Duplicating The Test

Dr. W.R. Hudson, Contributing Author / January 01, 2006

Yes, it is the AASHO Road Test—not the AASHTO Road Test—because it was conducted in 1958-61 before the name was changed to include broader “transportation.” It also produced more definitive pavement knowledge than any research study in history.

In the early 1950s Congress passed a bill authorizing the construction of the Interstate Highway System. The then-Bureau of Public Roads (BPR), now the Federal Highway Administration (FHWA), was charged with determining how to properly allocate the cost of construction. That is, how much of the costs should be paid by heavy trucks of varying weights and how much should be paid by smaller trucks and passenger cars. It was known that heavy trucks caused more damage than passenger cars, but it was not known how much more damage they caused.

Frank Turner, chief engineer, and Ted Holmes, director of Highway Research and Planning for the BPR, went to the Highway Research Board (HRB) for help in addressing this question. They met with William N. Carey Jr., assistant director of HRB, and Fred Burgraaf, director of HRB, and conceived the idea of conducting a large-scale road test to study the questions. Alf Johnson, then the executive secretary of AASHO, was brought into the project and sold the idea to all 49 states.

Carey added the pavement dimension to the study since he had been chief engineer of the WASHO Road Test. Holmes supported both pavement and the 210 studies since he was both director of planning (the cost allocation part) and research (the pavement research part). Therefore, Carey and Holmes conceived and added a detailed study of pavement performance, and the AASHO Road Test was born.

It cost $30 million in 1956 currency, about $500 million at today’s prices. Both pavement industry groups, the Asphalt Institute and the Portland Cement Association, became supporters of the study as did truck manufacturers and the American Trucking Association. A number of state DOTs contributed personnel to the project staff, but especially the Illinois Department of Highways provided the right-of-way and dozens of personnel to supervise construction testing along the proposed route for I-80. The U.S. Army provided drivers from the Transportation Corps because it would have been too expensive to use commercial union drivers for the round-the-clock truck operations required to apply the loads.

The results of the AASHO Road Test were profound, and many of the findings are still valid and have had lasting benefits for better pavement design and performance.

Two general classes of benefits resulted from the Road Test: (1) General benefits in pavement engineering that were developed or were greatly improved by the Road Test results, and (2) Technical findings about how to build and maintain pavements to produce longer pavement life and better performance. These were in addition, of course, to the information that was developed about the relative damaging effect of various axle loads to better allocate costs.

Enhanced results

Funding and conducting the Road Test sparked renewed interest in pavement engineering worldwide. The Road Test not only answered questions about allocating costs to various axle loads, it also produced a great deal of information about pavement engineering. More importantly it sparked a strong interest in pavement evaluation such as roughness measurements, distress surveys and deflection measurements, which have helped predict pavement performance and which ultimately resulted in the development of pavement management concepts originally and now asset management by transportation agencies.

The Road Test sparked considerable interest in the modeling of pavement performance and the development of mathematical equations to predict how pavements responded to climate and cumulative axle loads. It also proved to the pavement research community the importance of obtaining high-quality data for higher quality research and better results in civil engineering projects. This was coupled with the expanded knowledge about the use of factorial experiment design and good statistical data analysis to make sure that the answers were meaningful and could be proven beyond a reasonable statistical doubt. Such concepts are now widely used in pavement research worldwide.

Loaded answers

Most people do not seem to remember the poor quality of pavements and pavement engineering prior to 1958. Very little pavement research was being carried out and little was known.

Prior to the Road Test, the California bearing ratio (CBR) method was used for most flexible pavement design. There were gaps in this method that were filled by the Road Test studies. The most important factor was the development of load equivalency concepts that defined the relative damaging effect of axle loads and showed, for example, that a 30-kip axle load caused 10 times as much damage to the pavement as an 18-kip axle load (then the legal load) in most states. This load equivalency concept also was used to determine the relative cost allocation and taxation plans ultimately adopted by Congress.

Pavement failure was not well defined prior to the Road Test but the present serviceability index-performance concept developed at the Road Test solved this problem and has been widely used since. Failure is best defined as a level of serviceability (PSI) that is unacceptable to the riding public (a PSI level of about 2.5).

A major breakthrough developed at the Road Test was the layer equivalency concept, which showed beyond doubt that greater pavement thickness and/or stronger pavement layers produced pavement that lasted longer and served the riding public better. Based on these findings states began to construct thicker pavements with higher quality, stronger materials including cement-stabilized and asphalt-stabilized layers. The Road Test also proved that placing an improved layer under a portland cement concrete pavement greatly increased the life by reducing the pumping or ejection of the subgrade material from beneath the pavement due to the pounding of heavy loads in wet weather. It also was found that steel dowels placed across the pavement joints would reduce damaging pavement cracking at the corners of the slabs. This was shown to be particularly true when the joints were placed no more than 15 ft apart. Modern jointed concrete pavements still follow these proven concepts. Continuous reinforced pavements were not tested at the Road Test.

Since 1965, almost all pavement design methods used worldwide have made use of one or more of the results of the AASHO Road Test, and most still do. There have been improvements in theoretical predictions of certain factors and effects, but none of these invalidate the proven results obtained in 1960 at the Road Test. Work currently being completed in the development of the New Mechanistic/Empirical Pavement Design Guide provides a more complicated theoretical framework for explaining a pavement’s behavior, but does not invalidate the overall performance concepts from the Road Test.

Several good men

The AASHO Road Test was very successful because a strong team led by Carey, Walt McKendrick and Paul Irick was assembled, then laid out a precise plan and stayed focused on that plan to carry the research to completion. The team included Frank Scrivner, A.C. Benkleman, Fred Finn and many others. They did not allow personal aspirations and desire for promotion or political goals to divert the program. The BPR sent about 200 engineering trainees to work on the Road Test team, each for a six-month period. Among those trained there, three went on to become executive director of FHWA: Les Lamm, Dick Morgan and Dean Carlson. Many other trainees also filled key positions in FHWA over the years.

Carey went on to become executive director of TRB and Irick became the first TRB associate director for research and special programs. The Road Test also produced an unexpected bonus when the 120 trucks that had been purchased and used were sold after the test was completed. The resulting approximately $2 million was used to start the initial National Cooperative Highway Research Program. If you wish to know more about the Road Test all the results are published in Highway Research Board Special Report 61 (six volumes). TRB’s TRIS also contains hundreds of references to related studies.

About the Author

Hudson is a DC Greer professor emeritus at the University of Texas, Austin. He was an assistant research engineer at the AASHO Road Test.

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