I have seen recent reports about an attempt to improve the skid-resistance of a ‘tarmac’ surfacing in a particular area, which has resulted in making it more slippery. As I do not know the area in question or the methodology adopted, I cannot comment on this particular case. I would like, however to comment on the terminology used.

The British Standards Institution’s glossary of highway engineering terms defines “tarmacadam”, of which the term tarmac is a corruption, as “A road material consisting of aggregate and road tar, or a tar-bitumen mixture, mixed under controlled conditions, having a preponderance of coarse aggregate and a substantial proportion of voids”.

“Aggregate is the crushed rock… which, when held together by a binding agent forms a substantial part of such materials as concrete, asphalt, coated macadam or the like”. Aggregates can be dense-graded or gap-graded (macadam), terms which go back to the great pioneers Thomas Telford and James Loudon McAdam of the 1830s.

“Tar” is defined as “a viscous liquid, black in colour, having adhesive properties, obtained by the destructive distillation of coal, wood, shale, etc.”. Its residue is pitch.

Tar is not very user-friendly and is no longer easily available so that it has not been used to any extent in the last 100 years or so.

Old habits die hard and, after a century, we still keep saying ‘tarmac’

“Bitumen” is defined as “a viscous liquid or solid material, black or dark brown in colour, having adhesive properties, consisting essentially of hydrocarbons, derived from petroleum or occurring in natural asphalt, and soluble in carbon disulphide”.

Bitumen is relatively docile to use and plentifully available so that all black-top surfacing in the last 100 years or so has been in bituminous macadam or asphaltic concrete.

Old habits die hard and, after a century, we still keep saying “tarmac”, like “nugget” or “John Bull”, I suppose.

In my 50 years or so of involvement with road surfacings, I was once asked by the Gas Board to try and relieve them of a considerable amount of tar, which they had from the production of town gas. Try anything once. I chose a secondary road, part of the present Coast Road, but the result was a disaster. I had no lab facilities and it was all I could do to rescue my equipment. I had to lay a thick inert carpet to prevent vehicles getting bogged down in the quagmire. Never again!

I have also read about a suggestion to incorporate vulcanised rubber in the hot asphalt surfacing. I have tried this and have fared decidedly better. When I completed the Porte des Bombes flyover in 1957 I was anxious to avoid rumblings and vibrations on the bridge. I was approached by my good friend, George de Trafford, who had a Swiss friend who produced a rubber additive that was marketed under the trade name of Colplaste.

It sounded fine but the material had to be reduced to a liquid form that is technically known as a ‘cut-back’, in other words a solution in heated kerosene. The mixing plant and paving machinery belonged to the Air Ministry Directorate General of Works. Their chief, Wing Commander Charrington, was a peach and greatly supportive.

When the bridge was opened to traffic we could not feel any vibrations. What a relief!

Andrè Zammit is a chartered road engineer.

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