Three Degrees of Cold

Jan. 1, 2006

Thirty years ago, equipment manufacturers were working on designs to build a machine that would actually grind off a portion of the existing pavement so that a new surface could be constructed without removing the entire pavement structure. Little did they realize at the time the future impact their inventions would have on the highway construction industry.

Thirty years ago, equipment manufacturers were working on designs to build a machine that would actually grind off a portion of the existing pavement so that a new surface could be constructed without removing the entire pavement structure. Little did they realize at the time the future impact their inventions would have on the highway construction industry.

Today, cold-milling operations have changed the way the pavement designers approach projects. It is not necessary to remove and replace curbs and driveways so a pavement can be resurfaced. Full-depth asphalt pavements (perpetual pavements) are designed with a mill and overlay anticipated in the future without changing the pavement profile, loss of curb height or changes in the drainage characteristics.

Owners are able to specify and contractors are able to provide cold-milling services that save taxpayers millions of dollars over new construction and provide hot-mix asphalt producers with hundreds of thousands of tons of RAP for use in recycled asphalt mixes.

In order to be a full-service cold-milling contractor it is necessary to have a complement of types and sizes of milling machines. These machines are generally put into three different size categories: the small, utility-size mills; the mid-sized, half-lane machines; and the high-horsepower mills used for deep cutting and full-lane projects.

Size small

Today’s smaller, low-production mills are far more versatile than earlier versions of the machines.

Newer machines have a much shorter turning radius that allows them to trim around manholes and valve boxes in a single pass. They usually come equipped with automatic grade and slope controls which allow the machine to work with one operator.

Many machines also have the ability to change cutting drums, which allows for different cutting widths and depths. This versatility allows contractors to perform varied tasks such as pavement joint repairs and partial-depth patching with very little changeover time.

Some manufactures also are building mills that can be fitted with hard tires when extended traveling is required, such as a patching project, or tires that can be replaced with tracks when conditions require less ground pressure for greater stability.

Load-out conveyors are standard equipment on most utility mills today, making cleanup much easier and faster than machines that leave the milled material on the ground.

In addition to being able to change configurations, some of today’s machines have attachments designed to perform special tasks such as cutting rumble strips in both asphalt and concrete pavements.

Milled rumble strips are superior to the “rolled-in” type because they are more consistent, perform better and allow the newly constructed asphalt shoulders to be compacted to proper density without trying to roll in rumble strips after compaction.

Also, with the advancement of the milled rumble strips, some states are installing milled rumble strips on two-lane pavement shoulders as well as the traditional four-lane pavements to warn motorists when they are outside the driving lane. Some also are using rumble strips for centerline delineation on narrow pavements or to warn drivers of no-passing zones.

Recently, Dunn Co. installed centerline rumble strips on the very narrow Martin Luther King Jr. Bridge, which runs from East St. Louis, Ill., into downtown St. Louis, Mo.

Size medium

Probably the most significant changes in the milling industry are those that have taken place on the mid-sized, half-lane machines.

When manufacturers began building mills in the late 1970s most were in the 400- to 500-hp category with a fixed half-lane cutter system. The machines built today range from 400 to 900 hp and have the capability of changing cutting systems.

Half-lane mills are now set up to vary the cutting configuration by either changing the entire cutter housing or by removing the housing end plates and replacing the drum shell with a different width or texture drum.

Changing the cutter housing allows machines to be used for such things as coring for pavement widening or deep patching. Cutting widths can vary from 24 to 48 in. or custom drum widths can be built for special projects. These changes can be made in an owner’s shop in as little as a half day. In some cases, the width of the housing can be changed to allow expansion or reduction of the drum width to perform special tasks such as milling 8-ft or 10-ft shoulders using the same machine.

New machines, including four-track configurations, have a much tighter turning radius by using all-track steering. This allows even the larger, higher-production machines to cut short radii and cul-de-sacs often required in performing urban milling projects.

