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Rammed earth construction is built by compacting humid soil into formworks layer by layer, using clay fraction as only binder between the grains, without firing procedure. At this stage, the material is unsaturated, and capillary forces thus act to add cohesion between particles. Indeed, a peculiarity of this material is its high hygroscopic capacity, which allows water vapor to be absorbed and desorbed into the clay matrix. The variation of hydric state in the material leads to a variety of mechanical and thermal properties. Consequently, the complexity in studying this material is due to the strong dependence of many parameters on the moisture content and still few data are available in the literature on this subject for rammed earth materials. A soil used for an existing rammed earth building in Lyon is here characterized in order to determine its hygrothermal properties. Earth conductivity was firstly characterized in a reference dry state, then the influence of humidity variations was investigated. Sorption and desorption isotherms were determined to investigate the ability of the material to store and release humidity. Thermal conductivities were measured at different relative humidity, to put into evidence the influence of environmental conditions.

Experimental studies have been undertaken to understand the link between hydric state and mechanical behaviour of rammed earth. Prismatic rammed earth sample of 15 cm × 15 cm × 45 cm, structured as a wall element with several layers, were subjected to one dimensional drying in laboratory. The kinetic of drying was monitored by continuous weighing the sample and humidity measurement at a regular interval. Suction was then used to represent the hydric state. Simultaneously, unconfined compressive strength was performed after 0, 2, 3, 4, 6, 8 and 19 weeks of drying. Digital image correlation was used to determine the stiffness of samples. Results show an increase up to 60% of the maximum in compressive strength in the first 3 weeks of drying which is correlated with a sharp decrease in water content in the sample. The suction was correlated with the evolution of mechanical performances of the samples during the drying. [ABSTRACT FROM AUTHOR]

The influence of hydric state on the elasto-viscoplastic behaviour of a unstabilised rammed earth (URE) wall has yet to be studied in the literature. This paper presents an experimental campaign on a rammed earth wall. The aim is to evaluate the link between the mechanical properties (including viscosity) and the varying hydric state inside the drying wall after manufacture. Cyclic axial compression and stress relaxation tests were carried out for this purpose. A compression test was conducted up to 0.1 MPa, followed by a stress relaxation test. These tests were periodically performed over 32 weeks. In addition, the hydric state inside the wall was monitored by humidity sensors. The results show that both the elastic modulus and the dynamic viscosity coefficient increase as the structure dries. A dependence of the mechanical behaviour on time is therefore found in these samples in the transient state. This can occur when the sample is in the drying or wetting phase. As rammed earth is a material particularly sensitive to water, this result is crucial for the durability of earthen constructions. [ABSTRACT FROM AUTHOR]