272. Grout is thin-tempered mortar, composed almost entirely of cement and water. It is run into the joints, and is useful in filling crevices in masonry which cannot be filled with mortar.
CONSTRUCTION OF ARCHES.
273. The foundations being secured, and the piers and abutments being carried up to the springing line of the arch, the centres are carefully adjusted to their places and the arch is commenced. When the voussoirs begin to bear upon the centre (which is when the angle of the joint with the horizontal is greater than the angle of repose of one stone upon another), the frame is liable to change of form, (particularly when the arch is flat,) which must be provided for by counter loading the centre in various points as the work proceeds. Great care should be taken to make each stone point in the direction of the radius of the arch. To do this effectually, their thickness should be marked upon the outer rib of the centre. The line of the joint may then be fixed by a straight-edge placed both on the centre and the rib mark, or by a template so cut that when one side is level the other shall stand at the proper angle. Excess of weight upon one side of the centre causes a depression at that point, and a corresponding rise at the opposite side of the arch. Both sides being loaded, the haunches settle, and the crown rises. The point where the centre is first loaded will determine the point where the frame is to be temporarily weighted. Such precautions, however, need only to be taken in arches of over fifty feet span, unless the curve is quite flat. The keystone should be put into the proper place, but not driven until the rest is finished. The back joints are then closely wedged and cemented with thin tempered mortar, and the whole is left to set. The masonry of the spandrels is brought up to about one fourth the height of the arch, or enough to prevent by their weight any change of form of the curve. The centres are then struck and the soffit and voussoir joints cleaned and pointed. The facing and road-way may next be carried up; the parapet coping and drains finished off; and the whole pointed. Parapets are shown in figs. 127 and 128. The spandrels, fig. 129, may be carried up solid or hollow; their weight must be enough to stiffen sufficiently the arch. It should, at least, be carried up solid to the line c c c; the shaded mass being of well-cemented rubble. Above this the filling may be of masonry, solid or arched, or even of well-rammed layers of earth. The road-way should, in all cases, be well drained, that the water may not sink through to the masonry.
Fig. 127. Fig. 128.
Fig. 129.
The apparatus for handling stone (cranes, lewises, and derricks) is much better understood by inspection than by description.
Wherever walls support masses of earth, the thrust may be somewhat lessened by ramming the earth behind the wall in layers inclining backward. In laying up the courses each should be well cleaned and moistened before the mortar is laid upon it. When a stone has been once placed upon the mortar bed, it should not be moved at all laterally, but may be gently mauled on top.