20 Defects in bituminous roads you should know

Defects in bituminous roads are inevitable in bituminous roads. The following are the major defects associated with paved flexible pavements. The names in parenthesis against each defect are the various names by which the defect is also called or known.

1. Alligator crack

(crocodile crack, alligator crazing, fatigue crack, map crack, chicken wire crack)

Alligator crack is a crack pattern which forms as a series of interconnected, closely spaced sharp-angled, many-sided cracks which resembles  chicken wire or the skin of an alligator. 

Alligator cracks in asphalt pavements begin at the bottom of the asphalt surfacing where tensile stresses and strains are the  highest under a wheel load and spread to the surface. 

Initially,  the cracks appear as a series of longitudinal cracks which after repeated traffic loading then join up to form many-sided crack patterns.

Causes
Alligator cracks may be caused by

  • fatigue failure of the asphalt concrete layer  under  repetitive axle load applications.
  • overstressing of pavement structure due to excessive axle loads
  • inadequate structural capacity of pavement
  • shear failure in the underlying pavement layers due to insufficient layer strength, poor compaction or inadequate drainage.
  • loss of visco-elastic property of  the asphalt concrete due to embrittlement or age-hardening

2. Block crack (Ladder crack)

Block cracks develop when a series of longitudinal and transverse cracks interconnect to form an  approximately rectangular or square pattern with block sides sometimes exceeding 2 m in extent.

Causes
This pavement distress is caused mainly by

  • shrinkage of  the pavement sub-grade reflecting through a thin road construction
  • thermal contraction and expansion of the asphalt concrete layer
  • fatigue cracking in aged or  embrittled asphalt concrete.
  • use of hard asphalt binder in asphalt concrete mix
  • use of poor quality materials
  • poor construction

3. Longitudinal crack (Line crack)

This is a crack that runs approximately parallel to the direction of the roadway. It often occurs at the joint between adjacent lanes of the asphalt layer  and may occur as a series of parallel lines on the pavement surface.

Longitudinal crack in roads

Causes
Longitudinal cracks may be caused by

  • lack of  lateral support in embankment fill
  • volume changes in clayey or expansive sub-grade soils
  • differential settlement between cut and fill that reflects through the asphalt layer
  • poorly constructed lane joint in asphalt pavement
  • displacement of joint at point of pavement widening
  • reflection of joint crack in a rigid or an old asphalt pavement that has been overlaid.

4. Transverse crack

(Low temperature crack, thermal crack, contraction crack)

This is a crack that runs across the whole or part of the road cross-section perpendicular to the roadway direction. When there are many, they usually tend to be approximately equally spaced.

They normally occur when the temperature at the surface of the pavement drops sufficiently to produce thermally-induced shrinkage stress in the asphalt concrete layer that exceeds the tensile strength of the asphalt mixture.

Thermal cracks usually initiate at the top of the pavement and propagate downward through the asphalt concrete layer.

Causes
These cracks are caused by

  • thermal expansion and contraction of the asphalt concrete
  • asphalt age-hardening which causes binder embrittlement and loss of visco-elasticity of the asphalt concrete layer
  • use of hard asphalt binder in asphalt concrete mix
  • overheating of asphalt concrete mix during hot-plant manufacture
  • use of poor quality materials
  • poor construction

5. Slippage crack (also parabolic crack, shear crack)

This is a  crescent (half-moon) or U-shaped crack which develops in areas where vehicles brake, turn or accelerate.

The characteristic feature of this crack is that the top of the U always points in the direction that the forces are being applied.

Thus when the crack is caused by braking, the top of the U will point in the direction of travel whereas a crack caused by acceleration will have the top of the U pointing away from the direction travel.

Causes
The defect occurs when

  • soft binder-rich surface mixes are subjected to high stresses from braking  and acceleration.
  • a poor bond exists between the surface course and the underlying  base course
  • thin wearing courses are laid
  • the base course material has low strength (modulus)

6. Edge crack

This is a longitudinal crack that develops close to the edge of a pavement and which sometimes is accompanied by transverse cracks towards the shoulder. In some cases, settlement may occur within the narrow strip of pavement between the edge crack and the edge of the pavement.

Causes
Edge cracks may be caused by

  • lateral movement of the pavement material due to insufficient shoulder support especially in areas of high embankment fill
  • settlement or yielding of sub-grade or base material close to the edge of the pavement.

7. Reflection crack

This is a crack that develops when discontinuities or cracks in an overlaid asphalt concrete pavement propagate through the new overlay to the surface.  Such cracks may therefore  show as longitudinal,  transverse or any other type of crack pattern.

Causes
Reflection cracks may be caused by

  • Cracks or joints in an underlying rigid pavement
  • Low temperature cracks in an overlaid old asphalt concrete layer
  • Longitudinal cracks in an overlaid old asphalt surface
  • Fatigue cracks in the old asphalt surface

8. Pothole (also surface hole, chuck-hole)

This is a small (usually less than 0.9m diameter) bowl-shaped hole in the pavement surface with sharp edges and vertical sides near the top of the hole.

