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Surface Distresses
Properly designed and maintained HMA pavements can provide many years of
satisfactory service. However, like all pavements, HMA pavements can be
damaged by certain conditions. This page is intended to:
- Assist in identifying basic HMA pavement damage. HMA pavement damage
that is visible at the surface of the pavement is often called "surface
distress".
- Provide some insight into why particular surface distresses are
problematic.
- Provide some basic guidance into what kinds of conditions might lead to
certain surface distresses.
- Suggest some basic repair strategies.
This section is organized alphabetically by type of surface distress.
If you are looking for a specific type, scroll down and look at the thumbnails
to see which picture most closely matches the condition you are investigating.
Photos of each type of distress are accompanied by a description of the
distress, the reason it is a problem, some of the most likely causes and basic
repair strategy.
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Alligator (Fatigue) Cracking |
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Left: Alligator cracking near a parking
lot drain. The cracking is likely due to water infiltrating pavement cracks and
running down to the low point drain underneath the pavement, weakening and removing material
from under the already-thin HMA layer.
With little remaining support, the HMA will fail even under light
loading.
Right: Alligator cracking on a
poorly supported road widening. The road was widened by
about 2 feet, however the subgrade under the widened portion was not adequately compacted.
Therefore,
as it settled over time, the HMA layer settled with it and cracked. |
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Description: A series of interconnected cracks caused by fatigue failure of the HMA surface
under repeated traffic loading. As the number and magnitude of loads
becomes too great, longitudinal cracks begin to form (usually in the wheelpaths). After repeated loading, these longitudinal cracks connect forming
many-sided sharp-angled pieces that develop into a pattern resembling the back
of an alligator or crocodile.
Problem: Roughness, indicator of structural failure,
cracks allow moisture infiltration into the
base and
subgrade, eventually results in
potholes
and pavement disintegration if not treated.
Possible Causes: Inadequate structural support for the given loading,
which can be caused by a myriad
of things. A few of the more common ones are:
- Decrease in pavement load supporting characteristics
- Probably the most common reason is a loss of base, subbase or
subgrade support from things like
poor drainage or
spring thaw.
Water under a pavement will generally cause the underlying materials to
become weak.
- Stripping on the bottom of the HMA layer. The stripped depth contributes
little to pavement strength so the effective HMA thickness
decreases.
- Increase in loading (i.e., the pavement is being loaded more heavily than anticipated in
design)
- Inadequate structural design (i.e., the pavement was designed too thin
for the anticipated loads)
- Poor construction (i.e., inadequate
compaction)
Repair: A fatigue cracked pavement should be investigated to
determine the root cause of failure. Any investigation should involve
digging a pit or coring the pavement to determine the pavement's structural
makeup as well as determining whether or not subsurface moisture is a
contributing factor. Once the characteristic alligator pattern is
apparent, repair by crack sealing is generally ineffective. Fatigue
crack repair generally falls into one of two categories:
- Small, localized fatigue cracking indicative of a loss of subgrade
support. Remove the cracked pavement area then dig out and replace
the area of poor subgrade and improve the drainage of that area if necessary.
Patch over the
repaired subgrade.
- Large fatigue cracked areas indicative of general structural failure.
Place an
HMA overlay over the entire pavement surface. This overlay must be
strong enough structurally to carry the anticipated loading because the
underlying fatigue cracked pavement most likely contributes little or no
strength (Roberts et. al., 1996).
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Bleeding |
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Left:
Classic bleeding on a newly paved road shows up as a shiny reflecting
surface - especially in the wheelpaths where the aggregate voids are less.
Right:
Bleeding on a freshly paved
SMA job, most likely the result of mix design
or production problems. |
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Description: A film of asphalt binder on the pavement surface. It
usually creates a shiny, glass-like reflecting surface that can become sticky
when dry and slippery when wet.
Problem: Loss of
skid resistance when wet, unsightly
Possible Causes: Bleeding occurs when asphalt binder
fills the aggregate voids during hot weather or traffic compaction, and then expands onto the
pavement surface. Since bleeding is not reversible during cold
weather or periods of low loading, asphalt binder will accumulate on the pavement surface over time.
