Multicriteria analysis demonstrating the impact of shiv on the properties of hemp concrete

Abstract

The shiv derived from the shredded stem of the hemp plant [1], when mixed with a mineral binder and water, produces hemp concrete (or “hempcrete”), which is considered as an eco-material because of its remarkable environmental properties (favourable CO2 balance, easy management and disposal at the end of the product life, etc.) [2]. However, much like any agricultural product, its quality may be affected by the soil where the hemp has been grown, the weather conditions, and a variety of other factors. As these parameters are rather variable, the produced plant matter tends to be somewhat irregular, which can hamper the development of this type of building solution. The study at hand tests the characteristics of the hemp (particle size distribution, water absorption capacity, etc.), to assess the impact of the variability of these factors on the performance of the hempcrete made with the shiv. To begin with, the thirteen types of shiv used in the study were compared by multicriteria analysis, and then classified into three different groups.Next, nine types of shiv were selected, so that each of the three groups was represented. In identical conditions (i.e. the only variable being the type of shiv), specimen of hempcrete were made up in the lab for experimentation. The aim here was to validate the results of the study characterising the different types of shiv. The results show that there are three main types of mechanical response when the hempcrete is subjected to a compression test, where the material behaviour is characterised by small, medium or large strains. The large specific surface area of shiv is known to be responsible for weak binding at the interface between the binder and the plant particles, leading to total failure of the test piece. In addition, its high water absorption capacity, which is the likely cause of the “false set effect” associated with the elongated form of the particles, is identified as being responsible for the high rates of deformability found. It should be noted that in the latter case, the specimens behave like a succession of layers, which accounts for the significant levels of strain.By multicriteria analysis of the results for the hempcrete specimens, we found that they can be also classified into three groups, matching the three detected by the multicriteria analysis of the types of shiv from which they were made. Unlike the previously published literature, this study leads to the conclusion that shiv types with small particle sizes and high water absorption capacities do not produce the best performance.