Ferrocement : Introduction, Property & Application

 Ferrocement

Ferrocement, also written as ferrociment, ferrocemento, ferrocimento, and ferrozement, literally mean much steel rather than much concrete. The Committee 549 of the American Institute for Concrete submitted the following definition of the ferrocement subsequently adopted by the majority of the specialists:

     " The ferrocement is a type of reinforced concrete in thin elements currently constituted by micro) concrete of hydraulic cement reinforced with thick layers of continuous netting, in wire with a relatively small diameter. The net may be metallic or in other materials ."

A.E. Naaman defines ferrocement as reinforced concrete in guise of thin elements with
very high performance as regards the resistance to extension, the ductility, the resistance to impact. The same author enhances the possibility of using this material for executing several constructions with thin walls, with superior physic-mechanical characteristics.

The ferrocement displays a series of advantages as compared to reinforced concrete, among whom:

•   a wider range of elasticity 
•   greater resistance to extension 
•   better behavior at dynamic stress 
•   increased value of the breaking effort out of extension  

Historical Background of Ferrocement 

   
                    Although discontinuously used on the market, ferrocement is a contemporary material of reinforced concrete. According to Shah's published works, the Lambot was the first small boat made of ferrocement in 1849. Incredibly, this boat was still afloat in 1949, that is to say a hundred years after it was constructed. Since then, it has been exhibited in the Brignoles Museum. Pier Luigi Nervi, who considered ferrocement as a boatbuilding material, started using it in Italy during the last century. He also built ships, due to the flexibility and exceptional resistance of this material. One of these ships called "Irene", had 165 ton of displacement and her hull had 1.38 inch (35 mm) of thickness. According to Nervi, the ship was 5% lighter and 40% cheaper than a similar hull made of wood. Nervi also builds a shed with a span of 321.64 feet (98 m) for the Turin Exhibition in 1949. This shed had 1.58 inch (40 mm) thickness ferrocement prefabricated ribbed assembled shells.

Properties of Ferrocement

         The engineering properties of ferrocement structure are equivalent to normal concrete, and in some applications it performs better. The tensile strength of ferrocement is a result of the volume of reinforcement used in the structure. Apart from the volume of reinforcement, the direction of its use in line with the force direction and tensile stress direction is also important. The tensile performance of the ferrocement concrete or structure can be grouped into three, namely, the pre cracking phase, post cracking phase, and finally the post yielding phase. A ferrocement member subjected to upwards tensile stress behaves something like linear elastic material until the first crack appears. Beyond this, the member will enter the multiple cracking and eventually continuing to a point where the mesh starts to experience yielding. Once at this stage the number of cracks will continue to grow with the increase in the tensile force or stress. The specific surface area of ferrocement member or element has been found to influence the first crack in tension, as well as the width of the cracks. The maximum stress at first crack for ferrocement matrix increases in proportion to the specific area of the element. The behavior of ferrocement element under compression mainly depended on mix design properties.

The well distributed and aligned reinforcement has made the ferrocement to behave like steel plates. Ferrocement is also has other outstanding properties besides its engineering properties compared to normal concrete. Ferrocement exhibits a very easy mold-ability characteristic, that it can be used to produce any desired shape of structure. Besides that due to superior tensile behavior and water tightness, the material is widely used for lightweight construction and water tight structure as well as for potable structure. Some of the successful application of ferrocement includes boat, sampan, pipes, shell roofs, wind tunnel, modular housing, sandwich pools, permanent form of concrete structure etc.

  Application of Ferrocement 

Ferrocement is applied in many parts of a structure. Applications of ferrocement as various part of structure are discussed below:

a) Ferrocement in Slabs:

