AASHO Road Test
Encyclopedia
The AASHO Road Test was a series of experiment
s carried out by the American Association of State Highway and Transportation Officials
to determine how traffic
contributed to the deterioration of highway
pavements
. Officially, the Road Test was "...to study the performance of pavement structures of known thickness under moving loads of known magnitude and frequency." This study, carried out in the late 1950s in Ottawa, Illinois
, is frequently quoted as a primary source of experimental data when vehicle damage to highways is considered, for the purposes of road design, vehicle taxation and costing.
The road test consisted of six two-lane loops along the future alignment of Interstate 80
. Each lane was subjected to repeated loading by a specific vehicle type and weight. The pavement structure within each loop was varied so that the interaction of vehicle loads and pavement structure could be investigated. "Satellite studies" were planned in other parts of the country so that climate
and subgrade
effects could be investigated, but were never carried out.
The results from the AASHO road test were used to develop a pavement design guide, first issued in 1961 as the "AASHO Interim Guide for the Design of Rigid and Flexible Pavements", with major updates issued in 1972 and 1993. The 1993 version is still in widespread use in the United States. A new guide, originally planned for release in 2002 but as yet still under development, would be the first AASHO pavement design guide not primarily based on the results of the AASHO road test.
The AASHO road test introduced many concepts in pavement engineering, including the load equivalency factor. Unsurprisingly, the heavier vehicles reduced the serviceability in a much shorter time than light vehicles, and the oft-quoted figure, called the Generalized Fourth Power Law, that damage caused by vehicles is 'related to the 4th power of their axle weight,' is derived from this. The other direct result of the tests were new quality assurance
standards for road construction in the US, which are still in use today.
The road test used large road user panels to establish the "Present Serviceability Rating" (PSR) for each test section, as the test proceeded. Since panel ratings are expensive, a substitute key parameter "Present Serviceability Index" (PSI) was established. The PSI is based on data on the roads longitudinal roughness, patch work, rutting and cracking
. Later studies have shown that PSI is mainly a fruit of unevenness, with a correlation of more than 90% between the two. Uneveness was measured with a mechanical profilograph
, reporting a parameter called slope variance (SV). SV is the second spatial derivative of height. For a vehicle travelling at speed, SV is the exciting source to vertical acceleration; the second derivative in time domain of height. This makes very good sense, since 1 – 80 Hz acceleration is the parameter used when relating human exposure from vibration to perceived discomfort in the current ISO 2631-1 (1997) standard. Thus, SV is physically linked to ride quality
.
While the study is now quite old, it is still frequently referenced, though critics point out that its data is only valid under the specific conditions of the test with regard to the time, place, environment, and material properties present during the test. Extrapolating the data to different situations has been 'problematic'. Other studies have attempted to refine the results, either through further empirical studies, or by developing mathematical models, with varying success. The AASHO study is still the most often quoted study on the subject however.
Experiment
An experiment is a methodical procedure carried out with the goal of verifying, falsifying, or establishing the validity of a hypothesis. Experiments vary greatly in their goal and scale, but always rely on repeatable procedure and logical analysis of the results...
s carried out by the American Association of State Highway and Transportation Officials
American Association of State Highway and Transportation Officials
AASHTO, the American Association of State Highway and Transportation Officials, is a standards setting body which publishes specifications, test protocols and guidelines which are used in highway design and construction throughout the United States...
to determine how traffic
Traffic
Traffic on roads may consist of pedestrians, ridden or herded animals, vehicles, streetcars and other conveyances, either singly or together, while using the public way for purposes of travel...
contributed to the deterioration of highway
Highway
A highway is any public road. In American English, the term is common and almost always designates major roads. In British English, the term designates any road open to the public. Any interconnected set of highways can be variously referred to as a "highway system", a "highway network", or a...
pavements
Pavement (material)
Road surface or pavement is the durable surface material laid down on an area intended to sustain vehicular or foot traffic, such as a road or walkway. In the past cobblestones and granite setts were extensively used, but these surfaces have mostly been replaced by asphalt or concrete. Such...
