Minimum thickness of slab over slab

[joncroke]

Hello,

I have a 4" slab in my shop in excellent condition. Every summer there is a tremendous amount of pooling water condensation that is a real hazard. It is also causing the epoxy paint coating to come off. There was no vapor or insulating barrier installed when it was poured.

I intend to solve the problem by covering the slab with an approved vapor sheet, and then an approved insulating board (blue DOW Styro or similar) and then pouring another layer of concrete over this.

My question: what would be my minimumm thickness for the pour?? (Shop only has automobiles as the heaviest equipment.) Is 2" OK?? I was thinking of the fiber reinforced material to avoid metal screen for this top layer.

Any opinions/ advice greatly appreciated!

Thanks
Jon

 

[RiBeneke]

One would need more info to do a design, and you should probably contact a local specialist.

The compressibility of the insulation is an important consideration in the design, as this would allow the new slab to deflect over a small area under vertical load. Also consider whether an automobile might be jacked up using a jack on the floor.

My initial thought is that 2 inch is too thin, perhaps 3 inch may be ok.

 

[brianchaffee]

JON,

Unfortunately, slab stacking with a vapor barrier may or may not solve your moisture/water migration problem.

Before such lengths are committed too, it would be advisable to purchase a small amount of concrete impregnator known as a silane or siloxane.

This material when applied by simply rolling it on to the concrete will permeate the surface traveling into the inherent pores of the concrete and essentially plug these pores, prohibiting the rising moisture from reaching the surface.

This is not a topical sealer, and as a result will not contribute to lack of traction.

A simple search on the net will render a plethora of reputable suppliers for concrete impregnators.

Once applied, refer to the manufacturers Technical Data Sheet [TDS]for cure out, rather short, and then do a moisture test. This can be done by taping a 24" x 24" piece of six [6] mil plastic with duct tape on the floor where the moisture is most excessive and leaving for forty-eight [48] hours. If you see there is no moisture inside the plastic, your problem is solved.

Slab stacking is a tricky application, and you would be advised to consider a more definitive bond breaker besides the visqueen since whatever cracking or control joints that exist in the host substrate will for the most part transfer up through to your new floor slab.

I would refer to RiBeneke's thoughts of making the slab thicker in addition to complimenting the new concrete mix design with fiber, low slump, super plasticizer added at the job manually, wire mesh and proper finishing techniques.

You obviously have the clearance issue covered regarding door thresholds, etc., since you will be raising the floors current elevation.

BRIAN

 

[focke]

Vapor barriers aren't designed to stop water, but will stop rising damp.
where's the water/condensation coming from?
It sounds like you need drain tiles/curtain drains and sump pump installed to drain through the gravel under your existing slab.
Some people feel that water that gets past the seams of your vapor barrier is now trapped and can't drain out quickly.