•  For structures protection, reinforced concrete is commonly used. However, concrete structures subjected to dynamic loads due to explosion will respond differently than statically loaded structure and the structure now is subjected to a combination of blast and fragments loads. During blast and fragment impacts form small charges, the structure will shake, vibrate, severe crushing of concrete occurs and crater forms in the front of the concrete. On the other hand, if a large charge is used, large penetration will occur and will result in scabbing to the back side of the wall or perforation with risk of injury for people inside the structure.
• The use of the Ferrocement as a reinforcement to concrete slabs enhanced the perforation resistance and reduce the heat transfer through the thinner thickness of the steel mesh reinforced cement matrix.
•  Ferrocement slabs, fire tested, with 2 meshes lays showed less resistance to penetration compared to non-fire tested slabs. Increasing the volume fraction might cancel that observation. 
• The ferrocement layers can resist fire and crack propagation at 700°C for 30 minutes. Increasing in volume fraction increased the fire resistance effect slabs.
• In general, the ferrocement layers showed good stiffness, ductility and impact resistance. The impact resistance of the ferrocement was improved with higher ratio of volume fraction. The thickness of the slabs increases the absorbed energy. The impact resistance of ferrocement is improved with increase in number of meshes/layer. 
• Within the scope of this experimental investigation, there are two modes of failure observed (perforation and scabbing) the mode depends on the thickness of the slabs. Ferrocement would effectively resist the impact effects caused by a bullet impact. In perforation mode, increasing volume fraction does not significantly improve impact resistance. However, the spalling area is smaller compared to slabs with fewer numbers of meshes or without meshes. The advantages of using ferrocement can clearly be seen, when slabs fail in scabbing mode. It was observed that not only the missile perforation was prevented, but also the damage in front and in back face was reduced significantly. In scabbing mode, ferrocement layer catches the bullet and no concrete flies.

b) Ferrocement for Structural Beam Rehabilitation: 

• A large number of civil infrastructures around the world are in a state of serious deterioration today due to carbonation, chloride attack, etc. Moreover many civil structures are no longer considered safe due to increase load specifications in the design codes or due to overloading or due to under design of existing structures or due to lack of quality control. In order to maintain efficient serviceability, older structures must be repaired or strengthened so that they meet the same requirements demanded of the structures built today and in future. Ferrocement over the years have gained respect in terms of its superior performance and versatility, and now is being used not only in housing industry but its potentials are being continuously explored for its use in retro fitting and strengthening of damaged structural members. As per Ferrocement Model Code, ferrocement is a type of reinforced concrete commonly constructed of hydraulic cement mortar reinforced with closely spaced layers of relatively small wire diameter mesh. The mesh may be made of metallic or other suitable materials. Thinking ferrocement as a material to be applied to thin walled it is necessary to adjust the material properties to the construction type and acting forces in the structures, to obtain the proper strength, stiffness, cracking control, ductility and impact resistance. Water soluble polymers and aqueous polymer dispersions are often used to improve the properties of mortar. Polymer modified mortars are being used as a popular construction material because of their excellent performance. Polymer modified mortars are generally superior in the resistance to oxygen diffusion to unmodified mortar. Consequently the use of polymer-modified mortars as repairing and finishing materials can be recommended in order to inhibit the wet corrosion of reinforcing bars in concrete structures (Y.Ohamaet al. 1991). Even powdered polymer-modified mortars can be used in the same manner as those of aqueous polymer-modified mortar for practical application (Musarrat Ullah Khan Afridi et al. 1994). The water retention of the powdered and aqueous polymer-modified mortars increases with a rise in polymer-cement ratio, however the magnitude of the improvement depends upon the types of cement modifiers used, polymer-cement ratio or both (M U.K. Afridi et al.1995). W hen the cement mixtures are mixed with polymer, a large volume of air voids often forms. Jae-Ho Kim et al. 1997 established a technique involves pre-wetting the cement and sand with plain water before adding the polymer solution or dispersion. Another area of interest for possible future research would be to determine whether differences noted with the additives used still apply to mortars mixed to higher flows and what effect saturating specimens, initially dry cured has on the tensile bond strength. W et cured polymer mortars appear to have lower bond strengths than dry cured polymer mortars, indicating that the curing method has much to do with the strength gain of polymer-modified mortars (James Colville et al. 1999). During the hardening of cement, polymer can fill into the micro cracks, pores and cracks (J.M.Gao et al. 2002). Also it has been noticed that with increase in the addition of polymer the water absorption decreases remarkably when polymer cement ratio is small. But when polymer cement ratio exceeds 10% the change may become unnoticeable (Ke–Ru W u, Dong Zhang et al. 2002). Besides the studies to meet the requirements of the applications, attention is paid to the mechanism and means of polymer modification. Furthermore, this technique becomes particularly attractive for flexural strengthening in the negative moment regions of beams where external reinforcement would be subjected to mechanical and environmental damage and would require protective cover. Ferrocement laminates with skeletal bar can take significant role in strengthening reinforced concrete beams. For flexural strengthening, the ferrocement laminates were cast onto the soffits (tension face) of the beams without any change in width of the beams. As this technology emerges, the structural behavior of RC elements strengthened with polymer modified ferrocement laminates needs to be fully characterized. 
• The variations in compressive strength, split tensile strength and tensile strength of polymer modified mortar among mesh reinforcement with different volume fractions through the investigations of D. Rajkumar1 and B. Vidivelli2( 1Senior Lecturer, 2Professor, Department of Civil and Structural Engineering, Annamalai University, Annamalainagar – 608 002, Tamilnadu, India) facilitate for rehabilitation of damaged RC beams and on the analysis of the test results leads to the following conclusions: Better strength can be achieved by adding up 15% of SBR through 5% volume fraction of mesh reinforcement in the polymer ferrocement specimens. The flexural and compressive strength of polymer modified mortars are generally improved over unmodified mortar. While noticing the tensile strength establishes the fact that all the chosen volume fractions of mesh reinforcement hold best results. From this it is clear that, the strength properties are based on the provision of percentage of polymer, volume fraction and the arrangement of reinforcement. Youngs modulus under tension is greater than the value of Young’s modulus under compression. Polymer modified ferrocement with 5% volume fraction of reinforcement was higher in its Young’s modulus under compression and 6.43% volume fraction of reinforcement being higher under tension. Hence mortar contributes in its towards the cracking stage and the steel towards the multiple cracking and ultimate stage. Based on the other properties of ferrocement it has been concluded that it is a low cost and good material to restoring the load carrying capacity of the member. Hence it can be used as a rehabilitation material especially for beams without skeletal steel. Result from the test program shows that by incorporation of polymers, the mesh reinforcement with volume fraction 5% appropriate for compressive as well as flexural members and 6.43% precise for tensile members. The performance of the strengthened beams was compared to the control beams with respect to cracking, deflection and ultimate strength. The result show that all the strengthened beams exhibited higher ultimate capacity. A decreased in the volume fraction of reinforcement of the polymer modified ferrocement laminates from 5% to 3.55% resulted in a reduction in strength. The presence of these laminates has an inhibiting effect on the tensile cracks so as the crack spacing and crack width were reduced after strengthening. The deflections, the rebar strains and the crack width in the rehabilitated beams have reduced significantly compared to the perfect beams. Rehabilitated beams exhibit an increase of 85% in its overall performance compared to the perfect beams.

