ForewardThis manual is intended for both professional and amateur. Many of the smaller details and tricks of the trade will become more evident with an actual tank in progress. The reader is urged to have the manual available for reference during tank construction. Durable waterproof-ink field manuals from ferrocement.com have proven to be welcome on the job.

Versions in other languages may sometimes be more complete.

Chapter 1 is not necessary to read if the builder has no interest in calculating strength or estimating materials prior to construction. This chapter will be useful, however, when building a tank of different size than the sixty cubic meter example. It is also an important chapter for those interested in building ferrocement tanks as a business.

Chapter 5 contains a short explanation of how to finish an open tank. The model tank photographed for this manual was built as an open topped tank and then finished again as a roofed tank starting with chapter six. An open top tank is useful for anything from animal water troughs to aquaculture. Larger and deeper open tanks may be used for swimming pools or sewer systems.

Chapter 9 summarizes some of the key aspects of plaster application. It warns about leaving voids, and finding them by lightly tapping the newly applied plaster with a hammer. There are sufficient instructions to accomplish a finished tank yet one should realize that description of the art of plastering is an entire subject.

Beginners are often most comfortable with rubber gloves and hand application, which is actually the best method for complete penetration of plaster into the armature. The hand can push plaster into the armature with greater force than a tool, this is important to keep in mind. The amateur's main disadvantage is making the finished job look professional, it's mostly timing, and something only practice can provide. Less than perfect appearance does not reduce longevity or strength.

Ferrocement is the best choice for a reservoir. The author has witnessed young amateurs harnessed to a wheelbarrow hauling plaster up a hot mountain side to build a reservoir. Strong people pulled downhill on a long rope that passed through an uphill pulley and back down to pull

the heavy wheelbarrow upward. That tank was plastered with hands and finished with brushes instead of trowels, it was built in 1968 and remains in continuous use.

Should you have additional questions after reading this manual, email ferrocement.com

GlossaryArmature: Wire, reinforcing steel and fibers which reinforce concrete. May be tied, sewn or welded.

Center Post: A post in the center of the reservoir which may be temporary or permanent.

Concrete Mortar: Plaster, mortar, concrete plaster (herein). Cure: Hardening process. A chemical reaction occurs as the mixture becomes hard. This process requires time and water. Twenty-eight moist days is the standard. The time period is effected by temperature. Warmth accelerates the process. The process stops if the concrete is dry; it will not start again.

Ferro Cement: ferrocement, ferrociment, ferrocemento, ferrocimento, ferrozement.

Hog Rings: "C" shape which is compressed with a tool to "O." Large rings are used in agriculture to pierce pig nostrils in order to lead them by the nose. Approximately two centimeters is the size used used to attach wire layers together. Usually found at wholesale upholstery supply stores.

Layer: Strata of wire or reinforcing steel. Metal lath: Also called expanded metal. It is used on one side of a vertical wall and the surface of a ceiling. It can be used on both sides of a vertical wall but is difficult to force plaster through. Thin gauge, painted metal lath is best for ferrocement work. Galvanized expanded metal is less convenient but holds wet plaster equally well as thin gauge lath. This material is very sharp where it has been cut.

Pneumatic hog ring gun cannot attach heavy gauge, galvanized expanded metal.

Plaster Proportions: 2 - 3 dry measures of sand to one measure of cement. Test to for 550 - 850 kgf/cm2 concrete plaster.

Approximate reinforcing steel diameters:

#2 = 0.60 centimeter#3 = 0.95 centimeter#4 = 1.25 centimeters#5 = 1.60 centimeters

Reinforcing steel: reinforcing steel rod, re-bar, reinforcing steel bar (herein) Welded Wire: Manufactured in many wire sizes and spacing. Ten gauge wire creating fifteen centimeter squares is common for ferrocement. Ten gauge = 0.357 centimeters.

Wire Cloth: Galvanized welded wire available in various widths and grids. A grid of approximately 1.25 centimeters is used for vertical ferrocement walls, on one side. This grid allows easy penetration of plaster yet holds it well.

Premium quality welded wire is also available in this size. This wire is better steel and is not galvanized.

Sponge float: A trowel with a sponge surface. A wood float is a trowel made of wood.

Stanchion: A supporting post or pole.

Business Model:

Reservoir manual. Glossary. (edited versions on this website: Spanish, French, Portuguese and English)

Equipment: Plaster mixer, small truck, welding torches, roof support rafters, masonry tools, hoses, cutters, hog ring pliers, and benders.

Optional equipment: Arc welder, pneumatic hog ring gun, concrete pump nozzle (Though manual work --- mixer to cargo to application --- is the best method). A rented concrete pump facilitates lifting concrete to large roofs.

Felix Candela, photo section

Fundamental Business:

1) Construct water reservoirs, aqueducts, animal shelters, and work area shade. Fire safe structures. Earthquake safe homes.

Reservoir Manual, Chapter 7

Add to business use and sales inventory (between jobs):

2) Prefabricate reservoir hatches (for water and septic)3) Prefabricate various size septic reservoirs (for sale and installation).4) Prefabricate road drainage catch basins (for sale and installation).5) Large plant containers (for hotels and parks).6) Water for animals.7) Expand roof brace inventory.

House Book, Chapter 6

Chapter One: Site Preparation and Calculations

60 cubic meters

Water is heavy. Be sure to locate the tank on solid ground. Cut enough room for the entire tank to sit on solid ground if the tank is going to be on a hillside. Excavated soil is not good for a tank site because it will settle over time. Ferrocement water tanks last for decades and stable ground is important.

Enough area for working is also important, especially on the uphill side. Make the site large enough so dirt and rocks don’t fall into the steel armature. Contamination entangled in the structure is a problem to avoid during construction. The area made up of excavated fill is a good place for the access road to terminate and to store materials. If this is a large tank and the excavated material is a mountain of dirt poised to cause damage below during a flood year, then it should be placed on a cut bench cut of its own and be compacted for stability and safety.

Volume Calculation:

πr2h = volume (where π = 3.14, r = radius, and h = height)

The following example is for a tank of sixty cubic meters; height is 2.13 meters.

πr2(2.13) = 60 cubic meters (sixty thousand liters)

r2 = 60 cubic meters ÷ (2.13 x 3.14) = 8.971 m2

r = radius = 3 meters

2r = diameter = d = 6 meters

Strength Calculations:

Sixty cubic meters is used in this example because many ferrocement tanks have been built of this size and there have been no problems, even after twenty-five to thirty years. Tanks of this age in the 200 to 400 cubic meter class have likewise shown no problems. Two hundred cubic meters is somewhat more difficult to build and 400 cubic meters is the beginning of a heavier construction project size.

Convert the depth into pressure, measured in grams per square centimeter and calculate the circumference in centimeters.

πd = 3.14 x 6 meters = circumference = 1884 centimeters.

The pressure on a square centimeter (kg/cm2) = the depth of 2.13 meters = 0.213 kilograms per square centimeter.

This means that there is 0.213 kilograms of outward pressure on a one centimeter square at the bottom of the tank wall. Since the wall is 1884 centimeters around, the total outward force on the bottom centimeter of wall is 0.213 x 1884 = 401 kilograms.

The next step is to determine the strength of the wall as it resists this outward pressure. The concrete plaster is only considered as waterproofing for the steel in this calculation. All the strength is assumed to be in the steel. Add up the horizontal strands of welded wire and the bars which encircle the tank. Count the welded wire and the reinforcing bars separately since they are different strengths of steel. Reinforcing steel is 3515 kilograms of tensile strength per square centimeter and the welded wire is 6328kg/cm2.

There are five horizontal wires and two reinforcing bars in the bottom thirty centimeters of this sixty cubic meter tank. Ignore the welded wire bent to come up and out of the floor until further along the discussion. Standard welded wire is ten gauge wire on 7.5 centimeter squares. Ten gauge wire is 0.356 cm diameter.

