ESSENTIALS FOR A GOOD WOOD PAVEMENT The best method of laying a wood block pavement to withstand heavy traffic was so well set forth by R. S. Manley, at the last annual meeting of the American Wood Preservers Association, that we quote as follows: A creosoted wood block pavement should show no evidences of wear for many years, if the proper materials are used, and if they are assembled in the proper way.
The correct depth of base or foundation varies with the soil conditions; but the materials forming this concrete foundation, and the methods of mixing, are in such common use as to be standard and easily secured.
We are interested principally in the construction placed on top of the concrete. The principal causes of defects of more or less serious nature, are: (1) irregular or uneven surface due (a) to careless laying, (b) to shifting of sand cushion, (c) to breaking or settling of concrete. (2) Expansion difficulties due to the entrance of water into the blocks either by way of the joints or from below.
The first (irregular or uneven surface) is death to any paving material, because a depression in the surface holds water, and repeated churnings of 'wagon wheels in the depression are bound to cause an enlargement and deepening of the depression.
To avoid (a), the concrete should be mixed quite wet, and finished smoothly with a flat wooden spreader, which gives a surface practically as even and uniform as could be obtained by templet. On this should be spread from one-half to one inch of clean sand, making the sand cushion conform to the contour of the finished street. On this, place the blocks quite closely together; roll thoroughly until a perfect surface with no inequalities has been obtained, and until the blocks are firmly in place. It will require a great deal of rolling to accomplish this, but the end justifies the means. After this, fill all joints two-thirds full of hot bituminous filler of such melting point as is suited to climatic conditions; and spread a thin coating of sand thereon. The use of the bituminous filler is, in my estimation, the most important of all. It converts the street into an effective watershed which, without absorbing any of the water, directs it into storm sewers or other drainage paths. Should any water remain on the surface, the wind and the sun, both good evaporative agencies, will rapidly dissipate it.
Now you have an absolutely even surface waterproofed and converted into a watershed. This surface cannot be worn by traffic, because the pressure of wheels is even and regular, and there is no dropping or jolting of wheels entering and leaving low spots. The blocks are laid tightly together, so that there is no wearing at the joints. There can be no change in the sand cushion as long as the surface remains intact, a solid sheet, in fact, of wood block cemented together by the filler; and consequently the difficulty of shifting cushion is avoided. It is assumed that the concrete is sufficiently strong so that it will not break or settle. In planning the depth, any
error should be on the side of too great, rather than too little depth.
Expansion difficulties are eliminated by the use of bituminous filler, for there can be no expansion without absorption of water, and no absorption of water when all rainfall is conducted quickly to drainage sewers. In addition to this, it must be remembered that with the bituminous filler each block is surrounded by an individual expansion joint.
The other way of constructing wood block surface which is sometimes recommended, is to provide a mixed sand and cement cushion and sand-filled joints or interstices. The sand and cement cushion does not give the opportunity for absolutely smooth surface that the sand cushion gives, and is considerably more costly. The sand filler in the joints allows moisture to be absorbed in the pavement; and ultimately this moisture gets into the blocks, and trouble ensues. It is only on extremely heavy traffic streets that sand can be used as a filler without expecting some expansion difficulties sooner or later. The proof of the pudding is the eating; and the proof of theories of wood block construction lie in the actual occurrences on the street.
It can be stated without fear of successful contradiction, that every sand-filled pavement in the South has at one time or other given trouble from uncompensated expansion; that with equal confidence it can be stated that not one bituminous-filled pavement has given trouble from this cause.
Now, there have been objections put forward to the bituminous filler because of the belief that it would produce a sticky surface, disagreeable in warm weather; but if the proper filler is secured, and it is correctly applied, there can be no such objection. The suitable filler has a consistency of rubber, and can be taken in the fingers, bent and twisted without soiling the fingers. In applying this filler, a spreader with squegee attachment places the filler in the joints where it is needed, and not on the surface of the blocks where it is not needed.
It is proper also to use less creosote oil per cubic foot of timber when bituminous filler is used, for the primary function of the creosote oil in this case is to preserve against decay, instead of trying to make the creosote oil fill the double role of preservative and absolute waterproofer. No one familiar with preservative methods and their history will question the efficacy of sixteen pounds of creosote oil per cubic foot in preserving against decay for an indefinite period. We therefore see that bituminous filler can be used carefully, and without inconvenience because of stickiness.
to sum up, therefore, provide adequate, smooth concrete foundation; use enough sand to cover any inequalities in the concrete or depth of blocks (except in railway areas and on grades, when use sand and cement mixed); lay blocks tightly; roll until smooth; fill joints with bituminous filler; spread coating of sand; and turn on traffic.