Ferrocement

Ferrocement or ferro-cement is a system of construction using reinforced mortar or plaster, generally with no gravel, over an armature of metal mesh, woven expanded-metal or metal-fibers and closely spaced thin steel rods such as rebar. The cement is typically a very rich mix of cement and sand. The material is not concrete. It is used to construct relatively thin, hard, strong surfaces and structures in many shapes such as hulls for boats, shell roofs, and water tanks. It is widely used in thin-shell structures.

Quotes

My invention shows a new product which helps to replace timber where it is endangered by wetness, as in wood flooring, water containers, plant pots, etc. The new substance consists of a metal net of wire or sticks which are connected or formed like a flexible woven mat. I give this net a form which looks in the best possible way, similar to the articles I want to create. Then I put in hydraulic cement or similar bitumen tar or mix, to fill up the joints.
- Joseph-Louis Lambot (ca. 1841) translation by Gainor W. Jackson, W. Morley Sutherland, Concrete Boatbuilding: Its Technique and Its Future (1969) as quoted in "State-of-the-Art Report on Ferrocement", ACI 549R-97 (1997) ACI Committee 549.

[W]elded wire reinforcement (WWR) [was] formerly known as welded wire mesh or fabric. Welded steel wire reinforcement is the predominant form... A grid of orthogonal longitudinal and transverse cold-drawn steel wires is welded together at every wire intersection... The size and spacing of the wires can vary... based on the requirements... Welded wire reinforcement can be epoxy coated or zinc coated (galvanized). ...Plain and deformed welded wire reinforcement is covered in... ASTM A1064... Stainless steel wires are specified according to ASTM A1022... Epoxy-coated WWR... in accordance with ASTM A884... Galvanized WWR... with ASTM A1060... Even plain wire used in welded wire reinforcement has both chemical bond and mechanical bond to the concrete. The mechanical bond results from bearing of the welded cross wires against the concrete in the grid of reinforcement.
- Steven H. Kosmatka, Michelle L.Wilson, Design and Control of Concrete Mixtures: The guide to applications, methods, and materials (2011) 15th edition, pp. 141-142.

A thin shell is a special kind of vault whose geometry may include many shapes. ...a three-dimensional form made thicker than a membrane, so that it can not only resist tension as membranes do, but also compression. On the other hand, a thin shell is made thinner than a slab, which makes it unable to resist bending, as a slab does. In short, thin shells are structures thicker than membranes, but thinner than slabs.
Thin shells are made possible by the use of materials that work well under tension and compression. Masonry has no tensile strength... Only the availability of reinforced concrete and ferrocement made a thin shell possible.
- Michele Melaragno, An Introduction to Shell Structures: The Art and Science of Vaulting (1991) p. 115.

RECOMMENDATION 6: Ferrocement in Disaster Relief.
After fires, floods, droughts, and earthquakes... [t]ransportation is often disrupted... Supplies of bulky conventional building materials may be stranded outside the disaster area, whereas the basic ingredients of ferrocement may be available on the site or easily transported. The versatility of ferrocement also reduces logistical supply problems: wire mesh, cement, sand, and water can be substituted for the metal used for roofing, woods or plastic for shelters and clinics, asphalt for helipads, steel for bridges, and so on. Moreover, most ferrocement structures, though built for an emergency, will last long after the emergency is over. ...[F]errocement could be used at a disaster site for many purposes: Transport facilities, from simple boats to barges, docks, marinas, helipads, and simple floating bridges or short footbridges as well as road repairs. ...Food-storage facilities, quickly designed to local needs and quickly built, to preserve emergency food supplies. ...Emergency shelters such as, for example, the quonset type of roof, which is easy to erect and highly efficient. ..Public health facilities, such as latrines and clinics, built with ferrocement roofs and stucco-type walls of the same wire mesh and mortar. ...[C]adres of ferrocement workers could be trained in emergency applications and the supervision of local laborers at the disaster site.
- National Academy of Sciences, Ad Hoc Panel of the Advisory Committee on Technological Innovation, Board on Science and Technology of International Development, "Ferrocement: Applications in Developing Countries" (Feb. 1973) pp. 9-10.

[Ferrocement defined:] A thin walled construction, consisting of rich cement mortar with uniformly distributed and closely spaced layers of continuous and relatively small diameter mesh (metallic or other suitable material).
- United Nations High Commissioner for refugees (UNHCR), "Large Ferrocement Water Tank Manual" (July, 2006)

Ferro-Cement Boat Building Manual (1972)

U.S. Navy, Naval Ships Systems Command, NAVSHIPS 0982-019-1010, Vol. 3.

