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Frequently Asked Questions
Cement & Concrete
Technology Home > FAQs > Moisture sensitive floor coverings
Q: How do I design for moisture sensitive floor coverings?
A:The choice of slab
design for moisture sensitive floor coverings should include the
following considerations:
- Concrete water-cement ratio.
- Sub-base moisture conditions.
- Mineral and chemical admixtures.
- Concrete curing.
- Concrete drying environment.
For slab construction of this type, the water cement ratio should
be maintained at 0.4. The sub-base moisture conditions should be
considered to determine if a vapor retarder will be required or if granular cushion will be sufficient
to resist the entrance of moisture into the slab. A water reducer
may be considered for use with a low water-cement ratio concrete
to promote workability and to aid in ease of consolidation.
Curing practice may be altered to accommodate an
early dried condition (three day moist cure).Proper ventilation
and low relative humidity environment are recommended for the drying
conditions.
Moisture related problems are unique with every slab that is placed.
First you will need to consider the ground water and drainage conditions
for each site. This information will determine if a vapor retarder will be required. In most cases if a vapor retarder is not required, a 28 day air drying of the slab
should prove to be adequate as preparation for placement of the
floor covering.
Should retarder be required things get
a little more complicated. There are a number of different ways
to do this and each has its strengths and weaknesses. A vapor retarder
placed below a blotter layer (a layer of sand or granular material
used to allow moisture to evacuate the slab from both faces) minimizes
curling, yet may act as a moisture reservoir to promote higher vapor
pressures. A vapor retarder in direct contact with the bottom of the slab does
not provide this reservoir but forces the convenience water from
the initial placement to evacuate through the top of the slab only.
This may substantially change the water cement ratio in the upper
surface of the slab. This in turn may make for a weaker finished
surface for the floor and will increase the shrinkage rates at the
upper surface of the slab promoting curling.
Some designers have adopted the practice of using the vapor retarder
at the bottom contact surface of the slab, a low water cement ratio
with water reducers to control the workability of the concrete mixture,
and a mat of steel in the upper half of the slab to restrain shrinkage
and with that to control curling.
For more information see Concrete
Floors and Moisture (EB119).
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