Bio-aggregates Based Building Materials

This chapter gives the state of the art on the chemical composition of bio-aggregates and their interactions with mineral binders from the standpoint of bio-aggregate based building materials. The chemical composition of various bioresources included in bio-aggregate based building materials has been reviewed by comparing the results of 24 published references. This comparison highlights a large dispersion among the results of different references for the same bioresource, reflecting not only the effective variability of chemical composition due to agronomic, environmental and processing parameters but also the strong disparities in the results of common indirect gravimetric methods of biomass compositional analysis. However, the chemical composition of lignocellulosic particles can strongly impact their properties as bio-aggregates included in a mineral matrix. At early age, they can disturb the setting and hardening mechanisms of mineral binders; in the hardened state, they can modify the properties of the composite; and, finally, in the long term, they can influence durability.

The high porosity and microstructure of bio-aggregates are fundamental to their physical properties. Typically they have a low density and a complex pore structure. This has two principal effects. In the first instance, low density is associated with low strength, but also with low thermal conductivity. For this reason most bio-aggregates are not suitable for use as structural materials, but are eminently suited to act as a low density filler in composite materials conferring low thermal conductivity on the resulting bio-composite. The complex nature of their porosity results in a material that is able to readily adsorb moisture and humidity. This results in a material that has an exceptionally high moisture buffering capacity, a characteristic that is of great interest in building materials, because it tends to stabilise the internal environment of a building, thereby resulting in a much more healthy indoor environment. This chapter considers the range of methods that can be used to measure porosity and to characterise the microstructure of materials in general, and discusses how some of these techniques have been used on bio-aggregates. It also identifies opportunities to use novel techniques on bio-aggregates in order to improve our understanding of their porosity, pore size distribution, pore connectivity and microstructure, all of which are characteristics that are essential to the optimisation of the performance of bio-aggregates within the construction industry.

In “agro-concretes”, highly-porous plant-based particles are used, and 5 are responsible for massive water absorption.

In this chapter, a state of the art of Particle Size Distribution (PSD) measurement of bio-based aggregates and characterization methods is presented. Shiv particles coming from the stem of plants cultivated either for their fibers (hemp, flax, etc.) or for their seeds (oleaginous flax, sunflower, etc.) are very different from the mineral aggregates typically used in concretes. Owing to the structure of the stem of the plant they are made from, such aggregates are generally malleable, elongated and highly porous with a low apparent density. Irregular shape are generally observed, especially in case of shiv coming from fiber plant due to the shredding action of the decortication process. Such ground bio-mass lead usually to uni-modal size distribution that can be efficiently characterized using basic distribution models with two parameters. Starting from the standardized tools and techniques developed for mineral aggregates, other technics using image processing are investigated and discussed in the global perspectives of the effect of the PSD on the properties of the in-service building material.

Hemp is made of highly deformable particles. Depending on the water content, on the particle size distribution, and on many other material parameters such as initial porosity and retting of the processed stalks, the mechanical behaviour of shiv in bulk can change significantly. In a compaction process, the mass per volume of the raw material increases with the applied stress and some creep or relaxation effects occur as observed in wood based materials. Hence, the mechanical properties of the bulk impact the packaging of the raw material as the shiv density inside the final mix and the in-service properties of the composite material. In this way, the bulk compressibility is primarily useful to manage the building processes, from transport of the raw material, to mixing and casting.

This chapter gives the state of the art of previous studies on hygric and thermal properties of bio-aggregate based building materials. Firstly, hygric characteristics such as sorption isotherms, water vapor permeability and moisture diffusivity are given. The ability of bio-aggregate based building materials to moderate ambient relative humidity may be valued using moisture buffer value. Then thermal properties (thermal conductivity, thermal diffusivity conductivity and specific heat capacity) are reported. Finally, concluding remarks on hygrothermal behavior with simultaneous heat and mass transfer are provided, they underline that considering only thermal conductivity and specific heat capacity is not sufficient to evaluate the energy performance of bio-aggregate based building materials. The results found in bibliography mainly concern wood-based and hemp-based materials.

This chapter reports the state of the art of several investigations on behaviour of bio-aggregate based building materials exposed to fire. Discrepancies between fire reaction and fire resistance is highlighted in this chapter. Various results of fire reaction test performed on bio based materials are presented. Bio-aggregates are often in Class F while concretes range in class B1. In the case of fire performances, limited tests are performed and reported in the literature. Such test must be performed on wall or part of building to give the more realistic overview of the behaviour of building structure exposed to standard fire. In some of presented case studies, render and plaster play a key role in the fire resistance. EI 90 fire resistance appeared to be accessible with conventional technologies.

Used for several decades for building insulation, concretes containing plant aggregates have thermal, acoustic and hygrothermal properties that greatly improve the comfort of homes (Amziane and Arnaud 2013). Nevertheless, during their life time, they are submitted to a hydrothermal environment (humidity and temperature variations) that can change these functional properties and/or induce the development of microorganisms on their surface. The objective of this chapter is to present the state of the art on the evolution of the properties of these vegetal concretes after different types of aging in laboratory.

Production parameters affect hydration/carbonation and density of hemp concrete which consequently determine the strength and the hygric and thermal properties of the concrete. This chapter investigates the effect of production parameters including curing conditions (65% vs. >95% RH), time of demoulding and specimen geometry (cylinder vs. cube) on the concrete’s strength which relates to density and therefore to thermal and hygric properties. It studies hydration in the concrete’s microstructure and measures the 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 chapter 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 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.