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Because almost all concrete is mixed with
more water than is needed to hydrate the cement, much of the
remaining water evaporates, causing the concrete to shrink.
The amount of drying shrinkage that occurs in concrete depends on the
characteristics of the materials, mixture proportions, and placing methods. Restraint to shrinkage, provided by the subgrade friction,
reinforcement, or another part of the structure, causes tensile stresses to
develop in the hardened concrete. When these tensile stresses
exceed the concrete tensile strengths, cracking is caused.
Restraint to drying shrinkage is the most
common cause of concrete cracking. In many applications, drying shrinkage
cracking is inevitable. Therefore, contraction (control) joints are placed in
concrete to predetermine the location of drying shrinkage cracks.
Occurrence of drying shrinkage will defeat the
very purpose of grouting for applications such as grouting anchor bolts,
grouting machine foundations or grouting prestressed concrete ducts. This necessitated use of a cement that
will not shrink while hardening and thereafter.
Expansive cements are hydraulic cements which (unlike
portland cement) expand during the early hydration period after
setting. Expansive cement is used to
compensate for volume decrease due to shrinkage and to induce tensile stress in
reinforcement. The advantage of using
expansive cements is to induce stresses large enough to compensate for drying
shrinkage stresses and minimize cracking.
When the magnitude
of expansion is small but usually adequate to offset
the tensile stress (about 0.2 to 0.7 MPa) from restrained drying shrinkage, the cement is known as Shrinkage
Compensating Cement. ( Read
Now about Shrinkage Compensating Concrete) .
When the magnitude of
expansion is large, the cement is called Self-Stressing
and can be used for the production
of chemically prestressed concrete elements. (Read
Now about Self-Stressing Concrete)
Additional care is necessary to provide continuous moist curing for at least 7 days after placement to allow
for development of expansion , and the
structural design must be such as to ensure adequate expansion to offset
subsequent drying shrinkage.
Types of Expansive Cements. Three kinds of expansive cements are defined in ASTM
C 845.
- Type K: Contains
anhydrous calcium aluminate
- Type M: Contains calcium
aluminate and calcium sulfate
- Type S: Contains
tricalcium aluminate and calcium sulfate
Only Type K is used in any significant
amount in the United States.
CLICK
HERE to directly go to a sketch showing contraction/expansion
phenomenon in Type K Cement Concrete vis a vis Portland Cement
Concrete
CLICK
HERE to directly go to a graph showing comparison in
contraction/expansion of Type K Cement Concrete vis a vis Portland Cement
Concrete
Expansive cement contains hydraulic calcium silicates
(such as those characteristic of portland cement) that, upon being mixed with
water, forms a paste, that during the early hydrating period occurring after
setting, increases in volume significantly more than what the portland cement
paste does.Cements of this type are manufactured by careful use of an “Expanding
agent” and a “Stabilizer”. Also, proper selection of material &
controlled proportioning are required. Generally about 8-20 parts of
the sulphoaluminate clinker is mixed with 100 parts of OPC and 15 parts of
stabilizer.
Physical and mechanical properties of shrinkage
compensating concrete are similar to those of Portland Cement Concrete (PCC). Tensile, flexural, and compressive strengths
are comparable to those in PCC. Air-entraining admixtures are as effective with
shrinkage-compensating concrete as with Portland Cement in improving
freeze-thaw durability.
Some water-reducing admixtures may be incompatible
with expansive cement. Type A
water-reducing admixture, for example, may increase the slump loss of
shrinkage- compensating concrete (Call 1979). Fly ash and other pozzolanas may
affect expansion and may also influence strength development and other physical
properties.
Structural design considerations and mix proportioning
and construction procedures are available in ACI 223-83 (ACI Comm. 223
1983). This report contains several examples of using expansive cements in
pavements.
In Japan, admixtures containing expansive compounds are used
instead of expansive cements.
Expansive compounds are also available in the United States.
They can be added to the mix in a way similar to how fly ash is added to
concrete mixes.
Shrinkage- compensating concrete is an
expansive cement concrete which, when properly restrained, for example by reinforcing
steel, will expand an amount equal to or slightly greater than the
anticipated drying shrinkage. Because
of the restraint, compressive stresses will be induced in the concrete during
expansion. Subsequent drying shrinkage will reduce these stresses but
still a significant compressive stress (about 1-3.5MPa) shall remain
available.
Shrinkage
compensating concrete can be used to:
(1) Compensate for the volume decrease due to drying
shrinkage,
(2) Induce tensile stress in reinforcement (post-tensioning),
and
(3) Stabilize the long-term dimensions of
post-tensioned concrete structures with respect to original design.
Shrinkage-compensating concrete is used to
minimize cracking caused by drying shrinkage in concrete slabs, pavements, and structures.
Self-stressing concrete is expansive cement concrete in which the expansion,
if restrained, will induce a compressive stress high enough to result
in a significant residual compression in the concrete after drying shrinkage
has occurred.
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