By James A. "Buck" Durham, P.E. Structural Engineer - Durham and Wehrman Engineering,
Phone 256-353-0504.
It is nearly impossible to prevent cracks from developing in concrete slabs on grade,
especially exterior slabs. The reasons are explained herein.
First of all, whenever concrete changes from a semi-liquid or plastic (i.e. from fresh,
ready-mix concrete) to a hardened state (i.e. cured-out), it undergoes a volume
shrinkage. This volume shrinkage is primarily related to loss of water. Concrete
generally consists of sand, portland cement, gravel and water. The water combines
(chemically-reacts) with the portland cement to create the rock-like, hardened mass we
call concrete. During this process, much of the water is used up. In addition, a
significant amount of water is lost to either evaporation or seepage into the subgrade.
To help prevent rapid loss of water in freshly placed concrete, especially during
warm/hot/windy days, the finished surface of the concrete should be kept moist and
covered with plastic or wet burlap for several days. (This helps to maintain moisture to
slow down the moisture loss process.)
In spite of extended curing [in a moist state], the eventual loss of water in fresh
concrete will cause a significant volume change (reduction) from the ready-mix to the
final, hardened state. In relatively thin concrete slabs on grade, this volume change
results in a significant dimension change in the surface (plane) of the slab. As the slab
shrinks, frictional forces can be generated along its bottom face (due to contact with
the subgrade); this can set-up tensile stresses in the concrete. Also, if the concrete
loses moisture through the surface only, due to the presence of an underlying plastic
vapor barrier, the top face of the slab will shrink more than the bottom. This causes
the slab to curl up around the perimeter edges. Both of these conditions can lead to
cracking, because both (dimension shrinkage and curling) set-up tensile stresses in
the slab.
Tensile stresses are detrimental because, while strong in compression, concrete is
very weak in tension; i.e., it is relatively easy to "pull-apart" plain (un-reinforced)
concrete. Therefore, steel (wire mesh) is embedded in concrete to provide it with the
tensile strength needed to control crack formation. Effective steel reinforcement,
however, is more easily attempted than accomplished. Unless the wire mesh is held up
by closely-spaced chairs, it is not likely to remain properly positioned near the
top-center of the hardened slab (the position at which it is most effective). The
success of wire mesh in crack control depends upon whether the contractor/finisher
systematically reaches down through the concrete mix with a hooked bar and pulls the
steel mesh off of the ground and up into the plastic mix. Unfortunately, as is well
documented, test cuts consistently show that these precautions are not taken; more
often than not, the wire mesh is found lying on the ground, barely embedded in the
concrete. Under these circumstances, it serves no purpose. It is also necessary to
realize that larger slabs require larger steel for crack control. Engineers may be
mindful of this, but few concrete finishers are.
Once an exterior concrete slab on grade is placed, finished, and cured, it is subjected
to perpetual, direct weather exposure, including periodic (and sometimes drastic)
changes in temperature and humidity. Because concrete tends to absorb and/or give
off moisture according to the surrounding weather conditions, weather changes affect
the concrete's moisture content; and, along with any moisture content change comes a
volume (dimension) change. Concrete also expands/contracts with changes in
temperature. Hence, once hairline cracks form in concrete, the effects of Mother
Nature usually start to take their toll: initially small, invisible shrinkage cracks later
enlarge. Crack enlargement facilitates direct moisture penetration into the crack,
which can lead to isolated subgrade saturation or freezing expansion of trapped water
during extremely cold weather. (Everyone knows that water expands upon freezing;
hence, trapped water inside concrete cracks can pry cracks apart during freezing
weather.)
In addition, moisture that seeps into the subgrade beneath a slab on grade can affect
the behavior of some soils-especially highly plastic or silty-clays. Highly plastic clay
soils exhibit an electro-chemical affinity for water, actually incorporating water
molecules into their chemical structure (crystalline lattice). Thus, these soils are
typically called expansive clays. During the winter, which is the normal rainy season in
North Alabama, these soils take on moisture and expand in volume. Soil expansion can
lift up or heave a slab-on-grade. During the summer, when droughts and hot weather
frequently occur, the soils desiccate and shrink in volume, causing slabs to sink or
settle. Hence, changes in the subgrade moisture content can cause slabs on grade to
"move" throughout the year. Fine-grained soils, like silts and clays, are also very weak
when wet or saturated and easily yield (deform) under a load. Hence, placing concrete
slabs directly on top of highly plastic clay soils is a bad idea-especially for driveways
which support heavy vehicle wheel loads. There are many types of highly plastic silty
and expansive clays in North Alabama. They typically occur above/around limestone
bedrock deposits. Contact the county extension agent at the nearest USDA Soil
Conservation Service to learn more about the locations of expansive clay soils in your
area.
