Slurry Surfacing Overview
Slurry surfacing systems (Slurry seal and Microsurfacing) are thin emulsion cold mixed overlays which are recognized as cost-effective restorative and preventive maintenance techniques for roadways of most traffic levels, airport runways and taxiways, highways, shoulders, sidewalks, foot and cycle paths, and parking lots. They may also be used as wearing courses on emulsion stabilized bases or recycled roadways, bond courses, crack fill or combined with other techniques like chipseal (“capeseal”).
The systems combine finely-graded aggregates, bitumen emulsion, water and mineral fillers like cement or lime. Polymers, pigments, fibers, chemical additives and recycled materials may be included to enhance performance or processibility, or to improve the environmental profile. Bitumen emulsions used in slurry surfacing fall into the category of anionic or cationic slow-set grades, but need not pass the cement mix test. The bitumen content of the cured seals ranges from 5 to 16% by weight of aggregate. Slurry surfacing is normally applied on highways using a purpose-designed self-propelled mix-paver but may be prepared in more basic mixing equipment and applied manually for smaller scale applications.
The slurry ingredients are dosed and mixed in the paver then discharged into a spreader box which distributes the material over the roadway and maintains the profile and thickness of the seal. Application rates range from 3-14 kg/m2 for a single pass. Some specially formulated materials may be applied in multiple layers or in deeper deformations or potholes. Bond coat is sometimes applied before sealing to ensure that the new layer adheres to the existing surface.
Two slurry products can be distinguished – Slurry seal and microsurfacing. Slurry seal is normally laid no thicker than 1½ times the top size of the aggregate, which can range from 3-10 mm. Mineral fillers like cement or hydrated lime are commonly used, but are not always required. Slurry seals waterproof and seal the surface from water ingress and deterioration and provide a skid resistant, smooth running surface. Typically, slurry seal can be trafficked in 1-4 hours. “Quick-set” slurry seal can be opened to traffic within an hour and usually has a shorter mix time. Microsurfacing can be laid in multiple layers to correct significant deformations including rutting and is suitable for higher traffic or higher stress situations. Higher quality aggregate is required, and usually has a top size of 5-10 mm. The emulsion must contain minimum 3 wt% polymer by weight of bitumen and meet more demanding durability specifications than slurry seal. It is applied with a mix-paver. The mix sets within 15 to 30 minutes, and may be trafficked within 1 hour. Mineral filler is almost always included in the mixture. The appearance of cured slurry is similar to that of a thin hot applied overlay but with higher texture depth.
What can be achieved with slurry surfacing:
• Prevent water penetration
• Improve skid resistance
• Improve surface texture/surface drainage
• Rut-filling (microsurfacing mixes only)
• Correct effects of raveling
• Fill small cracks including alligator cracks
• Fill small potholes
• Correct flushed surfaces or stripped surface dressings
• Seal base courses
• Wearing surface on stabilized bases
• Bond coat
• Delineate bus lanes, cycleways, shoulders (optionally
with color)
• Provide good background for road marking
• Fill open graded mixes
Slurry seal and microsurfacing cannot be used to address structural problems or major cracking associated with the existing pavement.
Table 1: Slurry surfacing types
The history of slurry surfacing
While cold mixes with a slurry like consistency based on clay stabilized emulsions may have been used 100 years ago, and were further developed during the 1930s in Germany as “Schlämme”, the first slurry as we recognize it today is believed to have been based on chemically stabilized anionic emulsions and was in use by the late 1950s in both USA and Europe.
The beginning of the 1960s saw the development of continuous mix slurry machines and systems based on cationic emulsions, including what we would now describe as quick-set slurry or microsurfacing. Today the vast majority of slurry surfacing uses cationic bitumen emulsion.
The Redicote Slurry System, developed around 1962, was one of the very first cationic slurry systems and is still in widespread use today. Around the same time Thomas Swan & Co. Ltd. in the UK and Lancaster Chemical AZS division in the USA developed a range of slurry and microsurfacing emulsifiers, later acquired by Nobel Industries and marketed under the Catimuls tradename. In 1985, Scanroad International (part of Kenobel Industries) bought Slurry Seal Inc. in Waco, Texas, which since 1961 had been producing Young’s slurry pavers. Raymond Young was one of the pioneers in the manufacture of slurry surfacing pavers and emulsion plants and was the first president of the ISSA (International Slurry Surfacing Association), established in the 1960s. In the 1990s Scanroad continued to manufacture and develop slurry pavers (HD-10 and SR-10), emulsion plants, as well as other construction equipment in Waco, and also operated slurry seal and micro paving contracting companies in Europe and USA.
