Investigation over the parameters affecting the mechanical behaviour of small prismatic straw bales for use in construction

Straw bale construction is a building technique offering many advantages: it provides excellent hygro-thermal insulation; it ensures good performance against earthquakes; and it is sustainable. From a mechanical point of view, straw bales can act as a surviving cell for the building in case of calamity. For this reason, measuring the mechanical properties of straw bales is important for the comprehension of the behaviour of straw bale buildings. Despite the fact that the use of straw bales in construction has been constantly increasing since the ‘80s, the behaviour and the performance of straw bale buildings have not been fully assessed so far. Using a recently developed test methodology (Maraldi et al., 2016), an analysis of the performance of small prismatic straw bales under monotonic compressive loads has been conducted. Force-displacement curves obtained from the tests have been fitted with a two-parameter rheological model to extract bales performance indicators. Results show that bales laid flat are stiffer than bales laid on-edge. However, if the influence of bales initial geometry is disregarded by considering stress and strain instead of force and displacement, no difference in the elastic modulus between flat and on-edge orientation appears. Furthermore, bales stiffness and elastic modulus are dependent on the initial density of the bale. It has been also found that straw bales exhibit a typical deformation pattern which depends on the baling process and that the Poisson’s ratio does not remain constant along the longitudinal direction during loading, whereas it is null along the transverse direction.