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Concrete Technology
Home > Concrete Design &
Construction >Concrete Shrinkage
Concrete Cracks: A Shrinking
Problem?
By Scott M. Tarr
 While
often overlooked, the shrinkage potential of a concrete mixture
is perhaps the most important consideration for concrete used to
construct industrial floors on ground. All concrete, even shrinkage-compensating
concrete, shrinks. Because of a loss in volume, concrete shrinkage
can lead to cracking when base friction or other restraint occurs.
Shrinkage also causes curling/warping which can lead to a variety
of slab issues including decreased load-carrying capacity (structural
cracking) and joint stability problems such as spalling. Shrinkage-induced
cracking and curling/warping can also contribute to decreased vehicle
ride (a potential health and safety issue) and floor covering failures
(buckling caused by reverse warping). In short, shrinkage is a factor
in most concrete slab-on-ground performance issues. To increase
the performance of concrete floors, shrinkage must be better understood
and addressed.
Material Handling Equipment has Changed
Concrete shrinkage has become an increasingly important design issue
with industrial floors. One of the main reasons for this is the
evolution of the material distribution industry. As the industry
has evolved and the market has become increasingly competitive,
the efficiency of the operation has become an important consideration.
Material handling equipment has been developed to move more product
at a faster, yet still safe, pace. Racking equipment can safely
hold more load and is increasing in height. However, the size of
baseplates has not increased substantially, which results in greater
contact pressures and flexural stresses.
Another feature related to the evolution in material handling which
influences concrete floors is the wheel design for lift truck vehicles.
Wheels have evolved from large pneumatic tires to small solid castors
which increase the vehicle stability. However, these wheels apply
significantly greater contact pressures on smaller footprints which
increases the importance of joint stability in order to maintain
the effectiveness of joint fillers to resist joint deterioration.
Industrial slab design must become more sophisticated in order to
service these modern efficient operations.
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Concrete Mixtures have Changed
In addition to the changes in the material handling equipment, the
shrinkage of concrete mixtures typically used for slabs on ground
has increased over the past couple decades. There are many factors
which contribute to this increase such as high paste content and
the increasing use of admixtures. Also, the availability of good
quality aggregates is diminishing which has forced the use of aggregates
with properties which promote higher shrinkage concrete mixtures.
Another significant factor is the increasing demand for fast-track
construction. Many mixtures with rapid setting and strength gain
performance have an increased shrinkage potential.
Causes of Concrete’s Volume Changes
The shortening of concrete slabs can be caused by temperature decreases
or moisture loss. These two causes are also related to curling and
warping of slabs, respectively. Curling is the deformation of the
slab due to a difference in temperature between the surface and
the bottom of the slab (temperature gradient). Like most materials,
concrete expands and contracts with a change in temperature. If
the slab surface is cooler than the slab bottom, the surface contracts
causing the slab edges to curl upward. Slab “warping”
is the deformation of the slab surface profile due to a difference
in moisture between the surface and bottom of the slab (moisture
gradient). As with a sponge, if the slab surface is allowed to dry
and the bottom is kept moist, the edges will tend to warp upward.
Exterior pavement slabs typically have a permanent upward edge warp
and experience curling on a daily basis due to surface warming and
cooling cycles related to exposure to the sun. In general, the edges
of interior concrete floor slab panels warp upward due to a moisture
gradient through the slab depth.
Temperature Contraction. Concrete has
a coefficient of thermal expansion and contraction of about 0.0000010
mm/mm/°C (0.0000055 in./in./°F). After hardening, concrete
will contract as a result of cooling after the peak heat of hydration
(typically coinciding with drop in ambient temperature at night).
A 22°C (40°F) drop in temperature between day and night
can cause about 0.8 mm (1/32 in.) of contraction in a 3 m (10 ft)
length of concrete. This magnitude of shortening is sufficient to
cause cracking, especially at early ages when the strength of the
concrete is low. Therefore, it important to consider the potential
for early age temperature drops and install contraction joints properly
to control the location of cracking. Keeping concrete placement
temperatures as low as possible and close to the ultimate operating
temperature of the facility [target 26°C (80°F)] helps to
decrease the magnitude of the temperature drop.
Drying Shrinkage. After hardening, concrete
begins to shrink as water not consumed by cement hydration leaves
the system. This is known as drying shrinkage. Water above that
necessary to hydrate cement is required for proper workability and
finishability – the water is called “water of convenience.”
In general, the higher the additional water content, the higher
the shrinkage potential. For small, unrestrained concrete specimens
(prisms), a low ultimate shrinkage (strain) is considered to be
less than 520 millionths (at 50% relative humidity and 23°C
[73°F]). Typical concrete shrinkage has been measured at 520
to 780 millionths. However, for some mixtures, shrinkage exceeding
1100 millionths has been documented. Using concrete with a higher
drying shrinkage increases the risk of problems with the floor performance.
 Testing.
Concrete drying shrinkage can be measured in the laboratory. The
American Concrete Institute (ACI) 302 Guide for Concrete Floor
and Slab Construction recommends following ASTM C 157 Standard
Test Method for Length Change of Hardened Hydraulic-Cement Mortar
and Concrete. However, as drying shrinkage requires a period
of time to complete (typically over 6 months for lab specimens and
12 to 18 months for field slabs), it is unlikely that the testing
can be completed within typical pre-construction schedules. ACI
360 Design of Slabs-on-Ground suggests following the procedure
in ACI 209 Prediction of Creep, Shrinkage, and Temperature Effects
in Concrete Structures which includes a method of predicting
the ultimate drying shrinkage from early-age data measured at, for
example, 14 or 28 days. While the accuracy of the prediction improves
with later-age test data, the test can be used to compare potential
mixtures as well as confirm that the design of the slab and joint
spacing are appropriate for the concrete available. The design may
need to include mechanical load transfer devices in joints, steel
reinforcement, or, in cases where only high shrinkage concrete is
available, the use of post-tensioning or shrinkage-compensating
concrete.
