Raw earth material for sustainable construction: numerical modelling of shrinkage in earth plaster onto brick during drying.

Laboratoire GERS/SRO

After the monopoly position of hydraulic binder (Portland cement) during several decades, the traditional construction modes are coming back. In this context, raw earth material offers several opportunities for architects and builders in term of environmental benefits. Composed by only water and local mineral resources such as soil composed by sand, silt and a clay fraction, walls built with raw earth materials usually need to be covered by earth plaster for protection. During the plaster implementation, clay may induce shrinkage during the drying stage at the end of the manufacturing, conducting to damaging cracks. Not only the plaster formulation (clay nature, added sand fraction and water content) impacts the occurrence of plaster degradation but also the wall characteristics (porous structure) and the wall/plaster interface. The interface between earth-based support such as compressed earth brick (or more largely porous support such as fired brick, stone, concrete blocks…) and earth plaster needs to be better understood. Internship will contribute to define the compatibility between support and plaster, while the next step will be the control of the plaster deformation under moisture variation to prevent the fissuring while increasing the plaster durability by limiting erosion in contact with rustling water.

The internship proposes a numerical modelling work using parameters measured in previous experimental work. We would like to explore various aspects of numerical calculation to simulate cases close to practice in raw earth construction. The main work will be numerical but some punctual experiments may be realized to complete the set of parameters required in modelling or to validate modelling results (i.e. some dynamic image correlation (DIC) test, suction curve, material shrinkage during drying).

The different steps should be the following (note that the program may be modified after each step):

1. Bibliography on earth plaster from patricians and scientific point of view. Establish the various processes occurring in earth plaster/support during drying according to the imposed environmental conditions and establish the parameters that need to be measured especially for a modelling using thermo-hydro-mechanical coupling using COMSOL MultiPhysics.

2. The numerical study can be carried out according to the following steps:

• Analyzing the sensitivity of each parameter on numerical results
• Investigation in order to understand how to detect numerically the cracking phenomenon in plaster (in relation with tensile strength of earth plaster).
• Evaluation of the plaster shrinkage behavior and comparison of amplitude with experimental results.
• Exploring some multilayer systems with a plaster layer with coarse sand followed by a fine sand finishing layer (as met in practice on site), using experimental data from previous studies and material characteristics from the literature.
• The impact of the porous support of earth plaster will be studied too:  a porous rigid support such as a fired clay brick or a water-sensitive raw earth brick submitted to volume variation with water variation.
• Finally, the impact of the initial water content conditions (gradient in the porous support or environmental RH/T°C during implementation) will be numerically studied.

Master 2 student in the domain of geotechnics (soil mechanics, unsaturated soil) or material sciences (geomaterials) with numerical skill.