Popular Science Monthly/Volume 80/April 1912/Science in the Service of Highway Construction
|SCIENCE IN THE SERVICE OF HIGHWAY CONSTRUCTION|
NEW YORK CITY
IN a popular sense, a road is a means of communication by vehicle between different localities. To the citizen who ordinarily uses it, aside from considerations of its aspect and surroundings, the condition of the surface and the ease of traction over it have been the main considerations. 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 art of highway engineering, that is to say, of the construction of roads, was considered to have been developed to a high degree of perfection at the end of the last century, as evidenced by the magnificent system of broken-stone roads which were in existence at that time in France, more especially, and in England and other foreign countries, while in the United States successful systems of broken-stone roads had been begun in New Jersey, Massachusetts, Connecticut and a few other eastern states. Roads of this type, when constructed by engineers of experience, with suitable stone, of a proper thickness and with a sufficient foundation, properly drained, and when continuously maintained, were found to be adequate to support the severest kind of travel to which they were subjected at that time, at a cost which was not an excessive burden on the state or the taxpayer. The traffic consisted almost entirely of horse-drawn vehicles and the road surface was resistant to a degree which would carry this traffic without rapid deterioration. Roads of this character were known as water-bound macadam, a name derived from their resemblance to the broken stone roads constructed in England by the celebrated highway engineer John Loudon Macadam in the early part of the last century. Briefly, water-bound broken-stone roads, of the highest type known in the United States, and the only form built until recently to carry heavy travel, are constructed as follows:
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. 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. It can be readily understood that the rigidity and wearing character of a road can be no greater than that of the subgrade which supports its surface.
Upon the subgrade the road itself is built, or a further foundation may be constructed upon it, if the conditions seem to demand it, that is to say, there are two types of broken-stone roads, one commonly called a macadam and the other a telford road. In the former the broken stone is placed directly on the subgrade and in the latter, from considerations of the character of the subsoil or of that of the traffic on the road, on a further foundation constructed, as described by Mr. Austin B. Fletcher, as follows:
The foundation, whether of the macadam or telford type, should be properly drained, since the presence of water softens the subsoil so that the broken stone is forced into it under pressure, weakening the road and destroying the shape of the surface. This provision is very generally neglected in the United States. In addition ditches or channels must be provided on each side of the road to remove the ground water collected by the drainage system and to take care of the surface water which is thrown off the road by the crown, or camber, and the grades.
Upon the subsoil or telford foundation is placed broken stone between shoulders of soil or other suitable material, to prevent its lateral displacement. According to Macadam the stone consisted of pieces of uniform size, about two inches in diameter spread to a depth of ten inches and then compacted by the traffic which passed over the road. To-day quite a different procedure is employed. The stone is applied in two courses and in two sizes. The first or lower course consists of larger stone than that used for the surface, from an inch and a half to two and a half inches in diameter. It is carefully and uniformly spread to such a thickness that it has, when compacted, a thickness of from three to five inches, depending on the character of the foundation and that of the traffic which the road is to carry. 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. Each course is rolled separately until it ceases to move under the roller.
After the compression is completed a binder or filler of much finer stone is spread over the surface, all of it passing-an opening three eighths of an inch in diameter and a considerable portion being fine dust, for the purpose of filling the voids in the upper course of stone, closing up the surface and preventing infiltration of water through it. This is accomplished by the use of water, applied with a watering cart, which washes the fine material into the interstices in the stone. The road is then again rolled with the steam roller, to aid in forcing the filler into the surface, and to render it compact and waterproof. Skill is required in the manipulation of the roller to produce a surface of proper conformation and uniform density.
The method of constructing a water-bound broken-stone road, which has been described in a very general way, is still in use for work of this type, and was the only one employed up to the beginning of the present century, in building roads to meet the most trying conditions then existing. It was a very satisfactory form of construction and still is, under certain conditions and environment. The execution of such work was an art involving great skill and experience, but science contributed but little to perfecting it and placing it on a rational foundation, with the exception of methods of examining the character and availability, from this point of view, of the various rocks which are used in building roads, which were developed in France, and on a much more elaborate scale by the Massachusetts Highway Commission, and in the Office of Public Roads of the United States Department of Agriculture by Mr. Logan Waller Page, the present director of that office, who, with his assistants, has contributed largely to the application of science to the improvement of highway construction. These methods are of so much interest that they are worthy of description.
