Rust on wallpaper (concrete slab under wallpaper), new construction; with pictures

Hello dear people,

we are currently building a semi-detached house. Today we discovered rust stains on the concrete panels at the construction site. Some parts are already wallpapered – the rust stains are showing through the wallpaper.

The building substance is as follows: concrete at the four corners, in between [Kalksandstein]. The stains only appear on the concrete walls and partly on the ceilings.

Do you know where the stains come from or why there is rust, and what is normally done to prevent this? Seal before wallpapering? Should I be seriously worried about the substance and/or the statics (reinforcement is not visible)?

Thank you very much for your help!

Hello,

Rust occurring in this form is actually rare.
The usual case is rust that deposits on the formwork from the reinforcement and then remains visible after concreting. This rust does not look nice, is considered a discoloration during the assessment, but does not develop further. It can be sanded off and also painted over with the appropriate concrete paint.
This does not apply here.

The second case for rust is that the concrete cover was not maintained and the reinforcement at the voids (this is what these holes in the concrete are called) is not sufficiently protected and can thus rust. For this, you would need to remove the area once and see if the reinforcement then becomes quickly visible. If this is the case, the spots must be treated with appropriate agents (on the reinforcement). Furthermore, the concrete cover should then be checked randomly. However, I assume that this does not apply here, as usually dark discolorations appear on the concrete when the reinforcement is very close to the surface. Otherwise, the concrete looks well processed.

The third case (and in my opinion rare, as we do not have this problem in Lower Saxony where I live) is ferritic components in the aggregate of the concrete that begin to rust because they lie very close to the outside. This would explain the frequency and the appearance as circular rust spots. Therefore, I suspect this is the case with you.

My recommendation would be to remove the concrete in these areas, remove the product (if reinforcement is exposed, protect it), and seal the spot again with the appropriate product.
Just visually preventing the rust is, I think, the wrong way, because (in the worst case) the rusting aggregate lies too close to the reinforcement and also attacks it.

Of course, only an expert on site can provide more precise information. If you live near Hannover in Lower Saxony, I would be happy to take a look at something like this.

Best regards,
Jann Stöter (Concrete engineer)