Causes of House Cracks1
Most engineers
will recommend two things before repairs are begun to a house or building that
has been damaged by expansive soil movement. The first recommendation is that the building’s ground floor be surveyed
to ensure that the type and degree of deformity or distortion is identified.
The second
recommendation is that a period of time be permitted to elapse during which the
house is observed so that the range of movement or distortion is measured and
observed as well as to ensure that the movement is neither continuing nor
increasing. A one-year observation
period is often recommended when structural failure is not a
consideration. It makes no sense to rush
into a repair when continuing movement can render the repair ineffective.
Foundation Types1
Houses without Basements or Crawl Spaces
Slab-on-grade
(or just slab, for short) foundations usually are constructed of reinforced
concrete and can be inexpensive depending on the price of concrete at the time
of construction. The function of a
slab-on-grade foundation is not to resist or limit the amount of heave that
might occur beneath the slab foundation, but to move up and down with the
shrink and heave that might occur and to limit the distortion that the
shrinking and heaving soil might cause to occur in the superstructure of the
house. The amount of deflection or
distortion that the slab is designed to permit is usually a function of the
materials that will be used in the construction of the building being supported
by the slab and the materials used to finish the structure (i.e., provide the
finished appearance). Thus, total shrink
or heave is usually less a design consideration for slab-on-grade foundations
than the maximum expected differential deflection.
Typically,
there are three types of reinforcement used in slab foundations: welded wire fabric (WWF, or just wire), mild
steel (steel bars, or “rebars”), and post-tensioning (PT). None of the three types of reinforcement make
any significant contribution to the strength of the concrete. The purpose of the reinforcement in slab
foundations is principally to control cracking that will occur early in the
life of the slab as the concrete sets and becomes hard.
Post-tensioned
slabs are constructed with steel cables covered with a plastic sheathing
instead of using steel bars. The cables
extend through the sided of the forms. After the concrete is poured over the cables and has hardened, the steel
cables are stretched or tensioned and then fixed so that they cannot shrink
back to their original length. Post-tensioned slabs usually have larger cracks in them initially than
either welded wire fabric or steel rebar reinforce slabs, but upon tensioning
the cables, the concrete’s drying shrinkage cracks in the slab are squeezed
together and the slab’s concrete is placed in compression.
About Remedial Measures2
How Does a Slab-on-Grade Foundation
Work?
A slab
foundation is intended to work with the shrinking or heaving soil; it is not
intended to prevent the soil from moving. The slab foundation is intended to be strong enough or stiff enough to
limit the flexing or distortion that could occur in the superstructure as a
result of changing soil volume beneath the slab foundation. To construct a slab foundation so stiff or
rigid that it would not permit any movement would be cost prohibitive. If the superstructure is made of brittle
materials that are likely to crack with only a little bending, then the
foundation must be designed and constructed stiff to limit the bending to less
than that which the superstructure can withstand. On the other hand, if the superstructure
materials are very flexible, then the foundation can be less stiff and more
flexible.
What Is Structural Damage?
Typically,
structural damage refers to a situation in which a structural member is broken
or otherwise damaged to the point at which it can no longer carry or transmit
the magnitude of loads or forces that it was intended to carry. Under some conditions, a structural member
experiencing an excessive amount of deflection is considered to be damaged.
What Is Cosmetic Damage?
Damage that
does not result in structural damage is said to be cosmetic damage. Typical examples of cosmetic damage include
cracks in sheetrock walls or exterior brick veneer walls, cracks in concrete
floors or basement walls, separating fascia trim, cracks in floor tiles, and so
forth. Cosmetic damage does not impair
the structure’s load-carrying capacity.
Sometimes the
expected total amount of heave beneath the foundation is so great that it
exceeds the capacity of a slab-on-grade to perform it job correctly. Most structural and geotechnical engineers
consider 4 in. (100 mm) as the maximum amount of swell that should be permitted
beneath slab-on-grade foundations.
- So Your Home Is Built On Expansive Soils—A Discussion on How Expansive Soils Affect Buildings-2019 Second Edition—Prepared by the Shallow Foundations Committee of the Geo Institute of the American Society of Civil Engineers. Pages 21
- So Your Home Is Built On Expansive Soils—A Discussion on How Expansive Soils Affect Buildings-2019 Second Edition—Prepared by the Shallow Foundations Committee of the Geo Institute of the American Society of Civil Engineers. Pages 70, 73 and 19