This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.
Glass fibres are produced in a process in which molten glass is drawn in the form of filaments, through the bottom of a heated platinum tank or bushing, (Bentur and Mindess, 2007)
The primary concern is the durability of the glass fibres embedded in the highly alkalinity of concrete matrix. In relation to that, researchers have two ways to solve this problem. First alternative is developing new fibres which are durable in high alkalinity environment such as alkaline-resistant glass (AR-glass). Second approach is to use of less-alkalinity matrix and develop a polymer-modified mortar matrix (Balaguru and Shah, 1992).
Glass fibre is available in continuous or chopped lengths. Fibre lengths of up to 35 mm are used in spray applications and 25-mm lengths in premix applications.
Claims have been made that up to 5% glass fibre by volume has been used successfully in sand-cement mortar.
It is suitable for use in direct spray techniques and premix processes and has been used as a replacement for asbestos fibre in flat sheet, pipes and a variety of precast products, (FRC, 2008).
Glass fibre is extensively used in cementitious materials for architectural cladding, which the fibres are predominantly made of alkali-resistant glass (AR-glass). Beside, there has another typical type of glass fibres which normally known as borosilicate glass (E-glass) fibres. Table 2.4 shows the typical properties of selected glass fibres.
188.8.131.52 POLYVINYL ALCOHOL (PVA)
Polyvinyl alcohol is obtained from polyvinyl acetate which is readily hydrolysed by treating an alcoholic solution with aqueous acid or alkali.
The fibres for producing fibre reinforced concrete products have several advantages, as follows: high aspect ratio; high ultimate tensile strength; high modulus (2.95 x lo4 MPa); good alkali resistance; good chemical compatibility with Portland cement; good affinity with water; faster drainage rate; and no health risk from their use. The PVA fibres act very effectively to prevent micro-cracking from unstable crack growth in fibre-reinforced cementitious materials containing mesh layers, (Zheng And Feldman, 1995). Some properties of PVA fibres are shown in Table 10.9. (Bentur and Mindess, 2007).
PVA fibres have a positive effect on the bending strength of their composites, due to a good interfacial bond between the fibres and cement matrix. The good interfacial bond is attributed to a non-circular cross-section of the fibres, and hydrogen bonds between the fibres and the cement matrix. High chemical bond strength, due to the hydrophilic nature of PVA fibre, is a property characteristic of PVA. Polyvinyl Alcohol (PVA) fibres have been used in the construction of thin products as a replacement for Asbestos fibres due to their good mechanical properties and physical characteristics. (Suwannakarn, 2009).
Table 2. Properties of PVA, (Bentur and Mindess, 2007)
Polypropylene has been produced as a result of research and development in petrochemical and textile industry.
Polypropylene fibres were suggested as an admixture to concrete in 1965 for construction of blast-resistant buildings for the U.S Corps of Engineers . Considerable improvements in strain capacity, toughness, impact resistance, and crack control of concrete can be obtained through the use of polypropylene fibres. Polypropylene fibres are manufactured in various shapes and different properties.
The fibres can be incorporated into concrete as short discrete chopped fibres, as a continuous network of fibrillated film, or as a woven mesh (2). The form of the available fibre decides the method of fabrication. Each and every method has its own limitations. The choice of the method is guided by the volume percentage of the fibres that can be obtained during fabrication using a particular technique, (Brown et al, 2002).
Polypropylene is hydrophobic due to its chemical structure, which leads to reduced bonding with the cement, and negatively affecting its dispersion in the matrix. In addition polypropylene has a relatively higher Poisson ratio. Under tensile loading, the cross section of polypropylene fibres reduce rapidly and fibre surface is debonded from the matrix. [2-4].
Polypropylene fibres may increase flexural tensile strength, due to their ability to enhance the load bearing capacity in the post crack zone, but this increase is not that significant [2-4], (Bentur and Mindess, 2007).
It is used at present as discontinuous fibrillated material for the production of FRC by the mixing method, or as a continuous mat for production of thin sheet elements [2-4].
Polypropylene fibres provide better ductility and malleable than the other fibres. The fibres are resistant to most chemicals.
Figure 9. Effect of the content of rectangular polypropylene fibers on the loaddeflection
curve under flexural loading, (Yurtseven, 2004).
Carbon fibre is one of synthetic fibres and can be produced from different raw materials, such as polyacrylonitrile (PAN), or petroleum and coal tar pitch (pitch). Both processes involve heat treatments, and various grades of carbon fibres can be obtained with each, depending on the combination of heat treatment, stretching and oxidation. The manufacturing process from both precursors is outlined in Figure 2 .14, (Mallick, 2007). Carbon fibre usually has carbon content between 80% and 95%., (Siang, 2006).