πr2 = 0.1 square centimeters of steel times five wires = 0.5 square centimeters. Multiply this by 6328 kilograms per square centimeter = 3164 kilograms of tensile strength in the bottom 30 centimeters of wall. Divide by 30 to compute the welded wire strength in an average centimeter of wall. 3164 ÷ 30 = 105 kilograms of horizontal welded wire tensile strength per average vertical centimeter of wall. The same calculation is done for two horizontal wraps of #4 bar (1.27 centimeters).

πr2 multiplied by 2 multiplied by 3515 kilograms of tensile strength per square centimeter = 7030 kilograms of tensile strength in the reinforcing bar, in the bottom 30 cm of wall. Divide by 30 to find the average strength in a centimeter of wall. 7030 ÷ 30 = 234.

The total wall steel strength is 234 + 105 kilograms = 339 kilograms of tensile strength in the steel. There is an additional #4 bar in the floor-to-wall key which brings the steel strength figure to 456 kilograms.

The final step in comparing steel tensile strength to water force is to draw a circle and quarter it as pictured below.

Imagine all the water force as concentrated in one direction along arrow B. The small circle at A is an anchor. Arrow B pulls with a force of 401 kilograms, which is the total outward water force on the bottom centimeter of wall (calculated above).

Imagine next that the tank wall is infinitely strong except where the line CD cuts the tank in half. At points C and D the wall is the tensile strength of the steel calculations; 456 kilograms at C and 456 kilograms at D. Total wall steel strength the water must break is thus 912 kilograms. Steel tensile strength divided by water force is 912 ÷ 401 = 2.3; the wall steel is 2.3 times stronger than the water force.

Note 1: The welded wire coming out of the floor adds enough to bring the steel strength figure to almost 2.5 times stronger than water force, assuming that all the wires are at 45 degrees.

Note 2: An impression of just how strong ferrocement is for structures other than tanks is gained by reversing arrow B; push instead of pull. Well cured ferrocement easily has 550 kilograms of compression strength per square centimeter. If a structural wall is eight centimeters thick, points C and D would add 8800 kilograms to the 912 kilograms of steel strength. Arrow B must push with a force greater than 9700 kilograms to crush a one centimeter wide arc of ferrocement, at points C and D.

Economics (cost analysis):

Area calculations for 60m3 tank: Floor or roof area = πr2 = π32 = 28.26 m2

Wall area = 2πr(height) = 2π(3)(2) = 37.5 m2

Roof: The roof steel extends down the wall and the roof is also an arc.

Floor: To estimate floor steel add ten percent for waste and ten percent for the steel which extends beyond the circumference line before bending it to vertical position.

The result is (1.2)πr2 = floor area calculation for steel. Add a little more for roof arc and use (1.25)πr2 = roof area calculation for roof steel.

Floor or roof area multiplied by 2 (two layers of welded wire) = 56.5 m2. Multiply this figure by the factors discussed previously. 56.5(1.2)(floor) + 56.5(1.25)(roof) = 138.4 ≈ 138m2 of welded wire in the roof and the floor.

Conclude the welded wire computation by adding the wall.

There are two layers of welded wire in the wall. 37.5m2 multiplied by two = 75m2; add 10 m2 for wire overlaps and waste = 85 m2.

The total for welded wire is 138m2 for roof and floor plus 85m2 for the wall = 223 m2 of welded wire. The price of welded wire per m2 multiplied by 223 m2 = total cost of welded wire.

Calculation of reinforcing bars depends upon the spacing chosen between the bars and the length of a standard bar. Chapter two uses the grid space of 30 to 45 centimeters. Six meters is used further on in this book as a standard length. The method used to calculate reinforcing steel is to visualize a square with equal to the standard length of reinforcing steel. In this example it is a six meter square with an area of 36m2.

Nineteen bars creates a spacing of 33.33 centimeters across six meters. This equals thirty eight bars total. Divide 38 bars by 36 m2 = 1.05 reinforcing steel bars per m2. Add ten percent for waste and overlaps and there are 1.15 bars per m2.

28.26m2 (roof) + 28.26m2 (floor) + 37.5m2 (wall) = 94m2 (total).

1.15 bars/m2 multiplied by 94m2 = 108 bars of reinforcing steel at a 33.33 centimeter spacing.

This calculation at a 45 centimeter space between bars is 6 m divided by 45 cm, plus one bar = 14.33 bars. multiply this by two for the total bars = 28.66. Divide by 36m2 = .79 bars/m2. Add ten percent = .9 bars/m2. Multiply by the total area (94m2) and the reinforcing bars required equals 85.

Multiply the price of one reinforcing steel bar by the number of bars to compute the total cost of reinforcing steel bars.

Expanded metal for the inside of the roof and wall is wall plus roof areas multiplied by their use factors. 28.26(1.25) (roof) + 37.5(1.1) (wall) = 76.5 m2.

Concrete is best estimated at 7.75 centimeter thickness multiplied by the total area plus approximately five percent for waste. The floor is estimated separately and done first.

A small volume factor (0.2) for the joint between wall and floor is added to the floor estimate. 28.26 m2 (floor area) multiplied by 0.0775 m (thickness) multiplied by 1.2 = 2.6 m3.

Roof and wall is (28.26 m2 + 37.5 m2)(0.0775)(1.05) = 5.35 m3.

Summary (60 m3 tank):

Welded Wire...................223 m2

Expanded metal..............76.5 m2

Thin welded wire...........40 m2

Chicken wire (roof).......30 m2

Reinforcing steel bars....85 to 108Concrete:floor...............................2.6 m3

roof and wall.................5.35 m3

Other materials:Tie wire........................2 - 3 rollsWater seal (inside):Cement product...........70 - 100 kgglue.............................12 - 16 lHog rings....................3 - 5 kgPlumbingHinge and Latch

Color pigments, extra cement water seal product, and glue (if the outside is to be colored).

Chapter 2: Tank Floor

Mark the circumference of the tank:

Smooth and compact the actual tank area with hand tools when the tank site is completed. Place a stake in the center. Loosely tie a rope or a measuring tape with a wire loop on the end to the stake and scratch a tank-sized circle into the ground. It can be helpful to mark the circumference with flour or other white material. This circle will become a long-lived ferrocement tank.

Floor steel:

The following steel layout is for up to sixty cubic meters. Use #4 bar or larger and reduce spacing with increases of tank size and height.

Place one layer of fifteen centimeter square, 10 gauge welded wire on the circle. This is a standard construction grade welded wire which has almost twice the tensile strength of standard reinforcing bar. Place more pieces of welded wire until the tank floor circle is complete with the first layer. Overlap pieces of welded wire at least one square. If the welded wire is from a roll, flip it over so it can't roll back up, reinforcing steel bars will hold it down.

A grid of #3 bar is placed on top of the first layer of welded wire. A grid spacing of 30 to 45 centimeters has proven sufficient for this tank size. Reinforcing bars should extend forty to sixty centimeters beyond the circle on both sides. The bar can be cut with a torch, saw, or large bolt cutters. Overlap the bars at least thirty centimeters.

Trim the first layer of welded wire when the reinforcing bars are all in place and the grid is neat. The trimmed welded wire should extend 30 to 45 centimeters beyond the circumference line. Make trim patterns in squares. Do not leave sharp tails on the welded wire. Sharp wire can cause injury and blindness.

Hog ring pliers (above) and pneumatic hog ring gun are both are very helpful. They are purchased at wholesale upholstery supply stores. The regulator shown below can be used to deliver compressed air from a tank, purchase at welding supply store.

Note the absence of sharp wire tails when the welded wire is neatly cut in squares.

The wire extends less than 30 to 45 centimeters in the example because it is a small, 7.5 cubic meter tank. Extend beyond the circumference more fully for tanks above fifteen cubic meters.

A decision about roof construction is made before the floor steel is placed. If the water tank is the only one which will be built, that is, if no roof supports which can be used more than once are also being built; it can be helpful to dig a hole in the center of the tank to receive a center pole of at least five centimeter pipe when the floor concrete is placed. Although a wooden post sitting on the concrete floor is adequate, a strong center pole securely mounted in concrete is convenient, especially as tank size increases; build a tempory center pole hoist economically.

