Civil MDC

Standard Practice for Selecting Proportions for Structural Lightweight Concrete (ACI 211.2-98) 2

Standard Practice for Selecting Proportions for Structural Lightweight Concrete (ACI 211.2-98)

Description

Standard Practice for Selecting Proportions for Structural Lightweight Concrete. The purpose of this standard is to provide generally applicable methods for selecting and adjusting mixture proportions for structural lightweight concrete. These methods are also applicable to concrete containing a combination of lightweight and normal weight aggregate.

Discussion in this standard is limited to structural grade, lightweight aggregates, and structural lightweight-aggregate concrete. Structural lightweight-aggregate concrete is defined as concrete which: (a) is made with lightweight aggregates conforming to ASTM C 330, (b) has a compressive strength in excess of 2500 psi (17.2 MPa) at 28 days of age when tested in accordance with methods stated in ASTM C330, and (c) has an equilibrium weight not exceeding 115 lb/ft3 (1842 kg/m3) as determined by ASTM C 567. Concrete in which a portion of the lightweight aggregate is replaced by normal weight aggregate is within the scope of this standard. When normal weight fine aggregate is used, it should conform to the requirements of ASTM C 33. The use of pozzolanic and chemical admixtures is not covered in this standard.

2.1.1The principal factors necessitating modification ofproportioning and controlprocedures for lightweight-aggregateconcrete, compared with normalweight concrete, are thegreater absorptions and the higher rates of absorption ofmost lightweight aggregates.

2.1.2 Damp aggregates are preferable to dry aggregates attime of mixing, as they tend to absorb less water duringmixing and therefore reduce the possibility of loss of slumpas the concrete is being mixed, transported, and placed.Damp aggregates have less tendency to segregate in storage.Absorbed water is accounted for in the mixture-propor-tioning procedure.

2.1.3 When concrete is made with lightweight aggregatesthat have low initial moisture contents (usually less than 8 to10%) and relatively high rates of absorption, it may be desir-able to mix the aggregates with one-half to two-thirds of themixing water for a short period before adding cement,admixtures, and air-entraining admixture to minimize slumploss. The supplier of the particular aggregate should beconsulted regarding the necessity for such predampeningand for mixing procedure


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