Microsurfacing techniques include different products that should be distinguished from each other.
These are mixtures of a bitumen emulsion, mineral aggregates, water and specific additives in set proportions, mixed and applied evenly onto a clean, prepared surface. The first application is intended as a pre-treatment to seal and repair certain base layers before laying the wearing course. These applications are characterised by a graded material with a maximum particle size less than 4mm. The mineral aggregate materials form the skeleton that can be rolled or crushed or a combination of the two.
The binder is generally a standard bitumen emulsion.
The fines content is larger than 12% and the richness modulus is greater or equal to 4.
Microsurfacing pavements are essentially asphalt mixes for surface layers, designed to waterproof surfaces and provide very good adhesion. They are mainly characterized by:
- exclusive use of solid rock aggregates identical to those used in hot mix wearing course asphalt,
- a grading greater or equal to 6 mm,
- a fines content typically varying from 6 to 10%,
- a richness module less than or equal to 4,
- frequent use of a polymer modified bitumen emulsion. This modification can be done either in the base bitumen or with a latex during the emulsion manufacturing process,
- design of the emulsion and the choice of the aggregates, in order to obtain a fast cohesion built up and a short reopening to traffic.
Mineral aggregates, comprise 85 to 90% by weight of the mixture depending on the type of grading, have a strong influence on the performance. The best results are obtained with aggregates from 100% crushed rock, clean (low clay values) and with good mechanical properties.
Generally, continuous gradings such as 0/4, 0/6 or 0/10mm recomposed sands are used, with water added to avoid segregation during transport.
The fines play a dual role: they reduce mix segregation and determinethe rates of emulsion breaking and setting of the product. They also increase the stiffness modulus of the mixture placed.
Mineral additives (cement or lime)
Cement or lime is frequently added to regulate the emulsion breaking rate and to achieve the target aggregate gradation of the mix, but are generally limited to 1% (sometimes 2% in the case of cement), for obvious economic and technical reasons.
Some designs contain mineral or organic fibers. They can be used to facilitate the installation of microsurfacing in case of sloped surfaces, as the fibres reduce segregation and emulsion drainage loss.
It is required to pre-wet aggregates and filler to avoid segregation and to facilitate their coating with the emulsion. This pretreatwater generally contains a surfactant that makes coating even easier, improves binder-granulate adhesion and regulates emulsion breaking.
Generally, in order to better meet cohesion and durability requirements, the bitumen emulsions used in most microsurfacing designs are cationic polymer bitumen emulsions. The residual binder content in the mixture is usually between 5.5 and 9.5% by mass depending on the specific weight and size of the dry aggregates. The bitumen content of the emulsions can vary from 60 to 65%. The breakingrate of these emulsions is controlled in order to break as soon as possible after the mix has been poured, while maintaining the high stability required for storage and transport. The composition of these emulsions is determined according to the nature of the aggregates, the emulsifier(s) and the specific objectives (suitable coating of the aggregates, breaking and recirculation time).
The application of microsurfacing asphalt layers involves controlling the breaking rate of the mixture on site. For this purpose, a “control agent” is used, which can increase the breaking time of the emulsion sufficiently to allow the proper installation. The control agent is often identical to the emulsifier used in the design of the bitumen emulsion, due to its obvious compatibility with other components.
Application areas and limitations of use
Preventative and rehabilitation maintenance are the preferred areas of application.
The flexibility in grading curve, the possibilities of using modified binders or using monolayer or double layer designs allows this technique to adapt to different traffic circumstance and support surfaces. It can be applied onto hot asphalt mixtures, including open graded ones, surface dressings, cement concrete and paving stones after stripping.
The application areas (non exhaustive list) include :
- maintenance of urban road pavements and in open countryside, even under very high traffic loading,
- new wearing course on top of a base layer of asphalt or emulsion stabilised base course,
- maintenance of airfields, taxiways, etc..,
- bituminous layer adhesion systems,
- treatment of accident-prone areas
- coloured cold mixes (photos 13),
- retarding cracking systems,
- large car park surfaces,
- clogging of open graded asphalt mixes
The advantages of microsurfacing are:
* their extreme thinness (this is the thinnest asphalt mix technique),
* moderate rolling noise,
* minor inconvenience to road users and residents,
* an almost total absence of chip loss,
* good technical/economic compromise.
Given their performance properties, microsurfacings are most often used for maintenance purposes for :
- Waterproofing of the existing pavement protecting the underlying layers from water damage.
- Restoration of the pavement’s surface characteristics to meet safety and comfort requirements of users and residents.