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Modelling the Hygrothermal Behaviour of Hemp Concrete: From Material to Building

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Sustainable Agriculture Reviews 42

Part of the book series: Sustainable Agriculture Reviews ((SARV,volume 42))

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Abstract

Global warming, the scarcity of natural resources, the polluting emission is a major concern for the human community. The construction sector especially has a significant impact on the environment and has therefore a role to play in the development of innovative sustainable solutions. Bio-based materials are known to be an interesting solution to address energy and environmental issues. In particular, hemp, fast growing renewable raw vegetal, has the qualities to be a serious alternative to modern insulation solutions. Hemp wool using the fibres and hemp concrete using the shivs of the plant have interesting hygrothermal properties and a good thermal insulation level. Their porous, hygroscopic and permeable structure gives them high moisture transfer and storage capacities, improving the hygrothermal comfort felt by the inhabitants.

In order to answer to the future environmental and energy regulations, expanding the use of hemp in the building sector depends on a better knowledge of its hygrothermal behaviour and its response to climatic variations.

Hemp concrete presents a significant hysteretic behaviour. This complex behaviour influences the evolution of the moisture content inside the material which is a key factor of the evolution of the hygrothermal properties and transfer. This chapter reports that the recent consideration of numerical models suited to the hemp concrete hygric behaviour implemented in a heat, air, and moisture transfer model has improved the knowledge and prediction of the hemp concrete hygrothermal response. Especially, we show that the modelling of the temperature-dependence of sorption mechanism allows to better represent the effective response of hemp concrete subjected to real weather conditions. The predicted local daily variations of temperature and relative humidity through a wall are found to be consistent with the experimental ones. Moreover, we review experimental campaigns lead in situ which show that hemp concrete helps to maintain a good level of hygrothermal comfort.

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Abbreviations

BET:

Brunauer, Emmet and Teller

DSC:

Differential scanning calorimetry

DTA:

Differential thermal analysis

DVS:

Dynamic vapour sorption

GAB:

Guggenheim, De Boer and Anderson (at the origin of GAB formalism)

HAM:

Heat, air and moisture

HLC:

Hemp-lime concrete

MBV:

Moisture buffer value

REV:

Representative elementary volume

RH:

Relative humidity

SEM:

Scanning electron microscopy

cp :

specific heat capacity [J.kg−1.K−1]

Cφ :

Milly’s coefficient [K−1]

Dl :

isothermal liquid diffusion coefficient [m2.s−1]

Dl φ :

liquid diffusion coefficient under relative humidity gradient [kg.m−1.s−1]

Dl,T :

liquid diffusion coefficient under temperature gradient [kg.K−1.m−1.s−1]

Dl T :

total liquid diffusion coefficient under temperature gradient [kg.K−1.m−1.s−1]

Dl,w T :

isothermal liquid diffusion coefficient under moisture content gradient [m2.s−1]

Dv :

vapour diffusion coefficient [kg.K−1.m−1.s−1]

Dt :

total moisture diffusion coefficient [m2.s−1]

e:

thickness [m]

E:

molar bond energy [J.mol−1]

g:

mass flow density [kg.m−2.s−1]

h:

capillary height [m]

H:

enthalpy [J.m−3]

kg :

effective moist air permeability [m2]

Kl :

hydraulic conductivity [kg.m−1.s−1.Pa−1]

lv :

latent heat of condensation [J.kg−1]

m:

mass [kg]

M:

molar mass [g.mol−1]

n0 :

open porosity [%]

n:

total porosity [%]

NA :

Avogadro’s number [6.02 1023 mol−1]

p:

pressure [Pa]

qst :

isosteric heat [J.kg−1]

q:

heat flow density [W.m−2]

r:

pore radius [m]

rm :

mixing rate [−]

R:

perfect gas constant [8.314 J.mol−1.K−1]

S:

specific surface [m2.kg−1]

Se :

effective saturation [−]

t:

time [s]

T:

temperature [K]

u:

mass moisture content [kg.kg−1]

v:

air velocity [m.s−1]

V:

volume [m3]

w:

moisture content [kg.m−3]

α:

thermal diffusivity [m2.s−1]

γ:

dynamic viscosity [Pa.s]

δa :

air permeability [kg.m−1.s−1.Pa−1]

δp :

vapour permeability [kg.m−1.s−1.Pa−1]

δp,a :

air vapour permeability [kg.m−1.s−1.Pa−1]

θ:

contact angle [°]

λ:

thermal conductivity [W.m−1.K−1]

μ:

vapour diffusion resistance factor [−]

ρ:

bulk density [kg.m−3]

σ:

surface tension [J.m−2]

φ:

relative humidity [%RH]

∗:

equivalent

∗∗:

fictitious

0:

dry

a:

air

o:

open

ads:

main adsorption

adv:

advection

cond:

conduction

conv:

convection

cr:

critical

diff:

diffusion

des:

main desorption

g:

gas phase

l:

liquid water

lat:

latent

m:

molecular

p:

pore

sat:

saturated

sen:

sensitive

suc:

succion

t:

total

v:

vapour water

w:

water

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Correspondence to Yacine Aït Oumeziane .

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Aït Oumeziane, Y., Collet, F., Lanos, C., Moujalled, B. (2020). Modelling the Hygrothermal Behaviour of Hemp Concrete: From Material to Building. In: Crini, G., Lichtfouse, E. (eds) Sustainable Agriculture Reviews 42. Sustainable Agriculture Reviews, vol 42. Springer, Cham. https://doi.org/10.1007/978-3-030-41384-2_6

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