Market Infiltration

Load test performance opens agency specs for permeable interlocking concrete pavement

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Civil and pavement engineers can design permeable interlocking concrete pavement (PICP) with confidence for many municipal streets and parking lot applications thanks to findings from one of few studies globally to examine open-graded PICP bases’ structural response to wheel loads. Through full-scale PICP testing, University of California, Davis researchers have validated a 2011 Interlocking Concrete Pavement Institute subbase thickness chart and justified refinements by considering the number of days per year a subbase sees standing water. A resulting chart presents thinner subbases at the lower end of a 0–120-day range of exposure to standing water compared to the ICPI standard, which assumes high exposures to saturated subgrades and subbases in all design cases.

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Following placement of pressure plate stress gauges in open-graded, 18- to 37-in. thick aggregate bases, crews mechanically installed about 2,600 sq. ft. of permeable interlocking concrete pavement on a 96-ft. track for the UC Davis research.

Subbase thickness also depends on other factors such as the amount of soil support and anticipated wheel loads. In pavement research, UC Davis staff observe, the best way to know how long a structure lasts is by conducting full-scale accelerated load testing. Under such conditions, a PICP is loaded with many wheel passes until it exceeds allowable tolerances. UC Davis engineers conducted load testing with a Heavy Vehicle Simulator (HVS) to gauge in a month the effects of 20 years of loading. Most of the rutting from the accelerated loading occurred in the soil subgrade as was expected. While rutting occurred up to 2 inches, none of the concrete pavers cracked. UC Davis proposed design charts for subbase thicknesses that use 1-in. rutting as the failure criteria.

The university study began with a literature review that found little domestic or international research. Investigators then load tested local PICP projects with an 18,000-lb.-rated truck axle to better understand deflection and pavement strength. 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.

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Engineered to replicate in one month of test cycles the effect of two decades of traffic service, the Heavy Vehicular Simulator subjected the 96-ft. PICP track to an 18,000-lb. single axle loading.

The track facilitated the first full-scale load testing on PICP in the Western Hemisphere. It included three subbase thicknesses (approximately 18 in., 27 in. and 37 in.) instrumented to provide data on stresses while loaded and rutting. The weak clay soil subgrade was compacted and non-woven geotextile placed on the subgrade and sides of the excavation. Above the subbases was a 4-in. thick layer of #57 aggregate, 2 in. of #8 aggregate, and 3-1/8 in. thick concrete pavers with permeable jointing #8 aggregate. A concrete curb restrained the #57 aggregate, bedding, and pavers.

The UC Davis design charts go to one million 18,000-lb. equivalent single axle loads (ESAL) as also provided on the 2011 ICPI design chart. Revised charts will appear in an emerging American Society of Civil Engineers national standard on PICP as well as in an updated edition of the ICPI PICP manual. Both are scheduled for 2016 release.

Assisting ICPI in project funding were the ICPI Foundation for Education and Research, Concrete Masonry Association of California and Nevada, and California Nevada Cement Association. A copy of the UC Davis report is available from ICPI Technical Services Director David Smith, [email protected].

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