CONSTRUCTION OF WOOD PAVEMENTS.
As stated in Art. 63, the older types of wood-block pavement, in which the blocks were laid with open joints on a plank foundation or on gravel, are prac tically obsolete, and wood blocks are usually laid with close joints upon concrete foundations. This gives firm support to the blocks and admits of even wear upon the surface of the pavement. A durable base also has the advantage that when the surface layer is worn out, the pavement may be resurfaced without removing the foundation. The concrete base is con structed in the ordinary manner as described in Art. 47. It is commonly about 6 inches thick, although under specially trying conditions a somewhat greater thick ness is sometimes employed. In a few of the European pavements very heavy foundations, 7 or 8 inches thick, are employed; but these are exceptional and the 6 inch depth is usually found sufficient. Lighter foundations, 4 or 5 inches in depth may be used under favorable conditions and where traffic is heavy; but these pavements are usually employed upon streets of con siderable traffic, and in such situations very light construction is not desirable.
For-the purpose of receiving the blocks and affording them uniform support, a cushion coat of sand or a thin coating of cement mortar is placed over the con crete. The sand cushion when used is usually about inch in thickness and is placed in the same manner as in laying a brick pavement. When a mortar surface is employed, a coating of about } inch of mortar is floated over the surface of the concrete and brought to the exact form of the finished surface, the blocks being placed before the mortar sets and bedded into the surface of the mortar. This method, while used to a much less extent than the sand cushion, seems to give excellent results in maintaining a uniform surface where the work is properly done, giving more uniform support than the sand cushion.
The blocks are set with the grain vertical, close together and commonly in courses making an angle of 6o to 70 degrees with the curb line. In some instances the blocks are placed with open joints across the street of to inch. Most of the older work was
constructed in this manner, the wide joints being intended to give better foothold to horses, as well as to allow for expansion. Experience has shown that where impervious joints are secured, the expansion may be taken care of by an occasional expansion joint, such joints being placed along each curb and across the street at distances of z00 to 200 feet, a joint of one-half inch every no feet being usually sufficient in good work. Pavements constructed in this manner have the die.
advantage of being somewhat slippery and wide joints are frequently used where the grade of the street is more than 3 or 4 per cent. There are, however, in stances of wood-block pavements with close joints on gradients of as much as 8 per cent which seem to give satisfactory results.
Three methods of filling joints are frequently em ployed, sand, pitch and cement mortar being used. Sand filler is applied by placing a light coating of dry sand over the surface and brushing it into the joints, then covering the pavement with a layer of sand and opening the street to traffic. Pitch joints are made by the use of asphalt or coal-tar paving cement as used for brick pavements. They are made by spreading the hot paving cement over the surface and brushing into the joints. Care must be used to apply the cement at such temperature as will cause it to readily run into the joints and brush off all surplus cement. A light coating of sand is then placed over the pavement to take up and grind off the pitch left on the surface of the pavement, which may otherwise become objection able in warm weather.
When the joints are grouted with hydraulic cement, a mortar composed of one part cement to two parts sand is usually employed, mixed to a liquid condition so that it may easily run into the joints. This mortar is slushed upon the surface and broomed into the joints, and a light coating of sand is placed over the surface before opening to traffic. This sand is ground by the traffic into the blocks, tending to make the sur face more gritty.