As with any comparative test, the procedure for measuring shrinkage
in the laboratory must be duplicated exactly in order to compare
the results. Exact mixture proportions and resulting slump should
be reported for the batch used to fabricate test specimens. For
instance, if the project specifications allow a 125 mm (5 in.) maximum
slump, the test specimens should not be fabricated using concrete
with a 75 mm (3 in.) slump. The specimens should also be the appropriate
size as dictated by the maximum size of the coarse aggregate. Some
additional items to consider when analyzing lab results include:
- method of batching and mixing concrete
- method of consolidation
- method of curing
- length of curing (0, 7, 28 days)
- storage conditions subsequent to curing
- method of measuring beams
- point of “zero” measurement (length of mold,
initial beam length, length after soak).
Considering the potential differences between laboratories, it
is an obvious advantage to have the same lab perform all testing
on the trial mixtures being considered. That way, even if the standard
test method is modified, the same procedure is still followed for
comparative mixtures. For slabs on ground, reasonable modifications
to ASTM C 157 test may include altering the curing conditions from
a 28-day lime bath soak to a 7-day soak or moist cure since this
may better reflect actual construction conditions. However, since
the specimens have a relatively large surface area in comparison
to their size, care should be taken to prevent drying during the
curing period to accurately simulate anticipated construction conditions.
After curing, the specimens should be stored in air in a strictly
controlled drying environment as required by the standard procedure.
Laboratory shrinkage test results can be used to consider the potential
for joint/crack widening of the floor. However, concrete in a slab
on ground shrinks less than that measured in the laboratory. There
are several reasons why this occurs. First, unlike the laboratory
samples, a slab is restrained by the subgrade. Subgrade drag coefficients
are well-documented for different types of subbase material. Another
reason slabs shrink less than lab samples is because a floor in
contact with the ground does not dry the same amount as small beams
exposed to 50% RH on all surfaces. Typically, slab moisture is very
high (95% RH or greater) at the bottom. Even when ambient air conditions
are substantially below 50% RH, the slab interior remains at a significantly
higher moisture condition, especially when a vapor retarder is not
located immediately beneath the slab. The surface may dry but the
bottom remains moist which is why warping occurs.
Joint/crack widening is related to this moisture gradient as well.
Therefore, joints and cracks are typically much wider at the floor
surface than at the bottom. As a result, predicting, or even measuring
joint width at the slab surface is an ineffective method of determining
joint load transfer capability. In general, considering sub-slab
frictional restraint and actual in-place moisture condition, the
shrinkage of typical concrete (520 to 780 millionths) is actually
closer to 100 to 300 millionths (or even less at the slab bottom).
Experience with the performance of locally-available concrete mixtures
in floor slabs can provide good information to consider anticipated
joint performance.
Effect of Concrete Ingredients on Shrinkage
Many researchers1 have studied the factors associated
with the shrinkage of concrete mixtures. The most influential factor
is the type of coarse aggregate used. Hard, dense aggregate is able
to restrain the shrinkage of the cement paste. In contrast, using
aggregate with a higher compressibility can increase the shrinkage
of the concrete mixture by about 120 to 150 percent. Therefore,
locally-available materials play a critical role in the shrinkage
behavior of the concrete. The properties of aggregate from various
quarries should be considered if shrinkage is to be minimized. Some
recommendations include using a large topsize aggregate and optimizing
the gradation of the aggregate and combining aggregate sources to
minimize gap-grading and corresponding paste content of the concrete.
However, the overall benefit of these suggestions is dependent on
the aggregate properties used. If the aggregate is of poor inherent
quality, maximizing the size, gradation, and content may have little
effect on the concrete shrinkage. Likewise, blending a large aggregate
with poor qualities to a mid-size aggregate with good properties
may increase the resulting shrinkage behavior of the concrete mixture.
Other factors that have been found to have a significant impact
on the shrinkage of concrete mixtures includes the use of shrinkage-promoting
admixtures (such as accelerators), the use of dirty aggregate which
increases water demand and using a cement with high shrinkage characteristics.
The cumulative effect of these factors has been found to be multiplicative
and not additive. So, combined factors can easily increase concrete
shrinkage by several hundred percent. Therefore, the specific impact
of any set of materials should be determined by laboratory testing.
The shrinkage of a concrete mixture can have a significant impact
on the performance of floors on ground. With the increasing demand
for structural load-carrying capability and corresponding floor
performance, shrinkage has become a growing issue. As repairs and
maintenance can be costly, good joint performance is essential for
industrial concrete floors. Therefore, it is important for slabs
to remain in contact with the supporting base (minimal warping)
and the joints to have minimal widening. The shrinkage potential
of the concrete mixture must be well-understood so proper design
and construction methods can result in the expected long-term serviceability
for the owner. Even for commercial floors where heavy loading does
not occur, concrete shrinkage can result in warping relaxation subsequent
to installation of floor coverings or coatings. When surfaces are
reprofiled in preparation for installation of floor finishes, slab
distortion can result in delamination and buckling of the flooring.
Knowledge of potential concrete shrinkage can help minimize such
problems.
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The information above and further discussion of
this topic is included in the book Concrete
Floors on Ground
(EB075) by the Portland Cement Association.
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1. Tarr, Scott M., and Farny, James A.; Concrete
Floors on Ground,
EB075, Fourth Edition, Portland Cement Association, Skokie,
Illinois, USA, 2008, 256 pages.
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