The Paving of Roadways.
Page 21
Page 22
Page 23
If you've noticed an error in this article please click here to report it so we can fix it.
(Continued from page 277.) Wood Paving.
The first we hear of any wood pavement being laid in London was in the year 1839, which, according to Mr. T. Aitken, was laid in front of the Old Bailey on the Stead system. In this system the wood blocks were laid on a bed of gravel 3in. thick on ground previously levelled and well beaten. The wood blocks were of Scotch or Norway fir, and were hexagonal in shape, 6in. to 8in. across, and from 311n. to 6in. deep. The upper -edges of the blocks were chamfered, and, in some cases grooves were made across the surface. In several instances a concrete foundation of sin. thick was laid, consisting of small broken stones flushed with Ardwick lime or Roman cement. Round blocks were also used by Stead. Following this came De Lisle's .method; Carey's Pavement ; improved Wood, hailing from the United States; the Ligno-Mineral Pavement ; -Asphaltic Wood Pavement ; Benson's Pavement ; and Mowlem and Co.'s method. I need not enter into details of each of these systems, which all employed wood blocks as the surface pavement. It may be mentioned, however, that in De Lisle's a system of dowelling was used to hold the blocks together ; and that Carey's was practically the first wood pavement to exhibit good qualities of durability, etc. Pine blocks were used, 611n. to 71in. wide, and 131n. to isin. long, and from 8in. to gin, deep, a considerable waste of good material. The cost of the paving was 145. 2d. per square yard. The joints of jin. were grouted with lime and sand mortar, and the life of the paving was said -to have been nine years under heavy traffic, and eleven years under light. But as this was in 1841, we may judge of the values of the words "heavy" and 'light" traffic compared with the present-day demands. The Ligno-Mineral Pavement was introduced frc.ni France, and is said to be the first provided with a hard concrete foundation moulded to the curvature of the street. Blocks of oak, elm, beech, or ash, 3M. by 6in. and gin, long were used. This brings us as far down as 1872 to I875. Among the woods now in general use for paving are Deal, Pine, the West Australian hardwoods, Jarrali and Karri, and Blackbutt, a New South Wales wood
As regards the hardwoods now employed for paving, the two former are the better known at present, and the most widely used. All these woods belong to the Eucalyptus family, and
their tensile and crushing strengths are high. The average weights of Jarrah and Karri are respectively 651b. and 631b. per -cubic foot, as compared with 4lb. for British oak and 4o1b. for elm. The principal properties of the West Australian woods -are as follows :-Karri (Eucalyptus diversieolor) is a hard, heavy, pale red wood, with very little absorptive power; the fibres are strongly interlaced, and it is of great tensile strength. It contains tannin and other gum resins. Jarrah (Eucalyptus marginata) is a heavy hard wood slightly redder in colour than Karri. It has very little absorptive power, and is very durable and tenacious. It is consequently an excellent wood for paving. It also has considerable natural antiseptic properties, containing a red resin (red kino), 4 to 5 per cent. of kino-tannin, and a .particular vegetable acid, viz., oxalic, which is particularly disagreeable to the various forms of insects (ants, wood beetles, etc.) which generally infest woods ; and this property alone .renders it very valuable. Both these woods are, as stated, red in colour, and a common method of distinguishing them from one another is by burning, Jarrah leaving a black cinder and Karri a light coloured ash.
From the table already given, it will be seen where, among the paving materials in use now, these hardwoods appear. They are, of course, of very close texture, and their wear is thus considerably longer than ordinary deal. The life which has been given to these woods has differed considerably, according to their treatment and the conditions under which they have been used. The London County Council have fixed the period of to years for lbans for this description of paving, although many instances have occurred where they have been known to last longer than this on the roadway, and then have been taken up, cut down and relaid in another thoroughfare. An instance of this is recently recorded at Camberwell, where Karri blocks, having done service for ii years in Rye Lane, were taken up, -trimmed down to .in. blocks and relaid in Peckham Road. As a rule sin. blocks are used for road paving, though, from a long experience, I am convinced that a Ain. block is quite deep -enough, while in some places a 31in. depth would be sufficient. The length is from 6in. to gin., and the width 3in. This suggested reduction in depth would, of course, make the paving considerably cheaper, and would not affect the wear. The first cost of a 3in. by gin. by kin. Jarrah (taking the blocks at Lg zos.
