The slurry surfacing technique, which further includes slurry seal, was first utilized in the 1950s, mainly as a protective covering layer for airport runways.
Cases of slurry utilization in road maintenance have been multiplied with the emerge of rapid setting cationic emulsions and then emulsions modified with polymers for thick-layer slurry applications, which today is known as micro surfacing.
This technique was first used in Germany in the late 1960s. Today, the slurry surfacing by bitumen-based emulsions is often known as "slurry seal".
The slurry surfacing is used for the following applications:
• It is used to repair alligator cracks, block water penetration seals, and make the asphalt pavement impermeable.
• It prevents asphalt from fretting and raveling caused by loss of the binder and aggregates.
• Minor surface puddles can be restored to reach a uniform asphalt pavement structure and shape.
• By slurry surfacing, the asphalt surface will appear more uniform.
• It elevates resistance to sliding by turning the surface into a rough surface.
Micro surfacing can further be applied to fill the puddles on asphalt roads that are prone to deterioration.
Slurry surfacing in small and limited areas
The slurry is typically mixed mechanically and applied manually with a big brush to provide a uniform surface texture. It consequently protects surfaces and gives them a stunning appearance. Primers can additionally be utilized to add more strength, and colored slurries are applied to mark cycling lanes.
Thin layer slurry on the roads
These layers are usually 3 mm in thickness and applied continuously and seamlessly on the surface by machines. Thin layer slurry can be made fast and with minimal interruption. Due to its low thickness, this technique is employed only for areas with relatively low traffic.
Thick layer slurry on the roads
These operations typically include single-layer restorations by using 0 to 3.6 mm materials and emulsions modified with rapid (quick) setting polymers. The thick-layer slurry is normally more durable than the thin-layer slurry and is therefore suitable for use in high traffic areas.
Micro Surfacing
Micro surfacing is typically done by using materials ranging from 3.6 to 10 mm in order to create a new surface. The micro surfacing can also be applied in two layers. The first layer adjusts and restores the surface and fills cracks, while the second layer forms a dense surface with an appropriate texture.
Slurry in airports
The slurry technique has been applied for airport runways and taxiways for many years. Slurry surfacing is an ideal maintenance strategy for airports because it is relatively fast to implement and the materials used are small to minimize the risk of notable external material damage to jet engines.
Slurry surfing renders only a thin coating for an existing paved area. Therefore, it does not remarkably add strength to the road structure and is not load-bearing. And if applied on the wrong bed, it will not be much durable.
Slurry seals are usually applied on low-traffic roads such as residential roads, sidewalks, airport runways, car parks, low-traffic local routes, and so on, while micro surfacing can be worked on heavy traffic roads because it makes the surface stronger.
Slurry surfacing can also be used to cover surfaces where car wheels make much noise. This technique prevents the formation of cracks on the coated surface. Surface coating by slurry surfacing is called "cape seal".
The use of fiber-reinforced micro surfacing is growing. This technique seems to be helpful in the future in solving some of the problems of severely cracked asphalts. The colored slurry surfacing is also worked as a given product with different colors.
These coatings are applied in a very thin layer and can only be used for areas such as sidewalks and routes with low traffic.
Materials used in slurry
Materials used in slurry surfacing include bitumen-based emulsions, aggregates, mineral fillers, and water. A chemical additive can also be utilized to apply slurry surfacing at areas with variable temperatures. Materials utilized to make asphalt can also be used in slurry. In China, basalt and diabase, also known as dolerite or micro gabbro, are commonly used in micro surfacing. Other types of materials (e.g., granite, sandstone, limestone, etc.) can also be applied.
These materials must match the requirements specified for materials used in asphalt, but there are some other features to consider.
Purity
The sand equivalent (SE) test indicates the purity of the material which can be obtained from standard tests ASTM D2419-09 (ASTM, 2009b) and BS EN 933-8: 2012 (BSI, 2012). Aggregates are usually mixed with clay, which diminishes the duration of preparing the slurry mixture, but makes it difficult to disperse. The presence of clay can lead to long failure times, which delivers a material with poor cohesiveness. Accordingly, purity is a leading property for materials utilized. Materials will be more pure at higher SE values.
Surface activity
Materials hold a surface electric charge that remarkably influences the nature of the reaction between aggregates and bitumen-based emulsions. The reaction will be more intense at higher surface electric charges. For freshly crushed aggregates, for instance, the surface load decreases over time and much faster when the aggregates get wet.
The type of mineral in the aggregate further influences its surface electrical charges.
International Slurry Surfacing Association (ISSA)
ISSA has introduced three types of granulation Type I, Type II, and Type III, which are presented in the table below.
These classifications are widely used in the United States, China, and other countries.
The diameter of the smallest grains is critical in all three grades. This will provide a higher capability for filling. Therefore, it can provide the power to penetrate cracks.
These materials can be used to fill cracks on the surface of roads and repair minor to moderate surface imperfections. This type of granulation will provide the road surface with a fine texture because of using fine-grained materials. Therefore, it is commonly used on low-traffic roads such as residential streets, rural roads, and so on.
Granulation Type I is mainly intended to prevent weathering of the asphalt pavements.
Type II
This type of granulation can be used to fill surface pits and repair moderate surface imperfections, as well as to seal and create a durable asphalt surface.
Granulation Type II can create a rougher asphalt that can be used on roads with moderate to high traffic.
It can also be worked as a seal on previous flexible surfaces or fixed surfaces before applying the next layer.
Type III
In granulation Type III, materials and grains are larger than the previous two types and can provide a better surface texture. Therefore, asphalts created with this type of granulation show more strength and resistance.
This can add to the durability of the asphalt because larger sizes increase the thickness of the pavement. This type of micro surfacing is proper for heavy-traffic roads. It can also be used to fill grooves and cracks in the surface.
Granulations Type II and III are typically used for micro surfacing and Type I for slurry surfacing.
Emulsion
The majority of emulsions used in the slurry seal are of slow setting type and can be anionic or cationic depending on the type of rock material. Slurry surfacing by rapid (quick) setting cationic bitumen emulsions CQS-1h breaks earlier than normal slurry seals. We can only use emulsions modified with polymers and rapid (quick) setting cationic emulsions for macro surfacing. In China, it is known as BCR and is defined as technical instructions for macro surfacing and slurry seal.
In Europe, emulsions are applied according to the standard BS EN 13808: 2013 (BSI 2013).
Latex is often used as a polymer modifier for emulsions in micro surfacing. Certain polymers can enhance the adhesion and cohesion between the emulsion and the material. Therefore, micro surface layers do not easily detach from the road surface under heavy traffic.
Mineral fillers
Mineral fillers are normally Portland cement or hydrated lime, which are added to the mixture to control its consistency and setting rate. Portland cement is typically used up to 2% of the total volume of the slurry and 3% of the total volume of the micro surfacing. The volume of hydrated lime is usually 0.3% to 05% of its total weight.
The mineral filler should be viewed as part of the granulation program.
Water
The water utilized in the slurry should be clean and free of salt, mud, etc. The water-slurry ratio influences the mixing time and consistency of the slurry.
Additives
Additives are added to expand the time of slurry mixing at elevated temperatures. Setting-accelerating additives are sometimes used at lower temperatures. Additives are normally surface materials, i.e., materials that reduce surface tension.