Carbon fibres are commercially available in three basic forms, namely, long and continuous tow, chopped (6-50 mm long), and milled (30-3000 Âµm long).
Table 2.2 shows the properties of carbon fibre. The PAN carbon fibres are of higher quality (and higher cost), and are sometimes classified into two types, I and II, with type I having a higher modulus of elasticity and strength. The pitch carbon fibres have a much lower modulus of elasticity and strength, but they are much less expensive than the PAN fibres. Nonetheless, the pitch fibres still have superior properties with respect to many other synthetic fibres, hence and can be suitable for use in a cement matrix, (Bentur and Mindess, 2007). Although still more expensive than many types of fibres including glass fibres, (Johnson, 2001).
The properties of carbon fibres can vary over a wide range, depending on the degree of perfection, which is a function of the production process. The properties of carbon fibres depend on the raw material and manufacturing process, (Rao, 2007).
In general, attractive properties of carbon fibres include the following, (Chung, 1994):
High tensile modulus and strength
Low thermal expansion coefficient
Thermal stability in the absence of oxygen to over 3000c
Excellent creep resistance
Chemical stability conductivity
Low electrical resistance
The pitch fibres were developed in Japan, and much of the work on using these fibres in FRC has been carried out there.
The diameter of the carbon fibres is very small, 7 to 20 Î¼m. The strength of carbon fibres depends on the type of precursor used, the processing conditions during manufacturing, such as fibre tension and temperatures, and the presence of flaws and defects, (Campbell, 2004).
Carbon fibres are inert in aggressive environments, abrasion-resistant and stable at high temperatures, as strong as steel fibres and more chemically stable than glass fibres in an alkaline environment. Moreover, carbon fibres are low in density, especially when compared to steel fibre, their strength to density ratio is one of the highest among all fibre types. Carbon fibres can stop the free expansion of micro-cracks in the composites by enhancing the ductility.
The main drawback of carbon fibres has been its high cost, and low cost is essential for most applications of cements.
Error: Reference source not found
Modulus of elasticity
Elongation at break
Coefficient of thermal
0.5 to 1.2
7.6 - 8.6
0.1 to 0.5
30 - 32
2.0 - 2.4
Table 1.2 Properties of Carbon Fibres (Bentur and Mindess, 2007).
Fabrication Processes For CFRC
The fabrication processes for CFRC with long carbon fibres and the mats are hand lay-up and filament winding methods, which are almost the same methods as used for FRP (fibre-reinforced plastics). Fig. 2 represents the classification of fabrication processes for CFRC.
Briggs' review (1977) is a good reference on the fabrication processes for CFRC with the long carbon fibres. The fabrication processes for CFRC with short carbon fibres are the spray-up method in two-dimensional orientation, and casting, pressing, and extrusion moulding methods in three-dimensional orientation after uniform mixing. Of these moulding processes, the casting method is most widely applied. In the casting method, first the short carbon fibres are randomly dispersed in three dimensions inside the cement matrix by using a proper mixer, and then the mixed fresh CFRC is cast in various forms, (Ohama, 1989).
The mechanical properties have generally been evaluated for two types of composites, prepared either by hand laying of continuous fibres and mats, or by mixing of short fibres.
Most of the work with continuous, hand lay-up reinforcement has been carried out with the high quality PAN carbon fibres, while random short fibre reinforced composites have generally been studied using the lower quality pitch carbon fibres, (Bentur and Mindess, 2007).
A pozzolan is broadly defined as a siliceous or aluminosiliceous material that, in finely divided form and in the presence of moisture, chemically reacts with the calcium hydroxide (CH) released by the hydration of Portland cement to form calcium silicate hydrate (CSH) and other cementitious compounds. Equation 2.1 shows this reaction. Pozzolans and slags are generally categorized as supplementary cementitious materials or mineral admixtures, (Kosmatka et al., 2003).
2S +2CH â†’ C-S-H Equation 2.1
The use of pozzolanic materials as partial replacement for Portland cement in blended cements and concrete has become almost unavoidable due to energy saving concerns and other environmental considerations, such as CO2 emission related to Portland cement clinker production. The performance of supplementary cementing materials (SCMs) in blended cements and concrete mixtures depends on factors such as particle size distribution and specific surface area which controls water requirement, rheology and pozzolanic activity, (Uzal, et al., 2010)
Pozzolans additives commonly used in modern concrete construction include coal fly ash (aka pulverized fuel ash or PFA), ground granulated blast furnace slag(GGBS), silica fume(SF), and metakaolin (calcined clay). Figure 2.2 shows the particle size for some pozzolans.
