About a dozen visitors from NDOT and Granite Construction were at the University of Nevada, Reno on Friday, April 18, 2014 to observe a large-scale pavement testing experiment.
The test was the fourth in a series of 10 tests funded by the National Cooperative Highway Research Program (NCHRP) that are designed to better understand the impact of geosynthetics on pavement performance.
The goal of the research is to develop a methodology to quantify the influence of geosynthetics that can be incorporated into the new Pavement ME Design software developed by AASHTO, or the American Association of State Highway and Transportation Officials.
Pavement ME is state-of-the-art design software that builds on eight years of research by the NCHRP and other transportation agencies to help engineers design better pavements. This research will contribute to that effort by providing more information about how geosynthetic materials impact pavement performance.
"Right now, several assumptions are routinely made in the finite element studies for predicting the performance of reinforced pavements," said Elie Hajj, assistant professor of civil and environmental engineering and project principal investigator. "One of the main purposes of the large-scale pavement testing is to acquire a better understanding of the geogrid and geotextiles behavior under typical traffic loading so that when the modeling is done we can model effectively."
The research is highly innovative in that it tests pavement samples at a scale rarely seen. Most pavement tests are done in small-scale laboratory settings, which often do not fully approximate real-world conditions, or performed on road-size strips of pavement, at great expense and time. Instead, Hajj and his team repurposed a 6-foot tall, 8-foot diameter soil tank that was designed in 2008 for a separate research project but had since been in storage.
Data from instrumented full-scale field studies suggests the 8-foot diameter is well-suited to appropriately simulate traffic-induced loading conditions, thus enabling a comprehensive evaluation under a controlled environment in the laboratory.
"After running the numbers, it was the suitable size," said research associate professor Sherif Elfass, who designed the Tank for the original study. "So no modifications were deemed necessary."
While geosynthetic materials have been widely used for the past 30 years, limited research has dealt with methodologies to quantify their influence on pavement performance in a way that would allow incorporation in the field's standard pavement design and analysis procedures. Accordingly, the University's study caught the attention of a number of local transportation researchers and industry representatives.
The research will be testing both flexible (asphalt concrete) and rigid (Portland cement concrete) pavements. The pavement structure for Friday's experiment consisted of 6 inches of asphalt layer on top of a 10-inch reinforced crushed aggregate base on top of a 54-inch clay layer. A geogrid was placed in the middle of the aggregate layer and dynamic loading was applied while instrumentation captured data about five variables, including vertical stress distribution, deformation, confinement, and slippage condition.
These variables, according to Hajj, are not very well known and the research community is split on how best to address them in current models. These variables can shed light on the mechanics of the geosynthetic-soil interaction.
"These questions have been raised in so many different venues," Hajj said. "If we could provide an answer to these questions it would be a significant step forward toward a better modeling of a geosynthetically-reinforced base."
The research is part of the University's Western Regional Superpave Center (WRSC), one of only five such centers in the U.S. The current research team at the University of Nevada, Reno consists of Dr. Elie Y. Hajj (Principal Investigator-UNR), Dr. Raj V. Siddharthan (Key Researcher), Dr. Sherif Elfass (Key Researcher), Murugaiyah Piratheepan (Research Scientist), Jeffery Des Islets (Graduate Research Assistant), and two undergraduate Civil and Environmental Engineering students.
"The next step is to take and implement the test results, feel confident in the pavement models, and start applying the findings to reinforced pavements in the field," said Hajj.