Colonel Crompton s Manchester Paper.
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One of the important pre-show fixtures at Manchester, was Colonel Crompton's paper of, Saturday last on "Commercial Motor Road Traffic." • • This was read at the Midland Hotel, before an overflowing meeting of the Manchester Association of Engineers, and it was to be observed that many local owners of commercial motors attended as guests, or by invitation. An interesting discussion followed. The chairman of the C.M.U.A. made opening references to various historical achievements, including his early road experiences in India and elsewhere. He introduced the modern aspects of motor transport by quoting typical instances of point-to•point conveyance, and emphasized the value of eliminating terminal handling. We extract a portion of the references to the undesirable factors which are inseparably attached to horse-cum-rail methods :— Here we have five or six distinct operations, all of which take time and cost money, to be added to the actual cost of haulage. Apart from the actual cost of handling or transferring goods during such transport, a large proportion of goods suffer damage by frequent handling. This is particularly the case with the perishable goods which do not keep if the articles touch one another and the containing boxes or baskets are changed in their position during the handling. An allowance has to be made for such damage, so that it is well understood that the three items of packing, handling and damage in transit due to handling have always to form an appreciable and varying fraction of the total cost of transport. Now the door-to:door transport by road given by the motor wagon or van in most cases need be debited with these charges to the smallest extent; in fact, road transport in this particular shows to great advantage so that it presents advantages in all cases where such charges form a large fraction of the total cost. Here it will be seen what scope there is for the designing engineer to obtain the full advantages of carrying perishable foodstuff and similar materials in road vans specially designed to allow of easy loading, little damage during hauling and easy unloading. There is no doubt that in this way great economy can be introduced by suitable design and laboursaving devices.
I give prominence to this matter because I believe it will play a great. part. in the advantages we are likely to obtain from transport by road in developing food growing on much of the land not so used now on account of want of transport.
Colonel Crompton next turned to certain considerations affecting design and construction. He declared himself a strong advocate of rubber tires, and stated that the demand for speed necessitated them. The wider use of rubber would have its effect upon roadmakers, who were now able to produce asphaltic or similar surfaces suited to rubber tires. These roads would not wear the rubber as much as other classes of roads had done in the past. The better the road, • the less the wear on the tire. Proceeding to elaborate this view, Col. Crompton proceeded :•-
; In order that we may attack this problem systematically I propose to go in some detail into the road' improvement question, to show what I believe can be done tu improve their surfaces so as to reduce the wear and tear of the vehicles and their tires to the lowest limit, and then to consider the vehicles to see how they can be improved so as to damage the roads as little as possible and transport goods or passengers at the lowest possible cost.
The author had the following observations to pass in respect of the corrugation of some highways :
We find that at. the high speed at which road traffic must now be carried we cannot consider the wear caused by the vehicles is that due to pure rolling action of the wheels. At present speeds simple rolling action does not exist. In all cases the vehicle progresses by a series of short leaps or bounds, the smooth rolling being transformed into a harmonic percussive action or a number of blows delivered to the road of greater or less severity depending very much on the shape of the road surface. Even ith a steel wheel oil a steel rail true rolling rarely, if ever, exists, for although it is true that with rail and wheels in good order the difference
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between true rolling and harmonic percussive action is but small, even in this case such harmonic percussive action is now known to be the chief cause of the corrugation so often observed on tramway rails and in some cases on railway rails, but at any rate it is now always found on ordinary road surfaces.
I need hardly recall to you how soon this harmonic effect shows itself when a line of motor omnibuses has been running for a short time over an ordinary macadam road, although this road be smooth and level at the commencement of such a line of traffic; in quite a short time after the traffic commences well marked cross-corrugations or waves extending across the whole or part of the width of the road show themselves and necessitate reconstruction so that this wave formation may be resisted as well as we can do it.
We find that this harmonic wave-producing action always starts from an initial cause, generally from a projection, either a large stone or the top of a metal surface box in the readway. it also starts from the exaggeration of slight undulations originally left in the road at the time it was resurfaced_ It is therefore evident that our efforts must be addressed to producing our road surfaces as free as possible from such initial waves or promineeces.
