Effect of Testing Variables on the Hydration and Compressive Strength of Lime Hemp Concrete

This paper investigates the effect of testing variables including curing conditions (65% vs>95% RH), time of demoulding and specimen geometry (cylinder vs cube) on the compressive strength of hemp concrete. It studies hydration in the concrete’s microstructure and measures compressive strength development at intervals between 1 day and 1 month. Moulding time and curing conditions influence drying and therefore may impact binder hydration and consequently strength evolution. Specimen geometry may affect drying and can also determine how strain builds up in the concrete and thus when failure occurs. The paper concludes that curing hemp-lime concrete with hydraulic content (50%CL90: 50%CEMII) at high RH (>95%) lowers compressive strength (65.4% drop at 10 weeks). It is unclear why this happens, as the presence of water vapour during curing at high RH should enhance hydration and consequently increase strength. It was also found that delaying specimen demoulding increases compressive strength of the CL90:CEMII concrete (22.9% increase at 10 weeks), probably due to the presence of moisture for longer enhancing hydration. The NHL3.5 concrete shows the same trends although the results are not statistically significant probably due to lack of sensitivity of the measuring instrument at low strength values. The specimen geometry does not significantly impact the ultimate compressive strength of hemp-lime concrete however, it affects behaviour in compression. Initially, cylinders and cubes deform on load application up to a similar yield point. However, following this yield point, the cylinders fracture showing a more brittle behaviour while the cubes keep crushing to finally experience an additional stiffness produced by mechanical bridges being formed between opposing cell walls.