Its growth is accelerated by water collecting in the hole and traffic breaking the edges causing the hole to have a wider diameter with passage of traffic. 

Pothole on road

Causes
Potholes are caused by

  • abrasion and dislodgment of  surface material by the action of traffic
  • asphalt binder embrittlement and subsequent stripping or raveling of asphalt pavements.
  • localized shear failure of the pavement base due to improper compaction, improper aggregate gradation, use of unsuitable material
  • Infiltration of water into pavement

9. Bleeding (also sweating, fatting up, flushing)

This is the formation of a film of bitumen on a paved road surface, creating a shiny, glass-like, reflecting surface which becomes sticky during hot weather and slippery during wet weather.

Under hot weather conditions, it may cause passing automobiles to leave an imprint of their tyres on the pavement surface.

The defect is traffic-related and occurs mainly in the wheel tracks and may sometimes cause the tyres of passing vehicles to sound as if they are riding on a wet pavement surface.

Causes
Bleeding is caused  by

  • unstable mix or excessive amounts of binder in the asphalt concrete
  • low air void content in the asphalt concrete layer
  • stripping of aggregates
  • use of soft asphalt binder in asphalt concrete formulation
  • spillage of petroleum products onto the pavement surface
  • excessive application of tack coat

10. Shoving (also pushing)

This is a permanent longitudinal displacement of a localized area of asphalt pavement surface normally occurring in soft binder-rich surfacing materials and at locations of the pavement where heavy vehicles brake or make sharp turns. 

The defect may also occur in locations of the pavement where heavy commercial or truck traffic  slow down significantly below highway speed. The defect is almost always associated with slippage cracks and is easily detected at the edge lane markings where these lines bend outward in the vicinity of the defect

Causes
The defect may be caused when

  • the asphalt concrete pavement lacks adequate stability
  • the applied forces on the pavement surface due to braking or acceleration exceeds the shear strength of the asphalt concrete or the underlying layers
  • a thin asphalt concrete overlay is placed over granular bases and sub-bases.
  • the bond between an asphalt wearing surface and the binder course is poor or has been broken.

11. Corrugations (Ripples, washboard)

Corrugations are surface defects which manifest as transverse undulations that occur at fairly uniform  intervals along the surface of the pavement.

They are characterized by alternating ridges and depressions in a wave-like manner usually about 1-3m apart which if closely spaced may cause passing vehicles to vibrate.

Corrugations are prevalent on unpaved roads and on paved roads occur at locations or pavement sections such as at intersections where vehicles brake or accelerate.

Causes
Corrugations may be caused by

  • tender mix or lack of stability of asphalt concrete due to excess asphalt content or  use of soft binder
  • use of mix with excessive amounts of rounded aggregates or excessive fines
  • braking and acceleration actions of vehicles particularly at intersections
  • wavy compaction at the time of placement of asphalt mat
  • uneven spread of gravel layer and subsequent wavy compaction in gravel roads

12. Ravelling (Weathering, fretting, pitting, disintegration)

This is a pavement defect characterized by the progressive disintegration or the wearing away of the asphalt concrete layer from the surface downward due to dislodgement of  aggregate particles from the asphalt concrete matrix.

The ravelled surface has open rugous (pitted)  appearance and is characterized by loose debris on the surface of the pavement in the immediate vicinity of the distress.

Causes
Ravelling may be caused by

  • abrading of asphalt films from aggregate particles that were insufficiently or poorly coated with asphalt during mix production.
  • segregation of the asphalt concrete matrix at the time of placement of the asphalt mat so that there is a concentration of coarse aggregate without sufficient fines to aid continuous inter-particle contact and bonding.
  • poor mat compaction resulting in low asphalt concrete density
  • asphalt age-hardening and embrittlement leading to brittle fracture of asphalt films and subsequent dislodgement of coated aggregate from asphalt concrete matrix under traction forces.
  • use of high dust to asphalt ratio in asphalt concrete mix formulation.
  • use of hard grade asphalt for asphalt concrete manufacture.
  • low asphalt content in asphalt concrete.

13. Rutting

A rut is a longitudinal depression in the wheel tracks often accompanied by pavement uplift along its sides when pavement materials are displaced.

Causes
Rutting may be caused by

  • permanent deformation in any of the pavement layers or sub-grade as a result of continuous wheel tracking over a narrow width of the pavement.
  • continued densification of pavement layer materials  under traffic after initial compaction or as a result of plastic deformation of surfacing materials subjected to repeated trafficking or channelization.
  • insufficient compaction of pavement layers during construction.
  • improper asphalt concrete mix design for asphalt pavements
  • use of excessive amounts of rounded aggregate in asphalt concrete mixes in asphalt pavements
  • use of soft bituminous binder in asphalt concrete formulation
  • inadequate structural capacity of pavement.