Likely causes are:
- Excessive asphalt binder in the HMA (either due to a poor mix design or
manufacturing problems)
- Excessive application of asphalt binder during
BST application
- Low HMA air void content (e.g., not enough void space for the asphalt
to occupy), likely a mix design problem
Repair: The following repair measures may eliminate or reduce the
asphalt binder film on the pavement's surface but may not correct the
underlying problem that caused the bleeding:
- Minor bleeding can often be corrected by applying coarse sand to blot up
the excess asphalt binder.
- Major bleeding can be corrected by cutting off excess asphalt with a motor
grader or removing it with a heater planer. If the resulting surface is
excessively rough, resurfacing may be necessary (APAI, no date given).
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Block Cracking |
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Left:
Block cracking in a parking lot near the University of Washington.
Block cracking in parking lots is common because many parking lot pavements
experience only light traffic (e.g., no heavy trucks).
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Description: Interconnected cracks that divide the pavement up into
rectangular pieces. Blocks range in size from approximately 1 ft2
to 100 ft2.
Larger blocks are generally classified as longitudinal and
transverse
cracking. Block cracking normally occurs over a large portion of
pavement area but sometimes will occur only in non-traffic areas.
Problem: Allows moisture infiltration,
roughness
Possible Causes: HMA shrinkage and daily temperature
cycling. Typically caused by an inability of asphalt binder to expand and contract with temperature
cycles because of:
Repair: Strategies depend upon the severity and extent of
the block cracking:
- Low severity cracks (< 1/2 inch wide).
Crack seal to
prevent (1) entry of moisture into the subgrade through the cracks and (2)
further raveling of the crack edges. HMA can provide years of
satisfactory service after developing small cracks if they are kept sealed
(Roberts et. al., 1996).
- High severity cracks (> 1/2 inch wide and cracks with raveled edges).
Remove and replace the cracked pavement layer with an
overlay.
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Corrugation and Shoving |
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Left:
Corrugation on the uphill side of a street. Corrugations were
probably caused by vehicles starting on the uphill grade.
Right:
Shoving at a busy intersection. Conditions are bad enough here that
the shoving occurs not only near rigid objects but across the pavement in
general. |
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Description: A form of plastic movement typified by ripples (corrugation)
or an abrupt wave (shoving) across the pavement surface. The distortion is
perpendicular to the traffic direction. Usually occurs at points where traffic
starts and stops (corrugation) or areas where HMA abuts a rigid object
(shoving).
Problem: Roughness
Possible Causes: Usually caused by traffic action (starting and stopping)
combined with:
Repair: A heavily corrugated or shoved pavement should be
investigated to determine the root cause of failure. Repair strategies
generally fall into one of two categories:
- Small, localized areas of corrugation or shoving. Remove the
distorted pavement and
patch.
- Large corrugated or shoved areas indicative of general HMA failure.
Remove the damaged pavement and
overlay.
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Depression |
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Left:
Depression in an access roadway probably caused by subgrade settlement.
Note that the pavement has fatigue cracked significantly as it has settled.
Right:
Depression in parking lot probably caused by subgrade settlement.
Again, the pavement has a considerable amount of
fatigue cracking where it
has settled. |
Description: Localized pavement surface areas with slightly lower
elevations than the surrounding pavement. Depressions are very noticeable
after a rain when they fill with water.
Problem: Roughness,
depressions filled with substantial water can cause vehicle hydroplaning
Possible Causes:
Frost heave or subgrade settlement resulting from
inadequate compaction during construction. Repair: By definition,
depressions are small localized areas. A pavement depression should be
investigated to determine the root cause of failure (i.e., subgrade settlement
or frost heave). Depressions should be repaired by removing the affected
pavement then digging out and replacing the area of poor subgrade.
Patch over the
repaired subgrade.
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Joint Reflection Cracking |
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Left:
Joint reflective cracking on a low-use industrial road. This pavement
has been cracked for many years and will probably not get much worse.
Notice the grass growing in the cracks.
Right: Joint reflective cracking on an urban arterial.
Some joint reflective cracks on this pavement are crack sealed but these are
probably too advanced and too wide to be crack sealed. |
Description: Cracks in a flexible overlay of a rigid pavement. The cracks occur directly over the
underlying rigid pavement joints. Joint reflection cracking does not
include reflection cracks that occur away from an underlying joint or from
any other type of base (e.g., cement or lime stabilized).
