Specimen preparation for testing, lab versus field: the spatial approach

One of the main goals of laboratory testing is to predict the field behaviour of the investigated asphalt mix under traffic and climatic loading. It is well known that the mechanical behaviour of an asphalt mix can be influenced among others by the adding sequence of the raw materials, the mixer type, the mixing time and temperature, segregation in the laboratory or during transport and paving, the way of filling the mould, the way of compaction, the specimen dimensions, the side effect and the curing time after specimen preparation. So it is very important to know these influences for choosing the right way of specimen preparation to be sure that the mechanical properties of laboratory prepared specimens agree with the mechanical properties of field specimens. In the wake of functional specifications this is of extreme importance. In this paper the possible differences between laboratory and field specimens are discussed. Examples are given of influence of mixing, differences in pore structure, surface and side effects, density distribution over the height of specimens and influences of compaction methods on mechanical properties. Relatively new techniques like image analysis and X-ray are useful tools to get information about structure and density distribution of specimens. It can be concluded that only under certain conditions prediction is possible. Researchers should pay extra attention to specimen preparation to minimize differences between laboratory and field.