per thousand) laid on and including 6in. Portland cement concrete foundation, is about is. per yard super. As representative of the cost of laying down and keeping in repair 1,000 yards super. of this hard wood, I may again quote figures from a report issued by Mr. Oxtoby, the borough engineer of Camberwell, dealing with the life and cost of different pavements : First Cost.-r,000 yards super. 3in. wide by gin, long by 4in, deep Jarrah wood paving, upon and including 6in. Portland cement concrete foundations ... 4750 o o Contingencies ... so o o Total first cost ... 4800 o o Cost every to years.-I,000 yards super. Taking up old blocks and repaving with new sin. by gin. by din. Jarrah wood blocks on old concrete foundations re floated, at 125. 6d. 625 o o Contingencies 75 o o
£700 0 0
The repayments of principal and interest for 20 years will be : First to years, principal 4800 ; total interest for to years at 3, per cent., £165. Second to years, principal 4700; total interest for to years at 31 per cent., 4x44 7s. 6d. Total cost for 20 years for ',ono yards super, £1,809 75. 6d. Average cost per yard super. per annum for 20 years, is. 9.7d.
Then as to the sanitary qualities of hard woods and other woods. I cannot do better than insert here the conclusions arrived at by Drs. Norton and Hake, and given in the report published by the Agent-General for Western Australia. The following table shows the kinds and quantities of matters absorbed by the woods
ANALYSES OF AQUEOUS EXTRACT OF vaatoos PAVING BLOCKS AFTER USF. IN TRAFFIC.
• The figures given represent the quantities present in 100 parts by weight of wood.
"In other words, judging from the analytical results, we place Karri in the first rank, Jarrah in the second rank, and red gum in the third rank, while deal and creosoted deal constitute a :ourth class in order of efficiency. It seems apparent that, however excellent the creosote may be in preserving the wood, it dues not appear to affect the sewage absorption. This general classification is in fairly close agreement with the relative absorptive power of the woods, as indicated by the moisture contained in them, and with the total solids extracted by water."
Blackbutt, a New South Wales wood, has been to some extent employed in this country, but not for a sufficiently long period to enable us to judge as to its suitability for paving streets. The recorded dry weight of the wood is 561b. to 592,1b. per cubic foot, and the grain is coarse and dense. It is also said to be non-inflammable. The absorptive power of the wood is only about 5 per cent., while the expansion and contraction is said to be very slight. It is, however, a wood of even closer texture than Karri, and would, therefore, be more slippery and give less hold for horses than the West Australian woods, especially where the gradient is unusual. The Blackbutt tree grows to a height of from tooft. to isoft., and has a diameter varying between 21ft. to 4ft. From a paper read by Mr. R. W. Richards, late City Surveyor of Sydney, N.S.W., we find that during 1888, wood pavement was laid in Castlereagh Street, between Park
and Bathurst Streets, with a close joint. The timbers consisted
chiefly of lilackbutt and Tallow wood, and during the week ending February 27th, Leese, blocks were removed from portions of the carriage-way carrying the most of the traffic, which may
be taken as heavy and continual, also fast and slow, and upon
examination showed an average wear at the rate of x-3oth of an inch per annum. Blocks of Blackbutt and Hardwood from Kent Street, subjected to the heaviest class of traffic common to mercantile and wharf districts of the city for twelve years, showed wear at the rate of one-thirtieth of an inch per annum. These sections were selected as being typical of the most variable traffic conditions that Sydney affords. The loss in wear annually of these woods at Sydney being so slight would no doubt arise from the fact that the surface is treated once or twice each year with a dressing of distilled tar and sand—a method which I would strongly recommend in this country. A glance at the table of comparative values of different descriptions of paving given above will furnish a means of estimating their durability and economy. Hardwoods stand an easy first, softwood in the third position. From a sanitary point of view, hardwood follows asphalt, softwood again in the third place, which is also borne out by the independent researches of Drs. Norton and Hake. For quietness alone softwoods stand naturally first and hardwood second, and it is largely for this reason —as an authority points out—that softwoods have been adopted ; for safety, wood of both descriptions comes far before asphalt ; for ease of cleansing, hardwood stands before softwood ; and for traction resistance, although the two woods are grouped together, the hardest description must necessarily be first. There is, however, a consideration, which no doubt has some weight with certain authorities for selecting softwood, namely, first cost, but as a rule a maintenance clause is included in the contract covering a certain period, of from 6d. to is. 3d. per yard per annum over the whole area laid, which, while having the effect of reducing first cost, actually proves to be dearer than Karri and jarrah. Mr. P. II. Palmer, the Borough Surveyor of Hastings, whose experience with wood paving is very considerable, remarks, in regard to softwood, "It has been found in actual practice, even when creosoted, that owing to its coarseness of grain it takes up and holds a large amount of impurity due to the traffic and manure on the road. It is a difficult material to deal with in respect of expansion and contraction, and on the whole, whether creosoted or not, is much less sanitary than a hardwood paving. With regard to the life of softwood paving, this appears to vary very considerably in different places. There are instances where softwoods appear to have only lasted a very few years, and others where it has lasted for seven or more. This, of course, may be duo to a combination of circumstances, such as the nature of the traffic, width of the roadway, and principally to the careful selection and laying of the blocks." On the hygienic character of softwoods, Drs. Norton and Hake, whose valuable investigations I have referred to before, say ; " Softwoods have been used mostly in London on account of their being less noisy under traffic than the hardwoods, but the chief objection to them is that they wear away much quicker than the hardwoods, They are much more absorbent, and consequently soak up all the sewage in the streets, owing to their fibres being coarse and long; and owing to the wearing away of the fibres in hot weather innumerable particles of dust and debris ground down under the wheels of the traffic are thrown into the air. Moreover, they are not so easily cleaned as the hardwoods."
