Structural lightweight aerated concrete
Thesis (MScEng)--Stellenbosch University, 2013.
Cellular concrete is a type of lightweight concrete that consists only of cement, water and sand with 20 per cent air by volume or more air entrained into the concrete. The two methods used for air entrainment in cellular concrete are (1) the use of an air entraining agent (AEA), and (2) the use of pre-formed foam. If pre-formed foam is used to entrain air into the concrete the concrete is named foamed concrete and if an AEA is used the concrete is termed aerated concrete. Depending on the type of application, structural or nonstructural, cellular concrete can be designed to have a density in the range of range of 400 to 1800 kg/m3. Non-structural applications of cellular concrete include void and trench filling, thermal and acoustic insulation. Structural applications of cellular concrete include pre-cast units such as concrete bricks, partitions, roof slabs etc. Due to the high levels of air in cellular concrete it is challenging to produce compressive strengths that are sufficient to classify the concrete as structurally useful when non-autoclaving curing conditions are used. The autoclaving process combines high temperature and pressure in the forming process, which causes higher strength and reduced shrinkage. This process is also limited to prefabricated units. Non-autoclave curing conditions include moist curing, dry curing, wrapping the concrete in plastic, etc. However, now that the world is moving in an energy efficient direction, ways to exclude energy-intensive autoclaving are sought. It has for instance been found that the utilisation of high volumes of fly-ash in cellular concrete leads to higher strengths which make it possible to classify the concrete as structurally useful. Now, that there is renewed interest in the structural applications of the concrete a design methodology using an arbitrary air entraining agent needs to be found. The research reported in this thesis therefore attempts to find such a methodology and to produce aerated concrete with a given density and strength that can be classified as structurally useful. For the mix design methodology, the following factors are investigated: water demand of the mix, water demand of the mix constituents, and the amount of AEA needed to produce aerated concrete with a certain density. The water demand of the mix depends on the mix constituents and therefore a method is proposed to calculate the water demand of the mix constituents based on the ASTM flow turn table. Due to the complex nature of air entrainment in concrete, the amount of air entrained into the concrete mix is not known beforehand, and a trial and error method therefore had to be developed. The trial mixes were conducted in a small bakery mixer. From the trial mixes estimated dosages of AEA were found and concrete mixes were designed based on these mixes. The factors that influence the mix design and strength of aerated concrete include filler/cement ratio (f/c), fly-ash/cement ratio (a/c) and design target density. Additional factors that influence the strength of aerated concrete are specimen size and shape, curing, and concrete age. It was found that the sand type and f/c ratio influence the water demand of the concrete mix. Sand type and f/c ratio also influence compressive strength, with higher strength for a finer sand type and lower f/c ratios. However, the concrete density is the factor that influences the strength the most.