Figure 2.1: Particle size for some pozzolans (Uzal et al., 2010)
Pozzolans such as ground granulated blast furnace slag (GGBS) and fly ash (PFA) have relatively low water demands and can be used to replace up to 70% of cement in some concrete mixes. In contrast, silica fume has much finer particles with very high water demand, necessitating the use of high range water reducers or superplasticisers to even achieve a 10% cement replacement, (Vitro Minerals, 2008).
Figure 2.1: Particle size for some pozzolans (Uzal et al., 2010)
Shaheen (2002) indicated that using 10% of cement dust, SF, slag, grog, FA, and RHA in the mortar causes an increase in the compressive, tensile, and flexural strength as shown in Figures 2.2, 2.3, and 2.4.
Figure 2.1: Effect of Pozzolans in Compressive Strength, (Shaheen, 2002)
Figure 2.1: Effect of Pozzolans in Tensile Strength, (Shaheen, 2002)
Figure 2.1: Effect of Pozzolans in Flexural Strength, (Shaheen, 2002)
Pozzolans are added to reduce cost and to improve long-term strength and durability of the hardened mass. Although pozzolans help to improve the packing density of the solids, the primary role of the pozzolan is to provide additional CSH-phases through reaction with water and the calcium hydroxide contributed by the reaction of portland cement. This pozzolanic reaction is slow for most pozzolans that are used in high proportion. Thus, the benefits of the pozzolan are seen within the time frame of a week to several weeks after casting. Some of the finer, more highly reactive pozzolans, such as silica fume, are added in smaller proportions and help to improve early strength aswell as durability at later ages (Shannang and Yeginobali, 1995; Singh, 2000).
The very fine pozzlans particles, having size between 0.1and 10 Î¼m, can fill the gaps between OPC grains, while the larger pozzolan particles, having size between 10 and 100 Î¼m, can fill the gaps between fine aggregate grains. The result is a much denser matrix, (Middendorf et al., 2005). Some of the more common additives are described in this chapter below.
Influence of Silica Fume on Cementitious Materials
During the last decade, considerable attention has been given to the use of silica fume as a partial replacement of cement to produce high-strength concrete, (Almusallam et al., 2004).
Silica fume is the one of the most popular pozzolanas, whose addition to concrete mixtures results in lower porosity, permeability and bleeding because their oxides (SiO2) react with and consume calcium hydroxides, which is produced by the hydration of ordinary Portland cement. The main results of pozzolanic reactions are: lower heat liberation and strength development; lime-consuming activity; a smaller pore size distribution, (Mazloom et al., 2004). Silica fume used as an additive in a concrete mix has significant effects on the properties of the resulting material, (Chung, 2002). Silica fume is known to improve both the mechanical properties and durability of concrete. Silica fume is commonly used along with carbon fibbers in order to help the dispersion of the fibres in the cement mix, (Yogendran et al., 1987).
Khatri and Sirivivatnanon (1995) indicated that the addition of silica fume to Portland cement concrete slightly decreased the workability of concrete but significantly improved its mechanical properties. The compressive strength improved at all ages and the strain due to creep was lowered. However, the early age drying shrinkage of concrete was observed to increase with the addition of silica fume and the long term drying shrinkage of silica fume cement concrete was lower than that of plain cement concrete.
One of the benefits of using silica fume in Portland cement-based composites is its performance as filler in capillary pores and in the cement paste aggregate interface. The most well known effect of silica fume is the increase in strength, (Collins and Sanjayan, 1999), including the compressive strength, tensile strength and flexural strength. The strengthening is due to the pozzolanic activity of silica fume causing improved strength of the cement paste, the increased density of mortar or concrete resulting from the fineness of silica fume and the consequent efficient reaction to form hydration products, which fill the capillaries between cement and aggregate, (Chung, 2002).
Xuan et al. (2009) concluded that the addition of SF in concrete, the interfacial bond strength of near surface-zone aggregates measured by the pull-out test gradually increases.
Bhanjaa and Sengupta (2005) indicated that the optimum silica fume replacement percentages for tensile strengths have been found to be a function of w/cm ratio of the mix. The optimum 28-day split tensile strength has been obtained in the range of 5-10% silica fume replacement level, whereas the value for flexural strength ranged from 15% to 25%.
