ROADS OF THE FUTURE

Otago Daily Times, Issue 18752, 4 January 1923, Page 6

ROADS OF THE FUTURE

DISADVANTAGES OF TAR. INCREASING USE OF BITUMEN. THE IDEAL SURFACE. (Fhom Our Own Correspondent.) LONDON, November 24. Somewhere down near Arundel recently engineers discovered an abandoned grassgrown Roman road which was made in a similar manner to tho most up-to-date sections of roadway in England to-day. It was made of bitumen, and geologists were able to demonstrate that tho bitumen came from Egypt. In the ancient civilisation of Peru bitumen roads seem also to have been constructed. while the substance is mentioned in the Book of Genesis as being used as a cementing material in the construction of tho Tower of Babel. With the development of travel and commerce along the main roads of this country during the past decade engineers have had to improve on tho old water-bound metal roads, and they turned to tar for a surface dressing. It was a vast improvement for purposes of light motor traffic, but the traffic has been growing ever heavier and heavier, and the provision and maintenance of roads equal to the strenuous demands now made upon them has become an absorbing study to every road engineer. Sooner or later tho same subject will be a j very vital one in New Zealand, and no doubt • some idea of what is being done in this 1 country will be of interest. To tho ordinary pedestrian a road : s ; just a road. After conversation with a few !_cxports tho road over which one passes becomes a thing to he examined with interest ! From two such experts I have recently ! obtained a good deal of information. Tbe j one, an officer in charge of a Government I experimental station, spoke as on© without 1 any personal interest in any special form I of road-making. The other, whose business I it w-as to push the sal© of bitumen for a I certain world-famous oil firm, spoke natu | rally with a certain bias towards this latest I method of construction. | DESTRUCTION OF FISH. For a good many years complaints have | come from many quarters that tho drainings | from tho surface of tarred roade killed fisn j in adjacent streams, and some time ago tho ; Government set up an experimental station ! under tho joint Ministeries of Agriculture and Fisheries and Transport to ascertain whether tar was poisonous, and if so whether some suitable substance could be suggested which was not poisonous to fish life. It is under j stood that the experimental station has now I submitted a report which is so damaging to the use of tar that the Government hesiiates for tho moment to give it publicity. Some of the components of tar are phenol, quino line, napthalenc, and acridine, and it is this last-mentioned substance which is most poisonous of all. It is even suggested that acridine may be one of the causes of cancer. Rats painted with acridine have developed the disease, and it is maintained Ihat people who work wit’o tar are more liable to develop cancer. Some of the less poisonous chemicals in far may be separator!, but so far it has been found practically impossible to separate the acridine. Experiments have shown that tar in any appreciate quantity is invariably fatal to fish life, while on the other hand bitumen has not been injurious to fish, and so far as thosq carrying out the experiments are aware there is no poison in the substance. i Corporations and companies interested in the disposal of tar in this country are of course many in number, besides being extremely influential, so that any report detrimental to their industry is sure to be extremely unpopular. Nevertheless, public opinion is beginning to grasp some elementary facts—that tar is not especially cheap, its life is not particularly lengthy, it is by no means a good binding material, end its low melting point is a particularly objectionable feature in hot weather. These facta wore borne out by the Government official in question, who was also a civil engineer. His advocacy of bitumen a« a road-forming material waa oven more enthusiastic than the advocacy of the trad* representative, u.iumen roads have been constructed at the experimental station and subjected to verv severe tests. One has been down for two years and its present condition is even bettor than at first. This is owing to the tendency til© substance has of flowing very slowly into any holes or substances and. always finding for itself a level surface. Thus, should particularly heavy traffic make an impression, it also crushes the surface grit still finer, and the bitumen binds all the tighter and in a very short time readjusts itself to tha proper level. SELF-SEALING TENDENCY. This self-scaling tendency prevents any water from penetrating and no injury can be dona to tha roads by frost. In the case of tar. onco the surface is broken there is no self-sealing, and the water penetrates, the frost comes, and the road is further disintegrated. How long the bitumen road will last without attention has not yet been ascertained, as no roads laid in England in this way have yet required attention. Prom America, however, there cornea the testimony of Prolessor S. F. Peckham. who refers in his standard work on solid bitumens to tho bitumen-surfaced streets of Kansas City, which, notwithstanding the violent extremes of temperature, and the enormous traffic, have