Representing Salts in Wufi

[Toby Cambray]

Hello

We have an interesting project at the moment, a large old house in Scotland which has very high levels of salt content in the masonry. We believe the builders used sand from the beach near to the house, and possibly even used sea water to mix the mortar. The house is also very wet, having been abandoned for about 20 years; broken windows and eventually the roof partly collapsed, no heating etc. I’ve taken some samples which we are doing gravimetric testing on to quantify moisture content, and I have found a contact in the UK that will do Ion Chromatography for us to quantify the concentration of salts, and identify the different salt compounds.

I was wondering if there’s any way of simulating the effects of this in Wufi – some aspects could be represented, but not others as far as I can tell. The salt will affect the moisture storage function, which we could quantify by sorption testing which could increase the risk of deterioration of embedded timbers, by pushing up the moisture storage curve. I’m also concerned about the transport of the salt from within the wall to the surfaces as they continue to dry out – depending on the new internal finishes selected, and when there are applied (with respect to the drying process), I am concerned that efflorescence could continue to occur on the internal surfaces. We’ve been exploring the possibilities of salt-blocking plasters or poultice type rehabilitation plasters, the behaviour and safety of which depend not just on moisture transport but on how salt is carried too. As far as I know this is not something that could be represented directly in Wufi? Is there any way that we could account for or otherwise estimate it? Is it possible to extract moisture fluxes at specific nodes within the model – I know the kl, dl, kr, dr values in the simulation summary give an indication near to the surfaces, but is it possible to get a time series of this data at a specific point, somehow?

Thanks

Toby

[Manfred Kehrer]

Hello Toby,
you can access all moisture fluxes over time at all points where layers connect.
Typically, salt increases the liquid transport coefficients due to a higher fluidity. It also decreases the vapor transport coefficient due to a reduction of the pore volume.
Both phenomena you can measure, ASTM E96, ASTM C1794

[Toby Cambray]

Hi Manfred

Thanks for this, I should have looked in the help file first... This is useful function, I feel like I have unlocked an achievement in the game that is Wufi! Some of the dialogues are a little awkward to use though. I noticed you can only plot fluxes in Wufi Graph using data from Wufi 2D (I only have Pro 6) - are you planning to add this feature?

I don't have access to the ATSM's you mention, I assume they equivalent to BS EN 15148 and 12512? I would guess that to make the equivalent test you would use an appropriate saline solution instead of 'pure' water for the absorption test? And for the vapour permeability test - you would some how introduce the correct amount of salt to the specimen before beginning the cup tests?

It would obviously be a lot of work to do this properly - is there any research on how the changes in these parameters could be estimated (and existing materials in the database adapted accordingly?)

Thank you!

[Thomas]

Hi Toby,

WUFI can not take salts and their effects into account. This subject is much too complex to be treated by a simplified and practical tool which WUFI aspires to be. If you wish to try and use WUFI's non-salt results to help you in estimating the behaviour of the salt(s) in your walls, you are welcome, but I'm afraid we can not offer much assistance here, being no experts in this subject. As Manfred has already pointed out, WUFI will give you the liquid and vapor flux densities across layer interfaces, and if you want to know the moisture flux within a layer, you can simply introduce an "artificial" interface at that spot (the materials on both sides being identical).

Salt dissolved in the pore water will affect the transport poperties of both liquid and vapor transport, and the extent of this effect will depend on the salt concentration in the water and the amount of undissolved salt clogging the pores. There may even be new transport effects: Dissolved salt will diffuse through the water and thus transport itself even if the water remains stationary. And I would not be surprised if concentration differences gave rise to additional capillary transport (but I'm no expert here...).

The dissertation of A. Holm describes a few experimental and calculational experiments with salt solutions, concerning adapted moisture storage functions (Fig. 13) and water absorption and drying processes (Fig. 14). However, only "homogeneous" salt solutions with mainly constant concentration are considered.

A. Holm: Ermittlung der Genauigkeit von instationären hygrothermischen Bauteilberechnungen mittels eines stochastischen Konzeptes
(there should exist an English translation, but it seems I can only find a German version online.)

Kind regards,
Thomas

[Christian Bludau]

Hi Toby,
in here is a chapter about WUFI calcualtions with salt:

Rucker, P., Holm, A., & Krus, M. (2000). Determination of moisture and salt content distributions by combining NMR and gamma ray measurements. Proceedings materials week, Munich.
http://www.hoki.ibp.fraunhofer.de/ibp/p ... pub1_7.pdf

Best regards,
Christian