The reinforcing bar grid only needs to be tied enough to keep it in place until the second layer of welded wire is tied on top. The second layer is unrolled at a right angle to the first layer and offset over the first layer to create 7.5 centimeter squares.

Alternatively, the second layer of wire can be placed on a forty-five degree angle. The object is to make smaller and smaller areas of concrete without steel reinforcement.

Fasten the the first and second layers of welded wire together with tie wire or hog rings. Work from the center outward. Leave the outer thirty centimeters or so untied until the reinforcing bar and the welded wire are bent to vertical at the circumference line.

Bend the top layer of welded wire to vertical by placing the side of one foot to the circumference line and about two centimeters inside it. Position so the curve to vertical results in the upper layer being bent upward just inside the circumference line. It can become difficult to force the horizontal squares into a vertical circle. Additional material may be cut from the welded wire where resistance develops. For example: remove fifteen centimeters from every meter of circumference, this leaves shorter pieces to bend into vertical position. The finished vertical wall bend for this first layer should extend upward a minimum of thirty centimeters.

Now bend the reinforcing steel to vertical being careful to keep the bends just inside the circumference line. This bend can be done with hands and feet or with a piece of pipe slid onto the bar steel to bend it upward. A standard bending tool is also good to use for this bend. If two vertical bars are closer than about fifteen cm, cut one off at fifteen cm above the floor steel.

Tie a horizontal #4 bar to the verticals about nine centimeters above the floor steel. This is the horizontal key steel and will be cemented into a wall key when the floor is plastered. After this first horizontal wall steel is in place, go around the tank floor and make the bend of the inner welded wire as neat and squarely bent as practical. Light hammer blows can assist this task in occasional places which are difficult. Finish the wire ties which were left undone around the outer thirty centimeters of floor to make bending the reinforcing steel easier.

Bend the remaining layer of welded wire upward and vertical. The lower layer of welded wire in the floor becomes the outer layer in the wall. When removing squares from the outer layer it is important to do it so no gaps go through the now vertical layers. If a gap is inadvertently created, patch it with small pieces from the scrap pile.

Inventory clean scraps by size for rapid retrieval of often perfectly cut pieces.

Tie or hog ring all remaining steel and lift the finished floor armature onto 3.75 cm thick support blocks made of concrete.

Note: All usable scraps should have sharp tails removed before being placed in scrap inventory. This will eliminate frustrating tangles and possible injury. Scraps too small for use should be put into a a recycling container after being cut into small harmless pieces. Some scraps can be made into very fine welding rod and stored for later use.

Floor Steel Summary:

1) Bottom layer welded wire2) Middle layer reinforcing steel bar3) Top layer welded wire

Note:To make the floor 1.5 centimeters thinner: Support the steel armature with two centimeter support blocks. Add a fourth layer of poultry wire on top of the three layers shown.

Chapter Three: Plumbing and Floor Concrete

Plumbing: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.

Chapter Four: Tank Wall Steel

Floor-to-Wall Key:

Straighten uprights to perfect vertical with a level. This seemingly minor task is important for maintaining as close to a true circle as practical.

The wall starts out as a flimsy single layer of welded wire. Minor irregularities become part of the tank and tend to grow into unattractive flat spots or bulges. Flats and bulges are often next to each other and thus even more noticeable, especially on larger tanks.

Bottom Horizontal Reinforcing Steel Bars:

The first horizontal steel is a number four reinforcing bar. Place it as close to the wall key as possible and make sure it is level. A few of the highest spots in the key might need to be chiseled off here and there in order to place this first bar absolutely level. Use a sight level or a water level for best results. Mark the level line on the wall key vertical steel with soapstone. These first horizontal bars guide the placement of welded wire. Slight up and down variations in the first layer of welded wire translate into places where the welded wire is forced to lean in or out to compensate for any up or down variation of the horizontal wires. Large tanks require more precision than small tanks.

Inspect for verticals that are too far in or out even though they are vertical. Bend out of line verticals in or out and make them vertical again after the first horizontal reinforcing bar is in place and completely tied. The idea is to adjust vertical bars to meet the circumference without distorting a smooth circle. Vertical bars that were bent to meet the circumference are adjusted vertical with a level after the first horizontal circumference is firmly tied to all vertical reinforcing bars. Repeat this horizontal circumference bar procedure twenty centimeters above the first horizontal with #4 bar (1.25 cm). Check again for level and vertical.

Overlap the horizontal reinforcing steel bars thirty centimeters or more. For larger tanks, when the first bar is harder and larger, it is easier to lap weld and work with twelve to eighteen meter lengths. Manually bend the final lap joint to position the steel flat against the wall. Even the smaller bar sizes will occasionally resist all efforts to make a neat overlap which is flat to the wall surface. Manually bend protruding bars into the circumference arc with repetitive small bends rather than one bend.

The first tie of horizontal reinforcing steel is always made at the center of the bar. Start at the center of an eighteen meter bar and do the same with a one meter bar. Leave the last 60 to 100 centimeters of each bar untied. The loose ends of the overlap are then tied together; starting at the center of the lap joint.

Walk each end of the reinforcing bar toward the tank circumference, hold the ends in loosely with a two meter length of wire, then go back to the center and tie the horizontal steel to the next

vertical bar on either side of the first tie. Walk the ends in toward the wall further and hold them in place by moving or shortening the long wire. Then go back toward the center and make more ties to vertical steel. This job is either for one or three people; a person at each end of the horizontal reinforcing steel bar can replace the loose wire ties and make the job quicker.

Inspect for verticals that are too far in or out even though they are vertical. Bend out of line verticals in or out to maintain a smooth circumference line. Adjust them to vertical again after the first reinforcing bar is in place and completely tied. Now repeat the circumference wrap procedure twenty centimeters above the first horizontal with #4 bar (1.25 centimeters). Check again for level and vertical.

The above procedure is somewhat different than the accompanying pictures of a small open tank. The main difference is that the floor concrete is not be placed separately for this small project. The first inner layer of welded wire is attached to the floor steel. Higher walls and a roof will be added in later chapters.

An old partner and good friend once built a forty cubic meter tank without cementing the floor first. His plan was to cut down on expenses and he would have done so but for a strong wind which blew the almost finished tank armature off the mountain top. He and his crew eventually hauled it back up the mountain, banged it back to round, and plastered it. The tank is a good one and has been in service for many years with no problems.

Inside layer of welded wire (tank height of 2.13 meters):

Unroll and cut a piece of welded wire long enough to make a circumference plus an overlap. This is a nineteen meter piece for the sixty cubic meter tank. A good practice is to cut the fifteen centimeter wire tails off as you go. Imagine one of these rolls winding back up out of control, with wires fifteen centimeters long stabbing from the end.

Flip the nineteen meter length of welded wire over and roll it up loosely backwards to relieve wire strain from the factory roll (one meter plus diameter roll). Unroll and flip back over again. Re-roll even more loosely in the original direction and stand the roll up inside the tank, next to the wall. Loosely tie one end of the roll to horizontal steel. Unroll the standing welded wire around the inside of the tank wall until the approximate mid point is reached. Tie the welded wire to the upper horizontal bar. Use a vertical on the welded wire so it can slide up and down a little.

Use imagination to view this roll technique as welded wire inside a tank which is bein unrolled and attached to lower bars.

Place the fine steel metal lath inside the tank before the wall circumference of welded wire is completed. Now is the time.

Loosen the first tie and let the first half of the welded wire roll to a stable position. This step is crucial to tank roundness. Work in both directions from the center of the welded wire as pictured above with fine mesh instead of welded wire. If the key steel is vertical and the inside layer of welded wire is unrolled and attached level, the finished wall will be vertical and free of flat areas or bulges. Do not hurry this first layer of welded wire. Make sure it is tight against the vertical bars, which are inside the horizontal bars, pull the welded wire only to within 1.25 centimeters of the horizontal steel bars, which are 1.25 centimeters outside the vertical bars in the key. This will avoid distortions in the plane of the welded wire.

