Pavement failures [All the Essential things you should know]

Pavement failures

Pavements begin to deteriorate when opened to traffic after construction as a result of the cumulative effect of traffic loading and environmental factors. 

The figure below provides a pictorial representation of the state of a pavement defined by its present serviceability index (PSI) at any given time.

State of pavement

The present serviceability index  (PSI) is a qualitative rating of the current surface condition of a pavement. Newly constructed roads can be expected to have PSI values in the range 4.5-5.0  whereas roads in poor condition have values below 2.0.

As indicated in the figure, the deterioration of a road is generally gradual in the first few years but begins to proceed at a much higher rate with time.

With timely intervention in the form of pavement maintenance, it is possible to return the service condition of the road almost to the as-constructed state.

Pavement failure will manifest visibly in the pavement, at one time or the other,  as a defect in the structure or the surface or both. The defects affect the serviceability, appearance and structural capacity of a road.

Failure modes

There are three major failure modes that can be associated with pavement defects. These are:

  • Structural deformation
  • Cracking
  • Surface defects

Each of these failure modes has its own peculiar characteristics and manner of impacting the performance of pavements.

It must be emphasized, however, that it is not always the case that given pavement distress can be ascribed to one particular mode of failure since in general one mode of failure may induce or lead to other modes.

The type of pavement may affect the preponderance of one mode of failure over the others.

For example, whereas in rigid pavements cracking of concrete slabs is a predominant mode of failure, in unpaved roads the major failure modes are structural deformation and surface defects with cracking being virtually absent.

When related to the function of the road, the mode of failure will determine whether the failure can be described as a structural failure or a functional failure

Structural failure refers to all failures that include a collapse of the pavement structure or a breakdown of one or more of the pavement components of such magnitude as to make the pavement incapable of sustaining loads imposed upon its surface.

Functional failure on the other hand refers to all types of failures which will not allow the pavement to carry out its intended function without causing discomfort to passengers or without causing high stresses in the vehicle that passes over it due to roughness. 

a. Structural Deformation

To a very large extent, a structural deformation mode of failure has the highest potential to incapacitate a pavement structurally since it may cause complete collapse of one or more of the components of the pavement.

b. Cracking

This mode of pavement failure is associated more with paved than unpaved or gravel roads. In paved roads (mostly asphaltic and rigid),   cracking provides evidence of fatigue failure and/or failure under tensile stresses of the surfacing material.

Cracking rarely appears in unpaved roads and only so mostly in cases where fills in embankments undergo lateral movement and subject the soil material to tensile stresses and/or where layer materials undergo shrinkage and develop shrinkage or desiccation cracks.

c. Surface  defects

Pavement distresses that manifest as surface defects generally affect the ride-ability of the pavement and in some cases also the structural performance of the pavement.

Remedial measures for such defects are mostly confined to the surfacing and may not require any structural changes to be made to the pavement.

Characteristics of pavement failures

The following attributes describe the characteristics of   pavement failures

Location

The location of a distress is given in terms of two points along the length of the pavement. These points delineate a pavement section to which the subsequent description of distress pertains.

Generally for long sections, the description of distress is not precise. The pavement may then be divided into various short sections to allow for a more accurate description.

Modes and types

There are three ways in which distress occurs. These are called the modes of distress and they are; structural deformation, fracture or cracking, surface defects.

Each of these failure modes has its own peculiar characteristics and manner of impacting the performance of pavements.

It must be emphasized, however, that it is not always the case that given pavement distress can be ascribed to one particular mode of failure since in general one mode of failure may induce or lead to other modes.

The type of pavement may affect the preponderance of one mode of failure over the others.

For example, whereas in rigid pavements cracking of concrete slabs is a predominant mode of failure, in unpaved roads the major failure modes are structural deformation and surface defects with cracking being virtually absent.

Degree and  extent

The degree of a particular type of distress is a measure of how bad it is at a particular point on the pavement. For example the degree of cracking can be a function of the width of the cracks.

Position and spacing

The position of a distress is defined by the coordinates on the pavement with respect to either edge or the centerline and the chainage. The spacing of distress is an additional parameter indicating the geometry of the distress.

