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What is Full-Depth Reclamation?
Between mill-and-fill and total reconstruction, there’s
an economical,
long-life alternative that’s gaining popularity in North America
Everyone from highway engineers to
frustrated motorists has fantasized about a machine that would move
steadily down a road, gobbling up bad pavement in front and leaving a
trail of perfect pavement in the rear. No more construction backups. No
more breathlessly expensive rebuilds. No more pockmarked, rutted roads
waiting to break axles and bend wheels.
Although pavement recycling technology has not yet evolved to fulfill
that fantasy, the industry is getting closer.
Cold planing, cold-in-place recycling, and hot-in-place recycling are
accepted, widely used techniques for rehabilitating flexible pavements
with surface course imperfections. Each delivers a piece of the
road-renewal dream, leaving in the wake of its recycling train a smooth,
flat surface that is accomplished relatively quickly and inexpensively.
In recent years, another recycling technology has gained popularity in
North America — full-depth reclamation. It comes even closer to the
road-renewal dream because it gives pavement managers a fast, inexpensive,
long-wearing alternative to rebuilding roads that require major repairs or
total reconstruction.
While the other recycling technologies grind off a portion of the
surface course of asphalt and replace it, full-depth reclamation
penetrates the entire flexible pavement section and a predetermined
portion of the base material, uniformly pulverizing and blending them
together to produce a stabilized base course. Thus, FDR can correct
deficiencies in the base as well as the bound asphalt layers.
Full-depth reclamation technology can be utilized to depths of 12
inches or more; the most typical applications involve depths of 6 to 9
inches. As it pulverizes and mixes, the road reclaimer can also meter in
precise amounts of additives to further enhance the structural
characteristics of the stabilized base course. Its benefits start with the
fact that FDR completely erases deep pavement cracks, eliminating the
potential for reflective cracking. It also allows for cross-slope and
profile grade adjustments, and road widening is easily accomplished.
The Evolution
 At the
heart of full-depth reclamation is a small fleet of road reclaimers,
machines that use milling drums similar to those found on milling
machines, but which are designed to cut and mix at much greater depths.
Road reclaimers evolved from machines designed to handle mass production
soil stabilization work. Indeed, the only difference between many of today’s
reclaimers and soil stabilizers is the milling drum.
These machines have been in use in Europe and in North America for many
years, but their suitability for FDR, North American style, has evolved
with the advent of high-horsepower diesel engines. Powered by engines as
big as 800 horsepower, today’s reclaimers cut harder and deeper, mix
faster, and cover more ground than ever before. And with these
improvements, popular FDR applications have broadened from low volume
country roads to include city streets and medium volume roadways.
The classic application for FDR is a secondary or tertiary road with a
2- to 4-inch asphalt overlay on a compacted base. When the overlay is too
deeply cracked or rutted for a mill-and-fill remedy, full-depth
reclamation is the next cheapest alternative — and it can produce a much
longer-lasting solution.
The full-depth reclamation process is fast and straightforward. A
reclaimer pulverizes and mixes the asphalt and base material, creating a
strong new base. The reclaimer is typically followed by a grader, a water
truck, and various compactors. Minutes after the last compactor completes
its pass, the road can usually be opened to traffic until the contractor
is ready to apply the final surface treatment.
For some low-traffic roads, the surface treatment can be as simple as a
double chip seal. For higher traffic roadways, the FDR operation is
typically followed by an asphalt overlay, creating a new road that should
have much better wear and load-bearing properties than the old road. More
to the point, say FDR advocates, the new road is the equivalent of a
traditionally rebuilt road in terms of life expectancy, wear, and
load-bearing characteristics, but it costs a fraction as much and can be
completed with far less interruption of traffic.
Full-depth reclamation can be used to rehabilitate and improve gravel
roads, and it has also been used on major highways, including interstates.
There have even been cases where a road was first milled, to reduce the
bound asphalt depth to an appropriate thickness, so that FDR techniques
could be applied. This milling is also sometimes done to allow for proper
curb reveal on curb and gutter streets or to control grade prior to the
subsequent asphalt overlay.
The full potential of full-depth reclamation is still being defined,
but it has emerged as an important and valuable option for road managers
to consider as they search for budget-stretching solutions to the
thousands of miles of roads in Canada and the U.S. that can no longer be
cost-effectively repaired.
Reprinted from Better Roads Magazine
July 2001 |
