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Chapter Three: Plumbing and Floor Concrete


The outlet and the pipe for cleaning the tank are installed just prior to placing the floor concrete. The pipe for cleaning the tank is shown passing through the wall to floor key. It should be seven to ten centimeters in diameter. This pipe is positioned sloping downward gently and with its inner surface slightly below the finished top of the concrete floor surface. The outlet pipe is usually placed 5 -10 cm above the floor. Sediments which settle on the tank floor are then left behind. Outlets can be installed with the wall. A domestic water outlet at mid wall height leaves the bottom half full for fire emergency (through a second pipeline exiting near the bottom).

Place plastic pipe in the shade of the tank because sunlight decomposes plastic and makes it brittle. It is a good idea to glue one centimeter squares of pipe material on to the pipes so they do not spin if wrenches are used to connect threaded pipe fittings. Use a clamp and let the glue cure for a full day before removing the clamp. Brass pipe should be scored or have bumps brazed on it for the same reason.

If plastic pipe must be located on the sunny side of the tank, cover it with thin wire mesh or poultry wire mesh. Plaster it when the wall is being plastered. This will add many years before sunlight makes the plastic pipe brittle.

An alternative method of installing the pipes is to purposely spin the pipes periodically, before the plaster has become hard. Moisten and then remove these pipes when the plaster is hard enough to hold its shape. Standard tank fittings with a rubber washer on the inside can the be slipped through the holes and replaced whenever necessary. This option requires room for the gasket flange all the way around the pipe. Have the parts on site before the floor concrete is placed in order to be sure of a proper fit. Check the next day to be sure the concrete has not shifted and the tank fittings will pass through the holes. The holes can be made a little larger by using a piece of reinforcing bar as a rough file, before the concrete has become too hard.

Note: Pipelines and temporary water storage above the tank site are prerequisite to starting the tank.

The photo above shows the inlet pipe and a larger, screened and vented overflow pipe. A piece of window screen covers the hatch hole. Only half the sanitary hatch screen was placed for this photo so the hatch and hinge structure would remain visible. The screen is held in place with a rope or twine cinch and the weight of the finished hatch cover (shown opened and laying on a piece of plastic on the tank roof). An air gap of 15 cm below inlet and above over flow pipes will help ensure that possible tank contamination does not reach the inlet pipe or the water source. A third pipe at about the level of the inlet pipe would be used for electrical water level sensors.

Concrete (Plaster):

Before discussion of placing the concrete it is good to understand that the floor can be made thinner than described when using two centimeter support block for the steel. This is an option for smaller tanks under sixty cubic meters and is most important when the tank site is difficult to reach or the wet concrete plaster must be hand carried. To reduce the volume of concrete in the floor use two centimeter support blocks and add a layer of poultry wire on top of the last layer of welded wire. Inspect for wires that may protrude above the finished floor surface and fix them. The finished floor can be made as thin as 5.75 centimeters.

The cement to sand ratio is 2.7 to 3 sand measures for each measure of cement, except in a small area of the North Eastern United States where the author has found the ratio should be a maximum of two sand for each cement. It is always good practice to cure a series of varying sample test mixes to establish the ratio which yields hard plaster, paper cups are good for this. The concrete plaster must not contain excess water; for maximum strength and reduced shrink cracking. When a finger mark in the mix settles very slightly, and slowly, the water portion is correct. Mix the concrete plaster well; use a plaster mixer, wheel barrow, mixing tray, or have it delivered by truck.

It is not necessary to smooth the floor to the perfection of a slab in a garage or a dwelling, though if one has this experience it is quite easy to make the floor perfectly flat. Visualize dragging a long board across the concrete to level it. A long board slides on preset wood supports. Remove support stakes from the wet concrete at the correct time, fill the stake holes and trowel smooth. A tank floor need only be as smooth as one can make by raking the concrete flat and smoothing it with a trowel.

Re-trowel any shrink cracks. Use a stiff broom and water mist on shrink cracks when the floor is too hard to work with a trowel, if there are any cracks left. Keep the tank floor wet always.

Flood the tank floor when the concrete is hard to the touch and as soon as water won’t erode it. Fill it up to the top of the floor-to-wall key. Place black plastic over the wall key so it won’t dry in the sun. Hold the black plastic in place with shoveled dirt outside the tank and a few rocks inside. Tie the plastic to vertical reinforcing steel if it is windy. Wait three or four days to start the wall steel if possible. Then keep the floor wet during construction of the rest of the tank.

Twenty-eight moist days is the standard for a perfect concrete cure.

Tie wire is purchased at masonry supply stores in standard rolls of mild steel. These rolls are sized to fit a reel. If you do not use a reel, pull the wire from the center of the roll. Cut 7 - 8 centimeter lengths ± (3" ±) and place them in a belt pouch for rapid removal and tie work. Duct tape wrapped around the outside of the wire roll will preserve the roll shape and help avoid wire tangles.

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