—The powder must then be rammed with iron punners of not less weight than 10 lb. heated so as to prevent the adhesion of the asphalte. The ramming must be done lightly at first, so as to ensure equality of thickness, and afterwards augmented to heavy blows. Where the rammers are not available a T tool must be employed.
To meet some of the objections to compressed asphalte as a material for roadways the “Imperishable Stone Paving Blocks” have been introduced in America; they consist of asphalte formed into rectangular blocks under pressure of about one ton to the square inch, these are laid close together without any grouting, and a pavement of this description is said to combine all the advantages of wood and asphalte, though sufficient time has not yet elapsed to prove this.
In Salford, Manchester, etc., I believe “Woodward’s Patent Molten Ironstone Blocks” are used with some success where there is not any very exceptionally heavy traffic.
One of the principal reasons of durability in asphalte pavement is its elasticity, and it should be remembered that compressed asphalte does not begin to “wear” until all compression has ceased; this is the case with no other system of pavement—stone and wood both begin “wearing” from the day the traffic commences. Under ordinarily heavy traffic it may be estimated that it will take two years to complete the compression of asphalte, and the weight of a square foot of this pavement will at the expiration of that time be nearly the same as on the day it was laid, though the thickness is reduced during the first two years as much as it will be in the following eight.
Much is said about the advisability of good and dry concrete, but it may be as well to explain the reasons that necessitate so much care in the foundation. First, it should be always borne in mind that asphalte pavement is nothing more than a tough “carpet,” and has no power of itself of offering resistance to heavy traffic; consequently, if the substratum or concrete is not thoroughly solid and resisting, the weight of traffic will crush it, and the asphalte will at once give way in all directions. The concrete should be made strong enough to resist the traffic, and the asphalte is a simple covering to protect the concrete from direct contact with the wear and friction caused by the traffic. So much for the strength, but the dryness is of even still greater importance; for the best asphalte, laid by skilled workmen, on thoroughly first-rate but damp concrete, will rapidly go to pieces—a phenomenon takes place, which, although quite natural, is little realised by most engineers. When the hot asphalte is laid, the water is immediately sucked up and turned into steam, which tries to escape through the heated powder, and the result is that although the surface of the asphalte is smooth, the mass is really disintegrated from underneath by its bitter enemy “water,” and as soon as the surface begins to wear, the fissures formed by the passing of the steam appear on the surface and the whole pavement falls to pieces: thus accounting for some of the failures this description of roadway has met with under unskilled treatment.
This completes the subject of roadways; I will turn to that of footpaths in the [next chapter].
[90] In this respect the Seyssel is the best, being of a very fine grain.
[91] Vide ‘Minutes of Proceedings of Institution of Civil Engineers,’ vol. lx.
[92] Vide Mr. Howarth’s paper on ‘Wood as a Paving Material under Heavy Traffic,’ ‘Minutes of Proceedings of the Institution of Civil Engineers, vol. lviii. p. 35.