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Paving Without Asphalt or Concrete

Asphalt and concrete account for the vast majority of paving today. These materials are very different: asphalt is a mix of aggregate and leftover heavy hydrocarbons after more valuable, lighter fractions of crude oil have been extracted; concrete is a hardened, rock-like material usually made by mixing Portland cement with sand, coarser aggregate, and water. In the green design community, there is some interest in avoiding these conventional pavement products in favor of more natural, less energy-intensive materials using locally available aggregates and bio-based binders. Road Oyl™ Resin Pavement™ from Soil Stabilization Products of Merced, California is such an alternative.

Road Oyl, introduced in 1991, is an emulsion formulated from pine rosin and pitch in water. (An emulsion, for those whose high school chemistry is rusty, is a mixture in which globules of one liquid are suspended—but not dissolved—in a second liquid.) The pitch and rosin, which comprise roughly 50% of Road Oyl by weight, are coproduced with other timber products from southern pine in the southeastern United States. Pine pitch is a black, viscous “tar” derived from the distillation of wood; before the development of coal-tar pitch, pine pitch was used to impregnate building papers and felt, and to saturate hemp fiber for oakum. Pine rosin is the residue from distillation of turpentine oil from raw turpentine. The Road Oyl liquid is brownish in color with a mild odor. When rubbed between the fingers, it becomes extremely sticky as the water evaporates (and is hard to wash off!). It is shipped as a liquid by the 6,000-gallon (22,700 l) truckload or 55-gallon (210 l) drum.

For paving, Road Oyl is mixed with crushed stone—usually decomposed granite—that includes a range of particle sizes, from ¹⁄₂” (13 mm) or ³⁄₄” (19 mm) down to fines or filler that pass through a #200 sieve (75 µm). In the company’s recommended ³⁄₄” aggregate mix, one-quarter to one-half of the aggregate should pass through a #8 (2.4 mm) sieve. Ideally, the aggregate should not include organic matter or clay particles, though clayey mixes are sometimes used satisfactorily. The aggregate is typically wetted (2% to 4% water), then the Road Oyl emulsion is added at 6% to 9% by weight, depending on the application and the aggregate. The aggregate and emulsion mixture is then laid down using fairly conventional paving equipment and practices. In remote areas, such as along park trails, small-scale equipment and hand labor are often used.

After compacting or rolling and several days of cure time, Resin Pavement forms a hard, durable surface that contains neither petroleum derivatives nor cement. Unlike asphalt, which hardens by cooling, or concrete, which hardens through a chemical hydration process, Resin Pavement hardens through evaporation of the water carrier. And harden it does. The compressive strength of Resin Pavement typically exceeds that of conventional asphalt by more than threefold. It is also highly water-resistant and can withstand heavy wash and even flooding. In some applications, where greater compressive strength is required or the underlying roadbed is weak, the company’s EMC Squared® soil stabilization product is applied to the soil prior to laying down the Resin Pavement—soil stabilization products will be addressed in a future article. Because Resin Pavement is not softened by contact with hydrocarbon solvents (as asphalt is), this system is also commonly used around fuel storage facilities. The Road Oyl emulsion darkens the aggregate slightly but maintains the same basic look, which makes this system desirable in natural settings. Note that this is not a porous paving product; with the high content of fines (required for proper hardening), the cured pavement is highly impermeable.

Test data provided to EBN by the company showed Road Oyl to be quite clean compared with asphalt. As part of the U.S. Environmental Protection Agency National Estuary Program, testing was conducted by Analytical Chemical Testing Laboratory of Mobile, Alabama in the Spring of 1999 to verify the suitability of Road Oyl and EMC Squared for use around sensitive coastal estuaries. Total petroleum hydrocarbon (oil and grease) levels in runoff from newly applied Road Oyl pavement were measured at 2 parts per million (ppm) immediately after installation. To provide comparison, the laboratory reported on a study of stormwater pollution from new asphalt pavement in which hydrocarbon levels exceeded a company’s NPDES permit limit of 15 mg/l in six out of seven outfalls. This is not surprising, since the typical oil and grease content in asphalt drops from several hundred thousand ppm (greater than 20%) when new to about 30,000 ppm (3%) after aging for a period of years, according to Analytical Testing Laboratory.

Resin Pavement samples (as opposed to runoff) were also tested for 63 volatile organics (VOCs), 16 polynuclear aromatic hydrocarbons (PAHs), and 9 metals. None of these regulated substances were found (i.e., the levels of all were below the detection limits of the measurement apparatus). Newly applied asphalt, on the other hand, frequently contains measurable levels of many regulated VOCs and PAHs, according to the laboratory.

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