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August, í91o. THE VARSITY ENGINEER.

Notes on Road Construction.

Read on June 7, 191o, by Mr. W. Calder, Assoc.M.Inst.C.E., City Surveyor, Prahran, Victoria.

INTRODUCTORY. The Road Problem and its concomitant, " The Dust Prob-

lem," have of late years been so prominently before Road Engineers and the general public, and so much has been written on this question, that any attempt to deal with the subject in a necessarily short paper, will appear presumptuous, but as your Hon. Secretary expressed a wish that your Society be supplied with some particulars regarding the methods em-ployed in road construction in the Prahran Municipality, the writer has been induced to communicate some data and par-ticulars of the methods employed there, and though no method can be ,laid down as the best under all conditions of locality, traffic and climate, there are certain general principles in road construction which may be observed with some modification to suit local circumstances.

The few remarks here noted, which are specially applicable to city and suburban streets, are the result of the writer's ex-perience as a Municipal Engineer, confirmed by observation of methods employed in other lands.

Within the past five or six years, owing to the ever increas-ing number of rapidly moving self-propelled vehicles, the conditions of traffic have undergone a radical alteration. The ordinary dry or water bound macadamised road, which if well constructed and efficiently maintained, has hitherto been found economical and sufficiently good for the best country roads and suburban streets of medium or light traffic, no longer meets the newer conditions.

For slow moving, horse drawn vehicles, small inequalities of surface were of little consequence, and the dust produced from the wear of their surfaces was taken as a matter of course. Now all is changed. The public has become more exacting; the drivers of motor cars, with good reason, demand a smooth and even surface, as rough or rutty roads are destructive to their costly vehicles. We have likewise reached a sanitary age when the general public will no longer tolerate the nuisance and discomfort experienced through the clouds of dust raised by motor cars. The dust existed before, but it was only not-iced when raised by the winds. Since the advent of the motor car,'the dust is raised in dry weather even when calm.

Those who do not use the motors blame them for making the dust, and though they are undoubtedly most efficient dust

10 THE VARSITY ENGINEER. August, 1910.

raisers, the writer is of opinion that on well constructed roads with smooth, even and impervious surfaces, they cause far less damage than iron tyred vehicles drawn by iron shod horses, and that the dust raised by the cars is mainly produced by the disintegrating action of the former, combined with the influence of the weather. Metal studs and other non-skidding devices on the tyres of motor cars should, however, be pro-hibited.

The motor car has come to stay; so, it is to be hoped, has the sanitarian; and in the future self-propelled vehicles of one form or another will surely, to a great extent, take the place of others for traction, haulage of merchandise, and many pur-poses where the horse is now the motive power.

Nor is this to be altogether regretted, for apart from the pounding and screwing action of the shod feet of the horse on the road surface, he is responsible for another fount of dis-comfort and expense. Mr. W. E. Higgins, an English En-gineer, has estimated that a horse will produce from 7 to 8 tons of manure per annum, a considerable portion of which is distributed about the streets. During dry weather this manure becomes pulverised under traffic and is added to the dust pro-duced from other sources. To satisfactorily meet present day requirements, roads must possess the following qualities :-

I. Sufficient stability and durability to withstand the weights passing over them.

2. A smooth and even surface offering the minimum of tractive resistance and facilitating cleansing.

3. Comparative noiselessness under traffic. 4. Fair foothold for horses. 5. A surface impervious to moisture. This latter quality

has an important bearing on the life of the road. The problem before Road Engineers is to provide roads

possessing these desiderata at a reasonable cost, taking local conditions into consideration.

Of all known forms of pavement, asphalte and wood on a foundation of cement concrete most nearly comply with the above ideal conditions, but owing to their high initial cost, their adoption in this young country with growing towns and rapidly extending suburbs must be restricted to the most im-portant city and suburban streets, where the volume and na-ture of the traffic, class of residence or premises in the street, and the funds at disposal warrant their use.

WOOD PAVEMENTS. • The examples of wood pavements as laid with close joints

in the City of Melbourne are equal to any to be seen in other parts of the world, and this method of construction is so fam-iliar that brief reference only will be made here.

August, 1gro. THE VARSITY ENGINEER. ii

The earlier pavements were laid with red gum blocks, but as the supply of this excellent timber has practically ceased, we are mainly dependent upon Jarrah from Western Australia, Spotted Gum, Blackbutt, and Tallow-wood from New South Wales, and to a lesser extent, Tasmanian Blue Gum. Jarrah blocks are supplied in Melbourne at a cost of about f7/to/-per two, the hardwood blocks from New South Wales cost here about t/to/- per moo.

The cost of Jarrah blocks is about 8/3 per sq. yard, or laid complete with a six inch concrete base, 17/- to 18/- per sq. yard, which strange to say is slightly more than the cost of the same pavement laid in Great Britain.

The life is variously estimated at from to to 20 years, ac-cording to the volume and nature of traffic. Pavements of Red Gum blocks laid in Toorak-road and Chapel-street, Prahran, in 1889, are still in use, and when some of the blocks were removed for repairs in 1907, the actual wear was found to be only % inch. They are now failing from wet rot at the bottom where they rest on the concrete.

Well maintained, with a dressing of distilled tar and sand, it is a sanitary pavement and comparatively easily cleansed, can be laid on gradients up to I in 25, and as a sanitary and non-dust producing pavement, it ranks next to asphalte, but the discomfort of the necessary annual tar dressing is strongly objected to by the public.

Wood is still one of the most popular pavements in Eng-land, but Australian hardwoods are in many cases being sup-planted by Pine, Memel Deal, and other soft woods in a number of cities, particularly in London and Sheffield, the reason being that it is difficult to obtain large supplies of hard-wood blocks of uniform quality and texture, consequently the pavements wear unevenly. Also, owing to the hardness of the wood it becomes polished and greasy in wet weather, and the arrises wear off, leaving a rounded upper surface.

Soft wood, On the other hand, though less durable, wears more evenly, and affords a better foothold for horses. The soft wood is creosoted, and when laici is liberally covered with gravel, which is worked in by the traffic.

