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Pavement Distresses Flexible Pavement
Sponsored by: Minnesota LTAP Center Presented by: Michael Marti, P.E. SRF Consulting Group, Inc. Dan Wegman, P.E. SemMaterials This presentation follows the order of the SHRP Distress Identification Manual for the Long-Term Pavement Performance Project, SHRP-P Copies of the manual are available from FHWA. To order: TRB Bookstore Title: Distress Identification Manual for the Long-Term Pavement Performance Project Book Code: PP338 Published: 1993 Price: $20.00 Series: SHRP P Report Number: P338 Pages: 137 ISBN#: To Purchase Via Phone or Fax Charge orders may be placed by telephone ( ) or fax ( ). Be sure to include complete credit card information. SRF Consulting Group, Inc. 1
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Various Rating Manuals
Pavement Rehabilitation: A Guide for Minnesota Cities and Counties MN LRRB Distress Identification for the Long Term Pavement Performance Program SHRP
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Identifying the Distress is the 1st Step in the Process
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Identifying Distresses Includes:
Name Severity Amount “Rate what you see and not what you think.”
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Cracking Types Fatigue Thermal/Transverse Block Edge Longitudinal
Pavement Distress Cracking Types Fatigue Thermal/Transverse Block Edge Longitudinal Reflection Slippage Cracks occur from a variety of causes including stresses from axle loads, temperature changes in the asphalt pavement layer, or moisture and temperature changes in an underlying layer. It is important to accurately identify the type of cracking which a pavement exhibits in order to accurately assess the causes for the cracking and subsequently the proper repair techniques. SRF Consulting Group, Inc. 14
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Fatigue Cracking Causes Cures Insufficient strength
Pavement Distress Fatigue Cracking Causes Insufficient strength End of pavement life Too heavy of loads Cures Remove and replace Mill and overlay Coat hanger analogy -- Fatigue can best be described by a coat hanger. You can bend a coat hanger so many times, until it is sufficiently weakened so it can not longer bend and eventually breaks. The same is true of asphalt pavements. Asphalt pavements are correctly referred to as “flexible pavements”. An inadequate pavement structure (a “squishy” base--usually during the spring thaws) means the pavement will have greater strains (have to flex more), and will therefore withstand fewer flexes. The higher and the heavier the traffic, the more it will be flexed, and the sooner it will fatigue crack. Oxidation is the age hardening of the liquid asphalt. As asphalt oxidizes, it becomes less elastic, and less resistant to the flexing. The end result is “tensile failure”, where the asphalt is no longer able to withstand the stresses. Fatigue cracking is the signal of the end of a pavement’s design life. Ideally, preventive maintenance would have been done well before the formation of fatigue cracks. The problem is made worse by inadequate pavement drainage , which allows pavement layers to become saturated and lose strength. The asphalt pavement layers experience high strains when the underlying layers are weakened by excess moisture and fail prematurely in fatigue. Fatigue cracking is also often caused by repetitive passes with overweight trucks and / or inadequate pavement thickness due to poor quality control during construction. SRF Consulting Group, Inc. 20
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Thermal Cracking (Transverse)
Pavement Distress Thermal Cracking (Transverse) Causes Contraction and Expansion of Pavement with Changing Temperatures Cures Crack Filling Full-Depth Reclamation Thick Overlay Low Temperature, or thermal, cracks are transverse cracks which generally run perpendicular to the roadway centerline and are often approximately equally spaced. These cracks usually initiate at the top of the pavement and propagate downward through the mixtures. SRF Consulting Group, Inc. 15
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Block Cracking Causes Cures High Void Content
Pavement Distress Block Cracking Causes High Void Content Oxidative Hardening of the AC Thermal Cracking Cures Crack Filling/Sealing Seal Coat Full-Depth Reclamation Thick Overlay 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 materials 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. The asphalt pavement exposed to low traffic volumes does not densify sufficiently; hence, the asphalt pavement has a relatively high void content. This results in more oxidation and hardening of the asphalt, leading to thermal cracking. Block cracking can also be caused by oxidative hardening of the asphalt if mixed too long in the pug mill of a HMA batch facility, mixed too hot, or stored too long in silos. All these mechanisms make the asphalt cement especially susceptible to tensile strains which can exceed the tensile strain capacity of the asphalt mixture and cause the block cracking pattern. SRF Consulting Group, Inc.
