| 
|
|
| Summer 1996 Issue |
Vapor Barriers and Concrete:
Not Always a Good Mix
Vapor barriers are commonly used under concrete
floors to protect against moisture migration into buildings,
but improper use or installation of such barriers can result
in a variety of problems.
During the curing process, water bleeds out of newly placed
concrete. In the presence of a vapor barrier, typically polyethylene
sheeting, water which might normally migrate downward is forced
upward. This prolongs the bleed time and increases the amount
of water that collects at the surface. Excessive bleedwater
leads to a weaker concrete surface.
Vapor barriers also intensify problems associated with differences
in the drying and setting times between the surface and base
of the concrete, and with concrete finishing operations, such
as:
- Differential drying times can cause concrete slabs to
warp or curl.
- Along with the normal shrinkage that occurs as concrete
dries, differential setting times and surface moisture evaporation
can cause cracking.
- As the increased bleedwater is forced to the surface,
chemical reactions between cement alkalis and aggregates
are concentrated at the surface. These reactions can cause
popouts.
- Finishing concrete in the presence of bleedwater on the
surface can result in a soft, chalking surface.
- Finishing during differential setting conditions can help
activate surface blistering.
- Differential setting and inappropriate timing of finishing
can precipitate surface scaling.
Tips for the Best Results
To minimize these kinds of problems, several courses of action
are available. Of course, the most fundamental consideration
is whether a vapor barrier is even needed.
Generally, a vapor barrier is unnecessary if the building site
has no soil moisture problems, or if it has good drainage and
the water table is normally well below the floor elevation.
In such if the floor coverings and adhesives are cases, if the
floor coverings and adhesives are not moisture sensitive, a
compacted layer of granular fill at least four inches thick
may be used instead of a vapor barrier.
A vapor barrier may be needed if a nonporous sealer or coating
will be applied to the concrete, or if the flooring and adhesives
are moisture sensitive. A barrier should also be considered
if merchandise, supplies or equipment that can be damaged by
moisture will be in direct contact with the floor for extended
periods of time.
When a vapor barrier is used, a layer of damp sand at least
3 inches thick on top of the barrier will allow a downward path
for bleedwater. This will reduce moisture accumulation and chemical
reactions at the surface.
As for the concrete, the mixture should contain a cement factor
of at least 5.5 bags/cu. yd. Air-entertainment - 3%-5% air content
for interior applications, and at least 5.5% outdoors -- will
reduce the amount of bleedwater. Use of a water-reducing admixture
will reduce the mix water content.
Finally, concrete should be placed at the lowest practical slump
and allowed an extended period of moist curing to prevent surface
drying. Premature drying may cause the slab to curl.
|
