The Road Conference at Olympia.
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Abstracts from the Papers (continued).
How Ear is Skidding Due to the Lack of Proper Street Cleansing?
By E. SHRAPNELL SMITH,
Some engineering friends of mine, who are interested in motorbus developments, recently made a series of tests which enabled them to determine the co-efficient of friction for the rubber-shod driving wheels (locked) of a double-deck vehicle on dry macadam. This proved to be as high as 0.58 (45cwt. draw-bar pull, with an axle-weight of 77cwt.), and it is upon this result that I intend in part to base my short contribution to to-day's conference. It shows that the limiting angle of friction is 30°, which is equivalent to a gradient of 1 in 1.72. This ascertained figure will be used later in the establishment of a scale.
Careful search does not provide much direct evidence on the subject, and there is clearly room for fuller investigation, but the following table will give us certain data which, if admittedly generic, are reliable and from authoritative sources. Others might be quoted, but these will suffice for immediate purposes.
An examination of the above table discloses a marked and proportionate decrease in the co-efficient of friction, between any two of the bodies subjected to test, as the condition of their surfaces becomes more and more removed from the dry and the clean states. This broad fact is, of course, well known to engineers, and a very large number of records exist in connection with certain particular classes of sliding friction—e.g., lubrication research tests for different metals.
Leather on metal (dry), according to the tests recorded in Low's pocket-book, is within 4 per cent. of the special test of rubber on dry macadam, which I named at the outset, the coefficients being respectively 0.56 and 0.58. It is necessary to make some assumption in our present limited knowledge of the co-efficients for rubber under varying conditions, and I have adopted " means '? from the results quoted .above in order to establish my own deductions,
The following comparisons are then available :— APPROXIMATE SCALE SHOWING TENDENCY OF THE SMOOTH RUERFE TIRED WHEELS OF A MOTORBUS TO SKID AS ADHESION IS DIMINISHED.
I have advanced these figures as being a fair comparative statement, so far as present knowledge enables one to assess the various co-efficients for rubber upon asphalt, macadam and wood. It is hard to say how low the co-efficient of friction may fall for the worst conditions of asphalt in London, but Mr. Worby Beaumont informs me that he has found that a very slight push will start a rubber-tired vehicle upon its sliding career on a gradient of only 4°, when such an asphalt surface is "very greasy." This experience corresponds with a co-efficient of 0.069 (and angle of 4°), and a capacity to "hold the road" of 11i per cent, of that which exists in the case of dry asphalt, and it is in close agreement with my own deductions. It is interesting to note here that no road engineer usually cares to lay asphalt on a gradient of more than 1 in 30 (V), although I have seen it stated (Tillson) to be safe for as much as1 in 16 (V), but it is obvious that the reduction in adhesion due to the latter slope would too closely approach the average limiting angle of friction when "greasy, and would in extreme cases be greater than that angle. While the usual maximum cross-fall of a wood-paved London thoroughfare is 1 in 20, with an average of 1 in 36, I do not know the practice for asphalted streets, but it must be remembered that 0,027 is the necessary co-efficient for propulsion on the level, allowing for the proportion of load on the driving wheels, in the worst case, therefore, skidding must occur on a gradient of 1 in 30.
In conclusion, I hope this brief paper may prove to help to send home the need for more frequent hosing, and a more liberal 'use of the squeegee. Clean thoroughfares are of benefit to all classes of traffic, as well as to pedestrians, and I believe that the use of tank-wagons containing water under air pressure, so that a cleansing stream may be applied, at all hours of the day, with sufficient force thoroughly to cleanse the surface, and without inconvenience to the public, is the most obvious and ready solution of the skidding problem. I submit to this conference that street-cleansing methods should be revised and brought up to date as the proportion of motor traffic in our great cities becomes higher year by year.
THE USE OF TAR IN ROAD CONSTRUCTION.
By A. DRYLAND, Assoc.M.Inst.C.F.., County Surveyor
of Wiltshire.
Modern developments of traffic by self-propelled vehicles, deriving their propulsion from wheel friction with the road surface, have led to an idimense increase of the use of tar in various forms of application to prevent the disturbing effects of this frictional wear, and, incidentally, the raising and distribution of the dust particles present on all macadamised roads as commonly formed. The larger proportion of this dust is due to foreign matter either blown or deposited upon the highways, and to excess of binding material, but a considerable amount is caused by the wear due to common forms of traffic. There is no doubt that increased abrasion of the surface, and consequently more dust, is the result of the wheel friction propulsion before referred to, and fast-moving vehicles of that kind, as at present constructed, form most efficient dust raisers and distributors. It will be contended, no doubt, that horsedrawn vehicles are propelled by friction, by means of horses' hoofs, which is perfectly true, but that was so distributed and minimised by the muscular form of application that the dust trouble, previous to the increase of self-moving vehicles, was not of sufficient importance to cause any particular anxiety to the road engineer. The "wear and tear" and "dust problems" are now the important questions, troubling not only the road engineer, but all users of, and residents near, our important thoroughfares.
The solving of these problems does not rest entirely, or even chiefly, with the road engineer, because there must always be a considerable quantity of dust upon roads from other sources than wear, and motor engineers could and should do much by improvements in design to lessen the dust-raising powers of the modern motorcar ; but the devising of some reasonably perfect form of road surface, obtainable at a modest cost, would confer an immense immediate benefit upon the whole community.
