Preliminary views on the potential of foamed concrete as a structural material

Low self-weight (800 to 1600 kg/m³), high workability (flowing and self-compacting) and excellent thermal insulating properties (< 0·50 W/mK) make foamed concrete attractive for many construction applications. Indeed, it is now a well-established material in void filling and highway reinstatement uses. However, its wider use in structural applications has been inhibited by its technical and engineering unfamiliarity and a perceived difficulty of achieving sufficiently high strength (say > 25 N/mm²). This paper describes a laboratory study of the development of foamed concrete, utilising two types of fly ash, with the potential for use in structural applications. ‘Fine’ fly ash (i.e. to BS EN 450) was used to partially replace Portland cement and a ‘coarse’ fly ash (i.e. to BS 3892-2) to replace sand fine aggregate. In addition, the potential of polypropylene fibres in foamed concrete to enhance plasticity and tensile strength was examined. The key early age, engineering and durability properties were measured and these data show that foamed concrete is indeed viable for structural uses. To further demonstrate the concept, the results of full-scale pilot tests on conventionally reinforced foamed and normal weight concrete beams are also reported and their performance compared with the BS 8110 and Eurocode 2 requirements for serviceability. However, the characteristics of foamed concrete, particularly high drying shrinkage strain and relatively low tensile strength and stiffness performance, mean that straight substitution for normal weight concrete is not possible and more innovative structural forms will need to be developed.