A slurry seal is a homogenous mixture of emulsified asphalt, water, well-graded fine aggregate and mineral filler that has a creamy fluid-like appearance when applied. Slurry seals are used to fill existing pavement surface defects as either a preparatory treatment for other maintenance treatments or as a wearing course. There are three basic aggregate gradations used in slurry seals:

1. Type I (fine). This type has the finest aggregate gradation (most are smaller than the 2.36 mm (No. 8) sieve) and is used to fill small surface cracks and provide a thin covering on the existing pavement. Type I aggregate slurries are sometimes used as a preparatory treatment for HMA overlays or surface treatments. Type I aggregate slurries are generally limited to low traffic areas (ISSA, 2001a[1]).
2. Type II (general). This type is coarser than a Type I aggregate slurry (it has a maximum aggregate size of 6.4 mm (0.25 inches)) and is used to (1) treat existing pavement that exhibits moderate to severe raveling due to aging or (2) to improve skid resistance. Type II aggregate slurry is the most common type.
3. Type III (coarse). This type has the most coarse gradation and is used to treat severe surface defects. Because of its aggregate size, it can be used to fill slight depressions to prevent water ponding and reduce the probability of vehicle hydroplaning.