Asphalts, serving as cements or binders in bituminous pavements, behave essentially as purely viscous, mobile, liquids at high temperatures and assume varying degrees of viscoelasticity, plasticity, elasticity, etc., upon cooling. They stiffen to a semi-solid state at cold temperature and do not. exhibit a discrete melting or freezing point, Saal (1)* classifies bitumens into three types according to their rheological properties at normal temperatures. They are: 1) those which behave entirely or almost entirely like Newtonian liquids, 2) those which show elastic effects during initial deformation and which exhibit Newtonian flow thereafter, and 3) those which show almost complete resilience within a comparatively slight range of deformation, beyond which they exhibit various degrees of permanent deformation and cease to exhibit proportionality between stress and rate-of-strain. It has been shown (2) that asphalt behavior at cold temperatures is largely independent of time and is therefore similar to the behavior of an elastic solid.

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No. 214

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