Effect of glass fibre on reinforced concrete beams with metakaoline subjected to cyclic loading. Illustrate your essay with specific examples.

In search of habitat humans have explored even remotest of places on earth due to sudden and rapid expansion of human civilizations. In this process of progressive expansion, man has made cutting edge advancement in technologies of construction and has mastered the art to counter nature’s wrath over the years, by learning from his mistakes. The major contributing sources to this space crunch have been globalization, high productivity and growth leading to expansion of civilization and high rise in population. This has also led to building of structures in seismic prone zones and hence it necessitated a set design strategy for developing safe building structures. It has been quite popular belief to construct building structures which are able to survive strongest of earthquakes. But the construction of these structures is not only difficult but is also not considered as a viable economic option. In practicality structures are build on the philosophy that they can withstand moderate ground motions without undergoing any kind of physical damage or under occasional moderate ground shaking they can not only prevent structural damage but also minimize any kind of non-structural damage. It must also be kept in mind that the structure should be able to avoid collapse or serious damage in case of rare major ground shaking.
Inertia forces are generated in form of seismic waves traveling in all directions resulting from ground movements these are also called Earthquake loads. These waves in turn create certain ‘demands’ or test the structures in terms of their strength, ductility and energy. Their magnitude is highly variable which depends on the seismic activity in the area and the dynamic characteristics of the building. The basic design principle, which any earthquake-resistant structure must satisfy in order that it does not collapse, is that the “Seismic demand” should be greater than the “Computed capacity”. Where, ‘Seismic demand’ is defined as the measure of effect on the structure due to dynamic loads generated by ground movement. The amount of resistance that a structure can offer without failure is called as ‘Computed capacity’. For design purposes ‘Design Demand’ is computed which is the maximum value of seismic demand. In order to balance between cost and safety, it becomes a tough decision for the designer to choose an appropriate design. (Rai, 2000) Many factors such as geographical conditions, availability of raw materials and labor, equipments, political considerations, total cost etc becomes a major factor in consideration.
The materials used in earthquake resistant construction plays a major role in deciding the durability of the structure. The material used in the structural members should have following attributes: (i) High Ductility so that it can resist large loads. (ii) High strength / weight ratio is kept in mind when deciding on materials for construction. (iii) The material used should be uniformly Homogenous throughout, (iv) The Orthotropy should be maintained (v) Ease in making full strength connection. (N.I.S.E.E )
With the advent of new technologies and varied geographical conditions there exist a large number of local design practices for constructing earthquake resistant structures. But in general some basic design practices are common and followed in all design codes. These are common practices that have evolved over the years and are being followed world wide. While designing any earthquake resistant structure it should be kept in mind that (a) Structure members should not be brittle or weak. They should be able to resist the loads and able to deform considerably as magnitude of earthquake loads is quite high. (b) Bracing and shear walls should be provided in the structure on each side. By the help of shear walls the load gets distributed and is uniformly transferred from structure to the ground. (c) To prevent separation of members during earthquake, all members should be tied together to form an integrated unit. This helps in transfer of loads across the structure. (d) The stiffness and strength of the entire building should be compatible with the stiffness and strength of the soil foundation. The soil foundation should be able to take the load of whole structure without failing when subjected to large earthquake loads. (e) Superstructure should have relatively shorter spans than non-seismic-resistant structure and avoid use of long cantilevers. (f) Proper care must be provided for choosing the materials that are to be provided in the structure. They should be of desired quality, free from corrosion and effect of natural agents like rain, water, sun and acids. If needed there should be a provision of treating them before using them for construction with adhesives to retain their strength for longer duration. (g) Care should be taken while selecting a foundation. It should be strong and wet soils should be avoided. The structure should be well tied and connected to foundation to dissipate energy during the shaking of ground. (h) The seismic activity of area where building is constructed also plays major role in deciding the suitable design. Hence a suitable Factor of safety can be considered. (i) For better earthquake resistant structures the building on a whole should be symmetrical. Any kind of asymmetry in design will lead to torsion during earthquake which can prove to be fatal. (j) Simple regular shapes should be preferred and in order to prevent hammering or pounding damages between blocks a certain amount of space should be provided. (k) During an earthquake, significant damage can result due to instability of the soil in the area affected by internal seismic waves. The soil property depends on the mechanical characteristics of the soil layers, the depth of the water table and the intensities and duration of the ground shaking. If the soil consists of deposits of loose granular materials it may be compacted by the ground vibrations induced by the earthquake, resulting in large settlement. Thus proper care should be taken while selecting the soil surface for foundation.