As this wood absorbs a considerable amount of moisture, it should not be laid in its natural form. The failure of softwoods is, says Mr. Aitken, on this account doubtless traceable to the imperfect manner in which they have been laid with untreated blocks and wide joints. The absorption of moisture, is, to a large extent, prevented by creosoting.
The comparative expansion of creosoted against plain softwood blocks, af:er immersion in water for 48 hours, was stated by Mr. Buckham, Borough Engineer of Ipswich, to be as follows:
0 length of block ... Creosoted .oesi Plain .6
„ width e „ -57 3, .83 „ depth „ „ _ .15 „ .31 These would represent in a 3oft. carriage-way 21in. for plain blocks and practically :in. for creosoted blccks, if under same conditions.
Cement Concrete Foundation.
One of the most important matters in connection with any paving, is that of foundation_ More obloquy is brought on paving materials from thic, cause than from any other. A good foundation means a good roadway, whatever the material above it ; a defective foundation means a bad, irregular roadway, however excellent the paving material used, and I feel that I cannot press this matter upon your attention with too great an emphasis. No good roadway can be made without a rigid foundation, the material with which the street is paved being merely the skin or veneer to preserve the real roadway—the foundation underneath. Experience has shown that the best foundation is one composed of cement concrete, and it follows that the more perfectly this is done, the longer the life of the wood, granite, or asphalt veneer. But under this concrete roadway is another foundation, one of earth, which has frequently been disturbed for a considerable distance, and which, if not properly consolidated, will render all care afterwards of little avail, for the traffic follows the completed street so quickly that depressions must occur if this is not very well done, as the concrete is not sufficiently hard to resist the pressure. The real roadway, or the foundation of concrete, whichever you like to call it, should be perfect in form and as strong to resist compression as it can be made with good cement ballast. For wood paving it is imperative that the exact falls required should be given to the surface of the concrete. It should be even and true, and although not required to withstand wear and tear, should be impermeable to moisture. Wood blocks are cut to an exact gauge and laid upon the concrete without any intermediate padding, therefore the under level rules the upper. For asphalt and granite paving, these conditions are not quite so important. The asphalt can vary a little in thickness, and its perfect form depends on the skill of the workmen laying it, more than on the surface of the concrete underneath. And the granite blocks vary so much in depth that they have to be laid on a bed of sand of sufficient thickness to allow for their inequalities. The compressive strength of concrete is very great when it is allowed time to mature and harden, say seven days as a minimum. Proportions of six of clean gravel to one of Portland cement when hard will bear a pressure of eao tons to the square foot; 8 to 1, 75 tons ; to to x, so tons; and 12 1U 1, about 33 tons per sq...are foot.
Concrete is very weak in tension, being only about 1-roth of its compressive strength, a fact which shows how necessary it
is to have the ground well consolidated on which the concrete
is laid_ Engineers and surveyors should bear this in mind, and
should specify that small rods, with their ends turned up, should be placed as low in the concrete as possible, across the site of the trenches recently cut and filled_ Wrought iron has a tensile strength of 56,000lb. per square inch, and by placing these bars low down, and of a shape that cannot slip, we can make up for the deficiency in the tensile strength of the concrete, and make the lower half nearly equal in strength to the upper. Chalk and river mud are the constituents of the Portland cement, made on the Thames and Medway. An excess of the former is very likely to produce a cement containing caustic lime, and an excess of the latter either causes fusing in the burning, which makes the clinker useless for cement, or it is under-burnt and produces an inferior article. It is the clay that possesses the power of rendering the compound hydraulic. The strength of the concrete depends very much on the quality of the aggregate. It should be clean, angular, and hard, and of all sizes from one-fifth of the thickness of the floor to the finest grains of sand. Gravel, broken stone, igneous rocks, flints, sandstones, limestones, broken bricks, burnt clay, coke breeze, slag, are all used. To the Portland cement concrete expert the foregoing rules are axioms in his business, but how do these fit with the practice of our street contractors? Trenches are dug and filled in as soon as the pipes or cables are laid and a certain amount of ramming is done, but rarely sufficient to prevent subsidence afterwards, when the soil gets dry and the constant vibration settles it. On this the concrete is often laid with a very inferior cement—the aggregate composed of the dirty siftings and broken material from the old street excavations. The proportion of aggregate to cement is also often much more than it ought to be. The concrete has no time given it to harden before the wood, granite, or asphalt is laid, and the traffic at once allowed on it. Under these circumstances, is it to be wondered at that we get depressions on the surface of our newly-paved roads? The necessity for getting all paving work done quickly in our London streets is answerable in a great measure for the paving becoming uneven so soon after it is laid. Observe in the City how hastily the trenches are filled up and rammed. They must settle and sink when dry. Where repairs are going on, the old concrete is broken up and used for the new cement foundation, no matter how dirty it may be. The cement itself is often the cheapest obtainable, the newest and the quickest setting, therefore the most unreliable. This is immediately covered and the traffic allowed over it.
