Slurry Seal Emulsion

A slurry seal is a mixture of well-graded fine aggregate, mineral filler (if needed), emulsified asphalt, and water applied to a pavement as a surface treatment. It is used in both the preventive and corrective maintenance of asphalt pavement surfaces. It does not, nor is it intended to, increase the structural strength of a pavement section. Any pavement that is structurally weak in localized areas should be repaired before applying the slurry seal. All ruts, humps, low pavement edges, crown deficiencies, waves or other surface irregularities that diminish the riding quality should be corrected before placing the slurry seal.

Slurry seal, when applied to the surface of an older pavement, can be used quite effectively. It will seal the surface cracks, stop raveling and loss of matrix, make open surfaces impermeable to air and water, and improve skid resistance. Its timely application will help reduce surface distress caused by oxidation of the asphalt and embrittlement of the paving mixture.

Slurry seal has a number of advantages; some are listed below:

  • Rapid application.
  • No loose cover aggregate.
  • Excellent surface texture for paint striping.
  • Ability to correct minor surface irregularities.
  • Minimum loss of curb height.
  • No need for manhole and other structure adjustments.
  • In many cases, the relatively low cost of the treatment makes it practical to import aggregates for special effects, such as high skid resistance, color contrast and noise reduction.

The slurry is usually applied in a thickness of 3 to 6 mm (1⁄8 to ¼ inches). It comes directly from a traveling mixing plant into an attached spreader box that spreads the slurry by a squeegee-type action. The machine used for production of the slurry seal is self-contained, continuous-flow mixing unit. It is capable of delivering accurately to the mixing chamber predetermined amounts of aggregate, mineral filler (if required), water, and asphalt emulsion. It also discharges the thoroughly mixed materials on to the prepared surface. Certain basic features are common to all batch type slurry machines. They are truck-mounted units with separate storage tanks, bins, and metering systems for emulsified asphalt, water aggregate and mineral filler. The slurry machine has a continuous-flow mixing unit, either single or double pugmill, from which the slurry is discharged into a spreader box. The box is equipped with flexible squeegees and a device for adjustable width. Spreader boxes may be equipped with hydraulically-powered augers to keep the slurry in motion and help keep the mixture uniformly spread across the spreader box width.

The aggregate used in slurry seal must be clean, angular, durable, well grades, and uniform.

The three generally accepted gradings used for slurry mixtures are:

  • Type I is used for maximum crack penetration. Also, it makes an excellent pretreatment for hot mix overlay or chip seal. It is usually used in low density traffic areas such as light aircraft airfields, parking areas, or shoulders where the primary objective is sealing.
  • Type II is the most widely used gradation. It is used to seal; to correct severe raveling, oxidation, and loss of matrix; and to improve skid resistance. It is used for moderate to heavy traffic, depending upon the quality of aggregates available and the design.
  • Type III is used to correct surface conditions, as the first course of multicourse applications for heavy traffic, and to impart skid resistance.

Emulsified asphalt used in the slurry mix may be SS-1, SS-1h, CSS-1, or CSS-1h. The recently developed quick-setting (CQS) asphalt emulsion is being used when early opening to traffic is necessary.

Blending the slurry seal materials in varying proportions in the laboratory is a great aid in selecting the proper mixture. Correct blending should produce a slurry with a creamy texture that will flow smoothly in a rolling wave ahead of the strike-off squeegee. This slurry should be a semifluid, homogenous mass with no emulsion runoff.

Just before applying the slurry, the pavement surface should be cleaned of all dirt, dust, mud spots, vegetation, and other foreign matter. A tack coat of diluted emulsified asphalt of the same type and grade specified for the slurry may be required directly ahead of the slurry application. With relatively new asphalt pavements, the tack coat may be omitted. In this case, the surface should be pre-wetted by water fogging. The surface should be damp but with no free water in front of the slurry machine.

Special care must be taken with longitudinal and transverse joints to prevent excessive buildup of slurry (ridging) or to prevent streaking. It is best to make the joint after the first placed lane is either completely cured or is still in a semi-fluid condition. For good appearance and durability, a joint should not be made when the lane to be joined is only partially set, as tearing and scarring may result.

Quite often a drag is pulled behind the spreader box to improve the joint and overall surface appearance. Drags should be changed regularly. Hand squeegees and hand drags are used to improve joints and place the slurry in areas inaccessible to the machine.

Rolling a slurry seal is only needed in those areas where pneumatic-tired rolling will improve durability. Such areas include taxiways, runways, truck terminal yards, and intersections of heavily traveled roads. All of these are subject to power steering turns, brakes, or acceleration forces. For rolling, a 4.5 tonne (5 ton) pneumatic roller with 345 kPa (50 psi) tire pressure will be most effective. Rolling can start as soon as clear water can be pressed out of the slurry mixture with a pieceof paper, without discoloring the paper. In most cases, however, traffic will iron out the slurry and close any hairline cracks of dehydration. Rolling usually is not needed unless the thickness is more than 6 mm (¼″) or unless late season work is involved.

Slurry should be placed only when the temperature is at least 10°C (50°F) and rising and when no rain is expected. A newly placed slurry should not be opened to traffic until it has completely cured. As with rolling, traffic generally can be allowed on the slurry as soon as a clear water can be pressed out of the slurry mixture with a piece of paper without discoloring the paper. The traffic, of course, must be controlled somewhat as quick stops or accelerations and the turning of wheels while parked will cause damage to the slurry.

For slurry seal design the following sources are recommended:

ASTMD 3910 “Standard Practices for Design, Testing and Construction of Slurry Seal” American Society for Testing & Materials
1916 Race Street
Philadelphia, PA 19103-1108

A105 “Recommended Performance Guidelines for Emulsified Asphalt Slurry Seal Surfaces” International Slurry Surfacing Association
1101 Connecticut Avenue
Washington, D.C. 20036-4303

“Recommended Guideline for Slurry Seal” Asphalt Emulsion Manufacturer’s Association
3 Church Circle, Suite 250
Annapolis, MD 21401-1903