1.8.3 Pavement damage factors
Pavement performance models can be based on a single independent variable, such as time or traffic load (see Section 1.9) or multiple independent variables which usually include the traffic load variable. Where traffic load is used as an independent variable, the concept of pavement damage factors is typically used to represent the impact of traffic on the deterioration of the pavement.
The concept of pavement damage factors, or load equivalency factors, involving the computation of an equivalent damage contribution of axles of various load forms the basis for most pavement structural design models, and as an input to predictive models of pavement behaviour, where for flexible spray sealed unbound granular pavements:
- Load Equivalence Factor (LEF) = (Actual axle load in tonnes/8.2)n
- where n is approximately 4.
A total LEF per vehicle is computed by summing the LEF for each axle. Methods also exist for computing the LEF based on groups of axles, see for example Austroads (2012c).
The relevance of the above ‘pavement damage law’ can be shown by an example, where for a 25% increase in axle load from say 8.2 tonnes (defined as a standard axle load) to 10 tonnes, the equivalent LEF is approximately 2.2, or 100% above the value computed if damage was a simple ratio of the actual and standard loads. Also, summing the LEF for the total vehicle fleet produces the SARs, which is equal to equivalent standard axles (ESA) and cumulative ESA for sealed granular pavements, or cumulative SAR, over a defined time period for other pavement materials.
The concept of LEF is widely used to assess the relative damaging effects of different axle loads and axle groups on pavements (Molenaar & Sweere 1981). In Australia the damage factors are related to the fourth power of the actual axle load relative to a standard axle load (Kinder and Lay 1988) for sealed granular pavements. This relationship was based on the results from the American Association of State Highway Officials (AASHO 1972) Road Test carried out in the late 1950s in North America (Highway Research Board 1961), even though the Road Test was not conducted on sealed granular pavements which are the most common pavement type in Australasia, comprising some 90% or more of the road network.
A theoretical analysis using linear elastic theory applied by the Queensland Department of Main Roads (1984) to sealed granular pavements confirmed the applicability of the fourth power law to these pavements provided a specified vertical compressive strain was not exceeded in the subgrade. Work by Martin (2009), using sealed granular pavement performance data from the Accelerated Loading Facility (ALF), found the power law for roughness to be approximately four, while the power law for rutting was approximately three.
For flexible pavements with cementitious layers and rigid pavements, the damage factors are related to axle loads by powers greater than four (Austroads 2012c). These higher power laws occur when the specified tensile strain in the bound layers of the pavement is not exceeded and the specified vertical compressive strain is also not exceeded in the subgrade. Similar forms for damage factors are also used for rigid pavements in the USA (Larralde & Chen 1987).
The damage factors may be useful in assessing various pavement design options, but on their own they do not give a full picture of LTPP. This is because the underlying condition states used to define critical conditions, or end of life, are not explicit (see the ‘Structural performance indicators’ section below) and a number of the assumptions behind the derivation of the powers in the expressions for these damage factors are open to question. Nevertheless, the concept is generally well established and widely used in predictive modelling.