Too much moisture in concrete slab

[l3oertjie]

Is it possible to dry out a 300mm thick 30MPa concrete slab with a 25mm polystyrene layer and 75mm screed on top already in position? Area under question - approximately 70m^2. Being an HVAC specialist, the engineer asked me - all I can think of is to de-humidify and heat the air below the slab - apparently this is not an option and work has to be done from above..... Any suggestions?

 

[Ron]

It can all be done from above; however, it will be slow. Dehumidification is the best approach as this will pull moisture from the slab better than ambient temperature variation. I would measure the insitu relative humidity in the top 60mm of the slab for a starting point. Leave the measuring sleeves in place and measure periodically to assess the humidity decline and develop a curve to predict when it will be done. Check ASTM F2170 method for the insitu relative humidity measurements.

Once the RH is below 75 percent, the slab is considered acceptable for floor coverings.

 

[dik]

What is the source of the water?

Dik

 

[l3oertjie]

Dik,

Does bad builder count as a source....?

I think the waterproofing was done before the moisture could dry out from the slab. Not 100% sure. If I'm correct, we must assume that the slab has cured but not yet dried out to satisfactory levels before the builder sealed it on top with the polystyrene layer and screed - in principle the moisture inside the concrete is trapped - well, almost, since in can evaporate to below, but surely very very slow?

 

[dik]

and no 'blistering' with the polystyrene?

Dik