STREET CONSTRUCTION

Otago Daily Times, Issue 17871, 28 February 1920, Page 7

STREET CONSTRUCTION

THE TAR MACADAM SYSTEM. DISCUSSED BY ENGINEERS' C3ONFERENCE. «OME DIFFERENCE OF OPINION,

Practically the whole of Thursday forenoon's sittuuj of the conference of the New Zealand Society at Civil Enginoera was devoted to the oouftideration and discussion of two papers on "Tar Mooadam Stwsete and Jl'oobpatns." Tho first paper readi was that compiled by Mr A. Dudley Dobaon, Christchuroh City Surveyor. At the outset in his paper Mr Dobaon etaied that tarred gravel, tor forming the surface of footpaths, was adopted many years ago in Obristonuroh. Mr Samuel jtaart undertook a very considerable mileage of footpath tar paving, and established a successful business m laying down garden potha, tennis courts, yards, eto. Till 1898 no attempt waa mado to use tar on the roadways, but during his term of office as Mayor, Mr Eden George strongly advocated tarring. Several streets were tarred, 'but tho only surface that lasted was the work done in Cathedral square. 'This waa apparently well done, and kept in good repair by frequent repainting. In 1900, 11 chains of Caahel street were laid down with tarred metal, and ibis portion was in excellent condition to-day. During tho last nine years a wearing carpet had! been twice laid down in the centre of tho street, and it had been tar-painted and spread with grit about ©very two years. Nothing had been done till 1910, when tho City Council decided to commence to lay down tarmacadajn in the streets, the surfaces of which were consolidated and of a proper contour. As it was decided that the work roust be done out of rates, only a small portion could bo undertaken yearly. This _ work had been carried on steadily ever since, and now 14 miles of streets were tar-coated. Some of the streets had a substantial coat of tar macadam, while others, particularly in tho residential quartets, had merely been tarpainted and £rit-coVered year by year, until now a very good wearing coat existed. The results had been meet satisfactory. The cost of maintenance had been substantially reduced, tho dust nuisance lraach mitigated, and the tar macadam, even on the traction engine routes, was successfully carrying traffic. No street could bo harder than tho jnaterial of which, it was made, and the jrtone and tho tar mixed ooujld not be' expected to make such a solid surface as conerete, or wood blocks on a concrete foundation, such as were being constructed in othor cities, but the work which had been done in Christchurch showed that, where the conditions were suitable, excollent streets could bo made at least at a third of tho cost of tho methods which were usually adopted for. substantial work. Twenty years ago there was but little demand for tar in Christchurch, .and it lay so long in taniks at the gasworks that the bottom were partially dehydrated and the tax thiok. When this thick tar was boiled, mixed with metal or gravel and allowed to ripen for some months, during -which tim© some of the light oil would evaporate, very good work resulted, but when there was a large demand and the tar was used when new, more particUr larly after the making of water gas was commenced, tho tar was in a quite different condition and contained a good deal of water. Tarwas also obtained from the different town in New Zealand, and there was a great deal of difference in the various samples. To make good work it became necessary to standardise the tar. 4. still was therefore installed and experiments were conducted to ascertain the degree of heat most suitable to the conditions and climate of Christchurch. The object aimed at was to- get rid of the water and light oils, which if left in, would eventually evaporate and leave the tor macadam in a dry and ravelled state. At ;he same time if too much reduced it became hard, and not only cracked in tho work, but had to be used as soon as mixed, or the material would set in mixed stacks. A temperature of 230deg. C. was finally adopted as the most suitable. Coal tar was a substance of most complex constitution, and in distilline up to '230deg C. tho following substances, which were inimical to good work, were taken out of it:— Ammoniacal liquor, containing aqueous solution of ammonium, hydrogen carbonate, ammonium hydro-sulphide, and numerous other compounds, and light oil c f a, specific.. gravity of 0.935. It was to . bo noted that in "The Road Board" (London) specifications for tar for tarring roads, the desirable temperature was specified to be between 220deg