Pavement maintenance describes all the methods and techniques used to prolong
pavement life by slowing its deterioration rate. The performance of a
pavement is directly tied to the timing, type and quality of the maintenance it
receives.
Figure 1: Bituminous Surface Treatment (BST),
also known as a Chip Seal, in Vantage
Figure 2: Parking Lot Showing a Fog Seal
on the Right Side
Crack seal products are used to fill individual pavement cracks to prevent
entry of water or other non-compressible substances such as sand, dirt, rocks or
weeds. Crack sealant is typically used on early stage
longitudinal cracks,
transverse cracks,
reflection cracks and
block cracks.
Fatigue cracks
are most often too extensive to warrant filling with crack sealer; they usually
require an area treatment such as a patch or reconstruction.
Figures 3 and 4: Crack Sealing
WAPA Pavement Note on Crack Seals
Crack sealing is best done in moderate temperatures (spring or fall) and is
most effective if performed immediately after cracks develop.
Before
applying crack sealant, cracks should be routed out and cleaned.
A fog seal (see Figure 2) is a light application of a diluted slow-setting
asphalt emulsion to the
surface of an aged (oxidized) pavement surface. Fog seals are low-cost and
are used to restore flexibility to an existing HMA pavement surface. They may be able to
temporarily postpone the need for a BST or
non-structural overlay.
WAPA Pavement Note
on Fog Seals
Fog seals are suitable for low-volume roads which can be closed to
traffic for the 4 to 6 hours it takes for the slow-setting asphalt
emulsion to break and set.
An excessive application rate may result in a thin asphalt layer on
top of the original HMA pavement. This layer can be very smooth and
cause a loss of
skid resistance. Sand should be kept in reserve to
blot up areas of excess application.
A slurry seal is a mixture of
emulsified asphalt, water,
well-graded fine aggregate and
mineral filler that has a creamy fluid-like
appearance when applied. Slurry seals are used to fill existing pavement surface
defects as either a prefatory treatment for other maintenance treatments or as a
wearing course. Microsurfacing is an advanced form of slurry seal that uses the same basic
ingredients (emulsified asphalt, water,
fine aggregate and mineral filler) and
combines them with advanced polymer additives.
WAPA Pavement Note
on Slurry Seals
Slurry seals are not recommended for at least the first 6 months after a new
pavement is placed. Covering a new pavement too quickly with a slurry seal
may not allow the underlying pavement surface adequate time to set and harden.
A bituminous surface treatment, also known as a chip seal, is a thin protective wearing surface that is applied to a pavement or base course. BSTs can provide all of
the following:
A waterproof layer to protect the underlying pavement.
An anti-glare surface during wet weather and an increased reflective
surface for night driving.
WAPA Pavement Note on BSTs
BSTs are used more often in Eastern Washington than
Western Washington because of the generally lighter traffic volumes and because
the predictable
periods of good weather that BSTs require to achieve satisfactory results are
more prevalent in Eastern Washington. However, BSTs are used in Western
Washington (for instance, the City of Seattle has a BST program).
Non-structural overlays do not involve
extensive structural design and generally contribute little, if anything, to a
pavement's structural capacity. Non-structural overlays are generally thin
surface overlays of 0.5 - 1.5 inches that are used to (NAPA, 1995):
A loose classification of
non-structural overlays can be done based on traffic characteristics (NAPA,
1995):
Light volume/residential traffic. The primary objective in
light traffic areas is to retard
asphalt binder aging of the underlying
pavement. Since heavy traffic loads are not of great concern, overlays
are generally less stiff (resulting in a more
workable mix, increased durability
and flexibility and a potential for the overlay to reheal under traffic) and
use smaller-sized aggregates.
Heavy, high-speed traffic. The primary objective in heavy,
high-speed traffic areas is to prevent
rutting and provide good
friction.
Because of this, these overlays typically use larger angular aggregate and more
durable mixes such as SMA or
OGFC.
