Lightweight concrete is being used in the construction industry as a building material in its own right. Ultra-lightweight concrete can be applied as a filler and support material for the manufacturing of composite building materials. In the past various techniques have been utilised to reduce the specific weight of concrete like lightweight aggregates, mixing with foam and the inclusion of expanded polystyrene, polyurethane or polyisocyanurate. A novel development is the addition of a bio-degradable polymer into lightweight concrete to reduce specific weight, carbon footprint and negative effects in the case of fire.

This thesis is about the development of a stable and reproducible ultra-lightweight concrete. Expanded poly-lactic acid (EPLA) was used to assess the feasibility of a biodegradable polymer as a lightweight aggregate that will deliver advantages such as eco-friendly concrete and being a non-petroleum polymer aggregate. The properties of EPLA concrete were compared to concrete made with expanded polystyrene (EPS) with similar mix design proportions using ordinary Portland cement (OPC), ground granulated blast-furnace slag (GGBS) and magnesium phosphate cement (MPC). In addition, two types of lightweight aggregate, expanded perlite (EP) and expanded vermiculite, are investigated as fine aggregate in this study.

It was found that chemical reactions of EPLA in the highly alkaline environment of cement causes significant changes in the microstructure of concrete. A large amount of calcium carbonate was found as hydration products of EPLA concrete. Furthermore, EPLA aggregates shrunk and lost their strength, and the rate of degradation was much higher in moist conditions.

This investigation has shown that non-structural grade ultra-lightweight concrete with relative densities of 512.85 to 203.2 kg⁄m^3 can be obtained. The compressive strength of the concretes containing different ratios of EPLA and EPS aggregate varied from 4.62 MPa to 0.28 MPa. It could be demonstrated that the engineering properties such as density, compressive strength, tensile strength, elastic modulus and thermal conductivity of concretes containing EPLA aggregate decreased compared to EPS concrete. The application of EPLA causes changes in hydration products and increases in concrete porosity, which led to an increase in electrical resistivity and water absorption ratio. The shrinkages of EPLA aggregate and bond failures at the interface area of EPLA and the matrix caused the EPLA concrete failing in a more brittle way.

Fire resistance has also been investigated as an essential parameter of this ultra-lightweight concrete. The experimental results show that the carbon dioxide production and heat release rate of EPLA concrete is much lower than that of concrete containing EPS. Furthermore, the concrete containing EPLA aggregate shows the lower bond strength compared to EPS concrete when used as filler in composite sections. While the influence of ultra-lightweight concrete on the load carrying capacity of cold-formed beams was minimal, its effect on lateral torsional buckling was significant. After a comprehensive study on combinations of different binders to eliminate the degradation of EPLA beads in an alkaline environment, magnesium phosphate cement was found as a proper binder for concrete containing EPLA. Another solution to the problem of EPLA degradation was established by coating the EPLA. The experimental results from coated EPLA (CEPLA) show that coating is the best solution in the high alkaline environment.

A set of equations were developed to predict the compressive strength, thermal conductivity, elastic modulus, water absorption, bond strength and load-displacement behaviour of the investigated concretes. In addition, a new method is proposed for the mix design of ultra-lightweight concrete.