Tennessee has claim to some of the best roads in the country, and this year the state launched a program that represents a sea change in the way it will protect its substantial investment in this infrastructure.
The state-maintained highway system of more than 14,000 miles, which includes about 1,100 miles of Interstate, is consistently ranked among the top five in the nation. Additionally, the condition of its roughly 7,600 state-owned bridges and 12,000 locally owned bridges earns Tennessee a ranking of 11th in the nation for bridge quality.
"We have some of the smoothest roads in the country — we've won five Perpetual Pavement Awards," said Jay Norris, special projects coordinator for the Tennessee Department of Transportation. But he went on to say that the cost of maintaining this level of quality has been high due to rising construction materials prices and DOT's traditional focus on full-depth pavement reconstruction.
With Tennessee's population growth soaring and related number of vehicle miles travelled on the state highway system accelerating at an alarming rate, the DOT has been seeking more economical ways to protect and preserve valuable road pavements from the intensifying traffic pounding.
"We read the writing on the wall," said Norris, "and it said we had to do something different. We're very proud of our road system — our asphalt contractors have done a great job — and we have been trying to improve a specific number of lane miles each year under our 12-year pavement management program. But we haven't been able to do it." He noted that budget constraints are the main culprit. "We had to get more bang for the buck," he said.
Micro surfacing, which is designed to protect structurally sound pavement from further distress and oxidation, seemed to be a promising solution as an economical way to preserve pavements.
"In the '90s DOT tried several micro surfacing projects, and they performed well over the years. We found the treatment works best on roads with structurally sound pavements and higher ADT counts, say between 1,000 and 10,000 vehicles per day."
He added that when their roads exhibit cracks, they are usually not deep and emanate from the top down, not bottom-up, indicating cosmetic surface cracking and otherwise sound pavements.
Norris said that as a result of the successful earlier experience with micro surfacing, DOT's Chief Engineer Paul Deggs asked each of the agency's four Operating Centers to allocate 10 percent of their 2008 share of earmarked road maintenance funds to the application of this process.
Each center decides which of its roads to treat and what maintenance procedures to use, based on observations of road conditions and inventory data on file in the pavement management program managed by DOT's Materials and Testing Division. Design specifications of the pavement preservation processes they choose are subject to review and approval by Mark Woods, engineer for Material and Testing.
Tennessee's approach to pavement preservation falls in line with the recommendations of the Federal Highway Administration (FHWA), which is promoting pavement preservation as a way to extend the service life of the nation's roads. FHWA has expressed concern over the sustainability of construction materials and practices used to preserve infrastructure, and Tennessee DOT's use of emulsified asphalt technologies reflects that same concern.
In line with its heightened interest in pavement preservation, TDOT has been trying other surface maintenance treatments as well. One example: The agency has begun awarding chip seal projects — a process it last employed about 25 years ago. Norris said that chip seal is being applied on roads that experience less traffic than 1,000 vehicles per day. A latex polymer-modified asphalt emulsion is specified by DOT for the chip seal, while a light coating of non-modified asphalt emulsion, referred to as a fog seal, is sprayed over the chip seal for that "new-pavement" look, he said.
Still another pavement preservation tactic is the recent installation of thin-layer hot mix asphalt overlays on several small projects across the state, said Norris. As with chip seal, these have been placed on low-ADT roads.
While DOT has been trying chip seal and thin-layer hot mix asphalt incrementally on low-traffic roads ("We're taking baby steps right now," said Norris), it has embraced micro surfacing of high-ADT roads in a big way — in just one year.
In 2007, practically speaking, there was no statewide micro surfacing program, but for 2008, DOT awarded approximately $12 million worth of contracts to treat some 500 miles of roads across Tennessee. Among the four bidding specialty contractors was Vance Brothers, a Kansas City, MO-based firm that manufactures, distributes and installs liquid asphalt surface treatments, asphalt mix paving overlays and other pavement-related products.
Vance was awarded three of the micro surfacing contracts worth a total of $2 million, according to Tim Harrawood, the company's southern region manager. These included 30 miles in Decatur, Carroll and Weakley Counties; 12-1/2 miles in Perry County; and 5 miles in Wayne County.
The work crew employed a Bergkamp M1 mobile mix paver to apply the micro surfacing, a cold-mix material comprised of mineral aggregate, portland cement, water, and a latex polymer-modified asphalt emulsion. Micro surfacing can be spread in variable thicknesses — for example, to fill wheel ruts and provide leveling — and is designed to be a quick-traffic system that is ready to accept traffic within an hour. No rolling is required, and a tack coat is unnecessary before micro surfacing unless the existing pavement is concrete.
Harrawood said the aggregate for the micro surfacing was supplied by Vulcan Materials of Dixon, while the latex polymer-modified CSS-1HP asphalt emulsion was provided by the Memphis plant of Ergon Asphalt & Emulsions. Ergon formulates the cationic emulsion grade using SBR (styrene-butadiene rubber) latex polymer manufactured by BASF Corporation. According to Chris Lubbers, BASF Corporation's senior technical service engineer for construction polymers, latex polymer helps bind the asphalt, mineral aggregate and fines together.
"SBR latex allows early strength development in the micro surfacing mix and a quick return to traffic in an hour or less," Lubbers said. "It also improves resistance to vertical and horizontal deformation; increases abrasion resistance; and helps produce a tougher, more resilient mix," he added.
Vance Brothers applied the relatively small aggregate, of which 90 percent passed a No. 4 sieve, at the rate of about 22 pounds per square yard to produce a micro surface between 3/8-inch and 1/2-inch thick.
The contractor's micro surfacing crew finished the three projects in September. DOT will closely monitor the performance of the micro surfacing over time, but so far it has done well, according to Norris.
"The micro surfacing has done what we asked it to, and we're pleased with the product," he said. "We expect to do the same amount of micro surfacing next year," he concluded.
|Paul Fournier is a writer specializing in construction-related topics.|