We get questions about
concrete on a consistent basis, because
if you have
concrete as a building material, you will eventually find some
cracks in it. Homeowners fret about those cracks, especially in
new homes where any crack is seen as a flaw.
Cracks Not Always a Problem
Concrete is a wonderful
building material, flexible in application, and strong when cured
(dried in place), especially when fortified with steel reinforcing
bars (rebar). However, because it is a mix of pulverized rock and
sand glued together with water, it has a tendency to crack as it
dries. And, over time external stresses will also cause cracking.
of cracks are cosmetic, and not a threat to the structure?
cracks are a common occurrence when concrete dries, so you have to
over time to find out whether the crack is something you can
live with, or if it signals a need for remedial work. Here are
typical types of cracks you may have:
classic plastic shrinkage crack
When the concrete is still in
its plastic state (before hardening), it is full of water. As the
slab loses moisture while curing it gets a bit smaller. As the
concrete shrinks, the slab could crack in order to relieve tension.
Shrinkage cracks are common and can occur as early as a few hours
after the slab has been poured and finished. Usually they are not a
threat to the structure.
In hot weather a concrete slab
will expand as it heats up and pushes against any object in its
path, such as a brick wall or an adjacent slab of concrete. If that
object cannot flex, the resulting force might cause cracking.
You can see how temperature changes put stress on
Cracking can also result from
ground movement due to freeze/thaw cycles, or by growing tree
roots. During freezes, the ground can lift several inches, then
settle again when it thaws. If the slab cannot rise and fall
evenly, it will crack. Tree roots can also cause concrete to heave,
if slowly over time.
(also called Subsidence)
When the ground under the
concrete settles, cracks could form if the settling is uneven. It
can occur if the soil under the concrete gets saturated and soft,
and the weight of the concrete compacts the soil.
Settling can also be caused
the other way, if too much weight is placed on the concrete.
Concrete is strong once cured, but still has weight limits.
If the top of the concrete
slab dries too quickly, crazing cracks can form. These are very
fine surface cracks that resemble spider webs or shattered glass.
Crazing cracks can be unsightly, but are not a structural problem.
Crusting can occur when you
stamp a pattern on concrete that has dried too quickly. Stamping
literally shakes loose pieces of the concrete around the stamping
pattern. This is also a cosmetic flaw, not a structural threat.
cracks in concrete a threat?
If the cracks widen past about
an eighth of an inch, issues may develop: Water can regularly seep
into the wall or slab. This can lead to two problems:
- If water reaches
the rebar, the steel can rust. Rusting expands the steel, which further
cracks the concrete. Too much rusting and cracking can weaken
the area around the cracks. This is not always a threat to the
structure, however. It depends on where the rusting and
cracking is occurring. Remove the source of water to fix the
- If water is in the
crack when a freeze occurs, water turning into ice expands and puts
pressure on the sides of the crack. Over time this could
weaken the area around the crack.
We are great home inspectors ,
but we assess the condition of your concrete as it stands today.
whether a harmless-looking crack could lead to problems needs your
occasional inspection over time. If the condition
of the crack never degrades further, you may simply have a cosmetic
"flaw" that thousands of other homeowners
share with you!
Let us know if you need more
information like this to help you manage client expectations about
future concrete maintenance needs in a house they are
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