Problem: Allows moisture infiltration,
roughness
Possible Causes: Movement of the rigid pavement slab beneath
the HMA surface because of thermal and moisture changes. Generally not
load initiated, however loading can hasten deterioration. Repair:
Strategies depend upon the severity and extent of the cracking:
- Low severity cracks (< 1/2 inch wide and infrequent cracks).
Crack seal to prevent (1) entry of moisture into the subgrade through the
cracks and (2) further raveling of the crack edges. In general, rigid
pavement joints will eventually reflect through an HMA overlay without
proper surface preparation.
- High severity cracks (> 1/2 inch wide and numerous cracks).
Remove and replace the cracked pavement layer with an
overlay.
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Longitudinal Cracking |
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Left:
Longitudinal cracking on a city arterial. Its location in the wheelpath indicates that is probably the onset of
fatigue cracking.
Right:
Longitudinal cracking on a rural county road. Its location in the
wheelpath indicates that is probably the onset of
fatigue cracking possibly
caused by a thin pavement structure. |
Description: Cracks parallel to the pavement's centerline or
laydown direction. Usually a type of
fatigue cracking.
Problem: Allows moisture infiltration,
roughness, indicates possible onset of
fatigue cracking and structural failure.
Possible Causes:
- Poor joint construction or location. Joints are generally the
least dense areas of a pavement. Therefore, they should be
constructed outside of the wheelpath so that they are only infrequently
loaded. Joints in the wheelpath like those shown in third through
fifth figures above, will general fail prematurely.
- A reflective crack from an underlying layer (not including joint
reflection cracking)
- HMA fatigue (indicates the onset of future
fatigue cracking)
-
top-down cracking
Repair: Strategies depend upon the severity and extent of
the cracking:
- Low severity cracks (< 1/2 inch wide and infrequent cracks).
Crack seal to
prevent (1) entry of moisture into the subgrade through the cracks and (2)
further raveling of the crack edges. HMA can provide years of
satisfactory service after developing small cracks if they are kept sealed
(Roberts et. al., 1996).
- High severity cracks (> 1/2 inch wide and numerous cracks).
Remove and replace the cracked pavement layer with an
overlay.
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Patching |
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Left:
Two patches on an urban arterial placed after utility cuts.
Right:
A patch on an urban arterial used to repair a spalled joint reflective
crack. |
Description: An area of pavement that has been replaced with
new material to repair the existing pavement. A patch is considered a
defect no matter how well it performs.
Problem: Roughness
Possible Causes:
- Previous localized pavement deterioration that has been removed and
patched
- Utility cuts
Repair:
Patches are themselves a repair action. The only way they can be
removed from a pavement's surface is by either a
structural or
non-structural overlay.
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Polished Aggregate |
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Left: Two adjacent SMA
pavements at the NCAT
Test Track near Auburn, AL. The pavement on the right uses river
rock, which is more susceptible to abrasion, as its aggregate and is showing
some signs of aggregate polishing.
Right:
This close-up picture of a road surface shows aggregate wear after about 5
years of wear. Washington generally has high quality aggregate that
rarely suffers from excessive polishing. Notice the aggregate still
appears rough. |
Description: Areas of HMA pavement where the portion of
aggregate extending above the asphalt binder is either very small or there
are no rough or angular aggregate particles.
Problem: Decreased
skid resistance
Possible Causes: Repeated
traffic applications. Generally, as a pavement ages the protruding
rough, angular particles become polished. This can occur quicker if
the aggregate is
susceptible to abrasion or subject to excessive studded tire wear.
Repair: Apply a skid-resistant
slurry seal or
BST or
overlay.
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Potholes |
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Left: A pothole in the
surface course of an urban arterial. This pothole may have come about
from fatigue cracking that started in an excessively raveled area.
Left untreated it may eventually spread to the underlying pavement layers.
Right: A pothole
beginning to form from an area of excessive fatigue cracking. Notice
how just a few chunks of pavement are missing forming the genesis of a
pothole. Left untreated, adjacent pavement chunks will be dislodged by
traffic making the pothole larger. Additionally, the pothole makes it
easier for water to get into the underlying subgrade, further weakening it,
and speeding up the deterioration process.
