Surface Weathering

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Toby Cambray
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Surface Weathering

Post by Toby Cambray » Mon May 20, 2019 6:30 am -1100

Hello

I'm working on a concrete building about 70 years old. To inform material selection within Wufi, we took some core-samples and have completed absorption testing on them (as per BS EN 15148, with a few deviations e.g. the samples were smaller than the standard calls for). We noticed that the 15148 results were very different to results obtained from Karsten tube testing. We noticed all our 15148 tests were made with freshly cut sides downwards (in the water bath), so we re-tested with the weathered surface submerged, and the cut faces sealed with Vaseline, and the results were significantly lower, and in much closer accord with the Karsten tube testing.

It appears this difference is due to surface weathering - I would hypothesise that lichen, particulate pollution etc are filling the pore structure. this would affect the vapour permeability also, and perhaps to a lesser extent some of the other moisture-related properties.

A basic search does not yield any research on this topic, can anyone point me to any relevant information?

This factor could have significant implications for how we model masonry. Am I right in thinking that the materials within the database were measured from cut samples, i.e. no weathered face? This would make sense for the 'bulk' of the material. So should we model a thin layer at the outside surface to represent the less absorbent, less vapour open region? If so, how can we estimate the relevant properties and how thick should the layer be?

I have also realised that this effect could make the alpha-values derived from Karsten tube testing unreliable.

I'd welcome any thoughts, guidance or information on this!

Tobi
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Re: Surface Weathering

Post by Tobi » Mon May 20, 2019 9:13 pm -1100

Hello Toby,

I think you should differ between two aspects, the first one could be the "impact of weathered surfaces on material properties" like the a-value. Your assumption sounds possible for me, we already know that aged facades behave different then newly build ones. One important example since the early days of WUFI is concrete. The impact form air pollution and weathering seems to be an interessting effect for a research proposal.

The second aspect is, that you should be aware that the accuracy of the measurement with a karsten tube isn´t comparable with a labratory testing algorithm the difference could be very high on the same evaluated obeject/surface, the colleagues form BFH have investigated this issue, too. Two literature sources, unfortnately both in german but maybe google translate will help.

https://www.google.com/url?sa=t&rct=j&q ... A1yUg6itbZ

https://wufi.de/de/wp-content/uploads/sites/9/548.pdf

Hope this helps. Kind regards,
Tobias

Toby Cambray
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Re: Surface Weathering

Post by Toby Cambray » Mon May 20, 2019 10:26 pm -1100

Hi Tobi

Thanks for your reply. Yes, I'm aware the Karsten Tube method is not very accurate, but it's better than nothing (if only a little bit better...). We have used it in the past to guide the selection of materials, Thanks for the articles on this subject, I'll let you know how I get on translating them. We have done some informal experiments on this in my office too.

I have a contact with access to a micro CT scanner who is willing to put my samples through the machine, which will be super interesting - I will let you know the results of that if you are interested. She suggested that moisture content could cause any organic matter in the pores to expand, which further complicates the issue. Although this might, in theory, give the weathered surface a beneficial property of becoming more resistant to moisture in the winter and rainy periods - like a natural version of Intello!

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