Civil Engineering Reference
In-Depth Information
of approved granular, self-draining compactible subbase
material should be placed over the vapor barrier (or insu-
lation if present) (
ACI Committee 302
). Some contractors
find only 75 mm of granular sand over polyethylene
sheeting to be slippery, somewhat dangerous, and difficult
to keep in place while concreting. A 150 to 200 mm (6 to 8
in.)-thick subbase will alleviate this problem. The subbase
over a vapor retarder must be kept from getting saturated
by rain or construction activities to prevent excessive
vapor migration after the concrete slab is placed.
If concrete is placed directly on a vapor retarder, the
water-cementitious materials ratio should be kept low
(0.45 or less) because excess mix water can only escape to
the surface as bleed water. Because of a longer bleeding
period, settlement cracking over reinforcement and
shrinkage cracking is more likely. For more information
see
ACI (2001)
and
ACI Committee 302
.
Good quality, well-consolidated concrete at least
100-mm (4-in.) thick is practically impermeable to the pas-
sage of liquid water unless the water is under consider-
able pressure; however, such concrete—even concrete
several times as thick—is not impermeable to the passage
of water vapor.
Water vapor that passes through a concrete slab evap-
orates at the surface if it is not sealed. Floor coverings such
as linoleum, vinyl tile, carpeting, wood, and synthetic sur-
facing effectively seal the moisture within the slab; even-
tually this moisture may deteriorate latex adhesives
causing the floor covering to loosen, buckle, or blister.
To prevent problems with floor covering materials
caused by moisture within the concrete, the following
steps should be taken: (1) use a low water-cement ratio
concrete, (2) moist-cure the slab for 7 days, (3) allow the
slab a 2-or-more-month drying period (
Hedenblad 1997
and 1998
), and (4) test the slab moisture condition before
installing the floor covering.
In one commonly used test (ASTM F 1869), the mois-
ture vapor emission rate from a concrete slab is deter-
mined by taping a domed plastic vapor barrier with a
desiccant under it to the floor. After about 72 hours the
desiccant is weighed and the moisture vapor emission rate
is calculated. The slab is considered dry enough for
placing a flooring material if the moisture vapor emission
rate is below either 1.4 or 2.3 kg/m
2
/1000 m
2
(3 or 5
lbs/ft
2
/1000 ft
2
) depending on the type of floor covering
to be installed. Flooring-material manufacturers often
have their own recommended test and specified moisture
limits for installing their product. For more information
and additional tests for water vapor transmission, see
“Moisture Testing”
i
n Chapter 16,
Kosmatka (1985)
, and
PCA (2000)
.
Insulation is sometimes installed over the vapor bar-
rier to assist in keeping the temperature of a concrete floor
above the dew point; this helps prevent moisture in the air
from condensing on the slab surface. This practice also
creates a warm floor for thermal comfort. Codes and spec-
ifications often require insulation at the perimeter of a
floor slab. Placing insulation under the entire slab on
ground for energy conservation alone usually cannot be
justified economically. For more details, see
PCA (1985)
.
Forms
Edge forms and intermediate screeds should be set accu-
rately and firmly to the specified elevation and contour for
the finished surface. Slab edge forms are usually metal or
wood braced firmly with wood or steel stakes to keep
them in horizontal and vertical alignment. The forms
should be straight and free from warping and have suffi-
cient strength to resist concrete pressure without bulging.
They should also be strong enough to support any
mechanical placing and finishing equipment used.
Rain Protection
Prior to commencing placing of concrete, the owner and
contractor should be aware of procedures to be followed
in the event of rain during the placing operation.
Protective coverings such as polyethylene sheets or tar-
paulins should be available and onsite at all times. When
rain occurs, all batching and placing operations should
stop and the fresh concrete should be covered to the extent
that the rain does not indent the surface of the concrete or
wash away the cement paste. When rain ceases, the cov-
ering should be removed and remedial measures taken
such as surface retexturing or reworking in-place plastic
concrete, before concrete placing resumes.
Placing and Spreading
Placement should start at the far point of a slab and pro-
ceed toward the concrete supply source. The concrete,
which should be placed as close as possible to its final
position, should slightly overfill the forms and be roughly
leveled with square ended shovels or concrete rakes.
Large voids trapped in the concrete during placing should
be removed by consolidation.
Screeding (Strikeoff)
Screeding or strikeoff is the process of cutting off excess
concrete to bring the top surface of a slab to proper grade.
The template used in the manual method is called a
straightedge, although the lower edge may be straight or
slightly curved, depending on the surface specified. It
should be moved across the concrete with a sawing
motion while advancing forward a short distance with
each movement. There should be a surplus (surcharge) of
concrete against the front face of the straightedge to fill in
low areas as the straightedge passes over the slab. A 150-
mm (6-in.) slab needs a surcharge of about 25 mm (1 in.).
Straightedges are sometimes equipped with vibrators that
