Chapter 7: Slurry Seals

2.0 Materials & Specifications

The main materials used in slurry surfacing are:

• Asphalt Emulsion
• Water
• Aggregate
• Portland cement or other approved mineral filler
• Additives

2.1 Asphalt Emulsion

Asphalt emulsions are defined in Chapter 2 of this advisory guide as dispersions of asphalt in water stabilized by a chemical system. In the case of slurry surfacing, the emulsion may be cationic or anionic; however, cationic emulsions are the most common. Emulsions used in slurry seals are either slow setting (SS) or quick setting (QS). Common slow and quick setting emulsions include:

• CSS-1h
• CQS-1h
• QS-1h
• SS-1h

These emulsions are specially formulated for compatibility with the aggregate and to meet the appropriate mix design parameters. These emulsions are defined in Chapter 2 of this guide.

Emulsion specifications are based on standard emulsion characteristics, such as stability, binder content, and viscosity. In some quick-set slurry systems, polymer is added to the emulsion. The polymer enhances stone retention, especially in the early life of the treatment. The added polymer also reduces thermal susceptibility. Polymers also improve softening point and flexibility, which enhance the treatment’s crack resistance.

Emulsions are usually modified with latex, which is an emulsion of rubber particles. The latex does not mix with the asphalt; rather, the latex and the asphalt particles intermingle to form a sort of 3-D structure, as illustrated in Figure 2. The latex used is either neoprene or styrene butadiene styrene (SBR) for slurry seal. When modified with latex, slurry seal emulsions are referred to PMCQS-1h or, more commonly, LMCQS-1h (1).

Latex may separate from the emulsion due to the differences in density. If separation occurs, the latex must be remixed into the emulsion by circulation in the tanker before the modified emulsion is transferred to the slurry machine for application (3).

Basic emulsion requirements are shown in Table 1. Key requirements include the binder content and residual properties. The viscosity is of importance as is the storage stability to ensure that the emulsion can be used effectively in the field.

2.2 Aggregates

2.2.1 Aggregate Characteristics

The aggregate’s key physical characteristics for suitable incorporation into a slurry surfacing mix are defined by:

• Geology:  This determines the aggregate’s compatibility with the emulsion along with its adhesive and cohesive properties.
• Shape:  The aggregates must have fractured faces in order to form the required interlocking matrix (1).  Rounded aggregates result in poor mix strength.
• Texture:  Rough surfaces form bonds more easily with emulsions.
• Age and Reactivity:  Freshly crushed aggregates have a higher surface charge than aged (weathered) aggregates.  Surface charge plays a primary role in reaction rates.
• Cleanliness:  Deleterious materials such as clay, dust, or silt can cause poor cohesion and adversely affect reaction rates.
• Soundness and Abrasion Resistance:  These features play a particularly important role in areas that experience freeze-thaw cycles or are very wet.

When tested in accordance with AASHTO T 27 and AASHTO T 11, the target (mix design) aggregate gradation (including the mineral filler) shall be within one of the following gradation bands (or of one currently approved by the local paving authority.  The gradation for each type is listed in Table 2.

The primary difference among these gradations is the aggregate top size.  This indicates the amount of residual asphalt required by the mixture and the purpose to which the slurry is most suited.  The Type I slurries are the finest and are used for lightly trafficked roads or parking lots. Type II slurries are coarser and are suggested for raveling and oxidation on roadways with moderate to heavy traffic volumes.  Type III slurries have the coarsest grading and are appropriate for filling minor surface irregularities, correcting raveling and oxidation, and restoring surface friction.  Type III slurries are typically used on arterial streets and highways.

The role of fines (i.e., aggregate particles 75 µm and finer) in a slurry surfacing mix is to form a mortar with the residual asphalt to cement the larger stones in place.  The fines content is essential for creating a cohesive hardwearing mix.  Generally, the fines content should be at the mid-point of the grading envelope.  The general aggregate quality requirements are listed in Table 3.

2.3 Cement and Additives

In most slurry surfacing, Portland cement is used as a mixing aid, allowing the mixing time to be extended and creating a creamy consistency that is easy to spread. Additionally, hydroxyl ions counteract the emulsifier ions, resulting in a mix that breaks faster with a shorter curing time. Cement also has a fine consistency and, as such, absorbs water from the emulsion, causing it to break faster after placement. Fine materials, as previously discussed, also promote cohesion of the mixture by forming a mortar with the residual asphalt.

Additives other than cement vary and are features of particular systems. They act as retardants to the reaction with emulsions, either as a prophylactic, slowing the emulsifier’s access to the aggregate surface, or by preferentially reacting with the emulsifier in the system. Additives include emulsifier solutions, aluminum sulfate, aluminum chloride, and borax. Generally, increasing the concentration of an additive slows the breaking and curing times. This is useful when temperatures increase during the day.