The Bureau of Street Services as Custodian of the City's Street System
One of the Bureau's missions is to properly maintain all streets in a perpetual good to excellent condition. Streets in good to excellent condition are characterized as having good riding quality, drainage, and appearance. The total annual maintenance investment is four to five times less following preventative maintenance strategy than if streets were allowed to deteriorate to poor and failed conditions requiring major rehabilitation.
To accomplish this task a variety of minor and major maintenance techniques are employed by the Street Maintenance and Resurfacing Divisions which are responsible for maintenance and repair of 6,500 miles of dedicated public roadways (28,000 lane miles) and 800 miles of alleys (400 miles fully improved and 400 miles unimproved, of which 178 miles are unimproved dirt allies). It performs nearly all resurfacing and reconstruction of residential and major streets and alleys averaging up to 200 miles of resurfacing per year. It also cleans all improved 13,000 curb miles of streets and alleys, the pedestrian subways, tunnels and public walkways. Miscellaneous functions include bridge and stairway repair and bulkhead construction. The Bureau's two asphalt plants produce approximately 600,000 tons of asphalt annually.
Small Bituminous Repair (Pothole Repair)
Crack sealing is an early-interim preventive maintenance technique used for roadways. The Bureau uses a Polymer Modified Petroleum based product and slow setting asphalt emulsion product to seal cracks as they develop, thereby avoiding saturation of the sub-base through the infiltration of water and preventing development of a base failure.
- Repairing potholes and skin patching alligator cracked areas and ruts up to 50 sq. ft. is performed by a two-person crew. This repair is necessary as an interim measure to extend the roadways serviceability until major maintenance can be scheduled.
- Skin patching is defined as the manual placement of one inch of asphalt over an alligator cracked or rutted area.
Once a street is paved it comes immediately under attack by a variety of man-made and natural forces. Natural forces break down the asphaltic bond between aggregates by oxidation. The lack of an asphaltic bond between aggregates causes erosion of the fine aggregate particles away from the larger aggregates. This eventually causes surface erosion and allows for percolation of water into the sub grade causing base failures. Automobiles, large trucks, and buses continually flex the asphalt pavement eventually causing cracks permitting larger volumes of water to infiltrate the sub grade causing base failures. Also, buses and trucks generally have axle load ratings that exceed the original design limits of a roadway causing pavement failure. The Bureau employs the following service extending maintenance techniques to neutralize these destructive forces:
Slurry Seal & Flex Seal
Optimally, within three years of asphalt blanketing or resurfacing, a street should be slurry sealed. A slurry seal replaces eroded fine aggregate particles, seals minor cracks and provides approximately 1/8" to 3/8" wearing surface that lasts approximately seven years. Slurry seal is applied to residential streets with good riding and drainage qualities to keep the street perpetually in a good to excellent condition. A maximum of three slurry seals could be applied, extending the serviceability of the street by 21 years.
A maintenance blanket is the application, by an asphalt paving machine, of 1" to 1 1/2" asphalt wearing surface to a roadway. Once a roadway has deteriorated to the point of having poor riding and drainage qualities, the application of a maintenance blanket is required. The roadway is prepared for blanketing by profiling or milling to return good rideability. All cracks are sealed and flow line irregularities are repaired. Once the preparatory work is completed, the asphalt blanket is applied. Maintenance blankets extend the serviceability of a roadway at least 10 years.
Resurfacing is the placement by paving machine of 2" (plus or minus) of asphalt wearing surface over a prepared sub base. The roadway may have had up to 15% by surface area involved in base failures. Repair of base failures is required and involves removal of all sections of failed asphalt including all saturated sub base and replacement with a granular sub grade and asphalt base. A leveling course of asphalt may be needed to return proper shape to the roadway. Preparatory work may also include profiling, milling, and crack sealing. Resurfacing will, in effect, return a roadway to a new status.