Stretching a road maintenance budget with fine milling
- Less damaged road material needing to be removed
- Less repair material needed
- Milling crews can operate independently from paving crews
- With less material removed and less repair material required, the number of haul trucks is significantly reduced
- Project and contractor costs are more efficiently controlled, expanding the scope and number of repair projects where necessary
How It Works
As previously mentioned, fine milling involves swapping out a milling machine’s standard road-milling drum (bits spaced roughly 16 mm apart) with a fine-milling drum exhibiting more closely spaced bits, 0.3 inches (8 mm) apart or less. Drums also include conical tools, sleeves, and blocks in a range of available styles, including weld-on blocks and proprietary quick-change systems requiring no welding and, in certain applications, no bolts.
Customized drum design can be the key to giving users the productivity they expect. With a wealth of proprietary drum-lacing programs, Kennametal engineers can tailor drum designs to specific customer parameters, with such user-selectable design elements as cutting pattern, diameter, width, advance rate, and rpm. The program then determine the total number of tools required, total cutting width, wrap angle per side, radial spacing, vane angle, throat width, and estimated kicker clearance. Drum configurations for a number of pavement conditions and road machine OEM specifications are also available.
Such operating specs can greatly affect fine milling projects. For example, a faster machine advance speed generally yields a rougher texture while a faster drum rotation (RPM) yields a smoother texture.
Saving Millions in Georgia
The Georgia Department of Transportation (GDOT) recently wanted to repair a distressed open-graded friction course (OGFC) on 15.6 miles of I-75 south of Macon. This project had a dense graded surface mix layer with an overlying OGFC that had been in service for more than 10 years. Although the OGFC had begun to deteriorate, the underlying mix was still in good condition.
Milling only the OGFC and replacing it with GDOT’s porous European mix (PEM) was determined to be the most cost-effective approach. However, this rehabilitation practice had rarely been done in Georgia. The potential existed for poor bonding between the PEM and the conventional milled surface, permitting surface water to flow through the porous layer and become trapped in the valleys of the conventional milled surface, (usually with a ridge-to-valley depth (RVD) of 8 mm or greater). Because of these concerns, Georgia DOT typically placed a costly new layer of dense-graded surface mix over the conventional milled surface prior to the PEM.
Fortunately, GDOT had run tests with the National Center for Asphalt Technology to determine the underlying OGFC was in good shape and became aware of fine milling technology. For the GDOT project, an even finer-spaced micro-milling drum at 0.2 inches (5.08 mm) was used.
GDOT’s texture specification for micro milling on the project included controlling the depth of the micro milled surface to an accuracy of 1.6 mm and maintaining RVD of 3.2 mm or less. Target smoothness was 825 mm/km, not to exceed 900 mm/km.
The fine-textured surface achieved by micromilling along with less than half the previous RVD allowed GDOT to place a new PEM layer directly on the milled surface of the existing dense-graded surface mix, eliminating the need for an additional surface mix layer. GDOT estimated savings of $58,000 per lane mile or a total savings of approximately $5.4 million for the project.
NCAT assisted during the project by evaluating surface texture depths in the micromilled surface using its Ultra Light Inertial Profiler (ULIP) and measuring smoothness with its Laser Road Profiler (LRP). Results indicated that the RVD requirements were achievable with the proper application of a fine-milling drum.
GDOT has been pleased with the cost-effective strategy of using micromilling to remove distressed OGFC and replace it with PEM. The agency has also added software to the LRP to allow determination of RVD values while measuring smoothness.
With micromilling, the end result is a super-fine surface. This surface is for milled surfaces that are left open to traffic. This will provide the best readability. Some states and providences have readability specifications in the ranges that require this type of milling. This milling is normally done with shallow cuts, such as 25 mm to 50 mm deep. Speed is varied to obtain the proper ride spec.