The effect of moisture content and composition on the compressive strength and rigidity of cob made from soil of the Breccia measures near Teignmouth, Devon

Earth has been used as a reliable building material for many thousands of years. Recently there has been a world wide renaissance in the use of earth as a building material due to its architectural versatility and environmental sustainability. However, in the United Kingdom it is regarded by the majority of building professionals as either obsolete or a novel historical material. The utilisation of earth as a modern building material and the repair of historic earth building structures is retarded by the uncertainty of the knowledge of the properties of the material. This thesis considers earth building materials as composite materials containing a cohesive, low compression modulus binder fraction, a high compression modulus aggregate fraction, and a fibre fraction. The compression properties of a building material without fibre content (cob matrix material) are described in terms of the interaction between the binder and aggregate fractions, and moisture and the binder fraction. The effect of the moisture content of the material upon the compression failure mechanisms is described. Values of compression modulus predicted by a rule of mixtures equation are compared to experimental results for this material. The following mechanisms are proposed to account for the apparent discrepancy between the predicted and experimental results: • the effect of pore size distribution and the proportions of binder and aggregate fractions upon strain magnification within the material • the effect of an efficiency factor, primarily dependent upon the proportion of binder and aggregate fractions, which determines the degree to which the potential modulus of the material is realised. Time Domain Reflectometry is employed for repeated, real time, non-destructive measurement of the moisture content of an external cob wall. The results of these measurements are analysed and discussed. This thesis proposes that consideration of cob as a composite material has developed a paradigm which will enhance the level of understanding of all earth building materials, enabling the manipulation and accurate prediction of their structural properties. This will be an important contribution to the realisation of the significant sustainable qualities of earth building materials by the current construction industry.