Observations as to Various Causes of Wear and Deterioration of Roadways.'
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(Continued from page 603.) The principal cause of deterioration of roads and pavements is the lack of appreciation of the importance that should be attached to maintenance ; and this is especially the case in the United States. In probably 90 per cent, of the instances in which the materials are blamed, the seal causes of deterioration are the absence of the application of sound engineering principles in construction, and lack of anaintenance. New and questionable materials should only be used on experimental stretches of road. The question of patented pavements is a serious one in the United States, and will be until the patent laws are revised or the courts take a different attitude with regard to what is and what is not patentable in the nature of a pavement, Suburban Districts and Open Country.
By Laurence J. Hewes.
Chief of Economics and Maintenance, United States Office of Public Roads.
The author considers that what has been observed in America during the past five years concerning the wear of highways is largely of a descriptive nature and is supported by very little data. In the absence of method, observations have necessarily been qualitative rather than quantitative, and the results, from a scientific point of view, too often have been loosely presented, State-wide traffic censuses are possibly not justified economically, but in any case deductions as to wear must be made with due regard to the service performed by the road. With respect to macadam roads reliable statistics have been recorded by the Massachusetts Highway Commission. The method of measurement of wear is indirect ; the consumption of materials in short tons per square yard per annum being computed ; and the traffic records are those taken during August and October, 1909, at 237 stations. In many plates in New England water. hound macadam iroads have succeasfully withstood motor traffic: for a number of years, these roads having been in almost every instance laid down by men of exceptional skill and experience. The older, well-seasoned roads seem to stand motor traffic better than newly made roads, and the effects of seasoning should, therefore, be imitated during construction if possible.
The author regards the old-fashioned patch as a thing of the past, and holds that the water-bound roads must either be surfaced and treated with bituminous material or they must he allowed to wear until complete resurfacing is justified. Separate consideration is then given to concrete roads, bituminous-bound macadam roads, and brick roads, and to the causes of deterioration which are in each case the most important.
The paper concludes with a study of some of the effects of motor traffic, on roads, especially as regards the behaviour and the shape of pneumatic tyres.
Report of G. Lumet (France).
The author examines certain facts in the evolution of motor vehicle construction, which are favourable as regards wear of the roads and deterioration of the vehicle itself. The 'bestuse of the motive power, especially as regards its effect on the tires, is advantageous in reducing wear of the roads; and this, again, reacts favourably for the vehicle. The paper is divideci into two parts, dealing, respectively with the variation of acceleration and the variation of adhesion.
Variation of Acceleration.
The need for constant power through a wide range of angu lar speeds often leads to the restriction of maximum power. There is a clear tendency to an increase in the number of cylinders; four-cylinder cars are most numerous, and there are a considerable number of six-cylinder cars, and even eightcylinder cars have their backers. Two-cylinder cars are not largely represented. and the single-cylinder engine has almost been abandoned. Good balance gives suppleness, and absence of vibration.
The author briefly described leading types of clutches, and points out that the most usual number of speeds provided is four. Transmission by chains, has, in spite of its advantages,
' •' General Report, by W. Gibson Thompson, Editor of ',The Surveyor," to the Third International Road Congross. been abandoned except on industrial vehicles. The modern transmission is rigid, by a shaft carrying at its two ends one or two cardan joints on elastic mechanisms such as knuckle
joints. Special mention is made of a device called the "Elector." As regards the back bridge, carrying the differential gear, there is a tendency to arrange it so as to allow of the carrying of the back wheels. The object is to maintain the diametral plane of the wheels perpendicular to the road surface on cambered roads.
Braking the differential gear has a bad effect on the transmission, but it distributes the braking effect between both wheels. Brakes on the wheels are usually of the expanding type. An arrangement has been devised for braking all four wheels.
Variations of Adhesion.
