Structural Behaviour of Plastered Straw Bale Assemblies Under Concentric and Eccentric Loading

Abstract

The use of plastered straw bale walls in residential construction is growing as builders and owners seek environmentally friendly alternatives to typical timber construction practices. Straw has excellent insulation properties and is an agricultural bi-product which is annually renewable, and is often considered a waste product of grain production. This thesis presents new models for predicting the compressive strength of plastered straw bale assemblies subjected to concentric and eccentric load. A constitutive model for lime-cement plaster is adapted from a stress-strain model for concrete, available in the literature. Twenty-two cylinder tests on plasters typically used for straw bale construction were used to verify the constitutive model. The models for plastered straw bale assemblies were verified by testing plastered straw bale assemblies under concentric and eccentric compressive loads. An innovative steel frame test jig was designed to facilitate fabrication and testing of the specimens. Using this jig, 18 specimens of height 0.33 m, 0.99 m, 1.05 m or 2.31 m were subjected to concentric or eccentric compressive load until failure. The experimental strengths of the assemblies ranged from 23 kN/m to 61 kN/m, depending on the eccentricity of the load, the plaster strength, and the plaster thickness. Results indicated that the specimen height did not significantly influence the strengths of the specimens. The models predicted the ultimate strength of the assemblies to be, on average, 6% less than the experimentally determined strengths, with a standard deviation of 13%. The models were also used to predict the theoretical ultimate strengths for a number of plastered straw bale wall assemblies described in the literature. The fabrication techniques for these specimens were more representative of conventional straw bale construction techniques, and it was found that the experimental results were 30% of the theoretical strengths for assemblies with plaster strength less than 10 MPa and 6% of the theoretical strengths for assemblies with plaster strength greater than 10 MPa. Thus, to account for construction imperfections and potential alternative failure mechanisms, a reduction factor of no more than 0.3 for plaster less than 10 MPa is suggested in order to predict the strength of plastered straw bale walls constructed using conventional construction techniques. The results presented herein provide support for the use of plastered straw bale walls in residential construction and indicate the applicability of models based on the compressive behaviour of lime-cement plaster for modelling the behaviour of plastered straw bale walls under eccentric and concentric compression.