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Edge Cracking Causes Cures Soil Movement Beneath Pavement
Pavement Distress Edge Cracking Causes Soil Movement Beneath Pavement Cures Crack Filling/Sealing Full-Depth Reclamation With no shoulder, the pavement is “unbound”. Problem is exacerbated by moisture and poor drainage The cracking pattern is the same as fatigue--because there is insufficient structure under the edges of the asphalt pavement. SRF Consulting Group, Inc.
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Longitudinal Cracks Causes Cures Low Temperatures Crack Sealing
Pavement Distress Longitudinal Cracks Causes Low Temperatures Cures Crack Sealing Thick Overlay At joints between adjacent lanes: Longitudinal cracks can be induced by low temperature, since the density at the joint between paving lanes is lowest, resulting in low tensile strength. Typically the density of the asphalt mix near the longitudinal joint is at least 2 to 3 percent lower than the remaining pavement. Low tensile strength, coupled with the possibility of oxidative hardening of the asphalt due to high air voids in the joint area, makes the material particularly susceptible to racking and subsequent raveling. Poor adhesion between the asphalt and the aggregate can aggravate the problem adjacent to longitudinal cracking and cause more rapid deterioration. At the edges of wheel paths: Longitudinal cracks are the edges of the wheel path of a rutted pavement are most often caused by heavy loads or high tire pressures applied in the wheel path. As the asphalt pavement in the wheel path is densified by traffic, residual stresses develop in the asphalt pavement adjacent to the wheel path. When these stresses exceed the tensile strength of the asphalt, cracking occurs. The problem is aggravated in colder climates when heavy loads are applied to a cold asphalt mix over a soft subgrade. A relatively high deflection can occur, causing a longitudinal crack to develop in the brittle asphalt mixture. SRF Consulting Group, Inc.
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Reflective Cracking Causes Cures
Pavement Distress Reflective Cracking Causes Existing cracks or joints in the underlying structure Concrete slab movements Cures Crack Filling Extensive Pre-Overlay Repair Rubblizing Crack & Seat Saw & Seal Interlayer Thermal or fatigue cracks have a tendency to grow into new overlays. This growth is referred to as reflection. Old cracks on underlying concrete slabs open and close as the pavement expands & contracts with temperature. The slabs can also move up and down relative to each other as the slabs are rocked by traffic moving over them. Reflection cracks may be caused by: 1. Cracks or joints in an underlying concrete pavement 2. Low temperature cracks in the old asphalt surface 3. Block cracks induced by the old asphalt surface or those induced by subgrade soil cracking due to shrinkage whether stabilized or not 4. Longitudinal cracks in the old surface 5. Fatigue cracks in the old surface. SRF Consulting Group, Inc. 23
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Slippage Cracking Description Causes Cures
Pavement Distress Slippage Cracking Description U-shaped cracks in areas of braking, turning, accelerating Causes Poor bond between asphalt surface and underlying layer Cures Crack Filling/Sealing Full-Depth Reclamation These cracks show up in areas where vehicles brake, turn or accelerate. Slippage cracks form a distinctive U-shape with the top of the U always pointing in the direction that the force is being applied. SRF Consulting Group, Inc.
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Surface Defects Rutting Shoving Bleeding Polished Aggregate Raveling
Pavement Distress Surface Defects Rutting Shoving Bleeding Polished Aggregate Raveling Potholes Patching SRF Consulting Group, Inc.
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Rutting Causes Cures Low Air Voids Smooth, Rounded Aggregate
Pavement Distress Rutting Causes Low Air Voids Asphalt content too high Smooth, Rounded Aggregate Excessive amounts of natural sand Excess Dust Cures Remove/Replace 100 mm (4 inches) Micro-surfacing Low air voids usually mean an excess of asphalt cement (a “fat” mix). If the pavement structure is dependent upon asphalt cement with rocks in it, it will be more susceptible to rutting than if there is a strong aggregate skeleton. The idea is to have a strong stone on stone aggregate skeleton held together by the asphaltic binder, with sufficient binder to hold the mix together for a long period of time, to waterproof the pavement, and to resist cracking at low temperatures. Imagine a structure of marbles vs one of cubes. The marbles won’t stack and maintain a strong structure. Angular aggregates give a much stronger structure. SRF Consulting Group, Inc. 11
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Shoving Causes Cures Unstable Mix
Pavement Distress Shoving Causes Unstable Mix Braking, stopping or accelerating traffic Slippage between layers Cures Remove and Replace Mill and Overlay Thick Overlay The mix may be unstable because of low void content, poor aggregate structure, mix too soft or tender. Horizontal friction forces caused by braking or accelerating “push & shove” the asphalt surface. Shoving and corrugations occur when he applied forces exceed the shear strength of the asphalt surface or underlying layers. Shoving can occur when thin asphalt surfaces are paced over granular bases and subbases. Shoving and corrugations are primarily due to unstable asphalt pavement layers. SRF Consulting Group, Inc.
