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Experimental Work on Roads

25th September 1936
Page 84
Page 84, 25th September 1936 — Experimental Work on Roads
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Which of the following most accurately describes the problem?

rrlIE introduction to the paper by 1 Major F. G. Turner, B.Sc., A.M.Inst.C.E., is a review of developments in experimental work on roads. It mentions that this really began in 1929, when a road research laborator3i was built, and an experimental road constructed on the north side of the Colnbrook By-pass. The full-scale tests are .under the advice of a committee of engineers appointed by the Minister; the research work under a board appointed by the Department of Scientific and Industrial Research.

Resistance to Skidding.

Until a few years ago quantitative means for measuring the factors making for efficiency in road Construction were absent. The only measurement was by the engineer's eye Some time ago the Ministry, with the National Physical Laboratory, built a special motorcycle and sidecar for measuring the resistance of road surfaces to skidding. This has not been adopted by road engineers as part of their normal equipment, and the development of a cheaper apparatus would be welcome. There is also no instrument conveniently to measure road deformation. An apparatus has been built and will soon be brought into use. If this is as successful as is hoped, a great step forward will have been taken. There is also no means for measuring durability, or simple means for measuring lightreflecting qualities.

To complete records of full-scale tests, samples of materials used on experimental sections are sent to the laboratory. Tests are not made on proprietary materials or methods unless exceptional considerations be involved.

Some of the conclusions regarding the. effect of varying factors in design c34 are set out as follow :—What thickness of concrete is required? Slabs 6 ins. thick, with double reinforcement, carried satisfactorily traffic, which in 1929 was 12,000 tons per day and in 1935 16;000 tons per day, on a 20-ft. carriageway. The trouble was that the concrete cracked at the corners of the slabs. What are the effects of varying the water-cement ratio, and of laying concrete on (a) wet clinker, (4) dry clinker, and (c) waterproof paper? On dry clinker the concrete should be wet, on saturated clinker or waterproof paper as dry as is consistent with a good surface finish. What is the effect of steel reinforcement? The results indicate that the only advantage of a single layer is to prevent cracks from widening once they are formed; the use of two layers has a definite advantage in resisting the tendency to crack. No results that could be noted were obtained from surface treatment with silicate of soda.

It was found that sudden temperature changes, particularly such aa caused by rain after heat, tend to cause

cracking. Tests showed that abra.sion • is a negligible factor; the average .wear over the experimental road was only. .025 in. in four years.

Vehicle movement causes the slabs to rock. An 8-ton back-axle load usually causes a deformation of .01 in. of one slab relatively to its neighbour, but at one joint the deformationreached .03 in. Thus under the conditions which obtain on this road, simple butt joints are not satisfactory.

After six years' wear, the condition of the road is such that it can carry traffic for an indefinite period, but from the experimental point of view it has almost reached the end of its useful life.

A Non-skid Treatment.

As shown by tests on the Kingston By-pass, it is possible to lay a tar or bituminous surface which is eminently non-skid, but under traffic it may tend to become smooth and polished, and then require surface treatment. Application of tar and bitumen covered with hard-stone ehippings rolled in, is inconvenient to road users, and its repetition tends to cause the top layer of the road surface to become undesirably rich in tar or bitumin. These considerations have led to the introduction of what are called thin carpets, composed of balanced mixtures of stone and binder.

Until recently, it was thought that a surface must obviously be coarse and rough, but extensive research in America tends to show that " sandpaper" textures are better.

The position appears to be that with suitable materials and careful supervision, surface dressings can be carried out to give satisfactory results for three

years or more.


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