Road Science Tutorial
“NovaChip is a paving process that places a thin gap-graded (3/4- to 3/8-inch) mix over a liquid membrane – a polymer modified emulsion that is specially designed to seal the existing roadway and provide a strong bond with the NovaChip asphalt material,” the DOT says.
Similar to conventional HMA, NovaChip is easily produced at a HMA facility and placed with little difficulty, according to DOT. The main difference in the placement of NovaChip verses typical HMA is the use of a specialized paver, which allows the placement of the initial polymer modified “membrane” and NovaChip surfacing in a single pass.
The purpose of compaction during NovaChip placement is to seat the asphalt into the membrane rather than to obtain density. Compaction is partially obtained by vibration at the screed of the paver, and then by one or two passes from double-drum rollers operating in the static mode.
Pavers appropriate for NovaChip are similar to those for hot mix paving, but with an emulsion tank added for the initial placement. Immediately following placement of the polymer emulsion membrane, the ultrathin gap-graded hot mix asphalt is applied, which can be placed as thin as 0.5 inches (12.5mm) to 1.5 inches (38mm) thick. The emulsion cools quickly and rapidly bonds the asphalt to the pavement rapidly. Following the static compaction, the new surface can be reopened to traffic within 300 feet of the train.
The open-graded mix is seated by a tamper bar on the modified paver, and further seated by a roller which follows the paver. As the membrane cures as its temperature drops, the roller must work within a window of temperature range. Typically, traffic can be allowed on the surface treatment in as little as 20 minutes after placement.
The surface texture of NovaChip is very similar to that of a typical open-graded friction course, with the benefits of reduced surface water spray from vehicles, improved friction, and attenuation of noise at the pavement/tire interface.
NovaChip surfaces have significantly higher pavement surface friction numbers compared to dense-graded HMA wearing course, NCAT reported in 1997. It is marketed as an alternate to conventional chip seals, micro-surfacing, and open-graded friction courses.
Variations of NovaChip
A variation of this process involves placement of hot binder, immediately followed by an open-graded surfacing. For example, California’s rubberized open-graded wearing course consists of an initial sprayed layer of polymerized binder, deposition of 3/8-inch aggregate, and an immediate spray of rubberized binder, all in one pass by a single machine, followed by finish rolling. The wearing course typically is placed 5/8 up to 7/8 inch thick, depending on Caltrans specs, and on Calif. 101, was being placed 12.5-feet wide – a full lane.
A paver modified for this process will have an integral spray system that sprays polymer modified binder in front of the mix being laid, followed immediately by aggregate, and then a second spray of rubberized binder. The advantage is less mess, with no traffic or construction equipment tracking the binder before the aggregate is placed.
The first binder, the polymerized emulsion, is placed at 175 degrees F, while the rubberized binder comes out between 295 to 325 degrees F. Once the aggregate hits the oil below, it breaks it, and wicks up binder through capillary action through the aggregate mix, giving it its binding power. The immediate, subsequent application of rubberized binder from the modified paver coats the entire structure, penetrates downward, and melds with the bottom lift of binder, which retains its heat, creating a “hot-on-hot” near-monolithic wearing course that will hold up to the Central Valley’s punishing traffic and heat waves.
NovaChip cures within five to 10 minutes, while the rubberized wearing course takes a little longer, depending on how thick the wearing course is placed. One such spray paver has a tamper bar which compacts the mix behind the screed to 95 percent, while still keeping its open-graded character. Sweeping and sometimes crack filling are the only preparation needed ahead of a rubberized open-graded wearing course.
In addition to new techniques of bituminous two-lift paving, two-lift portland cement concrete has been placed in Europe and the United States. In fact, field examples of two-lift PCC construction are more numerous in the United States than conventional two-lift HMA paving.
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