The Forth Bridge/Asphalting
Asphalting.
The batter given to the sides of the structure brings with it one disadvantage, namely, that one-half of each lattice-girder flange forms with the vertical web a recess in which rain-water can lie and cause rusting. To prevent this, all places so situated that the water cannot of its own accord drain away are tilled with asphalt-concrete—that is, a mixture of asphalt, pitch, tar, and coarse gravel—to such an extent that all water will run off by gravity. Where this is not possible, or would lead to too much weight being put on, holes are drilled, and the asphalt so laid that the water is drained to them and away through them. The top members in central towers, all horizontal bracing girders, and all recesses in top and bottom junctions, are done in this way.
The whole of the inside of the skewbacks, except where the nuts of the foundation-bolts are situated, are also dealt with in the same manner, and the same at the bottoms of the vertical columns, the diagonal columns, and the struts in cantilevers; and in all cases pipes are fixed to lead the water to the outside. By this means it is hoped to prevent all rusting in the places where access for constant and thorough inspection, is not easily to be had.