ROAD SEALING

Evening Post, Volume CXVI, Issue 131, 30 November 1933, Page 14

ROAD SEALING

PROMISING PROCESS

FAST AND EFFECTIVE

BACK TO TAR PRODUCTS

An interesting method of road surfacing for secondary and residential streets is at present being applied on a number of city streets by the officers of tho City Engineer's department. The method enables tlic ground to be covered at a much faster rate, than previous treatments, it is economical, and appears to be very effective. Moreover, local products are used, for a return is made to tar and tar products for a part or the whole of the treatment, depending upon locality conditions. The heavy bituminous hot-mix programme is completed (except for small areas here and there), for the main city streets and the main traffic roads outside the city boundaries (under the City and Suburban Highways Board) are now all surfaced. Such treatment is expensive and is warranted only where the traffic volume is heavy and where a' sealed surface would be hammered to pieces in a comparatively short time. j Concrete pavements have not been adopted in any general way about Wellington, and the only straightout concrete road is the Graeefield Eoad (in the City and Suburban Highways Board district) though concrete founda: tions have been used to a considerable extent, as on the Hutt Eoad widening and in city streets where tramway track renewals have been carried out. LESSER TRAFFIC STREETS. .Following the completion of this main street and road programme, attention was turned to secondary traffic streets, in some of which a lighter surface of bituminous mix was laid, but which in the/ majority of cases were given a sealing treatment. Tar-sealing was used in some lengths, 'and imported bitumen and bituminous preparations and emulsions in others. The cost was very much less than that of heavy hotmix work. There were departures from standard methods in certain lengths, on which penetration and semi-penetration, methods were employed, but hot-mix and sealing were the standard processes, with, in the latter type, a swing away from tar products to emulsified bitumen, laid cold. The earlier. work of this kind necessitated the importation of emulsion, but latterly it has been manufactured in New Zealand. ' Following the completion of the first programme of tar or other sealing of secondary streets in the city, there still remained a big milage of still lighter traffic streets in the residential areas. The dust nuisance in such streets was in no way improved by the fact that other'streets had lost their dust (in fact, to the residents it naturally appeared very much worse), and experience of 'sealed streets suggested that even where traffic is light surfacing pays by reducing maintenance. Accord^ ingly, the council a few months ago approved a further programme of sealing which covers practically all. tho remaining traffic streets, and it is on those streets that the new process is being applied. PROCESS OUTLINED. Either tar or bitumen of the correct fluidity and power of penetration could be' used, and attention was naturally turned to local supplies. The preparation is not .straight tar, but is tar so treated as to take on special characteristics—as one of tho engineers put it, it is not tar, any more than soap is mutton fat, but the mutton fat is the prime ingredient of the soap.. There are several grades of the mixture; some cure quickly, others not so quickly; some penetrate deeply, others have greater binding power. They all smell pretty high; but that does not last. "M.C.L" was the grade most spoken of today when a "Post" reporter was taken round work in hand and completed iv tho Newtown and eastern suburbs districts by Mr. C. I. Kidson, who has charge of this new work. It. is used for the first treatment, a fluid tarry oil which is sprayed generously on the road surface. The surface may or may not be swept, but it is not scarified or loosened in any, way. , The preparation at a temperature of 120 degrees is applied by a gang spray, eight nozzles supplied by a fairly powerful pump, the container and spraying gear being towed steadily by a tractor. The spray delivery is fixed, and tho amount of preparation deposited is variod by the speed of the towing, tractor, which is fitted with a special speedometer for this purpose, but the amount delivered is always sufficient to leave the surface quite wet, though pools are to bo avoided if possible.' The mixture gradually soaks down, the heavier oils not getting far, the lighter,-brownish oils penetrating to a surprising depth. Twoinch stones dug out of a surface treated a week ago were found to be completely covered. TRAFFIC MUST KEEP OFF. Tor a few hours the treated surface is wet and unpleasant, and is to be avoid«l by traffic, so that there is a real meaning in the new notice boards, '' Beware, Liquid Bitumen.'' Gradually as the oils penetrate, the surface takes a leathery appearance, and is then quite tacky and will not pick up, though the penetration is still taking place. Streets are treated in halfwidths in order that traffic may keep off the fresh surface. The gang sprays cover a considet'able width, and about a mile and a half of half-width road can be treated with this first spraying in a working day. The dust is definitely laid, and it is considered probable that this one spraying, thanks to the remarkablo penetrative power of the mixture, will stand up to, traffic in residential streets for perhaps a couple. of• years without further treatment. ROAD SURFACE NOT LOOSENED. Subsequent treatment, as carried out on some lengths in tho eastern suburbs, will include a correcting coat of fine metal bound with one or other of the tar preparations, if there are irregularities to be made good, and a standard sealing coat, cither in tar products or emulsified bitumen style. . It is an interesting development already apparently proved' for roads where the surface is well consolidated, for it is. very rapid, it requires a minimum of preparatory road treatmentblade grading, brooming, but definitely no scarifying, for tho fullest use is made of the existing surface—and the first spray coat, which will probably be sufficient for the lightest traffic streets, will make subsequent sealing work more economical in material and, it would seem from the solidness of the sprayed lengths after the penetration action has been completed, a much more solid and lasting job. ■ Other interesting developments pointed out this morning were footpath renewal by the laying of a light coat of mix on the old rough surface, again making use of an already solid foundation, and the use of a special large brick for kerb and channelling work in Palliser Koad. The bricks are cheaper than concrete slabs, and are under test considerably stronger than concrete (actually double), though they are more subject to chipping. They make a more finished job than concrete slabs, and should look very well as an edging to a grass verge. One point is to be stressed iv rogavd to the new sealing process: liquid bitumen' means liquid bitumen. Traffic must keep off these surfaces until the gloss has disappeared; after that the mixture is quite stable.