Re: SB: Fly ash

[John Kingsley <johnk@magma.ca>]

Hi John,

Regarding map cracking, from a stucco handbook I have, it says:

  Map cracking usually caused by:
  a) inadequate sand in the mix,
  b) inappropriate gradation of the sand,
  c) excess water in the mix,
  d) inappropriate formula,
  e) lack of lath, or
  f) failure to gap plywood or OSB board.

Note: The stucco handbook is geared towards applying stucco
to stick houses, not SB houses.

My understanding is that the most common reasons for map
cracking is a) and c).

I've never heard that too much lime can cause map cracking.
Although d) above maybe implies the cement-lime ratio may
be important.  In another spot in the handbook, it says:
   The higher the portland cement, the greater the likelihood of
   plastic shrinkage cracks.
This is the opposite of what you say below.  Who knows?

I'm no plaster expert.  Just relaying what I have read in books.

On my 6:1:1 wall I had very little map cracking.  Although I did
add some poly fiber to the mix, perhaps this helped with the
cracking.

Regarding vapour permeability, I wanted my exterior wall
to be as vapour permeable as possible.  That's why I went
with the 1:1 portland:lime ratio.

My thinking is that no wall is perfect - every one will have
cracks of some sort.  And the water will eventually get in all
walls, no matter how it is built.  So build the wall as vapour
permeable as possible on the exterior so that the water vapour
can exit (when it heats up by the sun).  Note: this is from the
perspective of a someone who lives near Ottawa 
(not sure about other climates).

cheers,
John

John Salmen wrote:
> 
> I'm curious to hear more about this as my natural inclination is in the case
> of cement renders to eliminate the lime altogether and that is how I have
> worked with it. Traditionally a 'good quality' 3 coat stucco did not use
> lime at all or a very small percentage (10-15% of cement max) to improve
> workability. You use a cement stucco if you want the benefits of a cement
> stucco which are stiffness and impermeability. You don't want to detract
> from these properties as you compromise the inherent properties of the basic
> material and the wall. Too much lime in a cement mix increases permeability
> but also can promote map cracking (which is also a water entry source) so
> you end up with a potentially overly porous wall.
> 
> Now the properties of a lime render are different; flexibility and
> permeability. Is less prone to cracking in stress and can be self-healing of
> cracks - a whole different material with different benefits.
> 
> I'm definitely open to other opinions and information on this.
> 
> John

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