The AASHO Road Test was a series of

experiment An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight into cause-and-effect by demonstrating what outcome occurs whe ...

s carried out by the

American Association of State Highway and Transportation Officials The American Association of State Highway and Transportation Officials (AASHTO) is a standards setting body which publishes specifications, test protocols, and guidelines that are used in highway A highway is any public or private road ...

(AASHTO), to determine how

traffic Traffic is the movement of vehicles and pedestrians along land routes. Traffic laws govern and regulate traffic, while rules of the road include traffic laws and informal rules that may have developed over time to facilitate the orderly an ...

contributed to the deterioration of

highway A highway is any public or private road or other public way on land. It includes not just major roads, but also other public roads and rights of way. In the United States, it is also used as an equivalent term to controlled-access highway, or ...

pavements Pavement(s) or paving may refer to: Surfacing * Road surface, the durable surfacing of roads and walkways * Sidewalk, a walkway along the side of a road, called a pavement in British English * Asphalt concrete, a common form of road surface * Co ...

Methodology

The AASHO road test was to study the performance of pavement structures of known thickness under moving loads of known magnitude and frequency. The study was carried out from August 1956 to November 30, 1960 in

Ottawa, Illinois Ottawa is a city in LaSalle County, Illinois, United States, and its county seat. It is located at the confluence of the navigable Fox River (Illinois River tributary), Fox River and Illinois River, the latter being a conduit for river barges and ...

and has been used as a primary source of experimental data when vehicle wear to highways is considered, for the purposes of road design, vehicle taxation, and cost. The road test consisted of six two-lane loops along the future alignment of

Interstate 80 Interstate 80 (I-80) is an east–west transcontinental freeway that crosses the United States from San Francisco, California, to Teaneck, New Jersey, in the New York metropolitan area. The highway was designated in 1956 as one of the ori ...

. 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 is the long-term weather pattern in a region, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteoro ...

and subgrade effects could be investigated, but were never carried out.

Legacy

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. More recent versions of the guide are 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 (QA) is the term used in both manufacturing and service industries to describe the systematic efforts taken to assure that the product(s) delivered to customer(s) meet with the contractual and other agreed upon performance, design ...

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. Since panel ratings are expensive, a substitute key parameter present serviceability index (PSI) was established. The PSI is based on data on the road's longitudinal roughness, patchwork, 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. Unevenness was measured with a mechanical profilograph, reporting a parameter called slope variance (SV). SV is the second spatial derivative of height. For a vehicle traveling 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 ISO 2631-1 (1997) standard. Thus, SV is physically linked to

ride quality Ride quality refers to a vehicle's effectiveness in insulating the occupants from undulations in the road surface such as bumps or corrugations. A vehicle with good ride quality provides comfort for the driver and the passengers. Importance Good ...

. While the study is outdated, it is still used as a reference, 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.

Notes

References

External links

AASHO Road Test
at Federal Highway Administration website * {{DEFAULTSORT:Aasho Road Test Pavement engineering Tests