ROAD PAVING.

Auckland Star, Volume LXVI, Issue 49, 27 February 1935, Page 9

ROAD PAVING.

UNSOLVED PROBLEMS.

SLIPPERY SURFACE DANGER. ADDRESS BY ENGINEER. Rural highways and bituminous surface treatment wore the subject of a paper read by Mr. F. M. Hanson at the annual conference of the New Zealand Society of Civil Engineers to-day. Mr. Hanson said that ae a result of extensive research and careful stucly ample technical literature had been published in connection with the socalled higher types of bituminous pavements, but any literature on surface sealing was confined to typical specifications or rule-of-thumb method.-: of construction. This was unfortunate for New Zealand road cngineere, who were chiefly concerned with some inexpensive form of surfacing-. As methods and treatments applicable to America and Europe were not always suitable to Xew Zealand conditions, the roadbuilders of the Dominion had to solve j their own problem. Sealing Ideal—But Not for Long. Mr. Hanson said that there had been controversy as to whether the sealed road was the best road. It was a marked improvement on the gravel rood, but it presented objectionable features. For a short time after being laid the scaling provided an almost ideal surface, but the ideal condition was usually of short duration. Although climatic conditions made a non-skid surface much more necessary, both in Europe and many States of America, than in New Zealand, complete success at a reasonable cost had not been attained. The complaints against a dangerous, slippery- surface were world wide, and although considerable research had been made, no foreign country had developed a pavement or sealing which would retain really non-slip properties. Nonskid, non-glare bituminous surface sealing which did not "wave" or corrugate had been developed and wae being constructed in the Wellington district. Quite recently irregularities and corrugations which had developed in bituminous roads had been treated with a more even course to provide smoother riding qualities. This was known in New Zealand as honing or retreading, but unfortunately that type of treatment only improved and did not entirely overcome slipperiness. It was certainly not a non-skid surface, and thus the battle had only been partly

Mr. Hanson added that his remarks applied to a large proportion of the bituminous surfacing laid in Xew Zealand and other surfacing. Engineers had everywhere realised the drawbacks of incorrect proportioning of bitumen, but many engineers thought of percentage of bitumen in terms of aggregate voids and unfortunately there had Keen a practice to deckle upon the quantity of bitumen for sealing or of surface treatment by eve or rule of thumb.

The usual tar-sealing practice, both in Xew Zealand and abroad, left much to bo desired. Tests made by the Public Works Department at Wellington showed that chips produced from a rock of a lower crushing strength than 13,0001b per square inch were not satisfactory for scaling purposes, and some greywacke tested as high as 25,0001b to the square inch. Need of Proper Drainage. Mr. Hanson said that adequate drainage was one of the first matters to be considered when preparing a road for any kind of bituminous surfacing. On a recent tour over most of the scaled highways of the North Island it was observed that many miles of surfacing were failing on account of inadequate drainage. On some of the main liighways long sections of bituminous surfacing were starring and cracking up through the presence of water. Lack of sufficient drainage was undoubtedly the greatest cause of failure of scale! surfaces after being successfully laid, and it was incumbent upon engineers to give the question of drainage their closest attention. "The engineer who wishes to continue in favour must be able to furnish the maximum of paving surface at the minimum cost," concluded Mr. Hanson. '"The motorist demands smooth, dustless, non-skid, non-glare roads at a price within his paying capacity. The rule of thumb methods of the past cannot longer be countenanced. Bituminous surface scaling must be designed and proportioned just as scientifically as cement or bituminous concrete, and it must be laid under skilled supervision."