In addition to the adverse effects of shrink-swell soils beneath driveway slabs,
consider the problems that water beneath slabs can cause. Water that penetrates slab
cracks can puddle beneath slabs; then, whenever a heavy vehicle crosses the slab, it
will deflect under the wheel load and force water from the underlying puddle through
the cracks (this is called "subgrade pumping"). The water usually carries fine-grained
soil particles from the ground with it. After a sufficient amount of erosion has
occurred, a void typically forms beneath the slab; then, when a heavy wheel load
crosses the slab, it could cause an isolated portion of the slab to collapse into the
eroded hole, resulting in pavement failure.
Generally, closely-spaced control joints are installed in slabs to prevent/conceal
unsightly cracks. Either pre-formed or saw-cut, control joints create pre-planned
planes of weakness in concrete slabs, thereby predetermining straight (as opposed to
random, jagged) lines in the slab surface.
Whenever an engineer or architect is left out of a concrete slab construction project,
the concrete contractor is responsible for providing proper control joint spacing. A
rule of thumb states that control joints should be spaced at intervals (measured in
feet) equal to three times the slab thickness (measured in inches). In other words, in a
4 inch thick slab, control joints should be spaced at (4'' x 3 = 12') 12 foot intervals.
Moreover, control joints should be placed in areas of abrupt slab dimension change.
In order to be effective, saw-cut control joints must penetrate ¼ of the slab thickness.
Hence, a 4 inch thick slab must have 1 inch deep, saw-cut control joints. In narrow
pavements, like sidewalks, control joint spacings should range from what to no more
than twice the width of the sidewalk. If wider control joint spacings are desired, steel
reinforcement can be utilized to prevent random shrinkage cracking.
Another cause of concrete cracking is structural overload. Concrete slabs should be
placed on well-compacted beds of gravel. This helps to provide uniform bearing
pressure beneath the slab whenever it is exposed to surface loads. Heavy wheel
loads, for example, are transferred through the slab and onto the subgrade. If the
subgrade consists of weak or saturated soil, it will likely deform under the pressure.
This can set up bending stresses in the concrete slab which, in turn, can generate
adverse shear and tensile stresses. Under some heavy loads and poor subgrade
bearing conditions, concrete slabs crack. As stated, this lets water seep into the
subgrade and the application of future heavy wheel loads leads to a worsening
condition (see the preceding discussion of "subgrade pumping").
In summary, large, crack free, un-reinforced concrete slabs on grade are rare. To have
a long-lasting, aesthetically pleasing concrete slab on grade: utilize a high quality
concrete mix with proper air-entrainment (see section on surface defects), place the
slab on a well-compacted layer of gravel (subgrade), saw cut control joints at a
close-spacing (throughout the slab) within 12 - 24 hours of placement (in order to
provide pre-planned planes of weakness for crack control), then properly cure the
slab for several days. You'll find that, in a relatively short time, cracks will form at each
of the control joints. You can seal these cracks with a commercially-available concrete
crack sealer/caulking to preclude water entry. Other construction precautions include
utilizing a quality concrete ready mix during placement and adding the least amount of
water possible to the delivered concrete prior to placement.
Proper curing entails wetting the surface of the concrete after finishing, then covering
the slab with plastic or wet burlap. The concrete should be kept moist in this manner
for as long as possible-at least three to seven days. If this is impossible, spray the slab
with a commercial concrete sealer (according to directions of the sealer
manufacturer). You'll generally find that a very generous application of sealer is
required to form a thick layer of surface sealer, i.e., one capable of preventing
excessive moisture loss through the slab. Lastly, finished slabs should drain freely
onto the adjacent ground or designated areas which direct runoff away from the slab.
For more information you can check out the web site called Concrete Basics from the
Portland Cement Association.
A conscientious effort has been made by the authors to provide accurate information;
however, neither the authors nor Alabama Residential Inspection Services, LLC will
assume any liability for its use. Readers are advised to perform additional research,
seek other professional advice, and to act on the information provided, herein, very
carefully.
