News To Mill or Not to Mill? Pavement Milling Offers Long-Term Benefits in Pavement Management

When it’s time to repair pavement, pavement owners have a decision to make: 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 pavement milling?

Milling is the removal of the existing asphalt surface prior to repaving a road. A milling machine grinds away anywhere from one-half inch to six inches of the asphalt surface course, or riding surface. Removing the entire surface course—and the intermediate course, if there is one—down to the base course is called full-depth pavement milling.

Assuming the road has not sustained damage from water penetration or overweight loads, milling leaves behind a consistent surface free of cracks and voids suitable to receive a fresh layer of asphalt.

Is pavement milling required?

It is not necessary to mill before laying down fresh asphalt. It is possible to overlay new asphalt directly on top of existing pavement. Applying an asphalt overlay is cheaper than milling before applying an overlay, but the negative effects in most cases outweigh the short-term financial savings.

A common reason to apply an asphalt overlay is significant pavement cracking. When new asphalt is applied over cracked pavement, the asphalt eventually settles into the cracks, and the crack pattern returns. Subsequent layers of asphalt overlay will have the same problem.

Asphalt overlay applied over significant cracking has a lifespan of about 7–10 years. New asphalt applied on full-depth milled surface, on the other hand, has a lifespan of about 17 years without any other treatments, longer if preservatives are applied.

What happens when pavement is resurfaced without milling?

The most common alternative to milling is resurfacing by applying an asphalt overlay directly on top of existing pavement. Because a large proportion of municipal streets use curb and gutter construction, it is therefore necessary to decide how the new asphalt should interface with the gutter. There are three options to consider: asphalt lifts at the gutter pan, edge mill at the gutter pan, and paving into the gutter pan.

1. Asphalt lifts at the gutter pan

The first option is to apply additional asphalt lifts at the gutter pan. This raises the height of the asphalt directly adjacent to the gutter, forming a lip at the gutter pan. The benefits are that it doesn’t change the crown of the road or the capacity of the gutter. The drawbacks are that it creates a tripping hazard for pedestrians and uneven terrain for bicyclists moving into or out of the road. It can also wreak havoc with ADA compliance wherever there are curb cuts and ramps. These issues are 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. This keeps the height of the asphalt directly adjacent to the gutter the same but increases the crown of the road. The benefit is that this does not create hazards for pedestrians, bicyclists, or people with disabilities. The drawback is that multiple applications can cause the crown of the road to become too steep.

3. Paving into the gutter pan

The third option, and the least favorable, is paving into the gutter pan. The benefits are that this is the simplest and cheapest application and it does not create hazards for people. The drawback is that filling the gutter pan with asphalt causes a huge reduction in hydraulic capacity. Reduced hydraulic capacity means that wet weather events can cause localized flooding in the road and drainage issues on the surrounding properties.

Unless the situation is exceptional, pavement owners should avoid paving into the gutter pan.

Why choose pavement milling?

Milling and overlaying instead of just overlaying new asphalt pavement offers four benefits for pavement owners:

1. Milling extends the life of the road longer. As mentioned above, an asphalt overlay typically increases roadway life by 7–10 years, whereas mill-and-overlay increases roadway life by 17 years when untreated. Communities can go twice as long between mill-and-fill projects compared to asphalt overlay projects.
2. Milling allows structural issues to be corrected. In addition to grinding away any crack patterns in the surface course, milling also exposes any burgeoning structural issues such as water and soil intrusion and inadequate drainage. Addressing these issues early can prevent unnecessary degradation to the future riding surface.
3. For roadways with curb and gutter construction, milling doesn’t reduce drainage efficiency or cause ADA compliance issues. Asphalt overlays that extend into the gutter greatly reduce hydraulic capacity, increasing the chance that stormwater runoff will cause localized flooding. Asphalt overlays that stop at the gutter create a “lip” of lifted asphalt that can cause a tripping hazard for pedestrians and a jarring level change for bicyclists trying to move into or out of the roadway.
4. The cost of ownership is lower with milling. While the upfront cost of an asphalt overlay is cheaper than mill-and-overlay, when the cost is distributed over the life span gained, the reverse becomes true. An asphalt overlay that costs $14 per square yard that extends roadway life by 7 years costs $2.00 per square yard per year, while mill-and-overlay that costs $20 per square yard that extends roadway life by 20 years costs $1.17 per square yard per year.

What’s the difference between milling and not milling?

WithersRavenel’s Pavement Manager Rob Holland summarizes the difference between milling and not milling like this:

“The upfront investment in milling more than pays for itself down the road. 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.”

Holland knows the temptation can be great for communities to save money in the present by foregoing milling. He even acknowledges that there are times when an asphalt overlay is warranted, such as when a road exhibits high-severity surface defects with minimal cracking. But he’s quick to remind pavement owners that, at some point, the old pavement must be milled out.

A community can spend a lot of money on resurfacing in a small area, causing the total miles of road resurfaced per year to go down while the remainder of the roadway 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 says. “And that’s part of our philosophy, trying to help our clients save money and become more efficient over the long term.”

Can pavement milling be delayed?

WithersRavenel encourages clients to view their pavement system as an asset, one that can be tracked and managed to ensure maximum efficiency, sustainability, and cost savings over time. Because construction costs can go up quickly due to inflation, putting off milling because of the cost can make it significantly more expensive to do down the road.

With that said, there are times when it is prudent to postpone milling. For example, if your community is planning a sewer line replacement in two years, you can delay re-paving that section of road until the sewer work is finished. In that case, delaying re-paving avoids the expense and disruption of re-paving twice.

The bottom line

If pavement owners want pavements to last longer, perform better, and cost less over their lifetimes, milling is almost always a good choice.