Experimental investigation of the shear behaviour of hemp and rice husk-based concretes using triaxial compression

Abstract

Lime and hemp concretes are used as infill materials manually tamped in timber stud walls or more recently in the form of precast blocks. In either case, the structural design practice of wood frame construction associated with hemp concrete does not assume any contribution of the plant-based concrete whereas it may contribute towards the racking strength of the wall.This work is intended to evaluate the shear behaviour of two different bio-based concretes by means of triaxial compression.Hemp shives and whole rice husks were mixed with a lime-based binder according to the same mix proportioning and mixes were vibro-compacted in cylindrical forms. Then, samples were cured at 23°C – 65%RH before being tested under unconfined and triaxial compression. The triaxial shear test was performed after 60days of curing on unsaturated specimens under drained conditions at air pressure and for increasing effective confining pressure (from 25 to 150kPa).It was possible to estimate the shear strength parameters (peak friction angle and cohesion) of the two plant-based concretes. Comparing results, it appears a consistent value of cohesion but a different friction angle related to the binder and the aggregate contributions respectively. This leads to a first analysis of the relationship between the composition of the concrete (plant aggregates cemented with a binder) and its shear strength. Furthermore, the ductility of plant-based concretes is markedly enhanced as the mean effective pressure increases. Finally, the shear strength of plant-based concretes should be considered as part of the design practice of building envelopes.