In determining the suitability of a rock for the construction of a water-bound broken-stone road, its physical characteristics and its behavior under certain conditions are important. Before the development of means of determining these characteristics in the laboratory, the only way in which they could be arrived at was from observation of the behavior of the particular rock in a road when exposed to travel for a considerable period of time.
The important characteristics of a rock which enables one to judge of its suitability for road construction are (1) its resistance to wear or abrasion by impact, (2) its hardness or resistance to the displacement of its particles by friction, (3) toughness or resistance to fracture by impact, (4) the cementing properties or value of the rock powder or dust produced by attrition, when moistened, (5) porosity or capacity for absorbing moisture, the latter being closely associated, for the same kind of rock, with the specific gravity and (6) the structure or size of the grain of the rock, the character of the minerals of which it is composed, and the extent to which these may have become altered by weathering, upon which all the other characteristics of the rock will depend.
The methods of determining these characteristics have been largely originated and developed in the Office of Public Roads in Washington, and are described in one of its bulletins, No. 31, as follows:
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. The specimen is weighted so as to exert a pressure of 250 grams per square centimeter against the disk, which is fed from a funnel with sand of about 13 mm. in diameter. After 1,000 revolutions the loss in weight of the sample is determined and the coefficient of wear obtained by deducting one third of this loss from 20.
Toughness as here understood is the power possessed by a material to resist fracture by impact. The test piece is a cylindrical rock core similar to that used in determining hardness, and the test is made with an impact machine constructed on the principle of a pile driver. The blow is delivered by a hammer weighing 2 kg. which is raised by a sprocket chain and released automatically by a concentric electro-magnet. The test consists of 1 cm. fall of the hammer for the first blow and an increased fall of 1 cm. for each succeeding blow until failure of the test piece occurs. The number of blows required to cause this failure represents the toughness.
The cementing value, or binding power of a road material, is the property possessed by a rock dust to act as a cement on the coarser fragments comprising crushed stone or gravel roads. This property is a very important one, and is determined approximately as follows:
One kg. of the rock to be tested is broken sufficiently small to pass through a 6 mm. but not a 1 mm. screen. It is then moistened with a sufficient amount of water and placed in an iron ball mill containing two chilled iron balls weighing 25 pounds each and revolved at the rate of 2,000 revolutions per hour for two hours and a half, or until all the material has been reduced to a thick dough, the particles of which are not above 0.5 mm. in diameter. About 25 grams of this dough is then placed in a cylindrical metal die, 25 mm. in diameter, and by means of a specially designed hydraulic press, known as a briquette machine, is subjected to momentary pressure of 100 kg. per square centimeter. Five of the resultant briquettes, measuring exactly 25 mm. in height, are taken out and allowed to dry for 12 hours in air and 12 hours in a hot oven at 100° C. After cooling in a desiccator they are tested by impact in a machine especially constructed for the purpose. This machine is somewhat similar to that used in determining the hardness, and the blow is about the same, excepting that it is given by a 1 kg. hammer and the distance of drop does not exceed 10 cm.
The standard fall of the hammer for a test is 1 cm. and the average number of blows required to destroy the bond of cementation in the five briquettes determines the cementing value.
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.
The examination of a rock for structure, its mineral components and the degree to which it has become weathered, is carried out by preparing a thin section of such thickness as to be transparent under the microscope. Its characteristics are then readily determined by the methods employed by the petrographer for this purpose. The appearance of such a section, made from a trap rock of a kind used in the construction of a broken-stone road, is shown in an accompanying illustration, taken from the Bulletin of the Office of Public Roads, which has been referred to.