In America soft wood is also used for pavements. The blocks are 5 to to inches long (average 8 inches) 4 inches deep and 4 inches wide, and are heavily creosoted with creo-sote oil distilled from coal tar. In Minneapolis where the writer inspected some very excellent wood pavements during 1906, the specification for creosoting wood is very stringent and elaborate. The process, shortly, is as follows : —

The blocks are placed in an airtight chamber, where by the use of heat and vacuum all the sap and moisture is removed, the specified vacuum being 20 to 26 inches. While the cham-ber is under vacuum, the heated creosoting mixture is admitted


under a pressure of 125 lbs. per sq. inch until the blocks have absorbed 16 lbs. of creosote to each cubic foot of timber, or until the oil has entirely impregnated and thoroughly filled the pores of the wood, making it impervious to moisture and pre venting decay. The timber used for blocks is cut from long leaf, Yellow or Georgia Pine, Norway Pine, Washington Fir, or Tamarak, and the cost per sq. yard laid complete is 12/-.

Under this process the blocks are given a specific gravity greater than water, and one advantage is that inferior or even otherwise waste timbers may be utilised for the manufacture of paving blocks. In the same city, extensive experiments were also in progress for ascertaining the best method of laying the blocks to neutralise the ill effects of expansion. Sec-tions of the pavements were being laid with the courses of blocks at different angles with the street alignment with results not yet ascertained.

In Detroit, another populous American city, the rows of blocks are laid at an angle of 40 degrees with the axis of the street, and little trouble has been experienced there through expansion.

Wood paving blocks are not favored by Tramway En-gineers.

The following quotation is from W. Howard Smith, M.Inst.C.E., F.G.S., who states that :-

" Wood blocks do not, however, form a good material for tramway paving, and a tramway engineer seldom lays , them from choice. On the London County Council tramways no wood paving is admitted. Expansion and contraction are par-ticularly objectionable on tramways. The first tends to spread the gauge, while the latter by opening the joints allows water to penetrate to the' underbed.

For this reason, in a section of High-street, 50o yards in length, where wood paving was laid on the sides of the road, compressed rock asphalte was the material used between the tramway rails.

The main points to be observed in laying wood pavements are :—To ensure a solid, even, and true foundation for the blocks, the floating over the concrete should have the same con-vexity as desired for the finished surface. To provide for ex-pansion Mr. Mountain, M.Inst.C.E., City Surveyor, Melbourne, who has had a wide and extended experience with this class of pavement since 1880, has found the most satisfactory ma-terial to be pugged and tempered clay, and this practice has been followed with the latest wood paving laid in Prahran. The clay joints should break bond to prevent the tendency to longitudinal rutting by wheeled traffic. The timber should be thoroughly seasoned, and as the trees from which it is ob-tained are felled throughout the whole year, to meet the de-mands of the market, the only way to ensure this is to pur-

August, 1910. THE VARSITY ENGINEER. 13

chase the blocks in advance and store them. Before laying they are run through a machine containing a bath of hot dis-tilled tar. Formerly the blocks were cut 6 inches deep, but a depth of 5 inches is now the standard. 4% inches would be sufficient, as the actual wear is so slight, but as the difference in cost between 4% inch and 5 inch blocks is only 6/- or 7/-per moo, the saving in the timber is scarcely worth consider-ing. The blocks should be cut to an uniform depth and laid direct on the concrete bed, without a sand cushion.

ASPHALTE PAVEMENTS.

Immense areas of this pavement have been laid in the Old World and America. In Berlin alone it is stated on the au-thority of Delano (Proc. Inst. C.E., June, 1903) that 2% million sq. yards are laid. In the city of London, the greater part of the paving consists of 2% inches of compressed nat-ural rock asphalte on a 6 inch concrete foundation.

In this country it is little known. A sample section of 500 sq. yards was laid in Newcastle, New South Wales, in 1907, and the City Engineer reports that it has been found satis-factory, and the area is likely to be extended in that city. During 1908, an area of 3,50o sq. yards was laid in Commer-cial-road, Prahran, on a section of the road which was for-merly one of the worst in this city, and which was subject to considerable traffic. This pavement consists of a thickness of 2 inches of asphalte on a foundation of 6 inches of cement concrete, and has a guaranteed life of 20 years. The cost was 19/6 per sq. yard. The pavement was commenced in August and completed in October, 1908, and remains in every way perfect, having required no attention in the way of repairs.

Asphalte possesses many advantages. It is very durable and easy of repair, presents little tractional resistance, and as there is a total absence of joints, it is easily cleansed, forms no dust or mud, and on this account is, in the writer's opinion, the most perfect road pavement from a sanitary point of view. As it retains its camber it can be laid with a minimum of cross fall. Its one great disadvantage is its slipperiness during cer-tain conditions of the weather, especially under a slight or drizzly rain. On this account it is not popular with horse users. On the other hand it dries quickly after rain. Delano places the safe gradient at which it may be laid at 1 in 3o, but in the writer's opinion I in 5o is a prudent limit.

As great confusion exists even amongst Engineers, regard-ing the composition of the different materials known under the general name of "Asphalte," it may be here mentioned that true asphalte as defined by Delano is "a natural product, and consists on limestone rock impregnated with pure mineral bitu-men (mineral pitch) which is only found in its pure state in

14 THE VARSITY ENGINEER. August, T91o.

the rock which it permeates when in a state of vapor and under enormous pressure."

For street pavements the desirable percentage of the bitumen varies from 9 per cent. to 12 per cent., according to the locality, the lower percentage being suitable for tropical climates and the higher for temperate or cold climates, where the heat will not cause the bitumen to become soft and cause waviness or buckling in the road. In order to obtain the desirable composi-tion for different climates, the products of various mines are blended before the rock is ground, the principal mines in Europe producing the asphalte being the Val-de-Travers, Seyssel, and Servas.

The method of laying the asphalte is shortly as follows: In this country the ground rock is received in bags, the material being heated in special drying furnaces until it falls to powder. It is then conveyed to the road in carts covered to prevent loss of heat and spread on the prepared concrete bed, which should be dry, to the desired thickness—i% inches for light and 2 inches for heavy traffic. ' he powder is then beaten with heated rammers and rolled with a light roller, also heated, rammed again, rolled with a heavier roller and finally smoothed with hot irons. The pavement is ready for traffic as soon as has cooled. A mastic joint 2 inches in thickness, consisting of a mixture of bitumen and clean grit is laid on both sides of the roadway next to the paving to provide for any possible expan-sion and contraction of the material.

