Thermal and Moisture Content Monitoring of a Full-Scale Load Bearing Hemp Lime Arch Prototype

Abstract

Today, bio-sourced materials represent an important technological field of study, as they could sink atmospheric carbon dioxide into buildings. Little-processed construction materials would also reduce the environmental impact of the construction sector, which emitted more than 2.9 Mt of CO2 in 2020. Hemp-lime is a material that meets both these requirements. It is an insulating mix that can take different forms and be used in various parts of a building. The challenge is providing it with enough mechanical strength to make it loadbearing, at least to some extent. This research focuses on the construction and monitoring of a pointed arch, based on a previous experimental hemp-lime construction at Cardiff University in 2009, under the direction of architect David Lea. Since 2022, such an experiment on a possible loadbearing hemp-lime mix is being repeated at the Politecnico di Torino as part of a wider project called “experimental pavilions of vegetarian architecture”. The design and numerical analysis of the Cardiff prototype led to the modification of both the geometry and the composition of the mix using only pozzolanic air lime as the binder. The construction of the arch ended in December 2023. Observing the thermo-hygrometric conditions of this hemp-lime mix once in place is the main purpose of this article. A strong correlation is revealed between outdoor conditions with temperature and moisture content in the core of the arch. Building a full-size outdoor prototype allows for the avoidance of mathematical correction to the results obtained and allows the assessment the mix’s resistance in relation with environmental conditions. Due to some similarities of nature and function between lime and cement, many studies of lime mixes do not exceed a duration of 28 days, which cannot be considered the appropriate observation time for its curing. Therefore, we analysed this lime-based material for around 6 months, according to its own temporality and chemical kinetics. Through continuous monitoring at 10-min intervals, it was possible to highlight several significant aspects of rammed hemp-lime. The results show that the temperature within the mix is influenced by the outside temperature, but the sun exposure of certain areas drives up the corresponding temperature values more rapidly. Furthermore, while the absorption of water in the form of vapour is very rapid, desorption takes longer, as does re-establish a balance between the material and its context. Finally, solar exposure affects particularly 30-cm-thick elements, while elements that are 60 cm thick are not affected in the short term but only in long-term exposure conditions like season changes.