Phone 256-353-0504.
It is nearly impossible to prevent cracks from developing in concrete slabs on grade,
especially exterior slabs. The reasons are explained herein.
First of all, whenever concrete changes from a semi-liquid or plastic (i.e. from fresh,
ready-mix concrete) to a hardened state (i.e. cured-out), it undergoes a volume
shrinkage. This volume shrinkage is primarily related to loss of water. Concrete
generally consists of sand, portland cement, gravel and water. The water combines
(chemically-reacts) with the portland cement to create the rock-like, hardened mass we
call concrete. During this process, much of the water is used up. In addition, a
significant amount of water is lost to either evaporation or seepage into the subgrade.
To help prevent rapid loss of water in freshly placed concrete, especially during
warm/hot/windy days, the finished surface of the concrete should be kept moist and
covered with plastic or wet burlap for several days. (This helps to maintain moisture to
slow down the moisture loss process.)
In spite of extended curing [in a moist state], the eventual loss of water in fresh
concrete will cause a significant volume change (reduction) from the ready-mix to the
final, hardened state. In relatively thin concrete slabs on grade, this volume change
results in a significant dimension change in the surface (plane) of the slab. As the slab
shrinks, frictional forces can be generated along its bottom face (due to contact with
the subgrade); this can set-up tensile stresses in the concrete. Also, if the concrete
loses moisture through the surface only, due to the presence of an underlying plastic
vapor barrier, the top face of the slab will shrink more than the bottom. This causes
the slab to curl up around the perimeter edges. Both of these conditions can lead to
cracking, because both (dimension shrinkage and curling) set-up tensile stresses in
the slab.
Tensile stresses are detrimental because, while strong in compression, concrete is
very weak in tension; i.e., it is relatively easy to "pull-apart" plain (un-reinforced)
concrete. Therefore, steel (wire mesh) is embedded in concrete to provide it with the
tensile strength needed to control crack formation. Effective steel reinforcement,
however, is more easily attempted than accomplished. Unless the wire mesh is held up
by closely-spaced chairs, it is not likely to remain properly positioned near the
top-center of the hardened slab (the position at which it is most effective). The
success of wire mesh in crack control depends upon whether the contractor/finisher
systematically reaches down through the concrete mix with a hooked bar and pulls the
steel mesh off of the ground and up into the plastic mix. Unfortunately, as is well
documented, test cuts consistently show that these precautions are not taken; more
often than not, the wire mesh is found lying on the ground, barely embedded in the
concrete. Under these circumstances, it serves no purpose. It is also necessary to
realize that larger slabs require larger steel for crack control. Engineers may be
mindful of this, but few concrete finishers are.
Once an exterior concrete slab on grade is placed, finished, and cured, it is subjected
to perpetual, direct weather exposure, including periodic (and sometimes drastic)
changes in temperature and humidity. Because concrete tends to absorb and/or give
off moisture according to the surrounding weather conditions, weather changes affect
the concrete's moisture content; and, along with any moisture content change comes a
volume (dimension) change. Concrete also expands/contracts with changes in
temperature. Hence, once hairline cracks form in concrete, the effects of Mother
Nature usually start to take their toll: initially small, invisible shrinkage cracks later
enlarge. Crack enlargement facilitates direct moisture penetration into the crack,
which can lead to isolated subgrade saturation or freezing expansion of trapped water
during extremely cold weather. (Everyone knows that water expands upon freezing;
hence, trapped water inside concrete cracks can pry cracks apart during freezing
weather.)
In addition, moisture that seeps into the subgrade beneath a slab on grade can affect
the behavior of some soils-especially highly plastic or silty-clays. Highly plastic clay
soils exhibit an electro-chemical affinity for water, actually incorporating water
molecules into their chemical structure (crystalline lattice). Thus, these soils are
typically called expansive clays. During the winter, which is the normal rainy season in
North Alabama, these soils take on moisture and expand in volume. Soil expansion can
lift up or heave a slab-on-grade. During the summer, when droughts and hot weather
frequently occur, the soils desiccate and shrink in volume, causing slabs to sink or
settle. Hence, changes in the subgrade moisture content can cause slabs on grade to
"move" throughout the year. Fine-grained soils, like silts and clays, are also very weak
when wet or saturated and easily yield (deform) under a load. Hence, placing concrete
slabs directly on top of highly plastic clay soils is a bad idea-especially for driveways
which support heavy vehicle wheel loads. There are many types of highly plastic silty
and expansive clays in North Alabama. They typically occur above/around limestone
bedrock deposits. Contact the county extension agent at the nearest USDA Soil
Conservation Service to learn more about the locations of expansive clay soils in your
area.
