| Road Science
When Prevention Is the Cure
A systematic pavement preservation
program delivers more road-quality bang for the taxpayers’ buck.
by Tom
Kuennen, Contributing Editor
 A current pavement inventory, identification of conditions, and correct
timing of pavement preservation applications are the secrets to successful
preservation of hot-mix asphalt driving surfaces, according to a growing
body of experts and research.
Naturally, government agencies want to keep citizens happy by providing
as large an inventory of smooth streets as possible. But state, city, and
county road agencies alike have to remember that maintenance techniques
applied to pavements that are completely deteriorated beyond a certain
point are a waste of money.
A “worst-first” pavement maintenance philosophy tosses scarce
public funding at pavements that should be allowed to fail first, then be
reconstructed in an orderly, programmatic manner.
Asphalt pavements will perform well and deteriorate very slowly through
the first eight to 10 years of their lives, then fail rapidly as an
assortment of ills does them in. (see chart page 46).
The best way, experts say, to spend scarce maintenance funds is to
determine where the pavement is in its life-cycle “curve,” and apply
preventive maintenance techniques just before the period of rapid
deterioration sets in.
Unfortunately, the public agencies responsible for pavements ultimately
must answer to elected officials. It takes guts for a public works
supervisor to insist to a mayor or aldermen that the municipality will get
the best use of funds by allowing a street to fall apart before
rebuilding, despite what the voters say.
Support for preservation
 Experts agree pavement preservation is best executed in the framework
of a pavement management system that will enable a road agency to identify
pavement condition throughout its road inventory. Not only will such a
system help allocate where funds are best spent, but it will also provide
a database to prove the long-term benefits of pavement preservation and
justify additional preservation spending.
“The key is to have a good pavement management system that will allow
you to track the condition of your pavements over time,” says Mark
Buncher, Ph.D., P.E., director of field engineering for The Asphalt
Institute. “The best preventive maintenance practice will be indicated
by the pavement history in a pavement management system.”
Today, programmed pavement preservation is gaining major support from
some heavy hitters in the road and bridge community.
- The Federal Highway Administration delivers pavement preservation
guidance and moral support through its Office of Asset Management.
- The Foundation for Pavement Preservation, a public/private venture,
is funding and managing research into pavement preservation materials,
techniques, training, and best practices — and publicizing best
practices via outreach to governments and contractors. (visit at http://fp2.org).
- The American Association of State Highway & Transportation
Officials recently launched a web site devoted to agency asset
management and pavement preservation (visit at http://assetmanagement.
transportation.org/ ).
- The pending GASB 34 accounting regulations now being adopted from
coast to coast are compelling municipal and county governments to
reconsider their philosophy toward the transportation infrastructure
they possess, counting it as an asset that must be nurtured and
maintained, rather than just a network of streets and highways that
wears out and must be fixed.
A new way of thinking
 The theme of asset management supporting pavement preservation is
driving the new emphasis on preventive maintenance for the local and state
transportation infrastructures of the 21st century.
“Pavement preservation is at the core of all future highway programs,”
says Bill Ballou, president of the Foundation for Pavement Preservation.
“Without asset and system management, we can’t maintain highway
systems cost-effectively. We want pavement preservation to be a routine
undertaking for road agencies.”
“Pavement preservation is more than just a collective set of specific
pavement maintenance techniques,” said FHWA’s Office of Asset
Management Construction and System Preservation Team leader Jim Sorenson
at last year’s annual meeting of the International Slurry Surfacing
Association. “It is a way of thinking and the guiding force behind an
agency’s financial planning and proper asset management.”
And that leads to challenges in asphalt pavement preservation and
maintenance, Sorenson said at an FP2 conference in 2000. “The vast
majority (about 94%) of U.S. pavements are made of asphalt,” he said.
“The balance of the pavements in the high-service corridors are of
portland cement concrete. We make choices, and we need the competition
between the industries. But the mainstay of our pavements is black, and
that’s where we need to place much of our preservation focus.”
Sorenson adheres to the classic definition of pavement preservation —
application of the right treatment, to the right road, at the right time.
“We have to get the best bang for the buck out of those asphalt
pavements,” Sorenson said. “That means we have to do at least
something at the right time.” And to be able to respond at the right
time requires planning and budgeting, he said.
Asset management
Since 2000, the FHWA has promoted transportation infrastructure asset
management as a best practice for state and local road and bridge owners.
Asset management is a philosophy borrowed from the private sector, now
being applied to government agency-owned transportation infrastructure.
“Asset management is a business process that’s being introduced
into the highway community to allow agency top management — our policy
decision makers — to be cognizant of the investments we’re making in
our highway system,” Sorenson says.
“Highways are a big business, and every dollar that we invest in the
highway system must get a high rate of return. And in the United States,
in the postwar era, we can show those returns in the way our economy has
stabilized and developed.”
