphalt Mixture Design illustrated, Level 1 Lab Methods (FHWA-SA ), US Department of Transportation, Federal Highway Administration, Washington, DC, Professor Training Course Workbook in Asphalt Technology, National Center for Asphalt Technology, Auburn, Alabama, June 15-26,

6 William J. Davis William Davis is an Assistant Professor in the Department of Civil & Environmental Engineering at The Citadel in Charleston, South Carolina. He obtained a B.S. in Civil Engineering from the University of Alabama in After working as a consulting engineer for several years, he graduated from Auburn University with a M.S. in Civil Engineering in 1987 and obtained registration as a Professional Engineer in He continued working as a civil engineering consultant in Florida, California and Georgia and in 1997 earned a Ph.D. in Transportation Engineering from the Georgia Institute of Technology. Dr. Davis is a member of ASEE, ASCE, Institute of Transportation Engineers and Transportation Research Board. He serves as Secretary to the Executive Committee of the Urban Transportation Division for the American Society of Civil Engineers. Thomas R. Dion COL. Dion graduated from The Citadel in 1968 with a B.S. degree in Civil Engineering. He earned an MS degree in Civil Engineering from Clemson University in 1973 and became a registered professional engineer and land surveyor in the state of South Carolina in He became a full time faculty member of the Civil Engineering Department at The Citadel 22 years ago when he began teaching undergraduate students. Part of his departmental duties include being coordinator of the Civil Engineering Department s Capstone Design Course in Engineering Practice. Col. Dion is currently serving as Chair of the Research Unit for the Southeastern Section of ASEE. 6