THE ROADING PROBLEM

Otago Witness, Issue 3398, 30 April 1919, Page 6

THE ROADING PROBLEM

METHODS OF ROAD CONSTRUCTION. CONCRETE ROADS. The following concludes the report furnished by Mr B.» B. Couston, C.E.„ to the Otago Motor Club after an inspection of the system of road construction that is adopted in some parts of the North Island: In Auckland city two miles of street have bean laid down in concrete, the oldest pavement being little Queen street, where the concrete was laid in two courses founded on the old road bed. The foundation course six inches thick of seven to one concrete was followed by a surfacing course two inches thick of three to one concrete, the latter being screeded and worked up with steel trowels and lastly to remove the somewhat polished face, lightly broomed'. This work coat for concrete only 10s per square yard, and tifter some months the surface was coated with Union asphalt and screened beach shingle, at a cost of 5d per square yard. The .veet is in first-class condition and lias carried a heavy traffic remarkably well. Other streets seven and eight inches thick wore subsequently laid in one course of concrete, some with coating and some without. The cost was between eight and nine shillings per square yard. Exchange lane has a concrete pavement only 4£ inches thick and cost 5s 3d per square yard. There are very fine examples of bare concrete road adjacent to the wharves, and this work, eight inches thick, cost 9s per square yard. Symond street is now in course of con-sti-uction, and Mr Tyler, the assistant city engineer, showed me over the work, and explained every detail of construction. He is a strong advocate of concrete road construction, and pointed out that in almost lOvery case, concrete paving in Auckland had been laid in streets carrying heavy traffic, and that except those streets where gravel instead of metal aggregate had been used there was no sensible amount of wear.

Auckland practice for streets carrying heavy traffio is to use a thickness of concrete of seven to nine inches in one layer ■with camber of one in 36 and limiting grade of one in 18. The aggregate from which the best results are obtained is froni crushed stone, one part of i-inch to grains, two parts of l£-inch to 2-inch, and one part of coarse sand. A mechanical batch mixed is used in preference to hand mixing. Very great care is exercised in finishing and curing the surface. Screeding guides are set true to camber rigidly supported with iron stakes, and after the concrete has been roughly levelled with shovels, a stiff screed shod with iron is worked and reworked until the whole mass is compact, and the surface brought even and true.

This surface is, when firm enough, worked with steel trowels, and lastly lightly broomed and covered with damp bags for two or three weeks, and no traffic is allowed upon the pavement for three _ or four weeks after finishing. Clear-cut joints are provided for contraction, only SO to 50 feet apart, the joint surface being coated with 6oft eoap. A recent practice is to place a strip of tarred paper or felt between the bays to ensure a distinct joint and in order' to make the edge of the joint withstand the impact action of traffic, the surface of the point is coated with bitumen and sand.

So far as maintenance is concerned, not one shilling has been spent in Auckland on repairs of the concrete surface. Two of the streets have been coated with liquid asphalt and sand, and one has had the joints treated, and the cost of repairs to the asphalt surface have been less than one halfpenny per square yard for the whole period the concrete roads have been laid. A point worthy of note, and particularly interesting to local -bodies of Otago because of the comparative low cost at which gravel is obtainable in many localities, is that the use of beach shingle has been discontinued in Auckland and crushed stone used in preference. CARPETED CONCRETE ROADS OP WELLINGTON CITY. For Wellington City I have not the exact figures, but approximately a mile of concrete pavement has been laid in seven streets carrying heavy traffic. Molesworth and Rintoul streets wore the first constructed—in 1915, Mr Paterson, assistant city engineer, gave the details of construction and afforded the opportunity to view a practical exposition of the work on tha occasion of a former visit to Wellington. The concrete consists of ono cement to five of clean bank-run gravel containing a good proportion of sand. After about 14- days the surface is coated with tar and sand, and the road opened for traffic." Later on the road is finished with a "carpet" about an inch and a-half thiols, thoroughly rolled and cross rolled.

