The Blaw collapsible steel forms,asshown in Fig. 167, appear to be the only successful steel forms so far in general use. There have been many attempts to devise steel centering for column, girder, and slab construction, but no available system has yet been invented. The main trouble of those used is their liability to leak, tendency to rust, and liability to injury by dents in removing.

Fig. 166. Form for Construction of Horseshoe Shaped Concrete Conduit.

Fig. 166. Form for Construction of Horseshoe-Shaped Concrete Conduit.

Fig. 167. Collapsible Steel Center or Form.

Fig. 167. Collapsible Steel Center or Form.

The Blaw collapsible steel centering is in general use for sewer and conduit construction. This centering consists of one or more steel plates about 1/8- inch thick and bent to the shape required by the interior of the sewer to be constructed. The steel plates are held in shape by angle irons. When set in position, the sections are held rigid by means of turnbuckles, which also facilitate the collapsing of the sections. The adjacent sections are held together by staples and wedges, the former being riveted to the plates. The sections are usually made five feet long, and in any desired shape or size required for sewer or conduit work. When these forms are used to construct concrete sewers or conduits, the surface of the forms must be well coated with grease or soap to prevent the concrete from adhering to the steel.

3G1. Forms for Walls. The forms for concrete walls should be built strong enough so that they will retain their correct position while the concrete is being placed and rammed. In high, thin walls, a great deal of care is required to keep the forms in place so that the wall will be true and straight.

Fig. 168. Forms for Wall.

Fig. 168. Forms for Wall.

Fig. 168 shows a very common method of constructing these forms. The plank against which the concrete is placed is seldom less than 1 1/2 inches thick, and is usually 2 inches thick. One-inch plank is sometimes used for very thin walls; but even then, the supports must be placed close. The planks are generally surfaced on the side against which the concrete is placed. The vertical timbers that hold the plank in place will vary in size from 2 inches by 4 inches to 4 inches by 6 inches, or even larger, depending on the thickness of the wall, spacing of these vertical timbers, etc. The vertical timbers are always placed in pairs, and are held in place usually by means of bolts, except for thin walls, when heavy wire is often used. If the bolts are greased before the concrete is placed, there is usually not much trouble experienced in removing them. Some contractors place the bolts in short pieces of pipe, the diameter of the pipe being about 1/8 inch greater than that of the bolt, and the length equal to the thickness of the wall. When the bolts are removed, the holes are filled with mortar.