ARTIFICIAL ASPHALTE. In Germany, America and elsewhere, pavements are laid

consisting, not of the natural rock, but of a mixture of bitu-men, sand, and pulverized limestone in varying proportions according to the locality and the practice of the paving com-pany which lays them. The best known of these is Trinidad Asphalte, which consists of from io to 15 per cent. of As-phaltic cement, consisting of refined asphalte and flux, 63 to 8o per cent. of sand, and from 3 to 3o per cent. of finely pow-dered mineral matter known in America as filler. In America the asphalte pavements are almdst wholly composed of this artificial substitute for rock asphalte, in which country on the authority of Mr. E. P. Hooley, A.M.Inst.C.E., no less a quan-tity than 38 million square yards of this pavement are in exist-ence. The same authority states that Trinidad is not so hard or durable as European standard asphalte, but is slightly cheaper and less slippery.

MASTIC ASPHALTE. This is artificially compounded of powdered calcareous rock

asphalte and a proportion of refined natural bitumen or some-times of bitumen and powdered stone or grit in varying pro-

Commercial Road, Prahran. Paved with Compressed Rock Asphalt. October, 1908.

High Street, Prahran. Showing sides or road between Tramway Rails and side channels

paved with Tar-macadam. March, 1910.

August, 1g1o. THE VARSITY ENGINEER. 15

portions. This material is mainly used for foothpaths. In this city, several varieties of mastic asphalte laid by different firms may be seen in Collins-street. Also in Commercial and Toorak-roads, Prahran, these latter consisting of % inch of the mastic asphalte on a 3 inch foundation of concrete, which has proved quite a satisfactory pavement. The bitumen used in the composition of the artificial as distinguished from the natural rock asphalte is mainly obtained from the island of Trinidad. It should not be confounded with tar pitch from which it is easily distinguished by the smell when burnt.

STONE PAVEMENTS.

Stone pavements are the noisiest and at the same time the most unsanitary of pavements, owing to their rough surfaces and many joints which prevent efficient sweeping and cleaning. On this account they are unsuitable for city and residential streets. In this city bluestone cubes or pitchers are mainly used for the construction of rights-of-way and on roads of steep gradient where wood or asphalte pavements would offer insufficient foot-hold for horses.

In the manufacturing towns of Great Britain and Europe stone still forms the principal pavements for heavy traffic streets owing to its durability and comparatively low first cost. The setts are usually of granite 6 inches by 3 inches on the surface with a 6 inch concrete foundation, the average price in Great Britain being 14/- to 15/- per square yard. The gran-ites in general use are from Aberdeen, Guernsey, Norway, En-derby and Wales, the latter being the hardest and most durable. Welsh granite from Carnarvon on a foundation of cement concrete 8 inches deep is the material used for paving the Dock Road at Liverpool, which thoroughfare probably carries the heaviest traffic of any road in the world. Mr. John A. Brodie the distinguished Engineer for that city, states that on this road "the average two-horse load on four wheels amounts to from 7 to io tons, and where a common load behind a trac-

tiong engine may be taken as 35 tons, and where an exceptional

up to 1o0 tons on four wheels may be at any time ex-pected." The writer witnessed a section of this road being repaved in 1906. The material used for the jointing between the setts was a mixture of pitch, tar oil and sand with a small percentage of lime or cement. The jointing material is most carefully selected, the pitch and oil used for flux being required to pass a certain standard determined by analysis. Occasion-

especially in Rome and some other Continental towns the setts consisted of q. inch granite cubes. These pavements are less noisy than those constructed of larger surfaced stones, especially if the joints are filled with some bituminous or other mastic material.

i6 THE VARSITY ENGINEER. August, 1910.

TAR MACADAM ROADS. Considerable diversity of opinion exists regarding the merits

of roads constructed of tar macadam, and by some Municipal Engineers its use is not recommended on the ground that it is expensive and fit for light traffic only.

The writer's opinion is that it is an excellent material for use in residential streets of the best class, and in business streets of medium or even of moderately heavy traffic. This opinion is shared by some of the leading road engineers. of Great Britain, and many excellent roads of tar macadam or tac-mac (tarred furnace slag) may be seen all over that coun-try, notably in Edinburgh, Liverpool, Sheffield and Notting-ham. In the two latter cities this form of road construction has been practised for a period of between 4o and 45 years, and in Nottingham particularly, many miles of tarred mac-adam roads have been and are still being constructed. It has therefore stood the test of time in the old land, where the climatic conditions are much less favorable to its preservation than in this country.

Considerable areas of tar-macadam have been laid in the city of Melbourne with satisfactory results, and in his evid-ence before the Dust Conference held in Melbourne last year, Mr. Mountain, the City Surveyor, stated that it could be laid at a cost of 2/6 per square yard, which, for a fairly durable and comparatively dustless road, must be considered exceed-ingly moderate.

In the Municipality of Prahran we have I% miles, with a total area of 27,40o square yards, of tarred macadam of dif-ferent grades and methods of construction, and as all have proved more or less satisfactory, it is proposed to extend this class of work in the future.

The advantages of tarred macadam as compared with ordinary dry or water hound macadam, or surface-painted macadam.

i. Increased durability. 2. Ease of cleansing. 3. Economy in maintenance and watering. 4. Comparative absence of dust and mud. 5. It is a sanitary pavement. 6. Is noiseless. 7. Offers good foot-hold for horses, and its tractive re-

sistance is less. 8. It may be laid on almost any gradient, with variations

in the surface treatment. , One great disadvantage is that when the pavement has to be

broken up by plumbers for attention to gas or water services it cannot be made good without making a patch which causes a disfigurement, and for this reason the water mains in High-street were moved from the centre of the road and relaid be-neath the side gutters before the pavement was laici.

