30-40 means bitumen having penetration of a needle between
30-40 mm with standar weight on it at 25 deg Centigrade.
Lower the penetration is higher the viscosity.
The test is done with the apratus called Vicats apparatus.
the penetration caused by standard Condition i.e.
Standard Weight= 100 gm
Standard temp. = 25 Degree C.
Standard penetration time = 5 Seconds
Unis = 10th of mm to show the results and avoiding Errors
e.g. if the value is 30/40, the 30/40 is the bitumen
penetration value (tested in standard condition) lies
between 3 to 4 mm.