By installing a gradation beam ahead of the cutting drum, proper sizing of the milled material is much easier to maintain even when the entire surface is being removed. This sizing control allows hot-mix producers to use the RAP in recycled asphalt without additional processing. This further reduces the cost of producing recycled hot mix for the end user.

Manufacturers also are building high horsepower half-lane machines that can be outfitted with both half-lane and full-lane cutting systems for more versatility.

Cutting a full lane width in a single pass is very useful, especially when a single lift overlay has been specified. Not only is the full-lane cutter more productive, it also eliminates the problems associated with unmatched joints or different cross slopes in the center of a lane, which can cause material overruns and compaction problems when the pavement is overlayed.

Size large

Today’s high-performance machines are heavier and have greatly improved grade and slope controls, which increases productivity and provides a much smoother finished surface.

Full-lane machines are now available in widths from 10 ft to 14 ft wide and can be changed to meet the needs of various project requirements such as a 12-ft interstate inlay project or 131?2-ft, two-lane profiling job.

Full-lane mills also can be fitted with a “micro-milling” cutting drum which can profile existing asphalt or concrete pavements and leave a smooth finished surface for traffic without being resurfaced.

With the advancements in cold in-place recycling, some manufacturers are building machines that can be used for partial-depth recycling as well as normal cold milling. By adding a computerized additive control system and a special housing and cutter, a mill can now be a multipurpose machine.

Quick and concise

Other changes to milling machines over the years have had a significant impact on their operation and maintenance.

Most machines built today are driven directly from the onboard power plant through a gearbox and power band. The power band replaced the hydraulic drive or chain drive found on older machines. Power bands last much longer and are far easier to replace than chain drives. They also reduce damage to the machine’s drive system when they strike objects such as a manhole or an electric vault.

Machine drive systems also are designed to allow for the cutter speed to be increased or decreased by changing drive sheaves. This allows the operator to slow down the cutter where more torque is needed for deep cutting or to speed up the drum for shallow profiling work. In both cases the outcome is better machine performance and higher productivity.

Cutting tool manufacturers now produce cutting bits and bit holders designed to meet the needs of today’s higher horsepower machines. They offer specific tools based on the type of material being cut and the machine doing the work. Most machines today also have quick-change tool holder systems. Quick-change systems allow machine operators to change broken holders in the field without having to use a cutting torch or welder, which saves time and money. Several different types of systems are available including bolt-in-place or pressed-in holders.

Technology has greatly improved the electronics on new machines. When operated properly, these machines are capable of producing a finished product far superior to the capability of just a few years ago.

Manufacturers also are equipping machines with onboard computers that monitor machine functions, measure daily production rates and assist the operator or mechanic with troubleshooting malfunctions.

Contractors use milling machines to assist in other construction functions, too. They can be used to assist in earth and rock excavation, final trimming ahead of curb placement or paving operations. New and creative uses are being discovered regularly.

During construction of the new Red Tail Run Raymond Floyd Signature Golf Course in Decatur, Ill., Dunn Co. used its half-lane mill to cut out cart paths after the fairway had been built and seeded. This allowed the course architect to select the exact alignment for the paths with very little damage to the new course. More than four miles of paths were excavated and constructed in one week.

Biting with bytes

Significant improvements have been designed into the milling machines offered in today’s market, and as technology improves we can look forward to even greater changes in the future.

In the near future mills will be equipped with laser and GPS grade controls, allowing the milling contractor to download elevations and cross slopes from digitized construction plans. This will provide customers with a more precise end product than ever before.

A little further in the future, look for machines to be outfitted with belt scales on the load-out conveyor. By measuring the machine’s output the operator will be able to regulate truck loading and have an accurate measurement of daily production. This also will allow the contractor to manage RAP stockpile inventories.

As raw material costs increase, the use of cold milling and asphalt recycling will continue to grow and milling contractors will find new and better ways to serve the industry.

About The Author: Schwarz is vice president of Dunn Co., Decatur, Ill.

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