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Description: Small, bowl-shaped depressions in the pavement
surface that penetrate all the way through the HMA layer down to the base
course. They generally have sharp edges and vertical sides near the
top of the hole. Potholes are most likely to occur on roads with thin
HMA surfaces (1 to 2 inches) and seldom occur on roads with
4 inch or deeper HMA surfaces (Roberts et al., 1996).
Problem: Roughness (serious vehicular damage can result
from driving across potholes at higher speeds),
moisture infiltration
Possible Causes: Generally,
potholes are the end result of fatigue
cracking. As fatigue
cracking becomes severe, the interconnected cracks create small chunks of
pavement, which can be dislodged as vehicles drive over them. The
remaining hole after the pavement chunk is dislodged is called a pothole.
Repair: In accordance with
patching techniques.
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Raveling |
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Left:
Raveling possibly resulting from segregation, temperature differentials or
inadequate compaction.
Right:
Raveling that is likely the result of segregation and/or
construction-related temperature differentials. |
Description: The progressive disintegration of an HMA layer
from the surface downward as a result of the dislodgement of aggregate
particles.
Problem: Loose debris on the pavement,
roughness, water collecting in the raveled
locations resulting in vehicle hydroplaning, loss of
skid resistance
Possible Causes:
- Loss of bond between aggregate particles and the asphalt binder as a
result of:
- A dust coating on the aggregate particles that forces the asphalt
binder to bond with the dust rather than the aggregate
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Aggregate
segregation. If fine particles are missing from the aggregate
matrix, then the asphalt binder is only able to bind the remaining
coarse particles at their relatively few contact points.
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Inadequate compaction during construction. High density is
required to develop sufficient cohesion within the HMA.
- Mechanical dislodging by certain types of traffic (studded tires,
snowplow blades or tracked vehicles).
Repair: A raveled pavement should be investigated to determine the
root cause of failure. Repair strategies generally fall into one of
two categories:
- Small, localized areas of raveling. Remove the raveled
pavement and patch.
- Large raveled areas indicative of general HMA failure. Remove
the damaged pavement and
overlay.
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Rutting |
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Left:
Rutting in a busy arterial in the Greenwood area of Seattle. Ruts can
even be seen in the two-way left turn lane indicating that the rutting is
not due to heavy traffic.
Right: Rutting in
the parking lane. Likely, an overlay of the main road was extended beyond the original roadway
out to the curb . Extra HMA was used to make up the difference in
height but was not adequately compacted because the roller bridged over the
rut area (one side of the drum on the original pavement and the other on the
new mix nearest the curb). Subsequently, a large load or two (i.e., a
bus) drove over the poorly compacted area and caused a large rut. |
Description: Surface depression in the wheelpath.
Pavement uplift (shearing) may occur along the sides of the rut. Ruts are
particularly evident after a rain when they are filled with water.
There are two basic types of rutting: mix rutting and subgrade rutting.
Mix rutting occurs when the subgrade does not rut yet the pavement surface
exhibits wheelpath depressions as a result of compaction/mix design
problems. Subgrade rutting occurs when the subgrade exhibits wheelpath
depressions due to loading. In this case, the pavement settles into
the subgrade ruts causing surface depressions in the wheelpath.
Problem: Ruts
filled with water can cause vehicle hydroplaning, can be hazardous
because ruts tend to pull a vehicle towards the rut path as it is steered
across the rut.
Possible Causes: Permanent
deformation in any of a pavement's layers or subgrade usually caused by
consolidation or lateral movement of the materials due to traffic loading.
Specific causes of rutting can be:
- Insufficient compaction of HMA layers during construction. If it
is not compacted enough initially, HMA pavement may continue to densify
under traffic loads.
- Subgrade rutting (e.g., as a result of inadequate pavement structure)
- Improper mix design or manufacture (e.g., excessively high asphalt content,
excessive mineral filler, insufficient amount of angular aggregate
particles)
Ruts caused by studded
tire wear present the same problem as the ruts described here, but they
are actually a result of mechanical dislodging due to wear
and not pavement deformation. Repair: A heavily rutted pavement
should be investigated to determine the root cause of failure (e.g. insufficient
compaction, subgrade rutting, poor mix design or studded tire wear).