Center cut the bottom wire every one meter between verticals to relieve inward pressure from the key. Slightly bend these cut tails toward the key so they do not protrude from the final plaster surface.

Do not tie the inside layer too tight at first and avoid ties of vertical welded wire to vertical bars until the first layer is neatly in place. This step provides for slippage; when verticals are tied there can be no lateral slip. Lateral slip facilitates correct position. The task here is to avoid stressing the wire. Place it smooth. The main problem at this stage is introduced by unrolling the wire ahead and securing it with a temporary wire. Do not forget to loosen the temporary wire, which becomes too tight very quickly. The shortest distance between a temporary wire and the work area is a straight line; the tank is supposed to be round.

Vertical reinforcing steel:

Cut enough standard reinforcing steel bars into thirds. Space these bars between thirty-eight and fifty centimeters. Sixteen standard bars yields forty-eight two meter verticals spaced at about thirty-eight centimeters for this sixty cubic meter example tank. Number three bar is sufficient for finished tank strength but number four is better for supporting heavy, wet plaster with minimum sags and bulges. The same reasoning applies to spacing the verticals closer rather then further apart. The additional cost of a few extra bars sized a little larger than necessary is not significant.

Straighten any verticals which are slightly bent before placing them on the wall. Tie the verticals securely inside the first two bands of horizontal steel and against the inner layer of welded wire. These bars do not need to be spaced perfectly. Nor do they all need to be tied to the vertical bars in the key. Tie verticals firmly to the first two horizontal bars. Put a loose tie to the welded wire at approximately 150 centimeters of height.

Upper Circumference Bars:

The remaining horizontal bands of reinforcing bar are at thirty centimeter spacing. Place the highest bar just below the start of the wall to roof curve.

Skip one position upward. Place the first horizontal reinforcing bar sixty centimeters up from the first bars that were attached to the vertical steel in the key joint between floor and wall. Then go back down thirty centimeters and place the skipped bar. Now finish the higher horizontal wraps.

Never try to pull reinforcing bar steel into round with a tie to the welded wire, this will cause a distortion in the welded wire which will create a bulge or flat spot somewhere else in the tank wall.

Outside Layer of Welded Wire:

Unroll and cut three pieces of welded wire which are 6.75 meters long. Relax the tight factory roll by reverse rolling as was done for the inner layer of welded wire. These three pieces are sized for the sixty cubic meter tank. One may wait until two 6.75 meter pieces are in place and then measure to be sure the third piece is the correct size. One piece all the way around cannot be

used because the difference in radius between the inner and outer layers causes the pattern of fifteen centimeter squares to creep into alignment, instead of being staggered at 7.5 centimeters.

Adjust the fifteen centimeter squares of the inner and outer layers to make 7.5 centimeter squares. Position the outer layer down 7.5 centimeters relative to the inner layer. Center cut the bottom horizontal wire every one meter to relieve the outward pressure of the wall key.

Work from the center and tie the outer layer pieces only enough to hold them in place at first. Measure and place the last piece so that it overlaps forty-five to sixty centimeters on each end.

Complete the ties after the last piece of outer welded wire has been minimally secured in position. Work in a pattern similar to tightening the lugs of a tire. For example, divide the circumference into fifths and temporarily mark these locations with stakes. Tie the layers securely together at each spot starting at center height and working up and down. Call the first spot one, after it is complete, go to number three, and then five, before finishing number two and four. Gradually fill in untied areas from the center after all five division spots are tied. It is best to do this in the same five spot pattern and go around the tank several times (five is an arbitrary number, six or four is fine, too).

Do not tie the top fifteen to twenty centimeters completely because tails from the roof steel will extend into the wall steel thirty to 45 centimeters. Occasional ties may need to be removed at that time. Note that the top of the outer layer of welded wire is 7.5 centimeters below the top of the inner layer.

Bending Steel Circles:

Use caution when bending steel into circles. If the end escapes from the form it will be with a violent motion that can cause serious injury. Observers step back.

Insert at least fifteen centimeters into the bending form. Never let go of the steel! Slowly walk one way to bend the steel reinforcing bar around the circular form. Walk slowly backward to relieve the tension. Be careful! Danger!

Chapter Five: Top of Open TankFinish off the top of an open tank with an edge which turns outward. This gives extra strength around the top edge.

Notice that the horizontal wires are on the outside. This position gives maximum tank strength as well as a good place to wedge the 7.6 centimeter form board, which is used to bend the wires to horizontal. Center cut the top horizontal wire every forty-five centimeters, leaving three vertical wires to bend to horizontal together. These center cuts are a rare circumstance where the wire tails are not removed, there is a 7.5 centimeter tail on each side of each section. They will soon be covered by reinforcing bar.

Bend and trim vertical bars to fit the lip. Then place 0.6 or 0.9 centimeter bar around the outside of the lip. Sixty millimeters is adequate for the small tank pictured. Now cut enough fifteen centimeter wide strips of welded wire to go around the tank lip. The strip lengths should overlap at least 7.5 centimeters. Bend the fifteen centimeter wide strips over the same form board. Use the long wire as a catch to hold the wood board.

The board is of no further use when the fifteen centimeter strip has reached a ninety degree angle. Complete the bend by hand. Work along the strip two or three times to gradually complete the bend. Use a bouncing pressure with the palms of both hands. Listen for a click when the wires contact at the bottom of each downward push.

The short side of this piece is placed on the top. The long side goes down and forms the hypotenuse of a structural triangle.

Metal lath of thin gauge is painted black. It is much better for ferrocement work than the heavier gauge galvanized. Galvanized should only be used if the thin gauge is unavailable. Thin gauge is 4.5 mm.

The height of this open top tank is set so that a standard piece of metal lath can be cut into two functional pieces. One piece fits the inside wall up to the outward lip curve. The second piece covers the inside wall plus the lip; top and bottom (see last photo).

The bottom half of the lip is covered with lath. Thin welded wire is placed on top. Beware of sharp cut ends of metal lath. Flatten them inward with a tool.

Chapter Six: Roof SteelCheck and measure a good curve before cutting a center post. The eye can see a good curve as well as it sees color. A curve which is pleasing to the eye is usually a strong curve. Use a length of bar across the tank. Hold the center of the bar up and tie the ends loosely at the walls. Lift the center until the roof curve looks right. Measure the distance and cut a temporary post to fit.

Visualize curves under bridges and the curve will be good. The roof should be comfortable to work on. Don't make it too steep. The post should be about thirty centimeters higher than the walls for a sixty cubic meter tank with two meter walls.

Balance and temporarily tie a square meter of welded wire on top of the center post. This step was omitted in the photographs because the example is small enough to be self-supporting. Cut a piece of number four reinforcing bar which extends sixty centimeters past the center post and forty-five centimeters past the wall. Bend the reinforcing bar so it enters about thirty-five centimeters into the wall. Use fifteen to twenty-five centimeters for the bend radius. Remove

wire ties where the bars enter between the layers of welded wire. Slightly bend the top of the inner layer of welded wire inward to fit the curve.

The curve starts at the top of the outer layer of welded wire; that is 7.5 centimeters below the inner layer. The distance from the floor is two meters ±. This height is perfect for reaching onto the outer portion of the roof from low scaffolding. Low scaffolding is about seventy-five centimeters high.

Repeat the previous step and extend the the radius into a straight line diameter across the tank which is supported by the center pole and tied into the walls. Use thin sticks or bamboo and wire for temporary supports between the center pole and the walls. Do not tie too much at first. The reinforcing steel overlap must be loose enough to slide and adjust the circle at the top of the tank. Repeat again and make a second reinforcing steel bar rafter perpendicular to the first rafter bar radius.

