AASHO Road Test: The gold standard of testing

Driving Son No. 2's drum set back from Chicago, taking I-90/I-39 near Rockford, IL, reminded me of an engineering history landmark, not far south.


It's just off I-80 near Ottawa, Illinois: the remnants of a two-year pavement and bridge test undertaken prior to construction of the interstate highway system. It was sponsored by the feds and the American Association of State Highway Officials, (now AASHTO). Trucks operated by Army recruits drove around five loops from late 1958 into 1960.

The loops tested a wide variety of pavements and bridges. A sixth loop was left untraveled, as a control; the other five saw progressively higher weights of vehicles. Operations went on night and day, employing as many as 126 trucks. Some bridges failed after heavy use; some bridge designs held up so well that the organizers had to run super-heavy loads across the spans to hasten their destruction, since time was running out. In the 14 lane miles available there were more than 800 combinations of pavement and sub-bases. Here's a 50th anniversary article that appeared in Engineering News Record.


One outcome of the million-cycle study was to disprove a theory from the industry that a layer of gravel laid under rigid pavement could prevent the "pumping" of support material from under the pavement. The AASHO test led to pavement designs that held up much better. 

The Road Test shed light on the extent of damage caused by heavy truck axles, compared to cars. When a pothole in a highway causes havoc today, impacts from heavy trucks are usually a contributing cause.


But no test is accurate forever. Today's truck trailers have air suspensions, which cause less damage than old-style leaf springs for an equivalent axle load. New tests should include the effect of specialized hauling vehicles like concrete trucks, that can weigh as much as semis but pack that weight into a much shorter wheelbase, which can make a big difference on bridges. 

Tests should take into effect weigh-in-motion data that some states collect, which reveal how much weight trucks actually are carrying compared to what the law says they should carry. the first years of Montana's WIM scale data showed that nine percent of trucks on the major routes checked were overweight by an average of three tons before enforcement began.

One of the more obscure lessons from the Road Test is that as the speed of trucks rise, the stress they impose on bridge structures can go up dramatically if the deck surface is rough. Hence my comment on the I-94 bridge collapse in this post: roughness in the bridge-deck pavement caused by milling (namely, two-inch height differences) could have added stress that played a part in the collapse. Traffic speeds should have been held down.


Whether on a big scale or a smaller one, rigorous and realistic highway infrastructure tests are important because we need more information than computer models will tell us. We need experiments to know how well roads and bridges are going to hold up against the stresses of a new, leaner economy ... the Third Age of Industry.