The ideal road surface for such a purpose would seem to be one smooth and firm, yet slightly elastic, to give a good foothold, composed of materials sufficiently strong to give off the smallest quantity of dust by abrasion. The attempts at providing such a desideratum have been largely confined so far to the application of tar mostly in the following ways—viz., (1) tar concrete, (2) tar grouting, (3) tar painting and spraying. I will -deal shortly with each of these under their respective heads.
Tar Concrete.
For many years tar concrete has been used for roads, but until recently has been principally confined to providing surfaces for highways of light traffic in towns. The author, over twenty years ago, converted several miles of boulder-paved narrow streets, in a seaside town, to tar-macadam roads composed of broken flints from the seashore and burnt shingle obtained from the same source. Although the materials were by no means ideal, the purpose of providing a clean and comparatively noiseless roadway was served, and, with annual surface dressings of tar and fine shingle, and small repairs, those roadways are still, I believe, good and efficient. In several seaside towns tar concrete compounded of limestone has been in -successful use for many years as a road material. More recently tarred slag has been largely used, and with a considerable, although I believe varying, degree of success. Experiments on a considerable scale have been made with basalts, granites and quartzites as the aggregate for tar concrete. With some of these the author is aware there have been failures.
it has always appeared to the author that the adhesion of the tar to the aggregate was the secret of success in tar concrete. A good deal has been said and written about the "impregnation " of the material forming the aggregate by the tar. The author would like to elicit opinions as to this impregnation, his personal view being that a material that is sufficiently porous to absorb gas-tar to any considerable extent is not of a sufficiently durable character to withstand any considerable traffic. The author's view is that the principal requirement is a hard material which breaks with a sufficiently rough surface to form a good adherence for tar. Many of the hardest and finestgrained road materials are, in the author's opinion, unsuitable because of the -clean smooth surfaces left by fracture.
As to the preparation and laying of the concrete, the following are, in the author's view, essential points :— (1) An aggregate of the character before suggested, free from dirt or dust, with all moisture expelled by artificial heat. (2) Well-boiled gas-tar, that known as "distilled tat" being preferable, applied to the heated aggregate at a temperature closely approaching boiling point.
(3) The thorough mixture of the ingredients in such proportions as will ensure the coating of every particle without an excess of tar, the quantity needed varying with the kind of material and the gauge.
(4) The ripening of the material previous to laying for a period of four to eight weeks in bulk.
151 A firm and true foundation.
(6) A coarse aggregate for bottoming, not less than 21-inch gauge, and topping not smaller than li-inch for roads having any considerable traffic, with a total thickness of 4 inches at least.
(7; The material should be laid on a dry surface, be thoroughly rolled, and receive a top dressing of a small amount of clean stone or slag grit.
Varying success has attended the use of tar concrete, but the author thinks when it has not been successful it has been due to want of attention to some of the above-mentioned details, or to bad weather interfering with the work when in process of laying. There are, no doubt, limits to the ability of the best tar concrete to withstand traffic, just as there are to the best ordinary macadam. In the author's opinion it is more likely to be a success on open roads than in damp or perpetually shaded situations, excess of moisture being deleterious to the life of the tar.
Tar Grouting.
By this the author means the use of tar in some form in place of the usual slurrying of ordinary macadam toads. It has been used with considerable success, especially when followed shortly after by a surface dressing of tar and grit. It is of the utmost importance that the whole operation of laying the macadam and the grouting should be performed in dry, warm weather, when both the road and the material are perfectly dry, and the metal should be firmly consolidated before the tar is applied. The author has seen successful results obtained by this method with a top dressing of clean i-inch basalt chippings.
Under this head a reference might be made to the system of spreading the metal upon a layer of fine and rather moist tar concrete and rolling until this is forced upwards as a grouting. The author has not seen this carried out, but sees no reason why it should not meet with considerable success, especially if followed by a top dressing.
Tar Painting and Spraying.
For both these operations a very dry road is essential, and hot weather a great advantage. Moisture in the road surface will prevent proper adhesion of the tar. A newly-coated road surface is also necessary to any considerable measure of success. Once the surface had become worn and compacted the tar cannot penetrate sufficiently, and a thin film only is formed, which is destroyed by wet or frost. Another most important matter is the thorough removal of dust from the surface and between the stones to permit a sufficient key. All those who have had much experience in the application of tar know that it refuses to penetrate fine dry dust, and will realise the importance of thorough brushing. It has occurred to the author that if a suction process, similar to that in use in the "pneumatic cleaner," could immediately precede the application of the tar it would be advantageous. Perhaps some of the designers of tar-spraying machines may develop this in some practical form. The deeper the penetration of the tar, the nearer the success of the process of tar painting or spraying will approach tar-macadam, and the removal of the dust to a good depth is essential to penetration. Attempts at injection by force will, in the author's opinion, fail unless the dust is thoroughly removed. Within the limits of so short a paper it is impossible to deal fully with all the points and processes which have been tried with more or less success, and the author therefore desires to call attention to certain points with a view of eliciting discussion, and would suggest particularly as worthy of consideration the following points (1) The suitability of tar-macadam for heavy traffic and the best kind of aggregate to withstand it. (2) The size of aggregate and thickness required.
(3) The best top dressing.
(1) Tar grouting, and the increase of life of a macadam road thereby. (5) The hest means of tar painting or spraying, and the preparation of the road surfaces for the same. (6) Reliable figures as to the cost of carrying out the different processes. (7) The life of roads treated by any of these means as compared with that of ordinary macadamised roads laid with highclass materials in the best possible manner.
(8) Experiences as to disintegration of tar-treated roads and the cause.