07_chapter2.pdf

CHAPTER2 REVIEW OF LITERATURE 2.1 GENERAL Ferrocement is a highly versatile reinforced composite material made of cement mortar and layers of wire mesh, closely bound together to create a stiff structural form which posses high strength to weight ratio. According to the definition given by ACI committee 549 in its state-of-the-art report on Ferrocement (ACI 549 R-97, 1997) is a type of "thin walled reinforced concrete commonly constructed of hydraulic cement mortar reinforced with closely spaced layers of continuous and relatively small diameter wire mesh". Because of its excellent strength, cracking resistance and impact resistance, the ferrocement has been used for housing units, as roofing or flooring elements, construction of boats, water tanks, marine structures and grain silos etc. The strength is mostly derived from curvilinear and undulating shape hence it can span long distance with reduced number of costly supports. A ferrocement construction unlike other sophisticated construction requires minimum number of skilled labours and utilizes readily available local materials. Proper attention should be required to control the quality of construction; otherwise the purposes of thin shell ferrocement construction will be upset. To exploit the potential of ferrocement as a construction material, a proper understanding of material behaviour under different conditions is essential (Paramasivam et al., 2004). The most important advantages of ferrocement are that it can be fabricated into almost any desired shape to meet the need of the user. It is being extensively used in developing countries like India, Indonesia and Srilanka for (a) Housing applications (b) Marine application (c) Agricultural application (d) Rural energy application (e) Water and

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Sanitation application and (f) Repair and maintenance applications (Mansur et al. 1987, Paramasivam et al. 1990, Ganesan , 1994 ). For highly stressed structures like boats, barrages etc, steel rods along with the wire mesh is considered as a component of the reinforcement, imparting structural strength and stiffness whereas in most of the terrestrial structures wire mesh is treated as the main reinforcement. The reinforcement network should be securely welded or fastened together, so that it remains in its original position during the application of mortar ( ACI 549R-97 1997, Swamy 1984 ). In highly reinforced structures, the arrangements of the steel rod and mesh should be in such a manner that to allow adequate penetration of mortar so as to resulting a void free dense material. To obtain a good quality hardened mortar, the placing and compacting of the mortar must be followed by proper curing in a suitable environment during the early stages of hardening (Andal et al., 2003). The objective of curing is to keep the mortar saturated until the originally water filled space in the fresh cement paste has been filled to the desired extent by the products of hydration of cement. Ferrocement is ideally suited for thin-walled structures. In addition to this many other uses of ferrocement as a structural material are being explored throughout the world. These includes sunscreens and sandwich wall panels for high rise buildings (Mansur 1987, Paramasivam 1990) permanent forms for conventional concrete construction, biogas digesters, floor decks, swimming pools, water towers, small deck bridges, culverts and ferrocement encloser for geotechnical centrifuge. Recently ferrocement has been found to be extensively used in repair and maintenance of building (Romualdi 1987, lorns 1987). 2.2 COMPARISON WITH REINFORCED CONCRETE

The major differences between the ferrocement and the conventional reinforced concrete structural elements can be enumerated as follows (Swamy, 1984 ) . 6

a. Ferrocement elements are normally thin; thickness is rarely exceeding 25mm. But conventional reinforced concrete structures are relatively thick sections exceeding 100mm. b. Ferrocement normally contains a greater percentage of reinforcement. The reinforcements consist of large amount of small diameter wires or wire meshes discretely placed instead of reinforcing bars in reinforced concrete structures. c. Ferrocement matrix mainly consists of Portland cement and sand as mortar instead of concrete, which contains coarse aggregate. d. In terms of structural behaviour, ferrocement exhibits very high tensile strength to weight ratio and superior cracking performance than reinforced concrete structures. e. For the construction of ferrocement structures, formwork is often not needed. This permits economical construction of certain structures such as domes, wind tunnels etc.

2.3 ADVANTAGES OF FERROCEMENT Ferrocement is a suitable technology for developing countries due to the following reasons: I. Ferrocement structures are thin and light. 2

It requires little or no form work.

3

Its raw materials are readily available in most countries.

4

It can be fabricated to any desired shape.

