1.7.3 Parameters for pavement performance modelling
Pavement performance measures
There are many ways by which pavement performance can be assessed. Indicators of pavement performance can be very broad or detailed depending on the context of the performance assessment (OECD 2001). Measuring these performance indicators can either involve visual observation and the assignment of a simple rating (Institute of Public Works Engineering Australasia (IPWEA) 2015), or using technologically advanced equipment collecting pavement condition information continuously at high speed for later re-processing into pre-defined condition parameters (Austroads 2015a). The following two criteria (Roberts & Martin 1996) are useful for considering the aspects of pavement performance that are of most use to road asset managers, road users and other stakeholders influenced by road performance:
- the functionality of the pavement relates mainly to the surface conditions visible from the perspective of the road user, that is, how well does the pavement suit the needs and perceptions of the road user and the level of service offered by the pavement to the user
- the structural condition of the pavement is an indicator of the immediate and long-term performance of the pavement, as well as signifying when some form of structural intervention is required to maintain the existing level of service provided to the user.
Although they describe distinctly different aspects of pavement performance, the surface and structural conditions are likely to be quite interactive, particularly for sealed granular pavements which rely on the integrity of the bituminous surface seal to keep the pavement base waterproof and prevent its structural deterioration.
Bearing in mind the overall context of this Guide is to aid the efficient management of pavement infrastructure at a network level, performance measures embrace the following:
- distress measures, including the definition of specific surface defects, such as cracking, rutting, roughness and surface disintegration
- condition indices including functional, structural and composite indices, such as Present Serviceability Index (PSI) and Pavement Condition Index (PCI), which are often derived from the above individual distress measures.
- Well known distress measures are described in this Section 1, and referred to throughout this document, whereas explanation of the derivation, origins and application of the condition indices is provided herein.
In applying the above measures, the model user may choose to use any type of distress measure or selected range of condition indicators either explicitly or implicitly. On the other hand, many pavement design, evaluation and planning guidelines and systems acknowledge the concept of ‘end of service life’ without using performance measures. The concept of ‘end of service life’ (Austroads 2007a) relates to the initial derivation of the term and is often ignored by new users, but use of this particular concept is only valid if the link between performance and ‘end of service life’ is retained and understood. It provides a basis for updating or refining the results of previous research, and is particularly relevant where measures such as remaining pavement or surfacing life are used.
Consequently, a clear and agreed definition of the distress or condition indicators is required for each circumstance. This need is equally applicable where general visual descriptors, or ratings describing the scale of condition are used, such as very good to very poor, or sound, critical, failed, etc. Defined distress or condition indicators also provide a useful link to quantitative level of service measures.
Factors influencing the modelling of performance prediction
A wide range of factors, or variables, contribute to the initiation and progression of different pavement distress modes and differ in how they affect performance for both functional and structural distress performance models. They comprise general factors which are considered at the design stage of a new pavement and have a significant influence on initial performance, i.e. phase (1) in Figure 1.50 and in subsequent phases. Typical factors and parameters are listed in Table 1.16. Some of these factors do not change like performance parameters, but a number may require re-specification over time as a result of maintenance or improvement works carried out on the network, and changes in traffic numbers and loading. These factors also form part of the input data along with the performance measures as shown in Figure 1.51.
Where whole-of-life-cycle modelling is used, e.g. using deterministic type models (see Section 1.8), with the aim of predicting the change over time in various distress and structural dependent variables, including interactions and the effect of treatment strategies, the range of factors is expanded considerably. Re‑specification in terms of appropriate models and inputs, and the treatments employed, including description, layer thicknesses and properties is required. For some modelling frameworks, calibration factors will also be required as a basic input.
Where the initial surface conditions of a new pavement can be assumed based on typical standards of construction, those chosen at the point of application of a maintenance treatment or pavement renewal could be substantially different, with the potential to impact the effectiveness and durability of the treatment.
|Climate||Temperature, rainfall, Thornthwaite Moisture Index (TMI), etc.|
|Geometry/terrain||Terrain (flat/rolling/hilly), radius of curvature, grade, etc.|
|Traffic loading (including dynamic loads)||AADT, equivalent standard axles (ESA) or SAR, traffic composition, etc.|
|Subgrade type and condition||Californian Bearing Ratio (CBR) etc.|
|Pavement composition, material properties and surfacing type||Descriptive, layer thickness(es) and strength, durability|
|Structural capacity||Structural Number and Modified Structural Number (SNC)|
|Surfacing and age||Years or axle repetitions|
|Construction practice and quality||Descriptive|
To allow comprehensive modelling in the subsequent deterioration phases, the range of additional factors which need to be considered include, previous surface and structural condition and the definition of the new treatment, new pavement composition, quality, etc. Resets to current condition, post-treatment roughness and rutting, either through modelling, field validation or user defined values are also required.
Pavement performance measures are dealt in more breadth and detail in Section 1.8.