Nili and Afroughsabet (2010) concluded that the addition of silica fume into the fibrous specimens led to an increased in tensile and flexural strength up to 27% and 38% respectively, at the age of 28 days as shown in figure 2.1. The results shown that silica fume improved the fibre dispersion in the mixtures.
Figure Silica fume acting as "micro-filler" of the space between cement grains. The specific surface area of silica fume is approximately 20 000 m2/kg, 50 times greater than that of cement, (Spasojevic, 2008)
Fig. 2.1 Flexural strength and fibre volume fractions at the age of 28 days: (a) w/c = 0.46 and (b) w/c = 0.36, (Nili and Afroughsabet, 2010)
Khedr and Abou-Zeid (1994) found an increase in flexural tensile strength due to addition of silica fume. This increase was up to 20% and 33% with 15% and 20% of silica fume, respectively, at an age of 28 days.
Chung (2000) reported that the carbon fibre dispersion is enhanced by using silica fume as an admixture. Typical silica fume content is 15% by weight of cement. The silica fume is typically used along with a small amount (0.4% by weight of cement) of methylcellulose for helping the dispersion of the fibres and the workability of the mix, (Chen et al., 1997). Latex (typically 15Â± 20% by weight of cement) is much less effective than silica fume for helping the fibre dispersion, but it enhances the workability, flexural strength, flexural toughness, impact resistance, frost resistance and acid resistance.
Toutanji et al. (1999) found that the addition of 8% silica fume resulted in an increase in the tensile strength of mortar, but showed no effect on the tensile strength of cement paste. The replacement of cement by a higher amount of silica fume (16 and 25%) resulted in a decrease in the tensile strength of both cement paste and mortar. However, this reduction was higher in cement paste than in mortar. It was also confirmed that superplasticiser in combination with silica fume plays a more effective role in mortar than in paste mixes. This can be attributed to a more efficient utilization of superplasticiser in the mortar mixes due to better dispersion of the silica fume particles.
Li and Chung (1998) studied the surface treatment of silica fume with sulphuric acid by immersing the silica fume in sulphuric acid (96%) for 2 h, washing with water, filtering, and then drying at 150Â°C for 1-2 days. The Surface treatment of silica fume with sulphuric acid increase the tensile strength (by 12%), modulus (by 72%) and ductility (by 57%), abrasion resistance (by 20%), loss tangent (by 30-80%) and dynamic flexural storage modulus (by 80-120%), and loss modulus (by 160-300%) of cement paste or mortar. These effects are probably due to increased bond strength between silica fume and cement matrix and increased specific surface area of the silica fume.
Chung (2002) concluded that the use of silica fume as an admixture in cement based materials increases the tensile strength, compressive strength, compressive modulus, flexural modulus and the tensile ductility. Moreover, it enhances the dispersion of microfibers. The use of silane treated silica fume in place of untreated silica fume increases the consistency, tensile strength and compressive strength. Furthermore, the silane treatment increases the damping capacity and decreases the drying shrinkage and air void content.
YazÄ±c (2008) indicated that the high-performance of concrete can be obtained with high-volume fly ash content especially with 10% SF replacement. These mixtures have good mechanical properties, freeze-thaw and chloride penetration resistance. Moreover, these mixtures have also great environmental and economical benefits.
Behnood and Ziari (2008) reported that the replacement of cement by 6% and 10% SF increased the 28-day compressive strength approximately 19% and 25%, respectively. This is due to the reaction of the SF with calcium hydroxide formed during the hydration of cement that caused the formation of calcium silicate hydrate (C-S-H) as well as filler role of very fine particles of silica fume.
Pulverized fuel ash (PFA)
Pulverized fuel ash (PFA), also known as fly ash (FA), is an artificial pozzolan and creates significant benefits for environment and conserves natural resources and avoids landfill disposal of ash products.
FA is a waste product from the burning of pulverized coal in power stations. When coal is consumed at a temperature around 1500Â°, it is first ground to the fineness of powder. Blown into the power plant's boiler, the carbon is consumed leaving molten particles consisting mostly in silica, alumina and calcium. These particles solidify as microscopic, glassy spheres that are collected from the power plant's exhaust before they can "fly" away hence the product's name: Fly Ash. Concrete produced with fly ash can have better strength and durability properties than concrete produced without it, (Meyer, 2009).
Many previous studies showed that fly ash, as a pozzolanic material, is effective for improving the reological properties of the fresh concrete and the engineering properties of hardened concrete (Urban, 2003). These improvements are generally attributed to the physical and chemical effects. The physical process is due to the particles fineness of the supplementary cementing materials, having a diameter from 0.01 to 100 micron, that are much smaller than cement, thereby providing densely packed particles between fine aggregates and cement grains, and, hence, the reduction in porosity.
