THE ROADING PROBLEM

Otago Witness, Issue 3397, 23 April 1919, Page 17

THE ROADING PROBLEM

TARRED AND OONCItETE ROADS.

PRINCIPLES OF CONSTRUCTION.

The following is a continuation of the report supplied to the Otago Motor Club by Mr B. B. Couston, C.E., after an inspec : tion of the road system in the Taranaki provincial district: —

I do not propose to more than mention in this report that in some of the northern cities there are to bo seen excellent examples of more costly city pavements, such as wood block paving and Neuchatel asphalt, but -will give particulars later of the concrete road pavements of Wellington and Auckland.

My remarks to the present have, however, been confined to consideration of the one problem of top-course, which, though undoubtedly at the present time more troublesome than all other points connected with road engineering, is dependent on foundation and drainage, and these three oombined do not exceed, in importance any One of the cardinal features of grade, alignment, and section, which should rank equal and first in importance when considering any scheme for road improvement I shall, however, speak briefly of these matters, and only so far as they particularly relate to the roads inspected on my visit. GRADES. The road j that have been treated with tar in Taranaki do rot traverse what we would oall hilly country in the south. There are, for example, no escents like those over Kilmog and Mount Cargill, on the North road, or Lookout Point and Saddle Hill, on the South road; but there are inclines 20 to 25 chains in length on grades of about 1 in 12, where the road has been successfully tar-surfaced. The horses do slip and fall on these grades at first, as many did also on the level in Dunedin when asphalt was first laid down; but they soon accustom themseves to the new foothold, and drivers exercise more care in ascent and descent. On some of the roads the formation - has been wider ed on the hills, and_a shoulder of gravel a few feet in width provided to give foothold in wet or frosty weather. One feature of many of the North Island roads, both in ordinary and tarred roads, (a the excellent side track of earth provided for summer traffic. This is a material factor in reduoing the cost of maintenance for both classes of road, as. in the case of water-bound macadam, motors as well as horses take the summer road, and save the metal from ravelling in dry weather; and to the oase of tarred roads, the horses take the summer road, and save the tarred surface from the calking of their feet. Of concrete roads there are only a few trial sections on rural or suburban highways, and those I saw were on the level; but in Wellington and Auckland concrete roads have been laid, carpeted and otherwise, on grades, and horse traffic has become accustomed to their use. Auckland limits the grade to 1 in 18. Wellington has Molesworth street full width on a grade of 1 in 30, and between the train tracks on a grade of 1 in 17Jk ... In the question of alignment the' motorist —or, perhaps more truly, other traffio for fear of the motorist —is concerned principally with sharp curves, particularly on steep grades or at the foot of such grades. On the tarred roads of Taranaki the align; ment gave always "a