In addition to the adverse effects of shrink-swell soils beneath driveway slabs,
consider the problems that water beneath slabs can cause. Water that penetrates slab
cracks can puddle beneath slabs; then, whenever a heavy vehicle crosses the slab, it
will deflect under the wheel load and force water from the underlying puddle through
the cracks (this is called "subgrade pumping"). The water usually carries fine-grained
soil particles from the ground with it. After a sufficient amount of erosion has
occurred, a void typically forms beneath the slab; then, when a heavy wheel load
crosses the slab, it could cause an isolated portion of the slab to collapse into the
eroded hole, resulting in pavement failure.
Generally, closely-spaced control joints are installed in slabs to prevent/conceal
unsightly cracks. Either pre-formed or saw-cut, control joints create pre-planned
planes of weakness in concrete slabs, thereby predetermining straight (as opposed to
random, jagged) lines in the slab surface.
Whenever an engineer or architect is left out of a concrete slab construction project,
the concrete contractor is responsible for providing proper control joint spacing. A
rule of thumb states that control joints should be spaced at intervals (measured in
feet) equal to three times the slab thickness (measured in inches). In other words, in a
4 inch thick slab, control joints should be spaced at (4'' x 3 = 12') 12 foot intervals.
Moreover, control joints should be placed in areas of abrupt slab dimension change.
In order to be effective, saw-cut control joints must penetrate ¼ of the slab thickness.
Hence, a 4 inch thick slab must have 1 inch deep, saw-cut control joints. In narrow
pavements, like sidewalks, control joint spacings should range from what to no more
than twice the width of the sidewalk. If wider control joint spacings are desired, steel
reinforcement can be utilized to prevent random shrinkage cracking.
Another cause of concrete cracking is structural overload. Concrete slabs should be
placed on well-compacted beds of gravel. This helps to provide uniform bearing
pressure beneath the slab whenever it is exposed to surface loads. Heavy wheel
loads, for example, are transferred through the slab and onto the subgrade. If the
subgrade consists of weak or saturated soil, it will likely deform under the pressure.
This can set up bending stresses in the concrete slab which, in turn, can generate
adverse shear and tensile stresses. Under some heavy loads and poor subgrade
bearing conditions, concrete slabs crack. As stated, this lets water seep into the
subgrade and the application of future heavy wheel loads leads to a worsening
condition (see the preceding discussion of "subgrade pumping").
In summary, large, crack free, un-reinforced concrete slabs on grade are rare. To have
a long-lasting, aesthetically pleasing concrete slab on grade: utilize a high quality
concrete mix with proper air-entrainment (see section on surface defects), place the
slab on a well-compacted layer of gravel (subgrade), saw cut control joints at a
close-spacing (throughout the slab) within 12 - 24 hours of placement (in order to
provide pre-planned planes of weakness for crack control), then properly cure the
slab for several days. You'll find that, in a relatively short time, cracks will form at each
of the control joints. You can seal these cracks with a commercially-available concrete
crack sealer/caulking to preclude water entry. Other construction precautions include
utilizing a quality concrete ready mix during placement and adding the least amount of
water possible to the delivered concrete prior to placement.
Proper curing entails wetting the surface of the concrete after finishing, then covering
the slab with plastic or wet burlap. The concrete should be kept moist in this manner
for as long as possible-at least three to seven days. If this is impossible, spray the slab
with a commercial concrete sealer (according to directions of the sealer
manufacturer). You'll generally find that a very generous application of sealer is
required to form a thick layer of surface sealer, i.e., one capable of preventing
excessive moisture loss through the slab. Lastly, finished slabs should drain freely
onto the adjacent ground or designated areas which direct runoff away from the slab.
For more information you can check out the web site called Concrete Basics from the
Portland Cement Association.
A conscientious effort has been made by the authors to provide accurate information;
however, neither the authors nor Alabama Residential Inspection Services, LLC will
assume any liability for its use. Readers are advised to perform additional research,
seek other professional advice, and to act on the information provided, herein, very
carefully.
Concrete Slab Cracks
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