Yesterday’s road designs are displaying signs of overload volume distresses, mainly in the area of rutting and loss of skid properties. When a rutting distress first appears in an urban environment, it creates several problems such as stopping issues (hydroplaning), road-to-vehicle clearance issues and pedestrian issues. The loss of skid properties results in the lack of ability to safely stop. This can be caused by a number of factors from soft aggregates used in surface mixes of asphalt or concrete pavements or over-asphalted mixes.
Road maintenance managers have been looking for alternative maintenance repair methods for their toolbox of options that would extend the life of the pavements and restore the safety factors to the road surface. Precision milling has been the answer for the maintenance departments’ needs; it has allowed for the safe extension time until this section of road can be rehabilitated, if required.
Xacta’s points
In order to alleviate these issues and achieve a good precision milled surface, some preliminary engineering is required. Measurements of the existing road and depths of the ruts are required in order to establish the new profile and a direction for the water. A sound knowledge of the structural layers of the road composition and the measurements of the depths of each layer of pavement is essential. Any areas of deficient cover will result in having the total mix removed. The target should be to leave approximately 50% of the depth of old pavement wherever possible, while keeping in mind that the new road profile will be flatter and that the full depth of the humps to the bottom ruts will not be able to be removed. Any areas of deficient cover will result in the removal of the total mix and these areas will require immediate attention.
The Hard Rock Group uses a Wirtgen W1200F, a 1.2-m machine equipped with electronic slope and grade controls. The objective of XactaMilling, or precision milling, is to obtain a finished texture or pavement pattern that will not require an overlay. This fine-milled texture pattern is a result of a combination of factors:
- The standard milling drum is replaced with a drum containing approximately twice the number of teeth;
- The tip-to-tip separation of the teeth is set at a maximum of 8 mm; and
- The forward speed of the milling machine is required to be set in order to obtain a minimum of 1,830 contacts per sq meter.
The 1.2-m machine allows the flexibility to adjust the grade as required and to control the limits of removal to establish the best surface mat possible. It is not uncommon to have a final precision milled pavement profile with differing cross fall slopes from lane to lane. Normally the majority of the damage is found to be concentrated in the curb lane or the heavier traveled lane and thus the majority of the removal will be concentrated in that area. The adjacent lanes often have less traffic, which will result in less distortion and require reduced depth of milling.
The place to be precise
Precision milling has undergone trial testing by a large number of agencies. The Ministry of Transportation Ontario has used precision milling on its 400-series highways and on secondary highways such as Highway 7 and 35 in the Bancroft area, addressing a range of concerns from skid properties to pavement leveling on both concrete and asphalt pavements.
The city of Toronto has employed the service to establish a new slope in order to reduce rutting depths and prevent hydroplaning and winter icing. These roads in Toronto experience traffic volumes of 35,000 to 41,000 vehicles and 60 bicycles between the hours of 7:30 a.m. and 6:30 p.m. Canada’s capital city, Ottawa, utilized XactaMilling for a temporary ride improvement on an arterial road. A major reconstruction was targeted to take place within a few years. St. Catharines, Ontario, had a number of older residential concrete streets that had become uneven over time and had created a rough riding surface. The bumping of vehicles over the slabs caused vibrations that were noticed by nearby residences and XactaMilling was the solution to this problem.
This same condition—transverse bumping as a result of slab movement—tends to appear on older major highways of concrete and composite structure designs. These joints can be XactaMilled to provide a more comfortable and safe ride.
Details, details
The precision milling process is not a quick process with respect to conventional milling, and special attention must be paid to the type of crew and operator. Unlike conventional milling that tends to be more of a cut-and-produce operation, precision milling takes a more detail-driven crew. The field operation people will be required to make onsite design alterations, while keeping with the preliminary engineering. This type of operation requires more flexibility and patience and results in lower production volumes. These factors create a unique situation at bidding time and a very individualized project cost.
The advantage of using the XactaMilling method of repairs is three-fold. It minimizes the restoration time for the road surface, allowing the road to be back in full service immediately with minimal user delays. XactaMilling is an economical interim solution, because it will extend the life of a road. In addition the new milled surface doses not require an overlay; it is a finished riding surface.