For more information and tools to use, visit the FHWA’s Office of
Asset Management at www.fhwa.dot.gov/infrastructure/asstmgmt/index.htm.
Foundation for preservation
Research and educational support of long-lived pavements through
preventive maintenance is being undertaken by the public/private-funded
Foundation for Pavement Preservation.
Commonly known as FP2 (“FP squared”), the foundation is the nation’s
leading proponent of pavement preservation. Like FHWA’s Sorenson, the
foundation believes that proper pavement preservation means application of
the right treatment, to the right road, at the right time, and
communicates this principle to the top management levels of government
agencies, as well as to field personnel.
FP2 funds and conducts research, coordinates development of educational
courses and programs, sponsors symposia and workshops, issues external
publications, and identifies research problem statements for future work.
Established in 1992, FP2 is facilitating change in the transportation
infrastructure community, Sorenson says, by providing resources to advance
knowledge for improved asset management for maintaining and preserving
highway pavements.
“We are closely monitoring how reauthorization will support pavement
preservation,” says FP2’s Ballou. “While we are not a lobbying
group, we are acutely aware of the need to include pavement preservation
research funding, as well as system preservation funding itself.”
In 2001, FP2 began high-level talks with the FHWA and the American
Association of State Highway & Transportation Officials to readdress
pavement preservation research and policy needs, and to establish a focus
for a national program. And last February, FP2 brought FHWA, AASHTO, and
private sector stakeholders together into a “Strategic Partnership in
Pavement Preservation Research” initiative to identify common ground as
TEA-3 approaches. “We discussed reauthorization and how we could
participate and what our best options were,” Ballou says.
Sealer/binder research
Ongoing research on surface sealants and rejuvenators being coordinated
by FP2 likely will pay big dividends in the near and long term.
Sealers have been used in approximately two-thirds of the states, the
foundation reports. Rejuvenators are formulated to penetrate into the
pavement and enhance the properties of the asphalt binder of the existing
pavement.
To explore which treatments will work best under which climatic
conditions, the FHWA contracted with FP2 in February 2001 to evaluate the
effectiveness of spray-applied emulsified sealer/binders and rejuvenators.
This was a unique approach as it brought together a partnership between
industry and the FHWA. Although the FHWA provides the significant share of
research funding, the pavement preservation industry also contributes to
this effort through the foundation. FP2 conducts the research through its
partners and contractors.
As part of this research, a workshop and field application of test
sections was conducted in early autumn 2002 in southern Minnesota.
Preservation through maintenance
Preventive maintenance is a means to achieving pavement preservation.
“Preventive maintenance applies lower-cost treatments to retard a
highway’s deterioration, maintain or improve the functional condition,
and extend the pavement’s service life,” says retired Michigan DOT
pavement preservation engineer Larry Galehouse, P.E., L.S.
“With various short-term treatments, preventive maintenance can
extend pavement life an average of 5 to 10 years,” Galehouse says. “Applied
to the right road, at the right time — when the pavements are mostly in
good condition — preventive maintenance can improve the network
condition significantly at a lower unit cost.”
Beginning in 1992, Galehouse’s work at the Michigan DOT provided hard
evidence that preventive maintenance is a wise investment. According to a
study undertaken by the DOT and confirmed by an independent consultant, in
the long run Michigan’s preventive maintenance strategy is more than six
times as cost-effective as rehabilitation and reconstruction projects in
providing acceptable driving surfaces.
In Michigan, surface treatments for flexible pavement surfaces include
microsurfacing, chip seals, slurry seals, crack sealing, 0.75-inch
overlays of ultrathin hot-mix asphalt, and 1.5-inch hot-mix asphalt
overlays, Galehouse says.
Ills of asphalt pavements
Such procedures mitigate the degenerative problems that hot-mix asphalt
pavements endure. Some asphalt pavement ills — like raveling —
indicate the need for immediate pavement preventive maintenance. Others,
like alligator cracking, indicate a pavement that has deteriorated too
far, or a subbase that requires repair lest good money be thrown after
bad.
- Raveling is the separation of the surface aggregate from the asphalt
binder. As aggregate moves out of the mix, the surface becomes rougher
in texture. Aggregate that has “raveled” out of the matrix can be
found in gutters and close to drain inlets. The cause usually is
oxidation by sun and weather, as the asphalt binder is broken down by
the elements. This can begin early in a pavement’s life. Surface
treatments are indicated.
- Bleeding, also known as flushing, is the exuding of binder onto
pavement surface; it’s caused by excessive amounts of liquid asphalt
binder in mix or low air void content (densification) caused by
excessive traffic, and appears during hot weather. A seal coat can
help, or overlay of an open-graded friction course.