shown no signs of wear after a period of 17 years. The British Government exhibition "Tank” did no damage to tho bitu-men-sutraced streets of the American cities in which it demonstrated. Before proceeding further it may bo well to mention how bitumen is procured. It is, of course, a product of the crude petroleums drawn from tho oil wells in various parts of the world. As the benzines, kerosenes, gas. and flux oils all have boiling points lower than those of bitumen, these products boll off first when heat is applied, leaving tbe bitumen behind, a dark thick substance not unlika treacle, which, as its temperature decreases, solidifies to a hardness which is regulated by the extent of the distilling process. A USB FOUND FOR CLINKER. There are several methods employed in the formation of bitumen roads. However well the surface is constructed, the success of the road depends primarily upon efficient drainage and the stability and permanence of the foundation. Having these factors, the question then becomes one of final surfacing. But bitumen has been used very largely in street formation in some of the larger cities of England. Not long ago destructor clinker was a substance municipal authorities hao to pay people to cart away. Now a use has been found for it. Carefully-selected clinker is crushed and screened, and the half-inch material is used for the sub-crust of the roads. Tho clinker is heated on sand dryers to a temperature of 300 deg Fahrenheit, and is mixed with the heated bitumen in a steamjacketed mixer. For this sub-crust approximately 10 per cent, of bitumen is used. After mixing and whilst as hot as possible tho material is placed on the road which is to be surfaced. It is then spread to a sufficient thickness to give after rolling a consolidated depth of IJin. Clinker for the wearing surface consists of material passing iin mesh screen, and it is mixed with 15 per cent, of bitumen. Sufficient material for the wearing surface is carefully imposed to give a thickness after rolling of Jin, making a total of 2in for the two layers. Rolling is carried out until it is impossible to make further impressions, and then the surface is dusted over with cement or limestone dust, after which the road is ready for traffic. Similar streets have been laid down in London, the mineral aggregate consisting of hardstone. slag, limestone, or gravel, with graded sands mixed with about 9 per cent, of bitumen for a top-drcasing. In a number of localities country roads have been formed .in a very simple way. Ordinary hard rock, broken to 2in gauge, is evenly spread 3in in depth and lightly rolled. Two gallons of bitumen per super yard of road is poured on. A sprinkling of chippings is laid and rolling is continued, and a sealing coat is applied by means of pails and squeegees. FOR COUNTRY ROADS. Where something much less expeneive is required, such as in the case of the main country roads of Now Zealand, where the metal is substantial and the surface only requires attention, another process is quite effective. This is just a surface dressing. The road when dry. is swept clean of all dust. Bitumen is raised to a temperature of 350 deg F., and spread on the road with pails and squeegees, the quantity being one gallon to five or six square yards of road surface. Directly after the dressing is made chippings of J-inch limestone are sprinkled on, and those in due course become ground up in the bitumen. Ronds of this nature have been laid in some parts of England for two years only, but so far there has been no sign of their wearing. The great advantage of bitumen is that it retains its character for all time practically, whereas in (ho case of tar the various constituents evaporate rapidly ond leave only a crumbling carbon. NATURAL ASPHALT. With improved methods for gas making in England the qualify of tar has deteriorated a good deal, and tho present practice is to max bitumen with the tar to stiffen it up and bring it up to a uniform standard. In some parts of the world people have a preference for natural asphalt taken from the pitch lakes, but it is pointed out. that the natural asphalt has no advantage; <- the made-up mixtures of bitumen ■ • 1 g,: . ' These pitch lakes have been toi—ocl time by the oil bursting throu,.’: the car' - -’® surface In due course the lighter fvc'-t;. ■ I eve unrated, leaving the bitumen to mix u,. Tith tho debris and the eoil, tho proportion .

of mineral matter being about 36 per cent. Thus the natural asphalt is exactly the same as what is artificially made, but in the case of the former one is paying for tho trans port of a great proportion of mineral aggregate which could be more conveniently ob tained. THE PRICE. An instance is quoted of a certain urban district council with a tarred road beside a fishing stream having to pay £4OOO damages for destruction of fish. A certain firm dealing in bitumen has now substituted bitumen for the tar, and has in addition given the council an indemnity against any future damage to the fish. Finally, it is maintained that where 34d is spent for tar in road construction, only 4Jd need be spent on bitumen. Tho advantages in favour of tho letter point to its more general use. Whether u would be a paying proposition in New Zealand with the added freight is a question for others to decide.