From How to use cement for concrete construction for town and farm, by Henry C. Campbell:
Sometimes cisterns are built wholly or in part above ground, yet the natural place for such a structure is below ground. A cistern is nothing more or less than a tank required to keep clean water in storage without loss from leakage. It is therefore necessary that the structure be watertight. Cisterns have been built of such masonry as brick and stone but this cannot be depended upon to be watertight unless plastered, since leakage is almost certain to take place through mortar joints. For that reason concrete construction is perhaps more adaptable to the requirements than other materials. Steel tanks have been used for cisterns but from the very nature of the material it is subject to rust and cannot be regarded nearly as permanent as concrete.
Shape and Forms.
Since the advent of the commercial silo form used by rural concrete contractors in building concrete silos, many persons have had circular cisterns built. The home-made silo forms illustrated elsewhere in this book can be adapted to circular cistern construction if required, but unless one has already built such forms for use in constructing a silo, it is easier to build forms for a rectangular cistern.
In order to illustrate the principles of constructing a rectangular concrete cistern, the accompanying sketches have been fully detailed and show a cistern 7 feet square by 6 feet deep. A very advantageous detail of this cistern is the filter built on and as a part of the cistern cover slab. Rainwater enters this filter through the 6-inch tile drain shown and goes into the settling compartment containing the screen. This screen helps to prevent refuse such as leaves and other rubbish from going immediately into the filter compartment and thus clogging the filter material. The approximate capacity of this cistern is 70 barrels.
Materials Should All Be Ready Before Starting Work.
Before commencing to build a concrete cistern all necessary materials should be on hand. It is always well to have a slight excess of materials over, and above those required, to provide for slight loss due to waste in mixing and placing or to shortage through possible miscalculation of quantities required. The first thing to do is to lay out a square on the ground 8 feet on each side. If the earth is firm enough to serve as an outside form no other form will be needed. If, however, the earth has a tendency to cave, it will be necessary to make the excavation larger so that outside forms can be erected. As the concrete floor of the cistern is 5 inches thick the excavation should be made deep enough to allow for this and for the 3 feet of earth covering shown on the cistern roof. The cistern filter is 4 feet 8 inches by 3 feet 4 inches and covered with a reinforced concrete slab.
All necessary .forms should be built before commencing the excavation so if a sudden shower comes up forms can be quickly placed to prevent the earth from caving if it becomes water soaked. One-inch boards 4 or 6 inches wide, nailed to 2 by 4 inch uprights or studs placed 2 feet apart will make suitable forms. It will be noticed that two sides of the filter compartment have 6-inch walls which correspond to the wall thickness of the cistern, thus simplifying form construction in carrying this part of the work up into the filter. One-inch boards 4 by 6 inches wide nailed to 2 by 4-inch uprights or studs placed 2 feet apart will make suitable forms. The excavation as suggested should be made deep enough to provide for the small footing extension of the side walls, which extend below the floor slab. In this work it is expected that concrete for the side walls will be placed before the concrete floor is laid. Concreting of walls should be as continuous as possible to prevent construction seams or joints.
Horizontal reinforcing consists of 3/8th inch round rods spaced 6 inches center to center. The spacing of reinforcement for the various depths inside and out is shown to the left of section A-A in the section of concrete wall. Vertical reinforcing for the side walls should consist of rods long enough to permit of ends being bent over into the concrete roof or cover slab when this is the case. A plan of reinforcing for the roof shows in position a section of filter walls and the spacing of reinforcing rods for the cover slab, these rods also being 3/8th inch in diameter. Other sketches show details of the copper filter screen, the concrete filter slab on which the screen is placed, the removable cover for the filter compartment and the reinforcement for this cover slab. Vertical reinforcement in the cistern walls consist of 54-inch round rods spaced 16 inches center to center and turned 18 inches into the roof slab.
After the concrete has been placed for the side walls up to the bottom of cover slab the work may stop until the concrete has hardened sufficiently to permit removing forms, following which the concrete floor can be laid. A ½ inch beveled strip of siding should be set all around the bottom of wall at floor level against the offset of the footing and after the concrete floor has been placed and has hardened, these strips should be removed and the space left by them filled with hot tar to form a leak-proof joint. When the floor has hardened, which will require several days, studs can be set up to support the form on which the roof or cover slab concrete is to be placed. A hole should be left in this form, located to correspond to the location of the manhole in the filter so that after the roof has been concreted, entrance can be obtained to the cistern for knocking down the studs and removing forms.
Wherever reinforcement crosses or intersects it should be tied together with small iron wire so that rods will be held in their proper position and will not be displaced. Concrete should be mixed not leaner than 1:2:3. It should be of quaky consistency so that it will settle to all parts of the form and around reinforcing with slight paddling. Make certain that the concrete is thoroughly puddled around the concrete bricks or blocks used to support the forms at the bottom, at the same time taking care not to cover up these so as to prevent removing them when taking down forms. Wedging up the forms in this way at the bottom by placing these wedges under the studs allows the form to be dropped slightly and released when time to remove it.
Concrete should be placed as continuously as possible in courses not exceeding 6 or 8 inches entirely around and in the space between forms and should be well spaded next to faces so as to force back the coarse materials in the concrete and bring a film of mortar against the forms, thus resulting in a dense, smooth and consequently impervious surface.
If outside forms are not required, use care when placing concrete so as not to knock down dirt into it. If this happens porous pockets will be formed and probably leaks will result. Continuous concreting is desirable because in this way all concrete will be placed against fresh concrete, that is not hardened, and thus leaky construction seams will be avoided.
If an overflow opening is desired, arrange this at the proper level and connect it to a suitable outlet. The inlet pipe from the house drains should be placed as much below ground as depth of the structure will permit so as to prevent freezing. Two weeks after the last concrete has been placed it should be safe under usual summer weather conditions to remove the cistern roof forms.
Material used in the filter compartment for filtering the water consists of a layer of granular charcoal about 18 inches deep, on top of which is a 6 or 8 inch layer of clean well graded sand and gravel. A screen of ¼ inch mesh copper wire is placed over the pipe opening into the cistern in what has been already referred to as the settling compartment. This screen is held in position by the baffle boards as shown. It would be well to thoroughly wash out the cistern before filling with water for the first time although this will not be necessary unless the water is to be used for domestic purposes other than laundry work.