Effect of Plant Fibres on the Variability of Cob Materials

Earthen materials have different nature components and present a high variability comparing to conventional materials; researchers try to settle it down for a future normalization as environmentally efficient material. But there is a need in energy to do it, either directly (compaction, organic matter extraction, particle screening to get the best particle size distribution ...) or indirectly by including inefficient materials from an environmental perspective (cement, limestone ...). The aim of this study is to follow the variability problematic of cob materials by comparing and understanding variation level of the hygrothermal characteristics due to fibres nature and fibres content. We found that plant fibres (hemp, flax and hay) act as a stabilizer for dry bulk density; at 1% fibres substitute, hemp fibre composites show the highest coefficient of variation on the thermal properties (6.1% on thermal conductivity, 18.74% on specific heat capacity) but flax fibres show the highest mean values. Increasing hay stalk content induces the spread of the hygrothermal properties inside their range of variaation.