Stiffness of bitumen

Bitumen is a viscous material unlike other construction materials such as steel or concrete, which are elastic materials. In 1954, Van der Poel introduced the concept of stiffness modulus as the fundamental parameter to describe the mechanical properties of bitumens by analogy to the elastic modulus of solids (Van der Poel 1954). 

Stiffness modulus of bitumen (5, or often S, ), by analogy to the elastic modulus ( ) (Young s modulus), is the ratio of stress (o) to the strain (e). However, the stiffness modulus of a viscous material depends on the loading time (t) and the temperature (T). Thus, the stiffness modulus of the bitumen at a given loading time and temperature can be determined by the following equation  

The stiffness modulus is also affected when the grade of bitumen and its penetration index change. This has been demonstrated in Shell Bitumen (1990, 2003). 

The fluctuation of stiffness modulus of the bitumen with respect to loading time and temperature has a direct effect on the stiffness of the bituminous mixtures and performance of bituminous layers. 

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Both methods/procedures can only be applied when the stiffness of bitumen exceeds 5 MPa, and for the Nottingham method, it can be applied only to asphalts with VMA values ranging from 12% to 30% (Bonnaure et al. 1977 Brown 1980). 

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