The construction method chosen was the inverted wooden mold. For hulls up to 50 feet in length, and for utilizing unskilled labor, this method has been shown to be most efficient. ...The shape and fairness of the hull is first established and checked with the quick and easy-to-build wooden mold. ..The use of air-powered staple guns to fasten mesh and rods to the hull mold is a quick and efficient method and can be performed with unskilled labor. ..Lamination of the concrete skin is eliminated as the mortar is applied from one side only and vibrated through the hull shell reinforcing. ...Sagging of large unsupported areas is avoided. The men work from the outside of the hull and downwards.
- Introduction, p. ii.

One inch (25 mm), 21 gauge, hexagonal galvanized mesh was used. This mesh was the type manufacturers describe as "reverse twist," galvanized after weaving. Ten layers were applied... Four layers of mesh were stapled to the mold over 4-mil plastic sheathing.
Two more layers of mesh were stapled over 1/4-inch (6.4 mm) diameter vertical reinforcing bars which had been stapled on at 6-inch (152 mm) centers.
1/4-inch (6.4 mm) diameter reinforcing bars were then stapled longitudinally over this second layer of mesh. This layer of reinforcing bar was spot-welded to the first layer at approximately every second joint. This second layer of horizontal rods was applied on 3-inch (76 mm) centers.
The last four layers of mesh were hogring fastened to the outside of this last layer of rods.
- JOB 6 - MESH AND ROD REINFORCING - HULL, p. 14.

Clear plastic 4-millimeter sheathing was hand-stapled to the mold for two reasons: ...To stop the moist mortar from falling through the joints and gaps between the wooden battens planking the mold. ...To form a barrier between the wooden mold and the fresh mortar.
If no barrier were placed the wood would draw moisture from the new mortar and reduce its final strength.
- TASK 1 - Plastic Mold Cover, p. 14.

The first four layers of mesh were stapled to the mold over the plastic sheeting. Each length of mesh, already folded double, formed two layers. This first layer of mesh strips, 1-1/2 feet (457 mm) wide, was butted together. The second layer of mesh strips was laid out so as to cover the joints where the first layer was butted together, making a total of four layers of mesh.
- TASK 4 — First Four Layers of Mesh, p. 16.

The vertical rods were stapled firmly to the hull. An air-powered staple gun was used...
- TASK 9 - Placing Rods with Air-Power Staple Guns, p. 18.

Two more layers of mesh were stapled over the mold. Again 1" x 2" (25.4 mm x 51 mm) wide staples were used. The folded mesh was not lapped but just butted.
- TASK 12 — Two Layers of Mesh Between the Reinforcing Rods, p. 21.

The horizontal rods were welded on. Where a rod terminated on the hull it was lapped for six inches (152.4 mm) with another rod and spotwelded. All the rod joints were treated in this same way. The rods were stapled at approximately three-inch (76 mm) centers. Every second intersection of horizontal with vertical rods was spot welded. As there were two layers of mesh between the vertical and horizontal rods, the mesh was faired smooth in this small area. Care was taken not to burn too large a hole in the plastic sheeting where the rod welding took place.
- TASK 13 — Horizontal Reinforcing Bars, p. 22.

Wooden plugs of the same diameter as each through-hull fitting were cut out and placed on the mold in the exact position where the future through-hull fitting was to be installed later. These were cut from doweling and made one inch (25.4 mm) deep. A hole was drilled in the center of the doweling to ensure that the plug did not split when nailed to the mold. The mesh was cut away under the plug and trimmed neatly at the edges. Some attempt was made to place the doweling in a position clear of the intersecting rods.
- TASK 14 - Blanking-Out for Through-Hull Fittings, p. 22.

Starter rods for the stem, webs, bulkheads, bilge stringers, and engine beds were welded in place. These starter rods were placed at approximately six-inch (152 mm) centers. They were six inches (152 mm) long where they extended through the hull. Quarter-inch (6.4 mm) holes were drilled for these... The starter rods were lap-welded to either the vertical rods or the horizontals, depending on their position.
- TASK 15 - Starter Rods for Webs, Bulkheads, Etc., p. 22.

