Agriculture Reference
In-Depth Information
•
The large amount of air trapped in and
It is strongly advised that only those binders rec-
ommended for these specific applications be
used [
Règles Professionnelles de la
Construction en Chanvre
; see the site on con-
struction with hemp -
http://www.construction-
chanvre.asso.fr]
. Many tests have been
performed successfully on mixtures of air and
hydraulic lime. The results presented in this
chapter are from this work, ENTPE: National
School of State Public Works
(www.entpe.fr).
Classical trials to characterize hemp
cement and mortar are available today [ENTPE
and Professional Rules of the
Construction en
Chanvre
]. The object here is not to provide an
exhaustive account but simply to characterize
the essentials and relate these to the function
at the level of the envelope.
The material is characterized by an elasto-
plastic behaviour: demonstrating plastic distor-
tion with very little encouragement. It does not
show obvious fragility or break easily and can
be bent out of shape without snapping sud-
denly. Looking beyond this mechanical descrip-
tion, we can identify a pattern of behaviour
that is interesting for several reasons. Figure
17.7 provides a comparison of the results
obtained in tests to measure resistance to com-
pression for (i) the binding agent on its own,
(ii) various mixtures of hurds and the binding
agent (in various proportions) and (iii) hurds
alone. The binding agent on its own is a dense
material and ruptures suddenly into several
pieces after only a small amount of distortion.
The hurds, when tested on their own, demonstrate
between the particles ensures a low ther-
mal conductivity.
The porous aspect of the surface ensures
•
that these materials absorb noise well.
Finally, these same porous characteristics,
including pores of variable size (micropores of
the constituent materials and macropores
resulting from their chaotic orientation), influ-
ence water exchange strongly (i.e. the move-
ment of both liquid water and water vapour).
Mechanical behaviour
Hemp mortar and cement are also available as
composite materials associating a rigid matrix
of flexible granular particles. During the instal-
lation of such material, the chaotic arrange-
ment of the particles relative to each other
leads to the creation of pores between these
same particles (Fig. 17.6).
The mechanical properties of cement and
hemp are linked directly to the properties of the
binding agent. The choice of binder must take
into account three factors. It must first be easy
to create and should coat the different constitu-
ents easily. It should run smoothly through the
structure and, after setting, should provide the
mechanical properties that are essential to its
role as a mortar. It should be noted that the
physic-chemical interactions of the different
components are extremely complex, as the
plant material is far from inert in this mixture.
Fig. 17.6.
Test tube of hemp cement - surface view.