Shear Forces in Pavement Design

The Shear Forces in Pavement Design are important when designing a pavement from a structural perspective.

The mechanistic thickness is calculated from a number of factors including traffic volumes. However, the thickness design typically does not consider the governing forces of the traffic loading, whether it is in a free-flowing or high shear situation.

Shear forces are developed in a pavement structure, typically from braking, accelerating and turning under conventional wheel loads. These forces produce increased pressure to the bonding of the surfacing course to the base and can result in premature failure, particularly in flexible pavements.

Where a pavement structure has inadequate resistance to shear movements, typical failure mechanisms include:

• Slippage – Is characterised by crescent cracking of an asphalt surfacing, is typically found in areas of higher shear in dense graded asphalt mixes with high binder content and smaller stones sizes (<10mm nominal size)
• Delamination – Is characterised by localised stripping of the surfacing over asphalt or concrete bases typically in situations where inadequate bonding between bound surfaces is evident
• Stripping and Ravelling – Is the removal of the stone from a spray seal or asphalt surfacing otherwise known as ‘plucking’ under high shear movements

So as to provide increased resilience to shear forces, particularly in areas of intersection and roundabout approaches, consideration is typically given to the following, depending on severity:

• High Shear Asphalt Mixes – Asphalt mixes that incorporate stiff binders and more stringent material grading can provide greater performance in high shear locations
• Construction Requirements – In areas known to be subject to high shear forces, careful consideration is used to ensure adequate bonding between the surfacing course and the base. Including, sufficient binder agent, cleanliness and moisture levels on the base prior to placing the surfacing
• Suitable Pavement Configuration – Depending on the application and severity, certain pavement types may be excluded from use, including unbound granular pavements and spray seal surfacings.

Ultimately, when considering the effects of high shear in areas of concern the majority of distress mechanisms are related to either 1) inadequate bonding between the surfacing and the base or 2) poor mix stability or surfacing material.

Although there is no current design specifications to account for this failure mechanism, consideration must be given to the likelihood of high shear force acting in the proposed location of the pavement design.

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