All forms of cold mix can be stockpiled and specially formulated to meet the desired life of the stockpile. Their uses range from maintenance to base and wearing course paving and will allow for flexibility around unforeseen delays and varying weather conditions.
High-performance cold mixes are engineered with specific additive packages designed to work in all service/weather conditions and to suit a range of maintenance activities, permanent or temporary. These specialty patching mixes can be stockpiled or packaged and are ideal for potholes, utility cuts, railway crossings, and various maintenance practices for flexible pavements.
When designing a stockpile/maintenance mix a number of factors have to be examined and assessed to ensure a proper surface and/or maintenance patch will be placed that will perform for its service life. The following factors can have a tremendous effect on the performance of a stockpile/maintenance mix: RAP material, virgin aggregate shape and type, mix workability, coating ability, and residual asphalt content.
Aggregate Shape and Type:
The overall shape of the mix aggregate can influence the quantity of aggregate as well as the amount of asphalt emulsion to be used. The more graded the aggregate is, the higher the emulsion quantity needed. The compatibility of the emulsion used with the aggregate is critical.
The emulsion used should give the finished mix enough workability to be handled in cold temperatures, be homogeneous, and be designed for the intended method of processing, be it immediate use or in a long-term stockpile.
The asphalt emulsion should have the ability to coat the fine aggregate (material passing the 4.75 mm sieve) without the fine aggregate balling up. With some emulsions, the use of mixing water will aid in coating the fines.
Asphalt Residual in the Emulsion:
The quantity of asphalt residual in the emulsion can affect the quantity of emulsion needed to coat the aggregate. The lower the residual, the higher the quantity of asphalt emulsion needed.
During the design process, two critical factors to be controlled are a well-coated mix and adequate workability. A stockpile mix would typically require an emulsion content of 5.5 to 7.0% and a cutback asphalt content of 4.5 to 5.5%
Several factors have to be taken into account when choosing the emulsion to be used. The type and grade of emulsion required is affected by the aggregate, the coating ability, the compatibility, the mixing method, and the length of service and workability expected. The most widely used grades of asphalt emulsion used in stockpile mixes are MS-2, MS-2s, CMS-2, and CMS-2s. Cutback asphalts can also be used but, due to environmental regulations, these are becoming less common. In recent years, proprietary type emulsions have been developed for use in stockpile mixes.
The most common asphalt cutbacks used for stockpile mixes are MACPATCH HPB, SC-250 and MC-250. The SC (Slow Curing) grade is used for long-term stockpiling and the MC (Medium Curing) grade is used for short-term stockpiling. The use of cutbacks has been greatly reduced due to the environmental concerns of using cutbacks Volatile Organic Compound (VOC emissions.
The mix aggregate used in stockpile maintenance mixes can be processed or semi-processed crusher, pit, or bank run aggregates and can incorporate variable proportions of properly processed RAP. Most are graded from a maximum size of 25 mm, have very low clay content, and do not have an excessive amount of material passing the 75 µm sieve. They can also be blended as individual coarse and fine aggregates in order to produce the desired gradation.
In order to construct a proper, well-designed stockpile maintenance mix, the following guidelines should be followed:
- Determine the mixing process to be used (batch plant, continuous/drum plant, pugmill, pre-drying aggregate, not pre-drying aggregate)
- Determine the binder (emulsion or cutback) to be used
- Determine if the emulsion or cutback grade and type to be used is well-designed for the chosen mixing process
- Design a dense graded mix with the aggregate to be used on job
- Use a clay-free, hard, crushed aggregate with a well-graded appearance
- Ensure that the asphalt emulsion or cutback and the aggregate are compatible
- Ensure an adequate amount of emulsion or cutback is used
- Ensure that the aggregate is moist but not saturated
- Ensure that the mix is workable and the fines are well coated
- Follow proper mixing techniques
- Ensure the stockpile pad is clean and well drained
- Work only in weather suitable for the type and grade of emulsion being used