Plastic strain in flexible pavement design

Elaine Goosen, Stellenbosch University


Plastic strain causes non-recoverable, permanent deformation in pavement layers. The effect of plastic strain is generally referred to as rutting. Rutting may occur throughout the pavement structure in granular, stabilised and asphalt layers. Granular materials exhibit plastic deformation when loading occurs outside the elastic limits. The fatigue life evaluation of granular layers during design is generally sufficient to ensure adequate strength and rut resistance. Asphaltic materials may behave visco-plastically, whether as a surfacing or lower layer. Additional to the degree of loading, such materials' susceptibility to permanent deformation depends on the traffic speed, operational temperature, layer thickness and bitumen age. Opposite to granular material, asphalt is generally more susceptible to permanent deformation during its early life. During design, evaluating the effective vertical strain imposed on the layer is essential to specify an appropriate layer and mix type. Other interventions to counteract compliance under loading include the selection of high modulus bitumen. The theory of permanent deformation is linked to relative rutting in the pavement structure by observed plastic strain during triaxial testing. The influence of bitumen age and environmental factors on expected performance is discussed.


Elaine completed her PhD in Pavement Engineering at SU in 2021. She has experience in road consulting and construction supervision and is currently a lecturer at SU. Her research focusses on binder rheology and ageing.