"A-Value"

[chabriac]

Hello
I've already post in French section but I try again here (so excuse my poor english)
I'm trying to evaluate the comportment of rammed earth with Wufi 1D
I've got the moisture storage function but in order to generate the liquid transport function i need the A-value: the water absorption coefficient.

Is there an experimental method to calculate thsi A-value? Could I calculate it? What does it really mean.

I read lot of documentation, (Krus, Kunzel... ) but i couldn't find the answer

thanks for your help

[Thomas]

Is there an experimental method to calculate thsi A-value? Could I calculate it? What does it really mean.

Hi chabriac,

in order to measure the A-value you take a dry sample (or several samples) of the material with a simple shape (box-shaped, prismatic, cylindrical, ...). You weigh the dry sample and then put it into a tray of water, so that one face is submerged a few millimeters. If the sample is placed on a submerged metal grid or plastic mesh, the water has free access to the absorbing face.

After a while you take the sample out of the water, remove any adhering loose water with a damp towel and weigh the sample in order to determine how much water it has taken up. Then you put it back into the tray and repeat the weighing after appropriate amounts of time.

If you plot the amount of absorbed water (per m² of absorbing area) against the square root of elapsed time, you will usually find a straight line. The slope of that line is the water absorption coefficient or A-value.

If the sample is very thin, you'll have to work quite fast. If the material changes its properties during the wetting process, the plot will not produce a nice straight line. Then evaluate the A-value for a time which is appropriate for the duration of the wetting processes you expect to occur in your simulations.

If the experiment takes a longer time and evaporative losses through the sides could occur, you may seal all the sides of the sample, except the absorbing face, for example with some epoxy resin. (You may also leave the opposite face open to allow the displaced air to escape).

Our FAQ How to determine Material Properties mentions standard EN ISO 15148 as the relevant standard document. Google found a draft of prEN 15801 which may also be helpful. However, for using the data with WUFI it is not strictly necessary to comply with some standard. But the standard documents may provide convenient descriptions of the method. (And if your investigations are done for some testimony in a legal case, it may be useful if you can argue that your data comply with this or that standard...)

This report has a short description (including photos) of the method applied to wood samples.

Regards,
Thomas