Building A Concrete Pier
In A Basement Floor
A rectangular hole is cut in the basement
floor, lined with thin flexible foam, and concrete is poured in. Rebar
|Skill Level: 3 (Moderate)
||Time Taken: 4 Hours
This old farm house had a problem with sagging floors, so we decided to beef
up the first floor structure and add some steel lally columns in the basement.
Past experience taught us that we could not simply place the column on the floor
- that the floor would probably crack and sink into the soft dirt
So we decided to cut out two small sections of the floor and pour
new, reinforced pier footings.
||First we laid out the position of each footing and marked
the floor with a permanent marker.
Then we used the circular saw with a 7" dry-cut diamond blade.
Only we cheated. We sprinkled water on the blade with a
garden sprayer, to keep the dust under control. We made sure that the saw
was plugged into a Ground Fault Circuit Interrupter (GFCI) outlet, because
wet power tools can give a big shock.
|We can't say enough good things about this Black and Decker
diamond blade. It cost about $80 five years ago, and has done a lot of
cutting. The steel gradually wears down (this has lost about 1/4" off
And our old Black and Decker saw just won't die. It has seen
frequent use, for over ten years, and still works
fine. Cutting concrete is abusive on a saw, because the blade tends
to bind a lot and stall the saw. But this old machine takes a lot. (No,
we do not receive any compensation from Black and Decker.)
||It took a lot of hammering to break up the concrete after
making the cuts.
|On this pier we were able to dig down about 6 inches. We
lined the hole with blue sill-seal foam, just for an expansion joint.
We wanted an expansion joint so the pier would act independently from the
basement floor slab. From past experience we know that this slab is thin, weak,
and the soil underneath was not compacted prior to the floor being poured.
||We cut two 1/2" reinforcing bars (rebar) to a length
that was a little less than the diagonal measurement.
|The pier next to the chimney had a small problem: there were
big boulders underneath for a foundation, so we could not excavate very
deep. So we made a small box from scraps of wood, to extend the hole
upwards by 3".
||Like the other pier, we lined the hole with blue sill-seal
foam. Duct tape held the foam in place while we prepared the concrete.
|We mixed two 60 pound bags of Quik-Crete concrete mix in a
big plastic mortar mixing tub.
We knew that 2 bags would be enough because we measured the pier
dimensions and came up with around 1/2 of a cubic foot for each. (Multiply
Length x Width x Depth to get volume in cubic inches, then divide by 1728
to get volume in cubic feet.) Each 60 pound bag of concrete is very close
to 1/2 of a cubic foot.
This tub also works great as a jumbo summertime outdoor
dog water bowl / trough.
||We filled the hole about half way, spread
it out, and pushed a pair of rebars just below the surface. (Rebar should
be no closer than 1 inch from any edge of the concrete.)
|We added more concrete...
|When the form was filled with concrete, we used a small trowel to smooth
||Then we used a concrete edger tool to form a nice rounded
edge. This helps prevent chipping of the hardened concrete.
|This tool costs about $5. Well worth it for anyone who even
occasionally works with concrete.
||The last thing we did was to set a metal plate into the
surface of the pier. This metal plate came with the steel lally columns
that we will use to support the sagging floor in this old house.
We let the concrete cure for about 5 days before we applied any loads to the
We raised the floor by placing a hydraulic jack next to the lally column
(which went in the center, over the metal plate). This creates an off-centered
load that can really stress a footing, which is one reason to add a few pieces
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- Circular Saw with Diamond Blade
- Small Sledge Hammer
- Cold Chisel
- Concrete/Mortar Mixing Tub
- Foam Sill Insulation
- Concrete Bag Mix