August, 191o. THE VARSITY ENGINEER. 17

With regard to its durability. A section of / mile was laid in Commercial-road, east of Chapel-street in 1902, and has during 8 years required but little attention save for minor patching and tar painting of portions that appeared rough on the surface. Formerly this was one of the worst roads in the municipality. It required re-coating with metal every third year, and in wet weather the road scrapers and mud carts were familiar objects in the street. To all appearance this road is as sound as when first laid, the only defect noticeable being a somewhat uneven and wavy surface. The cost of recon-struction of this road was high, namely, 6/- per square yard, the reason being that the road bed had to be excavated to obtain the desired convexity, and as the foundation was wet the sub-bed was drained by means of stoneware pipe drains. The depth of the coat consisted of 6 inches of tarred 2;_ inch metal, with a binding of about rI inches thick of / inch tarred chippings, and sealed with a light coat of tarred top-pings, sufficient only to fill all surface interstices, laid on a foundation of 4 inches of cinders and 6 inches of ordinary dry metal. It is estimated that the increased economy in main-tenance and cleansing in this road has been 75 per cent., and the expense and inconvenience of the mud cart has no longer to be borne. This street carried what may be termed fairly heavy traffic.

In 1903 Greville-street for a distance of 19 and one-third chains was re-coated with ordinary dry metal, the binding being composed of tarred chippings, sealed with a light coat of tarred toppings, the additional cost of the tarred material and rolling being 1/4 per square yard. Only nominal repairs have been necessary in this road since the work was dome. This street carries light traffic only.

In 1907 a section of Toorak-road between Chapel and River-streets was re-coated with ordinary macadam. Trouble \vas experienced here formerly, as the road has a somewhat steep gradient, and the metal continually became disintegrated by the feet of horses, particularly adjoining the strip of paving along the tramway track. An improved method of treatment was adopted in this section. The dry 21 inch metal was first sprinkled with distilled tar, then a coating of tarred chippings spread over the larger metal before rolling. The surface was then sealed with tarred toppings. 'I'11e reason for this method is obvious. The tarred chippings wedged between the stones by the roller filled the interstices, and made the mass more homogeneous, thus preventing the moving and consequent ab-rasion of the stone under traffic. The only object of the surface dressing of toppings is to prevent the penetration of surface water to the body of the road. and on gradients steeper than say 1 in 18 or 1 in 20 it is inadvisable to make the surface too smooth, a surface dressing of dry toppings being sufficient.

IS THE VARSITY ENGINEER. August, Igio.

Recently when the construction of the Prahran-Malvern Tram-way rendered it necessary to re-construct High-street, the sides of the road for a total length of 83 chains between the Avenue and the Armadale Railway Station were constructed of tarred macadam by a method which the writer has not before seen followed, and which in his opinion is a great improvement on the former methods described.

Like most of the old roads, High-street was imperfectly formed with a high crown and irregular foundation, which rendered it necessary to excavate the old metal as well as the sub-bed.

The method of procedure was as follows :—After scarify-ing the old metal was forked over and .the clean metal, thus freed from stone dust and other dirt, laid aside for future use. The foundation was then excavated to the proper level, and when trimmed 'to shape, the old metal was spread and rolled with the steam roller. The sub-bed consisted of good dry loam. This was considered a sufficient foundation, and the old metal being partly pressed into the sub-soil by the roller pre-vented the sub-soil being squeezed up into the clean new ma-terial. A coat of 2% inch bluestone was then spread so as to form a 4 inch layer of stone after rolling, and this stone was sprinkled with distilled tar from rose cans. A layer of I/ inch metal to have a finished thickness of 2 inches was then spread and lightly rolled. Over this, a coat of tarred screen-ings to have a finished thickness of i inch was spread and rolled. In order to fill all interstices and seal the surface, suf-ficient tarred bluestone toppings free from stone dust were then spread and again rolled, and finally a surface dressing of clean coarse gritty sand was spread and the whole well rolled. The road therefore consists of a crust 7 inches in thickness (exclusive of the 2 or 3 inches of old metal rolled into the bed) of a well compacted and homogeneous body as free as possible from voids, which was considered sufficient with a reliable natural foundation such as described.

This finished road is as nearly perfect in appearance as any road can be not having a concrete foundation. It is smooth, even, and practically without the hollows or waves frequently noticed in tar-macadam, the surface appearance being like asphalte. The steepest portion of the road has a gradient of I in 21, and the cross curvature is about i in 35. The road was at no time during construction closed against traffic, in-cluding omnibus traffic, which was allowed to use the road as soon as completed. With the usual method of constructing tar macadam by laying and rolling the different grades of stone in well defined layers, there is a tendency for the topmost layers to peel or be ground off under traffic, and if compara-tively thin surface layers of fine grade are superimposed over a body of dry 2/ inch stone which contains many voids, the

August, 191o. THE VARSITY ENGINEER. 19

tendency is for the stone beneath to move and rock beneath the weight of traffic, and the inevitable result is a broken and uneven surface.

With cement concrete we are careful to obtain an aggregate composed of different sizes of stones so proportioned as to ensure a mass free from voids, and it would appear that similar reasoning should be followed in laying tar concrete.

It will be noted that only sufficient topping is used to afford a smooth and water-proof surface, and that the larger 1/ inch and % inch grades of stone are brought near the surface to form the wearing crust. The roller used is one of Aveling and Porter's to-ton rollers, with a Morrison's Scarifier at-tached. Many Engineers are of the opinion that a wavy sur-face is caused by the use of a heavy roller, and while a lighter roller would be most useful at certain stages of the process of laying the materials, the writer is of opinion that the hollows are most frequently caused by attempting to roll too great a thickness of loose and unbound metal, which is pushed in front of the roller wheels, or neglecting to thoroughly consolidate the foundation. For successful work in tar macadam it is essential that the different operations of the work should be carefully supervised by skilled men, that the tar should be dis-tilled, and that the metal and smaller gracies of stone should be thoroughly dry, as if wet or even damp, the tar will not adhere to it.

Of the pavement laid in High-street, a section 58 chains in length, having an area of 10,922 square yards, was done by the Council's own day laborers, and a section of 25 chains, with an area of 4.480 square yards, was constructed under contract. The total cost, including excavation and forming of the foundation, of the section clone by clay labor was f 1684 19/9, or 3/1 per square yard, as compared with 3/9% per square yard at contract rates.