14. Shear failure

This defect is the collapse of a component of the pavement structure in shear due to stresses exceeding the shear strength of the layer. The distress is recognizable by a depressed area and an up-heaved area.

Causes
Shear failure may be caused by any or a combination of the following

  • excessive wheel loads
  • inadequate base/sub-base compaction during construction
  • weak sub-grade layer
  • use of poor quality materials in pavement layers
  • inadequate pavement drainage

Edge failure

This is the progressive disintegration  or breakage of the pavement  structure from the  edge inwards, for paved roads usually where there are no kerbs and where vehicle over-run is common,  such that pieces of the pavement are broken, become loose, are dislodged and then displaced resulting in the reduction of the pavement across its width.

Causes
Edge failure may be caused by

  • lack of lateral support at the edge of the pavement in embankment zones
  • insufficient base thickness
  • base failure
  • excessive loads applied at the edge of the pavement
  • inadequate compaction at the edge of the road
  • inadequate  or poor drainage
  • narrow carriageway encouraging vehicle overrun at the edges

15. Depressions (sag, settlement)

Description

A depression is a localized pavement defect characterized by a grade that is slightly lower than that of the surrounding area and which highlights  the presence of a soft spot in the underlying pavement structure particularly in the sub-grade or a poorly compacted reinstatement of utility opening.

Causes
A depression may be caused by

  • failure of a soft spot in the underlying pavement layers
  • consolidation of a soft spot in the sub-grade
  • poor compaction after re-instating utility opening

16. Loss of skid resistance (Polishing, wear loss, loss of texture)

This is a surface defect characterized by loss of surface texture or lack of adequate friction between the pavement surface and tyres of vehicles to the extent that the surface looks shiny and becomes slippery during wet weather.

The defect is traffic-related and may sometimes cause the tyres of passing vehicles to sound as if they are riding on a wet pavement surface.

Causes
Loss of skid resistance may be caused by any or a combination of the following:

  • bleeding of the asphalt concrete surface.
  • polishing of surface aggregates by traffic action when aggregates with low polish or wear resistance are used in asphalt concrete.
  • use of small size aggregate and excessive fines in asphalt concrete mix.
  • loss of surface texture due to the embedment of coarse aggregates in asphalt matrix as a result of traffic action especially where soft or binder-rich mixes are involved.
  • use of rounded or naturally worn aggregates in asphalt concrete.

17. Loss of cover aggregates

De-bonding or breakage of surface dressing aggregates such that the waterproofing bituminous layer becomes prone to damage by vehicular action.

Causes
Cover aggregates on surface dressings may be lost by any of the following

  • use of dusty aggregates for the dressing
  • lack of strong bond between aggregates and bituminous binder
  • use of insufficient binder for dressing
  • erratic distribution of binder
  • binder embrittlement due to age-hardening
  • poor quality surfacing

18. De-lamination

This is the separation of the wearing course of the pavement from the underlying layer with subsequent loss of a discrete and large area of the wearing course at the affected area.

This is a surface defect that affects the structure of the asphalt pavement. This usually occurs where a very thin (veneer) layer of asphalt wearing course has been placed over a base course or a rigid pavement.

Causes
De-lamination may be caused by

  • loss of bond between the asphalt wearing course and the underlying asphalt base course.
  • the existence of a thin layer of dust on the binder course at the time of placement of the wearing course.
  • seepage of water through cracks to undermine bond between wearing course and underlying layer
  • the presence of weak and loose layer immediately underlying wearing course

19. Stripping

This is an asphalt pavement distress characterized by the loss of bond between the aggregates in an asphalt concrete matrix due to the displacement of the asphalt coating on the aggregate surface by water.

The defect typically begins at the bottom of the asphalt concrete layer and progresses upwards. 

The distress may sometimes be very difficult to identify because it can manifest as rutting, shoving, corrugations, raveling or cracking but is typically and visibly evidenced by partially coated or uncoated aggregates.

Causes
Stripping may be caused by any or a combination of the following factors:

  • presence of water in aggregates during asphalt concrete manufacture.
  • use of hydrophilic aggregate in asphalt concrete.
  • infiltration of water into the asphalt concrete layer.
  • poor drainage of the pavement base course.
  • poor compaction of asphalt concrete resulting in an open-textured surface.

20. Adverse camber

This is an irregular transverse profile of the carriageway where the camber or the cross fall is steeper than the current design parameters.

Causes
Adverse camber may be caused by

  • overlaying or resurfacing of the middle portions of a pavement excluding the edges in order to maintain kerb upstand
  • subsidence
  • widening or re-aligning a road without subsequent re-profiling

Conclusion- Defects in bituminous roads.

These are the most common defects in bituminous roads that you will most likely come across.

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