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CONTRACTING: THE ABCs OF SLURRY SURFACING |
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What Is Slurry Surfacing? Asphalt is an excellent way to waterproof and seal. For thousands of years it has been used for that purpose. The Babylonians used asphalt to hold paving stones together for road ways and water courses. When hard wearing aggregates are added a strong, abrasion resistant mixture can result. This is the basis of all road applications of asphalt. This is what slurry surfacing is, a mixture of aggregates and asphalt. "Slurry" itself is a rheological term which means completely dispersed. Slurry is a common method for moving solid materials. A slurry stays dispersed and usually involves water. Coal slurries are an example of this. So a slurry surfacing is a mixture of aggregates and asphalt applied in a slurry physical state. The origin of slurry as a roading method is debated despite the pat statements in many industry publications! The origin may have been the German "schlammer" of the 30s. But this was a hot mixture, rather like the gauss asphalt used today. Not a slurry in the rheological sense at all. One Californian company claims to have used the concept with asphalt emulsions, and asphalt emulsions are the real key, in the 40s. There is no doubt that the version of slurry surfacing " Microsurfacing" came from Europe via Germany but French claims that Ralumac is really a French product called "Sealgum" persist . The author makes no choices as this type of product has evolved considerably and the historical origin is not often a preoccupation of contractors! For practical purposes in USA in the 21st century slurry surfacing is a method of application of aggregates and asphalt to give the age old requirements of water proofing, sealing, dust abatement, abrasion resistance and pavement protection. Its variations and extensions , such as rut filling and surface profile correction are a result of improved materials, design and application techniques. As a technique the ISSA- International Slurry Surfacing Assocation- figures show that its on the rise! 
Microsurfacing Vs Slurry Seal: There is often great confusion on what the difference is! Sometimes Microsurfacing is presented as sort of superior slurry seal. Sometimes polymers are added to slurry seal and the product presented as Microsurfacing. Confused yet? Figure 1 shows what Slurry surfacing is - all slurry surfacing.  Figure 1 A slurry seal is a thin application,that is about 1-1.25 times the largest stone thick. A microsurfacing is a thicker application, 2-3 times the largest stone thick. That's it! The rest is detail associated with how this is achieved and what practical ramifications are produced! Example if its thicker it dries more slowly so you need to have faster set times. This leads to a whole technology of emulsifier types and use of specialist materials such as polymers and additives. So Slurry Surfacing is:
 Materials: Its obvious that the performance that is achieved is due to the materials used and their design.
Aggregate: The surface that you get is function of the stone grading and sizes. ISSA use three types I-III as shown in the table but larger and smaller gradings have been used.
 The aggregate must obviously be abrasion resistant, resistant to cracking under freeze thaw and not crush. But it must also be clean and react at the right rate with the emulsion. Emulsion: Emulsion type and compatibility is a key requirement.
Slow set emulsifiers include quaternary ammonium compounds , fatty amines or fatty acids. They usually have few compatibility and coating problems but will always be slow setting as they depends mostly on water evaporation for break or cure. They may be anionic or cationic. Traffic times of 3-6 hours are usual, even under good conditions. For this reason they may only be used for slurry seal applications. Quick set uses superior emulsifiers usually of the amidoamines family. Compatibility is more of an issue but can be solved in the mix design phase in most cases. These emulsifiers are cationic and react with the aggregates used. Traffic times are 1-4 hours, depending on conditions. They are suitable only for slurry seal. Quick set polymer modified systems have a polymer added. This may be done by blending a polymer with the asphalt prior to emulsification or else adding a latex polymer into the soap system or co-milling. ( post addition is sometimes used but this tends to slow the system and stability problems can also result). Traffic times are similar to unmodified quick sets but the presence of the polymer will allow these systems to be used for slurry seal or microsurfacing under favourable (warm conditions). Microsurfacing emulsions always have polymer and are based on very fast set systems. They may use an additive as a trigger to break. Most microsurfacing emulsifiers are immadazolines . They are more reactive with aggregates and compatibility and coating issues must be carefully addressed in the mix design. Additives: These are solid (cement or lime) or liquid ( surfactants) and are added via the water system to retard or accelerate the reaction. Polymers:
When extremes of weather are encountered the base stock of asphalt may be changed.
Design is about optimization of the binder content in the mix and addresses the following:
HOW IS IT MADE  The key feature of slurry surfacing application is that the machines are self propelled, either as truck mounted or with their own power. This allows rapid in place application. The material is mixed in exact ratios in a pugmill and applied via a box that is pulled behind the unit. Rubber texturing flaps are used to give a smooth finish. Boxes are designed differently. Slurry seal boxes may be a simple frame or have augers to distribute material. Microsurfacing boxes will have an extra texturing flap and augers to keep the material fluid in the box. For rut filling a special box is used that moved the material to the center of the rut.  All the slurry machine manufacturers make a range of boxes and can be contacted for details. Method: 
Slurry contractors will set up an application schedule including traffic control. Surfaces must be repaired, potholes filled and cracks filled before application. Preferably this is done well in advance, perhaps even the season before.Where is it Used?
The technique is well established and has been optimized and improved over the years. A powerful weapon in any road owners arsenal! Back to VSS Technology Papers Library |