The sine qua non of pavement research and design
By David Smith, ICPI Technical Director
Most progress in pavement design comes from failure. For accurate and predictable pavement design, pavements must be damaged and eventually rendered useless by repeated truck wheel loads to understand where that point lies. In fact, modern highway pavement design was originally based load testing from trucks conducted by the then American Association of State Highway Officials in the 1950s. The notion of 18,000 lb (80 kN) equivalent single axle load as the basis for loading in pavement design emerged from these tests. Among many things, this testing discovered Miner’s Law, i.e., doubling wheel loads increases pavement damage to the fourth power. The exponential relationship between wheel loads and pavement damage is why truck owners pay high road use taxes; trucks do the most damage.
Since the 1950s, machines were invented that apply truck wheel loads (or greater) without drivers and do this quickly. These large machines go by different names that all render 20 years of wheel loads in a matter of months; accelerated load facility, heavy vehicle simulator, etc. Often housed at universities connected to state departments of transportation, or housed by the latter, these machines have tested thousands asphalt and concrete pavements. This research via load testing is the norm for conventional pavements. Testing led to longer-lasting designs, most of it funded by tax dollars. Such research superbly uses tax resources because of the huge ROI to road networks costing billions since accelerated load testing is typically in the millions.
For permeable interlocking concrete pavements or PICP, accelerated load testing validated ICPI design tables for subbase thicknesses published in 2011. Load testing was conducted in 2014 by the University of California Pavement Research Center in Davis (see picture below). The design tables developed by the Center with help from mechanistic modeling provide for slightly thinner bases in some situations than those in the ICPI design tables. Accelerated load testing doesn’t come cheap: the testing at Davis cost about $400,000, co-funded by the ICPI Foundation, California paver manufacturers and the Cement Association of California and Nevada.
|Above: Load testing on PICP needs to continue,
and also expand to ICP.
Institutionalization from this industry investment include Caltrans PICP design tables in their pervious pavement literature, and in the ASCE national PICP standard. While the testing certainly confirmed that heavy trucks can repeatedly traverse PICP, additional accelerated load testing is needed using stronger subbases thereby expanding PICP use to busy urban streets, while storing and infiltrating stormwater.
While there has been accelerated load testing (mostly in the 1980s) of interlocking concrete pavements (ICP) here and overseas, they have taken mostly an experiential, empirical path toward validation of their structural capacity. Validation has come from millions of square feet used in airfield and port applications seeing wheel loads as much as 10 times greater than trucks. For road applications, some of the busiest downtowns have seen repeated bus and truck traffic. Downtown North Bay, Ontario and San Antonio, Texas are examples. Built in 1983, North Bay is likely approaching 4 million standard axle loads and San Antonio around 3 million, built in 1986.
While experience is informative, the interlocking concrete pavement industry might consider systematic full-scale load testing to undergird current structural design methods. A multimillion-dollar investment will put ICP in the same testing league that refined conventional asphalt and concrete pavements over the past several decades. ICP accelerated load testing will instill immeasurable confidence in designers, boost the industry’s technical credibility, and help lead to institutionalization by government road agencies and civil engineers. Like the PICP load testing, funds for ICP load testing will likely come from industry and not tax dollars, since there aren’t yet hundreds of miles of ICP roadways owned by municipal or state transportation agencies.
Success in expanding ICP road applications will come from taking ICP to the point of failure via accelerated load testing. Testing to failure is the sine qua non of pavement research and design. This can add further fuel to justifying lower life cycle costs from investing in ICP.