. Officially, the Road Test was "...to study the performance of pavement structures of known thickness under moving loads of known magnitude and frequency." This study, carried out in the late 1950s in Ottawa, Illinois
Ottawa, Illinois
Ottawa is a city located at the confluence of the Illinois River and Fox River in LaSalle County, Illinois, USA. As of the 2010 census, the city population was 18,786...
, is frequently quoted as a primary source of experimental data when vehicle damage to highways is considered, for the purposes of road design, vehicle taxation and costing.
The road test consisted of six two-lane loops along the future alignment of Interstate 80
Interstate 80
Interstate 80 is the second-longest Interstate Highway in the United States, following Interstate 90. It is a transcontinental artery running from downtown San Francisco, California to Teaneck, New Jersey in the New York City Metropolitan Area...
. Each lane was subjected to repeated loading by a specific vehicle type and weight. The pavement structure within each loop was varied so that the interaction of vehicle loads and pavement structure could be investigated. "Satellite studies" were planned in other parts of the country so that climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...
and subgrade
Subgrade
In transport engineering, subgrade is the native material underneath a constructed road, pavement or railway track. It is also called formation level.The term can also refer to imported material that has been used to build an embankment....
effects could be investigated, but were never carried out.
The results from the AASHO road test were used to develop a pavement design guide, first issued in 1961 as the "AASHO Interim Guide for the Design of Rigid and Flexible Pavements", with major updates issued in 1972 and 1993. The 1993 version is still in widespread use in the United States. A new guide, originally planned for release in 2002 but as yet still under development, would be the first AASHO pavement design guide not primarily based on the results of the AASHO road test.
The AASHO road test introduced many concepts in pavement engineering, including the load equivalency factor. Unsurprisingly, the heavier vehicles reduced the serviceability in a much shorter time than light vehicles, and the oft-quoted figure, called the Generalized Fourth Power Law, that damage caused by vehicles is 'related to the 4th power of their axle weight,' is derived from this. The other direct result of the tests were new quality assurance
Quality Assurance
Quality assurance, or QA for short, is the systematic monitoring and evaluation of the various aspects of a project, service or facility to maximize the probability that minimum standards of quality are being attained by the production process...
standards for road construction in the US, which are still in use today.
The road test used large road user panels to establish the "Present Serviceability Rating" (PSR) for each test section, as the test proceeded. Since panel ratings are expensive, a substitute key parameter "Present Serviceability Index" (PSI) was established. The PSI is based on data on the roads longitudinal roughness, patch work, rutting and cracking
Cracking
Cracking may refer to:* Cracking, the formation of a fracture or partial fracture in a solid material* Fluid catalytic cracking, a catalytic process widely used in oil refineries for cracking large hydrocarbon molecules into smaller molecules...
. Later studies have shown that PSI is mainly a fruit of unevenness, with a correlation of more than 90% between the two. Uneveness was measured with a mechanical profilograph
Profilograph
The profilograph is a device used to measure pavement surface roughness. In the early 20th century, Profilographs were low speed rolling devices. Today many Profilographs are advanced high speed systems with a laserbased height sensor in combination with a inertial system that creates a large...
, reporting a parameter called slope variance (SV). SV is the second spatial derivative of height. For a vehicle travelling at speed, SV is the exciting source to vertical acceleration; the second derivative in time domain of height. This makes very good sense, since 1 – 80 Hz acceleration is the parameter used when relating human exposure from vibration to perceived discomfort in the current ISO 2631-1 (1997) standard. Thus, SV is physically linked to ride quality
Ride quality
Ride quality refers to the degree of protection offered vehicle occupants from uneven elements in the road surface, or the terrain if driving off-road. A car with very good ride quality is also a comfortable car to ride in. Cars which disturb vehicle occupants with major or minor road...
.
While the study is now quite old, it is still frequently referenced, though critics point out that its data is only valid under the specific conditions of the test with regard to the time, place, environment, and material properties present during the test. Extrapolating the data to different situations has been 'problematic'. Other studies have attempted to refine the results, either through further empirical studies, or by developing mathematical models, with varying success. The AASHO study is still the most often quoted study on the subject however.
External links
- Design and Performance of Road Pavements, David and Paul Croney, p. 248, Google Print - discusses the road test
- Pavement Lessons Learned from the AASHO Road Test and Performance of the Interstate Highway System] - discusses engineering lessons learned from the Road Test