Structural lightweight aerated concrete
Date
2013-03
Authors
Van Rooyen, Algurnon Steve
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
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.
Description
Thesis (MScEng)--Stellenbosch University, 2013.
Keywords
Lightweight concrete, Air-entrained concrete, Theses -- Civil engineering, Dissertations -- Civil engineering