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The basement slab provides a smooth durable surface
for the basement floor. The slab also supports the
bottom of the foundation wall against earth pressures
(see graphic below).
Control joints are often not placed in the basement
floor. The reasoning is that the concrete will crack
(as all concrete does) but that it will be covered
with carpet and the cracks will go unseen.
The basement slab also provides lateral support for
the bottom of the basement walls. As earth pressure
pushes against the bottom of the wall the slab provides
support. the top of the foundation walls are supported
by the framed floor system.
slab can be placed, gravel fill will need to be placed
from the bottom to the top of the footing. This fill
needs to be compacted, unless it is a non-compactable
aggregate such as pea gravel. The contractor will
use a compaction machine such as a plate compactor
or a sheeps foot roller to compact the gravel and
ensure that settlement is minimized.
wire mesh is often used to reinforce concrete slabs.
The mesh is usually comes in 6'-8' rolls and has
a 6" x 6" wire grid. However is it becoming
more common to use fiber admixture in the concrete
The concrete is generally poured to a thickness
basement slab is poured following sub rough plumbing
and HVAC inspection.
Before the concrete
truck arrives the level of the slab is marked on the
side of the foundation walls with a chalk line. Screed
boards are placed in the middle of the floor at the
same height to ensure that the slab is level. The
concrete is then poured, leveled and finished.
be taken not to drive the concrete truck too close
to the foundation and especially not on top of the
backfill that was recently placed against the foundation
walls. Many foundations have been cracked, bowed or
have collapsed due to the pressure of concrete trucks,
bull dozers, and excavators.
After the concrete is placed it is leveled with a
screed board. A straight 2 x 4 is often used as a
screed board. Many concrete contractors will use a
power screed to level and settle large aggregate so
that the floor is ready to float and then trowel.
Years ago flatwork was tamped with grated mesh tool
called a "jitterbug". The tamping pushes
the large aggregate down into the slab and allows
the "cream" (sand, water and cement) to
rise to the surface. The cream allows the surface
to be finished hard and smooth.
The surface is then floated with a large metal float.
The float called a bull float is pushed across the
floor to smooth out any imperfections and once again
push the large aggregate down. As the cement hardens,
it is finished by toweling with magnesium metal and
flat metal trowels.
If hiring a contractor
for flatwork, check previous work for flatness and
for the job (usually figured by the square foot).
Price for change orders.
Hourly charge for work not covered by the contract.
Description of work to be performed.
Type and strength of concrete mix to be used. (compressive
strength, slump, etc).
and type of wire mesh or reinforcement to be installed.
Type of finish (smooth, broom, etc.).
sets up fast, there is no time for changes in the
project once the concrete is being placed. Preparation
is the key when pouring concrete. Once it sets, it
is expensive to remove and replace. Concrete crews
like to water-down the concrete so that is takes less
effort to spread. Adding a little water is okay as
long as the concrete retains the correct slump. Remember
that adding water significantly reduces the concrete
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