The Council possesses its own quarries, stone crusher and tar distilling plant, and the whole of the work of mixing was performed by hand. Recently a machine has been added to the plant for mechanically mixing screenings and toppings, by which means the cost of mixing tar screenings has been re-duced from 9/6 per load by hand to 3c1. per load by machine, and of toppings from i/7 per load by hand to 2d. per load by machine, but the apparatus was not installed until the comple-tion of the work in High-street. The Prahran Council is now inviting tenders for a 5-ton road roller, suitable for tar-macadam. With these and other proposed labor-saving ap-pliances, considerable economies will be effected in future operations of a like nature. High-street is a street of medium to fairly heavy traffic, and as there exist in the same street sections of wood pavement, compressed asphalte and tar macadam, all laid during the sanie season, there is opportunity

20 THE VARSITY ENGINEER. August, 191o.

for interesting comparisons regarding the relative durability and economy of upkeep of these various pavements under similar traffic conditions.

The reason of the increased durability of tarred macadam as compared with ordinary dry macadam or tar painted macadam, is that in the former, the stones are coated with a protective covering of a tough and adhesive material. In roads constructed of dry macadam, it is impossible to compact the mass by rolling and there will remain about zo per cent. of voids in the mass of stones. Under the action of traffic the stones are ground together, and the surfaces and edges worn away. The result of this action may be observed when the road metal is picked tip for repair or other reason. It is then found that the old road stones have a rounded appearance, the edges having been worn away. The grinding action is hastened should water have obtained access to the stone. Tar painting by sealing the surface of the road and preventing the admission of surface water certainly lengthens the life of the road by keeping a dry sub-bed and retarding surface disinte-gration by weather and traffic, but it does not prevent the abrading action which goes on beneath the surface. Also the adhesive action of the tough tar coating affords a lateral sup-port to the adjacent stones composing the road crust, and the angle of resistance to a surface load is considerably increased. If it be assumed that the line of pressure induced by a loaded wheel is at an angle of 25 degrees from the surface down-wards to the foundation, with a load on the wheel of io cwt., the weights upon the road bed would be approximately 26 cwt. per square foot, in the case of a 7 inch thickness of metal, and if it be assumed that with tar macadam this angle is increased to 4o degrees, allowing the same thickness of road crust, the weight per square foot upon the sub-soil would be reduced to 8.5 cwt.

That this increase in the power of tar macadam to distribute the load is not exaggerated but rather underestimated, may be realised by actual experiment or noted in every clay ex-perience. An ordinary tar paved footpath 3 inches in thickness may be torn up in sheets 3 or 4 feet in width, and these slabs will have considerable power to resist a cross breaking strain.

Again, a scarifier attached to a steam roller will tear up an ordinary dry macadam road with comparative ease to a depth of 3 or 4 inches, but recently when a tar paved crossing 3 inches in thickness had to be torn up on account of adjacent road works, the work was effected with the greatest difficulty by the same means.

It may be thought that unnecessary time has been devoted to this section of the paper, but the writer has been induced to give reasons for the faith that is in him, as he considers that this material has not received the attention it deserves at the

J


hands of many road making authorities. It has its limitations, but when it fails to meet the requirements of heavy continuous traffic the time has arrived for a more permanent form of con-struction with concrete as a base. It is an excellent inter-via between the cheaper roads and the more permanent, but much more expensive, pavements of wood and asphalte.

Mr. Thos. Aitken, M.Inst.C.E., author of the standard work on " Road Construction and Maintenance," considers that tar macadam has twice the life of ordinary macadam. The writer considers this estimate to be if anything under the mark.

In Melbourne, and in many other districts in Victoria, we have the raw materials at hand, a suitable stone and a supply of tar being all that is necessary.

The cost of tar macadam is only about 25 per cent. to 33 per cent. greater than ordinary dry metal roads, and for a material which is noiseless, sanitary, and practically dustless, the ulti- mate economy in cleansing, maintenance, not to speak of the gain in comfort to the road users, is surely an ample reward for the comparatively small additional cost.

TAR PAINTING, OR SPRAYING OF MACADAM ROADS.

The above treatment of roads is so well known and under- stood that it needs but brief reference.

This is the only practicable method within reasonable cost limits yet tried which has been found to effectively minimise the dust and mud nuisance, and which at the same time has been proved to materially increase the life of the road. This fact has been abundantly conceded by evidence and data given at the International Road Congress held in Paris during í9o8, and at the Dust Conference held in Melbourne last year.

The cost of mechanical spraying is given at id. to t/ci. per square yard. and by hand application the cost is from 1!d. to 2d. per square yard, according to materials used and local circumstances.

The writer is of opinion that the life of the road material must be increased quite 25 per cent. by its use. A portion of roadway in Prahran which was tar painted during 1905 has required practically no repairs since that date other than the annual surface dressing, while formerly the sanie section of road required re-sheeting about every third year. Between 8 and 9 miles of tar painting has been clone in this city. For successful results it is necessary-

1. That the road to be treated is in good condition of repair, with an even and perfectly dry surface.

2. That the surface should be thoroughly s cpt and freed from all dust and loose grit.

3. The work should be done in dry, warm, A%eather, and if possible, with the sun shining on the road.


4. The tar used should be applied hot and freed by distillation forni watery constituents and the lightest of the con-tained oils.

While tar painting of dry macadam surfaces is undoubtedly of very great benefit to the roads so treated, it is not to be com-pared with tar macadam roads either for durability or even for the lessening of the dust nuisance.

Where failures have resulted with tar macadam roads, they may be accounted for by one or other of the following reasons. Unskilful methods of laying, the use of unsuitable materials, carrying out the work in wet weather, attempting to coat wet stone with tar, or the use of inferior tar, and possibly the latter mistake is accountable for the majority of failures.

TAR. •

• As the necessity for using distilled in place of crude tar in road works has been frequently referred to in this paper, it is perhaps necessary to give • some reason for holding so strong an opinion on this point, which is based on the writer's prac-tical experience. in the distillation of tar for road purposes since 1899, when his Council installed its first tar still.

