An incremental approach to asphalt layer design

Semeshan Naidoo, Naidu Consulting (Pty) Ltd


Traditionally, pavement designs consisted of considering the design traffic loading over the structural life of the road together with a single conservative estimate of the layer moduli in order to estimate the bearing capacity of the pavement using mechanistic analysis. Only a single point in time is considered ignoring the changes in layer characteristics with time and temperature. Research for the South African Roads Design System (SARDS),the recent implementation of a performance-grade (PG) binder specification in South Africa and advances in computational capacity presents an opportunity to expand on the pavement design philosophy and consider environmental influences on pavement behaviour in the form of temperature.

In order to simulate these changes in pavement design, an incremental model based on time and consequently the material changes with time may be utilised. Time increments are chosen to represent appropriate variations in terms of material characteristics, one such characteristic is the dynamic modulus.

A temperature model based on temperature prediction algorithms developed by Viljoen (2001) has been automated for ease of calculation. The output provides a prediction of the pavement temperature for design purposes. Dynamic modulus models involve the generation of master curves. There are a variety of shape defining models and shift functions available for master curve generation. The master curve developed is based on a sigmoidal model, in accordance with SABITA Manual 35 (2020), with an appropriate solver function utilised for optimisation purposes. The resultant dynamic modulus model allows for the identification of asphalt layer moduli at the predetermined temperature and loading frequencies.

The predetermined temperature and loading frequencies are considered the design increments. Considering the life of a pavement as a series of time increments will highlight potential premature failures, allow for an improved prediction of pavement durability. Furthermore, the outcomes of the analysis may allow for the development of an optimised design solution which may lead to reduced initial capital investment and life cycle costs.

This purpose of this presentation is to create awareness and inform on the processes involved in carrying out the recursive, incremental design approach for asphalt layers.


Semeshan is a young Engineer who obtained his Bachelor of Science Degree in 2016.  He joined Naidu Consulting in 2017 and currently holds the position of Engineer with four years of industry experience specialising in Pavement Engineering.