TESTING PAVEMENTS

Evening Post, Volume CXIII, Issue 32, 8 February 1927, Page 10

TESTING PAVEMENTS

FINE POINTS OF ROAD WORK

SPEEDING UP ANALYSIS

Though anyone can lay down a concrete path or an asphalt ipath in his own backyard and survey it with due pride, until it commences to break up unexpectedly, the city engineers, have to step forward warily, for one bad crack in one good pavement will bring down upon them critics unnumbered. The amount of time and energy which is expended in looking into the finer points of road surfacing by the engineers is certainly not realised from a casual observation of road pavine work.

When the original Hutt road was being surfaced, strong faith in overseas specifications and a fair share of good luck combined to make a first-class job, but in later work things went wrong here, and there and were for the time being difficult to understand. The specifications were all correct, and those specifications were followed, as nearly as possible, which was not near enough, and so it was found that waves and cracks in surfaces which should have been as good as anything laid down elsewhere were due to minor variations from exact formulae, as to quantities and finenesses of metal, sands and "filler" dusts.

For some time the council carried on with a minimum of tests, having. no analysing chemist :o cheek over materials used or the results achieved in pavement laying, but the false economy of that policy was not long in making itself known! It was decided, therefore, to engage an analytical chemist who should busy himself with this one job, and in Mr. T. P. Rollings the council certainly found an enthusiast, and that is well, since to a, great extent the success of the two big paving schemes, city and suburban, depends upon his work.

LIKE A CREAM SEPARATOR.

Not only are the materials subjected to very careful tests before mixing, but from each length of finished surface samples are cut for analysis, to be broken down into their original bitumen, metal, sands and filler. Until recently this breaking down was carried out by an extraction test, the bitumen being distiNed, i.e., driven off by heat as vapour, and the residue being sifted and weighed. The distillation occupied about six hours, so that the rule was one day one test. Now the breaking down is done in from ten to twenty minutes, in an electricallydriven machine like a cream separator. The sample for test, broken into pieces about the size of walnuts, is placed iv a fiattish cup and is flooded with an evil-smelling liquid, carbon disulphide, a rapid solvent of bitumen. The cup is spun, up to 3000 revolutions per minute, and the liquid is thrown out, through filter paper, leaving the solids behind.

Several washings are necessary, and an extra few minutes' spin removes the last traces of moisture, leaving the metal and sand, in fact, bone dry. By weighing this residue and subtracting from the weight of the sample before treatment the quantity of bitumen thrown out is ascertained, and machine sifting and weighing give the exact quantities of metal, sands and filler in the sample. So exact is this test that the finest dusts .which are caught in the filter paper must also be weighed, and as it is impossible to retrieve them from the fibre of the paper, the filter is burnt in a crucible and the ashes are weighed. The weight of the ash of a standard (unused) pad is known, and subtraction again gives the weight of -filler trapped.

There are fine points, even in laying down 40 miles of paving.