Deep Rehab for Distressed Pavements
CIR can turn aged, deeply cracked pavement into a
rut-resistant base at a cost-efficient price.
Cold-in-place recycling can be a cost-effective solution for highway
agencies dealing with older roads that are still structurally sound and
have adequate drainage, but have pavement distresses too deep to be
restored by surface repair techniques like hot-in-place recycling or
simple overlays.
Commonly referred to as CIR, this process recycles pavements in place
without the use of heat at depths of 2 to 6 inches. By eliminating the
time and expense of hauling recycled asphalt pavement, and by minimizing
the amount of hot-mix asphalt needed for a wearing course, cold-in-place
recycling is often a faster, less expensive alternative to the traditional
mill-and-fill technology usually used to repair such roads.
 The classic application for CIR is rehabilitating cracked pavements
where the cracks run 2-inches deep or more. Surface treatments, like
overlays and even 2-inch mill-and-fill solutions, will be vulnerable to
reflection cracking in such applications, while CIR’s deep-treatment
capability can mitigate reflective cracking and produce a long-life road.
In the CIR process, the old pavement is milled, then crushed and
screened to size, and ultimately mixed with a liquid recycling agent as
well as new aggregate and other additives, if needed. A paver following
the CIR train lays the cold mix, and it is compacted as soon as the
emulsion recycling agent breaks and sets up, releasing the moisture from
the mix — usually 15 to 90 minutes after paving.
 Recycled cold-mix asphalt is more viscous than conventional hot-mix
asphalt and, because it has larger aggregate sizing, it has a higher Voids
in Total Mix ratio and can’t be compacted as densely as hot-mix. Its
physical characteristics are likened to open-graded pavements: while CIR
mixes are too porous to stand up to moisture and frost without a sealing
coat, they have excellent stone-on-stone contact which produces a rut- and
fatigue-resistant pavement, and they are more flexible than hot-mix
pavements which helps them stand up to freeze-thaw cycles.
The process is completed with the laying of a wearing surface over the
CIR mix — usually at least 1.5 inches of hot-mix asphalt, though
low-traffic-volume roads sometimes are treated with chip seals.
Under ideal conditions, the wear course is laid after the CIR mix has
cured — a process that generally takes from 10 to 14 days, depending on
the weather and the environment. Lime can be added to the CIR mix to
hasten the curing process. Some newer recycling asphalt emulsion
technologies are now being used to speed the curing process and to protect
against raveling; early tests have produced mixes that were sufficiently
cured for the application of the wearing course in three to seven days.
Ideally, the CIR pavement is closed to vehicle traffic during the
curing period, but in the real world, this is seldom possible. In those
cases, the Asphalt Recycling and Reclaiming Association says, the cold-mix
pavement can be given a light fog seal application to protect against
raveling. CIR pavements are also frequently re-rolled with a steel-wheel
roller during the curing process to remove minor consolidations that can
occur in the wheel paths as a result of traffic.
Cold-in-place recycling has gained acceptance with an increasing number
of highway departments because it provides a long-lasting solution to a
number of common pavement problems at a lower cost than conventional
rehabilitation methods. Since the CIR train occupies just 1.25 to 1.5
lanes of traffic at a time, the process minimizes inconvenience to
motorists. And because CIR is a relatively fast process — production
rates vary widely, but 2 lane miles per day is not unusual — the process
causes minimal interference with access to homes and businesses.
Applying CIR
While CIR is often used to treat cracked pavements, the Basic Asphalt
Recycling Manual lists more than a dozen types of pavement distress that
can be treated by cold-in-place processes, ranging from raveling and
potholes to rutting, corrugations, and shoving.
In addition to structural soundness and good drainage, pavements that
are good candidates for CIR must be at least 2-inches thick — 4 to 5
inches of thickness is preferred. Cold-in-place recycling usually treats
at least 70% of the existing pavement in order to mitigate reflection
cracking, and in some cases, full-depth CIR is used to treat the entire
pavement.
A detailed assessment of the pavement is vital to a successful CIR
project. The age and type of asphalt binder used in the existing asphalt
will affect what recycling agents and modifiers are selected. Similarly,
the characteristics of the aggregate, including size and angularity, and
the type and amount of patching materials are important considerations in
planning the project. A high incidence of patching materials affects the
homogeneity of the recycled mixture and may require special mix
considerations.
As a recycling technology, CIR pavement quality depends in large
measure on the quality of the original pavement, though modifications are
possible. For example, the province of Ontario has used CIR to treat
rutted pavements caused by excess asphalt cement content and low voids in
the original HMA. In these applications, the process was modified to add
new coarse aggregates to achieve optimum voids and stability control in
the binder course.
Other types of modifications can be used to achieve various mix
characteristics, and some modifications are required for deeper
treatments. Mixes treated with liquid asphalt emulsions or emulsified
recycling agents are generally limited to working depths of 2 to 5 inches.
Treatment depths can increase to 5 to 6 inches when additional recycling
additives like lime or Portland cement are added, or a chemical recycling
additive like type C fly ash.
