Water Cisterns Provide Farm Fire Protectión
Enable the Rural Fire District Department to do Effective Work*
FIRE strikes somebody’s farm every fifteen minutes. Within a 24-hour period it will visit 100 farm families, leave nine farm people dead and destroy a quarter of a million dollars worth of property† And this goes on every day.
To halt these losses, many farmers have banded together to form fire protection districts. But organized fire fighters need water quickly and in abundant supply. According to fire protection district records, from 2.000 to 3,000 gallons of water is a reasonably safe quantity to keep in reserve for fire fighting on the farm, but this amount is seldom available unless special storage tanks are provided.
An underground concrete fire cistern will store this water economically and efficiently. It can be placed conveniently near the farm building group, and when it is kept filled it is valuable insurance when fire strikes. It is so valuable, in fact, that many insurance companies give a credit in rates if 3,000 gallons of water is kept available at all times for fire use.
Building the Cistern
To determine the best location for the concrete cistern the farmstead layout should first be studied. The site chosen should be within 700 feet of the building group but not nearer than 100 feet to any building. A distance greater than 100 feet from large buildings is desirable to give the fire truck room to operate. A place that can be reached easily by the fire truck in all kinds of weather should be chosen. If reduction in insurance rates is expected, the recommendations of the insurance company should be checked before building is begun.
The plan described is for a square cistern with inside dimensions 11 x 11 x 4 feet deep. Maximum capacity is 3,630 gallons. Concrete side walls are 6 inches thick and are reinforced with wire mesh and steel bars. The cisten top is precast and is set in position after the concrete in the tank has hardened. Methods of construction are illustrated by photographs.
The location of the hole to be dug is marked out in much the same manner as for footings in buildings. Stakes or hatter hoards are set at the corners and strings are stretched to mark the boundaries of the excavation. Where the earth is used as the outside form, the finished hole must be exactly 12 feet square. Power equipment can be efficiently used in digging this size hole. If it is used, the hole should be dug slightly smaller than its final size. Any irregularities can then be smoothed out with a shovel. A plank frame is laid around the hole to serve as a guide for the final hand trimming and also to protect the edges of the bank against crumbling.
The total depth of the hole should be 5 feet 10 inches if the finished grade over the cistern is to be level with the undisturbed earth. This allows for 1 foot of earth over the cistern lid.
Forming the Tank
This form is made in sections and can be transported easily from place to place. Farmers in a community often share the cost and use of forms.
*Acknowledgement: From a bulletin entitled “Fight Farm Fires; Build a Concrete Fire Cistern,” published by the Portland Cement Association, 33 W. Grand Ave., Chicago 10, Ill. The bulletin contains many construction details not included in this article. Copies of the complete bulletin may be secured from the Association at the above address.
†Based on estimates from a report of the Subcommittee on Fire Loss Statistics, Farm Fire Protection Committee of the National Fire Protection Association, and issued through U. S. Department of Agriculture, Bureau of Agricultural Economics.
The form shown is for use when concrete is cast directly against the earth with no outside wood forms. If the soil will not stand in a vertical position without crumbling, a larger excavation and outside lumber forms are also required.
The tank form consists of these pieces: four wall panels, eight 2 x 6-inch wales, two 2 x 4-inch carriers and eight cross braces. Four of the cross braces are built of two 2 x 4’s spiked together in a T-shape; the other four braces are single 2 x 4’s.
The wall form is assembled by bolting the four panels together to form a box. Square-headed bolts are put through matched holes from the inside and nuts are tightened on the outside. The forms are then coated with a light oil. Braces and wales are not put in place until after the form is dropped into the hole. The form may be brought to the correct level by blocking up beneath the 2x4inch carriers resting on the plank frame around the hole.
The reinforcing steel for the tank is assmbled around the form before it is set in the hole. The 6 x 6-inch No. 6 gage welded wire mesh for the floor slab is slipped beneath the form and bent up on all four sides so that it will lap the side wall reinforcement by 6 inches. If the bottom mesh is made up of a series of strips, the strips should overlap each other at least 6 inches. Each strip will need to be 12 feet long. Two layers of 6 x 6-inch No. 10 gage mesh may be used if 6 x 6-inch No. 6 is not available.
A 4-foot wide strip of 6 x 6-inch No. 10 gage mesh 47 feet long is next cut off and stretched around the perimeter of the forms. The ends should be lapped and wired together. Also the bottom mesh should be wired to the side wall reinforcement. Spacers to hold the mesh 2 inches away from the form are set in position. These are removed later as concreting proceeds.
Three 1/2-inch round bars are placed continuously around the form, completing the steel assembly. These bars are spaced 1 foot apart and are wired to the mesh.
A mushy, workable mix should be used for the tank. A thin, soupy mix will not produce a watertight concrete and is objectionable because it will flow out beneath the wall forms.