Slight ruts (< 1/3 inch deep) can generally be left untreated. Pavement
with deeper ruts should be
leveled and
overlayed.
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Slippage Cracking |
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Left: Slippage crack at
an intersection due to stopping. The direction of traffic is from top
to bottom. Most likely caused by poor bonding with the underlying
pavement - possibly from a poor tack coat.
Right: Slippage crack
near a bus stop on the University of Washington Seattle campus. Based
on the direction of traffic flow, it appears to be caused by traffic
accelerating from a stop. Since this area of campus suffers from a
number of poor overlay bonding problems, this is most likely caused by poor
overlay bonding to the underlying pavement. |
Description: Crescent or half-moon shaped cracks generally
having two ends pointed into the direction of traffic.
Problem: Allows
moisture infiltration,
roughness
Possible Causes: Braking or
turning wheels cause the pavement surface to slide and deform. The
resulting sliding and deformation is caused by a low-strength surface mix or
poor bonding between the surface HMA layer and the next underlying layer in
the pavement structure.
Repair: Removal and replacement of affected area.
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Stripping |
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Left: A small pavement
core showing stripping at the bottom of the pavement section.
Right: A small pavement
core showing stripping at the bottom of the pavement section. |
Description: The loss of bond between aggregates and asphalt binder that typically begins at the bottom of the HMA layer and
progresses upward. When stripping begins at the surface and progresses
downward it is usually called raveling.
Problem: Decreased structural support, rutting,
shoving/corrugations,
raveling, or cracking (fatigue
and longitudinal)
Possible Causes: Bottom-up stripping is very
difficult to recognize because it manifests itself on the pavement surface
as other forms of distress including rutting, shoving/corrugations,
raveling, or cracking. Typically, a core must be taken to positively
identify stripping as a pavement distress.
Repair: A stripped pavement should be investigated to determine
the root cause of failure (i.e., how did the moisture get in?). Generally,
the stripped pavement needs to be removed and replaced after correction of
any subsurface drainage issues.
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Transverse (Thermal) Cracking |
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Left: A large transverse
crack caused by the pavement's contraction in cold weather. The crack
has subsequently been patched as a temporary fix.
Right: Another transverse
crack caused by thermal contraction during cold weather. This crack
has been sealed. |
Description: Cracks perpendicular to the pavement's
centerline or laydown direction. Usually a type of thermal cracking.
Problem: Allows
moisture infiltration,
roughness
Possible Causes:
- Shrinkage of the HMA surface due to low temperatures or asphalt binder
hardening.
- Reflective crack caused by cracks beneath the surface HMA layer
-
top-down cracking
Repair: Strategies depend upon the severity and extent of
the cracking:
- Low severity cracks (< 1/2 inch wide and infrequent cracks).
Crack seal to
prevent (1) entry of moisture into the subgrade through the cracks and (2)
further raveling of the crack edges. HMA can provide years of
satisfactory service after developing small cracks if they are kept sealed
(Roberts et. al., 1996).
- High severity cracks (> 1/2 inch wide and numerous cracks).
Remove and replace the cracked pavement layer with an
overlay.
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Water Bleeding and Pumping |
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Left: Water bleeding
through a pavement surface on a rural mountain highway. The subgrade
around the pavement is still wet from a recent rain indicating that the
water has come up through a porous pavement.
Right: Similar
conditions. |
Description: Water bleeding (left two photos) occurs when water seeps out of
joints or cracks or through an excessively porous HMA layer. Pumping
(right-most photo) occurs when water and fine material is ejected from underlying layers
through cracks in the HMA layer or out the sides of the HMA layer under moving loads.
Problem: Decreased
skid resistance, an indication of high pavement porosity (water bleeding),
decreased structural support (pumping)
Possible Causes:
Repair: Water bleeding or pumping should be investigated to
determine the root cause. If the problem is a high water table or poor
drainage, subgrade drainage should be improved. If the problem is a
porous mix (in the case of water bleeding) a
fog seal or
slurry seal
may be applied to limit water infiltration.
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