The carpet material consists _of specially prepared and is placed as hot a 3 possible on a dry, clean-swept surface, and, after rolling, is dusted with a sand and cement mixture, and at a later stage, and thereafter annually, top-dressed with tar and sand or road binder bitumen.

There is no question more debated amongst engineers of the concrete school than that of whether "carpeting" in some form is necessary as. a protective coat. In Wellington it was partly on account of the grades, but more particularly because of the excessive orossfall occasioned by levels of kerb and tram rails, that led to its adop-

tion. Molesworth street arid Courtenay place are perhaps the best examples in New Zealand of concrete roads with tar carpet finish, but in some of the other streets there were indications that the carpeting method had maintenance trouble of its own.

In city streets with a double line of tramway there are difficulties regarding maintenance that do not occur on rural highways, and in Wellington these are aocentuated by the particularly concentrated traffic of narrow streets.

The cost of concrete road 3 laid in Wellington, including the cost of tar pitch and asphalt carpet varies from about five to nine shillings per square yard. Blair street, of comparative recent construction, was laid down in two course work, six inches thick, top-dressed with tar and sand, and cost 7s 4£d per square yard. OTHER TOWNS. In Napier a quarter of a mile of concrete road has been laid in Emerson street. The concrete is 7in thick at centre and sin thick at sides. The gritting material for the tarred surface consists of equal proportions of clean fi.no gravel and crushed gasworks cinders. In Napier the local gasworks tar is distilled and used as a road binder and use is also made of the by-products, napthalene, for instance, going to the power house. An innovation in concrete work is the colouring of the flags of footways a light pink, the pigment being oxide of iron obtained from the electrolytic treatment of old petrol tins. Wanganui Borough has two concrete roads —the first, Victoria avenue, being laid down in 1914 of six to one concrete, with bank ' run gravel for aggregate. The thickness of slab is 6in, and the width of same 18ft. In the other street, Taupo quay, the slab is sin thick and the aggregate washed gravel and sand, the concrete being mixed three of the former to two of the latter to one of cement, and carefully floated and finished. The experience of Wanganui with gravel as the aggregate concrete is that such pavements, under present methods of construction and during the transition stage of traffio from horse drawn to motor driven vehicles require a wearing surface or carpet of some bitumistio material. Mr Stavely, the borough engineer, believes this to, be essential, and has covered the concrete with a carpet liin in thickness. The concrete cost of Taupo Quay was 6s lOd, and of the carpet 2s 5d; —total, 9s 3d per square yard. Kairanga County has, I think, the distinction of the one and only rural concrete road. A 'short length of bare concrete road 10ft wide was laid down bv a councillor who advocated this class of construction at a guaranteed cost of £29 a chain. The surface shows no sign of wear, but there is a slight tendency to break at the joints.

COST OF CONCRETE AND TARRED ROADS.

There is undoubtedly a considerable range in tho price of what appears to bo similar methods of construction, and this is partly due to the difference in cost of material and partly to the condition of the road before treatment.

If, however, we take for illustration a road 15ft in width the following prices may be said to represent the average cost of typical methods: (a) For tar-painting a well-consolidated macadam road in good condition, £3OO to £4OO per mile. (b) For tar-sealing a macadam road that

has been brought to good condition not less than' six months previous, £6OO to £IOOO per mile. (c) For reconstructing a macadam road with, say, 4in new metal on old macadam foundation with single or multiple penetration method of tar grouting, £ISOO to £2OOO per mile. (d) For constructing a tar macadam road of standard thickness with 3in to 4£in of tar maoadam wearing surface, £I6OO to £2400 per mile. (e) For a concrete road 6in in thickness, £3500 per mile. (f) Carpeting a concrete road lin to l^in of tarred screening, £9OO per mile. (g) Annual cost of resealing any of the above roads £IOO to £l5O per mile. RELATIVE ECONOMY.