The author points out that the lighter the group of parts not suspended, the less is its action under the, influence of unevenness of the road, and discusses the relations between suspended and unsuspended parts under the action of the load, of the driving power, and of friction. The more flexible the springs the greater will be the oscillations, and to reduce excessive oscillations both superimposed springs and oscillation brakes have been designed. The deadening of oscillations is proportional to the distance between the blades and to the amount of bend in the spring in its mean deflection. There are various devices for deadening oscillation while allowing of the use of flexible springs. Under the heading "The Connections" the author discusses, with acknowledgment of Professor 3farchis's work, the forces acting on the back bridge and transmitted to the chassis, and the various kinds of displacement that occur. He describes the following systems: Central Thrust, Thrust by Springs; • Thrust and Reaction by the Springs ; and Thrust by Latersl Rods ; and discusses various aspects of these.
Results of Observations.
In the trials which took place at Dieppe, in 1912, 47 vehicles took part. The road, on the whole, suffered very little and on the straight stretches it was in perfect condition. In places where drivers could force their motors there was a tendency to rut ; attributed to the power being too great for the load. " The road in this case served as the band of a brake for the absorption of the excess power not used in propulsion." The relation of weight to power should be such that the skidding of the driving wheels is impossible. Anti-skid devices, when the projections are of a suitable character, limit the skidding and the consequent destruction of the road. At these trials the stopping and starting of the cars, in front of the stand, did not seem to injure the road. It is clear that, great progress has been made in the design of clutches and in the suppleness of the engines. It must not he overlooked, however, that ruts were formed at the bases of bills where the change of speed takes place. At turnings, the heaviest vehicles swerved at the back, and light vehicles on all four wheels.
Report of Mr. Dryland and Others (Great BritainJ. Comparing the effects of motor traffic with those of horsedrawn traffic, Mr. A. Dryland thinks the former, driving by erip of the road, is more damaging than the latter. Mr. Wood thinks that it is not, or would not be, if the speeds were similar. The motor vehicle jumps out of depressions in an already worn road, and causes another depression a little further on. Mr. Wood and Mr. Beaumont think it important that there should be trained men on the road watching for and immediately attending to these depressions. Mr. Beaumont thinks that the axle weights of heavy motor vehicles are 40 per cent., too high. Mr. Dryland points out. the advantages of mixed traffic and the consolidating effect of the iron-tired wheels of slow vehicles; and specially condemns wheels of small diameter, in which other contributors concur. Attention is also drawn to the damage caused by heavy loads on narrow tires. Reference is also made to damage caused by horse-shoes having projecting heels and toes, and especially shoes provided with studs. The fact that roads badly damaged by heavy motor vehicles are often badly-made roads is noted. The increase in weights and the great increase in speeds are more serious causes of damage than the change from horse-drawn to motor vehicles as regards the method of propulsion. Mr. Wood finds that under urban conditions the rate of wear of wood paving is proportionate to the traffic. Mr. Morley points out that road surfaces are more affected by periods of excessive humidity than by consider. able rain. Mr. Wood emphasizes the importance of weather resistance. The failure of surface tarring to give good results in winter, especially under certain conditions, was pointed out by several members; Colonel Crompton dissenting, and expressing the opinion that surface-tarred, water-bound roads had been most successful where the tar coating had prevented the penetration of water to the stones immediately below it during the winter months. Mr. hlaybury considers that pitch grouting is better than tar-macadam (mixing method); but Colonel Crompton regards the latter, rolled while hot, as a strong rival of the former, though the choice may be a matter of local conditions and climate. Mr, Wood thinks that tar is unsuitable as a binder owing to its low melting point, and has obtained good results with an asphalte binder. Bituminous binders have proved economical in his borough, about a penny per square yard per annum having been saved by their use during the past five years. The effects of sun on street paving are sometimes serious (Mr. Morley, Mr. Spurrell, Mr. Dryland).
The sumniary of conclusions is as follows:
1. The conclusions to be drawn from the foregoing tend to .show, that the causes of wear'and deterioration of roadways by weather in towns, suburban districts, and open country, are similar under equal conditions of exposure to drying influences, but the damage from the traffic is relatively less in towns and suburban districts owing to the reduced speed, greater distribution of traffic, and the generally stronger road crusts.
2, That extreme heat adversely affects hard wood block paving and some asphahe and bituminously-bound surfaces. In the latter case difficulty can be avoided by the careful selection and preparation of the material.
