Constituents of Concrete

Concrete is formed by mixing cement, coarse and fine aggregate, water and possibly by the incorporation of admixtures [1-4]. Selection of the constituents is the first step for obtaining durable concrete, as their properties influence the behaviour 

For general use, i. e. when the environment has a mild aggressiveness and the strength requirements for concrete are in the usual range, most cements can be suitable. In that case the selection is made on the basis of cost or local availabihty. Portland and Portland-composite cements with strength class 32.5 or 42.5 are nowadays available as general-purpose cements. Although in several European countries Pordand-limestone cements can be found (Table 1.3), cements blended 

Luca Bertolini, Bernhard Elsener, Pietro Pedeferri, Rob P. Polder Copyright 2004 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3 527-30800-8 

When a high early strength is required, such as for post-tensioned structures or pre-tensioned precast elements, Portland cement with a strength class of 52.5 may be used. Blended cements are usually not suitable because of the slow rate of hydration, with the exception of Portland-silica fume cement and special (fast) slag cements. 

Special requirements on the chemical composition of the cement may be necessary for certain applications, such as those requiring higher resistance to sulfate attack, reduced heat evolution or where using aggregates susceptible to alkali-aggregate reaction caimot be avoided [4]. 

The concrete layer in a rigid pavement is the main structural element that receives and distributes the traffic loads to the underlying layer and provides a comfortable and safe surface to the traffic. 

An equally important function of the concrete layer is to withstand the thermal stresses attributed to expansion, contraction or warping the friction stresses that developed with the underlying layer and the stresses attributed to moisture changes of the concrete. 

Although the concrete layer is more durable than an asphalt layer, it requires frequent joint maintenance and restoration of the skid resistance property of the surface. The frequency of intervention depends on the quality/suitability of the material used and workmanship achieved. 

Joint maintenance, although time consuming, is generally effective restoration of the concrete surface s skid resistance though is a difficult task and needs special consideration and not always as effective. 

The materials used for the production of concrete are cement, aggregates, water and chemical additives. The aim of this chapter is not to give a detailed description of the concrete technology but to provide the reader the basic information and characteristics of concrete that will help in the design of a rigid pavement. 

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The alkaline constituents of concrete are present in the pore liquid (mainly as sodium and potassium hydroxides. Section 2.1.1) but also in the solid hydration products, e. g. Ca(OH)2 or C-S-H. Calcium hydroxide is the hydrate in the cement paste that reacts most readily with CO2. The reaction, that takes place in aqueous solution, can be written schematically as ... 

The sulphate attack with C-S-H gel decomposition and thaumasite formation is particularly harmful for the durability of concrete, because it occurs with the destraction of the most important binding constituent of concrete and leads to the collapse of this material. Because of the relatively low temperatures of thaumasite formation, the concrete foundations, and elements of the underground sewage systems, as well as the concrete road elements are especially susceptible for this type of destructioa In order to prevent thaumasite formation the classic approach should be applied, first of all the permeability of concrete should be reduced, and the use of cement with mineral additions should be considered. Bensted [273] suggested the lowering the C3A and alite content in cement. 

Table 9.2 Chemical requirements for fly ashes to be used as constituents of concrete...

Concrete aggregate—a hard, particulate material not reacting in a significant extent with the cement paste, with a maximum size of particles exceeding =5 mm, used as constituent of concrete mixes. 

Figure 3.2 Constituents of concrete-like composites, after Brandt et al. (1983).

In Figure 3.2 the main constituents of concrete-like materials are shown with their relations to the material structure. According to that scheme, fine aggregate grains have a double role in composite materials together with a binder they form a continuous phase around inclusions and reinforcement, but also these grains should be considered as inclusions with respect to... 

The replacement of cement clinker by GGBS may vary from 10% up to 70% or even more for pure slag cements, and different kinds of blended cements are available. The use of GGBS as a partial replacement of Portland cement has a significant ecological effect. The deposits of industrial waste are transformed into a constituent of concrete. The emissions of dusts and gases into the atmosphere from cement production are limited and the need of natural raw materials is proportionally reduced (Bijen 1996). 

Definition of important constituents of concrete are given in Selection and Use of Aggregates for Concrete, ACI Manual of Concrete Practice, Part I, American Concrete Institute (1970). 

Concrete, made from cement, aggregates, chemical admixtures, mineral admixtures, and water, comprises in quantity the largest of all synthesized materials. The active constituent of concrete is cement paste and the performance of concrete is largely determined by the nature of the cement paste. Admixtures are chemicals that are added to concrete for obtaining some beneficial effects such as better workability, strength, durability, acceleration, retardation, air entrainment, water reduction, plasticity, etc. Mineral admixtures, such as blast furnace slag, fly ash, silica fume, and others, are also incorporated into concrete to improve its quality. 

