A review is given of the work which has been done on the temperaturehyphen;viscosity relations of bituminous materials and it is pointed out that the methods commonly used for evaluating the changes of consistency with temperature are empirical and unsatisfactory. Viscosity measurements at temperatures ranging from 15deg; to 35deg;C have been made on fourteen asphalts of paving consistency by means of the alternating stress method described by Bingham and Stephens. It has been found that for these asphalts and temperatures a plot of the logarithm of viscosity against temperature gives essentially a straight line. An asphalt viscosity index is established which is defined by the relationship:A.V.I.equals;100lpar;10aminus;1rpar;,where ``a'' is the slope of the log viscosityversustemperature plot. To obtain numerical values for the A.V.I. it is only necessary to determine experimentally the viscosity at two temperatures (in the range where the straight line relation holds) and substitute in the expression:A.V.I.equals;100eegr;ceegr;d1sol;lpar;tcminus;tdrpar;minus;1,where eegr;cand eegr;dare the viscosities (in poises) attcandtd(deg;C), respectively. This gives the percentage decrease in viscosity of the asphalt for 1deg;C rise in temperature. The values obtained for the viscosity index for the asphalts studied varied from 19.2 for Venezuelan oil asphalt prepared in a batch steam still to 24.7 for a vacuum reduced Californian oil asphalt.
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