Theoretically the relative economy of the different constructions can only be expressed by the sum of the first cost and the capitalised cost of maintenance and renewal. But the general method adopted for comparison is to compute the annual cost per mile, allowing for interest and sinking fund over the number of years which the road is estimated to have life and for maintenance.

Unfortunately the life of a road and its cost of maintenance can only be approximated because of the unknown factor of fluctuation in amount and class of traffic that inevitably follow the improvement of a road and because we are at present in the transition period from horse-drawn to motor vehicles.

Moreover, although theoretically the road giving the lowest annual cost is the most economical one to build, financial considerations more often "than otherwise make it necessary to seleot some other type. It may be taken for granted that the county highway of the future will vary exceedingly in type depending on the intensity of the traffic, and will range from the highest form of construction down to the earth road and the gravel and waterbound macadam of the present day. These latter must necessarily contain the largest mileage for many years to come, and the importance of giving greater care and attention to the maintenance of suph roads cannot be overstated.

Results of observations of different types of road surface have shown that a good gravel road will wear well with 50. to 70 fight teams, 25 to 30 heavy teams with one horse, 10 to 15 heavy teams two or more horses, and 50 to 75 automobiles per day. The waterbound macadam will stand 175 to 200 light, 175 to 200 heavy one horse, 60 to 80 heavy two horse, but not over 50 automobiles at high speed per day, and that waterbound macadam, with a good surface coating of tar renewed annually will stand with 100 to 150 light, 50 to 75 heavy one-horse, 25 to 30 heavy two-horse, and 1500 or more automobiles per day. The limiting carrying capacity of gravel roads was repeatedly exceeded in pre-motor days, and these roads were then metalled, but the limit fixed for the macadam road was rarely exceeded, except in cities, until the advent of the motor, and it was for that reason that they so long- served their purpose. On those roads where the limit of 50 cars travelling at high speed is exceeded, we have • overwhelming evidence that some new form of wearing surface is necessary, and_ at the present time the choice is_ practically limited to some form of bituminous material and concrete. On many sections of macadam roads where it has become impossible to maintain ths wearing surf ace from, gravelling, it has been shown conclusively that a first expenditure of £3OO or £4OO, and an annual maintenance cost of £IOO to £l5O per mile per annum, or say, taking an average and giving the road only a life of five years, an expenditure of less than £2OO per year will not only give an excellent surface, but entirely save the wear of the macadam foundation.

On other sections where the traffic is more considerable, or where in view of future requirements a more permanent form of roadway has been thought desirable, a first cost of £6OO to £IOOO in tar-sealing and am annual maintenance cost as before has given a macadam road a life of 10 to 15 years, and the average as before is about £2OO per mile per annum. Setting aside altogether the great advantages derived from the improved surface, there can be no question that such a method is more economical than spending £IOO -to £l5O on a water-bound macadam road, and at' that figure being unable to annually replace the worn metal.

When it comes, however, to consideration of a road where nothing short of rebuilding will serve and where traffic considerations requh-e a more permanent type of pavement, from the point of economy there is little to choose between the tar macadam and the concrete road, as practically the annual costs of combined interest, sinking fund, and maintenance over a period of years may by assuming different ulti-. mate lives be shown to be the same.

I have purposely qualified tho statement, as I do not propose in this report to make comparison between the two types of con-

struction. The advocates for the concrete roads claim that at the end of the terra tho concrete remains as good as ever, whereas the bituminous macadam would require rebuilding. The advocates for the bituminous road argue that by annually resealing the life of tho road is indefinite, and even if the road required rebuilding old macadam could be used for foundation. My_ mission was to collect the data, not to point tho moral. Both types of tion represent so marked an improvement on former methods that it is invidious to draw comparisons. ROAD TRAFFIC CONTROL.

My report is incomplete without som« reference to» the question of road traffiq control.