c) Ferrocement in Water Tank: 

• Ferrocement tanks are lighter in weight (wall thickness of 500 liter is one cm) and higher in strength as compared to reinforced concrete tanks. It involves minimum skill, lesser project cost for fabrication, maximum utility and service-ability. Ferrocement tanks can replace costly steel/mild steel tanks. As compared to PVC and steel tanks, ferrocement tanks are Eco-friendly and the quality of these tanks is that here the water gets less heated in summer. 
• Ferrocement tank have diversified applications in addition to water tanks such as grain storage silos, septic tanks, animal feed, cup-boards, boundary wall, door shutters, segmented roofing sheets, man-hole covers (light, medium and high duty), irrigation channels, ferrocement earth quake resistant houses, flower pots etc. All these products can be fabricated in the same unit with the use of different molds.
•  While tanks up to 1000 L capacity could be fabricated at factory premises higher capacity tank could be fabricated casting the cylindrical surface in four segments which are later joined together with base in situ. These segments are transported in trucks carrying the material for large number of tanks.
•  Water tanks of capacity ranging from 200 L to 10,000 L are in great demand for storage of water due to the problem of water scarcity. These are being used in residential houses, governmental organizations such as housing boards, slum clearance boards, Public Health Engineering Departments, municipal corporations, rural community water supply etc. CSIR Labs have transferred this technology to more than 150 parties and ferrocement tanks are in use in various parts of the country. 

Conclusion:

                  Ferrocement is a labor intensive and a material saving technique has never been able to compete with reinforced cement concrete. However, innovative structures in different parts of the world have clearly indicated the unique, unmatched properties of this material and therefore the vast potential waiting to be explored. These special structures in the past include aircraft hangars & the famous Turin Hall built by Nervi. More recently the laminated Ferrocement technique developed by Martin Iorns by spraying an engineered mortar on layers of mesh holds a great promise.  This has been demonstrated in various offshore structures, bringing down labor costs and improving the Ferrocement matrix.


tags: ferocement, fero, camant,fro, cemnt.