Colonel Crompton' s Manchester Paper.
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{ Continued from page 361.) Passing from his interesting references to percussive action between road-wheel and roadway, Colone.1 Crompton .proceeded to explain some of the factors which must help the road engineer to determine which class of crust and foundation he will adopt, accovding to local conditions and intensity of traffic. We may emphasize, for the guidance of our readers, the fact that Colonel Crompton bases his views, not only upon his own technical experiments and observa, tions, both in his own laboratory and at the National Physical Laboratory at Tedderigton, but upon definite practice ift highway engineering, principally in the United States of America, and also, to a limited extent, in the United Kingdom.
Probably the most common case will be that. of superposing our new road surface on the top of that of the old road so that the old road really forms our foundation, and this ears generally be done in country districts, as the raising of the road surface entailed by this procedure does not cause appre ciable inconvenience. But where the roads pass through villages or inside towns, it is. not possible to raise the surface of the carriage way on account of the existing footways and their leerbs.
If the old road surface is of full strength and has carried its traffic without breaking up, the new bituminous 'Dwell layer might be reduced in thickness, and as it is advisable that the upper la.yer should be in no case less than 1 in. or preferably 11 in. in thickness, in such a case good results might be obtained with a total thickhess of new crust not exceeding 2'4' ins., i.e., 1 in. of coarse material, 1 in. of the fine wearing coat..
Thicknesses and Percentages.
Wherever the strength of the old road is insufficient, the lower coat must be increased in thickness, but it is probablo that in all cases sufficient strength is obtained by a lower coat of 3 ins., inakim, with the 11 in. wearing coat, 4,i ins. in all, and as the average specific gravity of the two coat works out at about 2.1 or 132 lb. per cubic foot, ittis found in practice that 1 ton of this bituminous concrete will, when consolidated, cover from 5 to 8 superficial yards, according to the requirements of strength and the traffic. On account. of this varying thickness, road engineers, in comparing prices, find it convenient to reduce the whole cosl. to a price per ton of either class of material, coarse and fine, rolled down in position and finished off ready for the traffic. Of the materials that have to be used, the binder is the most expensive, whether it be pitch or bitumen. Up to the present the cast of the bitumen has been practically double that of the pitch for equa.1 weights employed, but in either case the cost is very largely determined by the percentage that is used in the two coats, and this is one of the reasons why the material should be distributed between two coats. It is generally found thgt for the coarser aggregate of the lower coat from 7 per cent. to 8 per cent, by weight of binder is sufficient, whereas for the fine upper coat the prdportions vary according to the grading of the material employed from between 10 per cent. ti5 14 per cent., but in both cases it is the object of the designing engineer to cut down expense by producing the result he requires with a minimum percentage of binder. As the considerations I am now going to mention apply to both coats, but to the greater extent to the upper wearing coat, I will deal with this first.
The Composition of Ordinary Asphalt. We find in practice, if we take up a portion of rock asphalt pavement from the streets of Paris or of Loudon, where the surface has been formed from compressed rock asphalt, that the content in page bitumen will be quite small, varying from as low as 5i per cent. up to 8 per cent., the aggregate with which this is combined being mainly a very fine powder of lime or Magnesia carbonates. So far we have none of us succeeded in obtaining an equally stable and good wearing surface with an artificial mixture of bitumen except with the use of a considerably larger percentage of binder. In most cases the American specifications rewmmend that this percentage should lie between 9 per cent. and 14 per cent. I have spent a great deal of time in investigating this matter by laboratory. methods. I have supposed that efficiency c12 on the road depends on the consistency of the material to carry weight, and on its toughness to stand the percussive action of the wheels or of the horses' feet. I have imitated this by simple compression tests or by percussive tests in the form of slandardized hammer blows applied to the flat ends of test cylinders 1 in. diameter by about 1i in high, as such cylinders can be conveniently and readily moulded by 8imple means and the comparative tests for consistency and toughness can be easily made. I adopted these dimensions fur my tests because they had already been recommended dn American practice and have enabled me to compare my laboratory work with that of the American engineers. I may say further that the usefeletess of these laboratory tests has already been justified by the success of the pavements already made in London, which are standing remarkably well and which I naturally hope will continue to show improvement over others made up on rule-of-thumb methods, Interesting Laboratory Experiments.