5

The skills for ferrocement construction can be acquired easily.

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Heavy plants and machinery are not involved in construction.

7

It is relatively inexpensive.

2.4 TERMINOLOGY USED IN REINFORCEMENT

Two important reinforcing parameters commonly used in charecterising ferrocement are (i) Volume fraction of reinforcement and (ii) Specific surface of the reinforcement (ACI

549R-97 1997 , Ferrocement model code 200 I). 7

(a) Volume fraction of reinforcement: VR It is the total volume of reinforcement per unit volume of ferrocement. For a ferrocement reinforced with square meshes, VR is equally divided into VRL and V RT for the longitudinal and transverse directions respectively. Vn = V reinforcement

V composite

-------------- (2.1)

The volume fraction of reinforcement can be divided into longitudinal and transverse parts, that is Vn = V RL + V RT

-------------- (2.2)

(b) Specific surface of reinforcement: SR It is the total bonded area of reinforcement (interface area) per unit volume of composite. For a ferrocement unit square mesh, SR is divided into SRL and SRT in the longitudinal and transverse directions respectively. Sn = Total surface area of reinforcement

-------------- (2.3)

Volume of composite Sn can be divided into longitudinal and transverse components, that is Sn =SRL + Snr

-------------- (2.4)

The relation between SR and VR , when wire mesh are used in (ACI 549R-97, 1997). SR

4V R , Where

The total volume fraction of reinforcement VR in each direction should not be Jess than 1.8 percent (Ferrocement model code, 2001 ). The total specific surface of reinforcement SR in both directions should not be less than 0.08 mm2/mm3 . About 2 times these values are recommended for water retaining structures. In computing the specific surface of the reinforcement, any skeletal steel may be disregarded, but it should be considered in computing VR (Ferrocement model code, 2001 ). 8

2.5 FERROCEMENT FLEXURAL ELEMENTS The flexural behaviour and properties of ferrocement have been extensively investigated by number of researchers. Ferrocement roofs, floor slabs, beams etc. are gaining importance now a day. In these cases ferrocement element is subjected to flexural action. When compared to RCC element the tensile strength to weight ratio of ferrocement is large. The ultimate moment in the case of R.C.C is a property of the section since the matrix in the tensile zone of the bending element is fully cracked and do not contribute to the resistance at the ultimate stage, where as in the case of ferrocement, it is not so. 2.6 REVIEW OF STUDIES ON FERROCEMENT FLEXURAL ELEMENTS A large number of experimental and analytical studies have been reported in this area. The strength of flexural members can be predicted by analysing ferrocement as a reinforced concrete member using the ACI Building Code (ACI 318-08, 2008 ). Different analytical methods proposed by the researchers to predict the ultimate moment carrying capacity of ferrocement gives satisfactory correlation with experimental values. The design requirements of ferrocement includes volume fraction, specific surface, percentage of reinforcement, orientation of reinforcement and the severability requirements such as deflection and crack width (Balaguru et al., 1977 ). Expanded metal and square welded mesh in their normal orientations are most effective in flexure than any other types of reinforcement. But hexagonal mesh is particularly suited to complex doubly-curved section, square mesh to single-curved sections and expanded metal to planar sections. The maximum crack width in tension and flexural members for square mesh reinforcement can be predicted by using the approaches described in reference (Mansur, 1988). The research work carried out in the field of flexural behaviour of ferrocement elements in Asia, Europe and USA are discussed here in the following heads.

