I refer to the reports ‘More problems with the tarmac, this time at MIA’ (August 23), ‘Poor quality control cause of poor roads’ (September 1), and “Transport Malta will work on warped Pieta’ tarmac” (September 3).

In the first place I am irritated to see the continued incorrect use of the word “tarmac”. As I had explained in my article ‘Of tarmacs and rubbers’ (August 17), the term “tarmac” is obsolete as the material has not been in use, or available, for more than 100 years and would be very unsuitable for present day traffic.

Tarmac has tar as a binder, which is no longer available except in very small quantities. Besides it is complex and not at all user friendly. The macadam grading of the aggregate (gap grading) leaves a high proportion of voids which lead to deformation or “rutting” under heavy traffic.

The defects shown in the photos in the reports under reference would appear to be due to “creep” or lateral movement, especially on curves. There can be several causes for this.

In modern black top surfacing (asphalt) the binder is, almost invariably, bitumen. This is the residue from the fractional distillation of crude oil and is available in huge quantities in refineries all over the world.

On highly-trafficked roads, especially on bends, I would not like to use anything but hot asphalt, possibly with rubber additives

After the lighter fractions like aviation fuel, kerosene and diesel are flashed off, bitumen is left as the residue. Bitumen for asphalts is called penetration bitumen which is defined by the depth in tenths of a millimetre that a standard needle penetrates the bitumen sample under specified test conditions. The higher the pen number, the softer the bitumen.

In Malta, we have used 60/70 pen bitumen with very good results. For lightly-trafficked roads 180/200 pen bitumen has also been used. In the United Kingdom and in Italy 30/40 pen bitumen is very widely used for main roads.

In modern asphalt technology the “Marshall” method is in wide use. Aggregate grading, generally continuous, bitumen content and voids are rigidly controlled and “stability” and “flow” have to come within certain limits. Due to the variability of the aggregate the testing has to be carried out all the time.

In asphaltic concrete (hot asphalt) the bitumen is heated as well as the aggregate and the material is laid hot. The bitumen can also be “cutback” by means of solvents like kerosene and the mixing can take place at a lower temperature, which is cheaper and kinder on the aggregate.

If the bitumen, usually softer, is emulsified in water, the mixing can take place at ambient temperatures and cold asphalt is the result. This is weaker than the hot asphalt, the mixed material can be stored until the emulsion “breaks”. This gives considerable flexibility, especially for patching, but is a weaker material.

On highly-trafficked roads, especially on bends, I would not like to use anything but hot asphalt, possibly with rubber additives, and the road should not be opened to traffic until the asphalt has well set.

Andrè Zammit lectured on town planning at the University and practised in development control for over 30 years.

Sign up to our free newsletters

Get the best updates straight to your inbox:
Please select at least one mailing list.

You can unsubscribe at any time by clicking the link in the footer of our emails. We use Mailchimp as our marketing platform. By subscribing, you acknowledge that your information will be transferred to Mailchimp for processing.