Permeability and thermal conductivity of pre-cast lime and hemp concrete

Abstract

Lime and Hemp Concrete (LHC), used as building material, shows a wide range of densities and mix proportions and, as a consequence, various mechanical and thermal properties. The hemp shiv used as aggregate in such concrete is highly porous, with a low specific apparent density and a low thermal conductivity compared to the binder matrix. Shiv in bulk can be compressed to reduce significantly its volume since shiv particles are highly deformable. As a consequence, for same mix proportioning by weight, various densities can be achieved, depending on the casting process and the compaction applied on the fresh mix. This paper focuses on pre-cast elements made of compacted LHC. The fresh mixes are cast in a compression die with a controlled load. The binder and water contents are lower than conventional mixes cast in site. The pre-cast process has been accurately defined to achieve high reproducibility of blocks even if a slight density gradient occurs into the specimens due to wall friction during compaction. This process leads to lower void ratio into the LHC. The mechanical resistance is significantly improved while the increase in thermal conductivity is minor since the weight proportion of binder paste is lower. The ability of hemp concrete to regulate the indoor relative humidity is well known, and sometime not perfectly explained. The air permeability of such material gives a first indicator about the water vapor transfer ability of the LHC but any study has focused on its measurement. Our experiments show the changes in permeability and thermal conductivity induced by the compaction relative to this casting process for various mix designs.