This block will discuss the maintenance and rehabilitation of HMA pavements. The first module is pavement distress and the causes. To properly rehab a pavement it is necessary to understand what is causing the problem.
Prior to use of paved surfaces this was the primary distress in the nations roadways.
As a review an asphalt pavement can either be full-depth or an HMA layer over a crushed stone or gravel base. How they behave can depend on the manner in which the pavement is built. If water infiltrates the untreated base it can cause a weakening of the base and resultant failure of the pavement.
This slide shows the basic requirements for an asphalt pavement.
The desire in an asphalt pavement is to provide sufficient pavement structure to reduce the tensile stresses in the asphalt layer below the strength and to reduce the compressive stresses in the subgrade below the strength of that layer.
Distresses in asphalt pavements can be categorized into four different distress modes: fracture, distortion, disintegration and miscellaneous which covers all of those distresses that do not fit into the other three categories.
Fatigue cracking is often called alligator cracking because this closely spaced crack pattern is similar to the pattern on an alligator’s back. This type of failure generally occurs when the pavement has been stressed to the limit of its fatigue life by repetitive axle load applications. Fatigue cracking is often associated with loads which are too heavy for the pavement structure or more repetitions of a given load than provided for in design. Fatigue cracking can lead to the development of potholes when the individual pieces of HMA physically separate from the adjacent material and are dislodged from the pavement surface by the action of traffic. Potholes generally occur when fatigue cracking is in the advanced stages and when relatively thin layers of HMA comprise the bound portion of the pavement.
Fatigue cracking will generally start as longitudinal cracking in the wheel path and then progress to additional cracking until it form the alligator cracking pattern.
This picture shows extensive alligator cracking.
Another picture of alligator cracking. The cracking shown in this picture is probably due to poor support from the underlying layers – maybe caused by moisture in the subgrade.
The cracking will progress to this stage.
And finally to potholes.
Low temperature cracks are transverse cracks which generally run perpendicular to the roadway centerline and are often approximately equally spaced. These transverse cracks normally occur when the temperature at the surface drops sufficiently to produce a thermally induced shrinkage stress in the HMA layer that exceeds the tensile strength of the HMA mixture. These cracks usually initiate at the top of the HMA and propagate downward through the mixtures.
Note the regular pattern of the cracks and it appears that debris has infiltrated the cracks and is now causing a bump in the surface.
A close up of a thermal crack.
When a HMA layer cracks both longitudinally and transversely in approximately square shapes, this is referred to as bloc cracking. Typically, these cracks are caused by the same factors which cause low temperature cracking. The block cracking pattern most often develops on facilities which carry low traffic volumes. Because of the low traffic volume, the asphalt binder material has the opportunity to exhibit thixotropic hardening, i.e., it sets up and develops a type of internal structure which exhibits brittle behavior under thermal loading. Block cracking is more often seen in large paved areas, such as parking lots of airfield pavements, than on roads and streets. It can be very serious, especially if the cracks begin to exhibit raveling and other advanced stages of deterioration, such as development of secondary cracks.
An example of block cracking in a drive way.
A close up of block cracking.
Longitudinal cracks are individual cracks that basically run parallel to the centerline of the roadway. They most often occur at the joint between adjacent lanes of asphalt mixture or at the edges of the wheel paths in a rutted pavement. The cracks allow water to penetrate into the underlying layers, possibly softening nonstabilized layers and accelerating the development of fatigue cracks radiating outward from the longitudinal crack. The longitudinal cracks between adjacent lanes can be induced by low temperature, since the density at the joint between paving lanes is lowest, resulting in low tensile strength
Moderate severity longitudinal cracking in the wheel path.
A cause of longitudinal cracking is segregation. These segregated lines are areas of low density and will result in longitudinal cracking.
Edge cracking can be caused by an widening project or as in probable in this case a weak layer along the edge the pavement that could be the result of moisture getting under the edge and then traffic causes it to break off.
High severity edge cracking.
