Aggregates are composed of many different types of minerals. While many are excellent for slurry surfacing system use, some are not. Aggregates must be clean to ensure that the asphalt bonds to the aggregate particles and durable to ensure long-term performance. Aggregate size and physical property requirements will vary based on the gradation and type of slurry surfacing system (standard, modified, or micro surfacing) project specified.
Slurry surfacing systems require quality crushed aggregate such as granite, slag, limestone, or chat. Other high-quality aggregate, or combinations thereof, should be used. To assure the material is 100 percent crushed, the parent aggregate must be larger than the largest stone in the gradation. Quality aggregate must meet specified requirements for gradation, particle shape, and cleanliness or sand equivalent value.
The following laboratory tests are used to qualify the aggregate for suitability on the specific slurry surfacing system project. Other aggregate testing may include moisture effect on unit weight (bulking effect). This test is listed in the section on mix design and its importance is discussed in Section III.C.1, page 19, Moisture Present in the Aggregate Stockpile. Most importantly, the aggregate must be mixed with the emulsified asphalt and additives to determine compatibility before acceptance for use.
The aggregate’s key characteristics for suitable incorporation into a slurry surfacing system are:
1. Aggregate Gradation/Particle Size Distribution
In most cases the aggregate from a single source will not vary substantially during a project. Aggregate variation may be caused by different phases of the project occurring over a long period of time, location changes within the quarry or pit where aggregates are extracted, or gradation fluctuations.
Aggregate gradation directly affects the amount of emulsified asphalt required for a slurry surfacing system. Variations in aggregate gradation, even within the allowable specified ranges, will change the total aggregate surface area to be coated by emulsified asphalt. Thus, consistency is important when selecting an aggregate for a project.
Inspector’s Note: The inspector should be alert for unusual changes brought about by gradation fluctuation (especially on the fine aggregate sizes) which may cause enough variation in the gradation to invalidate the existing mix design.
By design, slurry seals are to be applied in a layer equal to the largest stone in the gradation. Slurry seal applied at less than, or in excess of, the recommended application rate, or in multiple layers, may create an unstable treatment leading to raveling, flushing, rutting, or washboarding.
Designer’s Note: For aggregate on the coarse side of the gradation specification, application rates should be higher. Otherwise, the largest aggregate particles will not adequately embed in the slurry surfacing mixture. Insufficient embedment may cause drag marks when larger stones are caught by the strike-off rubber. Raveling may result from insufficient embedment, as well. Conversely, for aggregate on the fine side of the gradation specification, application rates should be lower to avoid flushing of the surface.
ISSA specifies three standard gradations, each with a designated use for the surface condition to be addressed.
ISSA Type I: This aggregate gradation is used to fill surface voids, address moderate surface distresses, and provide protection from the elements. Although not a substitute for crack sealing, the fineness of this mixture provides maximum crack penetration. It is frequently used on airfields, parking lots, and residential streets where a tighter, finer surface texture is desired.
ISSA Type II: This aggregate gradation is used to seal/fill surface voids, address more severe surface distresses, and provide a durable wearing surface. It is the most common gradation used in slurry surfacing systems. It is often used on pavements to correct distresses caused by weathering and raveling while producing an adequate wearing surface for low, medium and heavy traffic volumes. This gradation is commonly used on airfields, parking lots, municipal streets, county roads and highways.
ISSA Type III: This aggregate gradation, which is the coarsest of the three ISSA types, gives maximum coefficient of friction and an improved durability due to the depth of the application, as the larger aggregate size increases the thickness of the mat placed. It is best suited to higher-traffic pavements such as expressways, major highways and arterials. When used in micro surfacing, this gradation is ideal for rut filling and re-establishing profiles with minor surface irregularities.
Table 1 – Standard ISSA Aggregate Gradations
Sieve Analysis Tests – [American Society for Testing and Materials (ASTM) C117 or American Association of State Highway and Transportation Officials (AASHTO) T11: Materials Finer than 75-μm (No. 200) Sieve in Mineral Aggregates by Washing and ASTM C 136 or AASHTO T 27: Sieve Analysis of Fine and Coarse Aggregates] This test is used to determine the gradation of materials proposed for use as aggregate. The results are used to determine compliance with the specified particle size distribution and to provide data for quality control of the supply of aggregate to the project
2. Particle Shape
100% crushed aggregate is required, allowing the fractured faces to interlock and resist movement. Slurry surfacing system mixtures with a higher percentage of flat and elongated particles (3:1 ratio) may exhibit lower shear stiffness, which could be problematic, especially for rut filling applications. If this test is required, refer to local specifications for methods and percentage maximums.
3. Cleanliness or Presence of Deleterious Materials
Aggregates must be relatively clean when used in slurry surfacing systems. Vegetation, clay lumps and excess dust or dirt affect performance by preventing proper asphalt-aggregate bonding. Micro surfacing aggregates must be cleaner than those used in conventional slurry systems.
Sand Equivalent Test – (ASTM D2419 or AASHTO T176: Sand Equivalent Value of Soils and Fine Aggregate)
This test assigns an empirical value to the relative amount of claylike material present in the test specimen. It evaluates the cleanliness of the aggregate, measuring the amount of expansive clays or organic materials.
4. Toughness and Abrasion Resistance
Aggregates undergo substantial wear and tear. In general, they should be hard enough to resist crushing, degradation and polishing during the application process and throughout the life cycle of the pavement. Aggregates not adequately resistant to abrasion and polishing may cause a loss of skid resistance.
Abrasion Resistance Test – (ASTM C131 or AASHTO T96: Resistance to Degradation of Small-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine)
This test indicates the hardness or toughness of the aggregate. It measures aggregate degradation of the parent rock under wear conditions. The results do not automatically permit valid comparisons to be made between sources distinctly different in origin, composition, or structure. Assign specification limits with extreme care in consideration of available aggregate types and their performance history in specific end uses.
5. Durability and Soundness
Aggregates that are durable (resistant to weathering) are less likely to degrade in the field and cause premature pavement distress. The soundness test determines an aggregate’s resistance to degradation by weathering, specifically by freeze-thaw cycles.
Soundness of Aggregates – (ASTM C88 or AASHTO T104: Soundness of Aggregate by Use of Sodium Sulfate or Magnesium Sulfate)
This test determines the aggregate’s resistance to weathering, giving an indication of its freeze/thaw characteristics.