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One of the most versatile tools in the road maintenance arsenal, Microsurfacing is a polymer-modified cold-mix paving system that can remedy a broad range of problems on today's streets, highways, and airfields.

Like its parent product, slurry seal, Microsurfacing begins as a mixture of dense-graded aggregate, asphalt emulsion, water, and mineral fillers. While conventional slurry seal is used around the world as an economical treatment for sealing and extending the service life of both urban and rural roads, Microsurfacing has added capabilities, thanks to the use of high-quality, carefully monitored materials, including advanced polymers and other modern additives.

A History of Expanding Service

Microsurfacing was pioneered in Germany in the late 1960s and early 1970s. German scientists began experimenting with conventional slurry to find a way to use it in thicker applications which could be applied in narrow courses for wheel ruts, and not destroy the expensive road striping lines on the autobahns.

When the scientists used highly selected aggregates and bitumen, and then incorporated special polymers and emulsifiers that allowed the product to remain stable even when applied in multi-stone thicknesses, the result was Microsurfacing.

Introduced in the United States in 1980, Microsurfacing now is recognized not only as the most cost-effective way to treat the surface wheel-rutting problem, but also a variety of other road surface problems. Microsurfacing is now used throughout Europe, the United States, and Australia and is making inroads into many other areas.

How is Microsurfacing Made and Applied?

Microsurfacing is made and applied to existing pavements by a specialist machine, which carries all components, mixes them on site, and spreads the mixture onto the road surface.

Materials are continuously and accurately measured, and then thoroughly combined in the Microsurfacing machine's mixer.

As the machine moves forward, the mixture is continuously fed into a full-width "surfacing" box which spreads the width of a traffic lane in a single pass. Or specially engineered "rut" boxes, designed to deliver the largest aggregate particles into the deepest part of the rut to give maximum stability in the wheel path, may be used. Edges of the Microsurfacing are automatically feathered.

The new surface is initially a dark brown color and changes to the finished black surface as the water is chemically ejected and the surface cures, permitting traffic within one hour in most cases.

Continuous-load pavers utilize support units which bring the materials to the job site and load the machine while it is working, thus maximizing production and minimizing transverse joints.

A Proven Problem Solver

Using various design mixes, techniques, and equipment, Microsurfacing can be used successfully in these situations:

• In quick-traffic applications as thin as 3/8 inch (9.5mm), Micro-Surfacing can increase skid resistance, color contrast, surface restoration, and service life to high-speed roadways (interstates and autobahns). Such projects are often reopened to traffic within an hour.
• Modern, continuous-load pavers can lay 500 tons of Microsurfacing per day, with no long traffic delays. This equates to an average 6.6 lane miles (10.6 lane kilometers) per day for surfacing applications.
• On airfields, dense-graded Microsurfacing produces a skid-resistant surface without loose rock that damages aircraft engines.
• As a thin, restorative surface source on urban arterials and heavy traffic intersections, Microsurfacing does not alter drainage; there is no loss of curb reveal.
• Microsurfacing is applied to problem sections of roads or runways to eliminate hydroplaning problems that occur during periods of rain. The Microsurfacing restores the proper surface profile and makes the area safe to use.
• Because Microsurfacing can be effectively applied to most surfaces at 3/8 inches (9.5mm) or less, more area per ton of mix is covered, resulting in cost-effective surfacing.
• Microsurfacing creates a new, stable surface that is resistant to rutting and shoving in summer and to cracking in winter.
• Applied to both asphalt and Portland cement concrete surfaces (usually preceded by a tack coat on concrete), Microsurfacing is often used to restore a skid-resistant surface to slick bridge decking with minimum added dead weight.
• Used as a scratch (leveling) course, to be followed by a surface course, Microsurfacing can provide transverse surface leveling.
• Because of its quick-traffic properties, Microsurfacing can be applied in a broad range of temperature and weather conditions, effectively lengthening the paving season. It is particularly suitable for night applications on heavy-traffic streets, highways, and airfields.
• Applied at ambient temperatures, Microsurfacing has low energy requirements. And it is environmentally safe, emitting no pollutants.
• Microsurfacing's life expectancy usually exceeds seven years.
• Capable of filling wheel ruts up to 1-1/2 inches (38mm) deep when the pavement has stabilized and is not subject to plastic deformation, Microsurfacing has the unique ability to solve this problem without milling.

A Product of Quality

Successful Microsurfacing incorporates carefully selected materials, scientific mix designs, advanced technical specifications, and problem field practices.

Microsurfacing begins with the selection of high-quality materials - asphalt, aggregate, emulsifiers, water, and additives - which must pass special laboratory tests, both individual and when combined, as a Microsurfacing system.

The International Slurry Surfacing Association's (ISSA) broad range of specialized mix design tests help to insure that the mixture has these Microsurfacing characteristics:

1. Is capable of being spread in various thick cross-sections (wedges, ruts, scratch course), which
2. After initial traffic consolidation, does not further compact (i.e. resists compaction) throughout the entire design tolerance range of bitumen content and variable thicknesses to be encountered, and
3. Maintains good macro-texture (high wet coefficient of friction) in variable thick sections throughout the service life of the Microsurfacing.

Successful Microsurfacing projects depend on strict adherence to technical specifications. Many users find it helpful to design their individual job specifications around those recommended by ISSA (Technical Bulletin A-143)

The resulting "mix design" and job specifications are carefully adhered to in the field, where ISSA member contractors use specialized job-calibrated equipment and thoroughly trained crews to maintain consistent quality control.