In 1858 A. Z. Wöhler by conducting experiments found out that if repeatedly a fix amount of nominal load is applied and removed to and from a part of a material (which is known as a “cyclic load”), then the part would be fractured after a certain number of loading-unloading cycles. He also found out that as he reduced the magnitude of the cyclic stress by decreasing the number of cycles, the earlier broken part will now be able to survive more cycles before breaking. This behavior pattern was known as “FATIGUE” as it can be implicated to the thought that the material got “wore out” (Epi-Eng website). The behavior pattern of concrete materials under reversal loads of such excessive sternness can be considered as a low-cycle fatigue phenomenon. Under such conditions the concrete might fail after several similar load applications even at those stress levels which are well below the point at which it would fail if it would have been applied in a single monotonically applied load test (Fang and Meyer, 1996). A structure which is subjected under repeated cyclic loads can lead to various areas failing under fatigue which must be accurately identified. As this is often very hard to identify in practice, analysis of a highly composite structure results in a high degree of uncertainty. Hence, generally it is resorted to experimental structural fatigue testing. The fatigue properties of reinforced concrete beams strengthened with glass and carbon fibers are also being investigated for this purpose. Earthquake loads can lead to deformation of various components of a Reinforced concrete structure in their inelastic zones. This extra energy developed in the structure is dissipated by the movement of base and pass on to ground (Barnes and Mays, 1999). Bertero and Popov, in 1975 suggested that the certain critical regions are formed in the members of structure which are the focal regions of these deformations. Hence there is a need to study of mechanical behavioral pattern of these members during ground movements. Study of stress and strain patterns for the materials is required for the finite element calculations of cyclically loaded structural parts which can be used to identify the parameters which are influencing the hysteric behavior of regions. (Filippou and Issa, 1988). Thus the knowledge of materials data under cyclic loading becomes increasingly important for designing and identification of material properties. These act as a comparison of material’s mechanical behavior and data can be used for the fatigue life estimation procedure known as the Local Strain Approach or Notch Strain Approach (Boller and Seeger, 1987).
lIn one of theories of reversed cyclic straining Masing suggested, (before the dislocations were observed) that the stress-strain curve which is measured from the point of reversal would be the cyclic curve scaled by a factor of 2 (Archuleta et al,1999). This is also called as Masing’s Hypothesis. The Stress-Strain response of most materials under cyclic loading is different than under single (monotonic) loading. But for fatigue analysis, it is necessary to consider the cyclic material behavior for strength and life calculations. Hence, fatigue in itself is a process of crack initiation and growth due to cyclic loading. This has been explained by the Bauchinger Effect which states that the material after being plastically strained in one direction in the stress-strain curve, the curve of the material in the opposite sense is lower as compared to the values in that cycle while it was in the previous direction. There are in general many types of fatigue loading. One of the types is called “zero-to-max-to zero”, in which a part which was not subjected to any load earlier is now acted upon by a load which is again removed later. Due to this exercise the part goes back to its no-load condition. Another type of loading is called a “varying load superimposed on a constant load”. An example of this could be the suspension wires in a railroad bridge. Here the wires are acted upon a constant static tensile load from the weight of the bridge, and also an additional tensile load when a train moves over it. Of all the cases the worst case of fatigue loading is known as “fully-reversing load”. In this type of loading a tensile stress of a known value is applied to an unloaded part and is released after that, after unloading a compressive stress of the same value is again applied and released. Fatigue failures generally start at the surface of the material and this could be because of mainly two factors i.e. in any solid material the most highly-stressed fibers are located at the surface (which can also be termed as bending fatigue) and there exists inter-granular flaws which are often found at surfaces resulting in tension failure.
