SLURRY SEAL By WILLIAM T. SUNLEY, Engineer of Local Roads and Streets

Slurry seal is a mixture of asphalt emulsion, aggregate and fillers used primarily tor maintaining existing roads. It is a durable, low cost treatment for everything from driveways to expressways.

The primary advantage of slurry seal is in the preventive maintenance of existing asphaltic concrete surfaces. Extending the service life of an asphalt pavement by applying slurry seals at the right time in the pavement life has been shown to be effective. An IDOT program with the acronym SMART (surface maintenance at the right time) has utilized this type of philosophy successfully in the past. Correcting the problems with an oxidized aged pavement surface that has lost its flexibility is more difficult and expensive than maintaining it with timely slurry seals.

The slurry seal is mixed by a traveling mixer which contains bins and tanks for the aggregate (usually crushed stone), asphalt emulsion, water and filler. The aggregate is fed into a mixer (pugmill) where the correct ratio of water and asphalt emulsion is sprayed onto the relatively dry aggregate which is continuously fed into the front of the pugmill. Mixing takes place rapidly by the paddles on the shaft of the pugmill and the slurry is run down a rubber chute into the spreader box. The spreader box has a rubber squeegee which controls placement and thickness of the slurry (usually about 3/8 inch).

There are three different types of slurries available: slurry seal, fibrated slurry seal and Microsurfacing. The typical most commonly used slurry is available in three gradations. The finest mixture would provide maximum penetration into pavement cracks but should be used in low traffic areas. The coarser, those with larger size aggregate, mixes would have increasing durability or wear resistance. An estimated cost for those types of slurries would be $0.80/sq. yd.

Another variation is the use of chopped polypropylene (plastic) fibers in the slurry. Excellent abrasion resistance and some crack bridging ability are provided by the fibers. The use of fibers does increase the cost to about $1.20/sq. yd.

Recent technology advancements in Germany has led to the introduction of Microsurfacing into the United States. Microsurfacing is a slurry seal which uses a toughened polymer modified asphalt (usually a natural latex rubber) with a quick-set emulsion chemistry. This material results in a highly durable and skid resistant surface that can be driven on in 30 minutes after placement. Microsurfacing is placed in two layers at a cost of about $1.85/sq. yd.

Being a more sophisticated mix, more than the typical truck-mounted traveling pugmill mixer pulling a spreader box with a squeegee is required to place Microsurfacing. Its extremely fast break (working) time of the emulsion requires controls on the mix to slow it down by adding the correct amount of chemical "retarder" for existing conditions. Also, the mix must be kept agitated in the spreader box. Best results have been achieved using large self propelled mixing machines which have devices that are capable of reloading materials bins while continuing to place Microsurfacing.

Pavement rutting can be remedied with Microsurfacing. Spreading equipment and mixes have been developed to fill wheelpath ruts up to 1 1/2 inches in depth. After filling each rut with the rutfill spreader box, a single layer of Microsurfacing is placed over the entire pavement at a cost of about $2.50/sq. yd. including rutfilling. Rutfilling has been stable under heavy traffic. The first installation in Illinois was placed on Washington Street in Peoria. The rutfilling withstood the daily stopping and turning traffic into the stockyards. The department has developed special provisions for each of these processes.

Slurry seal has little structural value. It is primarily a maintenance tool. It will not save a failed pavement. It can be likened to the old adage "a stitch in time saves nine." •

June 1993 / Illinois Municipal Review / Page 13