Chapter 8: Micro-Surfacing
2.0 Materials & Specifications
The main materials used in micro-surfacing are:
- Asphalt Emulsion (with polymer modification)
- Water
- Aggregate
- Portland Cement
- Additives
2.1 Asphalt Emulsion
Asphalt emulsions are (See Chapter 2 of Maintenance Technical Advisory Guide for additional information) dispersions of asphalt in water stabilized by a chemical system. In the case of micro-surfacing, the emulsion may be cationic or anionic; however, cationic emulsions are the most common. The most common emulsion is CSS-1h, however in some locations quick setting emulsions such as CQS-1h and QS-1h are used. However, only QS emulsions are used in micro-surfacing in California. For the use of MSE Micro-surfacing emulsion, see Reference 3. Those emulsions are specially formulated for compatibility with the aggregate and to meet appropriate mix design parameters.
Emulsion specifications are based on standard emulsion characteristics, such as stability, binder content, and viscosity. In all micro-surfacing 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 and, in the case of micro-surfacing, allow thicker sections (two to three stones thick) to be placed. Thicker sections allow micro-surfacing to be used for rut filling. Generally, micro-surfacing is not significantly resistant to reflective cracking.
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. Micro-surfacing is modified with either natural latex or SBR latex. When modified with latex, slurry seal emulsions are referred to PMCQS-1h or, more commonly, LMCQS-1h (1).
Figure 2: Micrograph of a Latex/Asphalt Cured Film (3) |
Common emulsions used for micro-surfacing are latex modified CQS-1h type emulsions and micro-surfacing emulsion (MSE) (1, 2).
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 micro-surfacing 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.
Table 1: Typical Emulsion Properties for Micro-Surfacing
Table 1: Typical Emulsion Properties for Micro-surfacing and Polymer Modified Slurry Quick Set (2, 3)
| Test |
Typical Specification |
Method |
| Residue |
62% min |
AASHTO T 59 |
| Sieve Content |
0.3% max |
AASHTO T 59 |
| Viscosity 25°C SSF |
15-90 |
AASHTO T 59 |
| Stability (1 day) |
1% max |
ASTM D244 |
| Storage Stability (5 days) |
5% max |
ASTM D244 |
| Residue pen 25°C |
40-90 |
ASTM D244 |
2.2 Aggregates
2.2.1. Aggregate Characteristics
The aggregate’s key physical characteristics for suitable incorporation into a micro-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 will 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.
Table 2: Slurry Surfacing Aggregate Gradings
| Sieve Size |
Percentage Passing |
| Type I |
Type II |
Type III |
| 3/8 (9.5mm) |
- |
100 |
100 |
| # 4 (4.75 mm) |
100 |
90-100 |
70-90 |
| # 8 (2.36mm) |
90-100 |
65-90 |
45-70 |
| # 16 (1.18mm) |
65-90 |
45-70 |
28-50 |
| # 30 (600-µm) |
40-65 |
30-50 |
19-34 |
| # 50 (330-µm) |
25-42 |
18-30 |
12-25 |
| # 100 (150-µm) |
15-30 |
10-21 |
7-18 |
| # 200 (75-µm) |
10-20 |
5-15 |
5-15 |
Table 2: Micro-Surfacing Aggregate Gradings
| Sieve Size |
Percentage Passing |
| Type II |
Type III |
Stockpile Tolerance |
| 3/8 (9.5mm) |
100 |
100 |
- |
| # 4 (4.75 mm) |
90-100 |
70-90 |
± 5% |
| # 8 (2.36mm) |
65-90 |
45-70 |
± 5% |
| # 16 (1.18mm) |
45-70 |
28-50 |
± 5% |
| # 30 (600-µm) |
30-50 |
19-34 |
± 5% |
| # 50 (330-µm) |
18-30 |
12-25 |
± 4% |
| # 100 (150-µm) |
10-21 |
7-18 |
± 3% |
| # 200 (75-µm) |
5-15 |
5-15 |
± 2% |
The primary difference among these gradations is the aggregate top size. This dictates the amount of residual asphalt required by the mixture and the purpose to which the micro-surfacing is most suited. The Type I slurries are the finest and are used for lightly trafficked roads or parking lots. Type II micro-surfacing is coarser and are suggested for raveling and oxidation on roadways with moderate to heavy traffic volumes. Type III micro-surfacing has the coarsest grading and are appropriate for filling minor surface irregularities, correcting raveling and oxidation, and restoring surface friction. Type III micro-surfacing is typically used on arterial streets and highways.
The role of fines (i.e., aggregate particles 75 µm and finer) in a micro-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. Recent work suggests that the distribution of the sub-75 micron fraction is critical to control the reaction rate in micro-surfacing emulsions (4). The general aggregate quality requirements for micro-surfacing are listed in Tables 3 and 4.
Table 3: General Aggregate Properties (2) and Aggregate Requirements (4)
| Test |
Micro-surfacing |
Slurry Seal Type I |
Slurry Seal Type II |
Slurry Seal Type III |
Test # and Purpose |
| Sand Equivalent (min) |
65 |
45 |
55 |
60 |
ASTM D 2419
Clay Content |
| Durability Index (min) |
55 |
55 |
55 |
55 |
ASTM D 3744
Resistance to wet/dry exposure |
| Abrasion (LA Rattler) 500rev |
35% max |
n/a |
n/a |
n/a |
AASHTO T 96
Resistance to traffic |
| Crushed Particles |
100% |
n/a |
n/a |
n/a |
ASTM D 5821 |
Table 3: General Aggregate Properties (2) and Aggregate Requirements (4)
| Test |
Micro-surfacing |
Test # and Purpose |
|
| Sand Equivalent (min) |
65 |
ASTM D 2419
Clay Content |
| Soundness (max) |
15% |
ASTM C88
(using NaSO4) |
| Abrasion Resistance |
30% max |
AASHTO T 96
Resistance to traffic |
| Crushed Particles |
100% |
ASTM D 5821 |
2.3 Cement and Additives
In micro-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 can act as retardants to the reaction with emulsions, either as prophylactics, 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 air temperatures increase during the day.
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