New Ulm uses a combination of bituminous road oil and crushed granite aggregate to seal the bituminous street pavements.
The road oil is rapid setting CRS-2 emulsified oil. Emulsified oil has had a chemical added to it so that it will mix with water. The water acts as a thinning agent to make the oil more fluid and easier to work with. After the mixture is heated to about 170 degrees Fahrenheit, and applied, the water will evaporate quickly leaving pure bituminous oil to seal the street surface.
Procedure & Equipment
After the street has been extensively patched to minimize any surface defects and swept to remove any debris and dirt, a seal coat may be done.
The first order of business is to cover any manhole covers and water shut of valve covers that are in the bituminous portion of the street. A "rosin paper" is laid down on the cover then a layer of sand or seal coat chips is put over the paper. This prevents the seal coat oil from penetrating and sealing down these utility covers.
Oil Distribution & Chip Spreader
Next, the distributor truck sprays the oil uniformly over the bituminous street at a specified rate of application (determined by many factors). Following close behind the distributor truck is the "chip spreader". This equipment spreads the seal coat chips uniformly over the oil that the distributor truck just put down.
The "chip spreader" and distributor must work in close unison for two reasons:
- The aggregate must be placed on the oil as quickly as possible to take advantage of the "soak" time for the chips.
- The "chip spreader must stay close to the distributor so that traffic is not able to get between the machines and drive in the fresh oil.
Following up the "train" of equipment are the two rubber tire rollers. These rollers orient the chips to lay flat and press them down into the fresh oil. After a short time the oil "breaks" or sets up and the chips remain permanently embedded in the oil. Within the process of seal coating is the coming and going of several dump trucks which are all hauling seal coat chips to the "chip spreader." The distributor is also coming and going because the tank must be filled several times from a larger delivery transport to complete a day of seal coating. All this action must be anticipated and coordinated.
The paper and rock is then cleaned off all utility covers leaving them clean and free of oil. Then, finally, the street sweeper sweeps up any excess rock from the sealed street and the seal coat project is complete.
Reasons for Seal Coating
There are two main reasons for seal coating streets.
The first is to seal the bituminous surface against the elements. The older a bituminous surface gets the more porous it becomes. This is due to the wearing away of the surface due to sunlight, rain and traffic. The rougher the surface becomes due to these factors, the more vulnerable it is and the more rapidly it deteriorates further. Water trapped in the surface and freezing at night and thawing during the day over a few days begin the formation of potholes. One method of avoiding this result is to routinely seal coat the streets to keep the water out.
The second reason to seal coat streets with a hard material like granite is to coat the surface with a wear resistant material. This not only minimizes damage done by normal traffic wear, but it also protects our streets from damage that might be done by scraping them several times a year with snowplows.
New Ulm streets are on an 8-year seal coat cycle. This means that the Street Department attempts to seal coat all eligible streets every 8 years. In the past, seal coating has been put off due to unexpected circumstances such as the windstorm of 1998 (the extensive cleanup used both the time and money that was designated for seal coating). Then there was the emergency repair of Center Street in 1993 (to make it last until it could be reconstructed in 1998/1999). Both of these circumstances reduced or eliminated seal coating in that particular year. Setbacks like these are unavoidable and take years to make up sometimes, however, every attempt is made in subsequent years to "catch up."
A street section may not be eligible for a seal coat because of its poor condition. It becomes very inefficient to seal coat a pavement if it needs to have extensive work done on it to get it ready for seal coat. These streets need to be reconstructed.
Bituminous paved alleys are included in the 8-year cycle of seal coating.
During a typical year of seal coating the Street Department uses about 50,000 gallons of emulsion oil and around 1,700 tons of aggregate. This translates to about 10 miles of streets. Alleys require much less material per block because of the narrow width. The number of alleys that are sealed each year varies but they are generally done in conjunction with the streets.
Oil and aggregate prices vary from year to year, which then affects the cost per square yard of seal coating. In the past few years it has gone from $0.37 per square yard in 1997 to $0.504 per square yard in 2000.