Cob: stucco question?

[Mike Wye]

Jay,
You start by saying "bearing all of this in mind" but your comments don't
suggest this.
< Correctly constructed dry stone walling may be strong enough
< relatively safe dry stone construction
I've never come across a dry stone wall that has been used as the foundation
or a plinth for a cob house. The foundations ( if any) and stone plinth were
always bedded in an earth or lime mortar or sometimes a mix of the two. Yet
the fields were often marked out with dry stone walls so these techniques
were well known but the people building cob houses in the last five
centuries didn't chose to use them for their homes. This could be a clue.
Pointing a wall with lime mortar doesn't stop water ingress - most stone
types are porous and will absorb rain water as will a lime mortar. The
foundations and stone plinth are also subject to rising moisture due to the
solid wall construction, if the water table is sufficiently high - it
certainly is in the most of the south west of England.
Failures in dry stone walling aren't devastating if their field boundaries.
The effects of failure would be more damaging if they were supporting your
home.
Mike



Mike Wye & Associates,
Traditional & Ecological Building Products
www.mikewye.co.uk
01409-281644
sales at mikewye.co.uk
----- Original Message -----
From: <jamie.ayres at m2.com>
To: <coblist at deatech.com>
Sent: Tuesday, August 14, 2001 4:46 PM
Subject: Re: Cob: stucco question?


> Bearing all of this in mind, and that correctly constructed dry stone
> walling may be strong enough to hold up a cob house, would it be safe to
> say that you could simply use a softer (less pozzolan) lime mortar simply
> for weather proofing?
>
> ie, instead of relying on an unknown factor for such an important job as
> holding the house up you could rely on the relatively safe dry stone
> construction and just point the wall with softer lime mortar that will
> prevent water ingress?
>
> Jay
>
>
> Jamie Ayres / M2 Communications Ltd / http://www.m2.com
> Any views expressed in this e-mail are not necessarily the same as those
> of my employer.
>
>
>
>
>
>
> "Mike Wye" <mike at mikewye.co.uk>
> Sent by: owner-coblist at deatech.com
> 08/10/01 08:50 PM
> Please respond to "Mike Wye"
>
>
>         To:     "Charmaine R Taylor" <tms at northcoast.com>
>         cc:     <coblist at deatech.com>
>         Subject:        Re: Cob: stucco question?
>
>
> Charmaine wrote,
> > Mike, as a matter of course would you recommend hydraulic hydrated lime
> > for all base stone work (foundations, etc?)  or is is priudent for owner
> > builders of stone bases mix their own clay-ash pozzolans into reg lime?
>
> Yes I would recommend an hydraulic lime mortar mix, by either route that I
> outlined, if the stone foundations/plinth are going to be permanently
> damp.
> How much pozzolan you add depends on how reactive the burnt clay portion
> is
> and how much compressive strength you need in the bedding lime putty
> mortar.
> The questions as to how a structure will resist earthquakes are academic
> if
> there isn't sufficient compressive strength in the
> foundations/footings/plinth to actually take the weight of cob above. Lime
> putty mortars have relatively low compressive and tensile strengths to
> start
> with. This is an advantage in many uses but not where the wall is
> loadbearing and the pure lime putty mortar's compressive strength would be
> insufficient. When adding a pozzolan its a question of how reactive is the
> pozzolan and how much you choose to add.
> At one end of the scale you can add a little pozzolan and barely change
> the
> characteristics of the lime putty mortar or you can add so much pozzolan
> that it reacts with every bit of available lime and makes a mortar as hard
> as any concrete. Where you want to be on that scale depends also on the
> location of your structure, the weight and thickness of the walls, climate
> its exposed to, position of water table etc etc. Historically there were
> wide differences in the lime/earth bedding mortar mixes used and their
> characteristics.
> We're aware of cases in England where people have used pure lime putty
> mortars for building and rendering externally without being aware of the
> history and existence of pozzolans and have "rediscovered" belatedly why
> they form an important part of lime building practice when they find that
> their pure lime mortar is still soft and uncarbonated months after
> application and then fails in the first frosts of winter.
> regards,
> Mike
> Mike Wye & Associates,
> Traditional & Ecological Building Products
> www.mikewye.co.uk
> 01409-281644
>
>
> > Mike said:When working with a lime mortar in foundations and stone
> > plinths
> > they traditionally used an hydraulic lime mortar either by
> > 1. adding a pozzolan to a lime putty/sand mix to set off some part
> > of the mix prior to carbonation.
> >     ( a pozzolan is a burnt clay such as volcanic ash, brick dust,
> > crushed
> > clay tiles etc which reacts with the lime to     form cementitious
> > compounds)
> > 2. using a natural hydraulic lime mixed with sand.
> > Permanently damp stone structures don't allow sufficient air and
> > hence
> > carbon dioxide into the mortar matrix to carbonate the lime to the
> > extent
> > needed.
> >
>
>
>
>
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