Chapter 7: Slurry Seals

1.0 Slurry Surfacing Description

This chapter provides an overview of commonly used types of slurry seals, including materials and specifications, mix design, project selection, details regarding construction, and a troubleshooting guide to assist the Engineer if problems arise during the placement of these mixtures.  In addition, it provides checklists at the end of the chapter with suggested field considerations for placing a slurry surfacing.

1.1 General Description

Slurry seals are a mixture of asphalt emulsion, graded aggregates, mineral filler, water and other additives.  The mixture is made and placed on a continuous basis using a travel paver (Slurry Surfacing Machine).  The travel paver meters the mix components in a predetermined order into a pug mill.  The typical mixing order is aggregate followed by mineral filler, water, the additive and the emulsion.  Typically, Portland cement, hydrated lime, limestone dust, fly ash, or other approved mineral filler meeting the requirements of ASTM D 242 can be used if required by the mix design and is considered as part of the dry aggregate. Figure 1 illustrates the process of slurry surfacing.

The resulting slurry material is a free flowing composite material that is spread via a spreader box over the existing road surface.  The consistency of the slurry material allows it to spread over the pavement, wetting it, and forming an adhesive bond to the pavement.

The slurry mixture contains asphalt emulsion that breaks onto the pavement surface through heterogeneous or homogenous flocculation.  The asphalt particles coalesce into films, creating a cohesive mixture. The mixture then cures, by loss of water, into a hardwearing, dense-graded asphalt/aggregate mixture that is bonded to the existing pavement.

A slurry surfacing does not add any structural capacity to an existing pavement; they are applied as a maintenance treatment to improve the functional characteristics of the pavement surface.  The types of slurry surfacing and the pavement characteristics they improve are discussed next.

1.2 Purpose of a Slurry Seal

Slurry seal is a thin surface treatment that is laid in a thickness equal to the largest stone in the grading of its component aggregate.  It may include either a conventional or polymer modified emulsion, and the slurry seal may be slow or quick setting.  The emulsion is usually cationic in nature, but may be anionic.  Slow set systems break mostly by evaporation; quick set systems have emulsifiers that react physio-chemically with the aggregate surfaces.  These quick set emulsions maintain a degree of chemical break.  For both systems, breaking and curing times are dependent on the environmental conditions at the time of application; at high temperatures, the emulsion in quick set systems breaks and cures very quickly such that the surface treatment can be opened to traffic within about one hour; slow set systems typically require several hours to break and cure.  In cooler conditions, the times before opening to traffic are longer for both systems.  For this reason, slurry seals should not be applied at night.

Slurry seal is used to:

• Seal sound, oxidized pavements.
• Restore surface texture by providing a skid-resistant wearing surface.
• Improve waterproofing characteristics.
• Correct raveling.
• Provide a new surface where weight restrictions preclude the use of heavier overlays (e.g., bridge decks).
• Provide a new surface where height restrictions are a problem (e.g., overcrossings)

Slurry seal should not be used to:

• Correct surface profile.
• Fill potholes.
• Alleviate cracking (with or without polymer modification).