- Rutting is longitudinal deformation of pavement; it’s caused by
high levels of repeated stress being applied to the subgrade below the
asphalt course, or by an asphalt mix without enough shear strength to
resist repeated heavy loads. Some say rutting can’t be fixed by
preventive maintenance treatments other than cold mill and overlay, or
hot in-place surface recycling; others say an asphalt thin surfacing
or microsurfacing will satisfactorily fill ruts while leaving a new
driving surface.
- Corrugation or “washboarding” is a transverse deformation of
pavement, and can be caused by poor compaction technique or a shifting
of the subgrade. It can’t be fixed by preventive maintenance
methods.
- Fatigue, or alligator cracking, is interconnected cracking with a
pattern resembling the hide of an alligator. Its primary cause is
attributed to base failure, or heavy traffic loading combined with
partial base failure, so it can’t be cured with a preventive
maintenance treatment. However, mild cases of alligatoring with
minimal base involvement can be treated with seal surfacing.
New look at fatigue cracking
This spring a contract was pending for new research into fatigue
cracking, as proposed by the National Cooperative Highway Research
Program. Project 1-42, Top-Down Fatigue Cracking of Hot-Mix Asphalt
Layers, would be a two-year, $400,000 project that will take another look
at why fatigue cracking happens.
Until recently, load-associated fatigue cracking of hot-mix asphalt
pavements that occur in the wheel path have been thought to always
initiate at the bottom of the HMA layer and propagate to the surface,
NCHRP says.
But new studies indicate that load-related fatigue cracks can also be
initiated at the surface of the pavement and propagate downward through
the HMA layer, NCHRP says. These studies say environmental conditions,
tire-to-pavement interaction, mixture characteristics, pavement structure,
and construction practices are among the factors that influence the
occurrence of this cracking.
Research proposed by NCHRP would identify mechanisms that govern the
initiation and propagation of top-down fatigue cracking, identify or
develop methods of lab-testing HMA mixes for susceptibility to surface
cracking, determine factors associated with top-down fatigue cracking, and
identify predictive models.
Preservation techniques
A variety of techniques and materials can be used to preserve a
pavement — if it is at the right stage in its life. These include chip
seals, fog seals, cape seals, slurry seals, microsurfacing, and thin-lift
hot-mix asphalt overlays. But before spending money on an application, the
engineer must determine if the treatment will actually prolong pavement
life.
“There are lots of different tools in the tool box to choose from,”
the Asphalt Institute’s Buncher says. “The pavement owner will have to
choose the best solution for the funds that are available.”
For example, most fatigue or alligator cracking indicates a failed
subgrade following years of weathering and traffic loads. The existing
fatigued asphalt and base must be milled or dug out, then rebuilt prior to
placement of a treatment or new lift of asphalt.
Rutted asphalt may be treated by a thin-lift HMA overlay, or by
microsurfacing. But a chip seal or slurry seal will do little to improve
the riding surface, much less prolong pavement life.
Simple raveling, on the other hand, may be treated by a number of
surfacing solutions, including the family of surface seals.
“The application of slurry seal will significantly extend the life of
existing pavements by protecting the under-surface from damage caused by
water seepage,” says the International Slurry Surfacing Association. “Improved
surface performance is an added bonus. A pavement maintenance program
using slurry seal will not only help to protect your pavement, it will
help to protect your paving investment.”
Simple pavement seals
There are four approaches to pavement sealing in common use today.
The classic chip seal is an inexpensive solution for an oxidized or
raveling asphalt pavement. An asphalt chip seal, also referred to as a
seal coat or a bituminous surface treatment, consists of sequential
applications of asphalt and stone chips, applied either singly or in
layers, to build up a structure that can approach 1-inch thick, according
to the National Highway Institute’s course of instruction, Techniques
for Pavement Rehabilitation.
Rubberized asphalt chip seals are a special type of chip seal in which
rubber (ground-rubber tires) is blended with the asphalt cement. “This
application has been used both as a SAM (stress-absorbing membrane) and a
SAMI (stress-absorbing membrane interlayer) to help reduce reflection
cracking, but it has also been used without overlays,” NHI says.
Fog seals are very light applications of an emulsion to the pavement
surface with no aggregate, according to NHI. “These applications seal
the surface and provide a small amount of rejuvenation, depending on the
type of emulsion used and the condition of the existing pavement surface,”
NHI says.
Cape seals are a combination of chip seal and slurry surfacing or seal.
“For paved roads, the chip seal is applied first and, between four and
10 days later, the slurry seal is applied,” NHI says. For unsurfaced
roads, an application of penetration oil (MC-70 or SC-70) is applied first
as a prime coat, followed about two days later by a chip seal and about
two weeks later by a slurry seal.
Slurry surfacings
A slurry surfacing, also known as a slurry seal, is a mixture of
aggregates dispersed in asphalt and applied in a slurry state.
