Here's the theory (very brief).
Cement and steel expand and contract with heat and cold at an equal rate. If there is enough steel spreading like capilaries in a human body, forces transmit evenly through the concrete. If force bends the concrete but does not stretch the molecules beyond that of heat and cold, concrete becomes like rubber. It's that simple.
Ferrocement structures have several advantages over those constructed of lumber or standard reinforced concrete, strength and flexibility are most obvious. Great strength contributes to the following three: 1) Almost zero maintenance, 2) No need for structural insurance, 3) Multi-millennia longevity. All three advantages are totally dependent upon good methods and materials. A qualitative advantage is increased design freedom gained by use of curvature as well as straight lines.
The requirement for good methods and materials is quite clear when comparing crumbled concrete bridges throughout the northeastern United States and ancient Roman concrete structures still standing. Safety nets and wood on the underside of bridges to catch falling material before it harms motorists is common in northeastern central north america, where freeway bridges have not yet been replaced.
One difficulty with ferrocement housing is the cost of engineering. This cost can be reduced if the builder prepares drawings for professional analysis. Chapter one includes area and site maps. Hygiene and water source are included also. Smaller sketches are cut and paste onto large field blueprints using either manual or computerized methods. Subsequent blue print pages are for foundations, utilities, etc.
Be aware that there is nothing new here-in. Modern materials are utilized along with techniques as old as mud and wattle; to wit, smear mud on sticks to make whatever structure is needed.
Sculptural freeform supported by an internal latice of steel reinforcing bar is the primary technique explored. Temporary support scaffolding is used for roof areas during construction.
The concept behind the three chambers illustrated by the septic tank drawing is known as The God Thanker Theorem, in this book. Prior to modern septic science the same idea was known as, The Three Hole System. This basic theorem mandates that an hygienic overview be placed up front in all plan documents. A more detailed chapter discussing actual construction of a ferrocement septic system is included; the septic chapter was first published in the the manual, ÒFerrocement Tank Construction.Ó
The scale of the three chambers illustrated is, approximately, L = N + 1 meters, where "N" is the number of people involved and "L" is the total length of the tank. Circled letters on the trench wall indicate one method of notation for pertinent data or associated project artifacts; geologic information or buried utility crossings, for example. The flat plain presentation morphs to a three dimension schematic to better illustrate the idea of horizontal distance from the drainage pipe to sloping terrain.
The first example is a multi-media structure of wood, stone, and ferrocement. It follows the rhythm of the painterÕs maxims involving darkest-darks juxtaposed with lightest-lights; here-in transposed to straightest-straights and curvy-curves.
The next example begins with a wonderful drawing of a life-time friend, Frank Robinson, now deceased. Though his drawing appears whimsical, OlÕ Frank was a task master during actual construction. He somehow managed to be stern with a smiling twinkle in his eye. Compare the above rectilinear floor plan and Frank's curved one below.
The foundation plan is trenched and ready for concrete footings. Frank Robinson was a unique architect who was educated as an aeronautical engineer during WWII. He was thus among few architects comfortable with calculations involving curved sheets of composite materials. Frank also used his professional talents to rescue adobe from prejudice and restore it to use in modern architecture. Frank Robinson was a many-faceted, fun-loving man who also helped keep a large neighborhood together as an accomplished master of ceremonies, stand-up comedian and great friend.
The curved white lines are plastic water pipe placed to position reinforcing steel where walls will be. Reinforcing steel verticals are tied to the pipe to hold them in position when the concrete is placed. The white post in the foreground is a temporary power and telephone center during construction. The slightly ragged trench leading from the white post toward the center marks the path of electricity entering the structure. A second utility trench for water and gas enters from upper left.
