Process

This is the reuse of pavement materials that have already served their first intended purpose. The process involves the combination of bitumen emulsion with reclaimed asphalt pavement materials , reclaimed aggregate materials, or both either in situ or in a central plant to produce cold mixed asphalts These mixtures may be used as base courses or surface courses depending on the mix designed and the application.

Emulsion Types

The emulsion type used depends on the type of mix designed . Firstly the aggregate gradings should be determined. The medium setting grades of emulsion are useful for coarse aggregate, i.e. open graded mixes. e.g. AMS, CMS, CMS-2 and CAM . VSS manufactures emulsions specifically for cold recycling that cure fast and mix well to disperse in aggregates.

These grades may be polymer modified to increase initial cohesion or improve crack or rut resistance of the final mix.

High float emulsions may be mixed with either dense graded or open graded mixes. In hot dry conditions coating is improved relative to medium setting anionic emulsions.

Slow setting emulsions such as CSS and ASS should be used with high fines content dense graded mixes. Polymer modification may be used to improve ini­tial cohesion, rut and crack resistance.

As water is often used in the process to prewet the aggregate the compatibili­ty should be determined as shown in section 3.8.

Rejuvenating oils ( aromatic oils) may sometimes be used in the process, types and amounts to be added must be determined from a proper analysis of the material to be recycled in terms of existing microviscosity and the target to be reached. Usually a limit of 4.4 Log Pa.s is figured to be the hardest binder able to be recycled without addition of rejuvenating oil. The target viscosity will depend on what virgin grade would have been used in that mix.

VSS Styrelf＆#8482; ERA＆#8482; is ideal for this type of application

Emulsion Quantity

The amount of emulsion to be added will depend on the mix design requirements.

Samples of the pavement should be crushed to size and the aggregate grading made up with virgin aggregate. The optimum binder content can be determined for that mix using conventional mix design techniques. Suggestions may be found in:

2nd Edition "A Basic Asphalt Emulsion Manual" 2nd Edition Part 3 1987.

Asphalt Cold Mix Recycling MS-21 . Chapter 3.

Grading envelopes can be adapted from hot mix grading

Advantages

Cold recycling can correct many types of pavement distress that involve both surface and base courses. By reusing aggregates and bitumen from the existing pavements reduces the requirements for new materials.

In place recycling reduces haulage costs. The addition of bitumen to existing granular base materials also increases structural strength and water resist­ance without adding to pavement thickness.

Emulsion allows good stone coating and cold application without the need to dry the aggregate first.

In place recycling is rapid and allows traffic to be reinstated in a few hours.

Constraints

Mixes curing is subject to climatic conditions. As water based materials are used the evaporation conditions are important. Cationic emulsions are less sensitive to weather than anionic (high float or conventional).

Laying work cannot proceed if rain is falling or is imminent. Rain increases moisture content above optimum and the mix requires aeration.

Central plants allow most rapid construction and control of application. Travel mixers are good in that only small areas are done at a time. Blade mixing, and similar mixed in place construction techniques expose large areas during mixing and is the most sensitive to weather.

Rolling of the mix cannot commence until the mix is stable enough to carry a roller, this requires good curing conditions.

Some of these problems may be overcome by the use of polymers in the mix.

Minimum temperature for placement of recycle mixes is 10 o C.

Recycle mixes based on emulsions gain stability by loss of fluids ( mostly water) lift thicknesses are limited by this. Factors affecting this include such things as mix water content, aggregate gradation and type, temperature, wind velocity , and humidity. Under best conditions lifts of dense graded mixes should be placed at no more than 75mm and open graded no more than 150mm.

Time between lifts is 2 - 5 days.