Experimental investigation of the biaxial load-bearing and deformation behaviour of wheat straw big bales for construction

Growing awareness of the impacts of anthropogenic climate change is driving the demand for sustainable alternatives to conventional building materials. Alongside modern high-performance materials, construction methods utilising traditional renewable raw materials and techniques are garnering increasing attention within society. Square straw bales can offer environmentally friendlier building solutions due to their excellent thermal properties and their ability to withstand mechanical loads when sufficiently compacted. This article focuses on experimental investigations into the biaxial load-bearing behaviour of vertically loaded wheat straw big bales intended for use in load transfer within buildings. The primary emphasis lies on analysing the short-term load-bearing behaviour under cyclic loads and the short-term creep behaviour under permanent loads. Utilising electromechanical and optical metrology, both vertical and horizontal deformation states are determined, forming the basis for establishing biaxial stress-strain relationships. It can be demonstrated that big bales display a notably stiffer behaviour in contrast to the small bales which have predominated scientific studies thus far. The stiffness characteristics are heavily influenced by bale density and the inhibition of pronounced lateral expansion behaviour. However, the creep behaviour is positively impacted by bale compaction due to mechanical preloading. Furthermore, the article aims to comprehensively document all pertinent specimen properties (such as appearance and straw structure), growth, manufacturing, and storage conditions (including environmental factors and pressing technology), as well as experimental parameters (such as test setup and metrology), test procedures (including load regimes and visual observations), and evaluations (such as stress-strain relationships and creep curves). This documentation aims to ensure traceability and reproducibility, facilitating comparisons with future test series and contributing to the development of standardised test methods for load-bearing straw bales.