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2.6.1 STUDIES FROM ASIA In the early 1940' s Pier Luigi Nervi (ACI 549R-97, 1997) resurrected the original ferrocement concept. After the Il0d World War, Nervi demonstrated the utility of ferrocement as a boat-building material. The International Ferrocement Information Center (IFIC) at the Asian Institute of Technology was established in Bangkok, Thailand in 1976. Now ferrocement is very extensively used in the production of structural elements in repair and maintenance work (Romualdi , 1987 ). Mansur and Paramasivam (1986) conducted an experimental and analytical investigation on flexural behaviour of ferrocement element to study the cracking behaviour and ultimate strength. Based on the concept of plastic analysis a method is proposed to predict the ultimate moment capacity of ferrocement and which is in good agreement with the experimental values. From the study it was established that the first crack moment and ultimate moment were increased with increase in matrix grade and volume fraction of steel. They also concluded that higher volume fraction of steel provides more effective control of crack width. Walkus (1986) made an attempt to determine the parameter to perform the testing of material of ferrocement, new laws for defining the properties of the structure and equipments for testing and measuring devices to determine the deformation and cracking. Atsushi Shirai and Yoshihiko Obama (1988) conducted an experimental study to check the improvement in flexural behaviour and impact resistance of ferrocement by the use of polymer. From their study, they found that first crack load, ultimate load and cracking resistance were increased and the occurrences of cracks and increase in number of cracks were considerably restrained by the addition of polymers. Impact resistance is also increased by the use of polymer modified mortar.

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Karunakar Rao and Jagannadha Rao (1988) proposed a theory for the computations of ultimate moment of ferrocement structural elements based on the experimental evidence for the crack patterns, extent and propagation of cracks. Desayi et al. (1988) proposed a bi-linear method to predict the deflections and another two methods for crack formation and ultimate moments of ferrocement elements. These methods are found to give satisfactory agreement with test data. Ganesan and Suresh Kumar (1988) carried out an experimental investigation to study the effect of discrete short steel fibres on the strength and behaviour of ferrocement structural elements of channel cross-section. The results indicated that the addition of steel fibres increases the strength and energy absorption capacity of ferrocement elements. Desai and Desai (1988) developed and tested ferrocement roofing elements of different shapes, in order to check the suitability and load carrying capacity for low cost housing. The study concluded that out of all the shapes considered, folded shaped ferrocement elements exhibited higher stiffness and ultimate moment carrying capacity. Also it was proved that the cost of ferrocement folded plate element is about 15 to 25% less than that of the asbestos cements sheets. Lohtia et al. (1988) conducted an experimental study on flexural behaviour of ferrocement slabs. They observed that cracking pattern for ferrocement slab is quite different from that of RCC slabs. In ferrocement slabs the cracks are numerous and much more fine and distributed over larger area than in the case of RCC slabs. Also it was found that ferrocement slabs have greater extensibility and higher reserve strength than RCC slabs. Rao and Rao (1988) carried out an investigation to evolve an acceptable and rational method for computing the ultimate moment of resistance of ferrocement channel units. Their study indicated that the analysis of crack pattern and deflections gave an insight 11

into the mechanics of the mobilization of moment of resistance of the ferrocement channel unit. Ganesan et al. (1988) conducted an experimental study on development and proof testing of a ferrocement roofing system. The development and proof testing of a composite roof I floor system consists of partially prefabricated ferrocement trough units and in situ concrete topping. The tests were intended mainly to study the flexural behaviour of partially prefabricated ferrocement trough unit as an individual element and also as a composite element with in-site concrete. The results showed that the structural behaviour of the ferrocement trough units as well as the composite units was found to be quite satisfactory. Naaman (1989) conducted a study on the different levels of technology used in ferrocement housing products. The study suggested that common housing requirements could be satisfied from a pool of about fifteen standard panel configurations. Box shaped panels were considered for the walls and lintels, while U shaped panels were considered for flooring and roofing. Vijay Raj (1990) conducted an experimental and theoretical study to predict the structural behaviour of large span bamboo ferrocement elements for flooring and roofing purposes. The study concluded that the large span bamboo ferrocement slab should have a thickness of 40mm to meet the requirements of roofing and flooring elements of reinforced buildings. Also they concluded that the cost of bamboo ferrocement slab was only 70% of the ferrocement slab. Mathews et al. (1991) carried out an analytical and experimental investigation of cracking load, ultimate load, deflection, crack spacing and crack width of a hollow ferrocement roofing system. The test results confirmed that the system has adequate strength, stiffness and other serviceability requirements for residential applications. The predicted theoretical values are in good agreement with experimental values.