Oakmoss. Extracts of oakmoss are extensively used in perfumery to furnisli parts of the notes of the fougnre or chypre type. The first step in the preparation of an oakmoss extract is treatment of the Hchen Evemiaprunastri (L.) Ach., collected from oak trees mainly in southern and central Europe, with a hydrocarbon solvent to obtain a concrete. The concrete is then further processed by solvent extraction or distillation to more usable products, of which absolutes are the most versatile for perfumery use. A definitive analysis of oakmoss volatiles was performed in 1975 (52). The principal constituents of a Yugoslav oakmoss are shown in Table 15 (53). A number of phenoHc compounds are responsible for the total odor impression. Of these, methyl P-orcinol carboxylate is the most characteristic of oakmoss. 

The properties of cured Pordand cement are affected by these four constituents of the manufactured Pordand cement. Tricalcium siHcate hydrates and hardens rapidly, giving rise to the initial set and eady strength. Increased concentrations of tricalcium siHcate causes an increase in the eady strength of Pordand cement concretes. Dicalcium siHcate hydrates and hardens more slowly, giving the cured concrete its strength increases beyond one week. 

Calcium Silicates. Cements aie hydiated at elevated tempeiatuies foi the commercial manufacture of concrete products. Using low pressure steam curing or hydrothermal treatment above 100°C at pressures above atmospheric, the products formed from calcium siUcates are often the same as the hydrates formed from their oxide constituents. Hence lime and siUca ate ftequendy used in various proportions with or without Portland cement in the manufacture of calcium siUcate hydrate products. Some of these compounds are Hsted in Table 6. 

There are no recorded data to indicate that materials of this type would alter the stiffness of the concrete into which they are incorporated. However, the fact that these materials are associated with the matrix/air interface, and not the cement hydrates themselves, would suggest that the physical properties of the bonding constituents of the hardened cement would remain unchanged. 

The effectiveness of each admixture may vary, depending on its concentration in the concrete, the time of addition in the mixing cycle and various constituents of the concrete. Although each class of admixture is defined by its main effect (i.e. water reduction, set acceleration), it may have one or more secondary effects (retardation of set, increased bleeding, air entrainment) and its use may result in side effects. Side effects are those modifications of properties produced in the concrete that, even though unsought, are both inevitable and independent of an admixture s main function. Prior to selecting an admixture for an intended application, these... 

Modem concretes often incorporate a mixture of chemical and mineral admixtures, each of which may interact with the various constituents of cements and influence cement hydration reactions. The admixture-cement interactions may in fact be viewed as the reaction between two complex chemical systems - the multicomponent, multiphasic inorganic materials in the cement and the organic compounds of multicomponent admixture systems. For example, lignosulfonate water-reducers are intrinsically complex mixtures of chemical compounds derived from the chemical degradation of lignin, while synthetic admixtures such as superplasticizers contain species with a broad distribution of molecular weights, reaction products, or other chemicals added for a specific purpose [125]. The performance of an admixture in concrete is highly dependent on many... 

Silicon is the most important constituent of igneous and many sedimentary rocks, occurring in combination with oxygen in feldspars, micas, quartz, sands and shales. The element is used in electronic devices, while silicon in combination with oxygen as silica and silicates finds application in concrete, bricks, pottery, enamels, glasses, optical fibers for telecommunications, and refractory (high-temperature resistant) materials. 

In recent years use of the oxide as a constituent of cement has been advocated,1 especially in Sweden. Thus, a mixture of Portland cement (60 to 70 per cent.) and white arsenic (40 to 30 per cent.) heated to 200° to 250° C. affords a hydraulic cement of normal setting time and of less solubility than ordinary cement, so that lime liberation is inhibited and the resistance to water improved. Wooden structures exposed to the action of sea water may be protected by spraying with a concrete composed of white arsenic, cement and sand in the proportions 1 3 12. The arsenic makes the mixture elastic and helps the cement to adhere to the wood. There is, however, danger in the too widespread application of arsenic in the directions described above. 

The constitution of alloys can be deciphered by several processes. One depends on measurement of the electromotive force of a battery consisting of the alloy and a plate of some resistant metal—for instance, platinum—compared with that of a similar cell made with one of the constituents of the alloy. To take a concrete example. Suppose a cell were constructed of a plate of copper and a plate of platinum dipping in some appropriate liquid, a certain electromotive force would result. Imagine a plate of tin riveted to the face of the copper plate, the electromotive force would now be that of the more electropositive metal, tin. If a plate of bronze be substituted, supposing it to contain free tin not in chemical combination with the copper, then the electromotive force will still be that of... 

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