F. and 240deg F., or 104.4deg C, to 115.5deg. C. Treated in this manner it was found that tho tar could be mixed with stone and allowed to lie 'n stacks without getting hard. At the same time, when laid on the streets in thin layers and rolled .?uffioiently, in a few days % could take heavy traction engino traffic without damage. It was considered that lie best work was obtained by machinerynixed tar macadam. In Christchurch a rotary mixer was established near the still, and it was found that by suitably timing tho delivery of metal from the quarries three men would mix from six to eight cubio yards per hour. The quantity of tar used per cubic yard was from six to six gallons and a-half. For screenings eight gallons were used per cubic yard. The stone required to be free from dust or the quantity of tar required would be much greater, espeoiallv with screenings. Dirty screenings would take from 10 to 12 gallons of tar per oubio yard, but the use • of dirty screenings should be avoided. The tarred material could bo used directly out of the mixer, or could be stacked for future use. The method of laying the carpet was that tho surface should be crowned to the proper section and heavily rolled. If the road was an old one, with a good consolidated foundation, it might be quite sufficient to lightly scarify the surface to removo all irregularities, and then heavily roll it till all loose material was well water-bound. A layer of 2i inch tar macadam from three inches to four inches thick should then be laid down and lightly rolled. A small quantity, of tarred 1£ inch stone was then spread on the first layer, just enough to fill all tho interstices in the 2JI inch metal. This should then be well rolled. A coat of seven-eighths of an inch of tarred screenings should bo laid down in a sufficient quantity to fill all the interstices in tho 1£- inch tarred metal, and when this has set a coat of five-eights tarred screenings should bo put on. No traffic should bo allowed upon the road for three or four weeks. After traffic has been on the road for a month or two it was advisable to heavily tar paint tho surface, cover it with fino grit and steam roll it. Tho rolling should not bo continued for any length of time, especially at first, or tho material would wave. Rolling every morning gavo excollent results. At tho present time a 3 inch finished and well rolled carpet would cost from 2s M to 3s per superficial yard. A similar paper compiled by Mr A. G. Walker (Auckland) was read by Mr Furkort. _ He described tho areas in Auckland laid down in bituminous or tar macadam. Somo of this paving, after being down for 18 months was found to have become defective adjacent to tho tram rails. It was also found that whero tho topping had worn off na a result of heavy vehicular traffic, tho remainder of the tarred material was very little better than water-bound macadam. The matter was then carefully investigated and the conclusions arrived at woro that futuro paving work would have to bo carried out with finer aggregate, and that a solution or compound not only of a better quality, but also ono that would facilitate tho laying of the pavement would have to be u-sod instead of coal tar. Bitumous macadam was mixed in the vicinity of the works, tho chips being dried on metal plates and after being mix-rd v/tth tho bitumen by hand, were laid hot. In spite of the mid© methods adopted this paving was very satisfactory. Mr Cyrus .7. R. AYilliams congratulated the authors of the papers on tho wav in which thoy_ had dealt with this question of street paving. He said tho society was greatly indebted to them for their works. Mr Walker had practically told them that tar-paving wa« a failure, and that bituminous pavements wero a success. Unfortunately, he was not present to explain why that should be so. He had not spent 40 years in travelling about different cities without coming to a conclusion about tar macadam. In Brisbane, some 30 years ago, it was tried, and tho general result was that it was of no uso for vory heavy traffic, and he thought tho experience of many years ago was still borne out to-day. He had drawn the conclusion that the effect of tar macadam paving depended upon the "skin," and unless that skin could be got in trood condition, then tar macadam was very little better than the system wo had before it was introduced. This brought him to anothor po'nt touched) or, bv Mr Dobs-on. Ho complains! that the Chriftohiirch streets had to be. made out of rates, instead of out of loan Jnoney,. The .reason of pkk was tbw h-4.

