Mill & Overlay Best Practices Every Local Agency Should Know

Table of Contents

• Before Milling: Find & Fix What's Underneath
• Setting Up the Milling Operation
• Planning Around Utilities & Manholes
• Expect the Unexpected After Milling
• Clean, Dry, & Tacked: Prepping the Milled Surface
• Don't Skip the Tack
• Paving the Overlay
• Joints Matter
• Talk It Through Before (& During) Paving
• Protecting Your Investment With QA

On paper, a mill and overlay can look pretty simple. Grind off the worn surface, put down a fresh layer of asphalt, and reopen the road. It's one of the most common projects a local agency will take on, and that familiarity is exactly why the details can be so easy to overlook.

The truth is, a new overlay is only as good as what's underneath it. Most of what decides whether it lasts or not happens before the paver ever shows up.

In our last blog, we walked through the surface prep checklist every local agency needs before paving. Now, we're taking a closer look at the mill and overlay process itself, from planning the milling operation to the last pass of the roller.

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Before Milling: Find & Fix What's Underneath

An overlay won't fix problems in the pavement structure below it. A weak base, failed patches, and poor drainage will find their way into the new surface sooner or later. That makes a mill and overlay for asphalt pavement one of the best chances an agency gets to deal with those issues.

Drainage deserves special attention. If water isn't being handled properly, it will eventually cause trouble. If the existing roadway already has drainage challenges, this may be the right time to address them.

Ideally, base repairs are wrapped up before milling starts, so the milling operation begins on a sound structure.

Setting Up the Milling Operation

There are a few different reasons an agency might mill. Sometimes the goal is to remove a specific layer of asphalt. Other times, it's to address issues in ride quality. Either way, the milling operation should be set up to hit that target consistently.

Start with depth. The milled depth should match the thickness of the overlay going back down, and it should be consistent across the project. An uneven milled surface means an uneven foundation for the new mat.

From there, think about how the finished surface needs to tie in with everything around it. Many local agency streets have curb and gutter, so the milling plan should account for:

• Curb and gutter elevations
• Center line and cross slope
• Driveway and intersection transitions
• Drainage, so you don't create a new problem while fixing an old one

Planning Around Utilities & Manholes

City streets, especially those closer to the urban core, often have utilities beneath the pavement and manholes right in the travel lanes. These are much easier to handle when they're planned for in advance.

When a full-size milling machine can't get close enough, smaller equipment can handle the handwork, such as a jackhammer or a skid steer with a milling attachment. In some cases, manhole collars can be lowered and covered so the milling machine can run straight through without stopping, then raised back to grade before paving.

What agencies don't want is to leave rough, unmilled patches around manholes and hope the paving crew can make up for it later. That rarely ends well.

Expect the Unexpected After Milling

Milling has a way of revealing things nobody knew were there. You might mill down into cracking in a lower layer that never made it to the old surface, or uncover soft spots that weren't showing up before.

The milling equipment itself can help here. It's heavy, and watching how the pavement responds as it passes over it works a lot like a proof roll—it points out soft areas that need a closer look.

When milling turns up new problems, now is the time to fix them. They won't magically get better once there's fresh asphalt on top.

Clean, Dry, & Tacked: Prepping the Milled Surface

If you've ever driven through a freshly milled street before it's been swept, you know it's basically a dust storm. Milling grinds up the existing asphalt, leaving a textured surface where dust and fines settle. Pave over that, and the overlay won't bond to the pavement below.

That's why the milled surface needs to be thoroughly cleaned before any tack goes down. TxDOT Item 354 outlines sweeper requirements, including a broom with water, a pickup apparatus, and a debris hopper.

Depending on the setting, agencies might use:

• A broom with water and pickup equipment to remove fines while keeping dust down
• A regenerative air vacuum sweeper, common in urban areas and often added to the plans as a general note
• Compressed air, where dust isn't as much of a concern

Keep in mind that water alone won't do the job. Once it dries, the fines are still sitting there.

Dust control matters, too—especially in urban areas. Milling dust can drift into oncoming traffic, neighborhoods, schools, and businesses. Nobody wants that in their front yard. The best time to set expectations for dust control is during scoping, before the project ever goes to bid.

