دانلود رایگان مقاله مکانیک straw bales برنامه های ساختمانی

Abstract

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.

4. Conclusion

The aim of the present study is to gain a better understanding of the mechanical behaviour of small straw bales, and identify the levers that might improve their use as a building material. For this purpose, an experimental study was carried out. The behaviour of the straw bales is found to be different if the bale is loaded on its edge or laid flat. For low densities, the bales laid flat are clearly stiffer than on their edge. Around 90–110 kg/m3 , the stiffness is equivalent in both configurations. When laid flat and loaded in uniaxial compression, the shape of a bale under deformation is controlled by the string length: the perimeter of the bale remains constant. The density of straw bales laid flat is not the only key parameter and we should also take the wisp density as well as the bulk density into account. The key parameter is then the ratio of bulk density over straw bale density, which characterizes the overall compaction state and solid volume fraction of the wisps. The shape of the bales, more particularly their slenderness, will influence the stiffness of a load-bearing wall made of straw: a high aspect ratio gives a greater stiffness. By using the Cooper-Eaton model, a critical compaction state could be identified for which the applied pressure in the baler ensures sufficient stiffness of the produced bales. This critical state could be identified by the ratio of initial to bulk density ρo/ρbulk of 2.15. This value corresponds to a critical compaction state, for which the applied pressure in the baler ensures a proper stiffness of the bale. The interaction between the baling process, the mechanics of straw packing and bales needs to be investigated further, particularly by developing the approaches of Leblicq et al. [16] applied to individual stems and Nona et al. [17] who worked on straw packing compaction and relaxation. These approaches should allow us to determine the entire loading path of the straw, from the crop field to the wall, by studying its compressive and relaxation behaviour.