Road Research Makes Steady Progress
Page 35
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The Road Research Board Reports a Year of Advancement, During Which Light Has .Been Thrown on Some Important Problems of Direct Concern
to Road Users
1VIANY topics embracing a wide iVirange of problems of vital interest both to road engineers and users are discussed in the annual report of the Road Research Board for the year ended March 31, 1938, which has lately been issued by the Department. of Scientific and Industrial Research (H.M. Stationery Office, 4s. net). The work dealt with in the report is carried oat at the Road Research Laboratory, Harmondsworth, Middlesex, and its object is the accumulation of the scientific knowledge that is essential to the economical and efficient construction and maintenance of the highways.
Impact forces, abrasion, due to the rubbing of tyres, and weathering are the main causes of wear in roads. The measurement of impact forces between a vehicle and the road has, during the year, led to important results. It now appears possible, the report states, even in the complicated case of a six-wheeled lorry, to predict, from a knowledge of surface irregularities and the vehicle characteristics, what forces will be imposed on a road.
Impact measurements have been checked by high-speed eine pictures of wheel movements and tyre distortions during the passage of vehicles over an irregular surface. Records were made on a private car travelling over an obstacle at 22.5 m.p.h. and 50 m.p.h. and of a lorry at 15 m.p.h. and 40 m.p.h., the pictures being taken at a rate of 1,000 to 2,000 per second.
Obtaining Intense Lighting.
In the ultra high-speed camera, the exposure given to each picture is of the order of 1/6,000 second, which demands a lighting intensity on the object to be photographed much greater even than is provided by bright sunlight. This lighting was provided by photographic flash bulbs arranged in a row a few feet from the path of the wheel. These bulbs were exploded by a succession of electric contacts placed on the road and operated by one of the front wheels of the test vehicle.
Skidding tests are to be made on road surfaces at speeds up to 75 m.p.h. by an apparatus towed behind a highspeed car.
The report refers to a method of improving the skidding resistance of those few concrete surfaces which, through faulty design or workmanship, are undesirably smooth. This consists in treating the surface with acid, and tests have shown that marked iniprovemeut is thus obtained. The cost of this method is likely to be considerably less than the cost of resurfacing.
The report states that investigations on the pressure exerted by pneumatic tyres on road surfaces have also developed in a way which has led to a better understanding of skidding
phenomena. It has been found that local pressures on small projections, such as the individual stones in a surface dressing, may amount to many hundreds of lb. per sq. in., and that the intensity of pressure depends, to a marked extent, upon temperature.
It has long been known that large seasonal variations occur in the skidding characteristics of road surfaces, and it now seems probable that changes in the physical properties of tyres may he responsible for a part of these variations. Work which is now in hand will make it possible to examine this hypothesis.
The Schichile of Investigations.
The normal sequence of investigations at the Road Research Laboratory is small-scale laboratory work, tests with road machines, and, finally, fallscale road tests. The road machines enable actual road conditions to he reproduced at the Road Research Laboratory and sections of roads constructed in various ways to be 'subjected to accelerated traffic tests.
In the largest of the three machines in use a full-sized lorry is driven electrically at speeds upto 40 m.p.h. around a circular track 110 ft. in diameter. Another small machine is now under construction in which conditions of temperature and moisture met with on roads can he reproduced artificially. Weather statistics are being collected from different parts of the country so that suitable cycles of sun, rain and frost can be worked out.
The value of road-machine tests depends entirely, the report points out, upon the degree to which the relation between such tests and tests on the roads themselves is known. The road machines are now fully occupied in an attempt to establish this relationship. In comparing road-machine tests of various road surfacings with tests on the open road, a knowledge of traffic' conditions is required. Special apparatus has been devised which automatically counts, throughout the 24 hours, the vehicles passing in both directions along a road.
This apparatus has been installed at the Col nbrook by-pass, outside the
Road Research Laboratery. The traffic, in passing over rubber traffic pads, inserted in the road—of the type usually used for actuating traffic lights —closes relays, which, in turn, operate a number of electric counrers. The pads are adjusted so a.s not to count cycIe.s.
It is nor only necessary to know the volume of the traffic, but how it is distributed across the width of the road. This is being determined by a " camera obscura " placed in a hut alongside the road. In this hut observers see a view of the road optically projected on to a white disc on which lines are drawn parallel to the kerb. Thus, without making any lines on the road itself, which might affect the normal flow of traffic, the sections of the road used by various types of traffic can he observed. What is called a wet road clock, depending on the changes of electrical resistance between metal strips laid in the road, records the length of time that a road is wet or dry.
An increasing amount of attention was paid during the year to the study of road foundations, and the whole position of research on earth works and foundations in relation to roads has been reviewed.
For measuring the temperature inside bituminous road carpets, during laying and rolling, an electrical thermometer is used with which a therrnojunction is formed at the point of a hypodermic needle. This instrument is similar to that used for measuring temperatures inside tyres.
Testing Surfacing Mixtures.
Among laboratory investigations described in the report now brought to the point of full-scale testing is work on bituminous surfacing mixtures. 'The simple forms of test devised for bituminous materials have been applied to the determination of the most suitable binder contents for different types of aggregates filler and binder. The addition of small percentages of chlorinated rubber (rubber treated with chlorine either in the form of a gas or a. liquid) to tar has been shown to be accompanied by certain improvements in properties. In general, it has been found that the addition of chlorinated rubber to Lar extends the temperature range over which the material remains plastic. In other words, it tends to prevent it becoming brittle in winter and soft iii summer.
In all cases, however, the true significance ot these results can only be fully appreciated when they are tested out under practical conditions on the road. Arrangements for such trials are in hand. To some extent they are already in progress in an experiment on the Colnbrook by-pass, in which 44 sections of differing compositions have been laid on the road, umier carefully controlle(I conditions.