SPS-2 Tech Day attendees examine pavement test sections on U.S. 23 in central Ohio.
SPS-2 Tech Day attendees examine pavement test sections on U.S. 23 in central Ohio.

Why and how do pavements perform as they do?

That was the opening question at a workshop and open house that took place in Delaware, Ohio on May 22, 2019. The event was an SPS-2 Tech Day, associated with the Federal Highway Administration’s (FHWA) Long Term Pavement Performance (LTPP) Program. That’s a hard question and finding an answer means that the program’s research has spanned decades and generated extensive data—with more to come.

Program Overview
The LTPP Program is the world’s most comprehensive study of in-service pavements. It was started in the late 1980s with two components: general pavement studies (GPS), which examined existing roadway sections, and specific pavement studies (SPS), that included multiple test sections at one location. More than 2,500 test sections have been investigated on in-service highways in the United States and Canada.

Test sections are typically 500 feet long, and detailed information is collected either by the FHWA or the state and provincial highway agencies. The information includes materials properties, traffic volumes and loading, weather, any rehabilitation or maintenance, and performance data.

The SPS-2 experiment examines the effects of subsurface drainage, warp and curl, dynamic load response, and more. The design factors studied include concrete strength, base type, drained versus undrained construction, lane width, shoulder type, and pavement thickness. Details of dowel bar and tie bar placement are also addressed. Site factors studied include coarse-versus-fine soils and climate zone.

All of this data is uploaded annually to InfoPave (http://bit.ly/2Eeajeg), a website where users can access the pavement performance data and findings from data analyses, the complete library of LTPP publications, and many additional pavement-related tools. LTPP data was the primary source used to develop AASHTOWare PavementME design software.

SPS-2 pavements were built from the early 1990s through 2000, with data collection beginning during construction. and continuing to the present day. Detailed construction reports and materials sampling and testing plans were developed for each project; these can be found on InfoPave.

Field evaluation at an SPS-2 Tech Day held in Washington.
Kevin Senn Field evaluation at an SPS-2 Tech Day held in Washington.

While the SPS-2 project was designed to study structural factors, the second piggy-backed study was launched in 2013 to examine pavement preservation strategies. Common pavement preservation techniques include diamond grinding or diamond grooving; partial or full depth repair; dowel bar retrofit; joint sealing or resealing; slab stabilization; longitudinal crack stitching; and buried treasure. The Washington State DOT leads this study, with Arizona, California, Colorado, Georgia, Kansas, and North Carolina participating. These states realized that given the age and relatively good condition of the SPS-2 test sections, pavement preservation techniques could be undertaken to learn the optimal timing for the use of pavement preservation treatments and the resulting improvement.

Local Learning: Tech Days
One important way of disseminating all this information is state-hosted SPS-2 Tech Days. Tech Days involve both presentations and field evaluation of SPS-2 test sections, which provides the opportunity to assess which design features have performed the best as well as which pavement preservation strategies are most effective.

A California highway being evaluated by Tech Day participants.
Kevin Senn A California highway being evaluated by Tech Day participants.

At a recent Tech Day in May 2019, attendees were able to examine pavement test sections on U.S. 23 in central Ohio, where approximately three miles of portland cement concrete (PCC) in the northbound lanes were part of an SPS-2 experiment to evaluate these five design features:

  • 8-inch versus 11-inch thickness
  • 550 psi versus 900 psi flexural strength concrete
  • 12-foot versus 14-foot travel lane widths
  • base types
  • drainage

For this stretch of pavement, the 2018 average daily traffic was 28,599 vehicles, with 16% being truck traffic.Some of the information shared at the Ohio Tech Day includes:

  • Jointed plain concrete pavement (JPCC) constructed on a layer of permeable asphalt treated base offered superior smoothness compared to sections constructed on lean concrete base or on dense-graded aggregate base.
  • JPCC also had low joint faulting and less cracking.
  • Wider slab sections showed less faulting than conventional width slabs.
  • Thicker slabs demonstrated better crack resistance than thin ones.
  • Recommendations were also made for dowel alignment: minimize longitudinal restraint and avoid misalignment and place dowels parallel to both the pavement surface and the longitudinal axis. Maximum shear load transfer capacity occurs when the dowels are centered longitudinally and placed at mid-depth.

For more information on SPS-2 Tech Days, see the December 2018 MAP Brief, Performance Experience and Lessons Learned from SPS 2 Test Sections of the Long-Term Pavement Performance Program (LTTP); go to http://bit.ly/2PeZI9h.

More to be Done: Studying Pavement Preservation
Pavement preservation is a critical part of keeping roads smooth, rideable, and structurally sound over the long term. The current recommendations on how and when to perform preservation techniques, however, assume all pavement types and strategies perform equally. This leads to over-simplification and anecdotal approaches in lieu of solid data and analysis.
Continuing SPS-2 studies will perform cradle-to-grave analyses of the test sections and the preservation treatments used, with a focus on pavement life extension and cost-effectiveness. The results will be used to establish pavement management system triggers.

To maximize the opportunities afforded by the research, road owners must allocate funding for planned preservation cycles and develop a feedback loop between members of the design, construction, maintenance, and administration teams. Intervention cycles need to occur earlier in the performance period than they have in the past, and owners and agencies must rigorously collect and analyze data to justify this schedule shift.

Even though it started in the 1980s, the LTPP project is far from over. States or other stakeholders who want to participate in the experiment—the world’s largest ongoing concrete preservation research project—can contact Jeff Uhlmeyer at [email protected], 360-709-5485.

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