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SURFACE TREATMENT INNOVATIONS
By WILLIAM T. SUNLEY, Engineer of Local Roads and Streets
Illinois Department of Transportation
Credits to: John Saner, Local Policy Development Engineer
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Bituminous surface treatments have long been the principal tried-and-true method of providing wearing surfaces on roads. Their familiarity has not only been reason for their success, but it also has limited the introduction of new ideas in the construction of surface treatments. This article will briefly touch on four ideas that hold promise in this area: precoated aggregates, sandwich seals, adjusted nozzles and a wet film thickness gauge for distributor calibration.
Precoating surface treatment aggregate to improve adhesion of the aggregate to the asphalt binder is not a new idea, but it is probably the single change that can be made to most improve surface treatment performance. The initial cost of precoating (about $3 per ton) has caused many to shy away from its use. This $3 cost is based upon limited work and is probably high. Lesser costs have been achieved by using less asphalt (for example, using PEP penetrating emulsion prime) in an emulsion mix type pugmill. Several studies have shown that the use of precoated aggregates has resulted in significant cost savings by the use of less aggregate during construction and by the extended service life. Precoating aggregate in the long run will provide the "most for your money".
An upside-down or inverted surface treatment also known as a "sandwich seal" is quite innovative. It comes to this country from France and is also sometimes called a "French Dressing". Basically, it can result in an A-2 surface treatment with only one application of bituminous material being used. To construct a sandwich seal, first a clean coarse aggregate is spread at about 80 percent of the normal rate (for example: 20 Ibs./sq. yd. instead of 25). A heavy application of bituminous material is then applied over the untrafficked coarse aggregate, followed by the second layer of a finer aggregate usually in an excessive amount. Exceptional friction or skid resistance properties with good macrotexture in the surface have been found on initial jobs. High speed curves may be an application where a sandwich seal could be of most benefit because of its high friction and coarse macrotexture. Another benefit is that no coarse aggregate is loose and flying, considerably decreasing the chances of headlight and windshield breakage. A sandwich seal may also help in absorbing asphalt bleeding up from an underlying asphalt-rich pavement by the use of a coarser aggregate in the first lift. Chris Isbell, former Superintendent of Highways of JoDaviess County, believes he can build a sandwich seal using less material and at less cost than an A-2 Surface Treatment.
In July of 1988, A. E. "Sonny" Kleiss, Township Road Commissioner in Tuscola Township, Douglas County, had the first sandwich seal in this country. A half mile of road was quartered into four test sections in
Page 20 / Illinois Municipal Review / May 1991
order to look at different material combinations in a sandwich seal. In the northbound lane a 5/8 inch limestone chip coarse aggregate with 0.45 gal./sq. yd. of HFE-90S was placed. A 1-inch chip with 0.50 gal./sq. yd. was placed on the southbound lane. The half mile was split north and south with a turkey grit (pass #4 —retained #10 sieve) being placed on the north quarter mile and a 3/8 inch chip being placed on the south quarter. All sections have gone through three winters with good performance in terms of visual inspections of durability.
The varying asphalt demand of the existing roadway surface often makes it difficult to spread the "correct" amount of asphalt for a surface treatment transversely across the road. The wheelpaths usually have an excess of asphalt due to chip loss from traffic wear. The Texas Highway Department has developed a method of machining standard nozzles so that the spray width remains equal but the volume of spray can be increased 20 or 30 percent. This is done by machining the vee-shaped grove in the nozzle deeper. These nozzles are placed in the spraybar so that the higher volumes are placed outside and between the wheelpaths. In one Texas job, 0.35 gal./sq. yd. of asphalt emulsion was applied in the two wheelpaths while the areas at the centerline, between wheelpaths, and at the edge of pavement received 0.47 gal./sq. yd.
A simple gauge has been developed to measure a distributor output. A notch gauge very similar to a wet film thickness gauge used to measure paint thickness was made and calibrated. A metal plate was machined so that asphalt film thicknesses corresponding to gallons per square yard (gsy) from 0.24 gsy to 0.46 gsy in 0.02 gsy increments could be measured. This gauge is easily made and used.
By using this gauge along with a flat piece of material to collect the liquid asphalt sample on the road, a quick, easy measurement of the spraybar output can be made.
For further information on this gauge or on any of the techniques discussed in this article, contact John Saner in my Office at (217) 785-5019. •
May 1991 / Illinois Municipal Review / Page 21