safe sight distance, and it is a point to be remembered that the better the surface the hfarher speed is the motorist tempted to travel; and before laying down a permanent paving the sharp ourves on a road should, as far as practicable, be elminated. GUIDE AND DANGER SIGNS. A minor-point that may be commented on here is' the more general provision of guide and danger signs in the north; some of the former giving the number of miles to each town indicated, and many of the former particularly specifying the danger, for instance, in Levin, at either end of the street, where the school is situate the sign reads "School: Drive Slowly," and one that can be taken two ways on the road to Day's Bay reads " Motorists, Beware of Children." I noticed also in one borough some Tery neat protective guard rails of reinforced concrete. "SECTION OF ROAD. The section Of a road is of concern to the motorist from the standpoints of safety and convenience and to the local authority from the standpoint also of economy. Under this heading the questions of crossfall, width of formation, and-of hard roadbed are considered, and these have a most important bearing not only on the cost of a new road, but on the comparative cost of maintaining a road with sufficient crown to shed the water in ordinary macadam and of remodelling such a road with sufficient crown for a tar-macadam surface,, A road is given "a crown or a crossfall to shed the water from its surface. A fall ' of one inch to a foot is the minimum at which an earth shoulder will shed water without too much maintenance; that is also the maximum agreeable driving slope. Three-quarter inch to a foot is the minimum crown that can be given a waterbound macadam road to maintain it satisfactorily, and half-inch to a foot on such a road having a tar flush coat; while quarter or throe-eighth inch to a foot is a satisfactory crown for a concrete road. The practice with most local bodies is to give much greater crown, principally because of the flattening that occurs under -traffic; and, secondly, because the shoulders of the road are only intermittently kept olear of grass, which quickly gathers the fine material disintegrated by the weather or won under traffic from the crown of the toad. The higher the crown the less convenient and safe is the road to motorists and the more will traffio travel in one small portion of ibs section. With the tarred or concrete road there • is no necessity for the high crown, On the contrary, every argument favours the use of the minimum slope required to drain the surface, and the small amount of crossfall on the tarred roads of Taranaki and on some of the concrete streets where oiroumstance has not made it necessary to conform to old levels is one of their most pleasing features. BANKING OF ROAD ON CURVES. A very important point that has bearing on crossfall is tho section of road to be used on sharp curves where the road should be banked or have the crossfall in one direction onlv. One seotion of asphalt road I saw had the surface falling about 2in to a foot right from tho edge of earth shoulders on the convex side, across the { boulder and hard road bed, then a steeper all of about 2in to the foot across the fthouldor on tho concave side. On the main

