Clifford Richardson

“ Whatever value a bitumen may have as a binding material for highway construction, is due to the presence and the character of the petrolenes. The solid material in itself has no binding power, but by its solution in or mixture with the petrolenes it gives to the latter their binding power, and also adds to their stability. ”

“ It is in this direction that science has proved itself of the greatest service to the highway engineer,
The destruction of the surface of a water-bound broken-stone road by motor traffic is due, according to experiments conducted by the Office of Public Roads, to the shearing or grinding action of the tires of the rear wheels of cars, which under the impulse of the engine, revolve at a slightly higher rate than that corresponding to the movement necessary to conform to that of the car over the road. ”

“ Public Roads in other states, were, and are to-day, entirely suitable and satisfactory for carrying horse-drawn traffic. When, however, self-propelled or motor vehicles became an important part of the traffic which these sufaces have to sustain, the latter have been found to be entirely unsuitable for the purpose.
The automobile has introduced an entirely new element into the road problem, and one which can only be solved by the application of the scientific methods. ”

“ From what has been said it can be seen that the rôle of the chemist in highway construction to-day, where bituminous materials are becoming so important an element of it, is an important one, that science can contribute much to the improvement of highway construction, and that these contributions should not be neglected where it is proposed to do the highest type of work, and to produce a highway surface which shall resist the heavy travel to which they have been subjected since the advent of the motor car. ”

“ The grades of the road are carefully studied and laid out in the most favorable manner by an engineer so as to make it as level and straight as possible with due regard to the economics of the problem. Corresponding to these grades the subsoil foundation or subgrade of the road is constructed either by cuts or fills, so that its surface is at a depth below the surface of the road as it is to be finished corresponding to the thickness of the compressed material to be built up thereon. ”

“ For the collection of these data and also, to a great extent, for their interpretation the highway engineer is dependent on the chemist, who is, therefore, becoming a considerable factor in successful road building.
Bitumen is a native material, that is to say, it is found in nature. The by-products of industrial operations, such as tar from the manufacture of illuminating gas and coke ovens, is not bitumen in the acceptation of the word as it was originally applied by the Latin writers. Coal tar is a bituminous substance merely from its resemblance to bitumen. ”

“ The subgrade must be so prepared, especially in fills, by the use of proper material and thorough rolling with a steam roller, that it is absolutely stable and rigid, and will not be thrown out of shape by frost. The preparation of the subgrade is one of the most important points in good road construction and, although it is purely a structural problem, it is too often neglected or passed over without sufficient consideration and care. ”

“ He has given little thought to the manner in which it has been constructed and has been, usually, quite indifferent to or ignorant of its cost originally or of that for its maintenance. As the use of the automobile has become so general there has recently been a very decided change in this respect. The movement for good roads has arisen and a general interest in the subject has developed. In what follows an attempt will be made, for the benefit of the general reader, to outline the development of the modern methods of highway construction, and to show how science has aided therein. ”

“ The relative proportions of sticky petrolenes and cohesive asphaltenes is the most important element in bitumens which are used in the construction of asphalt pavements and bituminous highways. It has been found that the asphalt cement, that is to say, a solid asphalt combined with a suitable flux to provide the proper consistency for practical use, if it contains less than 15 per cent, of asphaltenes will lack cohesiveness and stability or body, while, on the other hand, if it contains less than 70 per cent, of petroleum it will not be sufficiently adhesive. ”

“ On some of the roads near Boston automobiles furnish more than sixty per cent, of the traffic, and, during the summer, ninety per cent, of the vehicles used on the leading state roads were of the motor car variety. The conditions in both countries are seen to be the same. Lord Montagu, of Beaulieu, has calculated that the amount of gasoline used in motors in England in the year 1910 was sufficient, at 15 miles travel per gallon to represent a mileage of 600,000,000, and Mr. Maybury says: "What are we engineers doing to meet this revolution in traffic?" ”

“ The second or upper course, which forms the surface or crust of the road, is composed of finer stone, one half to one and a half inches in diameter, to a thickness of at least three inches when compacted. Great advances in highway construction have taken place since Macadam's day, in that steam rollers have been available for some decades for compressing and putting in place in a proper manner, the broken stone after it is applied to the foundation. ”

“ Since the advent of automobiles, particularly those capable of being operated at high speeds, it has become evident that $100 a mile a year is wholly inadequate for the maintenance of macadam roads, even if they be only of the width of the Massachusetts state highways, and that in order to keep such stone roads in perfectly good condition at least $300 a mile a year should be provided. ”

“ These facts are sufficient to show the damage that motor vehicles are doing to our roads of the water-bound type, but it must be remembered that if the traffic consisting of horse-drawn vehicles had in itself increased to the same extent as the number of motor cars now using our roads, the cost of maintenance would have increased to a large extent. Before 1900 there was no demand for trunk line roads to be used by horse-drawn vehicles in the same way that they are now used by motors.
The number of self-propelled vehicles is increasing every year. ”

“ Figures in the possession of the Massachusetts Highway Commission show that about 53 per cent, of the destruction of state highways is due to automobiles. In seven counties near London, England, the percentage of increased cost of maintenance, due to automobiles, has been recently reported to be from 22 to 77 per cent., and this condition is probably more or less the same throughout England. ”

“ The specific gravity, is the weight of the material compared with that of an equal volume of water, and is obtained by dividing the weight in air of a rock fragment by the difference of its weight in air and water. Given the specific gravity, the weight per cubic foot of a rock is found by multiplying this value by 62.5 pounds* the weight of a cubic foot of water. ”

“ Hardness is the resistance which a material offers to the displacement of its particles by friction, and varies inversely as the loss in weight by grinding with a standard abrasive agent. The test is made in the following manner: The test piece in the form of a cylinder about 3 inches in length by 1 inch in diameter is prepared by an annular core drill and placed in the grinding machine in such a manner that the base of the cylinder rests on the upper surface of a circular grinding disk of cast iron, which is rotated in a horizontal plane by a crank movement. ”

“ Cohesiveness.—Determined by the percentage of asphaltenes which the material contains, and by the residual coke remaining after ignition of the material in absence of air, which bears a close relation to the percentages of asphaltenes present. ”