Laboratory for Scientific Visual Analysis website states that the general reasons which can lead to a material failure are (a) Design deficiencies which are incorporated in design itself due to mistake in calculations of design engineers, (b) Manufacturing deficiencies which are sometimes introduced into the structure as geometrical or strain discontinuities, poor workmanship or improper manufacture techniques and material defects (c) Improper and insufficient maintenance, and Operational overstressing, (d) Environmental factors (example heat, corrosion etc.). Fatigue is also occurred due to corrosion. In this the corroded parts form pits which act like notches. It also minimizes the amount of material by corrosion which in turn reduces the strength and hence increases the actual stress in the fiber. One of other factors responsible for fatigue is called Decarburization, which is decrease in the carbon content from the surface of the material. The surfaces are most stressed due to bending and torsion effects. Loss in the carbon content makes the surface softer thereby reducing its resistance. The total additive effect of residual and design stresses are greater than the limit stress which was used in the original design calculations. Secondary stresses are not considered in the normal operating conditions and Fatigue failures.
Use of high strength concrete are generally preferred for construction of any High storey structures as it reduces the member size and self weight, also increases lateral stiffness and construction is comparatively easy. Ahn and Shin (2007) stated that many structural failures are occurred due to poor column resistance and can fail even in the inelastic strain region. In order to prevent the plastic hinging of columns which can lead to collapse of a building, it is advantageous to construct the earthquake resistant structure such that they fail due to yielding of beams thereby ensuring safety to the structure as a whole. The structure has to be tested up to the breaking point under earthquake response and it should be ensured that the models are computationally efficient to fairly predict the dynamic response of structures. Various experiments have been conducted to study the effect of cyclic loading on reinforced concrete structural sub-assemblages (Jirsa ,1977), on beams to columns connections (Durrani and Wight, 1982). The analysis of experimental results have stressed on importance of identifying the effect of earthquake loads on structure in critical regions. Generally safety and serviceability are the main design parameters which are kept in mind while designing Reinforced concrete (RC) structures (Kwak and Kim, 2002). Thus it becomes necessary to study cracks and deflection under reversal working loads acting on the structure. In many cases the inability to predict the true load results in the structure being stressed beyond its normal capabilities and structural limitations. Hence it becomes essential for the structural safety to determine the value of the ultimate load of structure with high precision.
Useful analytical results are obtained by conducting experiments in laboratories taking models which closely resemble the structure under consideration and real life conditions are created in the. Not only conducting experiments are quite consuming and costly but they also fail to predict accurately in situ interactions which are complex in nature. Thus there arise needs for analytical methods which can accurately predict the behavior of structure under dynamic loading conditions. Kwak and Kim, 2002 suggested that by studying the monotonic moment-curvature relation, the non-linear behavior of reinforced concrete (RC) beams can be stimulated. They proposed that for the structural members non-linear analyses with respect to materials used in the RC beams can be done using the basis of the proposed model but it was found out that the non linear behavior of RC beams can be efficaciously analysed using moment-curvature relation as it closely represents a beam element. The first of these mechanical models was introduced by Clough and Johnson who gave the concept of bilinear moment-curvature relation. One of the models which have been proposed for accurate study of RC beams subjected to cyclic loading take into account cyclic degradation (Chung et al,1988 cited Kwak and Kim, 2002). In recent developments trilinear, hysteric curve model (Roufaiel et al, 1987) and axial load effect (Assa Nishiyama, 1998) has also been proposed. But these models are unable to provide accurate results of structural behavior as they have shown limitations and there exists inherent deficiencies in the model itself, thus in there lies a lot of scope for research (Kwak and Kim, 2002).
Hasgur and Gunduz, (1996 cited in Gencoglu and Eren 2002) had defined ductility as the measure of the ability to absorb energy dissipated during earthquakes due to high amplitude and reversible deformations without losing their strength by reinforced concrete cross sections, elements and structures. Kurose et al. (1988) and Kitayama et al. (1991) have both studied the factors affecting the RC beam column joints and it was found out that shear strength and ductility of RC beam-column joints is increased as the compressive strength of concrete is increased. Pessiki (1990) and Otani et al (1985) have researched on the nature and detailing of joints when it undergoes reversal cyclic loads. It was noted that when large seismic forces act on RC frame structure it experiences large internal forces due to cyclic nature of earthquake loads due to which ductility of the structure depends on detailing of the reinforcements in the reinforced concrete. Also in various design codes use of closely spaced hoops has been advised in form of transverse reinforcement. Hence strength and ductility of reinforced structure is improved due to close spacing of hoop reinforcements.
