Durability of concrete and cement composites

Variations among concretes and -archetypical concrete-

Modern concretes are extremely varied in their characteristics relevant to the concerns of this chapter. To begin with, concretes designed for different applications are currently produced at widely different w:c ratios. The writer has extensive personal experience in the examination of concretes that were placed at w:c ratios of 0.8, 0.9, or even higher. At the other extreme, modern high performance concretes are successfully produced at effective water:cementitious material ratios as low as 0.25. Enormous variations in internal concrete properties result from this wide variation in w:c ratio. The chemistry of the Portland cement used in the concrete is another relevant factor; cements vary considerably in chemical characteristics that affect the developing pore solution chemistry and to a lesser degree, the paste structure. The specific content of the minor alkali-bearing components (mostly potassium sulfates or potassium calcium sulfates) in the specific cement strongly influence concrete pore solution chemistry. Cement alkali contents, usually expressed as equivalent % Na2O =(ˆ %Na2O +‡ 0.659% K2O), may vary from a few tenths of a percent to well over 1%. Other important variations in Portland cement chemistry often escape notice in considerations of permeation capacity. A peculiarity of cement chemistry is that very small differences in analytical CaO contents mark major differences in the relative proportions of the calcium silicate components C3S and B -C2S. The relative proportions of the two calcium silicates exert significant influence on paste microstructure, not least in terms of the relative amount of calcium hydroxide that is generated. Similarly, the proportions of the aluminum-bearing phases C3A and ferrite (nominally `C4AF’) in the cement influence the rate and extent of ettringite production.

In most countries the addition of small but sometimes potentially significant proportions of non-Portland cement components (such as ground limestone or fly ash) is permitted in Portland cement specifications. Furthermore, provision is also usually made for cements that contain larger proportions of non-Portland cement components. These additions certainly impact on the internal charac- teristics of the binders produced, as does incorporation of such materials as silica fume, fly ash, slag, metakaolin, and similar substances by concrete producers. Chemical admixtures induce additional sources of variation. While most chemical admixtures are designed to carry out a particular function in concrete, their direct or indirect effects on pore solution chemistry and on microstructure may sometimes be considerable. The growing importance of precast steam cured concrete elements introduces still further variations. Steam curing, even at moderate temperatures, strongly affects the development of both pore solutions and microstructure. The relatively new category of self-compacting concretes (SCCs) is also growing in popularity. The combination of viscosity modifiers, dispersants, and mineral fillers encountered in such concretes has significant effects on pore solutions and on paste structures, many of which are just beginning to be understood. The enormous numbers of different effects that these many variations can induce, individually and in combination, renders discussion of all of them in this chapter quite impractical, even if the information were available. However, in many cases, the effects can be viewed as inducing departures from a typical pattern that would be observed for `ordinary’ concretes. Accordingly, in the present chapter the writer will generally confine himself to concretes of the ordinary or typical pattern, with only occasional mention of some of the effects induced by some of the many possible variations. For brevity, such concretes will be referred to as `archetypical concretes’, an `archetype’ being defined in dictionaries as `the original pattern or model after which a thing is made’. Archetypical concretes are here taken to mean conventional, properly con- solidated field- or laboratory-mixed Portland cement concretes of w:c ratios of the order of 0.4 to 0.6, produced from `ordinary’ Portland cement with conven- tional aggregates, hydrated without steam curing, and incorporating either no other supplementary components or else only minimal contents of supplementary components and/or modest dosages of conventional chemical admixtures.

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