One-inch (25.4 mm) chain links were welded to the hull reinforcing cage where scuppers were to be placed. These links were aligned and welded in at deck level. ...[E]xposed steel pieces such as scuppers or screeds which require welding... should always be cleaned and protectively coated.
- TASK 17 - Scuppers, p. 23.

The last four layers of mesh were stapled to the hull mold. They were laid in the same way as the first layers. The mesh was fastened... as smoothly and as tightly as possible. It was clipped onto the horizontal rods with 3/4-inch (19 mm) hog rings. ...One-half inch (12.7 mm) hog ring staples which do a neater job could not be located ...All edges of the mesh were stapled down tightly so that no stray ends of mesh would penetrate later through the fresh mortar and thus interfere with the plasterers' work... Mesh over the chain link scuppers was clipped away and the ends fastened down neatly.
- TASK 18 — Last Four Layers of Mesh Reinforcing, p. 23.

The mortar used for the hulls was a mixture of clean, graded silica sand, ...Portland Cement Type II, and drinking-type water. This silica sand, of the grading and particle shape used in high-strength structures... The sand content used was... one 50-pound (22 Kg) bag of coarse grade, one 100-pound (44 Kg) bag of medium grade and one 50-pound (22 Kg) bag of fine grade. To this graded sand was added two 80-pound (31 Kg) bags of Portland Cement Type II and just sufficient water to make the mortar workable into the hull mesh reinforcing. ...There was one plasterer for roughly every 100 square feet (9 m²) ...Retarders or additives were not used. The sun shelters were moved into place ...
- JOB 8-PLASTER HULL, pp. 27-28.

First a heavy coat was applied all over the hull. Men stationed inside the hull mold began systematically vibrating the mold planking and checking the gaps between the planking for mortar penetration. Once the mortar had all been applied to the satisfaction of the men vibrating and checking, the excess mortar was then scraped back to the mesh. ...A new thin coat was troweled over the hull and allowed to start setting. When it started to set the hull was sponge troweled, the sponge trowel being used in a circular motion to smooth out surface irregularities. As soon as the sponge troweling was finished, the final steel troweling began. This was carried on until the hull surface had set up too hard to be worked on any further, and was as smooth and fair as the plasterers could make it.
- TASK 6 - Plastering the Hull, p. 32.

The hulls were steam cured for 24 hours at a temperature of 150°F (66° C). A steam pipe, perforated for its entire length, was placed under the inverted hull and a rubberized canvas steam tent drawn completely over. The temperature was carefully brought up to 150°F (66°C) in a period of four hours. Twenty-four hours were then maintained at this prescribed temperature until, finally, it was allowed to drop slowly to ambient temperature of 85° F (30°C).
- JOB 9-STEAM CURING, p. 35.

The hull was left untouched for 18 hours after the plaster finishing work had ceased. This allowed the hull to set-up hard enough for the men to drag the steam tent over it. It is not advisable to start steam curing too soon, as the jets of hot water from the steam pipe may wash some of the mortar off the hull while it is still green. Before steam curing began the wooden screeds were removed from around the hull sheer.
- TASK 1 - Before Steam Curing, p. 35.

"Ferrocement Roofing Manufactured on a Self-Help Basis" (July, 1977)

J. Castro, Universidad Autonoma Metropolitana Mexico, Journal of Ferrocement, Vol. 7, No. 1, pp. 17-27.

[These] low cost, easily built, high quality ferrocement roofings... offer an innovative solution to the serious dwelling problem affecting large numbers of people, especially in the marginal urban areas and rural zones of developing countries...

Ferrocement was chosen as the material for the proposed roofing because of its physical properties (strength in compression and tension, impact, permeability, etc.) and because it is cheap and easy to build.

[I]t was decided to develop a type of roofing based on prefabricated sections. ...With the partial results obtained in this stage, another part of the study could be initiated, i.e. to build this same type of element "in situ"... thus providing solutions for situations in which prefabrication is not appropriate...

The adaption of ferrocement precast roofings in self-help construction projects... permits the use of standard components which are easily erected without sophisticated equipment.

The construction of the mold simply consists of making a dome of well compacted earth, covered by a layer of well-finished concrete having a thickness of 8 cm [3.15 inches], with the shape defined by the trusses... used to [shape] the mold.