In Colonel Crompton's opinion, engineers have a very complex problem to solve, at the present time, in order to provide a carriageway that shall suit everybody. He considers that the road surface must be such as to maintain itself in a satisfactory condition throughout extreme vicissitudes of temperature, moisture, hot sunshine, frost or fog. It ought to be sightly elastic, and it must be completely waterproof, whilst the cost must not be excessive. Colonel Crompton favours the proper use of bituminous or pitch binders. His views as to their use may, here, briefly be stated thus : So far as I have been able to observe, the broad line of distinction between the coal-tar pitches and the earth-oil bitumens is that the former pitches within working temperatures on our roads have always the characteristics of a liquid. They possess surface tension and consequently,' have a natural tendency to reduce their surfaces and form spherical rounded masses or beads of pitch. Moreover, the viscosity of these pitches is very susceptible to temperature change, so that a coal tar pitch which is so soft as to run freely at the temperature of sunshine, 100 deg. Fah., in summer, will be as brittle as glass and be friable and chip in frosty weather. On the ther hand, most of the pure bitumens possess the opposite characteristics of solids. They apparently have no surface tension, no tendency to withdrawthemselves into spheres or spherical beads or to separate themselves from the, surface to which they act as the cementing medium. To some extent they are elastic; in fact, the bitumens may be described as 'hydrocarbons which are solid or jelly-like solids of low elastic nipclithis_ In other words, although the elasticity or resilience of the bitumen is not considerable, it is sufficient to make it extremely valuable as a material for road making, as will be seen when we consider in some detail the design of a read surtace.
From the time when wheeled transport was introduced the evolution of roads has been gradual. The first object was to prevent the wheels sinking into the road -surface when it is softened by water, and at a later date from pulverizing the surface into dust; and after mechanical transport was introduced we had to resist the formation of rhythmical waving. We found no difficulty in doing this with wood pavement as the concrete foundation is laid with straight edges to fixed points, and the same with the foundation for the later form of asphalt. pavements. Again, with the older form of asphaltic pavements which consisted of concrete foundation covered by 4 in. or 2 ins, of finely ground rock asphalt applied in this form of a heated powder, laid and beaten to form by hot iron beaters, any tendency to initial harmonic waving can be counteracted, The durability of these surfaces is :ksisSss!..n..te be very_ great, and it is not until these asphalt surfaces are considerably worn that troubles from wave formation are experienced. But when we come to reconstruct the surfaces of less heavily trafficked roads immediately outside our busy centres, cheaper forms of construction involving consolidation by rolling must be adopted and much mechanical ingenuity and personal skill is necessary to avoid initial waving. As in America, the deposits of native bitumen or the supplies of asphaltic oils from which bitumen can be obtained are very widely spread, whereas the rock asphalts are only available to a limited extent, the efforts of American engineers were directed at a very early stage to producing artificial asphaltic pavements, and to imitate the rock asphalt by graded sand cemented together by what they have termed asphaltic cement, but which the author prefers to call a pure bitumen Nv hich in viscosity, ductility, absence of surface tension and other physical qualities very closely irintates, if it is not actually identical with, the bituminous binder that we can ex.tract from the European rock asphalts.
Road surfaces were formed with sand and bitumen nearly 30 years ago, and although it was some time before their use became extended, yet fortunately some of these early roads still exist and afford us valuable data as to the durability of this class of material. 'Work in America with these sheet asphalt pavements was carried on to an increasing extent, but it was only about eight years ago that the causes of
success and failure were carefully looked into. Recently great progress has been made, the advantages of which have been felt on this side. You all know what a successful surface has been laid on the Thames Embankment. •I personally have had some hand in designing pavements of the same class which are withstanding successfully the severe harmonic traffic of the London motor omnibuses.
As the production of our roads at the lowest possible cost is of paramount importance, the first thing we had to consider was how to cut it down by using the cheapest materials to
give the requiredresult. With the American experience before us we found that we could best do this by dividing the crust of our reconstructed roads into two coats or layer's, i.e., a wearing crust composed of graded sand and bitumen supported by a lower layer of coarser bituminous concrete. In softie eases this may rest on the subsoil itself provided this is hard enough to carry it; in other eases, on the old macadam surface of the road that is being reconstructed, and in the case of extremely heavy traffic on a special hydraulic concrete foundation. The choice of this must be determined by circumstances.