Crude English coal tar from different localities may have a specific gravity varying from i.o86 to 1.206. The percentage of ammoniacal liquor contained in them may vary from i per cent. to 9 per cent., light oils from 2 per cent. to 20 per cent., creosote oils from 20 per cent, to 3o per cent., anthrecene oils from 7 per cent. to 20 per cent., and pitch from 3o per cent. to 6o per cent., the variations depending upon the kind of coal used, the method of gas production, and the temperature at which destructive distillation of the coal has taken place. The-valuable constituents contained in tar for road making pur-poses are the pitch and the heavy or dead oils (principally con-sisting of creosote, carbolic, and anthracene oils). These oils, together with the pitch, give the desirable qualities of viscosity ductility and adhesiveness necessary for the thorough coating of the stone with an adhesive and tough cementing medium. The watery constituents including Ammoniacal liquor and Naptha, are apt to evaporate or to volatilise under atmos-pheric influences, and if present in considerable proportions, they perish, the stone is bereft of its protective coating and dis-integration results. By distillation these perishable and vari-able elements in the tar are removed and by experience, even with rough and ready methods of testing, tars of varying quality may be reduced to some desirable standard, depending upon the purpose for which it is required. For the painting of macadam roads, fluidity is required to penetrate the road surface mid bind and seal the gritty particles in the interstices of the stone, and for this purpose the writer's practice is to remove the water and the most volatile of the light oils amount-


ing to approximately 6 to to per cent. For tarred macadam,

the whole of the watery constituents (about 5 to 5/ per cent.)

and the light oils (about to per cent.), or a total of from 15 to 16 per cent. of the original bulk of the tar is removed. For painting of wood pavements, distillation is carried still further as in this case it is desirable to produce a soft pitch, which, while retaining its tough and adhesive quality, will not be picked up readily by passing traffic, which is admitted as soon as the tarring and sanding is completed. and at the same time is not rendered brittle by the removal of the valuable heavy oils.

The tar used in the Prahran Municipality is obtained from the South Melbourne Works of the Metropolitan Gas

Co.,

which compares favorably with the better class coal tars ill Great L-'ritain. A sample of this tar, from which the whole o

f

the watery constituents and light oils had been removed by distillation for use in tar-macadam, was analysed, with the following result :-

Specific Gravity of Tar, 1.191.

Products of Distillation. Percentage.

Creosote Oils(containing Napthalene and Carbolic Acid) 25.30 Anthracene Oils .. .. .. .. .. .. • . .. .. .. • • .. .. 31.50 Pitch-Residue, grams per too c.c. .. .. • • • • • • • • . • 42.50

,Vote.—An addition of the various products will show all apparent excess. This is due to the fact that the sample of tar assayed was pleasured, while the pitch obtained was weighed.

In order to ensure continuity of work during the favorable summer months, a tar storage tank was erected in 1908, con-structed of reinforced clinker concrete, with a storage capacity of 20,000 gallons. The consumption of tar for road works for seven months ending 3oth April last was about 6o,000 gallons. Of this quantity 30,50o gallons were distilled. Of the distillate the ammoniacal liquor, which runs off first under a low tem-perature, is of no marketable value, but the light oil which comes later is sold at 8(1. to 8/d. per gallon for use in the manufacture of disinfectants under the name of phenyle, etc. Allowing for the value of this by-product, the cost of distilling the tar is about 1d. per gallon.

The majority of Engineers insist on the necessity of storing tarred stone, chippings, etc., before using for tar macadam, the object being to mature and toughen the mixture. While this is necessary when crude tar is used, the writer cannot realise that any advantage is gained by storage when the tar is properly distilled. The only apparent object to be gained by storage is to allow the water and light volatile oils to be hot rid of by atmospheric influence, and this object has already been accomplished by the tar still. On the other hand, storage adds to the cost owing to the necessity of an additional hand-

24 THE VARSITY ENGINEER. August, 1910-

ling (sometimes with the aid of a pick) when the material has lain for some time in a heap.

ORDINARY MACADAM ROADS.

For ordinary macadam roads little need be said here, fur-ther than that the best available stone is the most ecomomical in the long run. The usual depth of the metal coat is 9 inches, which for general purposes is sufficient but the depth must depend upon the amount and weight of the traffic to be pro-vided for, a sufficient depth being necessary to distribute the weight of a heavy surface load on the sub-bed so that the soil which forms the foundation of the road will not receive a weight sufficient to cause permanent depressions, which would of necessity soon be apparent on the surface. For binding, clean hard grit only should be used. It is false economy to use soft or inferior binding material, and for preference it should be of the same material as the metal used. If softer it will quickly form either mud or dust, and if much harder it will grind away the surfaces and angles of the road stones by friction. As a rule, the stones, especially for the surface or wearing coat, are broken too large in this country, especially for the finishing or surface layer. In Great Britain the stone is usually broken to a guage of from 1% inches to 2/ inches, and the smaller stones, which fit more closely together, with fewer voids, make a much more even and smooth surface. All surface interstices should be well filled with chippings gravel, or toppings, to seal the surface against water, which excepting traffic, is the principal enemy of roads, and its admission to the undercrust must be guarded against.

ROAD STONES.

