Evaluation of shear strength parameters of bio-based concretes by means of triaxial compression

Abstract

Recent decades have witnessed the emergence of plant-based building materials that combine crop residues with lime binders. This return to old building methods resulted in the development of hemp concretes. These ones are often used as infill materials manually tamped in timber stud walls. Precast blocks can also be manufactured by static loading or vibro-compaction of the freshly-mixed material. Such a process leads to improved compressive strength, rigidity and ductility. In either case, the structural design practice of wood frame walls associated with hemp concrete does not assume any contribution of the plant-based material whereas the latter may contribute towards the racking strength of the walls. In this context, it is necessary to study the shear behaviour of bio-based concretes since it is currently unknown. This work is intended to evaluate the shear strength 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 and 65%RH before being tested under uniaxial and triaxial compression. The triaxial test was performed after 60 days of hardening on unsaturated specimens under drained conditions at atmospheric pressure and for growing effective confining pressure (from 25 to 150 kPa). The results made it possible to estimate the shear strength parameters (peak friction angle and cohesion) of plant-based concretes. The predominant influence of the aggregate type on the friction angle was underlined and leads to a first appropriate analysis of the relationship between the composition of the material (plant aggregates cemented with a binder) and its shear strength parameters. The shear strength of plant-based concretes was found to be significant and should be considered for the design practice of building envelopes.