Its definition varies for the different types of distress but it always represents a linear measure. For example, for transverse cracks, the spacing indicates the average distance between the cracks.

Causes of pavement failures

Pavements begin to deteriorate when opened to traffic after construction as a result of one or more of the following:

a. Traffic loading

Flexing of pavement under repeated loading leads to fatigue, cracking and structural failure over weak subgrades and where the depth of carriageway construction is inadequate to provide sufficient spread of loads from vehicles.

b. Breaking and turning wheels

These cause the pavement surface to slide or deform and surface material is progressively lost due to abrasion.

c. Temperature

Extreme temperatures between day and night may cause thermal movement that tends to cause cracks to develop in a pavement surface.

High temperatures can cause stability problems in asphaltic pavements leading to rutting, ripple formation, corrugation, the flow of surface material, and in rigid pavements cracking, spalling, and blow-ups.

d. Moisture movements

Water is the greatest enemy of a pavement. When moisture has access into a pavement layer it weakens the shear strength of the layer materials and reduces the load-carrying capacity.

Moisture movement into the underlying pavement structure may also cause volumetric changes such as swelling and shrinkage that can lead to shrinkage or swelling stresses with the potential to cause cracks or heaving in both rigid pavements and structural asphalt layers.

e. Poor quality materials and construction

Excessive movement of flexible pavement which eventually results in uneven riding quality may be caused by poor quality pavement materials and poor construction.

f. Weathering

This leads to chemical changes in asphaltic concrete and Portland cement concrete and subsequent embrittlement especially in asphalt binder leading to cracking and progressive loss of matrix.

g. Rainfall

For unpaved roads, rainfall apart from causing weakening of pavement layer materials  causes the development of erosion gullies through the progressive washing away of materials from the surface by erosion.

Erosion gullies may be particularly pronounced on sections with steep slopes where because of the slope angle being greater than the surface camber surface runoff moves longitudinally along the road or diagonally across the road instead of transversely to the side ditches and drains.

i. Reinstatement of public utility openings

Digging across a pavement in order to install or access public utility services  is one of the greatest causes of pavement failures or surface irregularities.

Backfilling and compaction of the opening are never well done leading to lateral as well as vertical movements in adjoining areas so that in time the damage caused by the opening becomes extended well beyond the width of the original opening.

Defects in Bituminous roads

This has been discussed in my earlier post titled, “20 Defects in bituminous roads you should know

Defects in rigid pavements

Longitudinal crack
Longitudinal cracks are planes or lines of discontinuities that manifest in the rigid pavement surface as thin lines that run approximately parallel to the longitudinal axis of the pavement.

Causes
The cracks are generally caused by the same factors that cause the formation of cracks in general and include

  • contraction and expansion of  the concrete slab
  • warping stresses in concrete developed from temperature changes
  • inadequate sub-grade support
  • inadequate steel reinforcement
  • excessive  slab length
  • inadequate slab thickness
  • low concrete strength
  • lateral movement of base support or layer
  • expansion and shrinkage of base material

Transverse /diagonal crack(Contraction crack, thermal crack)
This  is a crack which runs across the whole or part of the rigid pavement slab transverse or diagonal to the roadway direction.

Transverse cracks normally occur when the temperature at the surface of the pavement drops sufficiently to produce a thermally-induced shrinkage stress in the rigid pavement slab that exceeds the strength of the slab.

Causes

  • contraction and expansion of  the concrete
  • warping stresses in concrete developed from temperature changes
  • inadequate sub-grade support
  • inadequate steel reinforcement
  • excessive length of slab
  • inadequate slab thickness
  • inadequate concrete strength

Faulting (Stepping, step-fault, step off)
This is a differential vertical displacement oftentimes at joints and cracks between abutting slabs resulting in step deformation.

Causes

  • differential settlement of adjoining slabs
  • loss of sub-base material through pumping
  • poor sub-base or sub-grade support
  • curling or warping of slab due to temperature gradient across the depth of the slab

Spalling
This is the breaking into angular pieces of concrete material from the edges of cracks, joints, and corners of the concrete slab or the flaky breakage of material from the surface of the slab.

