Chantilly, VA (PRWEB) May 13, 2015
The UC Davis research validates ICPI’s subbase thickness chart published in 2011 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 in all design cases. Subbase thickness also depends on other factors such as the amount of soil support and anticipated wheel loads.
In pavement research, the best way to know how long a pavement lasts is by conducting full-scale accelerated load testing. In other words, a pavement is loaded with many wheel passes until it exceeds allowable tolerances. Instead of taking 20 years to load all the wheel passes onto a pavement, UC Davis pavement engineers conducted load testing with a Heavy Vehicle Simulator (HVS) that accelerates loading; 20 years of loading is completed in months.
Most of the rutting from the accelerated loading occurred in the soil subgrade as was expected. While rutting occurred up to two inches, none of the concrete pavers cracked. UC Davis proposed design charts for subbase thicknesses that use 1 inch rutting as the failure criteria.
The comprehensive UC Davis study began with a literature review that found little domestic research and even less from overseas. The study then load tested some local existing PICP projects with an 18,000 lb (80 kN) 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 (29 m) long PICP test track over which the HVS could run truck tires and loads.
The test track included three subbase thicknesses (approximately18 in., 27 in. and 37 in. or 450 mm, 650 mm and 950 mm) instrumented to provide data on stresses while loaded and rutting. 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 four inch (100 mm) thick layer of ASTM No. 57 aggregate, two inches (50 mm) of No. 8 aggregate, 3 1/8 inch (80 mm) thick concrete pavers and with permeable jointing No. 8 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 18,000 lb. equivalent single axle loads as also provided on the 2011 ICPI design chart. The revised charts from UC Davis 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. As a result of validating performance up to 1 million ESALs, civil and pavement engineers can design PICP with confidence in many municipal streets and parking lot applications.
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. A copy of the UC Davis report is available upon request from firstname.lastname@example.org.
The Interlocking Concrete Pavement Institute (ICPI) represents producers, suppliers, contractors, design professionals and consultants and supports its members in assuring segmental concrete pavement systems are the preferred choice for sustainable and environmentally friendly pavements in North America. ICPI aims to increase awareness, use and acceptance of segmental concrete pavement systems in North America through the development of marketing and technical resources for design professionals, contractors and homeowners. To learn more about ICPI, visit http://www.icpi.org.