Fig. 138.—Blaw Standard Full Round Telescopic Sewer Forms, Showing Knocked-Down Sections Loaded on a Truck.
Courtesy, Blaw Steel Form Co.
194. Steel Forms.—These are simple, light, durable, and easy to handle. The engineer is seldom called upon to design these forms as the types most frequently used are manufactured by the patentees and are furnished to the contractor at a fixed rental per foot of form, exclusive of freight and hauling from the point of manufacture. The forms can be made in any shape desired, the ordinary stock shapes such as the circular forms being the least expensive. The smaller circular forms are adjustable within about 3 inches to different diameters so that the same form can be used for two sizes of sewers. The same form can be used for arch and invert in circular sewers. Fig. 138 shows the collapsible circular forms and the manner in which they are pulled through those still in position. Fig. 129 shows a half round steel form swung in position by chains and turnbuckles from the trench bracing, and Fig. 139 shows the free unobstructed working space in the interior of some large steel forms.
Fig. 139.—Interior of Steel Forms for Calumet Sewer, Chicago.
Sewer is 16 feet wide. Note absence of obstructions. Courtesy, Hydraulic Steelcraft Co.
195. Reinforcement.—It is essential that the reinforcement be held firmly in place during the pouring of the concrete. A section of reinforcement misplaced during construction may serve no useful purpose and result in the collapse of the sewer. In sewer construction a few longitudinal bars may be laid in order that the transverse bars may be wired to them and held in position by notches in the centering and in fastenings to bars protruding from the finished work. This construction is shown in Fig. 1. The network of reinforcement is held up from the bottom of the trench by notched boards which are removed as the concrete reaches them, or better by stones or concrete blocks which are concreted in. Sometimes the reinforcement is laid on top of the freshly poured portion of the concrete the surface of which is at the proper distance from the finished face of the work. This method has the advantage of not requiring any special support for the reinforcement, but it is undesirable because of the resulting irregularity in the reinforcement spacing and position.
In the side walls the position of the reinforcement is fixed by wires or metal strips which are fastened to the outside forms or to stakes driven into the ground. Wires are then fastened to the reinforcement bars and are drawn through holes in the forms and twisted tight. When the forms are removed the wires or strips are cut leaving a short portion protruding from the face of the wall. The reinforcing steel from the invert should protrude into the arch or the side walls for a distance of about 40 diameters in order to provide good bond between the sections. The protruding ends are used as fastenings for the new reinforcement. The arch steel may be supported above the forms by specially designed metal supports, by small stones or concrete blocks which are concreted into the finished work; or by notched strips of wood which are removed as the concrete approaches them. Strips of wood are not satisfactory because they are sometimes carelessly left in place in the concrete resulting in a line of weakness in the structure. Metal chairs are the most secure supports. They are fastened to the forms and the bars are wired to the chairs. In some instances the entire reinforcement has been formed of one or two bars which are fastened into position as a complete ring. This results in a better bond in the reinforcement, requires less fastening and trouble in handling, but is in the way during the pouring of the concrete and interferes with the handling of the forms.
196. Costs of Concrete Sewers.—Under present day conditions a general statement of the costs of an engineering structure can not be given with accuracy. Only the items of labor, materials, and transportation that go to make up the cost can be estimated quantitively, and the total cost computed by multiplying the amount of each item by its proper unit cost obtained from the market quotations.
A summary of some of the items that go to make up the cost of a concrete sewer and the relative amount of these items on different jobs is given in Tables 69 and 70.
| TABLE 69 | ||||
|---|---|---|---|---|
| Division of Labor Costs For the Construction of 96–inch Circular Concrete Sewer | ||||
| Classification of Labor | Classification of Work | |||
| Task or Title | Number of men | Total dollars per day | Type of Work | Dollars per foot |
| Superintendent | 1 | 6.00 | Excavation | 1.80 |
| Engineman | 1 | 3.50 | Sheeting and bracing | 0.58 |
| Hoister (engineman) | 1 | 2.00 | Bottom plank | 0.17 |
| Tag-men | 2 | 3.30 | Pulling sheeting | 0.45 |
| Earth diggers | 10 | 16.50 | Backfilling | 0.33 |
| On dump cars | 2 | 3.30 | Making and placing invert | 1.17 |
| Carpenter on bracing | 2 | 3.00 | Making and placing arch | 1.54 |
| Carpenters’ helpers | 2 | 3.30 | Laying brick in invert | 0.29 |
| Laying bottom | 2 | 3.30 | ||
| Moving pumps, etc. | 2 | 3.30 | Bending and placing steel in arch | 0.20 |
| Pulling sheeting | 3 | 5.25 | ||
| Mixing and placing concrete | 16 | 26.40 | Bending and placing steel in invert | 0.09 |
| On steel forms | 3 | 5.25 | Moving forms and centers | 0.62 |
| Water boy | 1 | 1.00 | Watchmen, water boy, etc. | 0.62 |
| Coal and oil | 5.00 | |||
| Total | 90.40 | Total | 7.86 | |
| Notes.—Trench was 12½ feet wide and of various depths. At depth of 12 feet the cost of excavation was $1.61 per foot. From Engineering and Contracting, Vol. 47, p. 157. | ||||