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Desayi et al. (1992) carried out an experimental investigation to study the first crack strength and modulus of rupture of light weight fibre reinforced ferrocement in flexure. From the study they found that volume fraction of steel fibre control the modulus of rupture and first crack strength of ferrocement elements. They also developed an equation to predict the first crack strength and modulus of rupture. Wail AI-Rifaie and Arsalan Hassan (1994) carried out an experimental and theoretical study on the behaviour of ferrocement one-way bending elements. Elements of different spans and widths were chosen to study their relative feasibility for adoption to roofing of small size residential houses. As the result of investigation it was concluded that one-way bending element undergo large deflection before failure and this is mainly attributed to the reinforcements. Also they found that, due to the increase in flange width, the ultimate load capacity is decreased but the ratio of ultimate load capacity to the first cracking load is increased due to the increase in span length. Mathews et al. (1994) conducted a study in planning aspects of ribbed ferrocement elements for low cost housing considering the principles of modular co-ordination. The study concluded that construction procedure can be made simple by the adoption of modular co-ordination and by the use of simple manufacturing plant; mass production for the elements can be easily achieved. Ganesan and Suresh Kumar (1994) had done an experimental investigation on prediction of moment capacity of fibrous ferrocement flexural members. The main objective of this study is to determine the effect of randomly oriented short discrete steel fibers on the strength and behaviour of ferrocement flexural elements. The results indicated that the addition of steel fibres increases the moment capacity of ferrocement flexural elements significantly. A method for the prediction of the moment capacity of the element was proposed in this study.

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Hossain et al. (1997) conducted a study to develop an analytical model for flexural and tensile behaviour of ferrocement plates in the pre and post cracking stages. From the study they established that two-way ferrocement slabs can take 10%-30% more loads than that of the one way slabs. Mansur et al. (2000) carried out at flexural test on thin walled ferrocement structural T, inverted T and symmetrical I-section to facilitate rapid assessment of flexural strength and expedite the design charts for structural section. They developed typical design charts and indicated the existence of considerable ductility for thin wall pandas suggesting that a rigid plastic analysis should be applicable to predict their ultimate moment capacities. AI-Kubaisy and Mohd Zamin Jumaat (2000) has carried out at a study on flexural behaviour of reinforced concrete slabs with ferrocement tension zone cover. The variances are percentage of wire mesh reinforcement in the ferrocement cover layer, thickness of the ferrocement layer and the type of connection between the ferrocement layer and the RCC slab. The results showed that the use of ferrocement cover slightly increases the ultimate flexural loads and first crack loads and reduces the crack width and spacing's. Seshu (2000) conducted an experimental study on ferrocement confined reinforced concrete beams. Their study concluded that confinement by ferrocement shell improve the moment carrying capacity by about 9 to 15% and also increases the first cracking moment. Rathish kumar and Rao (2000) carried out an experimental investigation to study the stress-strain behaviour of ferrocement confined reinforced concrete (FCRC) under axial compression by varying the specific surface factor and confinement index. Study concluded that improvement in ductility is proportional to the specific surface factor of ferrocement for a given confinement index.

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Mansur et al. (2001) conducted an investigation to find the shear strength of ferrocement structural sections. The study concluded that the flexural cracks occurred initially irrespective of the span/depth ratio but first cracking and ultimate load were decreased when the span/depth ratio was increased. Imam et al. (2002) undertaken a study to simulate deflection and stress behaviour of different type of roofing elements by finite element techniques. From the investigation they established that the numerical results are matching quite well with the experimental results. The principal stresses are less in the segmental shell element and this is most economical shape as a roofing element. Vijaya and Hedge (2003) conducted an experimental study on ferrocement confinement of concrete beams made of brickbat and recycled concrete aggregate. Their study proved that there was a significant increase in stiffness, strength and ductility of the beam due to ferrocement confinement. Andal et al. (2003) carried out a study on the flexural strength and impact resistance of ferrocement specimens cured under seawater and ordinary water. They found that due to the penetration of seawater in ferrocement elements, the sea water cured specimens showed a reduced strength in flexure and impact. Jaganathan et al. (2003) studied the suitability of polymer mesh as an alternative for reinforcement in ferrocement flexural elements. The results indicated that the four layered polymeric mesh reinforcement ferrocement element satisfy the IS requirements such as lower deflection, higher young's modulus, comparable flexural strength and reduction in crack width with increase in number of layers of mesh. Veerappa Reddy (2003) conducted a study on industrialised production of innovative ferrocement element. Study describes the industrial production of ferrocement elements such as sumps and septic tanks, precast modular community toilets and pretensioned undulated length sections for long spans. 15

