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Stone-Block

material, wear, nature, stone, pavement and sandstones

STONE-BLOCK pavements are commonly employed' where the traffic is heavy and a material needed which will resist well under wear.

Stone for this purpose must possess sufficient hard ness to resist the abrasive action of wheels. It must be tough, in order that it may not be broken by shocks. It should be impervious to moisture and capable of resisting the destructive agencies of the atmosphere and of weather changes.

Experience only can determine the availability of any particular stone for this use. The stone may be tested in the same manner as brick, and perhaps some thing predicated as to the probability of its wearing well under traffic; but the conditions of the use of the material in the pavement are quite different from those under which it may be tested, and any tests looking to a determination of its weathering properties are apt to be misleading.

Examination of a stone as to its structure, the close ness of grain, homogeneity; etc., may assist in forming an idea of its nature and value for wear. Observations of any surfaces which may have been exposed for a considerable time to the weather, either in structures or in the quarry, will be the most efficient method of forming an opinion concerning the weathering proper ties of the stone. The conditions of use in pavements are, however, somewhat different from ordinary expo sure in on account of the material in the pavement being subject to the action of water contain ing acids and organic substances due to excretal and refuse matter. A'low degree of permeability usually indicates that a material will not be greatly affected by these influences and also that the effect of frost will not be great.

Granite and sandstones are commonly employed for paving blocks and furnish the best material. Lime stones are sometimes used, but have seldom been found satisfactory. Trap-rock and the harder granites, while answering well the requirements as to durability and resistance to wear, are objectionable on account of their tendency to wear smooth and become slippery and dangerous to horses. Granite or syenite of a tough,

homogeneous nature is probably the best material for the construction of a durable pavement for heavy traffic. Granites of a quartzy nature are usually brittle and do not resist well under the blows of horses' feet or the impact of vehicles on a rough surface. Those con taining a high percentage of feldspar are likely to be affected by atmospheric agencies, while those in which mica predominates wear rapidly on account of their laminated structure.

Sandstones of a close-grained, compact nature often give very satisfactory results under heavy wear. They are less hard than granite and wear more rapidly, but do not become so smooth and slippery, and commonly form a pavement that is more satisfactory from the point of view of the user. Sandstones differ very widely in character, their value depending chiefly upon the nature of the cementing material which holds them together. In order that a stone may wear well and evenly in a pavement it is desirable that it be fine grained, dense and homogeneous, as well as cemented by a material which is not brittle and is nearly imper vious to moisture. Those sandstones in which the cementing material is of an argillaceous or calcareous nature are apt to be perishable when exposed to the weather. The Medina sandstones of Western New York and Ohio have been quite extensively used for paving purposes and prove a very satisfactory material for such use.

Limestone has not usually been successful in use for the construction of block pavements on account of its lack of durability against atmospheric influences. The action of frost commonly causes weakness and shiver ing, which produces uneven and destructive wear under traffic. There are, however, as wide variations in the characteristics of limestones as in those of sandstones, and there may be possible exceptions to the rule that, in general, limestone is not a desirable material for block pavement.