Civil Engineering Reference
In-Depth Information
Figure 3.12
Plastic membrane and steel mesh supported on bar chairs for a concrete slab
While studying building techniques at the Royal Melbourne Institute of Technology
in 1974, the subject of water penetration into concrete was studied in some detail. While
it is the general view that the plastic forms a moisture barrier, the lecturers at the institute
explained that it was not in fact the plastic that kept moisture out of the concrete slab. In
actual fact the moisture barrier is created by the concrete resting on the plastic that has a
glassy surface. As a consequence of this smooth surface the underside of the concrete was
extremely smooth, as all the fine particles in the concrete settled against the plastic. It is
actually the near glassy finish of the concrete that provides the moisture barrier. The
engineer that we employ and I are both concerned at some of the problems that this same
plastic can cause later in the life of the house. If, after the boxing is removed, the plastic is
cut off at the surface of the ground the plastic can actually form a well around the slab.
When the walls are wet from weather or watering, the water will run down the wall and
can go between the concrete and the plastic. This can lead to the house sitting in a pool of
water that the earth cannot draw away from the concrete floor.
Steel reinforcing for a concrete slab
Refer to your engineers' design to determine the steel required as reinforcing in your
concrete slab. There will be steel placed in the bottom of the trenches that is to be kept
away from the edges of the trenches. You will require steel mesh in the top layer of the
concrete slab. This mesh is to be located approximately one-third down from the top of
the slab. All steelwork is to be supported on bar chairs to keep it level with an even
distance from the plastic lining.
Concrete slabs and heat control
As mentioned earlier in the chapter (see 'Thermal mass, is it for you or against you?') it is
widely accepted that a concrete floor is better for heat control, however I have some