Don't Skip the Tack

A clean surface doesn't mean you can skip tack coat. The overlay has to bond to the pavement structure beneath it, and tack coat is what makes that happen. It only works on a clean, dry surface, so apply it right after cleaning (not days or weeks later).

The rule of thumb is simple: when in doubt, tack. That includes vertical faces, adjacent lanes, curbs, and manholes. Tack coat is important enough that TXAPA's 341M specification recommends paying for it by volume as its own bid item.

Paving the Overlay

Once the milled surface is repaired, cleaned, and tacked, it's time to pave, and all the usual best practices for hot mix asphalt paving come into play.

By this point, it's a little late in the game to ask whether or not you have the right mix. Still, it's worth confirming. The overlay should use a demonstrated mix with an aggregate size suited to the overlay thickness, so the crew can get good compaction and density. For a refresher, check out our blog on types of asphalt mix and their applications.

If milling left behind uneven or scabbed-out areas, it's also a good idea to include level-up in the bid package. That way, the surface lift goes down on a nice, uniform layer.

Joints Matter

Good joint construction is a big part of any paving plan, especially when a project adds width or changes the roadway layout.

Talk It Through Before (& During) Paving

The pre-pave meeting is where everyone gets on the same page about traffic control, daily paving volumes, the inspection and testing plan, truck counts, time of day, and weather. It's also the time to be curious and ask questions, like:

• How will each day's paving start and end, and how will you avoid a bump at the transverse joint?
• How will the milled surface be cleaned before tack?
• What happens if bad weather rolls in or the plant breaks down?

The goal is for the plant, trucks, paver, and rollers to work in unison. Consistent material delivery and steady paving speeds go a long way toward keeping mix temperatures where they need to be.

Once paving is underway, daily debriefs keep that communication going. At the end of each day, or at least on a regular schedule, the agency and contractor should talk through what went well and what needs to change for the next day. Good communication solves a lot of problems before they start.

Protecting Your Investment With QA

Inspection and testing should be included in the specification, and the pre-pave meeting should confirm who's doing each test and how often. That testing follows the mix from production at the plant to delivery on-site to placement behind the paver.

If compaction is a concern, temperature is the first thing to check (and the second…and the third). Inspectors should also keep an eye out for segregation and anything else that could affect the quality of the mat.

Some agencies accept contractor test results for acceptance, which is an option under TXAPA's 341M spec. In that case, the contractor's testing personnel should be certified and credentialed. Even so, it pays to have someone representing the agency on the project, whether that's in-house staff or a qualified testing lab hired on an as-needed basis. Owner-side QA adds another set of eyes and more data points, and it works alongside the contractor's quality control, not in place of it.

We sometimes hear that an agency can't afford its own QA. The reality is that you can't afford not to have it. Think of it like an insurance premium: nobody enjoys paying it, but a modest cost during construction protects you from a much bigger bill if a pavement fails early or a dispute ends up in court.

A successful mill and overlay for asphalt pavement starts below the surface. Repair the base, address drainage, set up milling to hit its target, and plan for what it uncovers. Then, you deliver a clean, dry, well-tacked surface paved with the right mix and a crew that's communicating. At the end of the day, quality is a team effort, and the agency and contractor are on the same team.

Have a project you'd like to talk through? Reach out to our team, and we'd be happy to sit down and chat. For more asphalt pavement industry insights, subscribe to our YouTube channel and tune in to our podcast, Road Maps for Texas Local Agencies.

FAQs

What is a mill and overlay?

A mill and overlay removes the worn top layer of an asphalt pavement with a milling machine and replaces it with a new layer of asphalt. It restores ride quality and extends pavement life without a full reconstruction.

How deep should you mill for an overlay?

The milling depth should match the thickness of the planned overlay and stay consistent across the project to ensure the new asphalt has a uniform foundation.

Do you need tack coat on a milled surface?

Yes. Tack coat bonds the overlay to the pavement below. The milled surface needs to be clean and dry, and tack should go down right after cleaning.

How do you clean a milled surface before paving?

Common methods include a broom with water and pickup equipment, a regenerative air vacuum sweeper, or compressed air. TxDOT Item 354 outlines sweeper requirements.

What happens if milling uncovers new problems?

If milling uncovers new problems, you should fix them before paving. Agencies should build contingency into the project scope and budget for patching and base repairs that turn up after milling.