Changes Mean Choices

TxDOT Looks for Better Roads, Adds New Mixes to Specification Book

By Mary Lou Jay

Hot-mix asphalt pavements are one tool TxDOT is using to give Texans the smoothest, safest roads possible. New mix types recently made the grade and were added to the department's specifications.

Dale Rand, flexible pavements branch director for TxDOT

There's no more one-size-fits-all for Texas' hot-mix asphalt pavements. After more than four years of research and testing, the Texas Department of Transportation has added three new mix types to its specification book.

"Item 341 was a dense-graded hot mix that we used for years," said Dale Rand, TxDOT's flexible pavements branch director. "The disadvantage was that we were using the same specification for an interstate as we were for a parking lot. We wanted specifications more specifically designed for certain applications.

"We took the best of what was going on in the other states, in Europe and around the world. We wanted to find mixes that would last longer and perform better than conventional mixes, and also that would have the lowest cost. We spent about four years in development of the project."

The new mixes include:

• Item 342, a permeable friction course designed to optimize road safety. Used only as a surface mix on high-speed roadways, this mix has at least 18 percent air voids, which means water will quickly run through it to an underlying layer and drain off. The mix reduces water splash and spray and thus the number of wet-weather accidents.
• Item 342 is made from a combination of seal-coat aggregates - some with tire rubber and fiber - with one-size aggregates that can range from .5 in. to No. 4 size. The rubber in the mix helps reduce road noise as well. >>
• Item 344 is the Texas version of Superpave, an all-purpose, performance-design mix that can be used for anything from base to surface courses. This mix is designed for use with a Superpave gyratory compactor, and the amount of asphalt can be adjusted depending on the type of application. (The higher the volume asphalt, the less susceptible the pavement is to rutting.)

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The mix can be made with six different aggregates, from large stones for base courses to fine stones for the surface courses.

The stone matrix asphalt mix is similar to the Item 342 mix but with all the voids filled by mineral filler. It features a high-asphalt, stone-on-stone content as well as fibers and/or tire rubber to reduce rutting.

Rand called SMA the "Cadillac" of hot mix, and it is usually the most expensive because of all the additives. With its excellent resistance to rutting and cracking, it works well as a surface mix on high-demand roadways.

Requiring smoother rides TxDOT updates it specifications book every 10 years, but this is the largest change in some time, Rand said. "One of the other major things we've done is implemented a new Item 585, our ride-quality specifications," he added. "We're now using an inertial profiler - a laser profiler - to measure the ride quality on all of our projects. We have bonus/penalty schedules set up as well. We want to get smoother pavements."

In addition, the department has taken steps to address specific problems with joint densities. "We are now requiring contractors to run quality-control tests to measure the density of the joint within the specification," Rand said. "They must do tests for both physical and thermal segregation."

He said that overall, TxDOT has made 50 to 60 significant changes in its specification books this time, all designed to provide smoother, longer-lasting roads.