Message from Arno Hefer

Wed, 22 May 2024 00:38:17 GMT

The RSA Asphalt Fatigue Transfer Functions or Failure Criteria published in the 1996 Review of the South African Mechanistic Design Method, are known to be conservative, resulting in unrealistically short fatigue lives. In addition, the available functions are only for conventional unmodified binders, whereas modified binders are now routinely used in asphalt paving. The need for well calibrated conventional and modified asphalt transfer functions have also become critical to assess risks associated with increased loading and tyre pressures. In light of the issues experienced with the 1996 South African asphalt fatigue functions, some international functions have often been used. Three of these typical functions are the Shell, Asphalt Institute and TRRL asphalt fatigue functions. In South Africa, the Shell function with a shift factor of 5 – as calibrated in a previous Austroads design guideline (2001 draft revision of 1992 guide) – has been considered as giving reasonable capacity estimates.

Denneman et al. (2011) developed and published basic transfer functions for typical South African asphalt mixes from laboratory fatigue test data as part of the Revision of the South African Pavement Design Method (SAPDM). Hefer and Johns (2023) related these functions to field performance of thin asphalt surfacings and published shift factors that also consider different levels of design reliability. The field data used to calibrate or validate the models are from the South African HMA Performance Database, also developed as part of the SAPDM research projects. Notwithstanding the importance of failure mechanisms related to top-down cracking and complexities introduced by ageing, the focus of the fatigue criteria included here, and their application, is classic mechanistic-empirical design with bottom-up fatigue cracking of thin (<50 mm) asphalt surfacings.

The RSA asphalt fatigue failure criteria included in the Rubicon Toolbox database has the following form with constants associated with different reliability levels and shift factors summarised in the table below.

Most modified mixes included in the HMA Performance Database were applied as overlays, and mechanistic analysis of these sections consistently gave low tensile strains due to the excellent support. Validation analyses indicated that shift factors approaching 1 relate to approximately 5% cracking. As part of a sensitivity analysis, applying shift factors developed for unmodified asphalt functions to modified asphalt functions produces unrealistically high structural capacities for the modified mixes. These analyses suggested that shift factors for modified mixes should generally be limited to less than 2.

The following figure shows relative outputs from different functions, including the original SAMDM AC and Shell functions mentioned above. From the figure it is evident that the Shell (SF=5) function is similar to the Medium Modified asphalt function, which explains why the Shell function has been considered to give reasonable estimates of the fatigue life of asphalt layers.

References:

1. Denneman, E., Anochie-Boateng, J., Ngapele, M., and Komba, J. (2011). 10th Conf. on Asphalt Pavements for Southern Africa (CAPSA). Champagne Sports Resort, KwaZulu-Natal, South Africa, 11-14 September 2011.

2. Hefer, A. and Johns, F. (2023). 13th Conf. Asphalt Pavements in Southern Africa (CAPSA). Champagne Sports Resort, KwaZulu-Natal, South Africa, 15-18 October 2023.

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Message from Arno Hefer