Rutting in a Pavement Structure – Rutting is a pavement distress mechanism that is a distress mechanism fluent with public road users.
It is a pavement distress mechanism that can significantly affect the ride-ability, pavement integrity and safety – a common symptom of ‘aqua planing’ on a road surface.
Rutting is typically also followed by surface failure in the form of fatigue cracking in asphalt and embedment in spray seals.
Typically rutting develops insitu from repetitious wheel loading which incurs permanent damage each pass. The contributing factors for rutting include:
- Subgrade Failure –This is considered the dominant cause of rutting and is generally a result of consolidation and or/settlement of the subgrade under repetitious loading which results in a ‘domino like’ effect, reflecting through the overlying pavement layers. The current mechanistic design procedure allows for subgrade rutting which estimates the allowable cumulative load induced critical strain repetitions based upon insitu subgrade support until the subgrade ‘fails’. The common method of reducing the susceptibility in a subgrade material is to provide additional cover (thicker pavement) so as distribute the load further and reduce the critical strain under each repetitious wheel movement.
- Mix Instability – Whilst theoretically, rutting can only occur in the subgrade, it is recognized that it can also occur in the unbound granular material and asphalt. This theory is based on the concept that the mix composition is ‘stable’ to resist compressive failure under repetitious wheel loading. This is particularly focussed around the intrinsic stone durability, crush strength, Particle Size Distribution and binder and filler content (in asphalt mixes).
It is recognized that old granular materials can be subject to rutting given that the stone material can deteriorate over decades of trafficking and fluctuating moisture conditions which can significantly reduce the structural capacity of the material. Where the addition of moisture is involved, unbound granular materials can also rut internally from segregation as a result of a build-up of pore-water pressure.
So as to reduce the likelihood of load induced rutting caused from material ‘instability’ within the pavement structure specifications and recommendations have been developed for mix compositions, crush strengths, durability, binder and filler contents so as to provide an expected level of service based on the material’s physical properties.
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