Along with advanced technological change come alterations in the implements required to take maximum advantage of the new technology. An example of this is North America ’s growing use of performance-grade pavements, especially stone matrix asphalt (SMA). This has also stirred a renewed interest in European-style screeds. A primary example of one of these is the Ingersoll-Rand ABG (Allgemeine Baumaschinen Gesellschaft mbH) high-density, double-tamping bar design.
IR-ABG markets four versions of its high-density Duotamp vibratory screeds. The fixed-width VDT 120 paves 9-feet, 10-inches wide, and the VDT 121 paves 8-feet, 2-inches wide. The VDT-V 79 has a hydraulically extendable screed from 8 feet, 2 inches to 16 feet, 5 inches, and the VDT-V 89’s extendable screed extends from 9 feet, 10 inches to 19 feet, 8 inches.
All four models feature Ingersoll-Rand ABG’s dual tamper bars which, combined with their massive weight, allow the paver to achieve high degrees of compaction at the screed and, ultimately, exceptional smoothness. A VDT 120 used with an ABG Titan 525 paver produced 92-percent compaction at the screed last year in an SMA project in Virginia . ABG’s high-density screeds are available with Titan models 326 EPM, 8820 and 525.
Recent modifications to the Virginia Department of Transportation’s (VDOT) specifications regarding the aggregate materials used for producing SMA bituminous asphalt, its paving placement and compaction, have fostered something of an unusual occurrence in the Old Dominion State .
Three of the state’s larger asphalt paving contractors have selected similar IR-ABG double bar tamper bar screed laydown machines to pursue comparable in-state SMA paving projects.
Interestingly enough, two of them, Mega Contractors Inc. and Lee Hy Paving Corp. are working on similar VDOT contracts on the opposite sides of the same road. The third, Virginia Paving Inc., is working on another comparable interstate highway project further north on I-95.
Executive Vice President Richard Schreck of the Virginia Asphalt Association, and one of the foremost proponents of SMA, has been quoted saying — “it’s not rocket science” — and, since his father is a certified professional rocket scientist, he should know. Schreck also compared producing high-quality SMA to being similar to creating a good ‘by the dot painting’ … “just connect the dots; all the dots, no shortcuts, not even one.”
The ABCs of SMA
Stone Matrix Asphalt is a gap-graded HMA that relies on stone-to-stone contact to provide strength, a rich mortar binder to provide durability and dependable laydown machines and modern asphalt compactors to reach optimum densities. SMA is a tough, rut-resistant, long-lasting and cost-effective paving mixture. The slight premium production cost (20 to 25 percent more than a typical dense-graded HMA because of the durable aggregates, higher asphalt content in a modified binder with fibers required) has proven to be well justified in light of SMA’s extended life cycle and reduced maintenance needs.
Although it has only been in general use in this country since about 1991, SMA has been put down in Europe for decades. According to the current thinking, the recognition for its development and introduction to the asphalt industry goes to Germany . In this country, Virginia , Maryland and Georgia have been given credit for being among the first to embrace its use. Although the long lasting mixture is used here and in Europe primarily as a rut-resistant surface course, Virginia has now implemented its use as an intermediate course underlying the wearing surface.
“In the mid to late 1980s, Virginia began some pilot SMA projects and Mega was one of the first contractors to do some of this preliminary experimental type paving with SMA. To date we (Mega) have probably placed more SMA tonnage than any other contractor in the state. In fact, one of the first SMA projects we did in the 1980s was a ‘mill & fill’ overlay on I-95 in front of a rest area. It is still in good condition with no further maintenance required since then and that is a heavily traveled stretch of highway,” says the president of Mega Contractors, Paul Owen Lanier II.