RAB Associates have prepared the following guide to "Bituminous Road Construction Defects: their causes, repair and renovation techniques". We hope you find the information of value and would welcome any feedback. If you wish to discuss anything further please do not hestitate to call us on 01629 814589 or email mike@rab-associates.co.uk
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: click here to see larger picture)

Sub-Grade:

  • Natural soil or fill material

The Sub-base:

  • Good quality granular material

The 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: click here to see larger picture)
Causes of Failure:

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. (See Fig 2 above).

"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 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": 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. Fig 3 below refers. (Need a solution for Thermal Cracking now? Then click here to jump straight to the solution).
(Fig 3: click here to see larger picture)

Load Associated Cracking:

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. Fig.4 refers below. (Need a solution for Load Associated Cracking now? Then click here to jump straight to the solution).

Reflective Cracking:

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. Fig.5 refers below. (Need a solution for Reflective Cracking now? Then click here to jump straight to the solution).

Shear Stresses:

Fig. 4 above refers. Vertical movements between concrete slabs induced by traffic loadings will result in shear, as will vehicle breaking, accelerating and tight radii turning. (Need a solution for Shear Stress Cracking now? Then click here to jump straight to the solution).

Winter Warping:

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. Fig 6 refers below. (Need a solution for Winter Warping now? Then click here to jump straight to the solution).

Permanent Deformation (Rutting):

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

Fig 7 refers below. (Need a solution for Rutting now? Then click here to jump straight to the solution).
Available 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

RAB Associates recommends "Permanite SAMI". Click here to read how RAB Associates specified and installed the SAMI solution on the A45 recently. To enquire further about the SAMI system please call 01629 814589 or click here to email us.

Reflective Cracking - The Solution

Permanite Permatrack "H" Inlaid System

Suitable For:

  • load associated cracking
  • shear stress
  • defunct roadbase

For those major severe cracks that have traveled through the entire roadbases.

Click here to 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. To enquire further about the Permatrack H Inlaid System please call 01629 814589 or click here to email us.

Rutting - The Solution

Permanite Permatrack Infill (with optional anti-skid)

Suitable for rutting caused by:

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

To enquire further about the Permatrack Infill System please call 01629 814589 or click here to email us.

RAB Associates offer:

  • Site Visit
  • Consultancy
  • Design
  • Technical Specification
  • Contracting Services

Throughout the UK


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