If you're a general contractor managing a paving project, milling is likely a subcontracted scope. Understanding what to specify, how to coordinate, and what to expect helps you manage the milling subcontractor effectively and avoid problems that affect your project schedule.
This guide covers the practical aspects of working with a milling subcontractor from the GC's perspective.
What to Include in Your Milling Specification
A clear milling specification includes: limits of work (plan showing exactly what areas to mill), milling depth (uniform or variable), surface texture requirements, disposal/recycling requirements for millings, and any special requirements around structures.
Specify whether depth is measured from the existing surface (uniform removal) or to a target elevation (grade correction). This distinction determines whether the milling contractor needs grade control references or can simply set a consistent depth.
Include requirements for transitions — how the milled edge meets unmilled pavement. A vertical edge (butt joint) is standard for areas that will be overlaid to the same limits. A tapered edge is needed where overlay limits differ from milling limits.
Coordination with Paving
Keep the interval between milling, cleaning, tack, and overlay as short as the approved sequence allows. If the surface is exposed to traffic or weather, it should be inspected and cleaned again before paving.
Communicate your paving schedule to the milling sub so they can plan production accordingly. If you're paving Monday, milling should happen Thursday or Friday of the previous week — not two weeks before.
Tack coat responsibility should be clearly assigned. Some milling contractors apply tack as part of their scope. Others leave it to the paving contractor. Clarify this in the subcontract to avoid gaps or double-application.
Coordination Gap
If tack coat responsibility isn't clearly assigned in the subcontract, you risk either no tack coat (delamination) or double-application (slip plane). Clarify before work begins.
Production Planning
Milling duration and daily production depend on depth, pavement hardness, machine size, sweeping, truck cycle time, recycling location, site access, and phasing. The project schedule should use a site-specific production plan.
Haul truck logistics often determine production rate. If trucks can't cycle fast enough, the milling machine sits idle waiting for empty trucks. Ensure your milling sub has adequate haul capacity for the project distance to the recycling facility.
Phasing, night work, restricted access, and repeated mobilization can reduce production and increase cost. The actual effect must be calculated from the site plan and proposed work sequence.
Quality Expectations
Acceptable milling quality includes: consistent depth within tolerance (typically +/- 1/4 inch for standard work, +/- 1/8 inch for precision), uniform surface texture without gouges or ridges, clean transitions at edges and around structures, and a swept surface free of loose material.
Verify depth at regular intervals during production — not just at the end. If depth is off, it's much easier to correct during production than after the area is complete. Spot-check with a ruler or depth gauge.
Document any areas where milling depth varies from specification. This information is needed for overlay quantity calculations and may affect your paving subcontractor's material order.
Milling duration and daily production depend on depth, pavement hardness, machine size, sweeping, truck cycle time, recycling location, site access, and phasing. The project schedule should use a site-specific production plan.
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Frequently Asked Questions
How do I verify milling depth?
Measure the milled surface against documented references such as curbs, adjacent pavement, survey control, or marked points. Sampling locations and frequency should follow the project quality-control plan and required tolerances.
Who is responsible for utility damage during milling?
Typically the party who failed to locate or protect the utility. If utilities were properly marked and the milling contractor hit them anyway, it's the milling contractor's responsibility. If utilities weren't marked, responsibility falls on whoever was supposed to arrange locates.
Can I use the millings on my project?
Possibly. Millings can be used as temporary surface material or base material in non-critical applications. However, using millings as structural base under new pavement requires engineering approval — RAP has different properties than virgin aggregate.