Since asphalt is a familiar term in road construction, most people associate it with the hot asphalt used for paving roads. However, the material discussed here is different. Cold asphalt, also called cold mix asphalt or Emulsion Aggregate Mix (EAM), is a mixture of asphalt emulsion and aggregates that can be placed without heating. It is mainly used for pothole repairs, patch works, shoulder repairs, low-volume roads, and emergency paving where a hot mix plant is not available or weather conditions make hot asphalt difficult to use.
The main ingredient of cold mix is asphalt emulsion. Asphalt emulsion is a mixture of bitumen, water, and a small amount of emulsifying agent (surfactant) that allows bitumen to remain dispersed in water. Depending on the manufacturer, an emulsion generally contains about 55–70% bitumen, 30–45% water, and a very small percentage of emulsifier. Because of the water content, it can be mixed and applied at normal temperature without heating.
Types of Asphalt Emulsion
According to the electrical charge of the asphalt droplets, emulsions are classified into two types:
- Cationic emulsion – positively charged.
- Anionic emulsion – negatively charged.
They are also classified according to their setting speed:
- Rapid Setting (RS) – mainly used for surface dressing and chip seals.
- Medium Setting (MS) – suitable for plant mixing and some maintenance works.
- Slow Setting (SS) – provides a longer mixing time and is commonly used for cold mix applications.
For most pothole repairs and site-prepared cold mix, Medium Setting (MS) or Slow Setting (SS) emulsions are generally preferred because they provide sufficient time for mixing and placing before the emulsion breaks.
Choosing the Right Emulsion
The selection of emulsion should always be based on the type of aggregate available on site.
Most siliceous aggregates have a slightly negative surface charge, so cationic emulsions are normally preferred because the positively charged asphalt droplets are attracted to the aggregate surface, resulting in better coating and stronger adhesion.
Some calcareous or limestone aggregates can also perform well with anionic emulsions. Therefore, the choice of emulsion should always follow the aggregate type and the supplier's recommendation rather than using one type for every project.
Preparing Cold Mix on Site
Although bagged cold asphalt is commercially available, many maintenance projects prepare the mix on site because it is more economical for larger quantities.
The exact proportions depend on the mix design, but in field work a practical mix generally contains well-graded aggregates with sufficient fines or sand to fill the voids. The residual bitumen content usually ranges between 4–7% of the aggregate weight, depending on aggregate grading and site requirements.
For example, suppose you have:
Aggregate = 50 kg
Sand = 5 kg
Total aggregate = 55 kg
If the required residual bitumen content is 5%, the mix requires:
55 × 5% = 2.75 kg of bitumen
If a 60% cationic emulsion is used,
Required emulsion = 2.75 ÷ 0.60 ≈ 4.6 kg, or approximately 4.5–5 litres, depending on the emulsion density.
The aggregate and emulsion should be mixed thoroughly with shovels or a mechanical mixer until every particle is uniformly coated. Medium-setting or slow-setting emulsions provide enough working time for proper mixing. If the manufacturer recommends, a small amount of additional water may be added to improve workability, but excessive water should be avoided because it delays setting and may reduce the quality of the mix.
Placing the Cold Mix
Before placing the cold mix, the pothole should be cleaned thoroughly by removing loose materials, dust, and standing water whenever possible.
The prepared cold mix is then placed directly into the pothole and spread evenly using a rake. Slight overfilling is recommended because the material will compact under rolling or traffic.
A thin layer of dry sand may be spread over the surface before rolling. This helps prevent the mix from sticking to the steel roller drum, especially when dry rollers are used.
After compaction, the repaired area may be opened to traffic once the material has gained sufficient stability. The exact setting time depends on the type of emulsion, weather conditions, and moisture loss. Under warm and dry conditions, the mix may become stable within a short time, while cool or wet weather can delay curing.
Workability on Site
Freshly prepared cold mix should be used as soon as practical. Unlike stockpile cold mixes produced in asphalt plants, site-mixed material has a limited workable time before the emulsion begins to break.
As the emulsion starts setting, the mixture becomes stiffer, darker, and more difficult to handle. Once this happens, compaction becomes less effective and the quality of the repair decreases. Therefore, the quantity prepared should match the amount that can be placed within the available working time.
Advantages and Limitations
Cold asphalt is not intended to replace hot mix asphalt for permanent road construction. Hot mix generally provides higher strength, longer service life, and lower cost for large paving projects.
However, cold mix has several advantages for maintenance work. It requires no heating equipment, can be prepared and placed with simple tools, and is particularly useful for emergency repairs, remote locations, and situations where transporting hot asphalt is impractical.
For site engineers responsible for maintenance works, cold mix remains one of the most practical and reliable solutions for repairing potholes and restoring traffic quickly until permanent resurfacing can be carried out.
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