“Slurry seal is a mixture of an asphaltic oil and water (emulsion)
and crushed rock aggregate that is spread over the street at about
one-fourth-inch thickness,” says the City of Austin (Texas) Street and
Bridge Division. “The slurry ‘cures’ when the water evaporates,
leaving only the asphalt to coat the crushed rock.”
Afterward, the asphalt acts as a binder to hold the slurry together and
bond to the existing pavement. The slurry seal protects the existing
street surface from the effects of aging and oxidizing and increases the
skid resistance.
“Slurry seals typically include some crack sealing on cracks
0.1875-inch and larger,” Austin says. “Prep work may also include a
minor amount of level-up and surface replacement areas depending upon
conditions; however, slurry seal is typically used on streets that are in
good to excellent condition.
“Slurry surfacings are designed in a lab, are proportioned by the
slurry machine, and laid down and cured so the asphalt-to-aggregate ratio
is maintained at the optimum value to assure uniform aggregate coating and
adhesion,” says Jeff Reed, president, Valley Slurry Seal Co.,
Sacramento.
Such surfaces use very large fractions of fines material, giving a very
high surface area and a lot of microstructure, leading to a sandpaper
surface and a high skid resistance, while maintaining a smooth finish,
Reed says.
Microsurfacing evolved from slurry
Microsurfacing is a more advanced extension of the slurry-surfacing
concept. Microsurfacing is described as a polymer-modified, cold-mix
paving system that can remedy a broad range of problems on today’s
streets, highways, and airfields.
Like slurry seal, microsurfacing begins as a mixture of dense-graded
aggregate, asphalt emulsion, water, and mineral fillers, says the
International Slurry Seal Association, but microsurfacing has added
capabilities, thanks to the use of advanced polymers and other modern
additives.
Introduced in the United States in 1980, microsurfacing is made and
applied to existing pavements by a specialized machine which carries all
components, mixes them on-site, and spreads the mixture onto the road
surface. These materials are continuously and accurately measured, and
then thoroughly combined in the microsurfacer’s mixer.
As the machine moves forward, the mixture is continuously fed into a
full-width “surfacing” box, which spreads the width of a traffic lane
in a single pass, the ISSA says. Also, specially engineered “rut”
boxes — designed to deliver the largest aggregate particles into the
deepest part of the rut to give maximum stability in the wheel path —
may be used to fill ruts. Edges of the microsurface mat are automatically
feathered.
Thin surfacings
The next step up from microsurfacing is a full-blown hot-mix asphalt
thin surfacing. This consists of a single layer of hot-mix asphalt
(minimum of 1-inch thick, but often 2-inches thick) used to level,
waterproof, and restore the original street shape and ride.
Thin overlays for pavement maintenance got a big boost this year when
the results of a National Cooperative Highway Research Project were
revealed at the 82nd meeting of the Transportation Research Board in
Washington in January.
There, a paper on NCHRP Project 20-50 (03/04) — LTPP Data Analysis:
Effectiveness of Maintenance and Rehabilitation Options — assessed the
relative performance of different pavement maintenance and rehab
treatments, including the influence of pretreatment condition and other
factors on treatment effectiveness.
The study was executed by Kathleen T. Hall and Carlos E. Correa, both
with ProTech Engineering, Inc., and Amy L. Simpson, now with PCS-Law. Data
used in the study were drawn from the Long-Term Pavement Performance
Studies SPS-3 experiment.
“In terms of roughness, rutting, and fatigue cracking, the most
effective of the maintenance treatments in the SPS-3 core experiment has
been the thin overlay treatment, followed by the chip seal treatment, and
then the slurry seal treatment,” they write. “The thin overlay
treatment was the only one of the four SPS-3 maintenance treatments to
produce an initial (albeit small) reduction in roughness, and the only one
of the four to have a significant effect on long-term roughness, relative
to the control sections.”
For rougher pavements, however, there was some evidence that chip seals
and slurry seals also had some effect on long-term roughness, rutting, and
cracking, relative to the control sections. “Slurry seals and crack
seals did not have any significant effect on long-term roughness, rutting,
or fatigue cracking,” the authors reported.
It should be kept in mind that the term effectiveness, as used in this
paper, refers to the magnitude of effect on initial and long-term
condition levels, and is not meant to imply anything with regard to the
relative cost-effectiveness of the different treatments. “The most
effective treatment is not always the most cost-effective treatment,”
the authors warn.
Options are vast
The options available to pavement engineers to help preserve pavements
are vast, sophisticated, and are getting better. But perhaps the engineers’
biggest challenge will be to move forward and apply true pavement
preservation principles to the infrastructure under their control.
Fortunately, support is building for pavement preservation, and they won’t
be alone.
Reprinted from Better Roads Magazine
June 2003 |