Toughness of the concrete is defined as the area under the load-deflection (or stress-strain) curve. The compressive strength of concrete in general is high but it is weak in tension which can be increased by reinforcing it with bars or fibers which could otherwise be brittle. This type of concrete is called as Reinforced concrete or ferroconcrete. In this type of concrete steel resists tension and concrete provides resistance to compression. One of the types of reinforced concrete is called Fiber reinforced concrete in which short discrete fibers are uniformly distributed and randomly oriented (Brown et al, 2002). Fibers are made up of different materials and can be of steel fibers, glass fibers, synthetic fibers or natural fibers. From experiments it was found out that by adding fibers to the concrete it greatly increases the toughness of the material. Hence, the fiber-reinforced concrete is able to sustain load at deflections or strains much greater than those at which cracking first appears in the matrix. The property of each type of fiber is different from other and varies with fiber materials, diameter, specific gravity, young’s modulus, tensile strength, geometries, distribution, orientations and densities (Daniel and Anderson, 1998). When peak stress is reached in the high strength concrete there is sudden failure of explosive nature when subjected to earthquake loads. When fibers are added there is shown little effect on pre cracking pattern but post-cracking response is raised by some amount thereby leading to greater ductility and toughness (Rao and Prasad, 2004.).
For using fibers in concrete effectively it should be having following properties. (a) The modulus of elasticity should be high than the concrete. (b) The volume of fiber content must be according to requirement and in proper content. (c) The fiber length must be sufficient such that there is good bonding between fiber and matrix. (d) The aspect ratio must be high or the length of fiber should be large with respect to diameter. The fiber plays an important role in improving the post peak ductility performance, pre-crack tensile strength, fatigue strength, impact strength and eliminate temperature and shrinkage cracks (Ziad and Gregory, 1989). Paul et al (1994) stated that the strength is calculated as per maximum resistance when fiber reinforced concrete is subjected to compression, tensile, flexural and shear loads. From experiments it was found that the compressive strength and strain corresponding to peak stress increased marginally when compressive strengths ranging from 35 Mpa to 84 Mpa (Ezeldin and Balaguru, 1992). Member deflection, crack spacing and width are controlled by Interfacial bond characteristics (Tighiouart et al, 1998 and Cosenza et al, 1997). Katz (2000) stated that due to insufficient shear strength between reinforcing bar and surface there is high probability of bond failure. This may also occur due to longitudinal splitting of concrete near reinforced bars (Ye and Wu, 2000). Using experiments behavior of fiber under cyclic loads was analyzed. The Load-deflection pattern was obtained for constant amplitude fatigue loading and static loading. It was found that by adding steel fibers the flexural strength was increased by a considerable amount. The test results indicated that the static load-deflection curve was able to determine the deflection of fiber reinforced concrete specimen at failure state under constant stress range fatigue loading (Zhang and Henrik, 1998).
When glass is used a fiber for reinforcement then it is called as Glass Fiber Reinforced Concrete. the basic properties of Glass fiber Reinforced concrete are that the Glass fiber has high tensile strength of about 2 – 4 GPa and elastic modulus of about 70 – 80 GPa but it posses low brittle stress-strain characteristics (2.5 – 4.8% elongation at break) and also low creep at room temperature. But it has been found that the Glass-fiber products when exposed to outdoor environment have demonstrated in loss of strength and ductility. In 1960, it was found that when 16% zirconia was mixed in glass fiber the final product was Alkali-resistant. Since 1970 glass fiber reinforced concrete had found major usage commercially (Cement and Concrete Institute Website). The advantages of using glass fiber are (a) The mechanical strength of glass filament has higher specific resistance than steel thus it make a better composite material, (b) The organic matrices are suited to glass fiber and is readily able to combine with resins and cement mineral matrices, (c) The conductivity of fiber is low and hence it can save heat, (d) It has a good abrasion resistance, (e) They posses good bending strength and can come in thin strips easily owing to their size and shape, (f) It is an environmental friendly fiber as it can be recycled easily. There exist large types of fiber which can be used into concrete according to individual requirements. From studies it has been established that the matrix comprising cement mortar is essentially alkaline in nature due to presence of free lime which is also alkaline in nature. Hence, the fiber which is being chosen for reinforcement must be necessary alkaline resistant. Therefore, while choosing the fiber the given specifications must be kept in mind and its suitability must be judged on basis of its mechanical and chemical compatibility with the cement matrix. Concrete Materials Research at Columbia University states that nowadays textile reinforced fiber concrete is being used for numerous applications. Different ways are being developed to incorporate mesh reinforcement for thin glass concrete sheets, partitions walls etc.