The reinforcement consists of two no. 2 bars along the edges, one of them straight and the other one with the necessary bends to provide the handles to lift and fix the dome to the structure. ...[T]wo layers of galvanized chicken wire, guage 22 with a separation of 13 mm are attached to the bars and directly mounted over the mold, one perpendicular to the other. ...[E]nsure a minimum overlap of 5 cm... and... ensure that these are stretched... to achieve the thinnest section possible.

The mortar used for the mix is made (using a mixture) of normal or puzzolanic cement and sand in a proportion of 1:1.5 by volume and with a water-cement ratio of 0.55.

After a couple of hours, the desired finish is applied (polishing or brushing), with the object of sealing the cracks or faults that may appear on the surface of the dome.

The curing of the shell is achieved by covering the surface with wet sand for a period of 72 hours.

An alternative construction method was also developed which did not require the use of any type of mold or form.

The best solution found was to form a double curvature surface... The curvature does not necessarily follow a pre-determined law, so that it may be checked roughly "with the naked eye".

[T]he smaller the thickness of the cover, the better will be its quality, which is why at the time of pouring, the meshes of the wire should be well stretched. Care should be taken that only enough mortar to cover the reinforcement is used.

One worker on one of the supports... either manually or with a trowel distributes the mortar over the chicken wire... Simultaneously, another worker from within the room... holds the mortar which is applied from the outside with a metal float or trowel in order that the mortar does not fall. Once this operation is completed, the required finish is applied both from the outside and the inside.

The central part remains [bare wire] and will be completed after 72 hours. ...[T]he worker can [then] climb on the previously cast portion, carrying out the same process ...[S]upport the dome until the mortar has cured in order to avoid deformations caused by the weight of the mortar and to guarantee curvature of the shell.

As eight of the shells tested failed as a result of the failure of the supporting concrete ties on the walls, it was decided to build samples which were very well reinforced... The ultimate load increased by 1.7 times for these shells...

The domes were very easily repaired by replacing the damaged mortar or mesh...

[T]he ferrocement roofings are practically waterproof and that they do not need any special treatment.

[T]o increase the load capacity of the domes and to avoid excessive deformations, it is necessary to provide the best possible anchoring at the edges.

[A] dome of any shape will amply comply with safety requirements. Because of this it is believed that it is possible to build domes in situ without specifying the shape of the dome, which makes skilled labor unnecessary.

Chicken wire mesh was recommended because of its ductility. It shows no oxidation problems as it is made with galvanized wire. It has reliable properties and is low in cost.

Ferrocement: Building with Cement, Sand, and Wire Mesh (1977)

by Stanley Abercrombie

The purpose of this book is to match an existing resource with an existing need. The need is shelter... simpler structures... that can be assembled quickly in the wake of a hurricane or flood... that can be built economically in undeveloped countries... that... provides pleasure in the form of self-made personal retreats...

This highly specialized, but by no means highly complicated building technique had been almost forgotten after its first use... in the middle of the nineteenth century until it was virtually reinvented in the 1940s by... Pier Luigi Nervi.

Ferrocement is used relatively little in the housing field because it is regarded as a labor-intensive... building technique. ...It is true that considerable labor is required to put together... sand, cement, and wire mesh... However, the elaborate temporary framework which consumes most of the labor in conventional work is often entirely eliminated... Even if we concede that ferrocement is impractical where labor is expensive... its use requires only time, not skill...

All ferrocement can be said to be reinforced concrete, but all types of reinforced concrete are not ferrocement.

[F]erroconcrete would be a more accurate term for our material, but that term is already in common use to describe... reinforced concrete work.

[F]errocement ...uses wire mesh, rather than heavy rods or bars, as the primary part of its metal reinforcement and which uses sand [in a mortar mixture] rather than a mixture of sand and stone ...as the aggregate in its concrete mix. ...The resultant product can be a shell of surprising thinness, durability, resilience, and, when properly shaped, strength.

The structural effectiveness of any reinforced concrete, including ferrocement, depends on the almost miraculously fortuitous fact (first discovered in 1870 by Thaddeus Hyatt) that steel and concrete have close to identical coefficients of expansion, swelling at exactly the same rate when heated, shrinking at exactly the same rate when cooled. Thus they may be permanently bonded together as a single material, utilizing the best structural characteristics of each: steel has the tensile strength... while concrete has the compressive strength...

Ferrocement... often acts more like steel than like a standard reinforced concrete. Hit with a hammer, it rings like a bell.

[F]errocement... may eliminate the need for separate layers of waterproofing.

