Asphalt Paved Streets (R. Hayter)

Recommendation:
That the September 29, 2003, Transportation and Streets Department report be received for information.

Report Summary

This report is in response to an inquiry regarding specifications for the production of asphalt mix used in street paving.

Previous Committee Action

At the August 19, 2003, Transportation and Public Works Committee meeting, Councillor R. Hayter made the following inquiry:

“The deterioration of asphalt paved streets has been a pet peeve of mine for a long time and led me more than 20 years ago to propose the addition of rubber from discarded tires to increase its durability.

I am pleased to see this finally taking place and, hopefully, the results will be good.

However, in my ongoing surveillance of asphalt paving in the city, I have made other observations:

  1. The consistency of the asphalt mix appears to differ considerably, with some being smooth with a fine texture, while other asphalt is rougher and appears to contain much larger aggregate.
  2. The pavement with the rougher appearance tends to break up sooner, with cracks often developing around sections where larger pieces of gravel are apparent.

I would like a report commenting on these observations, and specifically answering these questions:

  1. Do the specifications for the production of street paving asphalt specify the size of gravel permissible in the mix?
  2. Who inspects the asphalt mix to ensure it meets the City’s specifications? How many inspectors does the City have? Where are the tests done – at the production plant or at the delivery point?
  3. Does the size of gravel affect the durability of asphalt? In other words, does a mix containing smaller gravel compact better and does this provide increased resistance to cracking?
  4. What is the total amount and value of asphalt mix purchased in a year? How much of this is purchased from private companies and how much is produced by City forces? How many private companies are contracted by the City to provide this product?”

Report

The City of Edmonton, over the past several years, has been actively exploring innovative solutions for improved pavement performance. Some of these innovations have included the development of new asphalt mixtures for use on the roadway network. These newer asphalt mixtures have inherently different surface textures depending on the aggregate makeup for that particular mixture.

One of the mixtures being evaluated is a rubber asphalt mix, which contains crumb rubber particles that are blended with the asphalt binder component and then combined with the aggregate to create the rubber asphalt mix. The key anticipated benefits of a rubber asphalt mix are improved durability, improved crack resistance, improved noise reduction and improved skid resistance. The City is part of a three-year partnership between the Tire Recycling Management Association and several Alberta municipalities. The City of Edmonton has taken a lead role within the partnership with the laydown of over 70% of the asphalt rubber produced to date. As part of the partnership, several sections of rubber asphalt mix have been placed in locations that include 137 Avenue eastbound, from 127 Street to 113A Street; 111 Avenue, 124 Street to Groat Road; Stony Plain Road, from 156 Street to 166Street and 50 Street, from 13 Avenue to the South City Limits.

The second mix being evaluated is a Stone Matrix Asphalt mix. Stone Matrix Asphalt is a mix composed of a strong coarse aggregate skeleton and a binder mastic composed of a high asphalt cement content, cellulose or mineral fibres and a high percentage of mineral filler. The key objectives of this mix are improved rut resistance, improved skid resistance, improved durability, improved fatigue resistance, improved crack resistance and a reduction in traffic noise. A test section of Stone Matrix Asphalt was placed on 97 Street southbound in the curb lane from 135Avenue to 127 Avenue in the fall of 2002. The first-year results of this test section have been sufficiently successful, that the City of Edmonton will be placing additional Stone Matrix Asphalt mix at the following locations in 2003: Wayne Gretzky Drive, northbound approach to 118 Avenue; Yellowhead Trail and 121 Street intersection improvements and 170 Street and 107Avenue intersection. All three locations are high traffic volume locations, which have shown severe rutting over time.

Regarding the specific questions of the inquiry:

  1. Construction specifications used by the Transportation and Streets Department are contained within the City of Edmonton, Design and Construction Standards manuals. The specifications relating to the production and placement of hot-mix asphalt concrete are comprised of three sections: Section 02060 – Aggregate, Section 02065 – Hot-mix Asphalt Concrete and Section 02741 – Hot-Mix Asphalt.

Over the last decade, three main types of hot-mix asphalt have been utilized within Edmonton. They are designated as:

  • ACB (Asphalt Concrete Base) which is used as a base course material for major arterials, minor arterials, industrial/commercial roadways and collector roadways (maximum aggregate size of 25 mm);
  • ACR (Asphalt Concrete Residential) which is used for paving residential and non-bus route collector roadways (maximum aggregate size of 12.5 mm);
  • ACO (Asphalt Concrete Overlay) which has been used as a surface course material, since 1993, for major arterials, minor arterials, collector roadways and industrial/commercial roadways (maximum aggregate size of 12.5 mm). For each of the hot-mix asphalt types used by the City of Edmonton gradation bands/curves exist within the specifications to ensure that the aggregate gradation of the mixes are consistent from one supplier to another.
  1. The materials are tested to ensure that they meet the requirements of our construction specifications. Testing of the hot-mix asphalt materials is performed by the Transportation and Streets Department, both at the hot-mix asphalt plant and at the job site for the City of Edmonton’s capital projects.

For private development projects, private materials engineering and testing companies perform testing of the hot-mix asphalt materials. Plant checks are done on a regular basis to ensure the contractor/supplier is producing the hot-mix asphalt in accordance with specific batch requirements. In addition to laboratory testing, the Transportation and Streets Department currently employs a total of 10 Construction Supervisors and 21 Contract Inspectors who perform continuous field inspections on projects within the capital program to ensure that the work performed by the contractor meets the requirements of the construction specifications. A total of eight staff are also employed by the Transportation and Streets Department who monitor construction on private development projects.

  1. The maximum aggregate size does affect the durability of hot-mix asphalt but only to a limited extent. The durability of asphalt is dependent on a number of other characteristics, such as gradation, binder content, air voids, binder film thickness and mix stability.

Cracking of hot-mix asphalt is due to two phenomena, the first being reflective cracking and the second being low-temperature cracking. Approximately 30% of Edmonton’s roadway network consists of composite pavement sections - hot-mix asphalt over concrete base. Concrete, by nature, cracks as a result of shrinking during the curing process. These cracks are reflected back up through the hot-mix asphalt. Another 40% or our roadway network consists of hot-mix asphalt over cement treated base course gravel (soil cement). Cracks, occurring naturally during the curing of this soil cement layer, also reflect back up through the hot-mix asphalt.

The remaining 30% of Edmonton’s roadway network consists of hot-mix asphalt over base course gravel and oiled and gravelled roadways and are affected by the second phenomena which causes cracking in hot-mix asphalt – low temperature. This type of cracking is controlled by the type of binder used in the hot-mix asphalt. The binder used by the Transportation and Streets Department accommodates temperatures ranging from +56oC to –34oC.

  1. For the 2003 construction season, the Transportation and Streets Department has contracted to purchase a total of 285,000 tonnes of hot-mix asphalt at a cost of $17 million. This is broken down into 245,000 tonnes of hot-mix asphalt for use on capital projects and 40,000 tonnes of hot-mix asphalt on maintenance projects. This hot-mix asphalt is purchased from five private companies. The Transportation and Streets Department through its own asphalt plant, also produces a total of 12,000 tonnes of hot-mix asphalt at a cost of $0.73 million. This material is generally only produced during the winter months and is normally used for pothole repair and emergency utility repair restoration.

Background Information Available on Request

Construction Specifications:

  1. Section 02060 – Aggregates
  2. Section 02065 – Hot-Mix Asphalt Concrete
  3. Section 02741 – Hot-Mix Asphalt Paving

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