Concrete structurs

Páginas: 10 (2300 palabras) Publicado: 24 de enero de 2011
This Standard Practice describes methods for selecting proportions for hydraulic cement concrete made with and without other cementations materials and chemical admixtures. This concrete consists of normal and/or high density aggregates (as distinguished from lightweight aggregates) with workability suitable for usual cast-in-place construction (as distinguished from special mixtures forconcrete products manufacture).
Concrete is composed principally of aggregates, a portland or blended cement, and water, and may contain other cementitious materials and/or chemical admixtures. It will contain some amount of entrapped air and may also contain purposely entrained air obtained or air-entraining cement. Chemical admixtures are frequently used to accelerate, retard,improve workability, reduce mixing water requirements, increase strength, or alter other properties of the concrete. Depending upon the type and amount, certain cementitious materials such as fly ash, natural pozzolans, ground granulated blast-furnace slag, and silica fume may be used in conjunction with portland or blended cement for economy or toprovide specific properties such as reduced earlyheat of hydration, improved late-age strength development, or increased resistance to alkali-aggregate reaction and sulfate attack, decreased permeability, and resistance to the intrusion of aggressive solutions.
The technological developments have taken place,for the most part, since the early 1900s. The use of watercement ratio as a tool for estimating strength was recognized about 1918. Theremarkable improvement in durability resulting from the entrainment of air was recognized in the early 1940s.
Proportions calculated by any method must always be considered subject to revision on the basis of experience with trial batches. Depending on the circumstances, the trialmixtures may be prepared in a laboratory, or, perhaps preferably, as full-size field batches.
BASIC RELATIONSHIPConcrete proportions must be selected to provide necessary placeability, density, strength, and durability for the particular application.
Placeability.- property of concrete that determines its capacity to be placed and consolidated properly and to be finished without harmful segregation. It embodies such concepts as moldability, cohesiveness, and compactability. Workability is affected by: thegrading, particle shape, and proportions of aggregate; the amount and qualities of cement and other cementitious materials; the presence of entrained air and chemical admixtures; and the consistency of the mixture.

Consistency.- Loosely defined, consistency is the relative mobility of the concrete mixture. It is measured in terms of slump -- the higher the slump the more mobile the mixture -- and itaffects the ease with which the concrete will flow during placement.
Strength.- Although strength is an important characteristic of concrete, other characteristics such as durability, permeability, and wear resistance are often equally or more important. Strength at the age of 28 days is frequently used as a parameter for the structural design, concrete proportioning, and evaluation of concreteWater-cement or water-cementitious ratio [w/c or w/(c + p)].-The net water content excludes water absorbed by the aggregates. Differences in strength for a given watercementratio w/c or water-cementitious materials ratio w/(c + p) may result from changes in: maximum size of aggregate; grading, surface texture, shape, strength, and stiffness of aggregate particles; differences in cement types andsources; air content; and the use of chemical admixtures that affect the cement hydration process or develop cementitious properties themselves.
Durability.- Resistance to freezing and thawing, wetting and drying, heating and cooling, chemicals, deicing agents may be enhanced by use of special ingredients: lowalkali cement, pozzolans, GGBF slag, silica fume, or aggregate selected to prevent...
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