Micro-surfacing (MS) is a thin preventive maintenance layer produced by a dedicated machine that mixes polymer-modified emulsified asphalt, mineral aggregate, filler, water and additives in a precise ratio and spreads the slurry onto the existing pavement. A well-designed micro-surfacing cures enough to open to traffic within about 1–2 hours.
Unlike slurry seal, micro-surfacing is polymer-modified and may be used to fill wheel-track ruts on expressways. It is not a structural reinforcement layer: the existing pavement must already have sufficient structural strength and be in good condition with prior distress repaired.
Micro-surfacing uses a cationic, fast-setting modified emulsified asphalt. Key requirements: residue content ≥60%, penetration (25°C) 40–100 (0.1 mm), ductility (5°C) ≥30 cm, softening point ≥50°C, and polymer modifier content not less than 3% of asphalt mass.
Crushed aggregate must be hard and clean: crushing value ≤28%, Los Angeles abrasion ≤30%, polished stone value ≥42 BPN, sand equivalent ≥60%. Oversize particles must be screened out before paving.
By maximum aggregate size there are two types:
| Type | Max size | Typical use |
|---|---|---|
| MS-II | 4.75 mm | Secondary roads and medium-traffic overlays |
| MS-III | 9.5 mm | Expressways, primary roads and rut filling |
Typical single-layer cured thickness is 5–11 mm with dry aggregate of 5.5–16 kg/m² and an asphalt-aggregate ratio of roughly 5.5%–10.5%. The mix must meet: mixing time ≥120 s, cohesion 1.2 N·m at 30 min and 2.0 N·m at 60 min, wet track abrasion ≤600 g/m² (1 h) and ≤800 g/m² (6 days), and loaded-wheel sand adhesion ≤500 g/m².
Sequence: repair & clean the existing pavement → lay guide strings → spread the slurry → hand-finish defects → initial curing → open to traffic.
During work, check consistency (scratch test), asphalt-aggregate ratio (±0.5% by the three-control method), gradation and appearance. At acceptance the surface must be smooth, dense, uniform, free of raveling, streaks and tire marks, with skid resistance (pendulum value ≥50 on expressways) and even texture depth.
Micro-surfacing seals water out, restores skid resistance, fills ruts, smooths the surface and opens to traffic within hours—making it ideal for high-traffic highways where night closures and short work windows are essential.