RAB Associates

Bituminous Road Construction Defects:
Their Causes, Repair and Renovation Techniques

General Note: In order to design, or even assess, a proposed long lived repair technique, we need to understand some common major influences that cause premature failure of the roadway. These failures often start as being small in area, but unless quickly and correctly treated will ultimately lead to extensive and expensive total or partial major re-lay cost and traffic delays.

Overview

Design: UK highways are, in general, constructed in a simple layered structure, designed to distribute traffic loads efficiently, whilst minimizing the whole-life-cost of the road.

Fig. 1: Structural Layers in a Bituminous Pavement

• Sub-Grade:
  Natural soil or fill material
• Sub-Base:
  Good quality granular material
• Base Course:
  The base course being the main structural element, it is designed to distribute imposed loadings in order to prevent over stress of the underlying material. It should have a good stiffness and may act as a regulating layer. It should be structured to resist permanent deformation and fatigue cracking, which are caused by repeated loading, of any nature. Constructed usually of a dense bituminous material, but could be a weak concrete or cement bound material, it should be as impervious as possible.
• Surfacing:
  Comprises of two layers, lower level termed.
• Binder course:
  Its purpose is to help distribute traffic loads over the base course. Its thickness is aprox 60mm. and profiled to support the surfacing layer.
• Surfacing Layer:
  Must fulfill several requirements:
  • Resist deformation by traffic
  • Be Impervious; to protect the lower layers of the structure (exception to this is the use of porous macadam)
  • Be durable in resisting the effects of weather, abrasion and fatigue
  • Provide skid resistance and contribute to the pavement structure
  • Provide acceptable ride qualities

Failure Modes And Critical Strains In Bituminous Roads

Fig. 2: Failure and Critical Strain in Asphalt Pavements

Causes of Failure:

• Horizontal Tensile Strain in Bituminous Layer [See Fig. 2]
  The principle criterian for asphalt fatigue is the horizontal tensile strain at the ‘bottom’ of the bituminous layer. For Subgrade deformation it is the vertical strain at the top of the subgrade.
• Tensile Strength
  An important engineering property of bitumen is its breaking point. The tensile properties of bitumen are related to its stiffness modules, which in turn are inter-dependant on the ambient temperature. In practice, bitumen breaking point takes place under conditions of large stress, normally occurring at low temperatures, ie. materials are at “high stiffness” when the bitumen is at its lowest elasticity capabilities.
• Cracking & Splitting
  Cracking and splitting of the asphalt layer arises from repeated tensile strains. The maximum strains occur at the bottom of the bituminous layer as shown in the above drawing. The crack, once initiated, propagates upwards causing weakness to the structure allowing water entry, followed by freeze-thaw-damage.
• Thermal Cracking [See Fig. 3]
  During periods of cooling, bitumen mixes try to contract. However, asphalt within a road structure is effectively constrained and therefore cannot change its length. As a result, thermal stresses develop. Coupled with traffic imposed stresses, these ‘loads’ may exceed the breaking strength of the bitumen, resulting in a transverse thermal crack.
  See the solution below this page

Fig. 3: Plan of Typical Road Construction Showing Bay Layout

Critical Strains:

• Load Associated Cracking [See Fig. 4]
  When loaded, a bituminous pavement is subjected to tensile stresses. Cracks at the bottom of the bituminous layer can result if:
  • the stresses exceed the tensile strength of the asphalt mix
  • the structure is inadequate for the loadings
  • the subgrade changes through water ingress

  Repeated loadings will cause the crack to move upwards to the road surface.
  See the solution below this page

  

  Fig. 4: Traffic Induced Fatigue

• Reflective Cracking [See Fig. 5]
  Reflective cracking can occur when the daily thermal expansion, or contraction, of the cementitious under layer takes place. These movements induce high tensile strains in the surfacing immediately above cracks or joints. These ‘live’ cracks propagate upwards to the surface. However, cracks can also be initiated at the surface and move downwards to meet the cracks in the cement bound layer in addition to thermal induced stresses.
  See the solution that follows
  

  Fig. 5: Thermally Induced Fatigue

• Shear Stresses [See Fig. 4]
  Vertical movements between concrete slabs induced by traffic loadings will result in shear, as will vehicle breaking, accelerating and tight radii turning.
  See the solution below
• Winter Warping [See Fig. 6]
  During winter the upper surfacing, being much colder than the underlying layers, can cause a differential contraction. This allows the concrete slab to warp which induces tensile stress at the surface.
  See the solution that follows
  

  Fig. 6: Winter Warping

• Permanent Deformation (Rutting) [See Fig. 7]
  These types of problems maybe associated with any individual or combination of the following occurrences:
  • ‘Penetration Index’ of the bitumen was too low
  • bituminous mix had a low stiffness property
  • poor compaction at the laying stage
  • vertical compression stresses overcame the bearing capacity of the structure

  See the solution that follows

  

  Fig. 7: Surface Initiated Cracking

Available Remedial Repair and Renovation Techniques

All of the following materials and techniques have been tested not only in the laboratory but under monitored field trails throughout the UK. We believe that they represent long term solutions to all the problems and failures presented above.

• Thermal Cracking: The Solution

  Permanite SAMI

  Suitable for:

  • new tarmac type overlays
  • about to be placed over concrete slabs
  • planed and prepared Bitumastic Mixes

  Read how RAB Associates specified and installed the SAMI solution on the A45.

• Reflective Cracking: The Solution

  Permanite Permatrack “H” Inlaid System

  For those major severe cracks that have travelled through the entire road bases. Suitable for:

  • load associated cracking
  • shear stress
  • defunct roadbase

  Read how RAB Associates specified and installed the Permatrack H solution on the A454 Black County Road when its overbridges were suffering from cracked and leaking transition joints.

• Rutting: The Solution

  Permanite Permatrack Infill (with optional anti-skid)

  Suitable for rutting caused by:

  • sub-base movement
  • excessive compression stresses
  • poor compaction

Please visit our contact page for details on how to discuss your requirements further.