The application of the methods which have been described to the study of rocks for the purpose of determining their suitability and relative merit for road construction was the first contribution to and application of scientific methods to the subject. Before this road construction was merely an art, more or less skillfully carried on, based on experience, but purely rule of thumb in its execution and not founded on any rational principles. With the application of scientific methods as a means of determining the character of the stone to be selected the building of a water-bound broken-stone road was placed upon a much more satisfactory and rational basis. Roads of this type, so constructed, especially in Massachusetts and under the supervision of the Office of
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 thesehave 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. 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. It thus acts like a grindstone and loosens up the fine material which is necessary to cement the surface. This fine material in its loosened condition is picked up by the current of air produced by the rapid motion of the car and is blown away, forming the clouds of dust which is one of the most unpleasant features of the use of motor cars. Of course the greater the speed the greater the shearing action of the tires, the greater the amount of dust loosened and the greater the destruction of the road. As a matter of fact there is little or no damage done at speeds of less than thirty miles an hour. That the damage is due to the rear wheels alone is shown in instantaneous photographs of a car moving at ninety miles an hour over a water-bound surface. Practically no dust is to be seen about the front wheels while a cloud arises from-the rear tires. If the commonly accepted theory that the destruction of the road surface is due to the suction of the rubber tires, there should be an equal amount of dust stirred up by both the rear and front wheels.
The present condition of affairs is still further illustrated by the statement of the Massachusetts Highway Commission in its 18th annual report for the fiscal year ending November 30, 1910, which follows:
The strongest evidence of the fact that motor travel has injured roads of the water-bound broken-stone type is the increase in the cost of their maintenance, both in this country and abroad, since they have been used by automobiles, in regard to which a few data, among the large number available, are of interest.
At a conference of the governors and chief highway officials of the New England states, called together at Boston by Governor Guild, of Massachusetts, in 1909, Mr. Harold Parker, chairman of the Massachusetts Highway Association, stated that
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.
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.
Mr. Compton, county surveyor of West Cornwall, England, reported in 1910 that in 41 counties the cost of maintenance of broken-stone roads had increased in ten years, since the advent of the self-propelled vehicle, forty-one per cent.
Mr. F. C. Carpenter, county surveyor of the West Riding of Yorkshire, stated at the First International Road Congress at Paris in 1908, that the average cost of maintenance of the roads in his district in 1890 was $482 per mile, hut at that time had increased to $798, reaching in some cases as high as $3,900, while in others it was as low as $73. On the average it was $1,120 for urban roads and $584 for rural roads. He attributed the greatly increased cost in later years to the use of the roads by motor vehicles.
These conditions have been generally recognized elsewhere, both at home and abroad. The Route Rationales in France, reputed to be the finest roads in the world, especially those built of the softer limestones in southern France, have been so destroyed by motors that their maintenance, at any reasonable cost, as water-bound roads have become almost impossible.
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. Mr. Maybury, county surveyor of Kent, England, states that the increase in England in the year ending December 31, 1910, was no less than 36,935 and that this is more than likely to be maintained. In New York State more than 81,000 were licensed in 1910, in Massachusetts over 35,000. In the latter state more than one third of its vehicles are motor driven. 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 very general answer to Mr. Maybury's question is that some form of bituminous binder must be used, either in or on the surface of the road to enable it to resist the destructive action of the motor vehicle. This was the conclusion reached at the two International Eoad Congresses held in 1908 and 1910. The water-bound broken-stone road is a thing of the past on our main arteries of travel which are carrying the present enormous motor traffic.
In working out the problem of a new type of road construction in which some form of bitumen is employed as a binding or surfacing material science can, and is, taking an important part. Fortunately for the past twenty-five years or more, the native solid bitumens, the liquid forms and their surrogates, the tars, have been studied very thoroughly as to their character and in their application to the construction of street pavements. It is not, therefore, difficult, for one who has had an extended experience, to apply the knowledge gained thereby to the construction with the same materials, of country highways of broken stone. The contribution which science has offered to the solution of the road problem in this direction is, therefore, important, while it supplies at the same time means of controlling the uniformity of the binding materials in use and determining the fact that any particular bitumen is of suitable character for the purpose to which it is to be applied. 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.