road through Waitotara County there is a very good example of how tho crossfall can be thrown on a winding hill road with reverse curves. There is a clear line of demarcation between the throws to the right and those to tho left, and this lipe is run diagonally across the road, and is so marked that in ascending the hill after travelling a section of road with the throw to the right, you travel a short distance on the diagonal line before taking the next section of road with tho throw in tho opposite direction. The road itself suggests to the motorist the most convenient line of travel, and the banking is essentially a feature of the section WIDTH OF ROAD. With regard to the width of road, Taranaki generally has adopted 15ft as the width of tar-surf aced macadam; there are some by-roads where 10ft only has been tarred, and there are main roads where widths of 12ft, 18ft, and 20ft have been tar-surfaced. In rural boroughs through which the main road passes the centre section of road is generally > tar-grouted, and the remaining width of carriage-way tar-painted only. On either side of the tar macadam there aro shoulders either of old macadam or gravel, or in some cases of soil only. One matter to be noted here is that the hard road bed is almost invariably built up above the general love! of original _ surface ; that i 3 to say, - there is no benching, and this was pointed out to me as an essential feature for suocess of this class of road; The width of tarred surface must be governed . by the traffic the road has to carry. On a very wide road vehicles do not use the whole of the width, and the commonly travelled way, even with a perfeot surface, rarely exceeds 18ft and more vehioles use the centre width of 12ft to 14ft than any other pr t of the road. On the 15ft main roads there was an inclination for the tarred metal to break away at the sides, but on the 18ft roads this did not occur, and I therefore think for main roads the width of 18ft should be adopted, for secondary roads 15ft, and for by-roads 12ft. There are those that advocate that 18ft widths are not often required, and that the use of 12ft instead of 15ft or 18ft means a large saving and is good policy provided the narrower width serves the purpose. They advocate building the strong road just wide enough to comfortably take the heaver traffic, and if the natural shoulder material is not suitable treat the shoulders up to the 20ft width with gravel or waste stone, thus making them suitable and wide enough for the light turnout traffic. There are others that advocate the strong road sufficiently wide for safe clearance and no shoulder treatment. . These remarks dispose of the three main points of gn>de, alignment, and" section, and there remain for consideration the three features of drainage, foundation stone, and top course, which, combined, keep the surface intact and firm under traffic.. DRAjNAGE. The bearing power of the sub-grade and shoulders is increased by the surface and sub-surfaco drainage. I have dealt with the question of surface drainage in my remarks on section, and have also commented on the almost general rule of building up the hard road-bed in Taranaki. Otherwise it is rather from the absence of any drainage features than from their presence that any lessons are to be learnt. The subsoil in Taranaki is either of a sandy nature or clay of an open, friable nature, so that all surface water soon drains away. In many places on the Main road where the road crosses small depressions no culverts have been put in but the water is left to drain naturally away,_ and on fiat stretches of the road there is .an entire absence of the usual road ditch to which we are accustomed in the south. In cuttings where the road is founded 'on papa, the water-tables are obviously well attended, but on flat sections of the road reliance ifl placed solely on natural drainage and on the building up of the road above the side tracks, which is never less than six inches but more generally 18 inches to two feet. To excavate or box out the metal bed irrespective of the nature of tile subsoil or the natural drainage facilities is a common error in road-bed formation, and is altogether wrong where tar-treated roads are concerned. Several of the Taranaki engineers were formerly in practice in the South Island, and they were particular to emphasise the necessity for properly draining the roadbed where less favourable conditions exist, and they are agreed that whereas tarred macadam is a most successful top course for roads that are founded on sand, gravel, or other pervious material, and would be a success in many districts of the soxith where such conditions prevail, that for much of our formation, under drainage of the roadbed would be required and preferably also a bed of some pervious material between the clay and the foundation ston.e. FOUNDATION. The real foundation of a road is the earth sub-grade, and it develops its greatest power when dry, but more generally we speak of foundation as the lower course of stone, gravel, etc., used to distribute the concentrated wheel loads of heavily loaded vehicles over a safe area of sub-grade. The Main South and many parts of tho Main North road were laid on < a Telford base that is a foundation of pitchers set vertically on their broadest edge lengthwise across the road and forming courses and breaking joint with the next course and bound together by inserting and driving stones of # size and shape sufficient to wedge the pitchers together. This was blinded with broken metal, and when consolidated the wearing surface of ordinary macadam laid on top. But this road is the exception rather than the rule, and most roads have either been constructed with a sub-base bottom course of rough metal or gravel or more generally a coat of 6in to 9in of metal has been spread on the clay or earth sub-grade, and thin left to traffio to consolidate serves for both foundation and wearing surface. < Many of our roads on pitched foundations havo been so neglected that they have been down to the stone forming the foundation. These stones have been worn smooth by the traffic, and when the metal has been supplied the coating has been so thin that tho stone used for surfacing has been scattered or crushed by the traffio. Somo of our macadam roads that were laid down with 6in to 9in of metal have been worn down to 3in, in other cases almost worn to the clay. These roads must be built up from the foundation before they aro suitable for a bituminous wearing surface. • It la impossible to prediot the extent to which tho uso of motors for commercial purposes will increase, but that there will be a material Increase is certain, and there