Ramesh et al. (2003) conducted an experimental investigation on the behaviour of Hybrid Ferro-Fibre concrete under axial compression. Parameters varied are specific surface factor, reinforcing index of the fibre reinforced concrete. The results indicated that the combined use offerrocement and fibres has improved the ultimate strength, strain at ultimate strength and the ductility of reinforced concrete. The improvement is proportional to the specific surface ofreinforcement. Jaganathan and Sundararajan (2004) carried out an experimental and theoretical investigation to study the flexural behaviour of ferrocement slabs reinforced with 3-5 layers of polymer mesh as an alternative form of reinforcement. From the study it was observed that polymer mesh reinforced ferrocement slabs exhibit the same linear elastic behaviour up to the first crack load. Paramasivam et al. (2004) conducted a detailed investigation on the origin, suitability and application of ferrocement composite. The authors' discussed about the R&D works done on ferrocement element and its applications such as sunscreens, secondary roofing slabs, water tanks and repair material in building industries. The salient features of the design, construction and performance of some of these applications of ferrocement structural elements are highlighted. Bhaskar Desai et al (2004) conducted an experimental investigation on the flexural bahaviour of superplasticised partially cement replaced silica fume ferrocement elements with shear span to depth ratio and with number of mesh layers as variables. The study concluded that 10% ofsilica fume can be taken as the optimum dosage to replace cement for giving maximum possible compressive strength, split tensile strength and modulus of elasticity. Also found that the ultimate flexural strength increases with the increase in % of silica fume up to 10% for a given shear span/ depth (a/0) ratio and a given number of wire mesh layers.

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Hago et al. (2005) conducted an experimental study on ultimate and service behaviour of ferrocement roof slab panels. The study aimed at to determine the ultimate and service behaviour of ferrocement roof slab panels. The results showed that the use of monolithic shallow edge ferrocement beams with the panels considerably improves the service and ultimate behaviour of the panels irrespective of the number of steel fibres. Andal et al. (2005) carried out an experimental study on behaviour of ferrocement flexural members with polymer modified mortar. The main objective of this study was to determine the flexural strength of ferrocement element of size I 000mm x 200mm x 25mm with cement sand mortar 1: 1 and water cement ratio as 0.3. The variances used in the study are different volume fraction of reinforcement, different percentage of SBR polymer and different percentages of the Recrone 3S fibres. This study showed that 12.5% of SBR latex by weight of cement and a volume fraction of 3% gave the higher collapse load. Prem Pal Bansal et al. (2006) conducted an experimental study on effect of different bonding agents on strength of retrofitted beams using ferrocement laminates. Study done to determining the effect of retrofitting of beams using ferrocement laminates bonded with cement slurry, epoxy and shear connectors. The study concluded that, the third point loading on all the specimens showed reduced crack width, increased crack spacing, large deflection at ultimate load and a significant increase in ductility ratio. Prakash and Patil (2007) conducted an experimental study on effect of addition of silica fume on strength characteristics of fibrous ferrocement using round steel fibres. Study indicates that fibrous ferrocement is a combination of ferrocement and fibre reinforced concrete, shows better improvement in the mechanical properties such as toughness and impact resistance. This composite also shows higher compressive, tensile and impact strength.