3. That frost and snow are very detrimental to ordinary water-bound macadam and generally to surface tar treatment -on water-bound macadam, but are not appreciably damaging to properly-constructed tar or bituminously-bound macadam. 4. That variations of temperature, especially long-continued humid conditions, are especially detrimental to ordinary waterbound and sinface-tarred macadam roads: but that the damage is minimized in proportion to the effectiveness of the waterproofing. 5, That the effect of self-propelled traffic is only more damaging than horse-drawn where the road surface is unsuitable for the purpose of carrying such vehicles. 6. That a mixture of fast and slow traffic is of advantage as tending to distribution. 7. That other circumstances being equal the wear is proportional to the aggregate volume of traffic, but that wear is largely affected by distribution and speed.
8. That deterioration from the effects of speed is very great, and except with light vehicles on plain rubber tires on impervious surfaces is in proportion to the speed.
9. That the axle loads of heavy vehicles as at present permitted in the British Isles Cause destruction of roadways .and should in the interest of the public be reduced.
10. That similarly the small diameter of wheels is a very potent source of damage. 11. That a greater width of wheel than is required by the present regulations is desirable.
12, That rubber or elastic-shod tires and springs are desirable on all heavy vehicles. 13. That the slioeing of horses with catkins and steel-studded pneumatic tires are very destructive of road surfaces.
Report by J. M. W. Van Elzelingen.
Engineer-in-Chief of the Pouts at Chaussces, of the Province of Southern Holland. The Ilegne.
In his introduction the author points out that in more than half the Provinces of the Netherlands there are regulations for loads in relation to tire widths, and for maximum loads. In two Provinces the rules apply only during thaws, and in others the loads allowed during thaws are much less than those allowed at other times. The rules all relate to loads carried, and it, would be better, the author points out, to include the weight of the vehicle. The rules vary considerably in different Provinces, and if this is due in part to load circumstances, the chief reason is probably a lack of scientific basis. The paper begins with a mathematical study of the relations between loads, widths of tire, and diameter of wheels, as regards (1) the resistance of the road crust, and (2) the resistance of the subsoil. It is assumed that the vehicles are horse-drawn, and that the wheels have rigid tires, while the road crust is supposed to be quite elastic, and the distribution of the load on the width of the tire is assumed to he uniform. (NB.—The formula would, in many respects be applicable, within practical limits, to other conditions.) The reasoning leading up to the equations cannot be summarized, but it may be remarked that the author seems to have taken into consideration all practically important variants and to have followed sound mathematical and physical data. From his formula he deduces the following theories:— 1. In relation to the resistance of the road crust, the load of wheel tolerated is in direct proportion to the width of the tire and the square root of the diameter of the wheels. 2. In relation to the submit; the load of the vaheel to be tolerated is : (a) Independent of the width of the tire and of the diameter of the wheel, it' the width of the tire is less than 2 v tan a (see below), but in direct proportion to the pressure to he tolerated on the subsoil, the square of the thickness of the load crust, and the square of the tangentof the angle of distribution. (b) Independent of the diameter of the wheel if the width of the tire is greater than 2 v tan a, but in direct proportion to the width of the tire, the thickness of the road crust, and the pressure to be tolerated on the subsoil and the tangent of the angle of distribution.
In (a) and (b) above thickness of road crust ; a = angle of distribution L= half the angle at the apex of the cone of distribution.
In draft;no regulat;oos it is necessary to take into consideration both existing regnlations and theoretical formula. The nature and state of the mid crust indicate the pressare to he tolerated per unit width of tire. For a given road crust the nature of the subsoil fixes the total load on the wheel, or even the total maximum load. The loads to be allowed on a road crust will depend on the force causing a given amount of indentation and the maximum pressure to be tolerated on the crust, in the author's opinion the latter might be :—For good metalled roads, 20 to 25 kilogs. per square centimetre, or 285 to 355 lb. per square inch. No exact figures can be given for indentation in relation to load, and experiment in this direction is desirable. The author thinks that. the winter loads should be about half the summer loads and that in thaws the normal winter loads should be again halved. As moat's pressure on the subsoil, the allowance for peaty or alluvial soil may be from 0.8 kilog, to 2.4 kliegs. per sq. c'metre. or 1.6 kilog. per sq. centimetre (22.5 lb. per sq. in.) on the average. This would give a maximum total wheel load of 2000 kliegs (roughly 2 tons) or, say 8000 kilogs. or 8 tons fur a four-wheeled vehicle. This would he reduced to 4 tons in winter and 2 tons ie ' the short period of thaw." (Clearly, this relates to a climatic condition of the Netherlands, and is not intended to be applied in frequent thaws such as those of the English climate.)