The tank roof is now divided into quarters. Adjust the walls to as vertical as practical by pushing and pulling the roof bars in the center before tying them. The goal is to make the tank cylinder as vertical as it can be. If forcing the wall to vertical under one of these first roof bars causes the wall to be less vertical elsewhere, split the difference so both areas are as vertical as possible. One should understand that this compromise is not a mark of poor work unless the tank is so visually crooked as to catch the eye. This somewhat imprecise nature of ferrocement is a key reason for its absolute price advantage within the size range of fifty to five hundred cubic meters, plus.

Now make three circles of number three bar. The circle diameters are 30, 75, and 120 centimeters. They do not require perfection. Tie these concentric circles on top in the center. Use number four bar for larger tanks.

This is the time to put some pre-cut lengths of welded wire inside the tank, and the inner lath if it is not there already. Then cut four pieces of welded wire three meters long. This will cover much of the tank ceiling. Extra pieces of are better than less. Excess material can be easily removed through a temporary door cut in the wall after the roof steel is in place. It will be easier if all the pieces are reverse rolled to relieve the tension from the tight factory roll.

The roof grid is thirty centimeters or less. Sixty bars of steel extend out of the wall and into the roof of a sixty cubic meter tank, space them at thirty centimeters. Only four bars reach all the way to the center; they are already in place. Now cut eight pieces of number four bar which are ten to fifteen centimeters short of the center. These pieces extend fifty to sixty centimeters past the wall. Bend and test one before cutting the rest; it should extend thirty or forty centimeters into the wall and be ten to fifteen centimeters short of the center.

Divide each roof quarter section into approximate thirds with two of the eight roof bars just made. These bars are placed on the nearest thirty centimeter circumference mark. They will not divide the roof quarter sections into precise thirds. Slide the bars under the reinforcing bar rings and over the square meter of welded wire sitting on top of the center support post. Twist and push the bent tail ends into the wall. Remove any wall ties which makes this difficult. If too much steel crowds into or over the center, trim in place with a cutting torch, or, working on the ground with a metal saw.

Temporarily support the arc of each radius rafter with thin wood, bamboo, or small plastic pipe.

Now make three circles with diameters of 1.8, 2.4, and 3 meters. Place these circles on top of the tank radius bars. Inspect once again for perfection at the top of the tank. If one of the radius bars is problematic, loosen and re-tie it. Each pie shaped piece in the sixty cubic meter example is now about 1.5 meters wide at the tank circumference.

Continue filling in with larger rings and shorter rafters until the roof reinforcing bar steel is a thirty centimeter grid. Notice that some remaining rafters are much shorter. Use reinforcing bar from the scrap pile to finish the shorter rafters and larger rings. The shorter radius bars only extend far enough toward the center to maintain the thirty centimeter grid.

Bend the inner and outer welded wire in the wall to fit the curve. Put a "Z" shaped bend every forty or fifty centimeters along the highest horizontal wires. This reduction in circumference will pull the top horizontal wires into the curve and can be precisely controlled. The "Z" bends can also be undone slightly as needed. Keep these bends in the plane of the tank wall so bumps of steel do not protrude toward the finished plaster surface. "Z" bend maker is illustrated below.

Place the welded wire on the ceiling now. Start at the center of each piece and work outward. Overlap the original square meter of welded wire about thirty centimeters. Remove the temporary supports to make room for placing the welded wire. Re-install the temporary supports when each section of welded wire is in place. This will maintain a uniform roof curve. The first

pieces of welded wire should divide the tank ceiling into quarters. Continue until the interior ceiling is complete.

The next step is to put a top layer of welded wire on the roof. It is difficult to maintain an even spaced pattern because of the roof curvature and the patch method used to place the ceiling wire. It is best to position the top roof layer of welded wire at a forty-five degree angle to the inner ceiling wire for these reasons.

Cut a piece of welded wire long enough to go all the way over the roof and extend down the wall at least thirty centimeters on either side. Tie a piece of rope to the end of the cut piece and roll the rope up with the wire. This rope is pulled from the far side of the tank to unroll the welded wire and place it on top.

Cut a door in the tank wall when the welded wire on the roof is complete and well supported. The door should be larger than pictured.

Each layer is rolled back and tied to the tank wall, rather than cut completely out as pictured. The outer layer of fine wire should be cut on both sides and bottom and then rolled upward. The outer layer of welded wire is cut on one side, top, and bottom to be folded to one side. The inner layer of welded wire is cut at the top, bottom and on the opposite side as the outer cut so that it folds open in the opposite direction of the outer welded wire.

Reinforcing steel bars are then cut out of the way. Pieces of wood can be tied to the door sides if the tank wall distorts excessively when the door is cut open. The door is folded back into place and patched at the seams at the last moment during plaster application; proper sized replacement reinforcing steel with sufficient overlap should be ready for that event, bend the horizontal pieces slightly to match the tank wall curvature.

Be sure to remove sharp wire tails that will cause minor cuts or serious injury to those using the door during tank construction.

Chapter Seven: Thin Steel and HatchMetal Lath (Thin Gauge Expanded Metal): Begin with the wall. Attach the lath to the wall with wire or hog rings. This is not the final tie. Securely in place is enough for now. Metal lath sheets are about 2.4 meters of length. Since the wall is slightly less than two meters, the metal lath will curve up into the interior ceiling. The radius is smaller approaching the center of the tank, this causes the top corners of the metal lath to overlap too far. It is nearly impossible to force plaster through two layers of metal lath. Cut the top corners to make overlaps of two centimeters. Finish walls and then do the ceiling. Use the temporary supports to help hold the lath in place. When the interior lath is complete, put a layer of poultry wire on the outside roof, extend it down the wall about thirty centimeters below the curve.

Outer layer of fine wire: The outer layer of fine wire may be welded wire with 1.25 cm squares, metal lath, or two layers of poultry wire (with the wires of the second layer bisecting the holes of the first layer). If lath is used, try and keep all the opening directions going the same way. This seems a small detail yet examination of the openings will reveal that one direction is wider and will thus allow for easier plaster entry than the other direction. It doesn’t matter structurally which way the lath is placed but it is convenient during the rush of plaster application if the holes match directionally.

Cut the outer layer of fine steel to reach the beginning of the roof curve. The poultry wire on the roof is sufficient to hold the wet plaster in place at the curve. A roll of 75 centimeters wide and a roll of 120 centimeters wide fine wire cover the outer wall, for example.

Finish Tie the Steel Armature: Loose outer layers of fine wire are exposed by sags and bulges of heavy plaster. Not much can be done at this point. There are two ways to secure the fine steel so it won’t bulge. One is with hog rings. Hog ring pliers or a pneumatic hog ring gun are required using this method. These may be obtained from an upholstery supply store. Tie the underlying welded wire, where it is loose, with loops that go all the way through the tank. If the outer steel is tied well to the inner welded wire and the inner welded wire is loose, there will be a large sagging thick spot where the inner wire is loose. Bulges absorb many kilograms of cement.

If hog rings are not available, stitch the seams of metal lath. Use a slightly thinner wire than tie wire if it is available. One person passes the wire through at an angle and another passes it back, also at an angle. Tighten every third or fourth stitch. After all the seems are tight, stitch around and around the tank. Work upward to the wall curve. Space each pass around the tank 20 to 25 centimeters above the last. Although this method takes the most time, the result is excellent when the stitching is tight.

Stitching also works well for the roof, reduce the spacing to fifteen centimeters or less. Another procedure is to tie a third layer of welded wire tightly beneath the inside ceiling lath. The metal lath is supported well by an extra layer of welded wire underneath it. Finish plaster the ceiling to cover this layer of welded wire.

The Hatch:

Locate the hatch near the inlet pipe to facilitate maintenance of plumbing parts and observe water flow. Make the opening large enough to remove the ladder. If the ladder base is 70 cm make the finished hole size 76 cm and the rough opening 80 cm. To accomplish the example size, start with a 83 centimeter circle of #3 bar wired to the roof. Trim steel and bend welded wire in a convenient way. Plan for at least a four centimeter cement curb around the finished hole.