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Anila kumar et al. ( 2007) carried out an experimental investigation to study the effect of addition of SBR polymer in different proportion to polypropylene fibre reinforced concrete. The results indicated that the addition of SBR polymer to the fibre required concrete increases the workability and increases the compressive strength, tensile strength and flexural strength. Prakash and Patil (2007) carried out an investigation on the effect of sustained temperatures on the strength properties of fibrous ferrocement containing steel fibres. The study concluded that the compressive strength, flexural strength and impact strength of fibrous ferrocement can be enhanced either by increasing percentage of steel fibres or by increasing specific surface area of welded mesh and chicken mesh in sustained tern perature. Sudhikumar and Prakash (2007) conducted a test to find the compressive strength, flexural strength and impact strength with slurry infiltrated fibrous ferrocement with superplasticiser. The result of the study indicated that there is increase in compressive, flexural and impact strength with increase in the partial replacement of metakaolin with increasing temperature. Patil and Prakash (2007) made an attempt to study the effect of replacement of cement by fly ash on the strength characteristics such as compressive strength, flexural strength and impact strength of fibrous ferrocement using flat steel fibers. The percentage of flat steel fibres were varied from 0% to 2% with the increment of 0.5%. The cement was replaced by fly ash in different percentages like 5%, 10%, 15%, 20% and 30% etc. Prem Pal Bansal et al (2007 ) conducted a study on shear deficient RC beam initially stressed to a prefixed percentage of the safe load, are retrofitted using ferrocement. To increase the strength of beam in both shear and flexure, the wire mesh is placed at an angle 45° to the longitudinal axis of the beam. From the study it was concluded that the safe load carrying capacity of rectangular RC elements retrofitted with ferrocement laminate is significantly increased with mesh oriented at 45° . 18

Patil and Prakash (2007) carried out an experimental investigation in effect of addition of silica fume on strength characteristics' of fibrous ferrocement using round steel fibres. Study concluded that the addition of round steel fibres in ferrocement composite shows better improvement in some of mechanical properties, such as toughness and impact resistance. Laigude Atual et al. (2007) conducted a study of micro structural analysis with different combination of wire mesh reinforcements. Panels' of different combinations were tested for flexure under uniformly distributed load and its deflection and cracking patterns were observed and studied. 2.6.2 STUDIES FROM EUROPE The use of ferrocement as roofing for large span has been successfully used in many European and South American countries. Large ferrocement roofs have been constructed in Italy. Onet and Magureanu (1993) made an attempt to study the flexural behaviour of ferrocement beams under long term loading. Their results indicated that the long-term deflection influences the behaviour of beams much more than the instantaneous one. Mattone (1995) conducted a study on design and testing of a ferrocement roofing element. This study proposed operational stages for the development of a simple roofing element and equipment necessary for the small-scale production. Gurdev singh and Guang Jing Xiong (1995) performed a study on rational assessment of flexural fatigue characteristics of ferrocement for reliable design. Study based on a stress-life (S-N) plot and a new method proposed based on a probability -stress-life (P-S­ N) for the design of the flexural fatigue characteristics of ferrocement. Study concluded that a rectangular stress distribution is relatively more reliable and economical for

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predicting stress when designing ferrocement against fatigue by usmg the P-S-N relationship of wire tested in the air. Ramesht and Vickridge (1996) made an attempt to develop a computer program FAOFERRS to predict the ultimate moment of ferrocement under flexure. The study proved that the FAOFERRS program has in good agreement with the experimental values and it is easy to use. Nedwell and Nakassa (1999) carried out an experimental investigation into high performance ferrocement. The results showed that stainless steel and silica fume improve the first crack load, increase the number of cracks and decrease the crack width m addition to the cost effectiveness. Pankaj et al. ( 2007) carried out a study on mechanical behaviour of ferrocement composite- numerical simulation. Authors proposed an anisotropic elastoplastic models to simulate the mechanical behaviour of ferrocement plates. The study indicates that the mortar ferrocement layered model with orthotropic ferrocement layers performs the best. Also determined that, a single set of material properties can be used to simulate the behaviour of ferrocement plates under in-plane as well as out of plane loading. 2.6.3 STUDIES FROM USA Naaman and Homrich (1986) proposed a general methodology for the analysis and design of ferrocement flexural elements. The proposed method and the developed design charts are very simple and could be used to predict the flexural resistance of ferrocement element. Balaguru et al (1990) proposed an analytical model to study the ductility of ferrocement flexural element. The study concluded that the type of reinforcement distribution and reinforcement ratio affects the ductility only to a small extent but ductility increases for