The author concludes :—
1. That in drafting such regulations the nature of the crust and that of the subsoil must be considered.
2. That the former determines load per unit width. 3. That the latter determines the maximum total load.
4. That trials should be made to check the relations cited in this report, especially the pressure necessary to cause an indentation of given depth in the crust. 5. That good consolidation of the road metal gives a wide angle of distribution, and, therefore, a large area of support cm the suLsoil.
Note by General Reporter.
It is clear that no useful purpose could be served by an attempt to make a general summary of the papers contributed to this sub-section. In diagrammatic form we may represent the subject by a hexagon, surrounded by six other hexagons. One side of each of these six bears upon the central subject, two sides are bounded by neighbouring subjects, and three sides, though needed to complete the influencing subject, have no direct or indirect hearing upon the central idea. We may regard the surrounding hexagons as transposable, so that each of the influencing subjects may be bordered in turn by every two of the other influencing subjects. Thus, " materials of the crust" shown above as in relation with "traffic" and "climate," would also have to be considered in relation to, for instance, " maintenance" and "engineering." This con caption is intended to convey the idea that each of the influencing subjects would, in an exhaustive study of the central idea, be considered in its relations with every pair of the other influencing subjects as well as in its direct bearing upon
the central idea. For every set of conditions, that is, for every district and, as an ideal, for every road, there would be a representative grouping which would be the dominant grouping under which, failing or pending an exhaustive study, the subject would be best investigated in any particular area.
Under present conditions in England, for instance, the choice of materials of the crust, is mainly influenced by considerations depending upon climate and traffic; but elsewhere, possibly in the United States, it might be more important to study materials in their relations with traffic and administration, while in India 'materials' should perhaps be more especially studied in its relations with " climate " and "engineering," as the dominant grouping.
It is to be noted that an influencing subject may be capable of a good or a bad influence, or may be one the influence of which is good, its application being a question of antount. while conditions of sub-soil and climate may be considered as absolute, their importance in our study being mainly on account of the indirect effect which they have upon the central idea through other influencing subjects. This diagram serves to suggest the manner in which the central idea has been approached by the author of each paper. One writer may confine his attention to a particular hexagon, and mainly to that side of it which bears directly upon the central idea. Another will study the hexagon not only in its bearing upon the central idea, directly, but as influencing and being influenced by one or more of the other hexagons. We may find by reading a particular paper that the author has placed his hexagon, successively, between different pairs of the other hexagons, and has drawn some general conclusions as to the resultant of its influence on the central idea. It is obvious that when a subject is dealt with in contributions from different sources without preliminary partition of the ground of study, the papers as a group are not likely to be symmetrical about a centre. Some parts of the subject will be dealt with much more fully than other parts, and it is in order to supplement the papers in themselves by a note on the papers regarded as an organized attack upon the subject, or as lacking in organization, that the present note is written.
Turning now to the papers themselves, we find that considerable attention has been paid to traffic and the design of vehicles, to materials of the crust and, to a less extent, methods of construction ; that climate and weather conditions are discussed in a paper relating to urban streets and roads, and that in one paper administrative and social conditions form the principal theme. Of the six representative subjects into which we have somewhat arbitrarily divided the influencing group, two have received very little attention ; "engineering and the engineer" is in one paper studied in relation to administration and other subjects, but maintenance, though references are made to it, has, in spite of its importance, been almost entirely neglected by the authors of the papers. It is possibly significant of a good deal that hardly any attention has been given to the heavier engineering works of road-making, or to the road regarded as an earth work. The paper contributed by the English group of highway engineers (Mr. Dryland and others) is mainly a study of traffic effects in the United Kingdom. It includes useful comparisons between motor and horse-drawn vehicles and important opinions as to tire widths and wheel diameters; while some attention is paid to the relations between traffic and the materials and methods of crust construction, as well as to traffic in relation to weather conditions. As represented diagrammatically the "materials" hexagon has been placed between traffic and weather, but the weather hexagon has not been studied in relation to the other influencing subjects.
