General Factors For Rehabilitation of a flexible pavement High way

Table 1 shows the as-constructed road material quantities per lane-kilometer (lane-km¹) for each of the road classes, assuming a California Bearing Ratio of 3.²

Note: lane width set at 3.75 m
 

2.1.1 Rehabilitation of Flexible Asphalt Pavements

Asphalt overlay is required for flexible asphalt pavements at year 25 for a Class 1 roadway and at years 18 and 35 for Class 2 roadways. The study assumed that the Class 1 roadway, a rural secondary highway, was due to receive its 25-year asphalt overlay and that the Class 2 roadway, an urban arterial road, was due to receive its 35-year asphalt overlay (see Table 2).

The rehabilitation procedure consists of grinding the entire pavement surface, applying a tack coat to the milled surface, laying a new hot-mix asphalt surface by machine, and compacting the hot-mix asphalt using smooth steel wheel rollers and pneumatic wheel rollers. The materials include tack coat and hot-mix asphalt.

In the case of the Class 1 roadway, the first 25-year overlay is assumed to involve removing 40 mm of the existing asphalt and replacing it with one 50-mm lift of hot-mix asphalt. In the case of the Class 2 roadway, the 35-year overlay involves removing 80 mm of the existing asphalt and replacing it with 100 mm of hot-mix asphalt (placed in two lifts). The rehabilitation material quantities are shown in Table 2.

The study assumed that the hot-mix asphalt used to rehabilitate rural roads (Class 1 roadways) was produced in portable plants erected in a suitable location and that the asphalt used to rehabilitate urban roads (Class 2 roadways) came from permanent manufacturing plants. Typically, raw aggregates and asphalt binder are delivered by truck to the production plant, with the asphalt binder usually delivered hot in a liquid state and used while still hot. The aggregates for the hot-mix asphalt are dried and heated prior to mixing. After the hot materials are mixed, they may be stored in an insulated silo or loaded directly into trucks for delivery to the site.

The delivery trucks discharge the hot-mix into an asphalt paver, which spreads the material on the road. The hot-mix is compacted and smoothed by steel wheel and rubber tire rollers. The number and size of the rollers depend on the weather, the material quantities and the type of hot-mix, but it is common to have two steel wheel rollers and one rubber tire roller. When a tack coat is used, the emulsion is often delivered to the site in a truck equipped with a spray bar to spread the liquid material.

2.1.2 Representative Material Hauling Distances

Hauling distances for materials can vary substantially across the country, depending on the availability and proximity of materials. To account for variations in material hauling distances and thereby standardize energy-use reporting among the participants, the survey used pre-determined material hauling distances (Table 3).

*The zero value reflects industry practice to reduce hauling requirements by locating the portable asphalt mixing plant next to an aggregate source.

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2.2 Survey

The survey was limited to elements of road building over which the participants exercised a certain amount of control (e.g. the survey did not look at energy use in the manufacture of bitumen). As a result, the survey design had to break down the process of road rehabilitation into discernable parts to determine the energy used for different activities. The survey focused on the three main road rehabilitation activities:

	mixing aggregates and hot asphalt emulsion (bitumen) at the asphalt plant
	hauling materials and transporting personnel to the site
	using heavy machinery at the site

It sought to gather data on participants’ energy use, as well as information on their energy conservation practices. The survey was mailed to each of the five participating road building companies. The Institute summarized the data received and returned the individual summary, as well as the complete results, to all participants for their review. Therefore, participants were able to compare their data with the group average to see where they stood. This process made it easier to spot data outliers and to correct inaccurate data.

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Footnotes:

1. A lane-kilometer is one lane in width (a width of 3.75 meters was used for the study) for a distance of 1000 meters.

2. The California Bearing Ratio (CBR) test is an empirical test first developed in California, USA, for estimating the bearing value of highway sub-bases and sub grades. The CBR scale reflects a soil’s bearing capacity based on its moisture and density. A CPR of 3 indicates a poor soil classification and will affect the design of flexible pavement road structures.
 

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