Briefly, my laboratory investigations point to the great importance of the size and shape of the grains of mineral aggregate that are used. I first used seed obtained front various sandpits in the vicinity of London, or dredge(' from the Thames, and I found that not only was it difficult and expensive to grade the sand into the sizes of grains required to give the best results, but that the grains themselves are so rounded that they give small facilities for adhesion of the binder to the surfaces to be cemeettd together. I decided therefore to use artificially-produced sand and have succeeded in" obtaining what I require by pulverizing by impact methods, that is by the use of machinery where fragments of stone descend and come into contact with high-speed hammers which successively sub-divide the pieces of stone until the desired degree of fitness is obtained, and further what is of cosinmercial importance is. that the fineness and grading of the differentsizes can be obtained with sufficient accuracy at one operation by the simple means of driving the revolving crusher' by an electric motor which, as you all know, admits of accurate and easy speed regulation.
Necessity for a Filler in a Binder.
At an early stage the Americans found out that the aggregate to which the bituminous binder is to be added, should contain ins addition to graded sand a certain proportion of extremely fine pulverized Mist, which they call the "filler,'' end that better results !re obtained by using filler in nearly equal amount to the 'binder itself than by omitting it. My feyis experiments have shown that they are correct in this had that. the filler itself .plays a very important part. To take a very simple case. If we wish to mese up an artificial 8-h&A asphalt ciontaitheg 10 per cent, of bitumen it is better that this should be in the form of 10 per cent, of bitumen and 10 per cent, of filler aud 80 per cent, of sand, than that it should be 10 per cent, of binder and 90 per cent. of sand. Tine reason for this is somewhat obscure, but I think we may suppose that, the ca-use of it is somewhat analogous to that of the house painter who finds that certain mineral powders mixed with his oil cover a greater serface than others, so that in our case by combining extremely finely powdered filler with the bitumen we get a bituminous paint of great covering power and consequently increase the surfaces of the aggregate that we can effectually cover and cement together with a given weight of bitumen; I have made a considerable number of experiments to determine how the quantity of filler earn be varied with advantage and have produced some remarkable results. Some of the specimens which I am oxhibitiug here to-night snow that by adopting certain desta which are produced in industrial processes in the North of England, as a filler to take the place of the more expensive Portland cement or lime powders previously used, I have actually increased the toughness of my .specimens measured by the number of blows they will stand without cracking, as much as 50 per cent., and with no. increase of cost.
Foot-hold and Non-skid Surfaces.
One feature that most be borne in mind when designing a Theet asphalt pavement is that it must contain a certain proportion of grains of sand sufficiently large to make it wear to a gritty surface El) as to afford good foothold for men and horses, and to possess non-skidding qualities for motor
vehicles. It is found that the addition of quite a low proportion, not exceeding 10 per cent, of grains of sand lying between 10-mesh and 20-mesh sizes, produces i this result and there is no advantage in increasing it but an absolute disadvantage as it makes the surface more permeable to moisture and reduces its wearing quality.
No More Rule-of-thumb in Road-making.