An important part of the assessment phase is determining whether or not
the structural capacity of the pavement needs to be improved and, if so,
by how much. CIR mixes have a structural layer coefficient ranging from
0.25 to 0.44, depending on what stabilizing additives are used. When even
more substantial strengthening is needed to deal with heavier traffic
loads, the remainder of the capacity is achieved by increasing the
thickness of the hot-mix asphalt overlay.
While CIR is not a remedy for pavements with structural or base
failures, it can be cost effectively deployed on roads where these
deficiencies are less than 10% of the project area. In these cases, the
failed areas are removed and repaired by deep patching techniques — and
the causes for the pavement failure are remedied — before the
cold-in-place recycling process is launched.
Limitations
The main limitations to CIR are that it requires a dense, rigid sealing
course, and that it is not a solution for distresses related to structural
or drainage problems.
There are logical economic limitations, too. Shallow or superficial
pavement problems can be more inexpensively treated with other processes.
And there are potential limitations for partial-depth recycling in that
enough original pavement has to be left in place to support the high wheel
loads of the CIR equipment that follows the milling machine. As a rule of
thumb, ARRA says there is little chance of breakthrough if you have 2
inches of remaining pavement and at least 6 inches of granular base.
The process
Cold-in-place recycling trains range in size and sophistication. The
traditional single-unit train is based on a machine that mills the
pavement to the specified depth and cross slope, sizes the recycled
asphalt, and blends the recycling additive. One of the unique features of
the single-unit recycler is the use of a down-cutting milling head —
most mills cut on the up cycle. The down-cutting milling head lets the
operator control particle size by adjusting the forward speed of the
machine.
More sophisticated single-unit trains use a self-contained CIR machine
that mills, screens the recycled asphalt, and crushes oversized particles,
then blends the recycling additive and the mix in a pugmill.
A two-unit train links the milling machine with a pugmill mix-paver
which blends the recycling additive and places the mix; this process adds
precision to the injection and mixing of the recycling additive.
Multi-unit trains are designed for high production and optimum process
control. They have separate machines for milling, screening and crushing,
and mixing, providing more control over the size of the recycled asphalt
than the other methods.
Single-unit and two-unit trains are often deployed in municipal areas
because their compact dimensions are less intrusive than the multi-unit
trains. Single-unit trains measure about 70 feet in length, while
multi-unit trains can stretch 150-feet long or more.
After the mix is placed by a paver, it is compacted with heavy
pneumatic and double-drum vibratory steel rollers. Because the CIR mix is
more viscous than conventional hot-mix asphalt, heavier rollers are
required. According to the Basic Asphalt Recycling Manual, some
contractors start breakdown rolling with two passes of a static steel-drum
roller because it can improve final smoothness, prevent edge distortion,
and prevent excessive distortion of the mat by the pneumatic roller.
Breakdown rolling continues with pneumatic rollers of 25 tons or more
and continues until the machines walk out of the mix. Intermediate rolling
follows, using 12-ton or larger double-drum vibratory steel wheel rollers.
Finish rolling is done with the vibratory steel-wheel rollers in static
mode to remove roller marks.
The cold mix is ready for a wearing surface after it has cured, though
experts say it is better to overlay a partially cured mix prior to winter
shutdown and let the curing take place in the next paving season rather
than leaving the mix exposed to the elements all winter.
The expected service life of CIR pavements varies according to the
materials used, job specifications, and many other factors. The Basic
Asphalt Recycling Manual says that a CIR pavement with a surface treatment
such as a chip seal will typically have a service life of six to eight
years. A CIR pavement with a hot-mix asphalt overlay will have a service
life equal to the life expectancy of the overlay, which can be anywhere
from seven to 15 years.
What experts say
There is an abundant and growing body of evidence to support the
quality and cost efficiency of cold-in-place recycling as a road
rehabilitation technique.
One of the public agencies most experienced with the technology is
Ontario’s County of Wellington which used CIR for the first time in 1990
with a 1.5 kilometer project (0.9 miles) and now has nearly 150 miles of
CIR-treated roadways.
In a 2000 analysis of the county’s experience, construction manager
Rick Dale, C.E.T., summarized the performance of the CIR roads as
excellent. Dale’s review indicated that Wellington had experienced
performance problems on just two sections — a total of 7 miles — of
the 131 miles of CIR-treated roads the county had at the time, and in one
of those sections the problems appeared to be caused by an inadequate
surface pavement.
Similarly, New York’s Fulton County commissioned its first
cold-in-place recycling project in 1990 and has since made the technology
a primary option in its pavement rehabilitation program. Highway
superintendent Bronson Moore says that nearly half of the county’s 140
miles of highways have been recycled with CIR since 1990. Moore and Fulton
County are credited by the Asphalt Recycling and Reclamation Association
with helping to pioneer CIR innovations in such areas as road shoulder
recycling and the use of custom emulsions, polymers, and rejuvenators.
Many other cities, counties, states, and provinces have successfully
used cold-in-place recycling for everything from country roads to
interstate highways, and the list is growing as the availability of CIR
contractors gradually increases both through the creation of new
businesses and the expansion of existing firms.
Reprinted from Better Roads Magazine
July 2002 |
