|
|
What Causes Them?
How Can They Be Reduced?
When Should You Be Concerned?
Cracking in Concrete Walls
Cracks in concrete
walls and slabs are a common occurrence. They appear in floors, driveways, walks, structural beams,
and walls. Cracking can not be prevented but it can be significantly reduced or controlled when the causes
are taken into account and preventative steps are taken. Most cracks should not be a cause for alarm.
Causes of Cracks
Cracking can be the result of one or
a combination of factors, all of which involve some form of restraint. Some examples
include:
• Drying Shrinkage -- This occurs as water used in the mix design evaporates.
• Thermal Contraction/Expansion— Due to temperature changes.
• Subgrade Settlement (or Expansion)--
Resulting from poor soil conditions or changes in soil moisture content.
• Differential Bearing Capacity— Harder soils under part of the foundation can cause stresses as the building “settles in.” • Applied Stresses—Forces
such as building load, earth load, hydrostatic pressure, or heavy equipment operated to close to the wall .
Types of Cracks
Tremendous forces can build up inside the wall due to any of these causes. When the forces exceed the strength of the material, cracks
will develop. Each of these causes normally leave a “signature” in
the type of crack it creates.
The vast majority of cracks are of little concern
by themselves. Shrinkage and Temperature cracks are most often vertical to diagonal. They typically emanatefrom a corner of a window, beam pocket, or other opening. Cracks
of this type are called reentrant
cracks.
These are very common and, unless they leak or show significant lateral displacement,are of no structural concern. Cracks
which are horizontal are most likely caused by an appliedload. Vertical
cracks which are significantly wider at the
top or bottom could indicate heaving
or settlement. With these cracks
it is very likely that the crack itself is not the problem, but rather the result
of an external problem such as poor drainage, overloading, etc.
Minimizing the Problem Contractors can employ
several methods of reducing the occurrence and width of cracks. • The first is the use of proper concretemix
designs. A mix with sufficient strength using the minimum amount of water necessary to distribute the concrete
throughout the wall without voids should be used. The type and amount of cement,
as well as course and fine aggregates, can also have a large effect on the amount
of shrinkage.
• A
small amount of temperature
steel reinforcement will reduce the width of cracks that do occur.
•
Control joints are intentional weak spots designed to induce shrinkage
or thermal cracks in pre-determined locations. These can be very effective if waterproofed carefully.
• Rapid water loss and extreme temperature swings while the concrete is in the early stages of curing should be avoided where possible.
• Careful backfilling
is mandatory. Typical basement walls are not designed to act as retaining walls. They must be secured with
the basement floor at the bottom and the floor deck at the top, or be braced adequately, before being backfilled. The use
of heavy equipment near the wall should be restricted and carefully considered.
• Anchoring the deck in accordance with local building codes,
including the use of anchor bolts/straps and blocking, is very important.
Improper anchoring has been the cause of a number of failures. When Should You Be Concerned? Temperature and shrinkage
cracks in walls or slabs are likely to occur in nearly all structures. When the
width of a crack exceeds 1/4” in width; when they show 1/4” in lateral displacement; when water leaks through the cracks; or you find long horizontal cracks, it is probably time to seek professional assistance. The contractor that built the wall, or your local CFA
member may be able to help you. This
information was produced by the Concrete Foundations Association and is brought to you by your wall contractor. Concrete Foundations Association 113 First Street West P.O. Box 204 Mount
Vernon, Iowa 52314 Phone: 319-895-6940 Fax: 319-895-8830 Web Site:
www.cfawalls.org email: esauter@cfawalls.org March 2005
|