TxDOT Measuring Options for Highways

A Divided Highway Project is First Comparison by the Department

By Sean Donahoe

The Texas Asphalt Pavement Association has been lobbying TxDOT for several years to build a roadway that would demonstrate the durability and cost-effectiveness of an asphalt concrete pavement design - called perpetual pavement - in a side-by-side comparison with a traditional continuously reinforced concrete pavement.

The idea has come to fruition on a stretch of State Highway 114 just north of Fort Worth in Wise County from Rhome to the Denton County line.

"The Hot Mix Association approached us about doing a project in 1996," said TxDOT district materials engineer Richard Williammee.

TxAPA was formed in 1944 by a group of highway contractors to improve the quality of hot-mix-asphalt pavements and their usage. In 2001 the organization changed its name to the Texas Asphalt Pavement Association from the Hot Mix Association of Texas. The organization has a strong relationship with TxDOT and recently developed the Quality Hot Mix Asphalt Pavement Awards Program in conjunction with TxDOT.

Among the association's members is the firm of Duininck Brothers of Prinsburg, Minn., winners of the 2003 Sheldon G. Hayes award given by the National Asphalt Pavement Association. The company is the paving contractor on the 4.5-mi. stretch of SH-114, a heavy-use highway. The section is located east of U.S. 81 heading eastbound toward Dallas from Wise County to the Denton County Line and features 2.25 mi. of perpetual pavement preceded by 2.25 mi. of CRCP.

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Perpetual pavement is classified as a bottom-up form of asphalt-concrete pavement composed of a moisture-resistant rich bottom layer with multiple lifts of performance and/or stone-matrix-asphalt mixes. This type of structure is not subject to traditional bottom-up fatigue damage, but it will eventually experience surface distresses and will require surface renewal.

The structure of the ACP section of SH114 begins with 8 in. of lime-treated subgrade followed by 3 in. of ACP (19 mm max. aggregate size, rich bottom layer), 13 in. of ACP base (25 mm max. aggregate size) and 2.5 in. of ACP (19 mm maximum aggregate size). The surface layer consists of 2 in. of stone-matrix-asphaltic concrete pavement, the layer that sees the most wear and tear and will eventually require maintenance.
The CRCP section will be typical, beginning with 8 in. of lime-treated subgrade followed by 4 in. of ACP (19 mm maximum aggregate size) and topped off with 12 in. of CRCP.

The overall goal is to compare the costs and performances of the two sections, and in the process find ways to improve the quality of perpetual pavements.
"The high percentage of heavy loads traveling eastbound made SH-114 a good choice," said Bill Nelson, TxDOT area engineer for Wise and Jack counties, who is supervising the project.

TxDOT officials say the estimated construction costs are comparable between perpetual pavement and CRCP on the SH-114 project. A major goal of the project is to find out if perpetual pavement has a comparable life-cycle cost to CRCP.

TxDOT has been using perpetual pavement on Texas highways since the 1980s, but the SH-114 project is the first apples-to-apples comparison performed by the department.

The department is going beyond the standard testing requirements on SH-114, enlisting the services of a partner agency, the Texas Transportation Institute, for advanced testing.

Tom Scullion, professional engineer and program manager of the Pavement Systems program at the TTI, is working with TxDOT's Fort Worth District on monitoring and testing throughout construction on SH-114. TTI is an official research agency for

TxDOT and the Texas Railroad Commission based at Texas A&M University in College Station.

Scullion and TTI are using a variety of different methods in assisting TxDOT to ensure quality construction.

"TxDOT and the contractor were not happy with the temperature variations observed during the first day, so they changed operations the next day with much better results," Scullion said. "Thermal segregation is a well-known problem in Texas, especially with asphalt layers placed in the cold winter months. The cold spots in the mat are difficult to compact and often result in small porous areas that can let water into the lower layers."

TxDOT employed an infrared thermal temperature bar developed by >>
TTI to test temperature uniformity of the asphalt. Duininck Brothers and TxDOT were not satisfied with cold spots that appeared behind the paver. This "truck-end" segregation traditionally occurs at the end of every load of asphalt.

Duininck adjusted its methods, utilizing a special materials transfer device, which made it possible to remix the asphalt before putting it in the paver. Temperature profile results improved, and these improvements resulted in the achievement of a uniform density in the finished mat.

TTI and TxDOT are using ground-penetrating radar to verify asphalt-pavement thicknesses and uniformity, as well as running advanced lab tests on the different layers to ensure rut resistance.

Soil sulfates have traditionally been a problem for highways in the Dallas area. No sulfates were found during the geotechnical investigation during the design phase for this project; TTI personnel have not found any during project construction.

To measure pavement stiffness, TxDOT personnel are employing a falling weight deflectometer, a nondestructive device used for structural testing on pavement-rehabilitation projects and to detect pavement structure failure. The device applies dynamic loads to pavement surfaces similar to a heavy moving vehicle.

Dr. Soheil Nazarian, a professor at the University of Texas at El Paso, is assisting the department by performing seismic testing. Nazarian is associated with the Center for Transportation Infrastructure Systems, which coordinates basic and applied research on the nation's transportation infrastructure.

According to the Asphalt Pavement Alliance, a national coalition of state organizations (including TxAPA) committed to furthering the use and quality of hot-mix asphalt pavements, ACP perpetual pavements can last 50 years or more with traditional maintenance. TxDOT has expectations in the 30-year range.

"The plan for the ACP section is to remove and replace the top two in. every 15 years," said Andrew Wimsatt, TxDOT project engineer. "We are hoping that the bottom layers of the ACP pavement structure will perform much longer than 30 years, but we are not sure if that will be the case.

"Since the deterioration of the bottom layers may not result in surface distress for a long period of time, we will be conducting nondestructive testing on a periodic basis to determine if those layers are deteriorating."

The first phase of pavement will be opened to traffic in late June or early July. The project is scheduled to be completed in spring 2005.