Reading Mr. P. H. Palmer's interesting and practical report on wood paving in "The Street" some time back, I was struck with his experience of the expansion of cement concrete. No doubt the cement used by the contractor is often new and very coarsely ground, and would in itself expand, not having been air slacked, which experience I can confirm myself.
The Wear of Roadways.
In a paper read by Mr. Worby Beaumont in 1905, before the Municipal and County Engineers, referring to the wear of roads, he remarks that : "The wear of roadways is produced more by horses' hoofs than by wheels of vehicles, and as Sir John MacNeill showed, the wear is greater when lighter vehicles are being driven at high speeds than in the case of horses drawing heavy loads in heavy vehicles." He then gives a table showing the effects on the road by both horses and vehicles:—
The wear and tear of every vehicle of speeds of eight miles an hour and upwards is enormous, as compared with what it need be. It is the speed that wears the vehicles, and therefore the roads. It is the light vans, carrying r to sA tons on zin. tyres, and with trotting horses, that do more mischief than any other class, excepting the omnibuses.
Jointing.
There is a considerable difference of opinion as to the proper form of joints for wood paving—some advocate clIse joints, that is, butting the blocks together so closely as tc get a continuous and unbroken surface; whilst others consider blocks should be laid Ain. to Ain. open joints. My own experience is that blocks should be laid as closely together as possible, consistent with the fact that sufficient space is allowed to ensure that the tin. pitch or cement grouting thoroughly penetrates the joints all round the blocks, thus providing a watertight surface. Blocks for close jointing of course need special care in cutting, or we get rocking blocks. In a paper read by Mr. W. N. Blair on the jointing of wood pavements before the Municipal and County Engineers (1893-4), he remarked that he had laid roads varying from 22ft. to 481t. in width, with yellow deal, pitch pine, jarrah and Karri, with Ain. and r-zoin, joints, regulated by a lath of that thickness and lin. wide, laid on edge in the bottom of the joint, arid left there, the cross joints being left open the same width, but without lath. The channel joints have been from ?sin. to and the whole of the joints filled with a hot
compound of pitch and creosote oil, proportioned suitably to obtain a freely ductile mixture at a temperature of about 6odeg. Fahrenheit, the proportions being usually tent, of pitch to rA or gallons of oil, some variation being necessary to meet the differences in quality of the pitch. This compound is poured over the paving out of a bucket, and a rubber squeegee is used to push forward the fluid, so that as little as possible remains on the surface. In all work done in this way no case occurred of any pressure being exerted on the kerbs. It is, of course, admitted on all hands, that an open joint greatly reduces the life of the wood, the fibre being allowed to to spread and disintegration following. It is also recognised that joints foster dirt and dirt causes danger to traffic.
The general practice in Sydney is as follows, and may well be recommended for general use :—A puddle of clean clay sin, in thickness, or planks in lengths of from loft. to 2011. by 3m. by 6in. (that is the full depth of the block) are placed between the curbing, and parallel thereto, throughout the whole length of the wood pavement ; by this means expansion that may follow rain or climatic changes is provided against. It has been found that a further means of successfully preventing expansion is to topdress the whole surface of the pavement with distilled tar and dry sand; this work is carried on in dry weather, and during the hottest season. The coating gives a smooth and even. wearing surface, affords sure foothold for horses, and renders the pavement impervious, thus minimising expansion. To conclude, I will recapitulate a number a the qualifications necessary for a perfect roadway :—Impermeability, durability, foothold, ease of traction, adaptability to all gradients and all classes of traffic, noiselessness, non-manufacture of mud or dirt, eLse of cleansing, ease of repairs, economy in first cost and maintenance, and resistance to influence of climatic changes.