19 Chapters
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Chp-1

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Chapter

1

Introduc tion

Ever since civilizations first started to build, the human race has sought a material that binds stones into solid formed mass. The Romans mixed lime (i.e., burned limestone) with volcanic ash from Mount Vesuvius that produced structures of remarkable durability. During the Middle

Ages, the art of making hydraulic cement (cement that hardens when it comes in contact with water) became lost and it was not until the year of 1824 that the hydraulic cement (now commonly known as Portland cement) reappeared when it was patented by a Leeds builder named Joseph

Aspdin. The name “Portland cement” was given originally due to the resemblance of the colour and quality of the hardened cement to Portland stone (limestone quarried in Dorset). The most widely used modern construction material is concrete that is made by mixing Portland cement with sand, crushed rock and water. Man consumes no material except water in such tremendous quantities.

Concrete is neither strong nor tough as steel, so why is it the most widely used engineering material? There are number of reasons. Firstly, Concrete possesses excellent resistance to water.

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Chp-6

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Chapter

6

Chemical Admixtures

Admixtures for concrete are defined as materials other than hydraulic cement, aggregates or water that are added immediately before or during mixing. The most important admixtures are ones added to accelerate or retard setting, to decrease the quantity of water needed to obtain a given degree of workability, or to entrain air in order to increase freeze-thaw resistance of concrete.

RETARDING ADMIXTURES

Admixtures that retard setting are of value for concreting in hot weather, oil well cementing and other purposes. Many organic materials have this property; sucrose, glucose, calcium citrate and calcium lignosufonate are examples of retarders. The retarders most widely used in practice appear to be hydroxyl-carboxylic acid and their salts. Because of low concentration, these are commonly added as solutions. They act by adsorption and hence are used in low concentration. The time of initial set increases with the content of retarder and generally decreases with temperature and cement content. The increased retardation occurs especially with cements having high C3A content because once some C3A has hydrated, it does not absorb retarder and the retarder is available for action with calcium silicates. In a typical case, addition of 0.1% sucrose by weight of cement might increase the initial setting time by 10 hours, while a 0.25% addition might delay it by 6 days. High dose of retarders can permanently “kill” setting of cement.

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Chp-14

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Chapter

14

Fiber Reinforced Concrete

Fiber reinforced concrete (FRC) is defined as concrete made with hydraulic cement, fine or fine and coarse aggregate and discontinuous discrete fibers. For structural applications, steel fibers are used as complementary reinforcement to increase the cracking resistance, flexural and shear strength, impact resistance and ductility of RCC elements. Fibers are used in cementitious materials in order to improve the characteristics in the hardening or the hardened state.

Historically, fibers have been used to reinforce brittle materials since ancient times; straws were used to reinforce sun baked bricks, horse hair was to reinforce plaster and more recently, asbestos fibers are being used to reinforce Portland cement. Patents have been granted since the turn of the century for various methods of incorporating wire segments or metal chips into concrete. The low tensile strength and brittle charter of concrete have been bypassed by the use of reinforcing rods in the tensile zone of the concrete since the middle of the nineteenth century.

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Chp-16

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Chapter

16

Additional Topics in

Concrete Technology

Several physical, chemical and mechanical techniques are being applied in research of concrete over recent years. They provide important information including characterization of raw materials, cured concrete, quantitative estimation of products of hydration. Information on the specialized techniques are scattered in literature and hence latest knowledge on the various methods are compiled and presented in this chapter.

MERCURY INTRUSION POROSIMETRY

Mercury intrusion porosimetry (MIP) is used for determining pore size distribution of cement paste, cement mortar and concrete. The method is based on the relationship between pressure and corresponding volume of pores filled with mercury. MIP is relatively straight forward and generally yields reproducible pore size distribution. Parameters such as total porosity, threshold diameter, mean pore diameter, permeability, diffusion coefficient and retention factor can be deduced from the distribution.

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Chp-15

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Chapter

15

Non-destruc tive

Testing of Concrete

The need for non-destructive testing (NDT) of concrete may arise due to variety of reasons.

These are: assessment of structural integrity or safety following material deterioration, structural damage caused by fire, fatigue or overload, adequacy of members suspected to contain unspecified material, fault in design, and monitoring long term changes in material properties and structural performance. NDT is generally defined as not impairing the intended performance of element or member under test, and when applied to concrete is taken to include methods which cause localized surface zone damage. All NDT methods can be performed directly on the in-situ concrete without removal of a sample.

Broadly speaking, there are two classes of NDT methods. The first class consists of those methods that are used to estimate strength. The surface hardness, penetration resistance, pullout, pull-off, break-off, and maturity techniques belong to this category. Some of these are not truly

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