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thin section. The literature shows that ferrocement flexural elements of different shapes can be very effectively used as roofing I flooring element in developing countries. Hani et al. (2004) have done an experimental and analytical investigation on ferrocement concrete composite beams made of reinforced concrete overlaid on a thin section of ferrocement. Various types of beam specimens with different types of meshes are tested under a two point loading system up to failure. Study concluded that the proposed composite beam has good ductility, cracking strength and ultimate capacity. In this thesis an attempt was made to obtain an optimum ferrocement sections. So a brief review on optimisation techniques was done. A literature review of studies done on optimisation of ferrocement elements and reinforced cement concrete elements are given below: Many researches have been carried out in various optimisation techniques to optimise the cost of construction of different shaped ferrocement elements and for the design of reinforced concrete structures (Rajeev et al., 1998). Goldberg (1989) has given an introduction to Genetic Algorithm (GA) approach for engineering optimisation. This has significant applications in structural optimisation problems. Many investigators made use of this approach for optimising the shape and cross-section of structural elements. Rajeev and Krishnamoorthy (1992) extended the application of GA into discrete design variables for optimising the minimum weight of steel trusses, considering the deflection and buckling strength as constraints. Syam Prakash, Rajeev and Mathews (1995) carried out an analytical study to obtain an optimal design methodology for ribbed ferrocement roofing/flooring elements using GA. The variables considered were the cross sectional shape and the details of reinforcement.

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The study concluded that the GA based design optimisation methodology provides techniques for modelling in a realistic manner which leads to rational solution. Kalyanmoy Deb (2004) has published a book on an introduction to Genetic Algorithms. The author identified that a simple genetic algorithm is an optimisation technique that relies on parallels with nature, and a simple analogy can be made with a mathematical problem, made up of many parameters. These parameters can take the place of a chromosome in the mathematical analogy of a real chemical sequence. Author suggested the steps and the operations of the simple genetic algorithms. Govindraj and Ramasamy (2006) have published a paper on optimum design of reinforced concrete rectangular columns using Genetic Algorithms. Their study concluded that the optimum design model using GA provides an ideal technique to model practical design considerations such as predefined discrete variations in breadth and depth of column sections, detailing and placing of reinforcement bars. They have proposed a new optimisation technique which is less mathematically complex. Pranab Agarwal and Anne M. Raich (2006) carried out a study on design and optimisation of steel trusses using GA parallel computing and human computer interaction. Authors conducted study on a hybrid structural design and optimisation methodology that combines the strength of GA to evolve optimal truss systems. The application of GA to the design and optimisation of truss system supports conceptual design by facilitating the exploration of new design alternatives. Castilho and Lima (2007) conducted an analytical study on cost optimisation of lattice reinforced joist slabs using Genetic Algorithm with continuous variables. The results indicate that the GA method is a viable optimisation tool for solving cost optimisation problem for lattice reinforced joist slab.

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2.7 BEHAVIOUR OF FERROCEMENT UNDER FLEXURE The following are the assumption made in the flexural theory of ferrocement ( Paul and Pama 1978, Swamy 1984 ). Plane sections remain plane and perpendicular to the neutral axis, i.e. the strain in

I.

mortar and reinforcement is directly proportional to their distance from the neutral axis. The behaviour of reinforcement is elastic perfectly plastic i.e. for stress less than

2.

yield strength, steel stress is proportional to strain and after yielding stress in steel remains constant at the yield strength,J;,. Tensile strength of mortar is neglected in flexural strength calculation of cracked

3.

beams. 4.

Maximum usable compression fibre mortar strain is 0.003.

5.

For strength calculations at ultimate load, the parabolic stress-strain distribution of mortar can be approximated to a rectangular distribution.

The behaviour of ferrocement in flexure can be analysed using a typical load deflection plot. The load deflection curve can be divided into three regions or stages namely: (Paul and Pama 1978, Md. Zakaria et al. 1997 ) (a) Pre-cracking stage,( b). Post cracking stage and ( c) Post-yielding stage a. Pre Cracking stage Ferrocement has the highest stiffness in the pre-cracking stage. In this stage mortar contributes to both compressive and tensile resistance of the composite. The strength and stiffness of the beam can be calculated using the classical bending theory.

