Effect of compaction pressure on the risk of thermal instability of compressed earth bricks

Earthen materials, one of the oldest known building materials, are also one of the most sustainable materials with promising properties to meet the environmental and economic challenges currently facing the construction industry. For its use in buildings and structures, this material has been studied in almost all of its thermo-hygro-mechanical aspects. However, the fire behavior of such materials is not well known and poorly documented. For this reason, the present study focuses on the fire behavior of compressed earth bricks compacted with variable compaction pressure in order to evaluate the risk of thermal instabilities during fire exposure. Different bricks were made from a mixture of earth and water with compaction pressures of 5, 15, 30 and 50 MPa. The bricks were set to an equilibration period at 75% RH until they were stable. According to previous research, this chosen relative humidity level corresponds to an unfavorable configuration with regard to the risk of thermal instabilities of compressed earth bricks. Each material configuration was subjected to two fire tests of 30 min. The results obtained showed that the risk of instability increased as the level of compaction increased. Further residual nitrogen permeability tests were performed to analyze the results obtained. It was found that the microstructure of the material upon its compaction plays an important role in the build-up vapor pore pressure during heating. This pore pressure was certainly responsible for the thermal instabilities observed.