BEHAVIOUR OF PLASTERED STRAW-BALE ASSEMBLIES SUBJECTED TO THREE-POINT BENDING

The search for more sustainable construction methods has renewed interest in straw-bale construction. Rectangular straw bales stacked in a running bond and plastered on the interior and exterior faces have been shown to have adequate strength to resist typical loads found in one- and two-storey structures. The straw bales provide excellent insulation, while possessing low embodied energy compared to conventional insulation materials.

The structural behaviour of a load-bearing plastered straw-bale wall subject to uniform compressive loading has been the focus of a number of studies reported in the literature. However, in a typical building wall, there will be numerous locations (such as around window and door openings) where the load paths produce areas of concentrated stress. The behaviour in these regions cannot necessarily be predicted using tests from uniformly loaded wall assemblies.

This paper describes experiments on plastered single bale assemblies subjected to three-point bending. These assemblies develop shear and flexural stresses, and so simulate the stresses that exist around door and window openings in a wall. The specimens were rendered with lime-cement plaster, and were either unreinforced, or contained steel “diamond lath” mesh embedded within the plaster. The specimens were pin-supported at various centre-to-centre distances (L) ranging from 200 mm to 500 mm. The height (H) of all specimens was constant at 330 mm. This gave a range of H/L values of 0.66 to 1.65.

Two distinct types of failure were observed. For tests with H/L < 1, failure was due to flexural tension cracks in the plaster which propagated through the depth of the plaster skin. For tests with H/L > 1, failure was due to crushing of the plaster in compression under one of the loading points.

It was shown that models based on simple mechanics were able to adequately predict the assembly strength. In particular, analysing the assemblies with H/L < 1 as simple beams, and using the transformed section concept to deal with the straw and steel mesh, was adequate for predicting their strength.

The results suggest that current practice for straw bale construction is generally appropriate. To avoid tensile cracking of plaster due to flexure, regions around doors, windows, and other openings should be designed such that H/L > 1. In regions where H/L < 1, the use of steel reinforcing mesh can increase the plastered bale strength by 30% on average.