Asphalt Mix Instability – Drivers and Consequences

Asphalt Mix Instability – Drivers and Consequences, Whilst not currently addressed in mechanistic pavement design procedures and methodologies, the performance of Dense Graded Asphalt material relies on the intrinsic mix stability and the resistance to plastic deformation under repetitious wheel loading.

Common failure indicators and mechanisms including rutting (in the AC layer) and shoving are typical signs of mix instability and can result in significantly increased road maintenance costs. These methods of failure typically produce safety issues relating to a reduction in the effectiveness of surface drainage and the potential for aqua planing.

What does the term ‘instability’ mean in an asphalt mix?

Mix instability is a form of failure that results from internal plastic flow of the asphalt and results in deformation under conventional wheel loading (which may not necessarily reflect the performance of the underlying pavement layers or subgrade support)

How is this caused and what are common drivers?

The stability of a mix really comes down to confounding factors and the proportion and characteristics of each relevant constituent during the mix design process, including:

1. a) Stone Size, Geometry and Flakiness
2. b) Binder and filler volume
3. c) Binder Type, viscosity and resilience to viscoelastic change under temperature

The progression of pavement technology and the introduction of mix specifications has significantly reduced the likelihood of poor mix design, however there are instances in which the improper application can produce the same result.

Fine AC Mixes in heavily trafficked applications: Given the low shear strength of fine graded mixes (AC10 and below) relative to coarser mixes, they are prone to instability – particularly when placed in layers exceeding 4 times the nominal stone size or where used in applications other than a wearing course (i.e. in a base or subbase course).

Excessive Temperatures: Arguably the most influential factor, excessive temperatures are considered the most common driver to reduce the stability of an AC mix as a result of the binder softening and the migration of material within the mix under traffic loading. Given the infrequency and of these events (in Queensland), designing for cumulative deterioration of mix stability can be very difficult and hard to predict.

For this reason, where this is likely to occur – particularly in high traffic environments, the use of heavy duty asphalt mixes (including EME2) are commonly preferred which typically incorporates more stringent mix designs and temperature resistance binders including the commonly adopted A35P binder.

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