To mill or not to mill
When a municipality chooses to make repairs to its pavement, a decision must be made: to mill or not to mill. The choice may seem like a simple bottom-line consideration, but there are several long-term effects to consider.
What is milling?
Milling is the removal of the existing asphalt prior to repaving a road. In lieu of milling, a community may choose to overlay asphalt on top of existing pavement. But, the negative effects of an overlay in most cases outweigh any short-term financial savings.
When you get to a place where you need to overlay, in most cases you’ll have a significant amount of cracking (of pavement). And whenever you overlay on cracked pavement, whatever crack pattern you have is going to come back. When that crack pattern comes back, your pavement doesn’t last as long.
If you put in new asphalt on a full-depth milled surface, there’s a good chance you will get 17 years or more out of it, with no other treatments. But when you overlay pavement with a significant crack pattern, you might get 7-10 years.
The importance of curb and gutter
The presence of curb and gutter are considerations when making pavement milling decisions. Many municipal streets have curbs and gutter. Curb and gutter promotes efficient water runoff and creates a barrier for safer pedestrian and vehicle traffic. When milling is not used for an asphalt overlay on street segments with curb and gutter, there are three options to consider.
1. Lifts of asphalt at the gutter pan
The first option is applying additional lifts of asphalt at the gutter pan. This forms a lip at the gutter pan, which creates a tripping hazard for pedestrians and is dangerous for bicycle traffic. Along with causing issues for pedestrians and bicycles, this application also wreaks havoc with ADA compliance. The issue is compounded when multiple lifts of asphalt are applied.
2. Edge mill at the gutter pan
The second option is to edge mill at the gutter pan and increase the crown of the road. This will work for multiple applications until the crown of road becomes too steep.
3. Paving into the gutter pan
The third option, and the least favored, is paving into the gutter pan. Unless the situation warrants, a municipality should avoid paving into the gutter pan of the curb and gutter. With the filling of the gutter pan and the loss of curb face caused by asphalt buildup, the hydraulic capacity is greatly reduced. A reduced hydraulic capacity can create drainage issues for the surrounding residences.
Why mill and overlay?
So why mill and overlay instead of just overlaying pavement? A client can expect significant cost savings in the long run by pavement milling, as your roadway will often last twice as long between paving cycles.
You also don’t have to worry about damaging the gutter pan or losing drainage efficiency. When we mill down to the existing base, remove the crack pattern, and fix any structural issues, the newly applied asphalt will last longer. It should be noted that there are times when an asphalt overlay is warranted (ex. high severity surface defects with minimal cracking); this should be determined at the project level.
The upfront investment in milling more than pays for itself down the road, said Rob Holland, WithersRavenel’s Pavement Manager. It’s a ‘false economy’ if you don’t remove the asphalt before you resurface. It’s a false economy if you pave into the gutter pan and go, “oh I’m saving $4 per square yard by not milling.” But then you are paying for a street that is going to last half as long or less than if you had milled the first time.
Overlaying without milling also changes the drainage, interferes with driveways, and ADA compliance. In most cases, there really are no redeeming qualities to not milling.”
The temptation can be great for communities to save money in the present by foregoing milling.
Or, as the U.S. Federal Highway Administration put it in a recent report: “Today’s users are, in effect, consuming the infrastructure of their children.”
And that’s why Holland recommends that communities mill their old pavement before repaving. Because even if you overlay asphalt repeatedly, at some point the old pavement must be milled out.
A municipality can spend a lot of money in a small area, the yearly resurfacing mileage goes down, and then the remainder of the network rapidly moves down the deterioration curve. This ends up costing the municipality a significant amount of money in the future to dig out of the hole.
“It’s not saving you money, it’s costing you long-term,” Holland said. And that’s part of our philosophy, trying to help our clients save money and become more efficient over the long-term.”
Pavement as an asset
WithersRavenel encourages clients to view their pavement system as an asset, one that should be tracked and managed to ensure maximum efficiency, sustainability, and lower costs over time. We provide clients with visual evidence reflecting pavement management decisions and by delineating costs over time. And, costs can go up quickly due to inflation, making milling costs that you’ve pushed into the future significantly higher.
It’s all about S-T-R-E-T-C-H-I-N-G out the life of your pavement, Holland said. The longer the pavement lasts, the lower the annual cost of ownership.
Additional benefits of pavement management include understanding how your entire roadway system interacts with water, sewer, and stormwater assets.
For example, if you know your community is planning a sewer line replacement project in two years, you can delay pavement of that section of road to the same time. This can help better manage costs by saving you from an extra repaving.
The bottom line though? If you want your pavement to last longer, milling is almost always a good choice.