In the calculation of asphalt cement temperature susceptibility according to the following formula
A=log (pen at T1) – log (pen at T2) / (T1- T2)
Why we convert it to the following expression
A=log (pen at 250C) – log 800 / (25- TR&B) ?
You don't have to convert. These are indices proposed to evaluate the temperature susceptibility of asphalt. Your first equation is the general one (two penetrations at two different temperatures). It is Pfeiffer and Van Doormaal who found that most asphalt
had a penetration of about 800 dmm ( deci-millimetre) at the ASTM softening point (Ring and ball) temperature. Replacing T2 in the above equation by the ASTM softening
point temperature and the penetration at T2 by 800, you will get your second equation.
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