is clearly need for more rigid road foundation. A road is no stronger than its foundation, and a high grade wearing surface will accentuate rather than conceal faulty foundation work. Here I might remark that in any progressive district of the north, the practice so common in the*6outh of leaving traffic to consolidate a road would not be tolerated, and that even those counties who still adhere to ordinary water bound macadam are equipped with modern road rolling and scarifying plant. CONCRETE ROADS. In America the various types of eandplay, gravel, macadam and bituminous roads had been tried, and every type of laboursaving machinery employed to reduce the cost of maintenance and their engineers adopted ooncrote in preference to other material largely on account of the low first cost of construction. Their first roads were constructed during the transition period from horse-drawn to motor vehicles through which we are now passing, and inevitably mistakes were made, and road 3 in many oases did not withstand the traffic they were subjected to. The advantages claimed for the concrete road are that ft is not slippery in any kind of weather; that it can bo laid on any grade suitable for heavy hauling; that it is of low tractive resistance, offers a (rood foothold for horses, has a neat appearance, is sanitary, and can be repaired at low cost, and, unlike many types of paving that require a large plant to prepare ana lay, concrete is familiar to all contractors, is easily manipulated, and is therefore open for the keenest competition. Its disadvantages are its white, glaring appearance, its tendency to develop unsightly craoks, and the fact that, under heavy traffic, holes develop where the concrete mixture is not quite uniform; and even if no holes develop there is abrasion under heavy steel tired traffio and the impact of horses' hoofs. It is important to recognice at the outset that there are limitations to concrete as there are to other forms of paving, and that it should not be placed in competition with the various forms of asphalt or wood block paving for many streets on which such pavement ought to be laid. The revolution in traffio conditions through the change from the horse to the automobile is very greatly favourable to the use of concrete construction and on roads now becoming increasingly numerous, where automobiles form the great bulk of the traffic a concrete paving should, if laid in the best manner, show most excellent results. COST OF A CONCRETE ROAD. The average American cost for a concrete pavement is five shillings per sqare yard, but for most of the roads and city streets of New Zealand the cost would be, at present-day prices eight or even ton shillings per square yard. Taking the prioe at 8s this would work out at about £2350 per mile for a 10ft roadway, and £4220 for an 18ft road. The' annual cost of mamtenance I gather from various sources would be about £4O per mile. Taking interest and sinking fund at 64 per cent, for an 18ft road in concrete the annual cost is £274 interest and £4O maintenance; total, £314. Many of our metalled roads are at present costing £l2O to £l5O per mile per annum to maintain, and every year shows an increase in the annual cost, and the results obtained are not satisfactory. From the engineer's point of view, and also from that of the user of the road, the concrete pavement is to be commended, but until some system of equitably distributing cost, between the user and the taxpayer becomes the law of the land the first cost of concrete roads will prevent their adoption by any but the most wealthy ccunty councils, and, unlike America, where the country led and the towns fo-.owed, the cities ■ and boroughs of New Zealand have first adopted the _ concrete pavement, and country local bodies wait their experience before undertaking works of such, magnitude HIGH-CLASS MATERIAL AND WORKMANSHD? ESSENTIAL. There is no class of concrete work that calls for greater care in the selection of material than that of pavements. The conditions under, which concrete is mixed and placed are different. The nature of the structure itself is different. The conditions under which it must develop its service and fulfil the purpose of its existence are vastly more trying than those of any other type of concrete construction. Hard and fast rules should be laid down in regard to the aggregates, for these are the factors in successful construction which should never be deviated from. If, in certain localities, the proper quality of material is not available without importing, and concrete js_ prohibitive on account of the vesulting high co3t, it should not be attempted to build a concrete road. The question of proper coarse and fine aggregate is paramount in concrete road constructic n The requirements of broken stone are that it shall have a high coefficient of wear, and shall be the run of the crusher that will pass a IAJn ring and be retained on a ijin screen The stone must be free from dirt or any deleterious matter, and must contain practically no soft, flat, or elongated particles. The requirements for cement and sand are the usual standard specification, sand being graded frorn i'm downward. The use of pit-run gravel is not .approved. The proportions used vary considerably, some engineers specifying 1-2-32, others l-2 2 -42. and others 1-3-5. AMERICAN PRACTICE. The following are some standard clauses from American specifications regarding the laying of the concrete: Before placing the concrete the sub-grado is thoroughly wet, and the concrete is placed to a depth of 6in at the side and oin at the centre. A heavy strike-board is then used to compact the concrete and give the road its proper form, and this board is followed by a lighter one. When the strike-board comes within 3ft of a joint it is lifted and placed alongside the joint. Work then proceeds away from the strike-board. After water disappears from the surface of the concrete it is floated smooth with a wooden float. Conorete is mixed by a machine of a type that will insure accurate proportions of the ingredients, and the materials mixed wet enough to produce a concrete of such consistency that _ will flush readily when tamped, can be handled without causing a separation of the coarse aggregate from the mortar, and which will not creep or sag out of place when deposited and lightly tamped. After the addition of water the. mixture la handled rapidly to the place of final deposit, and under no circum