A ferro cement hatch can be either hinged and locked with a hasp, or locked down with two hasps. If hinged, use a large hinge. Weld or tie extra welded wire and steel to the hinge where it attaches to the roof and hatch, do the same thing to the locking hasp. The hatch begins with a circle of #3 bar which is about ten centimeters larger than the completed roof hole. Then fabricate a small version of the tank roof with hinge and hasp attached. The dome shape of the hatch will accomodate the plaster curb around the roof hole. Tie hinge and second locking hasp part to the roof. Reinforce these areas with welded wire patches and reinforcing bar scraps. A ferrocement hatch is heavy, the hinge and locking hasp should be large. The hatch is plastered after the tank has cured for a few days.

The tank is ready for plaster when all the fine steel has been placed. Observe the stiffness of the armature. It is strong enough to fill with water. The plaster waterproofs the tank and protects the steel: when it shrinks during cure, the roof will lift upward and the supports will hang from the ceiling. This power marks the tank strength and illustrates why ferrocement is so strong.

A 45 by 60 centimeter piece of welded wire is the inside lowest layer (in the above photo). It has been darkened to make it more visible. A person on top of the tank ties wires carefully passed upward by a companion working below. It is an effective way to support the inside layer of expanded metal in the ceiling. This method creates a thin roof without sags, it can help save many kilograms of cement.

The hinge mechanism is made of plastic pipe cut in half. It is taped to surround #4 reinforcing steel as shown to directly the left. The hinge bar is shown bent into the ceiling in the above photo. A prefabricated hatch can be plastered and cured before building the tank; it would also be installed at this point of armature construction.

The stake of wood supports the hatch lid two centimeters above the curb. This distance simulates cement thickness (Approximately one centimeter over steel). Complete the hatch and curb similar to the pictures in the last half of chapter five.

Chapter Eight: Roof Support

Brace the roof to hold the wet plaster when everything else is finished. Five by ten centimeter posts pushing 2 x15 cm boards against the ceiling will hold the wet plaster when there are enough supports to make the roof rigid. There will be several people working on the roof to plaster it. The roof should feel hard, almost like standing on solid ground. If it is soft, it will sag when the plaster is applied. The roof supports also help hold the walls vertical and straight. Leave enough room between the outer supports and the wall for plaster work. Wire the supports up so they stay in place during construction.

Now is the time to make a strong center post if one was not cast into the floor concrete. A 10 x 10 cm post is strong enough to hold up wet plaster, 5 x 10 is sufficient to place the roof steel. A base is required to hold the center post vertical and steady. This is easy enough to nail together. Make the center post easy to remove when the tank is finished.

It is not uncommon to return to a tank the day after it has been plastered and find the supports hanging from the ceiling. This occurs because the cement plaster shrinks as it cures. The shrinkage around the walls can cause the roof to lift upward as much as five centimeters. Tie the support structure loosely to the ceiling so nothing falls and damages the fresh and relatively soft plaster wall. This effect is not so obvious in smaller tanks but is an indication of the live strength within the composite material known as ferrocement.

Place a circle of forty-five to sixty centimeters of steel or plywood on top of the center pole. For those who venture into construction of tanks as a business, this top piece for the center pole should be steel made in two halves which bolt together through a hole in the top of the center pole. Put holes in the circular plate for tie wires to secure the long rafter supports which extend as spokes from the wall to the center support. For smaller tanks or construction of a single tank, a 2 x 15 centimeter board will work fine. These boards also bend nicely to the shallow roof curve and make good support rafters when wedged upward against the roof steel with stronger support wood such as 5 x 10 cm. Be sure to secure them to the roof with loose wire ties so they don’t damage the wall if they fall.

Ferrocement tank construction is a good business in areas where tanks are needed for water or grain. There is no way to make a tank better or less expensive in the size range of 50 - 500 cubic meters. This is especially true in areas where the water is highly corrosive and steel tanks last only a few years. Price your product near what installed steel tanks sell for and there will be enough profit to build a good business enterprise. Smaller ferrocement tanks are more expensive than other construction methods but are still competitive when the replacement cost of other tank types is included over their life cycle. One ferrocement water tank will last through replacement of several steel or plastic tanks even in areas with minimally corrosive water.

Note: A very thin ferrocement covering can be put on plastic tanks to shield them from the sun and they will last indefinitely.

Those who plan construction of numerous tanks or other structures with roofs will eventually make adjustable metal roof rafters and support stanchions which are removed and taken to the next project. Visualize a wire umbrella frame with the support stanchions extending straight down to the floor. Stanchions are made of square steel tubing which is supported by a screw jack. Such jacks are easily made from 2.5 cm threaded steel stock. Weld a seven to ten centimeter square of plate steel welded on the bottom for a foot. A nut and washer push upward against the bottom of the stanchion to make a tight fit of the rafter support spokes against the ceiling.

The center post for larger tanks is simply a larger version of the one pictured. The stanchions are made of a lower piece using three centimeter square tubing. A piece of 2.5 centimeter tubing slides inside for course adjustment and is set to the desired length with a pin extending through; this secures the adjusted length. For larger tanks use three centimeter square tubing inside 3.8 centimeter outer tubing. There are many combinations of outside measurement and tubing wall thickness such that a smaller size will slide inside a larger size, some steel supply stores have the right sizes and some don’t (the one’s that don’t often say there are no sizes that fit).

Square steel tubing is usually supplied in lengths of 6.5 meters ±. Short stanchions are made by cutting these lengths into quarters, cut the lengths into thirds or even halves to support higher ceilings at the center of large tanks. Combinations of these square tubing sizes are also used to construct the rafter support system. A pin of #3 bar at the top of the stanchion fits into a hole in the bottom of the rafter to maintain a secure lock on position. The roof weight can be quite large during plaster application.

A complete discussion of rafter support systems is too lengthy for this small book, yet it is worth mentioning that the rafter supports can be assembled with circumference chord connections. These can be straight pieces which slide over smaller stubs of square tubing welded to the rafters. One or two of these circumference connections should be of adjustable length. The pattern of the assembled support structure will look very much like the reinforcing steel bar

pattern in the roof armature. All support rafters need not reach to the center when building very large tanks but it is simpler to do so on sizes below 200 cubic meters ±, assuming three meter walls in the 200 cubic meter range.

Notice that when the single rafters are connected to each other there is a structural integrity to the rafter supports which is independent of the tank armature.

Hinged tubing tails at the outer circumference of the roof support system are helpful for tying the rafters to the wall steel armature. This complete roof support design introduces a major cost saving since most of the roof armature can now be built on the ground and lifted as a single piece on top of the rafter support structure. Roof armatures which are pre-built on the ground are upside down because the ceiling metal lath is placed on top. The only remaining work when the pre-assembled roof armature is placed on the roof supports is to join it to the wall steel with short reinforcing bar pieces in the wall to roof curve. These pieces should extend forty-five centimeters into the roof steel and be tied securely. Cover the reinforcing bars inside and out with welded wire to complete the wall to roof curve and the union of roof and wall. Finish the inside of the curve with metal lath making sure lath overlaps are not too large and then the outside with poultry wire.

This roof construction technique efficiently utilizes the extra layer of welded wire beneath the ceiling metal lath discussed previously. The tank builder who has roof support technology is ready to build roofs for just about anything. Support stanchions are placed two meters ± apart around the outer circumference. Two meters is a good place to start for spacing stanchions along the rafter toward the center. All this depends entirely on how one designs the overall support system, how large the tank is, and how perfect one wishes the roof to be. Safety is also a consideration. No workers should be inside large tanks when the roof is being plastered.

Additionally, for very large tanks, the roof plaster can be placed for about one third of the top at the center and then 60 cm wide plaster spokes can extend into the walls. This way requires a second plaster day but is safer if one has doubts about the support structure.

A final note about the roof rafters and building very large tanks involves the floor. Large tanks are quicker to build if the floor extends underground like a bowl. The amount of water in the bowl shape of the floor reduces the size of the tank but introduces the problem of a slippery slope under the stanchion feet. Eliminate this problem by casting flat topped steps which ring the floor at the proper radius measurement for the stanchion feet. It is easier to make entire circular steps on the proper radius than to figure out where to place a small step for each rafter stanchion to stand on.