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b. Post Cracking Stage The post cracking stage starts with the occurrence of the first crack. This stage extends up to the point when the extreme fibre of reinforcement starts yielding. The load deflection behaviour in this stage of loading represents the behaviour of ferrocement in field conditions where almost all beams are cracked in the tension zone but the stress in the extreme tension fibre is well within the yield strength. The moment of resistance of the beam can be calculated by using the classical bending theory as in the case of pre-cracking stage. However, since the section is cracked, some modifications have to be made. This is because, after cracking, the tensile force contribution of mortar is negligible compared to the contribution of reinforcement.

c. Post yielding stage The post yielding stage corresponds to the stage when steel starts yielding and the cracks are in the process of widening. The section attains ultimate moment capacity at the end of this region. The ultimate tensile strain can be taken as the strain of the mesh reinforcement at ultimate condition. The compressive strain at ultimate condition may be taken as 0.006. The details of evaluation of cracking moment and equations relevant to flexure are given in Appendix.A.

2.8 COMMENTS ON REVIEW OF LITERATURE From the review of literature the following points are noted, (i)

Large number of studies on strength and behaviour of ferrocement elements subjected to tension, compression and flexure have been carried out in the past. 24

(ii)

In most of these studies smaller scale specimen models have been used in the investigation. Only some attempts on large scale or prototype elements have been considered. However it may be noted that in order to exhibit the realistic behaviour of the structural elements it is advisable to consider prototype structural elements.

(iii)

Attempts have been made to obtain a relation between strength parameters such as first crack load and ultimate load with variables like volume fraction and specific surface of ferrocement.

(iv)

In all the previous investigations the fine aggregate used for making ferocement mortar was ordinary river sand. However it may be noted that as river sand is becoming more and more scarce, alternative to· river sand to be explored for optimum cost of ferrocement elements.

(v)

In the past, attempts were made to obtain the overall dimensions of ferrocement flexural elements which are purely based on available laboratory facilities, strength and stiffness concepts.

(vi)

No attempts have been come across to obtain optimum cross sectional shape of ferrocement elements with regard to (a) Flexural strength (b) Deflection (c) Cracking and (d) Cost.

(vii)

The earlier attempts were restricted to studies on strength and behaviour of individual elements. However, investigations on the combined action of elements are not reported. It would be more meaningful if studies were done. on the strength and behaviour of roofing or flooring structure which consists of number of elements.

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(viii)

The matrix of ferrocement consists of cement, fine aggregate and water. No attempts were made to improve the properties of ordinary cement mortar by the special process like polymerisation, which will improve many of the engineering properties like tensile strength, fracture toughness and ducti Iity etc. of the ferrocement composite.

(ix)

In the earlier studies, the combined effects of optimum cross section and polymer modification on prototype ferrocemnet elements were not carried out.

From the above comments it may be noted that attempts on (a) Optimisation of ferrocement structural element (b) the effect of polymer modification on strength, stiffness, cracking behaviour and ductility of ferrocement elements (c) the combined effect of the above have not been carried out. Hence there is a gap in the existing knowledge which shows that there is a scope for research to fill the above gap.

2.9 SCOPE FOR THE PRESENT INVESTIGATION

The main objectives of the study are, (i)

To carryout the investigation, to obtain the optimum cross sectional shape of ferrocement flexural elements having the following cross sections, (a) Channel (b) Trapezoidal and (c) Corrugated. As these cross sections are often used in the building industries, this shape can be considered.

(ii)

To study the strength and behaviour of polymer modified optimum ferrocement structural elements.

(iii)

To evaluate models for predicting (a) First crack load (b) ultimate load (c) energy absorption capacity and (d) ductility.

(iv)

To study the combined effect of ferrocement channel elements and to obtain the number of connection bolts.

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2.10 METHODOLOGY The scope of this present investigation will be carried out as per the following methodology: (i)

To carryout the preliminary investigation to fix the optimum percentage of polymer content and fine aggregate to be used in the study.

(ii)

To carryout the optimisation techniques in Genetic Algorithm to fix the optimum cross section of ferrocement flexural elements.

(iii)

To carryout an experimental study on optimised prototype ferrocement elements under third point loading with the different volume fraction of mesh reinforcement and% of polymer content and

(iv)

To develop models mentioned in the scope based on the test results obtained in the experimental programme.

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