Wood-block pavements are constructed of blocks set with the fibers vertical, so that wear comes upon the ends of the fibers and has no tendency to split pieces off from the blocks. These blocks are usually from 6 to 12 inches in length, 3 to 4 inches in width, and 3 to 5 inches in depth. Some engineers require the blocks to be of uniform length but a variation of from about 6 to Io inches is more common and seems desirable because of the greater freedom in obtaining timber for the purpose. A depth of 31 inches, or at most 4 inches, is sufficient and there seems to be no advantage in greater depth, as the block would become unserviceable and need to be renewed before this depth would be worn away. The blocks are cut from planks of uniform thickness, and are set in courses across the street, the blocks in adjoining courses breaking joints with each other.
Kinds of Wood. Wood for pavements should be close-grained and not too hard. It should be as homo geneous as possible in order that the wear may be uniform, and soft enough that it may not wear smooth and slippery. To give good service in wear the wood should be penetrated by water as little as possible and show good resistance to decay under the action of the weather.
Wood for this use should be sound and well seasoned. The blocks should always be subjected to careful in spection. All sapwood needs to be removed in order to lessen the liability to early decay, and blocks con taining shakes and knots should be rejected.
In Australia hard-wood blocks have been quite extensively used and are reported as giving good service, although they are admitted to be somewhat slippery in wet weather. Australian Karri and Jarrah woods are employed, and it is claimed for them that they show unusually great resistance to wear and are not soon affected by decay.
In London, where wood pavements have been very extensively employed, Swedish yellow deal is com monly placed at the head of the list of woods in value, yellow pine and Baltic fir being also largely used and considered good in use. The Australian woods above mentioned have also been used to some extent in Lon don, and are said to have given very satisfactory ser vice, showing greater resistance to wear than deal or pine, although somewhat expensive. Deal treated with creosote is extensively used and seems to give the best satisfaction. In Paris, teak, karri, and pitch pines are
frequently employed, although treated native pines are more commonly used and have been found to give good service.
In the United States the introduction of these pave ments has been limited by the high cost of suitable timber, and its extension is largely dependent upon obtaining satisfactory materials at lower cost. South ern long-leaf yellow pine has been most extensively employed, but is so much in demand for other purposes that properly selected timber for the purpose is too expensive. Norway pine, tamarac, and fir are also used for paving blocks to some extent. Experiments upon the Southern black gum have seemed to indicate that it may be used to advantage when properly treated. This wood is less valuable for other purposes, as it has a tendency to warp and is subject to decay, and is being used for paving to considerable extent. The gum,being subject to decay, needs to be properly treated in order to satisfactorily resist wear in a pavement.
The specifications used for blocks in New York City are as follows: "The material to be treated shall be wood blocks, which may be either of Southern long-leaf yellow pine, Southern black gum, Norway pine, or tamarac, not less than 90 per cent of heart, of a texture permitting satisfactory treatment as hereinafter specified, and is to be subject to inspection at the works in the stick before being sawed into blocks.
"All blocks shall be of sound timber free from bark, loose or rotten knots, or other defects which would be detrimental to the life of the block or interfere with its laying. No second growth timber will be allowed.
"The paving blocks cut from the lumber above specified shall be well manufactured, truly rectangular and of uniform dimensions. Their depth (parallel to the fibre) shall be three and one-half (3i) inches. Their length shall be not less than six (6) nor more than ten (to) inches, and their width shall be not less than three (3) nor more than four (4) inches, but all blocks used in any one contract are to be of the same width and of the same timber. Their depth and width shall not vary more than one-eighth a) inch from the dimension specified for any one contract."