bad a Mayor, in the person of tho luto Mr Taylor, who wag vory enthusiastic ovor tar macadam. Ho tried to persuado the people to borrow £150,000 to make tar macadam streets, telling thorn that one© tho streets woro mado they would never require to lie touched. Thoy mistrusted Mr Taylor's prediction, and, unfortunately, his proposals novor reached the s tago of being put to the voto; and thoy would not havo been carried had they ibeon put to the voto, because tho people got such a horror of building roads on tho suggestion that onoo they were built they would nover havo to be touched. The distrust by the people had been proved by the various tar macadam stroots that hud been made in Christchuroh. Some had boon more or lesa a success, whilo others had boon a failure, owing to not being sound at tho foundation. Mr Holmes said 1 ho would havo liked to havo had somo idea of the rate i>er hour paid for labour, tho cost of tho materials used, and tho details of the plant employed in connection with the work at Christchurch and Auckland. Otherwise it was impossible to comparo tho cost at tho present time, when all our values were in tho meltingpot, with the cost of the work under review. Ono point that was not dealt with by either paper, and that was the quality of tho materials used. In Christchurch and in Napier they had tho advantage of having a most excellent shingle, consisting of tho strongest particles of rocks washed down from the hills.

Mr M'Curdy: What rock is it? Mr Holmes: Tho remains of slate rock and also quartz nuggets. Continuing, he said that Auckland had stone from Hauraki. and very excellent stone was obtained from Onehunga and Kaipara. With material like this for their foundation their streets could not but be a success, provided attention was paid to the proper proportion which waa essential in carrying out the work on tho streets, and to which Mr Dobson had directed attention. Wellington, in a way, was very much handicapped, as they had no good stone. The City Council tar dressed somo of the streets, the particular portion being between the wharf and the post office. They did it in the usual way by putting on a coating of tar and dusting it with sand. This did splendidly until one hot day a cart passing along started to pick up the whole surface and carry it away on the wheels. The cause of that was that the stone material immediately under the surface had become orushed by constant traffic on it. and consequently had formed a layer of dry sand, which completely destroyed tho adhesive properties of the material below. Unless they had rock below stronsr enough to preserve the surface, then tho thing became a failure.

Mr Howitt said that to his mind, especially in this country, if they got a good foundation tar macadam roads were the best. If everybody advocated tar macadam roads tho question was. "How loivi was tho tar going to last?" He asked Mr Dobson if he favoured watering tho streets, and whether it affected them in hot weather. He did not water his tar macadam streets, but kept them well brushed, which he considered the best way to maintain thern,. Mr Staveloy said the only experience ho had had with tar macadam had been in W«inganui, but he had not been able to do tho work anything like as cheap as Mr Dcbson had done. His cost was more in keeping with Mr Walker's. They had + o pay more for labour and material in the North Island than was paid in the South. Mr Jickell said he did not want to draw particular attention to tho Palmerston North streets, but he did say that so far as his knowledge and observations went !io had never seon any streets that were better. In laying down these the whole roadway was sacrificed and a largo amount of material taken away. The street was reformed, and a new coat of metal placed on it. This was six inches thick at tho sides and 10 inches at tho centre. In some instances the crown was cut down some 18 inches and rolled until a good foundation was secured. Inch and a-half river bed shingle was applied and tarred. This was rolled with a llcwt hand roller at first and then in about a week or 10. days a 15-ton roller was employed on it for about a week. Ho said the result was a clean, smooth road that was equal to any concrete road, and that was still smooth after 10 years. For side streets, whero the traffic was light, he advocated tar-sealed roads. He was off opinion, too. that watering a tar macadam or tar-sealed road injured it.

Mr Oaven, who is visiting the dominion from America, was invited to speak. Ho said he regarded it as important that local conditions should be studied to find a standard. That had been done in other countries. Ho thought, too, that the Government should put in a testing laboratory, in which stone and other materials could be tested for engineers. Mr W. D. R. M'Ourdie (Dunedin Oity Engineer) did not agree with all Mr Dobson had said as to methods. For example, they had, not found it necessary here to boil tho tar. He described tho new works projected. As to concrete, he thought it quite unsuitable where there was any horse traffic. Ho could show them a stretch of road in tho Bouth of the city whero one portion was tarred maoadom on concrete, and the other tarred macadam on spaul, and he would defy them to tell the difference.

Mr Winter said, in respect to laboratory testa of material, that New Zealand already had such a laboratory in the Canterbury School of Engineering. Mr Dodds (Hastings) said that in his town they had been unable to procuro tar in any quantity, and residual asphalt had been used for a bituminous macadam road. This road had been down four and a-half years, and had proved exceedingly satisfactory. The County Council had now requested that they extend the road two and a-half_ miles, to join up with Havelock. This it was proposed to do in a way that would probably bo carried out on many of the county main roads. Concrete for the county roads was much too /expensive. A tremendous amount of money had been spent on tho foundations of the arterial roads, and to take out these foundations in order to put in concrete would be a waste of money. What had been done already could be used as foundation, with a wearing surface of bituminous macadam or tarred macadam.

Some members brought up the point of tho hardness of a concrete road on horse traffic.

Mr Holmes said he did not think they needed to consider tho horso traffic as something likely to remain a relatively big factor, for if the price of petrol dropped to pre-war figure tho horso would become a back number.

Mr Cyrus Williams said that they would have to consider the question of shoeing horses with rubber—that would obviate the difficulty.

Mr Dobson, in tho course of his reply, said that it would bo a difficult thing to po into details, and what ho had attempted to do was to give the scientific results of tar scaling. Votes of thanks were accorded to him and to Mr Walker for their papers.