cFrom experiments it was proved that though fiber and fiber volume controlled the post cracking behavior and ductility of the concrete, fiber reinforced concretes are found to be discontinuous, anisotropic, heterogeneous, multi-phase systems. It was found that the fiber matrix has interfacial bond micro-cracks and many inbuilt flaws due to change in volume and fabrication process. The discontinuities created throughout the geometry are center for future cracks and fractures in the structure (Morshedian and Nouri, 1995). Addition of Admixtures has been found to counter these difficulties effectively. By definition Admixtures are chemicals or minerals that are added to a concrete mixture apart from other basic ingredients like concrete, cement, aggregate and water. Apart from decreasing the overall cost, the admixtures have found wide acceptance as they own certain properties by which they modify the existing physical, appearance or performance characteristics of concrete like workability, curing temperature range, set time or color etc. when mixed. One of these types of mineral admixtures is Metakaoline. It was found that randomly oriented fibers increased tensile strength and retard crack propagation and prevents corrosion in High performance reinforced fiber concrete. Gutiérrez and Delvasto (2005) from experiments found out that permeability of composite can be decreased by mixing Pozzolona. It was showed that by adding Metakaoline there is an increase in strength of fiber reinforced concrete to an order of 20% and 68% by addition of only 15% Metakaoline. Fiber reinforced concrete had shown improvement in mechanical performance and durability due to significance decrease in the coefficient of capillary absorption and chloride penetration when the admixture is incorporated in the matrix. In case of Glass Fiber Reinforced concrete the current trends is of using microsilica as an admixture for composite material. However, it was shown that Metakaoline can replace microsilica as the admixture as it is cheaper and posse’s similar properties. An advantage of using Metakaoline is that the microstructure of Glass reinforced concrete matrix is positively influenced by lower production of portlandite. (Holesinky et al, 2005). At higher temperatures of around 600-800 °C it was found that concrete reinforced steel fibers and polypropylene with metakaoline retained 45% and 23% of their compressive strength. Also after being exposed to higher temperatures the loss in stiffness was higher than the loss in compressive strength. On replacing 20% of cement with Metakaoline the concrete showed higher compressive strength and relatively lower specific toughness in comparison to 10% silica replacement. (Poon et al, 2004)
Concrete are prepared from aggregate and a binder which holds the aggregates together. In ordinary Portland cement concrete there is typically about 75% calcium silicate and the binder used is paste. Due to hydration of Portland cement concrete Calcium Silicate Hydroxide and slaked lime or Calcium Hydroxide is formed. The Calcium Silicate Hydroxide formed acts as binder and Calcium Hydroxide provides resistance to corrosion. But due to excess present Lime in concrete it undergoes alkali silica reaction and provide harm to structure by serious cracking and deterioration of concrete. Thus to prevent the structure from harmful effects of excess lime Metakaolin which is an industrial form of Pozzolans is added that reacts with free lime and converts it into additional Calcium Silicate Hydroxide. Davidovits J. Geopolymer Institute website states that “The word metakaolin signifies another type of kaolin”. Metakaoline are formed by dehydroxylization of clay mineral called kalointe. When kaolinite is heated to about 100-200 ° C it looses most of the adsorbed water but at the range of 500-800 ° C kaolinite losses water due to dehydroxylization, this process is also called as calcining. Metakaoline is formed on heating beyond this temperature of dehydroxylization and a two-dimensional order in the crystal structure is retained. The process is an endothermic reaction and requires large amount of energy to remove hydroxyl ions which are bonded in the matrix. A research was done on the structure of metakaolinite by using wide_angle X-ray diffraction_scattering technology (WAXS) and the results showed that metakaolinite is a hemicrystal which is disordered in c-axle and composed of non_crystalline phase and a little crystalline phase which is subjected to change under exposure to higher temperatures. (Wang, 2001) The advantages of using Metakaoline are (a) it increases compressive and flexural strength of concrete, (b) It reduces permeability of structure, (c) There is reduction in efflorescence as free calcium is not available readily in concrete, (d) There is increment in resistance to chemical attack, (e) Durability of concrete is increased (f) superior workability as paste feels creamy or buttery on trowel and finishing, (g) Improves appearance and sustainability of concrete. (The Engelhard Website).