stances is concrete used that has partially hardened. The concrete is deposited to such depth that when tamped and properly finished the pavement has its specified thickness and the upper surface true and uniformTo assist in bringing the street surface to correct grade and crown stakes are drivon at centre and quarter points of roadway at intervals not greater than 25ft, with their tops at the elevation of th» finished pavement, or, if preferred, with a well-defined mark at this elevation so that during the laying of the concrete it is possible to stretch a line from a stake to the one in advance for the purpose of determining any irregularities. The conorete is deposited in strips transverse to the roadway, and when interruptions occur of Buoh length a* to permit the setting of concrete, the pavement is finished against a board set vertically and at right angles to the street, so that the depositing of concrete may be resumed without disturbing that already in place. The pavement is finished by thorough hand tamping, until the mortar flushes freely to tho surface, then lightly tamped and broomed with fiber tho final markings having a neat and uniform appearance transverse to the roadway. At all times during the construction of the pavement nothing is allowed upon It that will disturb the concrete while setting, and the street is not thrown open to traffio for at least 30 days. As soon as the concrete Is finished it is protected from the wind and sun by canvascovered frames, and as soon as possible after the concrete becomes hardened it is covered with wet bags or earth and kept wet for 14 days. EXPANSION JODNTS. At intervals of 25ft to 30ft transverse cleavage planes are constructed by placing a board in a vertical position across the roadway and on the rear side of tho board a layer of heavy building or roofing paper. This board is held in place by stakes until the concrete has been deposited for a distance of at least 6ft in advance when it is removed and the place is occupied with concrete and finished in the same manner as the rest of the pavement. In city work longitudinal joints are provided along the curbs. In some pavements patent " sandwioh" join,ts are used, and th© edges of the concrete are thoroughly. " tooled" with a jointer having a small radius and short sides. The spacing of joints has been increased in some instance* to as great as 100 feet intervals. Steel protecting plates have been tried at joints, but there was a tendenoy for the concrete to chip, requiring an almost immediate tarring of the joints. It is believed that the most satisfactory joint will prove one of merely felt or paper, with a slight rounding of the joint which will be filled by the pounding down of the felt. The value of expansion joints is debatable. It is the belief of many authorities that if expansion joints are omitted, cracks will form at approximately the same points at which the joints are customarily placed and that such cracks can then be maintained in the same manner as joints. The early maintenance cost and tho construction costs are thereby reduced. Recent specifications provide for expansion joints at points only where work is interrupted. It seems logical to suppose that concrete never again occupies as much space as when it is first placed on the street in a wet mass, and that the only serious temperature effects to be considered are those of contraction. , It is little more trouble to repair a transverse crack which forms across a road than to maintain a transverse joint. Where transverse joints are placed it is difficult to make the joint so smooth chat it will not be felt when a oar passes over it. Trouble has also been experienced where transverse joints are used with one slab rising above the adjacent one. BARE CONCRETE v TAR-CARPETED CONCRETE ROADS. Concrete .roads may be separated into two classes, those consisting entirely of concrete and those protected by a wearing surface. American engineers are divided in opinion, but the majority still favour the true concrete road, and this school are experimenting in many ways, with surface treatment. They argue that real defects are only such things as form actual or potential hindrances to traffio, and that so-called defects are of small account if easy and cheap maintenance is lossible. They admit that most of the wear on concrete roads is at joints, that small holes occur from local defeote, that flaking is caused by ooncrete being too wet or poor floating, and that longitudunal cracks, occur through bad drainage and consequent dropping of the slabs at tho sides. In some instances pavements ,are now bfeing reinforced throughout, expanded metal or other light fabric being employed, placed two inches from the top surface. One city is laying a one-oounse pavement six inches thick, using a wet mixture of rather lean proportions. All available means are used to secure the greatest possible density. Immediately after being placed the concrete is brought to the proper crown and levelled with shovels, and then thoroughly floated with long-handled floats weighing about 181 b each. Following this floating, a dry mixture, one part cement and one part fine aggregate is sprinkled over the surface in quantity sufficient to absorb all surplus water. • The pavement is then floated again with still heavier floats until the added dry mixture is thoroughly wot aid incorporated with the surface mortar, leaving the surface wet and slushy. The entire process of levelling, floating, placing the dry mixture and refloating is accomplished within 10 to 15 minutes after the placing of the conorete. A third and final floating is given as soon as sufficient set

