Success from Failures
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 on load testing from trucks conducted by the American Association of State Highway Officials in the 1950s. The notion of an 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. This 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 quickly apply truck wheel loads (or greater) without drivers. These large machines go by different names—accelerated load facility, heavy vehicle simulator, etc.—but all render 20 years of wheel loads in a matter of months. Often housed at universities or state departments of transportation, these machines have tested thousands of asphalt and concrete pavements. This research via load testing is the norm for conventional pavements. Testing, most of it funded by tax dollars, led to longer-lasting designs. Such research superbly uses tax resources because of the huge ROI: accelerated load testing costs millions; road networks cost billions.
For permeable interlocking concrete pavements (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). 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.
Institutionalization from this industry investment include Caltrans PICP design tables in their pervious pavement literature, and in the ASCE national PICP standard to be released later this year. 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 withstanding 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.