What is Microsurfacing?

Microsurfacing is a surface treatment of polymer-modified asphalt emulsion, crushed aggregate, mineral filler and water, spread on an existing pavement. A machine mixes the materials on site and spreads them in a layer over the old surface.

What it contains

The emulsion carries polymer, at least 3% by weight of the residual asphalt in the South Carolina and Minnesota specifications. Both name latex, and Minnesota also names SBS. Additives control the set time. The South Carolina emulsion must cure so that traffic can run on the new surface within one hour.

South Carolina sets these mix design limits, each tested to an ISSA technical bulletin:

• Residual asphalt 5.00 to 10.50% and mineral filler 0.5 to 3.0%, both by weight of dry aggregate.
• Wet track abrasion loss at most 538 g/m2 after a one-hour soak and 807 g/m2 after a six-day soak.
• Lateral displacement at most 5%, and excess asphalt at most 538 g/m2.
• Cohesion at least 12 kg-cm at 30 minutes and 20 kg-cm at 60 minutes.

These are one agency’s road values. The ISSA A143 guideline itself is a paid document and was not opened for this page.

How it differs from slurry seal

Minnesota DOT calls microsurfacing an advanced form of slurry seal. It has the same basic ingredients plus polymer additives. The FAA slurry seal item P-626 also lists polymer, so the polymer alone does not tell the two apart. The set time and the mix design tests do.

Rut filling

Germany developed microsurfacing in the late 1960s to fill wheel ruts in asphalt on the autobahn. South Carolina places rut fill and scratch courses before the surface course. It leaves a leveling course under traffic for at least 3 hours and does not cover rut fill for at least 12 hours. A rut is permanent deformation in a pavement layer or the subgrade, so a treatment that fills the rut does not remove its cause.

On airfield pavements

AC 150/5370-10H has no item for polymer-modified microsurfacing. Its nearest items differ:

• P-626 is an emulsified asphalt slurry seal with polymer. It covers all pavements at general aviation airports serving aircraft of 30,000 lb (13,600 kg) or less. At airports serving aircraft of 60,000 lb (27,216 kg) or less it covers all pavements except runways.
• P-629 has a “micro-surface” option, but it is a thermoplastic coal tar emulsion for airports serving aircraft under 60,000 lb. Some authorities prohibit coal tar.

AC 150/5380-6C lists a surface treatment as the repair for weathering and for loss of skid resistance. For rutting it lists replacement or grinding and milling. AC 150/5320-6G section 4.2 says timely surface treatments extend the functional life of flexible pavements.

The distress decides the repair, so a condition survey of rutting, raveling and weathering comes before the choice of treatment.