Mechanics of straw bales for building applications

Straw bales are seen as increasingly viable for building insulation and even for the construction of small load-bearing straw houses in the last decades, especially in view of the need to seek low environmental footprints. Straw bales can be used as load-bearing structures but they are currently mainly used as a filler insulation material associated with a timber structure. Up to present, very few studies are available concerning the mechanical behaviour of straw bales in buildings. This study aims at investigating the behaviour of straw bales and leads to recommendations for required bales densities. This allows to derive compression models which describe their behaviour in a wall. Therefore, the results show that, in the density range 90-110 kg/m3, the elastic and strength characteristics are similar whatever the position of the bales (laid flat or on their edge).The behaviour of the straw bales is found to be in correlation to the straw wisp density and to the initial wisp packing into the bales. The bale should therefore be considered has a system consisting out of the straw and the polymer links: when laid flat, it exhibits a particular type of deformation under single compression leading to a constant perimeter. In this position, the mechanical properties are controlled by two factors: (1) the packing density induced by the machine during the baling process, and by the elasticity and creep of the links; (2) the solid volume fraction of straw wisps, conditioned by the agronomic parameters and the pressure level in the baler.