A steel grid for the floor slab is placed after the trenches have been filled with concrete. The concrete color is so close to soil color it is difficult to see that the slab will sit on well-swept, clean concrete footings. The wooden rails are used as guides to support boards that plane the surface to level. These boards are known in the trades as, screed boards. The reinforcing steel is supported off the ground by small concrete blocks. The finished slab will be 10 centimeters thick with the steel in the center (3.5Ó+). Electrical conduits also cross under sand beneath the reinforcing steel.
Notice that a concrete encased steel pipe has replaced the wooden center stake. A pole such as this is very handy throughout ferrocement construction. Five centimeter inside diameter is sufficient in this case (2Ó). A support pipe this size is too small for larger projects when walls and roof are done together using a system that the roof plaster is placed first. The construction plan for this home yields finished, full-strength walls before the roof concrete is placed; lateral sway is not present even with tens of tons of fresh plaster and a work crew of twelve, fifteen, or twenty.
Never underestimate the danger of a failed center pole. Imagine yourself four to five meters high, atop a roof 12 - 14 meters wide. Concrete trucks are lined up. Diesel engines are screaming. A large work force is suddenly wide-eyed and frozen. The structure has swayed. If it goes down all will be swallowed by a giant ferrocement amoeba curing harder with each passing minute.
Patient gravity has become a rattlesnake. What would you do? Circa 1980.
ÒNobody move!Ó Point. ÒYou! YouÕre closest to the edge. Go first! Easy does it; down the ladder.Ó Nod slowly. Point again, use nose this time. ÒYouÕre next. Slide like a snake with a smooth touch of hustle. Down.Ó One by one. Etc. Sigh. ÒOkay, partner. ItÕs you, me, and the money riding on the job. What are we going to do?Ó
ÒHowÕs this? ThereÕs almost half-a-truckload in the center. LetÕs pour spokes which connect with the crew at the walls. Then finish the rest of the roof in a week or so?Ó
Ferrocement is a quick and easy method to build foundation forms in difficult areas. Such forms are also successfully used to build magnificent supporting girders.
Three different structures have been used as examples thus far. Others will be used to illustrate various construction details.
Here is the finished slab ready for the next construction phase. The slightly curved stand pipe left of the center pole is the septic system vent pipe for the kitchen sink. It rises inside the outer wall before passing through the roof. All utility lines which reach inner positions are complete. Rebar stubs mark wall locations. The drawing on the next page is included to provide visual reference and definition for the photo. Layout of FrankÕs design work has been rotated and sized to fit the page.
Reinforcing steel stubs protruding from the finished floor slab are ÒLÓ shaped. The bottom of the ell, approximately 15 centimeters long, is tied securely to horizontal steel bars at the bottom of the foundation trench. These bars are elevated and isolated from contact with the ground by small concrete blocks like those previously shown supporting the floor steel grid. Plastic water pipe was used to make curved wall lines for the upright re-bar wall anchor stubs only because it was readily available. Bamboo or thin wood is also suitable for this purpose. Vertical wall anchor steel extends at least 45 - 50 centimeters above top-of-slab.
One might logically wonder about footings which were dug into solid stone for this ferrocement home. Frank Robinson knew quite clearly that the combination of floor, walls and roof add up to such great strength that the foundation was not required as a supporting structural component. He also knew that the cost of a little extra concrete would be less than convincing the local government that a foundation was not needed. His eyes twinkled merrily as he justified the foundation as a way to keep the structure in place during an earthquake. We joked together about mooring the house with chain rather than using a foundation.
I once climbed atop a large boulder and watched waves and ripples swirl in the soil while riding out an earthquake which tipped over a nearby railroad train. The strong earthquake may be less dramatic for a ferrocement structure than a moderate day on the high seas for a ferrocement boat.
The next drawing illustrates the importance of consideration for vent pipe location. I spent over an hour one afternoon looking for the source of sewage odors with my brother, at his home. We eventually determined that the odor came from the vent pipe on his roof. He solved the problem with a long upward extension. Though unsightly, it eliminated malodor. Keep in mind that cool air flows downhill and puddles.
