What is a cracked slab?
The term “cracked slab” is
not a technical term. You will not
find it in any engineering text we are familiar with.
The term “cracked slab” is used by lay people and foundation repair
contractors. I frankly do not like
the term and believe it should not be used.
Language can be used in a way that helps us understand and solve problems
and language can be used to obscure and confuse.
In my opinion, the term “cracked slab” serves no legitimate purpose.
The term “cracked slab” can
be understood literally to mean a slab that has cracks.
But if this is what term is taken to mean, then it conveys no useful
information at all. All concrete
exhibits cracking. It is a
characteristic of the material that it cracks.
There is, in fact, no difference between a “cracked slab” and a
“concrete slab” since all concrete slabs
have cracks.
The other meaning of the term “cracked slab” is a slab that has
failed. But this usage, in our opinion, is illegitimate. Slab-on-ground
foundations do not fail in any normal sense.
These are ground-supported structures.
They are not elevated structures that can collapse and fall down like,
say, an elevated structure such as a bridge.
Slab-on-ground foundations are most realistically understood and
evaluated, not in terms of “failed” or “not failed”, but in terms of
performance and specifically in terms of degrees of performance. To quote
Donald P. Coduto, P.E.
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“A common misconception, even among some engineers, is
that foundations are either perfectly rigid and unyielding, or they
completely incapable of supporting the necessary loads and fail
catastrophically. This
‘it’s either black or white’ perspective is easy to comprehend,
but it is not correct. All...foundations
have varying degrees of performance that we might think of as various
shades of gray.” (Foundation Design - Principles and Practices, by Donald P. Coduto,
P.E., page 10).
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The term “cracked slab”
presumes that slab-on-ground foundations are best understood and evaluated in
terms of “failed” and “not failed.”
We consider this perspective to be wrong and misleading.
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If my floor tile has cracks, is
my slab cracked?
I take this question to mean
that if there are cracks in the floor tile are there corresponding cracks in the
slab surface? The answer is that it
depends. There are usually one of
two situations that present themselves for evaluation.
I discuss each of these below:
 | There are one or more individual tiles that are cracked but no cracks
that run from tile to tile: In
this situation, the most reasonable explanation is that there are probably no
crack in the slab surface under the cracked tile.
If there is such a crack, it is almost certainly not an active crack.
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| There is at least one crack in the tile that runs from tile to tile: When
this is the case, there is almost always a crack in the slab surface that
mirrors the crack or cracks in the floor tile.
The cracks in the slab are usually restrain-to-shrinkage (RTS) cracks are
not considered a major structural defect. |
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What is a corner or wedge
crack?
The large majority of
slab-on-ground foundations will develop what are called corner cracks or wedge
cracks. The name comes from the fact
that these cracks develop at or very close to the outside corners of the
foundation and frequently form a wedge at the corner.
These cracks develop as a result
of the expansion of the brick veneer when it is warmed by the sun.
When the temperature of the brick veneer rises, the brick veneer wall
expands in length and pushes or slides against the slab surface.
At the end of a brick veneer wall at an outside corner of the slab, there
is nothing to push back and the concrete cracks at each side of the corner
forming a wedge. Builders will
usually place a piece of plastic between the bottom of the first course of brick
and the slab; this reduces the friction force when the brick expands and slides
against the slab. This has the
practical effect of reducing the cracking on the slab at the corners but it by
no means eliminates the corner cracking.
These cracks do not indicate
anything unusual about the foundation. If
the cracking at a corner becomes very bad, the concrete wedge may become loose
and even fall off. In extreme cases,
the wedge will no longer adequately support the brick veneer; when this happens,
the corner will need to be repaired. This
is a concrete repair and not a foundation repair.
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corner or wedge crack |
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When does a crack become a
problem?
We can divide this question two
questions:
| When does a foundation crack become a concern? The Shallow
Foundation Committee of the American Society of Civil Engineers has
published some guidelines for evaluating cracks in slab-on-ground
foundations based on the width of the crack. Their
recommendation is that if a crack is 1/16th inch wide, it should
probably be looked at by an engineer. They also state that
cracks that are 1/8th inch or less do not typically indicate that
the foundation is not capable of performing as intended. Also,
the Shallow Foundation Committee points out that the presence of
cracks in slab-on-ground foundations "does not indicate a
life-threatening, dangerous condition."
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| When does a foundation crack become a serious problem?
Cracks due to foundation bending can result in
significantly more flexibility in the slab. This means that
the foundation will not do as good a job as it was intended to do to
mitigate the amount of damage the soil distortion causes the
house. If the flexibility caused by foundation cracking
results in structural damage to the house frame so that the frame
structure can no longer carry normally imposed loads safely, then
the foundation crack is a serious problem. Also, if a crack
increases the flexibility of the house so that doors necessary for
an emergency exit cannot be opened and closed by a small child, that
too is a serious issue that must be resolved. Baring either of
these two situations, a foundation crack may be a legitimate concern
but it is not a serious problem from a structural engineering
perspective.
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If my floor tile pops loose, do
I have a foundation problem?
Foundation movement, even
foundation movement that is well within a normal, expected range, can crack floor
tile. But it is virtually impossible
for foundation movement to cause well-bonded floor tile to pop loose.
Floor tile is bonded to the slab surface with what is called thin-set.
This material does an excellent job of binding the tile to the slab
surface if it is applied properly. The
key is for the slab concrete slab surface to be clean and free of any
contaminants. The underside of the
tile must also be clean but this is usually not hard to achieve.
The slab surface is another matter.
It is easy, during normal construction operations, for the slab surface
to get material on it that prevents a good, permanent bond.
When this is the case, the tile may eventually come loose.
In summary, this problem is
almost always a bonding issue and not a foundation movement issue.
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