Does concrete crack? Is this normal for a foundation?
Concrete without cracks is nearly impossible to find much less
achieve. The very nature of concrete is to crack somewhere, it is a
mixture of concrete, aggregate, and water.
However improperly placed reinforcing and control joints are the leading factors to major cracking (3/8" and greater) in which case need to be treated.
Cracks are generally caused
by the rapid evaporation of the water. Most cracks are aesthetic at best and present no load failure.
Unless you are looking at a large crack, then you should contact a
knowledgeable and experienced person involved in foundations. They would
be able to give you a better explanation.
Do I need to have a soils report?
How thick should I pour my slab foundation?
Some basic tips for a solid slab foundation
Concrete cracks, is it normal?
Three primary purposes for reinforcing slabs
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There are three primary purposes for reinforcing slabs-on-grade and they
are:
Shrinkage Control
Shrinkage control is the greatest design concern with slabs-on-grade.
Steel reinforcing along with joint spacing offer two primary elements
that can be effective in controlling shrinkage cracks. The most common
mistake among designers and engineers is not providing enough of steel
reinforcing for a slab-on-grade.
Temperature Control
Concrete is brittle — it is very susceptible to additional cracking and
fracturing by dramatic changes in weather. The change in temperature is
commonly referred to as Temperature Gradient. Steel reinforcing offers
the best assistance because it essentially has the same coefficient of
thermal expansion, thereby expanding and contracting at the same rate as
the concrete.
Moment Capacity
Slabs-on-grade design procedures usually provide a designer with the
slab thickness. The thickness of the slab is a function of loading,
subgrade, modulus of rupture for concrete, and slab stiffness.
As you can there are three primary design considerations for a every
foundation:
Steel Reinforcing
Joint Spacing (Crack Control lines)
Slab Thicknes
Having a firm understanding of each and their relationship is key to any
successful project.
Types of Reinforcement
There are 3 types of reinforcement that we utilize in our foundation
designs:
Rebar is a steel deformed bar ranging in size from #3 (3/8") to #18 (2
1/4") diameter. These bars come in two grades. Grade 40 and Grade 60
Typically all bars over #5 are specified as grade 60 and like wise #4
and #3's are grade 40. Grade refers to the yield strength of the bar.
Your only concern here is, that when placing your order your specify the
correct grades.
WWR (Welded Wire Reinforcement) is a smooth or deformed welded wire
reinforcement that is available in mats or rolls. With its cost
efficiency, WWR is frequently the reinforcing steel of choice for
slabs-on-grade. However, WWR’s full benefits of controlling cracking and
reducing maintenance can only be realized when it is positioned
accurately and properly placed on supports. One of the primary causes of
under-performing slabs-on-grade is the inadequate positioning or
complete absence of supports(1).
Supports are imperative to the accurate use and design of WWR in any
slab. This firm will not specify/design nor sponsor the use of rolled
wire reinforcement in any of our foundation designs. It is the opinion
of our principals in charge that rolled wire reinforcement is the most
difficult reinforcement to place properly. It is also our opinion that
properly placed (Rolled) WWR happens less than 5 out of every 100 slabs
poured. See Foundation Problems for more information.
Mesh Fiber is a synthetic or steel (hair like) fiber reinforcement that
is mixed into the concrete at the mixing plant. It offers great
reinforcement against (shrinkage) cracking in the concrete. However, a
common mistake many make is to eliminate steel reinforcing all together.
While this may be acceptable in some scenarios (slab portion), steel is
still a must in the footings. The mesh typically cannot handle the heavy
service loads alone. A better solution is to use mesh along with WWR and
Rebar. Mesh fiber adds about $7-$15 per cu yard of concrete. Mesh is
typically used in high performance specs.
(1)Tech Fact 702-R-03, Wire Reinforcement Institute
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