[A]t the new Sydney Opera House... [the] famous sail-shaped roofs (built of conventional reinforced concrete) have been covered with tile-surfaced panels of ferrocement which serve as waterproofing...

[B]ecause of its intrinsic hardening process which continues indefinitely, good concrete gets better and better, imperfect concrete (with flaws that invite erosion and corrosion) gets worse and worse.

[B]efore... crude beginnings of work with conventional reinforced concrete, work with ferrocement had already begun. ...The ferrocement technique seems to have been first used by ...Joseph Monier and, apparently... independently, by ...Jean-Louis Lambot. ...Lambot called his invention "ferciment" and used it to build boats... He constructed his first boat in 1848...

Although... two of the first patents for reinforced concrete of any type... were for ferrocement, that particular type of reinforced concrete was generally underutilized—in fact, forgotten—until Pier Luigi Nervi's work of the 1940s. ...The turning to ferrocement... was based on the logical use of a known fact: the structural behavior of reinforced concrete is most effective near the points of its reinforcement. ...Nervi was first to ask the question... why not distribute the metal reinforcement so evenly that all the concrete is in immediate proximity to it?
On this theoretical foundation Nervi performed the experiments which led to his establishment of ferrocement building technique as we know it today.

[B]y the end of 1943 Nervi's firm was at work on three 150-ton transport boats, their hulls completely of ferrocement, and one 400-ton vessel, largely of ferrocement.
The first construction was interrupted by the war, and it was not until 1945 that Nervi's method resulted in... [t]he Irene... a motor boat with a 165-ton displacement. On a supporting frame of 1/4" steel rods spaced about 4" apart, Nervi spread eight layers of wire mesh, four on each side of the rods, which were tied tightly together and plastered by hand with a rich cement mortar. The resultant ferrocement was 1 3/8" thick (about the same thickness as Lambot's boat). Other than the rods sandwiched into the mesh, no formwork was needed.

In 1947 [Nervi] built his first ferrocement structure on land, a storage warehouse... 35' x 70' and all four... walls [and] roof were of ferrocement 1 3/16" thick, their thinness made structurally feasible by their corrugated shapes.

The following year Nervi designed and built a 41' ketch... the Nennele... the hull's total thickness was less than 1/2".

Nervi's first used of ferrocement in an important public structure was in the 1948 Exposition Hall in Turin. ...The great corrugated roof is... ferrocement panels 1 1/2" thick tied together by ribs of conventional poured concrete...

In the 1953 Milan Fair building and in the 1959 Flaminio stadium... Nervi used ferrocement corrugations... in strikingly cantilered roofs. A further use by Nervi of the material has been in... smooth, lightweight forms into which conventional poured concrete could be molded...

Most of the more recent use of ferrocement have been by others, but it is to the insight and pioneering work of Pier Luigi Nervi that they owe their successes.

"State-of-the-Art Report on Ferrocement" (1997) ACI 549R-97

ACI Committee 549

Ferrocement is a form of reinforced concrete... [utilizing] closely spaced multiple layers of mesh or fine rods completely embedded in cement mortar. It can be formed into thin panels or sections, mostly less than 1 in. (25 mm) thick... Unlike conventional concrete, ferrocement reinforcement can be assembled into its final desired shape and the mortar... plastered directly in place without the use of a form.
- 549R-2

[T]he early work of Lambot... was one of the first applications of reinforced concrete, but [was] also... a form of ferrocement. His patent on wire-reinforced boats that was issued in 1847... This was the birth of reinforced concrete, but subsequent development differed from Lambot’s concept. The technology of the period could not accommodate the time and effort needed to make mesh of thousands of wires. Instead, large rods were used to make what is now called conventional reinforced concrete, and the concept of ferrocement was almost forgotten for 100 years.
- 549R-4

Portland cement is generally used, sometimes blended with a pozzolan. The filler... is usually a well-graded sand capable of passing a 2.36 mm (No. 8) sieve. However, depending upon the... reinforcing material (mesh opening, distribution, etc.), a mortar containing some small-size gravel may be used. ...Addition of short and discrete fibers of different types favorably affects the control of cracking and the capacity... to resist tensile loads. ...[R]elatively short and slender (l/d = 100) steel fibers may be randomly distributed in hydraulic cement mortars... the overall effect being to increase tensile strength and improve the shear resistance...
- 549R-5

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