I have dwelt at some length on this new and promising art of preparing the surfac.es'of,our roads as it is somewhatnew and will be interesting to many of you. There is no doubt that a great deal of our future work will depend on-the, experimental work now going en, and as is usually the case the more minds that can be directed to this art, themore complete and rapid will be the progress. Moreover, I believe that the days of rule-of-thumb road making are past, and that road engineers must adopt modern methods, viz., laboratory work and 'laboratory testing confirmed, of course, by testing out on the road, by the adoption of means by which the specifying engineer can from time to time test for himself or through his subordinates to see that the material used is closely in accordance with his specifications as the mechanical engineer would insist upon when carrying out the construction of Arn/Cilinery or ef iron or steel structures of any kind. You have doubtless most of you heard of the means of connecting laboratory work with the actual tests on the roads themselves which have been introduced atthe National Physical Laboratory in the shape of aroad machine which is a means of testing to destruction a circular path on which is laid the material to he tested either quantitatively or in compmvison with standard materials laid in the same track. The slides shown will give you a general idea ct the construction of the machine, which is is circular framework carrying eight wheels driven by electric power ; the wheels can be loaded to any desired extent; the speed can be varied within desired limits and the rate of wear or the nature of deformation of the surface can be accurately noted. This machine has been at work for nearly a year and is already yielding promising results, results which I think will conneec the similar tests made on material in the laboratory with the experiments on the road itself. Later on, no doubt, on the same machine, we shall be able to carry out, tests to see the effect of various road surfaces in wearing away the surface of cribber tires, and I believe that the machine will befound as valuable to the community as the private naval tanks/and the recentlyput-down public naval tank has been to naval architects, which have resulted in large economics measured by fuel consumption or by efficiency in obtaining increased speed at the lowest cost for fuel. 1 believei this will be found to be equally the case with the road machine either in its present form or as it may be developed DS time goes on.
The foregoing extracts from Colonel Crompton's paper, which was read before the Manchester Association of Engineers, will show haw carefully and explicitly he stated the case, in respect of which 1„,:e may term the now era, and method of road construction. 'We may add, in leaving that section of his paper, that evidence is not wanting to show that
wonderful economies will be effected, as compared with even up-to-date systems of waterproof road construction as we know them to-day, by the adoption of the methods which have been indicated.
Improving the Vehicles: Multiple Axles.
The following brief concluding portion of the author's paper was devoted to a few points which will be read with interest, if not with. oancur?ence, by owners and manufacturers. We quote these in full:—
I have dwelt at considerable length on what is to be gained by improved surfacing of our roads and left but little time for the question of improving the vehicles themselves. Just now when the manufacturers who are turning out motor wagons, vans and ears of all classes are so full of orders I shall not be regarded with much favour by their managers if I venture to point out that agood deal remains to be done to the vehicles themselves to make them carry goods or passengers, at the lowest total cost., i.e., when the running cost of the vehicle is added to the cost of the upkeep of the roads. All railway engineers know that two-axle locomotives Or even two-axle wagons are looked on with disfavonr on account of such two-axled vehicles punishing the rail-ends and joints to a far greaterextent than when multiple-axle wagons are used. It is practically certain the same applies to the roads and that. the tendency tc wave deformation, which is everywhere observable, could be minimized by providing some Of the vehicles with more than two axles.
A moment's consideration will show you why this is the case. When two-axled vehicles are need they always tend to accentuate and increase wave.formation by punishing the hollows and failing to wear down the high places. With a three or fourZxle vehicle the reverse is the case the crests are punished and the hollows arc eased.
I have already done something in this respect by developing a three-axle roller, a_7.sliderof which I will throw on the screen ; this roller has undoubtedly rolled road surfaces remarkably free from undulations or initial waves. The Renard train of which you have all heard introduces the same principle of three '-axles on all the vehicles, and in addition makes the whole of these driving axles, so that the tangential strains, due to propelling the vehicle, -exerted on -the surface of the road are considerably diminished.
Apart from this, the vehicle designer can help the road engineer by increasing the diameter ofhis wheels as the tendency to deformation is greatly increased when'small wheel
diameters are„used. Unfortunately this increase of wheel diameter somewhat increases the cost of construction and militates against easy loading, especially when the vehicles have to be loaded over the sides, but 'there is little doubt that there is a very strong feeling on the part of road engineers that the weight on axles Min, wheels of small diameter has been overdone; that even those sanctioned by the Local Government Board are too great and as they are frequently exceeded the roads are punished to an unfair extent, and that better results for everyone would be obtained if the loads were divided over a greater number of wheels and if the wheel diameters were somewhat increased.