STONE BLOCK PAVEMENTS Stone blocks are commonly employed for pavements where traffic is heavy. The material of which the blocks are made should possess sufficient hardness to resist the abrasive action of traffic, and sufficient toughness to prevent them from being broken by the impact of loaded wheels. The hardest stones will not necessarily give the best re stilts in the pavement, since a very hard stone usually wears smooth and becomes slippery. The edges of the block chip off, and the upper face becomes rounded, thus making the pavement very rough.
The stone is sometimes tested to determine its strength, resistance to abrasion, etc.; but, as the conditions of use are quite different from those under which it may be tested, such tests are seldom satisfactory. However, examination of a stone as to its structure, the closeness of its grain, its homogeneity, porosity, etc., may assist in forming an idea of its value for use in a pavement. A low degree of permeability usually indicates that the material will not be greatly affected by frost.
Materials.—Granite. Granite is more extensively employed for stone block paving than any other variety of stone; and because of this fact, the term "granite paving" is generally used as being synonymous with stone block paving. The granite employed should be of a tough, homogeneous nature. The hard, quartz granites are usually brittle, and do not wear well under the blows of horses' feet or the impact of vehicles; granite containing a high percentage of feldspar will be inju riously affected by atmospheric changes; and granite in which mica predominates will wear rapidly on account of its laminated structure.
Granite possesses the very important property of splitting in three planes at right angles to one another, so that paving blocks may readily he formed with nearly plane faces and square corners. This property is called the rift or cleavag(°.
Sandstones of a close-grained, compact nature often give very satisfactory results under heavy traffic. They are less bard than granite, and wear wore rapidly, but do not become smooth and slip pery. Sandstones are generally known in the market by the name of the quarry or place where produced as "Medina," "Berea," etc.
Trap rock, while answering well the requirements as to durability and resistance to wear, is objectionable on account of its tendency to wear smooth and become slippery; it is also difficult to break into regular shapes.
Limestone has not usually been successful in use for the construc tion of block pavements, on account of its lack of durability against atmospheric influences. The action of frost commonly splits the blocks; and traffic shivers them, owing to the lamination being vertical.