Of various methods used in practice for improving the durability and making high performance concrete, metakaoline has found itself in the core of new revolution. It is able to react readily with the calcium hydroxide in the cement paste and convert it into stable cementitious compounds thereby refining the microstructure of concrete. This leads to decrease in permeation properties. There has been limited research done on various physical properties of concrete made with metakaoline and thus it becomes difficult to develop a comprehensive mix design procedure. Basheer et al (1999) investigated by conducting experiments that metakaolin when is used in different dosages is able to change properties of both the fresh and hardened concretes. In their experiment they considered different range of concrete mixes with various proportions of metakaoline in it. Wide range of concrete samples with varying strength, workability and permeability were generated by varying aggregate-binder ratio and water-binder ratio. It was found out that metakaoline increased the compressive strength and reduced slump for a set of aggregate-binder ratio and water-binder ratio values. There was meager change in terms of air permeability when it is used as a replacement for ordinary cement. The results showed that for a replacement level of 10%, for high level of cement content the sorptivity of concrete is higher, whereas, on other hand there is negligible improvement when the cement content is low. It was researched that high reactivity metakaoline (HRM) is able to show concrete compressive strengths as high as 18,000 psi which is 124 MPa. Due to this special property of metakaoline it has found it applications in construction of longer spans, stronger structures and smaller sections leading to a more efficient concrete construction. Holesinsky et al (2005) performed experiments on glass fiber reinforced concrete with metakaoline under dynamic environmental conditions and concluded that even after 90 days the flexural strength of composite had retained its value. For finding long term implications accelerating durability test was performed. In this test the composite mixture underwent rigorous cyclic change on all major factors which influence the mechanical properties of the composite was done. It was concluded that the composite retained its properties for long time even under dynamic conditions.
One of the most scaring and destructive phenomena of nature is an earthquake and its dreadful aftermath. An earthquake occurs due to the movement of Earth leading to sudden release of stored strain energy in the Earth’s crust that in turn creates seismic waves. In past many severe earthquakes have shake the earth leaving behind them huge losses of human lives and property and more importantly a lesson to be learnt.
From the days of usage of ordinary Portland cement for construction we have come a long way in the techniques of construction. Now fabrication with admixtures and fiber reinforcements has paved the way for high strength composite concretes which have enhanced durability and workability than ordinary cement. It has been found that the glass fiber used in concrete degrade due to alkali conditions in the concrete. To counter the alkali effect metakaoline admixture was proved to be useful. It reacts with the free lime produced and thus there is very little free lime available to form crystals. But the total cost of composite concrete increases. It was found that glass fiber reinforced concrete mixed with metakaoline admixture is able to retain its flexural strength and properties even undergoing dynamic changes. The high cost of fiber metakaoline composite necessitates the development of low cost high strength composites.
With the advent of computers and engineering capabilities analyses is becoming increasingly easy but problems are now becoming more complex and ground motions more severe posing challenge to scientific community to invent better techniques and materials of construction so that in future dangers of earthquake can be mitigated. There is lot of research left to be done on properties of metakaoline and a need to develop better cyclic loading models which can imitate real life conditions. Today we face with a serious challenge to undertake counter measures to control the unpredictable hazard to life and property.