Pavingexpert – AJ McCormack and Son – Tarmacadam, Bitmac and …

Penetration Grades Bitumen, the binder used in Bitmac, comes in a range of "penetration grades". There are specific tests that determine just what is the "penetration grade" of a given bitumen, but, rather than go into a lot of very technical and insomnia-curing detail, it is best to think of 'pen' as being a measure of "bitumen softness", of how far a point load will sink into a block of pure bitumen at normal temperatures.

The pen grade of the bitumen binder has a direct bearing on the 'hardness' of the bitmac produced using that bitumen, and so we have a range of penetration grades, from 15pen, which is pretty damned hard, to 450pen, which is relatively soft. A 'softer', more pliable binder produces a softer, more pliable bitmac. Imagine trying to push the blade of a Phillips screwdriver into a bitmac surface; some bitmacs will offer far more resistance than others, and a lot of that difference is dependent on the 'pen grade' of the bitumen binder. So, some bitmacs are harder, or softer, than others, and the degree of pliability can be tailored to suit the requirements of the job in hand.

As a sweeping generalisation, the harder grades (70pen or less), are used as 'asphalts', while the softer grades (100 pen or more, are used as binders for bitumen macadams (bitmac). Bitumens of different pen grades can be blended to create binders with a specific pen grade.

There are different grades of cutback (you knew there would be, didn't you!) ranging from slow-curing, through medium-curing to fast-curing. Medium-curing cutback is a popular choice for hand-laid work, as it gives a good degree of workability for a reasonable amount of time, but still gives a trafficable surface in a matter of hours.

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Pavingexpert - AJ McCormack and Son - Tarmacadam, Bitmac and ...