USE OF COLLAPSIBLE FORMS By actual use concrete culverts have demon strated their economy. A concrete culvert can be constructed by the farmer, and will be more serviceable than one made of any other material. Its first cost is low and there is practically no further expense. No repairs are necessary. Properly constructed, freezing does not affect it, thawing does not damage it, and it becomes harder and harder every year. It solves the problem of an unobstructed bore, for no joints are necessary, and there are no projections to catch obstructions that might clog a clay tile or corrugated culvert. We have already, in con nection with the subject of "Forms," referred briefly to several types of collapsible metal forms and the method of using them in the con struction of small culverts.
The accompanying table gives figures from which to make estimates, giving the necessary thickness of top, bottom, and side walls for various-sized culverts, the amount of material needed, etc. These figures are for culverts eighteen feet long. On longer or shorter ones, make your estimates in proportion. In the table you will find the squared dimensions of water way for which different sizes of cylindrical moulds, such as the "Overturf," will be the equivalent; also the thickness of the sides, bot tom and top walls, the width to dig the trench, and the amount of sand or gravel, and of cement to be used.
The costs of completed culverts as here esti mated are figured on what are considered aver age conditions, with cement at 65 cents per hun dred pounds, and gravel at $1 per yard. The cost of this kind of concrete work is close to 18 cents per cubic foot of concrete used; and, as cement in the proportions mentioned does not increase the bulk of gravel, to estimate the cost of any sized culvert completed, multiply the esti mated amount of gravel in cubic feet by 18—this will give you the cost complete. Multiply by 11 to get the cost of cement; by 7 for cost of material on the ground and placed.
The figures in the table are made on a basis of 18-foot culverts, with coping to extend two feet from waterway through culvert, and con crete in proportion of one part cement and six parts gravel or sand.
If wooden forms are to be used, excavate trenches for foundation to a depth below frost and 2 feet 8 inches wide; and at the upper end of the culvert, connect the two foundations across with an 8-inch wall the height of invert. This is called an apron, and will prevent scour ing. Build invert 8 inches thick, having the top on a level with the bed of the stream. Next build forms for the wall, with one straight form strong enough to support the arch, and well braced, and the other form as the thickness of the wall requires.
For convenience in keeping the road open for traffic, and the saving in material for forms, we suggest making only nine feet of the culvert at a time. Should this suggestion be accepted, proceed as follows: Make three semicircular forms the size re quired, out of stuff, and set them in place three feet apart. Fasten joist 2 inches by 4 inches by 9 feet on them. This is called lag ging. Set the form thus made on large wedges supported by top of form marked "Sill." Grease forms well, and fill with concrete of a rather wet consistency, and tamp thoroughly every 6 inches, taking care not to disturb the form. Let stand until thoroughly dry, about 28 days; and then knock out the wedges, lowering the semicircular form, which will be easy to remove.
Should the culvert be made all at one time, enough semicircular forms should be con structed to support the lagging at least every 3 feet.
Reinforce the concrete with expanded metal, placing it so that it is inches in from the under side of the arch, and extending down through the walls. All concrete should be mixed one part Portland cement, three parts sand, six parts broken stone. Should wing-walls be required, they should be built at the same time as the foundation, should go to the same depth, and be reinforced, the reinforcing con necting with that in the walls. The width of these walls should be left to the judgment of the man in charge of the work.