One of the resolutions passed at the Ia» ternational Roads Congress was—- " That all regulations for the control o| road traffic should be based on the principle of allowing the speed practicable to each different kind of vehicle, consistent with public safety and general convenience, and the normal wear of the road." In New Zealand local bodies are em* powered to regulate what might be termed light traffic on the first-named principle only,- that is, consistent with-public safety and general convenience, and they may, it thought advisable for the safety of the public prohibit the use of a road or portion* of it for motor traffic altogether. With regard to heavy traffic, they are given greater powers, and although it is not so expressed in the statute, the underlying principle u regulating such traffic to "the normal wear of the road," but the power of control ia practioally limited to the winter months of tho year.

I hold that all traffic, whether heavy or light,_ should bo subject to regulation. The question of the extent to which and under what conditions, roads are damaged by motors, has occupied a prominent place in all recent discussions on road problems, and where it has been established that' substantial damage has been occasioned by these vehicles, the reasons for the damage are well understood, and it is generally agreed that the effect of self-propelled vehicles is only more damaging than those drawn by horsß when the road is unsuitable for the use of such vehicles.

Light motor cars do little damage to roads when the surface is smooth and sealed with some waterproof coating, such as tar. Neither do they appreciably damage a well* formed clay or closo gravel road. On well made waterbound macadam roads with smooth and even surface there is little damage, with one important stipulation—namely, that the cars are driven at a moderate speed. But very considerable damage takes place on this class of road when subjected to a considerable amount of motor traffic driven at high speed. The damage is caused' by the removal of the small, gritty particles of stone forming the blinding, which serves to keep the surface stones in place, and takes nlace more particularly in summer when the roads are dry. These small particles are not, as is popularly supposed, removed by the suction of the pneumatic tyre, but by the shearing action, caused by the driving wheels of the car. This shearing or tractive force tends to tear the surface, thus loosening the gritty particles forming the bond between the stones. The damage is greater if the road is out of repair and the 6urface uneven, _ tho force of the impact of the wheels in the hollows causing depression* to become deeper, especially during wet weather, when these depressions contain water.

Heavy motor vehicles, especially those shod with metal tyres, are a great source of damage not only to weak macadamised but, unlike the light motor car, also to gravelled roads. This damage is caused by shearing and impact. The effect of shearing is duo to the tractive force of the driving wheels, and the damage is proportionate to the weight and speed of the vehicle, while the force of impact, or the pounding force, increases with the square of the velocity. For example, if one vehicle carrying a load of five tons travels at a speed of "four miles a» hour, and another, similarly constructed and loaded, travels at the rate of 12 miles per hour, the force of the wheel impact, and conse* quently the damaging effect upon the road, will be in the ratio of 1 to 9'. Damage from impact does not occur to any appreciable extent with pneumatic tyres. The fact is now conceded that ordinary macadam roads are not suitable to with*stand the continuous traffic that may be expected in the vicinity • of populous centres of either light high speed traffic or heavy motor vehicles for commercial purposes.But when it is considered that ordinary macadam will, for reasons of economy and force of circumstances, be the material upon which reliance will have to- be placed for the construction' of many of our country roads, it is more desirable to regulate the traffio which use these roads before they. Buffer serious damage than to legislate after the damage has taken place. Only on certain main thoroughfares radiating from the city and carrying a'con* siderable motor traffic it is reasonable to expect that any special provision will be made in the near future to render them suitable for and less liable to damage by motor traffic.

In outlying districts it -would not' only be wasteful to construct roads to a standard above present traffic requirements, but an injustice to the many backblock settlers -who would welcome a metalled road of any description as a luxury. These comments are not made with & view to discouraging or unduly restricting the use of motor vehicles, which these daya have become a necessity for pleasure as well aa for business purposes, but to advocate that all reasonable means be employed which have the effect of reducing the damage to roads by excessive speed in thg caeo_ of light motor cars, or by excessive loading or defective design of heavy commercial vehicles.