The Results Are In
Two years in the making, the University of California Pavement Research Center in Davis recently released a report on full-scale load testing of permeable interlocking concrete pavement (PICP). The report proposes revised design charts that reduce the thickness of a subbase thickness chart published in ICPI’s 2011 manual, Permeable Interlocking Concrete Pavements. The revised charts from UC Davis provide more cost-effective thicknesses.
The subbase thickness in the 2011 ICPI manual are calculated using the flexible pavement design methodology in the 1993 Guide for Design with Pavement Structures by the American Association of State Highway and Transportation Officials (AASHTO). Well-known among pavement engineers, this book provides methods for calculating dense-graded base thicknesses under roads. The methods do not cover calculations for open-graded bases for permeable pavements. Conservative adjustments to AASHTO methodology were made to develop the 2011 design chart for open-graded materials.
The UC Davis research validates ICPI’s subbase thickness chart while refining it by considering the number of days per year a subbase sees standing water, i.e., 0, 10, 30, 60, 90, and 120 days. The resulting charts present thinner subbases at the lower end of this range of exposure to standing water when compared to the ICPI chart which assumes high exposures to saturated subgrades and subbases. Subbase thickness also depends on other factors such as the amount of soil support and anticipated wheel loads.
In pavement design, the mix of anticipated wheel loads over the pavement’s life are combined and equalized into 18,000-lb axle loads called ESALs or equivalent single axle loads. This load unit provides a consistent measurement that also relates to pavement rutting. To give an idea of an ESAL, an over-the-road tractor-trailer typically exerts 3 or 4 ESALs with one pass over a pavement. The PICP loaded at UC Davis received over 2.5 million ESALs using a machine simulating truck wheel loads. Pictures of the Heavy Vehicle Simulator (HVS) are above, as well as a video of it running dual truck tires on the test track.
In pavement research, the best way to determine how many ESALs define pavement lifespan in years is to conduct full-scale accelerated load testing. In other words, a pavement is loaded with many wheel passes until it fails or is no longer considered useful. UC Davis pavement engineers conducted load testing with an HVS that accelerates loading, accomplishing 20 years of loading in just five and a half months. For PICP, failure is defined as excessive rutting, typically over 1 in. (25 mm). Interestingly, none of the concrete pavers cracked while the pavement was loaded with truck tires at the UC Davis testing facility while rutting to over 2 in. UC Davis design charts for subbase thicknesses use 1-in. rutting as the failure criteria.
The comprehensive UC Davis study began with a literature review that found little domestic research and a paucity from overseas. The study then load tested some local existing PICP projects with an 18,000-lb truck axle to better understand deflection under it and the pavement strength. The deflection data was used to estimate the stiffness (elastic modulus) of each pavement layer by conducting computer-based mechanistic analysis modeling that correlates modeled and measured stresses, surface deflections, and permanent strains (rutting) to pavement layer strengths. This data was also used to determine subbase thicknesses for full-scale testing at a 96-ft long PICP test track over which the HVS could run truck tires and loads.
The test track included three subbase thicknesses (approximately 18 in. or 450 mm, 27 in. or 650 mm and 37 in. or 950 mm) instrumented to provide data on stresses while loaded and rutting (see diagram below). The weak clay soil subgrade was compacted and non-woven geotextile was placed on the subgrade and sides of the excavation. Above the subbases was a 4-in. (100 mm) thick layer of ASTM No. 57 aggregate, 2 in. (50 mm) of No. 8 aggregate, 3 1/8-in. (80 mm) thick concrete pavers and permeable jointing aggregate. A concrete curb restrained the No. 57 aggregate, bedding and pavers. The figure below shows a cross section of the test track. The aggregates were granite quarried from the foothills of the Sierra Nevada Mountains.
The testing represents the first full-scale load testing on PICP in the western hemisphere and one of a few studies globally that examines the structural response of open-graded bases to wheel loads. The UC Davis design charts go to one million ESALs, the maximum loads also provided on the 2011 ICPI design chart. The revised charts will appear in an emerging ASCE national standard on PICP as well as in an updated edition of the ICPI PICP manual. Both are scheduled for release in 2016.
The project was funded by the ICPI Foundation for Education and Research, the Concrete Masonry Association of California and Nevada, the California Nevada Cement Association and the Interlocking Concrete Pavement Institute.