Installing hinged rafters

This method illustrates applying the metal lath to the first layer of welded wire before placing it on the rafters in factory widths. This technique is simpler than making the entire roof on the ground before putting it on the rafters.

Chapter 9: PlasterPlaster Material: Good plaster depends on clean sand and water. If either contains reactive chemicals, plaster quality is reduced. Use sand that experienced plasterers know produces a quality product. Refer to page 17 for mixing proportions.

Roof: Start at the roof top center and work outward. One person stands inside the tank to watch progress for those working on top. No plaster can be applied from below at this time. Excess water flows and drips down and will cause fresh plaster to fall. Wait until the ceiling has become stiff before attempting to plaster it. Brush roof top with a stiff broom to remove shrinkage cracks. Apply water to the roof as a fine spray. Don’t let the roof become dry in the sun. Concrete pumps make roofs easier.

Wall: Begin the wall plaster on the shade side. Work both directions around the tank. Hand application is from both inside and outside the tank. Quality control at this point is a wall without voids. Do not apply a thin layer of plaster which does not penetrate and surround all the steel. Experienced plasterers are more likely to work too fast and leave voids than less experienced people who pack the plaster in with their hands. Have extra gloves available so that workers will use there hands for packing. Hand pack means use the hands to pack the plaster into the steel armature. Discover the full extent of voids by tapping the wall with a hammer and observing how the plaster settles and makes the void visible for hand pack. Do not finish trowel the wall too much at first, excessive vibration causes water to work to the surface, this, in turn, causes the plaster to slide off the wall.

Plaster Finish: Finish the inside ceiling when the plaster is firm enough to sponge float irregularities and accept more plaster. For wall finish, wait until the plaster has become firm, then use a wet sponge float to finish and smooth the surface. Start where the wall was first plastered. Use a plain sponge to brush off larger grains of sand and small pieces of gravel. The sponge float and sponge work best if they are rinsed and cleaned frequently. This technique is for both inside and outside. Apply more plaster in areas where steel shows through the surface. Use sponge float and sponge technique on any shrink cracks which may appear.

If the plaster is finished well, the tank should be water tight. A good routine is to seal the inside with a cement based sealer before the tank is filled. Use a push broom and a mixing tray to apply the sealer coats. These materials require one day to cure. Apply in the early evening and start on the shady side of the tank. Small leaks will eventually be sealed by minerals in the water as the water evaporates and leaves the minerals behind.

Cure: There are many ways to keep the tank wet until it cures (28 days = 100%). Old blankets and a hose work well if someone is present. A battery powered water timer can be used to turn a sprinkler on and off. Black plastic over wet blankets holds moisture well, it also brings the temperature up and increases the rate of cure. The idea is to maintain moisture in the plaster. Once the plaster has become dry, the state of the material changes and it will not absorb water. Typical concrete used for a floor or foundation has a compression strength of about 210 kgf/cm2. Ferrocement cured well ranges from a low of 450 kgf/cm2 upward to as much as 840 kgf/cm2. These strength figures are a good indicator of how long the structure will last. Good concrete protects the steel from the elements and the structure lasts a long time with no maintenance. The effort put into a good concrete cure will be appreciated for generations.

Mechanical Application: Pumped ferrocement is a mixture of 505 kg of cement per cubic meter of sand. The mixture is pumped through a rubber nozzle. Compressed air throws the plaster against the steel armature. This is the tried and true mechanical technique used in the trades, though there are smaller plaster sand pumping systems available. A strong, active crew on the inside holds sheets of plywood up to the steel as a backing so the plaster doesn’t simply pass on through. A second crew is required to clean and rotate backing plywood as it grows heavy with deposited plaster. The nozzle operators must also be fairly strong. Concrete sand has some grains that are almost gravel size. Some of these bounce back and accumulate as waste.

Supervision is a more intense part of the job as application speed increases. Clean up is also a bigger job. The quantity of voids decreases with nozzle operator skill and attention. Mechanical application becomes more practical with project size and where trained labor is either too costly or simply unavailable. The pros and cons of mechanical application are subjects of debate. Pumped plaster yields a fine tank.

Chapter 10: Coloring & Water sealInner waterproofing and outer coloring are both done with a cement base product which cures in one day. One such product is known as Thoroseal, it is available in grey or white. Grey base is best for earth tone colors.

Two to four liters of white glue are diluted and then mixed with each bag of sealant (22 kilograms ±). (Don't use latex glues in sunlight areas). About three bags are required for a sixty cubic meter tank. Two bags should be sufficient for a second inside layer. This project is possible without the glue but occasional bonding problems may occur.

A white or colored first layer inside a water tank makes it easier to see a second grey layer. The inside seal of colored open reservoirs or swimming pools starts with a first coat of grey, then a second color layer. Do not work too hard on colors in swimming pools as they will change over time.

The following paragraphs describe a multi-colored, single outer layer, to visually blend a water tank with the environment, for example. The same procedure is used to apply a second layer, which is timed for as soon as the first layer is sufficiently durable to be unaffected by broom bristles or other application tools (16 to 20 hours).

Mix the main batch of grey base in a paddle wheel plaster mixer. Separate approximately 1/4 into various buckets (for multiple colors). Lighter colors without red are mixed with a wand propeller on a drill motor in these buckets. (White base Thoroseal isn't quite right for rock colors but will make pastels, if desired).

The main color is then mixed in with the material still in the plaster mixer. (Finger paint on a hot muffler or something else hot to see a quick dry approximation of cured colors).

When the base color looks right, pour 1/4 to 1/3 of it into buckets. Now mix blacks, dark browns, and other dark colors with the electric drill motor and mixer attachment. (A stick will work but some colors resist mixing and turn into lumps, a few unmixed lumps burst under the broom or brush bristles and add interesting streak effects).

The color pallet now contains buckets of non-reds, a mixer loaded with base color, and additional buckets of darker colors, which were mixed on top of the base color. Use kitchen measuring cups and other measuring containers. Take notes, write all colors and measurements on paper. File the notes in a safe place. Use standard colors if possible.

Schedule the work to begin application during late afternoon, approaching evening. A foggy or cloudy day is ideal. Pour some base color into a wheel barrow and begin working on the shady side of the tank or other structure. Dip the stiff bristle push broom in base color and begin the fun. Work both ways around structure toward the sunny side, hold off working on the roof until the sun is very low.

Use a mixing tray and a second push broom if there is a second base layer color. The mixing tray is placed near the wall and brooms are used in the same way to seal the inside. The large push broom quickly moves material up and out of the mixing tray and onto the wall. Use smaller hand brushes for other small volume colors and to work the material into rough spots.. (An occasional green or brown (etc) pure pigment in glue or dry should be experimented with here and there).

Return to the starting point and mist with water to keep the thin layer moist. Avoid rivulets of excess moisture which will wash away the newly applied material.

Pigments are best from barrels. These colors have names like "raw umber", "sienna" or other familiar artist's paint names.

Maintain moisture until the night becomes too dark to see. Return early in the morning and apply moisture until evening. Now the waterproof layer is permanent (at least for 30 ± years).

The finished water tank may show some minor leaking even after application of water seal layers on the inside and outside. Tank builders often refer to this as, "sweating." Do not worry about minor leaks. The water is moving so slowly that evaporation leaves minerals behind which gradually solidify into cement like material which eventually stops all moisture from escape and makes the tank totally water tight.

Note that the natural seal of evaporating water does not occur on outside rooftops. Rainwater rinses microscopic cracks clean rather than plugging them up with deposited minerals. A completely waterproof ferrocement roof can be accomplished during construction by fog misting the top and continuously using a stiff bristle broom on any shrink cracks until the plaster becomes too hard to effect with mist and broom.

