Design & Specifications
The requirements of this paragraph as well as those of 2.2.5 should be repeated in the specifications if more than one type of concrete is required by the design.
2.2.1 Expanded shale aggregate has been used in lightweight concrete for practically every type of structural application. It has been furnished at all compressive strength levels common to construction practice today, including high strength of 5000 and 6000 psi (34.4 - 41.4 MPa) or more when required 2.2.2 The weight of this concrete will range from 70 to 120 pcf (1120 to /1920 kg/m3) depending upon the strength, air content, aggregate density and mix proportions. The equilibrium density is normally from 5 to 10 pcf less than the fresh density of the concrete as placed. The amount of weight loss in curing is primarily affected by the moisture content of the lightweight aggregate when it is batched. Also, expanded shale lightweight concrete will gain strength and lose weight even after the classical 28 day requirement. This is due to the absorbed moisture, and is especially significant in pumped lightweight concrete. The concrete may gain 500-1000 psi (3.4-7 MPa) and may lose 2% or more in density, in a period between 28 and 98 days, at which time it approaches density equilibrium. If the design conditions will allow extending the time for strength and/or density requirements, it will allow a more economical mix to be used. Consult the lightweight aggregate producer in your area for specific information relating to his aggregate.
2.2.3 The Architect-Engineer should require that the mix be designed for the specified compressive strength and density with a slump, which will enable the concrete to be placed and finished efficiently and economically. This slump should be specified. If the concrete is to be placed by pumping, certain considerations such as pre-saturation of lightweight aggregates, use of admixtures and minimum cement content may be necessary. The Aggregate Producer can supply the Architect–Engineer recommended mixes for special placing conditions. The workability of structural lightweight concrete is comparable with that of normal weight concrete having 1 to 2 in. (25 to 50 mm) greater slump. Shape and dimensions of forms, placement of reinforcement and other specified job conditions vary. However, the following are generally satisfactory; for slabs and beams 3 to 5 in. (80 to 130 mm). For columns and walls 2 to 4 in. (50 to 100 mm).
2.2.4 Air-entrainment in structural lightweight concrete, as in normal weight concrete, improves durability, workability and reduces bleeding. Generally the aggregate producer recommends air entrainment to achieve workability with minimum slump. For durability, 5 to 8 percent air is required; for workability, 4 to 6 percent is generally satisfactory. The Architect-Engineer should specify the amount of air content required for the most economical mix and for the specific application.
2.2.5 In establishing batch proportions, trial mixes should produce concrete with an average compressive strength higher than specified in 2.2.1. The aggregate producer generally follows one of the procedures that are outlined in ACI 211.2, Recommended Practice for Selecting Proportions for Structural Lightweight Concrete. The degree of over design required depends on variability of test results. See AC1318, Building Code Requirements for Reinforced Concrete, Section 4.4. It is recommended that the Architect-Engineer obtain from the aggregate producer a recommended economical cement content, slump, air content and density for the strength of concrete required by the structural design and for the finishing qualities desired.
2.2.6 In designing structural lightweight concrete mixtures, the question is often raised regarding the effect of aggregate absorption. Pre-wetting requirements in specifications to provide for this characteristic of the aggregate often lead to difficulty in control. Consequently, these specifications do not contain pre-wetting requirements. The mix and the control of the mix proportions should be established and maintained by the producer's quality control personnel or a qualified laboratory based upon the recommendations of the aggregate producer. With this approach, the variations that exist in batching arrangements, ready-mixed plant layouts, weather, as well as in aggregate properties, can be taken into account to produce the most efficient and economical procedure.
2.2.7 7The modified curing cycle for lightweight concrete is reasonable due to the large amount of absorbed water in lightweight aggregate concretes as compared to normal weight concretes. This curing cycle is standard practice for the expanded shale industry, and is recognized in ASTM C330.
2.2.8 8The density, slump and air content determinations at intervals specified enable the Architect-Engineer to maintain uniformity in the concrete mixture. As long as there is no appreciable change in the density, i.e., not more than 2 pcf (32 kg/m3) variation from the established fresh density, there is reasonable certainty that the correct proportions are being batched. Variations greater than 2 pcf (32 kg/m3) indicate that some change has taken place in air content, weight of aggregate or batch weights, thereby resulting in a variation of yield. When this occurs, proper adjustments in the mix should be made at once to bring concrete quality back to that specified.
