By Brian Knight
I was in my basement applying some 2x4 furring to a concrete block wall in
preparation for insulating and hanging drywall. I noticed a crack in a
horizontal mortar joint about half way between the floor and the ceiling running
nearly the length of the wall. I realized that this crack was an indication that
the block wall was bowing inward. I don't know how long this had been going on,
or how much pressure it took to cause the problem. What I did know was that I
had to take some steps to stop the wall from bowing more.
There are two common methods traditionally used to repair this problem. One is to dig the dirt away from the
wall, twist some augers into the surrounding soil and use tie rods to pull the
wall back into place. The other method uses steel I-beams that are installed on
the inside of the wall. They are attached to the basement floor and the floor
joists to prevent the wall from bowing inward. Properly designed, the I-beams
withstand the bending load on the wall. While both methods work, they each have
obvious disadvantages and neither method sounded appealing to me. Digging soil
away from the house is not good for my garden and I was not enthusiastic about
having I-beams attached to my basement wall.
My idea on how to stabilize the wall was to use a variation of hardware bonding
that Gougeon Brothers has been advocating for years. I bonded a ˝"
concrete reinforcing bar (re-bar) into a slot milled into a 2x4 furring strip.
The wood furring would provide a space for insulation and electrical wiring and
also provide a nailing area for the drywall. The steel re-bar would provide the
tensile strength the concrete wall was lacking. But my idea would work only if
the steel could be bonded to the furring, and the furring to the wall.
The theory behind my basement wall fix
Looking at my concrete block wall, you can see that it is being forced inward. The
outside surface of the block is the compression side of the wall. The inside
surface is being stretched—this is the tension side of the wall. Concrete has
very poor tensile strength, which is why cracks are appearing. The mortar is
failing in tension. The goal of my stabilization project is to create a tension
member that provides the tensile strength lacking in the concrete.
Bending stresses
My basement wall is bowing inward. The bowing or bending stresses are the result of
a combination of forces. If you stand in the middle of a plank supported by two
sawhorses, the plank bends under your weight. The top surface of the plank is
being compressed. The bottom surface is being stretched which puts it in
tension. The compression load is at its maximum at the top surface of the plank
and gradually transitions to a tensile load that is at its maximum on the bottom
surface of the plank. At the center of the plank, there is neither compressive
load nor tensile load. This is called the neutral axis and no load is exerted
here.
The material furthest from the neutral axis is required to handle more load than the
material closer to the neutral axis. Engineers design beams with this in mind.
For example, look closely at a laminated wood beam. The wood with the
straightest grain and the fewest knots is used in the top and bottom plies of
the beam, and the poorer grades of wood with more and larger knots are grouped
at the middle of the beam. There is more stress near the surfaces and less
stress nearer the neutral axis so the best quality wood is located where it will
do the most good.
When my basement wall was built 30 years ago, a reinforcing rod was installed
vertically in the core of the wall every 4 feet or so. Unfortunately, this put
the steel at the neutral axis of the wall, not the optimum location for
reinforcement.
My fix for the wall, in effect, moves the location of the reinforcing rod from the
neutral axis of the wall to a point that maximizes its usefulness as a tension
member. The further the steel is from the neutral axis the more effective it is.
Look again at the diagram above. As the steel re-bar is bent in the middle of wall,
it tries to become longer (tension). However, it cannot move because the
furring/steel re-bar assembly is glued to the wall. The tension in the middle of
the wall translates into shear (sliding) force with most of the shear load
occurring at the ends of the board. There is very little outward thrust near the
ends of the furring since the top and bottom of the wall are anchored to joists
and the floor.
To determine how much shear load the wood-to-concrete block joint would take, we
made a sample and tested it. The sample shown in the photo (below
left) consists of a concrete block with two pieces of 2x4 bonded to it with
epoxy (105/206/404). After allowing sufficient time for the epoxy to cure, we
loaded the sample in our hydraulic test frame and slid the 2x4’s off the
concrete block. We recorded the load when the sample failed and calculated the
pounds per square inch of effort necessary to break it. The total load exerted
on the test sample when it finally broke (below right) was over 30,000 pounds,
an astounding 1480 pounds per square inch. This was much higher than any of us
here at GBI expected.
This was good news for my repair idea. An 80" long 2x4 has 280 square inches of
glue area. The shear strength of my cement block as determined by the shear test
described above is 1480 psi. That means that each 80" furring strip is
capable of resisting almost 250,000 lb. of force if the force is evenly
distributed. As stated above, the force steadily increases as distance from the
middle increases. Even if you consider just the top and bottom 8", each
furring strip will handle a load pushing out in the center of the 2x4 of 82,880
lb. I have one 2x4 located every 2' on the wall. That should withstand a lot of
pressure.
As mentioned, I have no idea of the load being exerted on the wall. The wall has
stood for over 30 years without a catastrophic failure. I know the new furring
strips will support an enormous load, and I feel quite confident that I have
stabilized the wall. I will monitor the bow in the wall by occasionally standing
a straight edge at a given point along the wall and measure the deflection
present in the wall. If the wall bulges more, the gap at the ends of the
straight edge will increase. If the increase occurs at an alarming rate, I will
have to dig outside the wall and stabilize it in a more conventional manner.
Installation method
Before installing the 2x4 furring strips, I first prepared the materials by milling a
5/8" wide x 5/8" deep slot in the face of each 2x4. Then I pre-fit the
2x4’s drilled holes for Tapcon™ concrete fasteners and removed dust and
loose material from the wall. I cleaned and roughed up the epoxy-coated re-bar
with steel wool. The installation proceeds as follows:
1.
Apply a wetout coat of epoxy to the block wall in the area where the furring
will be installed and to the back of the 2x4-furring strip.
2.
Apply epoxy thickened to a mayonnaise consistency with 404 High-Density filler
to the back of the furring strip. Make sure there is enough to fill all gaps and
squeeze out a little.
3.
Fasten the 2x4 furring to the wall with Tapcon™ concrete fasteners. Because
the wall bowed inward, fasteners only at the top and bottom of the furring
provide enough clamping pressure in the middle of the strip. A small amount of
epoxy squeezed from the joint all along the 2x4, indicating good contact and the
correct amount of epoxy.
4.
Apply a heavy bead of epoxy in the slot with a caulk gun. Use an 810 Fillable
Caulking Tube filled with 105/206/404. Thicken the epoxy with 404 to a
non-sagging consistency.
5.
Insert the 1/2" re-bar into the thickened epoxy in the slot and fasten it
with 1-1/4" drywall screws—about 4 or 5 per bar. Catch the rebar under
the head of the screw. The rebar will be just below the surface of the 2x4.
6.
Smooth out any squeezed out excess epoxy with a plastic spreader. Fill the
spaces between furring strips with Styrofoam™ and install the drywall. Check
the wall with a straight edge from time to time, just in case.