The Performance Characteristics of Asphalt

The elastic modulus of asphalt is a key performance indicator in how asphalt will perform under wheel loading.

When utilized in the mechanistic design procedure, a performance factor ‘k’ is adopted which incorporates two factors, including a) asphalt moduli and b) the percentage bitumen or binder in the mix.

Ultimately, this ‘k’ factor is then assessed against the load induced tensile micro-strain to determine the number of standard axle repetitions until failure in the layer.

As a material, asphalt typically comprises a number of constituents, including graded aggregate, filler and bitumen and performs in a visco-elastic nature. This visco-elastic behavior means that the flexural response to loading is not only dependent on load, but the period of loading and the temperature.

Given the visco-elastic behavior, the period of loading (reduced time) and temperature can significantly affect its structural and fatigue properties.

Temperature

The temperature related performance of asphalt comprises three zones:

1. Viscos Zone (High Temperatures)
2. Elastic Zone (Moderate Temperatures)
3. Brittle Zone (Low Temperatures)

An increase in temperature (typically above 60°C) encourages the viscos behavior in the bituminous matrix, reducing the asphalt stiffness, resulting in greater stresses being placed on the underlying layers.

The intermediate temperature allows asphalt to behave in an elastic nature which is preferred in a pavement structure to provide both structural and fatigue resistance characteristics. This zone can vary between asphalt mixes but is considered to be below 60°C and above 10°C.

When asphalt is exposed to low temperatures, the bituminous matrix can behave in a brittle nature where the reduced flexibility can increase the susceptibility to fatigue cracking.

Period of Loading

The period of loading is recognized to play a role in the performance characteristics of asphalt. An increase in the period of loading (slower vehicle speed) can increase the time dependent loading in the asphalt and result in greater load induced strains and susceptibility to fatigue.

The complexity of asphalt as an engineering material extends throughout a number of other factors including:

• Binder Type, Stiffness and Content (%);
• Stone grading and durability; and
• The mix homogeneity

To incorporate these factors in to the general mechanistic design procedure, the modulus of an asphalt material can be determined by resilient and dynamic modulus testing.

It is recognized however, that in practice, the insitu conditions may not be representative and consideration is often given to the daily temperature fluctuation and the temperature at which the peak wheel loading occurs.

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