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Bleeding Causes Cures Too high asphalt content too rich a plant mix
Pavement Distress Bleeding Causes Too high asphalt content too rich a plant mix improperly constructed seal coat too heavy prime or tack coat Cures Sand blot Micro-surfacing Seal coat Bleeding or flushing, as it is sometimes called, is most often caused by too much asphalt in one or more of the pavement layers. A surface treatment that had too high an application rate, or that has lost its cover aggregate may also be responsible. Spillage of oils on the pavement can force the asphalt to the surface, as well. SRF Consulting Group, Inc.
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Polished Aggregate Causes Cures Soft aggregate Heavy traffic Seal coat
Pavement Distress Polished Aggregate Causes Soft aggregate Heavy traffic Cures Seal coat Micro-surfacing Thin Overlay The cause is particle wear on a microscopic level. See section on skid resistance for more information. SRF Consulting Group, Inc.
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Raveling Causes Cures Asphalt binder unable to hold aggregate in place
Pavement Distress Raveling Causes Asphalt binder unable to hold aggregate in place dusty aggregates segregation low in-place density aged asphalt binder Stripping Cures Seal Coat Micro-surfacing Thin Overlay A coating of fine dust on the aggregate means the asphalt sticks to the dust, not the aggregate. When the mixture is not homogeneous, “lean” spots occur where there is not enough asphalt binder to bind the aggregate in place. When the asphalt age hardens, it may not be “sticky” enough to provide the necessary adhesion. Low in-place density (not enough compaction) can mean insufficient cohesion of the mixture. Moisture damage can strip, or delaminate the asphalt from the aggregate surface. SRF Consulting Group, Inc.
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Potholes Causes Cures Raveling of cracks Moisture damage Freeze-thaw
Pavement Distress Potholes Causes Raveling of cracks Moisture damage Freeze-thaw Insufficient strength Cures Patch Potholes occur in periods of heavy rainfall, when water penetrates through cracks and softens the granular base course. In northern climates, potholes occur during the spring thaw. Cracked pieces of the pavement lose bond to surrounding asphalt surface, and are dislodged by traffic. Once started, the pothole grows rapidly. Fine materials is pumped through the cracks so that the underlying base support is weakened, resulting in removal of the adjacent material by traffic. Potholes can develop along any crack that occurs in a pavement. Longitudinal or transverse cracks can deteriorate at locations where the wheel continually crosses the crack. Since all unsealed cracks allow water to penetrate, base softening occurs which leads to either raveling adjacent to the crack or fatigue cracking on the edges of the crack. Once raveling or fatigue cracking occur, the result will probably be a pothole if the distress is not treated. Potholes can be minimized if good surface and sub-surface drainage is provided. SRF Consulting Group, Inc.
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Patch Deterioration Causes Cures Poor bonding to existing pavement
Pavement Distress Patch Deterioration Causes Poor bonding to existing pavement Improper compaction Poor materials Propagation of original distress Cures Remove and replace Re-patch Assess original distress Patching technique is important, including: 1) preparation of surface, 2) method of applying patch 3) compaction of patch 4) profile of patch relative to pavement surface Many patches are made without proper preparation of the surface to be patched, resulting in poor bonding. Trapping moisture, dust or dirt can cause poor bonding. The properties of the material used for the patch will also determine its durability, as will the construction techniques and quality. Applying a patch on a distressed surface without fixing the problem which caused the distress in the first place may result in a failure of the patch. SRF Consulting Group, Inc.
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Miscellaneous Distresses
Pavement Distress Miscellaneous Distresses Lane-to-shoulder Drop off Water Bleeding and Pumping SRF Consulting Group, Inc.
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Lane to Shoulder Drop off
Pavement Distress Lane to Shoulder Drop off Cause Differences in materials used for lane and shoulder Differential settlement Insufficient compaction of base Cures Wedge/leveling overlay Micro-surfacing Occurs when the outside shoulder settles as a result of pavement layer material differences. SRF Consulting Group, Inc.
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Water Bleeding and Pumping
Pavement Distress Water Bleeding and Pumping Causes Cracks Inadequate drainage Cures Full-depth patch Crack seal SRF Consulting Group, Inc.
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