Causes
Spalling may be caused by

  • use of poor quality concrete
  • corrosion of reinforcement bar
  • loading at the edges of joints and cracks

Blow-up (Buckling)
This is a localized upward movement of rigid pavement slabs that occurs at a transverse joint or crack. The occurrence of the defect is mostly accompanied by transverse edge shattering.

Causes
Blow-ups may be caused by

  • expansion of adjacent slabs under hot weather conditions
  • inadequate provision for expansion joints between slabs

Pumping (Blowing)
This is a process accentuated bypassing traffic by which a mixture of water, clay, and fine-grained soil is pumped out or exudes from underneath a rigid pavement slab through cracks,  joints, and the edges as the slab is repeatedly caused to deflect downwards by passing vehicles.

Causes
Pumping may be caused by

  • infiltration of water into sub base through cracks and joints in the concrete slab without the possibility of drainage
  • high fines content in sub-base material

Scaling (Ravelling, abrasion, disintegration)
This is the progressive loss of mortar and aggregate from slab surface

Causes
This surface defect may be caused by any or a combination of the following;

  • use of poor quality aggregate
  • inadequate curing of concrete
  • poor concrete quality
  • overworking of surface during construction

Defects in unpaved roads

The following are the major defects or failures associated with unpaved roads.

Rutting
A rut is a surface defect and is characterized by 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 the gravel layer under traffic
  • insufficient initial compaction of pavement layers during construction.
  • loss of gravel material from within wheel tracks
  • inadequate structural capacity of pavement
  • loss of strength due to inadequate drainage

Erosion gullies
These are channels that run almost longitudinally or sometimes diagonally on the surface of gravel roads created by the erosion of surface material by rainwater running on the surface of the gravel or earth road. Erosion gullies most often follow wheel tracks and characteristically turn diagonally to join side drains.

Causes
Erosion gullies are caused by

  • the erosion of surface material by rain water running on the surface of the gravel pavement.

Slippery Surface
This is a surface defect characterized by slipperiness during wet weather causing danger to vehicles on steep slopes and sharp curves.

Causes

  • use of clayey soil in forming base course
  • complete loss of gravel cover on clayey sub-grades

Loose gravel
This is a surface defect characterized by the presence of loosely compacted gravel or gravel in windrows along the center of unpaved roads, wheel tracks, and/or along the shoulders in parallel orientation to the direction of traffic.

Causes
The defect is caused by

  • inadequate compaction of the gravel layer
  • use of gravel materials with insufficient fines to bind the coarse particles during placement and compaction
  • segregation of coarse aggregates from fine particles by traffic action and subsequent whipping of coarse aggregates to the centre and roadsides.

Defects in shoulders and roadsides

The following are the major defects associated with pavement shoulders and roadsides.

Shoulder drop-off
This develops when there is an elevation difference between pavement edge and shoulder such that the riding surface of the pavement stands high above the shoulder level.

Causes
This defect may be caused by

  • erosion of the shoulder if it is unpaved
  • settlement of the shoulder relative to the pavement
  • building up the pavement without due adjustment to the level of  the shoulder

Excessive shoulder height
This is a defect typical of gravel roads and characterized by the shoulder having an accumulation of gravel such that the road edge is lower in elevation than the shoulder. This defect may be seen as the opposite of a  shoulder drop-off.

Causes
This pavement defect may be caused by any of the following

  • formation of gravel wind-row on the shoulders of pavements that were formed with materials containing excessive amounts of gravel
  • deposition of detritus and eroded material on the shoulder by rainwater
  • poor construction and improper maintenance

Vegetation overgrowth (Vegetation encroachment)
This is the growth of vegetation on the sides of the roadway such that vegetation or weeds overrun the shoulders and encroach upon the edge of the roadway sometimes causing a reduction in sight distance.

Causes
Vegetation encroachment results when

  • there is lack of routine maintenance

There are long intervals between periods of grass cutting particularly in areas with high rainfall and rapid vegetation growth.

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