Development of microstructure in compacted earthen building materials

The earthen construction material rammed earth (RE) is effectively a manufactured unsaturated soil. The sources of strength in RE are presumed to be suction, interlock and cementation. Previous work (Jaquin et al., 2009) has proved the link between the first of these features and strength, and cementation is also reasonably well understood. What remains unclear is the nature of the particle interlock, which is controlled by the material microstructure (i.e. the particle and void size distributions and the arrangement of water held at menisci or in bulk) and so the compaction process by which the material is made is of major significance. In this paper we present initial results from laboratory testing on artificial RE materials, to investigate microstructure formation during compaction. Use was made of so-called "transparent" clay materials, and a specially constructed compaction chamber to enable particle movements to be viewed more clearly. Novel use of a document scanner allowed images of the compacted material to be acquired. Particle Image Velocimetry (PIV) software was used to determine particle displacements from the scanned images. Various difficulties with the procedure are outlined and some results are presented showing the movement of particles and the consequential development of microstructure.