DPC plastic under surface bed and in the wall?

[BauTomTom]

Hallo Struc.Eng's

how important is it actually to provide a DPC plastic under the surfacebed? The detailing books actually shows that the DPC should go then actually even from under the surfacebed into the wall. Lets say maybe 400mm above the ground level.

How important is it actually and when is the reall perpouse? Do you need it even in dry areas?

BauTomTom

 

[Ron]

I will assume you are discussing a vapor barrier placed under a bedding material onto which a concrete slab is cast. I'm not familiar with your terms "DPC" and "surfacebed".

You must first understand the need for a vapor barrier. It is two-fold. First is to provide protection against excessive loss of moisture from the concrete during the initial set and curing into the subgrade. If you place the vapor barrier beneath the bedding layer, this will not be accomplished unless you wet the bedding soil before placement, which can cause a few other problems.

Another reason for the vapor barrier is to prevent vapor migration from the soil into the concrete slab. It makes no difference where you place the vapor barrier (directly under the slab or under the bedding layer) for this purpose; however, I'm not convinced that there is a great deal of detrimental vapor transmission into the slab from the soil unless the water table is within a foot or so of the slab bottom.

Once you understand the purposes of a vapor barrier, you can then decide whether it is needed and where it should be placed to best suit your project's needs. Also, you might consider that in some areas, a vapor barrier is required by the building code or at the least perceived by the inspectors to be required. Check you local code requirements.

Vapor barriers can cause numerous problems in the construction of slabs and in the performance of slabs. Be careful how you specify them and how they are allowed to be placed and protected at construction.

 

[BauTomTom]

--- to ron ---
Yes this is correct this are the two reason which I know to prevent moisture immigration from the soil into the slap and to keep the moisture in the fresh concrete for the hydration process, where I actually thought that this maid be the main reason, which could be provided as well with the wetting of the surface prior concreting. This would save the plastic.

Now I read somewhere that it is also needed to give the surface bed (bottom floor) the possibility to move, to slide on the sub surface. Can this me? Apparently it helps for cracks. Hhhmm…

You say “…Vapor barriers can cause numerous problems…” really? Which once?

I attached a detail picture which shows you that they take the vapor sheet also up into the wall. Is this really necessary? In case the water would migrate up the wall so then it would come right at the plastic out again. It would make more senses to (if really necessary) placed the plastic much more deeper, or?

Bau Tom Tom

 

[Ron]

No. It is not necessary to continue the vapor barrier into the wall.

Yes, the vapor barrier does provide some relief from subgrade restraint to prevent cracking; however, if saw-cuts are placed properly and at proper times after the slab placement, the restraint should not be a big issue.

 

[hokie66]

I think two things are being confused here. "DPC" is an acronym for "damp proof course", which is used to prevent rising damp in masonry walls. A vapor barrier is placed under slabs. The detail shows them being combined, which is not normal practice.

 

[BauTomTom]

-- to hokie 66 --

you right, actually this should be two different things but it also wouldn't make sensse if you woudl stop with the vapor just at the wall and provide some other DPC in the wall. Through this gab the moisture could actually imigrate to the inside of the building.

I don't know if this two are areally imprtant becuase how often do you see that nothing is provided

BauTomTom