Determination of the Low Temperature Bitumen Cracking Properties: Fracture Mechanics Principle Applied to a Three Points Bending Test using a Non Homogeneous Geometry

The current specifications for low temperature cracking properties of bituminous binders are mainly based on the bitumen's rheology. These methods allow to rank the binders when the physico-chemical compositions are similar, like for pure bitumen. However, cracking propagation and initiation depend also on the binder's microstructure. For these reasons, fracture mechanics principles seem to be promising to rank and to understand the cracking phenomena on all the binders, from pure bitumens to highly modified bitumens. Several publications give interesting results using a three points bending test on a pre-notched bitumen bar. A new promising geometry is proposed by Hesp. Whereas the calculus of the stress intensity factor and the fracture energy from three points bending test is well known when the bar is made with a homogeneous material, equation given by ASTM standard couldn't be applied with the geometry proposed by Hesp. This paper gives a method based on finite element analysis to determine the geometric factors in order to be able to obtain intrinsic fracture parameters for the heterogeneous bar in the three points bending test. The calculus is restricted to linear elastic state. Impact of the modulus and the Poisson ratio on the results is evaluated. Tests on homogeneous and heterogeneous bars are performed at low temperature when bitumen has an elastic behaviour. Finally, the validity of the geometric factor previously calculated is evaluated.