A CITY OF CONCRETE

Sun (Auckland), Volume 1, Issue 43, 13 May 1927, Page 8

A CITY OF CONCRETE

Auckland and Its Highways

Twelve years ago the first concrete road in Auckland, and one of the earliest in New Zealand, was constructed in Little Queen Street, a city thoroughfare that bears a share of the heavy waterfront traffic. From that humble and experimental start developed the extensive use of concrete throughout Auckland city and district, with the result that among engineers Auckland is now known as the home of concrete.

On a wide network of Auckland’s concrete roads the ratepayers of the city and the adjoining local bodies have invested thousands of pounds. Not everywhere is concrete accepted as the best of road-making materials, and when what some regard as an experiment assumes such vast and costly proportions, the citizens who put up the money not unnaturally exhibit close interest. in the results achieved. Hence the alarm of the Avondale Borough Council, which was told that disquieting fissures had appeared in a newly-formed concrete highroad, is easily accounted for. . To engineers, however, the appearance of cracks in concrete rarely causes uneasiness. Susceptible to the influence of variations in temperature, concrete has a habit of expanding and contracting. Though the movements and variations of its bulk are imperceptible, it is constantly “on the creep,” and cracks readily develop. Avondale’s Alarm Avondale has been agitated by the fear that the fissures in its road may have been due to a poor foundation. The formation on which the road through the district has been constructed is largely pug clay, which alters its bulk in wet or dry weather. Traffic on a road so inelastic as concrete is liable to cause cracks under these conditions, but engineers state that cracks may appear in concrete without regard to the type of foundation supporting the road. One of the reasons why naked concrete is not altogether ideal as a roadmaking material is, according to Mr. John Dawson, well-known Auckland civil engineer, the fact that it is too rigid to take up expansion and contraction caused by variation of the temperaturq. Allowing for Auckland a range of temperature of 100 degrees, which is a moderate estimate, the roads constructed hereabouts are affected by that much variation in the heat of the atmosphere.

Into this business there enter elements of advanced mathematics, and the technicalities of scientific engineering. The “Co-efficient of Expansion”

Some have called concrete an unscientific material because of its refusal to adapt itself to the fluctuations of the thermometer. Under varying temperatures it is subject to what engineers call a “co-efficient of expansion,” the exact co-efficient being .00000 - of its bulk. The proportion is minute —excessively so according to popular standards —but in a 50ft. slab of concrete, with a range of 100 degrees in the temperature, it may account for over half an inch of expansion and contraction in the entire bulk of the slab.

Such is the phenomenon that explains the Avondale fissures, or at least the majority of them. The difficulty is aggravated because concrete, a nonconductor of heat, takes the heat or cold unevenly through its bulk, so

. that the distortion is confined to its exposed surface. x But all this does not completely mar the effectiveness of concrete as a paving surface. Some, like Mr. Dawson, believe that concrete cannot yield its fullest service unless a protective coat of bitumen is applied. Requiring only modest maintenance, a bitumencarpeted concrete road may be as near an everlasting surface as man has yet discovered, for even under severe traffic stresses the basic road block proves to be as enduring as the rock of ages. Wedded to Concrete Examined yesterday, the surface of Little Queen Street showed little evidence of 12 years’ wear and tear. The bitumen carpet is patchy, but that can easily be renewed. At the same time other compositions have shown themselves to be equally lasting. Duin»m paved its main roads with pure Trinidad asphalt, a material of such plasticity that it absorbs its own expansions and contractions, and at the same time provides a durable wearing surface.

In the opinion of southern engineers, Auckland is wedded to concrete. With the Grafton Bridge, one of the largest concrete structures at. the time it was built, and dozens of towering buildings ih the same material, Auckland is a concrete city, and is recognised to be the largest user of concrete in the southern hemisphere. With its elaborate loading programme completed, it will have one of the world’s finest paving systems —all in concrete, and from north and south it will be reached by long white ribbons of concrete highway. High Cost of Concrete

Cost, everywhere else but in Auckland, has always been a barrier, and for a long time, on account of the expense, the Main Highways Board was reluctant to subsidise road construction in concrete. Even now it does not extend its unqualified approval, but the scale on which work in concrete is now being prosecuted south of Auckland, indicates that its objections were not insuperable. Costs of concrete roads built round Auckland have been in the vicinity of £B,OOO a mile, a tall figure, reduced, in more recent contracts, by the introduction of up-to-date plant. The cheapest concrete road built up to the Highways Board’s standards was formed outside Napier, where the presence of beach shingle deposits alongside the road allowed the successful tenderer to quote £ 5,000 per mile as his price, but in spite of the favourable conditions he was a loser by the time the job was finished. Consolation for the local bodies who shoulder high expenditure incurred by construction in concrete comes with: low maintenance costs as the years i roll on. For whatever the argument 1 about the economic value of concrete roads, where high standards of construction were observed there can be no doubts about their permanency. —J.G.M.