A simple method of spraying the water seal and color layers is available for those who make a business of building ferrocement structures. Eighty Liter ± agricultural chemical mixing tanks are available. Tanks of this type have a wide mouth hatch for pouring in material and can be pressurized with compressed air, which pushes the thin sealing material through a garden hose. One person sprays the material on walls and ceiling while others spread it with push brooms and smaller brushes.

Expanded metal is a sufficiently fine screen for separating out lumps which might plug the sprayer. Pour mixed material through expanded metal to remove lumps before attempting to pressurize and send liquid through a 15 to 25 meter garden hose. A two centimeter diameter garden hose is sufficient if the plumbing bend out of the bottom of the pressure tank is 2.5 centimeters in diameter. Garden hose with a 2.5 centimeter diameter is available but not necessary. Run water through spray apparatus to clean tank and hose.

Spray material onto structure surfaces using the thumb on the end of the hose, just like spraying water on a garden. Wear a thin dish washing type glove to avoid abrasive wear on skin.

There are several commercial products to accomplish this job, many have official stamps of approval. The web caretaker knows of individuals who have used ferrocement tanks waterproofed as described since the 1960's, with no apparent problem.

Chapter 11: Septic Tank Design and ConstructionThese design graphics illustrate a ferrocement septic tank which was built in 1975. It has been used continuously and remains in perfect condition. Contrary to the experience of many well respected sewage plant engineers and operators, the cement has not deteriorated. Perhaps this is due to the hardness of the mortar, described in chapters one and nine; 550 to 850 kgf/cm2 for the ferrocement plaster mix versus 175 kgf/cm2 for standard concrete mix (8,000 to 12,000 psi vs. 2,500 pounds per square inch). The oldest ferrocement septic system known at ferrocement.com was built in 1968.

Wall thickness varies between 3.8 to 7.5 cm (1 1/2 to 3"). The light grey area represents floating organic matter. Sludge is mostly composed of microbial remains heavier than water; it is a darker grey color at the bottom. Large arrows indicate water flow direction. Small arrows indicate flow direction for gasses of decomposition.

This septic tank did not receive grey water from laundry, sink, or bath drains. It was emptied in the year 2000; estimated remaining capacity was approximately fifty percent after 25 years of use. The bottom sludge was used for garden fertilizer and the top mat of organic matter was left in the tank. Biodegradable toilet tissue was always used and no chemical or bacterial products were introduced.

The first chamber is 55 percent of the empty volume. The second chamber is 30% and the third is 15%. When the first chamber is half full of sludge and floating organic matter, the relatively clear water in between is approximately the same volume as the clear water volume in the second chamber.

Most septic tank designs include only two chambers. The third chamber adds to protection of the drainage area, which is an expensive and difficult part of septic system construction.

Sewage water remains in this tank for an average of 45 days; it is held in the first chamber for an average of two weeks when the chamber is half full. Most codes require only one day retention time of clear volume in the first chamber (at 50% full). This insufficient time limit has caused contaminated ground water in many urban areas of the United States. Home owners are required to install municipal sewer systems after this occurs. A new source of domestic water is also often necessary.

Septic systems are not practical in areas of dense human habitation unless the discharge is directed to community treatment facilities rather than individual drainage areas. There is almost always enough area to use grey water for garden and landscape irrigation where septic systems are successfully utilized.

Local codes do not usually require mixing of grey water and sewage water within the plumbing system before it leaves the structure. Plumbing systems that do not pollute grey water with sewage are environmentally wise. Grey water is a larger quantity than actual sewage water. The additional plumbing cost to keep them separate is not large. Check local building codes for grey water utilization.

Discharge from individual or community septic tanks can be directed to a municipal system at much less cost than sewer systems which carry all grey water and organic matter. Sewage

collection accounts for 70 to 90 percent of municipal sewer system installation when all waste water and raw organic sewage are combined and then sent directly to a central location. Toxic chemical dumping through the sewage system is also easier to locate when sewage sludge is harvested from individuals.

Individual septic tanks overflow to a drain field as illustrated below. The width (X) times the length (Y) provides an area of percolation, which is determined by soil tests for absorption rate. The distance (Z) indicates the gravel bed beneath the drain pipe, 10 - 15 cm is common. The trench is filled with gravel to a similar distance from the surface. Connected arches are often used to create a cave at the bottom of the trench. This technique replaces both pipe and gravel. Plastic arch pieces sufficient for a residential septic system easily fit in the back of a small truck.

Depth to the drain pipe (P) must be sufficient to position the drain pipe below roots which will clog it. The leach field cannot be in an area of deep rooted trees. The depth (P) is also related to the slope of the terrain, a hillside leach field requires greater depth so that water does not surface on the downhill side (Ferrocement House Construction, p. 2).

Percolation tests for soil absorption rates follow procedures which may vary in detail from one locale to the next. The basic idea is to determine an absorption area that will be sufficient for the planned usage. This is usually accomplished by digging a preliminary hole large enough to work

in. The preliminary hole is located near point (L) on the drawing. It is dug to full depth and is large enough to work in. A smaller test hole is then dug in the bottom of the work area. This hole is filled with water and the absorption rate is measured. If there is variation of soil type in the computed length (Y), additional absorption test pits are dug at either end.

The primary reason for varying methods of calculating a total absorption area is that anaerobic organisms grow on the bottom and walls of the leach area as it matures to equilibrium with the area soil life. These anaerobic organisms die and are consumed by aerobic organisms in the soil. The growth of this living layer is on the trench bottom and walls, where gravel meets soil. It slows water flow out of the leach field and also accomplishes final purification of the waste water. Soil temperature and biology have a large effect on this layer and explain much of the regional differences in formulas employed for calculating the total required leach area.

If leach area calculations are based on a usage factor which includes all grey water and raw sewage, the leach area will be larger than necessary (should grey water be subsequently utilized for irrigation). A method of dividing the water flow so that half the leach area is "rested" while the other half is used can be installed then if desired. Install an outflow pipe from the septic tank so that water can be directed to drop pipes for either separated half at the leach field mid point (Y). A clay barrier in the gravel between the two halves improves separation efficiency.

Ferrocement.com advises using grey water for irrigation in the surface soil and in sunlight whenever possible. Surface soil life and sunlight rapidly convert grey water to biological usefulness. As long as this water does not pool and breed mosquitos it is relatively safe. Local building codes will provide detailed information for each locality.

Construction of a septic tank in the ground is similar to the open top tank except there is no need for an outer layer of fine wire. Corner pieces are made as shown on page 28. Having a supply of these ready will greatly speed up fabrication of the armature.

Number three bar on a 30 - 45 cm grid between two layers of 15 by 15 cm square welded wire is an adequate steel schedule. The reinforcing steel bars in the chamber separation walls need not be bent so they overlap into the outer walls, the welded wire corner pieces are adequate attachment for inner chamber walls.

Two centimeter thick support blocks, with wires in them for attaching to the armature, are sufficient to keep the steel from being in contact with the soil. Ferrocement objects with straight walls are easiest to build using welded wire sold in flat sections rather than rolls; this eliminates the need to straighten pieces cut from a roll.

A two coat cementiscious water seal product that cures in one day was used inside the septic tank illustrated in this chapter

.

Sanitary tee can be inspected and cleaned through small hatch from above.

The small ledge deflects strong currents so the bottom sludge is not disturbed.

Looking down on the entrance plumbing and the dispersion ledge below it.

The square opening at the top is the hatch, it can be wood or ferrocement.

Hatch covers are very simple, they only keep dirt from falling inside.

The upper white pipe is for return flow of gasses to the venting system.

A secondary small hatch for manual clean out of the entry fitting is recommended.

A turned down ell for water flow to the second chamber helps keep downward drifting solids from passing through.

Steel is omitted on the near side for photography.

The estimate table has been filled in during various correspondences. A materials list for the 50 - 60 cubic meter range (15,000 U.S. gallons) can be found in chapter one of the tank building manual.

The estimate table has been filled in during various correspondences. A materials list for the 50 - 60 cubic meter range (15,000 U.S. gallons) can be found in chapter one of the tank building manual.