15 May 2001
(a) General. The aggregate as well as the emulsion used in a slurry seal should be given
close attention. All aggregates must be clean, and the particles should be crushed to produce an
angular shape. Aggregates that contain plastic fines should not be used. These fines absorb excessive
amounts of emulsion, leaving inadequate amounts of binder on the remaining aggregate. The fines also
promote low-wear characteristics and premature break of the emulsion. Better performance can be
expected from slurry seals that are produced using crushed aggregate. Furthermore, natural sands
such as dune, river, and beach sands, and other rounded aggregates tend to have poor skid resistance
and wear characteristics and therefore should not be used in slurry seal coatings.
(b) Gradations for aggregates. The aggregates should be dense graded so that the
particles will lock themselves together. Table 4-4 shows the gradations for use with slurry seals.
Gradation type 1 is normally used for filling and sealing cracks in a pavement surface and it will provide
a thin wearing surface. Gradation type 2 is probably the most generally used, and is used to fill voids,
correct moderate surface irregularities, seal small cracks, and provide a wearing surface for traffic.
Aggregate gradation type 3 assures a thicker seal and provides a coarser surface texture. This
gradation might be used as the first course in a two-course slurry seal surface treatment.
Slurry Seal Aggregate Gradations
9.5 mm (3/8 inch)
4.75 mm (No. 4)
2.36 mm (No. 8)
1.18 mm (No. 16)
600 Fm (No. 30)
300 Fm (No. 50)
150 Fm (No. 100)
75 Fm (No. 200)
(3) Mineral filler. When stability or segregation problems occur, mineral filler at a rate of 0.5 to
4.0 percent by weight of the total mixture may be required to overcome the problem. When mineral filler
is needed, portland cement or hydrated lime is most often used in slurry seals. The filler is used to
improve the mix stability, that is, suspend heavier aggregate particles throughout the slurry seal mixture;
to reduce segregation of materials; and to meet gradation requirements. Care should be taken to insure
that the fines content, including mineral filler, does not exceed the gradation limits. Excessive fines or
mineral filler can cause shrinkage cracking to occur in the seal coat.
(4) Water. Water controls the workability of the slurry seal mixture. The mixture should
contain enough water to produce a smooth, creamy, homogeneous fluid like appearance. If too much
water is used, the resultant mixture will be soupy, and segregation or bleeding of the mixture will occur.
On the other hand, if not enough water is used, the slurry mixture will be stiff and will neither spread
smoothly nor perform satisfactorily. Only potable water should be used in a slurry seal mixture.
d. Design. The method of developing a JMF for slurry seals selects the optimum asphalt content
based on a desired film thickness of asphalt and the absorption characteristics of the aggregate. The
water and mineral filler content requirements are determined by a cone test, and the wear characteristics