Reflection cracks are caused by discontinuities in the underlying layers which propagate through the HMA surface due to movement at the crack. Reflection cracks may be caused by: . Cracks or joints in an underlying concrete pavement; . Low temperature cracks in the old HMA surface; . Block cracks induced by the old HMA surface or those induced bye subgrade soil cracking due to shrinkage whether stabilized or not; . Longitudinal cracks in the old surface; or . Fatigue cracks in the old surface. The name reflection cracking indicates that the crack is initiated by action in the underlying layers that produce stresses in the HMA surface which exceed the strength of the material.
This slide shows the mechanism for reflection cracking.
Ruts are depressions which occur in the pavement’s wheel path as a result of traffic loads. Some negligible amount of rutting occurs in HMA surfaces due to the continued densification under traffic after initial compaction during construction. In fact, it is quite common for void contents of HMA surfaces to be reduced from 7 or 8 percent after construction down to 4 or 5 percent after the first 2 or 3 summers of traffic, due to densification. In a 4-inch thick HMA pavement, this densification results in a rut depth of approximately 0.12 inches. Perhaps the most common recent cause of rutting is associated with the HMA layer, especially on routes with heavy loads and high tire pressures. Much of this rutting can be attributed to improper mix design. Some of the most common mistakes when designing heavy duty HMA mixtures are: 1. Selection of an asphalt content that is too high. 2. Use of excessive filler material (minus 200 material); and 3. Use of too many rounded particles in both coarse and fine aggregates in the HMA. Probably the singly largest contributor to rutting in HMA is excessive asphalt content.
An example of rutting.
The major problem with rutting is the depression which can collect and hold water that will result in a hydroplanning problem for traffic.
Severe rutting on an airfield taxiway.
Corrugations are caused by shear flow of the mixture or slippage between the layers. The shear flow may be a result of an unstable mixture, i.e., void content too low for stable behavior or the applied loads too high forcing aggregate particles to move over each other, producing shoving or corrugations. Most cases of unstable mixtures show up first in areas of the pavement where traffic moves slowly or where stopping occurs. Therefore, in rural areas, truck lanes where the commercial vehicles slow down significantly below highway speed are often the first or, in some cases, the only area where shoving occurs. If there is a horizontal curve, these areas will experience shoving as a result of the horizontal friction forces between the road surface and the tire which are required for the vehicle to negotiate the curve. In urbanized areas intersections are typically the first place where excessive rutting, shoving, or corrugations occur because of the slow moving traffic and the additional horizontal friction forces caused bye braking and acceleration. Bus stops are often the first locations to exhibit premature distortion in the form of shoving. Shoving can be easily detected at edge lane markings where these lines bend outward in the vicinity of the shoving.
Corrugating in the intersection.
Slippage cracks typically occur as a result of poor bond between the HMA surface and the underlying layer. These cracks most often show up first in areas where vehicles brake, turn, or accelerate. Slippage cracks form a distinctive U-shape with the top of the U always points in the direction that the forces are being applied. When these cracks are caused by braking, the end of the slippage cracks (top of the U) will point toward the rear of the vehicle. The only repair for areas with slippage cracks is removal and replacement with new material, being careful to ensure that a proper bond between the old surface and the new HMA material is obtained. A lack of bond may also show up quickly in areas free from braking or acceleration.
An example of slippage.
Another example of slippage cracking.
Stripping is a distress that is characterized by the loss of bond between the aggregates and the asphalt cement which typically begins at the bottom of the HMA layer and progresses upward. When stripping starts at the surface and progresses downward it results in ravelling. Stripping is one of the most difficult distresses to identify because the surface manifestations can take numerous forms: rutting, shoving, corrugations, ravelling, or cracking. Therefore, the only accurate way to determine if stripping is the cause of the observed distress is to open up the pavement structure and look at the material removed from the cross-section.
Ravelling is the progressive disintegration of a HMA layer from the surface downward as a result of the dislodgement of aggregate particles. The opportunity for dislodgement is precipitated by a loss of bond between the aggregate particle and the asphalt coating. A number of condition can lead to ravelling: 1. A coating of fine dust on the aggregate thick enough that the asphalt film sticks to the dust rather than to the aggregate. Surface traction forces abrade the asphalt film and then dislodge the aggregate. 2. Segregated spots in the surface layer where most of the fine aggregate is absent. 3. Low in-place density in the asphalt surface course. A badly ravelled pavement can pose a safety problem if the abraded material leaves a depression in the pavement surface deep enough to pond water, which could cause hydroplaning. Additional problems associated with loose debris on the highway may have to be dealt with, depending on the rate of progression of the ravelling problem. Loose aggregate on the surface reduces skid resistance and can be picked up by the tires of moving vehicles and thrown into the path of passing or trailing vehicles.