Bitumen is a mixture of hydrocarbons and their derivatives and may be gaseous, liquid, a very viscous liquid, sometimes called a maltha, or a solid. These hydrocarbons may be representatives of very different series, each having its own peculiar character, both chemical and physical, or a bitumen may be made up of hydrocarbons of different series. The value of any bitumen or combination of bitumens for road construction depends on the series of hydrocarbons and their derivative more particularly the sulphur compounds, of which it is composed. The consistency of such material and its suitability in this regard for use as a road binder is further dependent on the relative proportions of liquid and solid bitumens of which it is composed. It is, of course, the province of the chemist to determine these characteristics for all bitumens proposed for use in road construction, and to interpret the result in the light of practical experience.
From the point of view of their solubility or insolubility in petroleum naphtha the liquid and solid bitumens in use in bituminous highway construction are composed of two components, one of which has been arbitrarily named as a class petrolenes, soluble in naphtha, and the other, asphaltenes, insoluble in naphtha. The one consists of the liquid and the other of the solid components. 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.
The value of a bitumen as a road binder will further depend upon the character of the petrolenes which it contains. If the petrolenes are of a sticky nature, the particular bitumen will be adhesive and ceraentitious, whereas if they are merely oily and not sticky, the material will lack in cementitious properties. The asphaltenes impart cohesiveness as distinguished from adhesiveness, and supply body or stability, as has been said, to the binding material. As an example it may be cited that the heavy residuum left on the distillation of paraffine petroleums in the preparation of burning and lubricating oils, consists of practically 100 per cent, petrolenes, but these petrolenes are oily and not sticky and adhesive. The same is true of any of the preparations from paraffine petroleums or petroleums containing a considerable amount of hydrocarbons of the paraffine series. It is not in itself a suitable binding material for highway construction as appears, not only for the reasons given, but by its behavior in actual use. The petroleums derived from the asphaltic oils, on the contrary, such as those of Trinidad, California and Mexico, are of a much more sticky character, and are not only in themselves, when reduced to a proper consistency, more suitable for a binding material, but are particularly desirable when used to soften solid and harder bitumens, known as asphalts, which possess great cohesiveness, but are wanting in cementing properties. 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. Even with the proper proportion of petrolenes and asphaltenes a bitumen may still be valueless as a cement, if the petrolenes are not of a proper character, that is to say, not sticky. These are all facts to be determined by the chemist, and his contributions to the subject have been of the greatest importance to the development of bituminous highway construction. The characteristics which he determines may be summarized as follows:
1. General Characteristics.—The series of hydrocarbons of which the bitumen is composed for the purpose of comparing it with those in standard materials.
2. Purity.—The amount of bitumen apart from the mineral or other matter, with which it may be contaminated, to regulate the amount of it which should be used under various conditions.
3. Adhesiveness.—Arrived at from a determination of the specific gravity of the bitumen, its solubility in naphtha, the amount of paraffine scale which it contains, this being evidence of the facts that paraffine petroleums are present in the material or absent, and its ductility or extent to which a small test piece can be elongated under tension without fracture.
4. 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.
5. Consistency.—Determined by the depth to which a weighted needle will penetrate into the material, under a definite weight, at a definite temperature, during a definite period of time.
6. Viscosity.—Determined by the rate at which the material will flow through an aperture of definite size, at a definite temperature, in a definite period of time.
7. Capacity to Resist Temperature at Which it Becomes Sufficiently Liquid to ie Used in Actual Construction.—Determined by the volatilization of the material when exposed for a definite length of time in a definite amount, to the high temperature at which the materials would be used.
8. Safety.—Determined by the temperature at which the vapor arising from the material at high temperatures, such as those used in manipulating it, will flash or take fire.
Determination by the chemist of the above characteristics and comparison of them with well-known standards enables him to say whether the bitumen in hand possesses those which have been recognized as desirable in similar materials which have been subjected to service tests in actual work with successful results.
The methods of making the above determinations have been elaborated during the last twenty-five years to such an extent that they may be relied on for the purpose for which they are used, although they will, no doubt, be improved in the future, as they have been, from time to time, in the past. At present they are sufficient, not only as furnishing data which will form a satisfactory basis for arriving at the character of any bituminous material, but also as a means for control of the uniformity of any supply which may be selected, and for regulating its use in actual highway construction.
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.