has taken place and is continued as long as the setting will permit. This application of the dry mixture to the surface of the wet concrete is claimed to overcome one of the chief practical difficulties in the laying of concrete roads and pavements. If the concrete is mixed wet ■ enough so that it will compact well without forming voids, it ia apt to be so wet that in floating the pavement smooth, surplus water comes to the surface in such quantity as to prevent first-class work. By adopting the plan of sprinkling a dry mix on the wet concrete, the latter can be mixed wet enough so that it will run freely through the pipe of the distributor and spread easily on the Voids in ,the concrete will not occur with such a wet mix; and if the surplus water is absorbed by the dry material there is no chance of trouble from laitance. It is to be noted that this dry mix is spread immediately after float'ng the concrete, so that there l| no chance for the concrete to take any ink tial sec before the dry mix is applied. The # advocates of finishing a road with a bituminous carpet axe divided between those who argue that it is more economical to construct a concrete highway with a base of 4in to sin of concrete and a bituminous carpet of 2in than to construct a 6in to 7in concrete pavement, and those who consider it to construot tha concrete road to full depth in the first instance and after a period ■ of four to fivayears apply a bituminous carpet, which they claim adheres much better if applied then than when the road was first completed, and also saves the cost of surfacing for that period. The following are examples of carpeting* from American practice: (1) On the concrete is a bituminous carpet. Asphaltio oil is applied after thoroughly cleaning the concrete" surface and directly after stone screenings and sand u are uniformly spread upon it in sufficient quantity to combine wirh the oil without leaving an excess of screenings or sand on the finishod road surface.. (2) The concrete is of cement —grouted broken stone made by placing a layer of stone on the rolled subgrade, thoroughly rolling it and then filling the voids with Portland cement grout and rerolling tha wet mass to consolidate it. The top wearing surface, about lin thick, is a mixture of cement concrete and an emulsified asphaltio cement. One oyd of |in crushed rock, * oyd of olean sand. 1661 b of cement, and 57 gallons of asphalt emulsion known as Bi-co-mac, diluted with 20 per cent, water. (3) A concrete base 4in or sin in thickness with a wearing surface of 2in of asphaltio concrete. WIDTH OF CONCRETE ROADS. For a single line of traffio the width recommended is 10ft -with a continuous gravel or macadam shoulder 3ft or 4ft on each side of the pavement four or five inohea in depth. The turn-out method of construction was employed in some roads, but owing to the desire of the motorist always to be hurrying along, difficulties ■were encountered in attempting not merely to retain but to stop a vehicle long enough to make a turn-out for another vehicle to pass. For double track concrete roads the width recommended is 18ft, with 2ft shoulder on each side, and on this class of track, after trying both gravel and earth shoulders, the latter are considered preferable. A typical class of conorete pavement designed for motor trucks is a base 24ft wide and eight inches thick, 1,2, 4 concrete, resting on a cushion of decomposed 6tone five inches thiok and 40ft in width, with shoulders Bft wide on each side of the concrete base, also of decomposed stone. These remarks, so far as my reading goes, cover standard American practice, and I shall now explain to the best of my knowledge the New Zealand methods of construction. Mr J. E. Menzies, tengineer to Vincent County, accompanied mo on my visit north, and we made a point that one of us should see every example of concrete road in the towns visited, and subsequently compare notes.