In Mr. Elzelingen's paper we find ourselves wholly within the traffic hexagon, and mainly occupied in a study of wheels and wheel loads. A distinction is drawn between summer
Graphic representation of the and winter con inter relation of sectional ditions, regarded as papers at the Road Congress. absolute in effects.
c16 M. Lumet's paper is wholly within the traffic hexagon, being a study of the design of motor vehicles, as directly affecting the wear and deterioration of roads. M. Lumet deals with details of construction which have been little studied, some of them not at all, in relation to the wear of roe de.
M. Franze's paper, relating to urban roads (in Germany), includes a study of weather conditions in relation to materials, aed a study of traffic in relation to materials, and methods of crust construction.
In M. Pietsch's paper, relating to country roads, the weather hexagon is dismissed in a few words, the relation of this hexagon to others being, seemingly, regarded as fixed. The remainder of the paper is a study of traffic, and especially of the effects of motor traffic in their direct bearing upon the central idea.
M. Duhm discusses the subject of wheels and loads, not only in its direct bearing upon the central idea, but also in relation to the road crust materials and methods of construe. tion, as well as, on the other hand, to administration. There is a reference to certain climatic features of a particular area, but the relations between traffic and climate are not further dealt with.
Mr. Hewes's paper relates primarily to country roads, but also contains references to urban conditions. He takes two influencing subjects, " materials and methods" and "traffic," considering each in its direct bearing upon the central idea, and also paying some attention to the relations between the two subjects themselves.
In Mr. Connell's paper attention is specially directed to "engineering and the engineer" in direct bearing upon the central idea as well as in relation to administration and economies, and to materials and methods of crust construction. Some attention is also paid to methods and materials themselves and to the relation between the choice of materials and matters of administration and social economics. Maintenance is also considered, in its direct bearing upon the central idea, and in relation to "engineering," and the importance of considering climatic conditions is noted, though the subject is not actually dealt with.
In conclusion, the general reporter hopes that the above considerations may be regarded as more practically useful than a series of attempts to comment separately on these contributions of experts, each written with special knowledge, and some of them embodying the experience and opinion of a considerable number of competent authorities; and he has but the following conclusions to form a basis for discussion, these Conclusions.
1. Weather conditions are amongst the most powerful influences which cause deterioration of roads, and that the destructive effect of weather can be minimized by effective waterproofing of the road surface.
2. Any considerable volume of traffic consisting of either heavy motor vehicles or high-speed light motorcars has a seriously damaging effect on water-bound macadam roads. The damage caused is affected by the balancing of the vehicle; the ratio between propelling power and adhesive weight ; the weight of unsprung portions of the vehicle ; the continuity of action of the brakes; the system of springing; the type of tires employed, the diameter of the wheels, the width of the rims and other factors.
3. The damaging effect of heavy motor vehicles can be minimized by the use of wheels of large diameter ; tires of a width properly adapted to the weight of the axle load; rubber or elastic tires and suitable springs and that all reasonable means of reducing the damage to roads caused by such vehicles should be enforced.
4. Light motorcar traffic does not cause serious or exceptimed wear or damage in the case of properly-made macadam roads which have been properly treated or bound with tarry, bituminous or asphaltic materials. As regards horse-drawn vehicles it is desirable also to study the relations bete een load, width of rims and diameter of wheels and more especially the system of shoeing horses.
5. There is still a great lack of precise information in regard to the various causes of wear and deterioration of roadways and that it is desirable to collect more information compiled on carefully-devised scientific methods standardized so far as possible for the purposes of comparison, and to make further systematic study of these causes. The International Permanent Commission is charged with the preparation of a programme of observations, studies and experiments.
[The earlier exeernts and abridgments of the summary reports to the International Road Congress appeared in our issues of the 3rd, 10th and 31st July.—En-1