Architectural Precast

[Michael Hurd]

Within the WUFI material database there are various concrete materials available for selection. If I want to model a building that has a 6" thick architectural precast skin, do you have any recommendations as to which concrete might provide the best simulation of this type of material?

Most of the time it is not possible to know the exact concrete mix that a particular precast plant will use, do you think the lack of the exact concrete mix information will make much difference in the calculations?

[Achilles Karagiozis]

Within the WUFI material database there are various concrete materials available for selection. If I want to model a building that has a 6" thick architectural precast skin, do you have any recommendations as to which concrete might provide the best simulation of this type of material?

Q1. Can you match the densities of the concrete with what you might be employing ?

Most of the time it is not possible to know the exact concrete mix that a particular precast plant will use, do you think the lack of the exact concrete mix information will make much difference in the calculations?

Q2. Moisture transport properties are highly sensitive to the additive that are introduced into the concrete, the cement to water ratio and the stone size distributions. The answer is yes they do depend.

[Michael Hurd]

Achilles, I wouldn't have a clue as to what density a common architectural precast panel would be.

Are there any plans for expanding the materials database in the near future?

[Thomas]

Are there any plans for expanding the materials database in the near future?

Dear Mr. Hurd,

we are well aware that choosing the appropriate material from the database (or even deciding whether one of the available materials is appropriate for the case at hand) is a recurrent problem for many WUFI users. We occasionally have these problems ourselves.

Since the ability to run realistic hygrothermal simulations is a relatively recent development, there is as yet only a limited body of "practical wisdom", "reliable rules of thumb" and "practitioner's gut feeling" in this area. Providing simplified yet reliable guidelines for practical use is an ongoing field of research.

Currently, we can only offer what is in the database. We do not have the resources to measure the properties of all possible material variants on the (international) market(s). We hope the database offers a more or less representative sample of common materials. We will add more materials as the data become available, but it is a slow process. And even if there were hundreds of materials in the database, there is still the question of which material to choose in this or that case. I can currently only offer some general advice:

Please note that the detailed hygrothermal properties of precast concrete panels (or any other material) are only relevant if they play a key role in the hygrothermal behavior of the complete wall assembly. For example, if the leakage rate of the concrete walls of a water reservoir is to be estimated, if the water vapor infiltration through concrete cellar walls is to be assessed or if the risk of frost damage to some concrete construction exposed to rain must be investigated.

If your construction were to contain some concrete panels for structural reasons only, then the hygrothermal behavior of the construction might not depend markedly on the exact properties of the concrete as long as these remain within some permissible range. In this case, you could choose almost any type of concrete and still get realistic simulation results.

In other words: the question is how sensitive the results are with respect to variations of the input data (input data being material properties, weather conditions, surface transfer coefficients etc.). If the results are not sensitive w.r.t. some material property, you may use any value for this property within some reasonable range. If the results are very sensitive, then care must be taken to choose an appropriate value for that property.

Do the results for your wall sensitively depend on the choice of a particular type of concrete? To find out, you can run tests with the different types, and you can even vary some material properties beyond the range represented by the materials in the database. This will be some work, but it will give you first-hand experience of how the results depend on particular properties. If the results are more or less the same for any type of concrete, it does not matter which one you choose. If it turns out that they depend critically on, say, the water absorption coefficient being below some limiting value, then it must somehow be determined which value this coefficient has for the panels that will actually be used; there is then no way around that (but you can show your WUFI results to demonstrate that it is important).

Our experience so far shows that in most cases there is no need to be overly timid when selecting material data, at least when well-established constructions or small variations thereof are investigated. After all, the established types of construction must automatically pass a weeding-out process which leaves only those which do not fail immediately when the type of concrete used is slightly different, when slightly different boards of wood are applied, when the paint coat has a slightly different permeability etc. Only constructions with some inherent hygrothermal robustness can be durably built without extremely tight and expensive control that the material parameters be within a narrow preassigned range.

This may be different for new kinds of construction, but then you can simulate this weeding-out by playing around with WUFI, changing material data (or other input data) and watching the results. This will also give you some valuable experience and "feel" for what hygrothermal processes are going on in your construction.

Regards,
Thomas

[Michael Hurd]

Thomas, thank you for all of your recent replies to my questions. I really appreciate your input.