Concrete should be placed in a continuous operation if possible to avoid construction joints. A layer about 1 foot deep should first be placed in the forms the whole way around the tank wall. This layer should be spaded thoroughly so that it is worked well into and around the corner at the floor. Then the 6-inch thick floor is placed. The reinforcing mesh in the floor should be lifted occasionally to keep it from being pushed to the bottom by the weight of the concrete.
Concrete in the walls should be placed in horizontal layers about 6 inches deep and kept at a uniform level the whole way around the form. This prevents uneven pressures and keeps the forms from sliding. Layers are built up by dumping concrete at various locations around the form. Concrete should not be placed in large quantities at one point and then worked for long distances in the form.
Thorough spading along the surface of the forms and around the reinforcing steel is very important. This spading helps to make a dense, smooth, watertight wall free from rock pockets and honeycombing. The inside of the form should also be tapped with a heavy hammer or sledge at frequent intervals to release air bubbles which may be trapped in the concrete next to the forms.
When the wall form is completely filled, the top is struck off level with a woodfloat and the concrete is left to harden. Forms should not be removed for two to three days after casting. The forms are stripped by removing the cross braces and wales, then loosening and removing the bolts at each corner. Excessive prying against corners of the concrete wall should be avoided.
Making Joists and Lids
Concrete joists and lid slabs may be precast on any convenient level surface. The concrete floor of a building or level bed of sand is satisfactory. If a concrete floor is used, building paper, oil, or some other bond-breaker must be used to prevent the fresh concrete from adhering to the floor.
Four joists 5 1/2 inches wide, 9 1/2 inches deep and 11 feet 6 inches long are required. Each joist is reinforced with two 5/8-inch round bars near the bottom and one 1/2-inch round bar near the top. These bars are each 11 feet 4 inches long. A 1 1/2-inch layer of concrete is put in the bottom of the joist, then the two 5/8-inch round bars are laid in. Concrete placing continues until the surface is within 1 3/4 inches of the top of the form. At this level the 1/2-inch round bar is laid in. Then the form is filled and the concrete is struck off level.
Joist forms consist of 2 x 10-inch outside forms and 1 x 10-inch dividers. Braces across the forms at the third points hold the dividers from bending while the concrete is being placed. The forms are held tightly together with a bolt or twisted No. 9 wire at each end.
Eighteen precast lid sections are needed. These are 3 5/8 inches thick, 1 foot 11 1/4 inches wide and 4 feet long. The odd dimensions result from the use of finished lumber for forming. Each lid slab is reinforced with a 22-hich wide strip of 5 x 6-inch No. 6 gage mesh. After a 3/4-inch layer of concrete has been placed in the bottom of the form, the strip of mesh is laid in and the form is filled with concrete. Two of the sections are constructed with 10-inch drain tile risers built into them. A double layer of mesh should be used in the sections which have the risers. One of the tile risers is to receive the suction hose of the fire truck. The other riser is for emergency use in case of a large fire where more than the 3,000-gallon supply of water is needed. Water can then be emptied into the tank through one riser at the same time that the suction hose of the fire truck is withdrawing water from the other riser.
Forming the lid slabs consists of 2 x 4-inch perimeter forms and 1 x 4-inch divider strips. Slots to receive the dividers are made by nailing pieces of 1 x 4-inch to the 2 x 4-inch forms.
The lids and joists should not be moved for at least five days after placing. During this time they should be kept continually moist. This may be accomplished by covering them with wet burlap or wet straw. Fresh concrete should be protected from freezing in cold weather.
The joists are hauled from the casting site to the tank, then set and mortared in position in the seats of the tank wall. Two joists are placed at each location. Farm tractors can handle these units easily with the hydraulic lift or the manure loader. Each joist weighs about 625 pounds; each lid about 350 pounds.
Mortar (1:3 mix) is spread on the joist top and on the tank wall, then the lids are set on. Backfilling should be done gradually to avoid impact of earth on the lid.
If reasonable care is taken in mixing and placing concrete the fire cistern should require no coating to make it watertight. A recommended concrete mix for this cistern is: 1 part portland cement, 2 1/4 parts sand, and three parts gravel or crushed stone. Not more than 5 gallons of water for each sack of cement should be used if the sand is in an average moist condition. If the first batch mixed according to these proportions does not have the workability desired, the amount of aggregate may be adjusted to obtain desired workability in succeeeding batches. The proportion of water to cement should not be changed. Concrete should be mushy and workable but not soupy.
Portland cement should be free of lumps that cannot be pulverized in the hand. Water should be clean—drinking water is usually satisfactory. Sand should be clean, hard, durable and well graded with all sized particles ranging from a very fine up to 1/2-inch. Gravel or crushed rock should range in size from 1/4-inch to 1 1/2-inch. Particles larger than 1 1/2 inches should not be used.