It has also been reported that fly ash can reduce the hydration shrinkage of cement paste effectively (Gao and Zhou, 2005); damage due to autogenous shrinkage can be significantly reduced in concrete or cement paste when fly ash is added, (Zheng and Wang, 2009).
When FA replaces with cement in concrete mixture, fresh concrete workability is increased, early strength and shrinkage of hardened concrete are decreased, (Atis, 2003), and permeability is decreased, (Sengul et al., 2003) due to filling the micropores of concrete (Atis, 2004). All these variations on concrete properties, caused by FA, depend on the mineralogical composition and fines of FA. In addition, FA positively affected durability of concrete and mortar like freeze thaw resistance (Unal and Uygunoglu, 2004), sulphate resistance (Tosun et al., 2003), alkali silica reaction, and abrasion resistance (Topcu and Canbaz, 2007).
Another study by Haque and Kayali (1998) showed that, Concrete produced with fly ash showed 20% higher strength values compared to the control specimens.
Jaturapitakkul et al. (2004) also produced high-strength concretes with the replacement of cement by FA between 15% and 50%. And optimum results were achieved by a 25% cement replacement ratio.
Figure 2.5: Particle size distribution of fly ashes. (AydÄ±n et al. 2010)
Silicon dioxide, SiO2
Aluminum oxide, Al2O3
Ferric oxide, Fe2O3
Calcium oxide, CaO
Magnesium oxide, MgO
Titanium oxide, TiO2
Potassium oxide, K2O
Sodium oxide, Na2O
Sulphur trioxide, SO3
LOI (1000 _C)
Table2.4: Chemical composition of Fly Ash ( Siddique, 2003)
Research by Snelson (2005) reported that a Portland cement replacement of 30% with PFA is required to achieve a good sulphate resistance.
Snelson and Kinuthia (2010) concluded that the splitting tensile strength decreases as the Portland cement replacement with PFA increases. This is typical behaviour of strength development observed by researchers who have worked on replacement of PC with processed PFA.
(Kazberuk and Lelusz, 2007) showed that after 180 days the concretes containing 20 % of fly ash, related to cement mass, increased a compressive strength about 25 % .
Ground Granulated Blastfurnace Slag (GGBS)
Ground granulated blast furnace slag (GGBS) is one such pozzolanic material (termed by a few as a supplementary or complimentary cementitious material) which can be used as a cementitious ingredient in either cements or concrete composites, (Babu and Kumar, 2000).
GGBS contains around 30-40% active silica by weight. It is added at relatively high addition rates of up to 70%, (Purnell, 1998).
Blastfurnace slag is a by-product of iron manufacture and is chemically very consistent.. This material is rapidly cooled to form a granulate and then dried and the large size were grounded to a fine white powder, which has many similar characteristics to Portland cement which is named as ground granulated blast furnace (GGBS), (Cheng et al. ,2005). The material consists principally, of calcium alumino-silicates together with some magnesium, sulphur compounds and a small amount of alkalis. The granulated material when further ground to less than 45 micron. Mean particle size between 5 and 30 micron. Its specific surface is more than 350 m2/kg, some even more than 800 m2/kg. Figure 2.3 shows the particle size of GGBS.
GGBS is in use for a reasonably long period due to the overall economy in their production as well as their improved performance characteristics in aggressive environments, (Babu and Kumar, 2000). GGBFS has a positive effect on both the flexural and compressive strength of concrete after 28 days, (Cervantes and Roesler, 2007).
GGBS is a common addition to cementitious composites. It has been demonstrated that GGBS improves the general performance of concrete, decreasing chloride diffusion and chloride ion permeability, reducing creep and drying shrinkage, increasing sulphate resistance, enhancing ultimate compressive strength, and reducing heat of hydration and bleeding. It has also been suggested that GGBS may increase concrete durability. (Pavia1 and Condren, 2008). It also contribute to reduce the heat release of cement hydration at early ages, which is very important for controlling the temperature rise of mass concrete and hence reducing the risks of thermal cracking, (Zhong, 2010) . It is known that GGBS can substantially reduce the alkalinity of cement pore solution (Canham et al., 1987; Duchesne & Berube, 1994).
Oner and Akyuz, 2007 concluded that the compressive strength of GGBS concrete increases as the GGBS content is increased up to an optimum point(55-59%), after which the compressive strength decreases. In this study GGBS used in some mixes.