Civil MDC

Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete 2

Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete

Description

Concrete is composed principally of aggregates, aportland or blended cement, and water, and may containother cementitious materials and/or chemical admixtures. Itwill contain some amount of entrapped air and may alsocontain purposely entrained air obtained by use of an ad-mixture or air-entraining cement. Chemical admixtures arefrequently used to accelerate, retard, improve workability,reduce mixing water requirements, increase strength, or alterother properties of the concrete (see ACI212.3R).

De-pending upon the type and amount, certain cementitiousmaterials such as fly ash, (see ACI 226.3R) naturalpozzolans, ground granulated blast-furnace (GGBF) slag(see ACI226.1R),and silica fume may be used in con-junction with portland or blended cement for economy or toprovide specific properties such as reduced early heat ofhydration, improved late-age strength development, or in-creased resistance to alkali-aggregate reaction and sulfateattack, decreased permeability, and resistance to the in-trusion of aggressive solutions (See ACI 225R and ACI226.1R).

2.2 The selection of concrete proportions involves abalance between economy and requirements for placeability,strength, durability, density, and appearance. The requiredcharacteristics are governed by the use to which the concretewill be put and by conditions expected to be encountered atthe time of placement. These characteristics should be listedin the job specifications.

2.3 The ability to tailor concrete properties to job needsreflects technological developments that have taken place,for the most part, since the early 1900s. The use of water-cement ratio as a tool for estimating strength was recognizedabout 1918. The remarkable improvement in durabilityresulting from the entrainment of air was recognized in theearly 1940s. These two significant developments in concretetechnology have been augmented by extensive research anddevelopment in many related areas, including the use ofadmixtures to counteract possible deficiencies, developspecial properties, or achieve economy (ACI212.2R). It isbeyond the scope of this discussion to review the theories ofconcrete proportioning that have provided the backgroundand sound technical basis for the relatively simple methodsof this Standard Practice.


Leave a Comment

Your email address will not be published.