Accuracy of liquid transport coeff. from absorption coeff.

[Michel]

I have made a dipping test with a plain brick.
The water uptake was measured and I have deduced the water absorption coefficient (kg/m2h0.5).
Now I simulate this experiment with Wufi. It computes two values of the capillary diffusivity (at 100 and 80% RH). From these values, it computes the absorption and I can calculate the water uptake.
Measurements and calculations are of the same order of magnitude, however they differ by 100 to 300%.
Of course the diffusivity curve assumed in Wufi is exponential. In reality, it is different.
Do you think that the accuracy cannot be not better when the calculation uses the absorption coefficient and not the full diffusivity curve?
Thank you and regards

[Manfred Kehrer]

Michel,

the difference of measured and calculated water uptake is too high so something is not correct. Did you enter the Absorption Coefficient in WUFI in kg/m2s0.5 ?

[Michel]

In fact, the dipping test was made with a rendering (IRB H985 Z5).
After 4 hours dipping, the measured water uptake was 0.38kg/m2. A coefficient of 0.19kg/m2h0.5 was deduced or 0.0031kg/m2s0.5.
For the Wufi computation, most of the other parameters were measured. The following values were used: density 1370, porosity 0.48, specific heat 850*, conductivity 0.3*, vapour resistance factor 17, free saturation 160kg/m3, reference water content 23kg/m3 (RH 80%).
The Wufi uptake was 1.7kg/m2 after 4 hours.
I do not know the composition of the rendering but there is surely a water repellent. Can it be the cause of the discrepancy?
* not measured

[Manfred Kehrer]

Of course a water repellent rendering will make a difference. I think this is the problem. You might have to model this rendering as well.