Flexible Pavement Distress

Lecture 2

Alligator or Fatigue Cracking

series of interconnecting cracks caused by fatigue failure in BASE under REPEATED traffic LOADING, cracks propagate to surface

Block Cracking (Thermal Cracking)

BLOCK cracks divide asphalt surface, NOT load induced, mostly COLD shrinkage in hot mix asphalt

Binder in Flexible Pavement Distresses

• binder selection base on local day/night temperatures

• cracks can potentially heal b/c of binder

Joint Reflection Cracking from Concrete Slab

occurs on asphalt surface over a JOINTED concrete slab along transverse and longitudinal joints, where pavements are WIDENED, caused by moisture and movement in concrete slab

Lane/Shoulder Drop-off or Heave

difference in ELEVATION between traffic lane and shoulder, due to consolidation or settlement

Longitudinal Cracking

Longitudinal or traverse cracks to CENTERLINE, caused by poor construction, shrinkage, and reflective cracks, TOP down, most common distress in GA

Tire Contact Stresses

• vertical stress distributions = quite low

• traverse stress distributions = quite high

Radial vs Bias Ply Tires

• Radial (more common) = rigid tread, flexible wall

• Bias Ply = flexible tread, rigid wall

Water Bleeding

water seeps out of joints/cracks/porous HMA layer

Water Pumping

water and fine material is ejected from underlying layers by moving loads

Water Bleeding/Pumping Cause

porous pavement as a result of inadequate compaction during construction (paver & compactor must be closer together), high water table, poor drainage

Rutting (Consolidation/Instability)

depressions that form in the wheel paths

Causes

• rounded aggregate (Mississippi)

• low air voids

• insufficient thickness

• soft binder

Consolidation Rutting

repetitive application of wheels loads due to reduction in AIR voids or permanent DEFORMATION of subgrade/base

Asphalt/Traffic Speed Relationship

• stiff at high speeds

• relaxes at slow/stopping speeds

Instability Rutting

material issue within first 2 inches, asphalt gets “SHOVED” over

Binder Bleeding

excess asphalt BINDER on surface of pavement, not enough air voids

Binder Bleeding Causes

• excess prime or tack coats

• excess asphalt mixture

• drain-down

• contamination with diesel

Slippage

crescent or half-moon shaped cracks, 2 ends points into the direction of traffic, cause by thin layers & issues with tack coat

Swell

upward bulge caused by swelling soils or PCC blowup

Corrugation

ripples at traffic acceleration zones

Depression

localized elevation difference due to soil or built up

Potholes

broken pavement due to fatigue cracking or freeze/thaw

Ravelling and weathering

stripping of aggregates