Differential Stiffness Distress – The stiffness characteristic of a pavement material is a key indicator into it performs under repetitious wheel loads.

It can also affect the likely failure mechanisms and alter the load transfer geometry of a pavement composition, particularly if the material is changing from unbound to bound in nature.

The concept of stiffness within a bound material is developed from the relationship of deflection of the material under a nominated load within its elastic zone.

The pavement stiffness is typically based on a number of attributed including:

– Material Composition (volume of binder, filler and aggregate)

– Binder Type

– Material homogeneity and uniformity

Aside from affecting the load transfer geometry in materials, stiffness can contribute to premature pavement distress through the concept of ‘differential stiffness’. This concept is most always evidenced around the interface of a flexible pavement configuration overlying a rigid material or a piece of rigid infrastructure (culverts, bridges, causeways etc).

Where materials with significantly different stiffness’s longitudinally join each other, the lateral movement of the more flexible material can result in transverse tensile cracking, particularly in areas subjected to considerable trafficking. Whilst typically these constraints are typically unavoidable, a range of reactive treatments are generally adopted which are aimed at bridging the stiffness ‘gap’.

These measures of reducing the likelihood of premature transverse tensile cracking developing at the rigid interface, the use of strain alleviating fabric strips and/or asphalt reinforcement are commonly placed either directly on the joint or overlapping the structure, ultimately bridging between the differences in stiffness and providing additional flexibility to the rigid structure.

The effect of differential stiffness can be exacerbated by the effect of settlement, particularly in areas of embankment and new construction. Trafficking and vibration of overlying traffic can also lead to premature settlement of fill materials, which often comprise bridge approaches and culvert crossings. Transverse tensile cracking can also occur in these areas regardless of traffic volumes as a result of consolidation of subgrades and areas subject to significant moisture fluctuations.

Where the impact is deemed to be unavoidable, consideration can be given to crack mitigation so as to reduce the moisture ingress of the cracked surface. This can be achieved by undertaking routine crack sealing operations and implementing of subsoil drainage below the anticipated crack location – at the interface prior to undertaking pavement works. Additional measures to avoid potential failures include the use of a high stiffness, moisture resilient pavement material (polymer modification or the like).

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