K Ramamurthy

Aerated concrete is characterized by the presence of large voids deliberately included in its matrix to reduce the density. This study reports the investigations conducted on the structure of cement-based autoclaved aerated concrete (AAC) and non-AAC with sand or fly ash as the filler. The reasons for changes in compressive strength and drying shrinkage are explained with reference to the changes in the microstructure. Compositional analysis was carried out using XRD. It was observed that fly ash responds poorly to autoclaving. The process of pore refinement in fly ash mixes is discussed with reference to be formation of Hadley grains as well as fly ash hydration. The paste-void interface in aerated concrete investigated in relation to the paste-aggregate interface in normal concrete revealed the existence of an interfacial transition zone.

Cost reduction in roofs/floors has been achieved through the adoption of alternate techniques like filler slab, which works on the principle of filling a part of concrete in the tension zone with cheaper substitutes. Even though there are sound theoretical bases and proven methods of construction, there has been no systematic analysis of cost-effectiveness such a system can offer for a range of spans, imposed loading and support condition. This paper discusses the details of a cost-effectiveness analysis conducted on two-way filler slabs with Mangalore pattern tiles as filler units. The cost and weight reduction that can be offered by such a system in comparison to the conventional slab for different parameters as listed above are brought out.

Aerated concrete is relatively homogeneous when compared to normal concrete, as it does not contain coarse aggregate phase, yet shows vast variation in its properties. The properties of aerated concrete depend on its microstructure (void-paste system) and composition, which are influenced by the type of binder used, methods of pore-formation and curing. Although aerated concrete was initially envisaged as a good insulation material, there has been renewed interest in its structural characteristics in view of its lighter weight, savings in material and potential for large scale utilization of wastes like pulverized fuel ash. The focus of this paper is to classify the investigations on the properties of aerated concrete in terms of physical (microstructure, density), chemical, mechanical (compressive and tensile strengths, modulus of elasticity, drying shrinkage) and functional (thermal insulation, moisture transport, durability, fire resistance and acoustic insulation) characteristics.

The process of drying of many materials is accompanied by dimensional changes, which induce cracks in the structure. This phenomenon is particularly significant in aerated concrete owing to its high total porosity and specific surface of pores. The factors influencing drying shrinkage of aerated concrete are widely different from that of normal concrete because of the presence of coarse aggregates in the latter. This paper discusses the results of statistically designed experiments conducted to ascertain the influence of composition on the drying shrinkage of non-autoclaved and autoclaved aerated concrete. Detailed single factor experiments were conducted to assess the influence of basic constituents of the mix on drying shrinkage whereas fractional factorial experiments were used for the interaction effects and the influence of some additives. It was observed that increase in lime-cement ratio and fly ash content increases drying shrinkage. Significant shrinkage reduction is obtained by autoclaving, suggesting that drying shrinkage is predominantly a function of the physical structure of the hydration product.

Alternatives to conventional reinforced concrete (RC) slab like voided or filler slabs are based on the principle that a part of the concrete in the tension zone is either eliminated or filled with a relatively cheaper material, resulting in reduction in cost and/ or reduction in self-weight and better thermal insulation. This paper discusses details of a systematic cost-effectiveness analysis of one-way slabs with filler blocks and partial prefabrication system and the results are compared with that of conventional RC slab. The study is aimed at assessing the relative structural performance of alternate roof/floor slab systems adopting limit state design concept with a view of evolving design tables to serve as a ready reckoner for designers. A comparison of the relative cost and reduction in self-weight of these systems has been made for establishing their range of applicability.