The Visco-elastic Behavior of Confined Thin Films of Bitumen in Tension Compression

Abstract

The behavior of confined thin films of a bitumen under sinusoidal loading both in shear and in a direction normal to the plane of the film (tension/compression) has been investigated. For small strains, behavior in tension/compression like that in shear, is linear and thermorheologically simple and the temperature dependence of the rheological parameters is the same in each case. In tension/compression, the degree of confinement can be defined by the ratio of the radius of the confining plates (r) to the thickness of the film (d). For confinement ratios greater than about one, the longitudinal complex modulus |M*| was found to be greater than three times the complex shear modulus 3|G*| (the value expected for shear behavior). The complex modulus ratio |M*|/3|G*|, where |M*| and |G*| are measured at the same frequency and temperature, was found to be approximately proportional to the square of the confinement ratio over the confinement ratio range 4.5 to 45. This relationship should be predicted by the theory of the purely elastic situation. The energy loss factor under sinusoidal loading, π/2 tan φ (where φ is the angular phase difference between stress and strain), was found to be different in tension/compression from that in shear. As the confinement ratio increases, the energy loss factor decreases more rapidly with frequency than for shear conditions. To indicate the temperature/frequency region for this decrease, the behavior where tan φ=1 was evaluated and found to correspond to an |M*| value of about 8×108 dynes/cm (the same value is obtained in shear for tan φ=1). The particular relationship (for the bitumen tested) between the temperature, the frequency, and the confinement ratio when tan φ=1 was determined for the range of frequencies and temperatures covered experimentally. The theoretical implications of the results and their relation to the deformation, fracture, and fatigue behavior of bitumen bonded road surfacing materials are briefly discussed.