A ravelled surface.
Ravelling caused by fuel spillage in a parking lot.
Close up of a ravelled pavement.
Low skid resistance caused by a bleeding pavement. Or, by a surface treatment where the aggregate has not stuck to the surface.
Bleeding probably by too much asphalt in the HMA wearing course.
Bleeding caused by poor surface treatment.
This picture shows a close up of a flushed asphalt surface.
Rehab & Maint Distress 4• Drainage, drainage, drainage• Sufficient thickness and internal strength to carryexpected traffic loads• Adequately dense to prevent penetration of moisturefrom underneath, sides, and surface• Top surface which is smooth; waterproof; skidresistant; resistant to wear, distortion, anddeterioration by weather and deicing chemicalsBasic Requirements forFlexible Pavements
Rehab & Maint Distress 8Also called alligator or map cracking• Loads too heavy for the pavement structure• Too many repetitions of load• Inadequate support (pavement layers/subgrade)• Poor drainage• Very stiff binder in surface course• Occurs in wheel path, potholes develop in advancedstageLoad Associated (Fatigue)Cracking
Rehab & Maint Distress 15Transverse cracks occur at regular intervals• Asphalt too stiff at low service temperatures• Rapid chilling of the road surface• Highly temperature susceptible asphalt binder• Subgrade type (more on sandy)• Age of the pavementNon-Load Associated (Thermal)Cracking
Rehab & Maint Distress 21Occurs parallel to the center line of roadwayCenter Line Joint• Joint between adjacent lanes• Density gradient across the joint• Low density - low tensile strengthEdge Cracking• Lack of lateral (shoulder) support• Settlement of widened pavement• Poor drainage (ditches)Longitudinal Cracking
Rehab & Maint Distress 22Longitudinal cracking in the wheelpath
Rehab & Maint Distress 23Streaks ofSegregationSources of Longitudinal Cracking
Rehab & Maint Distress 26Discontinuities in the underlying layers propagate throughthe HMA surface due to movement of cracksDiscontinuities• Cracks or joints in underlying PCC pavement• Cracks in soil-cement base course• All types of cracks in the existing HMA pavementCauses• Thermal (movement of joints/cracks)• LoadReflection Cracking
Rehab & Maint Distress 27Shearing and Bending Stresses inHMA OverlayStress at the tip of the crackShearing stressAC bendingstressOld PCC or ACpavementAC overlayTip of the joint orworking crackVoidABC
Rehab & Maint Distress 36• Associated with slippage mechanism• Poor bond between the surface and underlying layer• Too much or too little tack coat• Steep grades/intersections• Acceleration/braking exert surface traction forces• U-shapedSlippage
Rehab & Maint Distress 40Breaking of adhesive bond between the aggregatesurface and asphalt binder usually in presence ofmoisture• Inadequate pavement drainage system• Mixes with excessive air voids (poor compaction),and low asphalt content• Incompatible aggregate surface and asphalt binder,asphalt binder displaced by water• Excessive pore pressure induced by traffic• Spontaneous emulsification of binder by waterStripping
Rehab & Maint Distress 41Progressive disintegration of HMA surface as a result ofdislodgment of aggregate particles in the mix• Lack of sufficient cohesion due to low in-placedensity• Lack of fines in the matrix, especially in coarse,segregated areas• Aging of asphalt binderRaveling
Rehab & Maint Distress 45Friction between the tire and road surface. Quantified byFriction Number (FN). Critical when surface is wet.Causes for Low FNs• Flushing/bleeding of HMA surface• Inadequate microtexture- aggregate prone to polishing• Inadequate macrotexture- no drainage channels- causes hydroplaning- important at high speedsSkid Resistance