WAPA Pavement Note on Thin Lift Paving
Thin lift paving, such as that seen in non-structural overlays presents
several construction concerns (NAPA, 1995):
Thin lifts require less HMA per foot of road length than thick lifts.
This can result in high paver speeds (in excess of 70 ft/min) making it
difficult for rollers to keep up.
Thin lifts will cool quickly. This can result in
little time available for
compaction before the thin overlay reaches
cessation temperature
(sometimes as little as 3 to 5 minutes).
Thin lift construction produces greater
screed wear. If the
lift depth is less than about twice the maximum aggregate size, the HMA may
tear under the paver screed. Very thin lifts (less than 1 inch)
can be damaged by the screed dragging large particles.
Thin lifts are more sensitive to
vibratory rolling.
Incorrectly chosen amplitude, frequency or roller speed can result in
aggregate breakage and damage of the bond between the
overlay and the existing pavement.
Density control is difficult. Thin lifts provide fewer
options for aggregate particles to rearrange under compaction. Thus, mat
densities will tend to be less uniform than those associated with a
thicker lift. This should be recognized if pay is in any way tied to mat
density.
Patches are a common method of treating an area of localized distress.
Patches can be either partial or full-depth, although typically HMA pavement
patches are full-depth. A high quality HMA patch can be considered a
permanent repair although many patches are done as emergency
repairs in poor conditions (e.g., cold, rainy) and therefore are only considered
temporary repairs. Patching material can be just about
any HMA or cold mix asphalt material as well as certain types of
slurries.
Typically some form of HMA is used for permanent patches, while cold mix is often
used for temporary emergency repairs.
WAPA Pavement Note on Cold Mix Asphalt
Cold mix asphalt is a mixture of aggregate and
asphalt emulsion or other
proprietary asphalt binder product. Once mixed, the water in the emulsion
will evaporate off leaving the aggregate coated with asphalt and ready to use.
It is called "cold mix" because, unlike hot mix asphalt (HMA), cold mix asphalt
is blended, transported and placed at near-ambient temperatures.
Generally, cold mix asphalt is used as a temporary patching material because,
although it will not provide the long-term performance of HMA, it
can be placed without the stringent quality control of
HMA.
Pothole Patching
Pothole patching probably receives the greatest amount
of public attention. Pothole patching procedures cover a wide range of methods and intentions from permanent
full-depth patches to temporary on-the-fly patches. However, potholes are
the result of pavement failure and therefore any patch is considered temporary
until the underlying cause is determined and corrected. Two general
patching procedures
are described next.
Figure 5: Semi-permanent
Pothole Repair
Figure 6: The Same Repaired
Pothole One Year Later - Still Performing Well
Semi-Permanent Pothole Patch (see Figures 5 and 6) (from FHWA,
1998)
Remove all water and debris from the pothole.
Square up the pothole sides so they are vertical and have in-tact pavement
on all sides.
Place the patching material into the clean squared-up hole. The
material should mound in the center and taper down to the edges so that it
meets flush with the surrounding pavement edges.
Compact the patching material starting in the center and working out
toward the edges. Compaction can be accomplished using a vibratory plate
compactor or a single-drum vibratory roller. Check the compacted
patching material for a slight crown. This is done so that subsequent
traffic loading will compact it down to the surrounding pavement height.
Place the patching material into the pothole without any preparation or
water/debris removal.
Compact the patching material using the patching truck tires (usually 4 to
8 passes).
Check the compacted patch for a slight crown. If a depression is
present add more patching material and compact.
Although it may seem that the semi-permanent technique would produce a higher
quality patch than the throw-and-roll technique, the FHWA's Long Term Pavement
Performance (LTPP) Study found that the "throw-and-roll technique proved just as
effective as the semi-permanent procedure for those materials for which the two
procedures were compared directly" (FHWA, 1998). Since the semi-permanent
technique is more labor and material intensive, the throw-and-roll technique
will generally prove more cost effective if quality materials are used.
