To mill or not to mill? Full-depth pavement milling offers long-term cost benefits in pavement management
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 that must be considered.
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, according to WithersRavenel Director of Pavement Management,Steve Lander, PE, 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),Lander said. 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, maybe 10 years tops.”
The presence of curb and gutter needs to be considered when making pavement milling decisions. Many municipal streets have curbs and gutter, which 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.
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, Lander said. 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.
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.
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, said Lander. With the filling of the gutter pan and the loss of curb face caused by asphalt buildup, the hydraulic capacity is greatly reduced, which can create drainage issues for the surrounding residences. That’s your first go – then what happens if the pavement gets overlaid again? And again? There are some municipalities that have paved right to the top of the curb. Then what are you going to do? When you mill at this point and the gutter pan was properly tacked, you end up damaging your gutter pan when milling out the asphalt.”
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, Lander said. 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, Lander said.
You can overlay more streets than if you have to mill, he said. It gives your network a false bump in your Pavement Condition Index (PCI); it looks like your system is in great shape. And then you keep overlaying until you can’t. You’ve depleted your network’s overall value. You’ve kicked the can down the road.”
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 WithersRavenel’ Lander and Holland, recommend 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.
I’ve had examples where communities are milling out 8 inches of asphalt, Lander said. You can imagine what that does to the unit costs for milling. A municipality ends up spending 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.”
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 tell the story by delineating costs over time. And, as anyone in 2022 knows, 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, thereby minimizing the overall road closure and better managing 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.
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