The qualities of a good stone for road purposes are sufficient density to prevent absorption of water, hardness and tough-ness to resist the grinding away of the surfaces and edges of the stone under traffic;

and crushing by heavy weights passing over the stone, or the pounding action of the shod feet of horses, and the cementing value of the gritty particles and dust worn off the stone by attrition or friction of the adjoining surfaces. In some stones this dust has a high cementing value, and with the addition of water forms a .binder which holds the road stones in place in the road. Flints and quartz, being of a brittle nature, are usually unsuitable road stones. The basalt obtained in the Melbourne district, though not a par-ticularly liard stone, is a good useful all round stone, and the grit worn from it has considerable cementing value. Scrap-ings removed from the roads during wet weather have fre-quently been used for making cricket pitches and footpaths in outlying localities, where it sets well when rolled.

v

i August, igio. THE VARSITY ENGINEER. 25

One of the best road stones in England is the Clee Hill Basalt, from the Dhu Quarries, Shropshire, which is described as an Olivine Dolerite. This stone appears to possess all the desirable qualities of a road stone. It is extensively used all over the Midland Counties of England, the output being about 300,000 tons per annum. Its absorption of water is o.6 lbs. per cubic foot, and its weight is 178 lbs. per cubic foot. It is hard, tough, and of high cementitious value. Many granites make good road stones. The Welsh Carnarvon granite, as used on the Dock Road, Liverpool, though much harder and tougher, and said to be the best of paving stones, is not so suit-able for macadam, owing to its low cementing value. The writer knows little of the relative values of road stones in this country, and as Municipal Engineers have neither the time nor the •apparatus for conducting scientific tests, and as the work is one of national importance, it would be well if this work could be undertaken by the University on the lines adopted by Mr. E. J. Lonagrove, M.Inst.C.E., in England, and Mr. L. W. Page, Director of the Office of Public Roads, Wash-ington, U.S.A., who have separately and jointly done much valuable research work in the nature of attrition tests, etc. A valuable table containing very full data regarding the above-mentioned tests has been published in " The Surveyor and County Engineer" of date April 15, 191o.

BITULITHIC PAVEDI ENTS.

Since 1901, when it was first laid, a material known as Bitulithic Pavement has come into vogue in U.S.A., and as evidence of its popularity as a road material in that country, no less an area than 2.880.000 square yards were laid in one year (1906), and Mr. W. P. Judson. M.A.S.C.E., states that "at the close of 19o7. 6/ million square yards, equal to 422 miles of 30 feet roadways, has been laid in the United States of America and Canada."

Mr. Rust, the City Engineer of Toronto, states that pave-ments of this material laid in that city in 1904 have up to the present given satisfactory results, the cost in Toronto being 10/- per square yard. The pavement is either laid on a con-crete base or on a foundation of broken stone 6 inches thick, which latter is rolled and then flushed with a bituminous com-position. The wearing surface consists of stones of varying grades from i/ inch gauge down to dust, sand, and a to per cent. addition of bitumen. The different grades of stone are carefully proportioned so as to have a minimum of voids after compression under a roller. And it is claimed that by this system the percentage of voids is not more than to per cent., so that by the addition of bitumen up to io or 12 per cent., a solid mass free from voids is obtained. After preparation the

26 THE VARSITY ENGINEER. August, 191o.

mixture is taken to the roads and while hot it is compressed by rolling to a finished thickness of two inches.

At the Franklin Inst., . in 1906, Mr. G: W. Tilson, C.E., Chief Engineer of the Bureau of Highways, New York, U.S.A., said of the pavement, " The bituminous concrete is nearly as dense as a block of solid stone with a surface that offers as little resistance to traction as asphalte, but one that is not slippery."

Other road authorities have also reported favorably of this pavement, but as the process is patented, the fact of its being a monopoly is against it. Bitulithic pavement as used in America compares with the place taken in Britain with tar macadam, but is of higher grade and more expensive.

GRADIENTS. The question of gradients is altogether a question of locality,

the purpose of the road, and f. s. d. In city streets the longitudinal gradient in main roads should not if possible exceed I in 20.

CAMBER. The fall from the centre to the sides of a road should be

sufficient only for the surface water to run easily to the side channels, the amount of cross fall given depending on the nature of the pavement. The least fall is necessary for asphalte and the greatest for macadam roads. In the majority of the old roads, the high crown is a common fault, and this renders road improvements an expensive undertaking, as in most cases it means reconstruction when extensive improvements are to be carried out. The writer usually adopts a modification of the Liverpool standard cross section. Mr.' Hooley gives the desirable rise from side to crown for the different pavements as follows :-

Asphalte .. .. .. I-8oth to I-Tooth of width. Wood . . I-both to i-both of width. Stone Setts .. .. 1-60th to 1-80th of width.

Mr. H. P. Boulnois, 1VI.Inst.C.E., considers the desirable cross fall for the different pavements should be for

Compressed Asphalte .. . i in 6o Wood I in 50 Granite Setts .... • • • . • . .. .. .. .. .. .. i in So

Tar Macadam .. . .. .. .. .. I in 45 Ordinary Macadam .. .. . i in 40

In France the standard cress gradient for paved roads is I in 5o, and the sane camber is favored in Belgium, but in order to ensure perfect lateral drainage with these slight cross falls, the road surface must be maintained in perfect condition. There should be a._ slight camber all the way from the centre to the sides.

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Some writers of literature on road construction advocate a cross section having an arc of a circle in the centre of the road with straight lines from thence to the side channels, but the wear of traffic tends to make the sides concave, and in this case water would not escape to the side gutters. On the other hand, a cambered haunch will during rains prevent the storm water from spreading over the sides of the road, particularly hr there are no underground drains and frequent gully gratings

to carry off surface water.

FOUNDATIONS. When so many excellent text books by well known author-

ities are available for reference, it is unnecessary to enter into detail here regarding this point, further than to accentuate the necessity of exercising the sanie care and judgment in prepar-ing a sound road foundation as prudence demands for other works.

The nature of the foundation to be adopted will depend on the funds at disposal, the class of traffic to be provided for, the nature of the subsoil and the kind of surface proposed to be

used. It should be remembered that the formation of the surface pavement of whatever nature is simply to take the wear of traffic, and should the foundation be insufficient to carry the

superimposed appear

roon without surface, and the maintenance

sofm

tatrue in-

evitably apP and even camber will become impossible. With first-class pavements, such as wood or asphalte, which

cannot be laid under a cost of from 8/- to ion- per square yard, and where it is imperative to maintain an even surface, it is obvious that a rigid foundation of cement concrete is alone ad-missible. In this country the usual 6 inch concrete bed has up to the present been found sufficient. In the case of such pave-ments requiring a concrete foundation :--Where recent excav-ations have been made for drains, pipe tracks. etc., provision should be made by bridging over these trenches with addi-tional thickness of concrete, resting on solid ground to guard against future subsidence. For macadam roads of light or medium traffic the natural sub-soil is usually found sufficient, especially if this be of sand, dry loam or gravel. The metal bed should be carefully formed to the required convexity and thoroughly rolled. In wet localities, where the sub-soil con-sists of plastic clay. a layer of clinker, ashes or other such readily obtained material should be spread and rolled in the foundation to prevent the sub-soil from being squeezed up amongst the road stones.

DRAINAGE.

In wet localities careful provision should be macle to thor-

28 THE VARSITY ENGINEER. August, 19X0.

oughly drain the road bed. In the Prahran district we are fortunately placed as regards the soil, which usually consists of a sandy loam overlying a firm and generally dry sandy clay. It is therefore seldom necessary to provide sub-drains.

The modern tendency to secure and preserve a water proof road surface renders sub-drainage less necessary than form-erly, and only in cases where sub-soil water is known to exist, or is likely to collect, is under-draining resorted to as in the case of Malvern-road already referred to.

ROAD MAINTENANCE

As regards maintenance of macadam roads, it may be said that where the road is in such condition as to necessitate ex-tensive repairs, it is generally advisable to effect these with a continuous sheet of metal, of a depth according to circum-stances, and unless the surface of the road be very uneven, it is usually necessary to scarify the sides only in order to give the edges of the new coating a grip on the old surface. Small holes and hollows should be macle good by picking up the old surface surrounding the hole and repairing with I% inch to 2 inch gauge stone, rolled in and lightly bound with clean grit.

As this paper has already exceeded the limits of the writer's time, and he is afraid also of your patience, it will be con-cluded with a repetition of the oft-quoted adage, "A stitch in time," which holds good as regards general road maintenance.

DISCUSSION.

In reply to questions, Mr. Calder said that even given un-limited funds, the man was not yet born who could completely eradicate the dust nuisance, which could only be ameliorated, even if all the streets of the city were paved with non-dust producing pavements. Much of the dust in the city was im-ported. Horse droppings were accountable for a great deal of the filth in cities, and he had seen clouds of dust blown from the ploughed fields on the Werribee Plains. Our hot north winds in summer were irresistible as dust raisers; they would raise Cain, let alone the dust.

The reason why tar painting was unsuccessful on roads of uneven surface was that water accumulated in the -hollows, which softened the tar, caused it to perish ; then vehicles and horse traffic disintegrated the road crust.

Brick was largely used for road pavements in America, and he had seen excellent brick pavements in Richmond, N.Y., and in Minneapolis, but these were laid on a foundation of cement concrete. There is a tendency for uneven wear in brick pave-ments if the paving bricks are not burnt to a uniform hard-

•


ness, and as brick paving laid on sand costs here as much as stone setts, the latter would be preferable where procurable, as inequalities in the stone pavement could be adjusted by raising the sunken stones with a bar, and grouting with sand and water. If this were done with bricks, the arrises would be chipped and broken away by the bar.

If the expense of a concrete foundation was' thought neces-sary, it would be advisable to pay the small additional cost of wood instead of brick for the surface paving.

It was not insanitary to lay water mains under the side gut-ters. The water mains were under pressure, and the joints caulked with lead, so there was no danger of surface water en-tering the pipes. Pillar hydrants were now fixed on the kerb stone instead of the old type of flush box hydrants, which were certainly liable to pollution by surface drainage when the pres-sure was low in the water mains.

Kleinflaster road pavements were extensively used in Ger- many and were generally well spoken of by German Engineers as being satisfactory and fairly economical. They were laid either on a foundation of cement concrete, rolled stone foun-dation, on a bed of sand or sometimes an old macadamised road was used as a foundation. The surface paving consisted of random stones of any shape laici by hand with their flat surfaces upwards. well rammed, and grouted with sand and water. The only road he had seen laici with this form of pavement was Church-street, \Vest Liverpool, where Mr. Brodie, the City Engineer, was laying an experimental section. In this case, the foundation was of broken stone, well con-

solidated and brought to a comparatively even surface ; the surface paving stones were being set by hand in a series of rings, with irregular shaped stones 3 inches to 4 inches sur-face measurement. The spaces between the outer circles of these rings were afterwards filled in with similar stones so that in the completed pavement, there were no continuous joints.

Granite or other hard stone setts on a foundation of con-crete was in his opinion the best material for paventeitt to carry heavy traffic on a steep gradient.

l -lard granite became polished on the surface, but though the individual stones were slippery, a horse seldom came clown, as the joints afforded him foot-hold to recover himself. lie advocated setts not wider than 3 inches or 4 inches in prefer-ence to wide surface stones, which were alike slippery and noisy.

The best way of joining pavements of different materials is one of the most difficult problems to be dealt with by road Engineers, owing to the different wearing qualities of the

possible it was always advisable to paving materials, but if avoid longitudinal rows or string courses in lines parallel to

3o THE VARSITY ENGINEER. August, 1910.

the centre of the street. Wheeled vehicles invariably formed grooves along the longitudinal joint. This would be noticed in the case of the string course of wood blocks defining the margin of the i8 inch strip of paving outside the tramway rails. On this account no string course was laid in the wood pavement in High-street to marl: the 18 inch strip. of pavement outside the tramway rails. The blocks were laid in courses at right angles to and abutting on the rails. In some cases it was possible to dove-tail or leave broken joints where pave-ments joined, but where a concrete pavement adjoined mac-adam, in practice he would ram sufficient tarred stone or chip-pings alongside the concrete as the macadam became worn below the surface of the concrete. This would require to be renewed at intervals. He would not lay marginal stone courses to border tar paved crossings on macadam roads, as the stones being harder than the adjoining macadam, wheeled vehicles invariably caused hollows to form in the macadam, where they left the stones and bumped into the hollows. In such cases he would rely on the tarred material alone. It might wear away at the edges, but the wear would be more gradual and there would be no sudden drop in level.

Foliage wind breaks would certainly assist to prevent dust, but this was only possible in wide streets, such as St. Kilda and Sydney-roads, the majority of suburban streets being too narrow for plantations.

Our prevailing winds were either from the north or south —in 9 days out of io throughout the year the wind carne from these quarters, and plantations should be made at the north and south ends of the main streets where practicable.

In laying out new towns the streets should not be laid out in the direction the prevailing winds.

1

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