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COMBATING SHRINKAGE WITH INTERNAL CURING
– A RESEARCH REPORT (PCA R&D SN2620)
 Shrinkage
in concrete is related primarily to the cracking tendency in concrete
structures. Cracks affect concrete’s compressive strength,
durability performance, and aesthetic quality. Whether in the form
of drying shrinkage, autogenous shrinkage, plastic shrinkage, or
other shrinkage mechanism, combating the volume instability of concrete
can be quite challenging for engineers and contractors.
With the increasing prevalence of high-strength concrete in concrete
construction, reducing autogenous shrinkage and the potential for
self-desiccation (internal drying) is an important research avenue.
High-strength concrete, in general, utilizes a larger cement content
and lower water-to-cementitious materials ratio. Combined with inadequate
curing, the shrinkage and cracking potential can be quite large.
The purpose of Hoa Lam’s doctoral research (PCA
R&D Serial No. 2620) was to explore different internal curing
methods and the use of shrinkage-reducing admixtures (SRA) for the
mitigation of autogenous and drying shrinkage.
Internal curing ensures a continued supply of moisture from within
the concrete for the development of cement hydration with age. Utilizing
lightweight, porous aggregates or superabsorbent polymers (SAP)
saturated with moisture, a relative humidity (RH) conducive to cement
hydration can be supplied to the concrete during its early life.
Internal curing ensures a high RH within the pore structure of concrete
that is prone to self desiccation, which extends hydration and results
in increased strength and durability performance. SRAs are chemical
admixtures that reduce the surface tension of water in the capillary
pore system, thus relieving the internal stresses that amass during
drying.
The experimental procedure involved testing various mortars and
concretes comprised of lightweight aggregates, SAP, and SRA for
shrinkage, tensile and compressive strength, degree of hydration,
as well as for rapid chloride penetration (RCPT). Based on analyses
of the test results, the following conclusions represent some of
the major findings:
- SAP, when used at 0.3 to 0.6% by weight of cement, proved
to be effective in mitigating autogenous shrinkage but provided
only marginal reduction in drying shrinkage. As a consequence, however,
a reduction in the compressive strength was observed. Although,
this strength reduction was accompanied by a significant tensile
strength increase. In addition, due to the differences in chemical
composition of cements and aggregates, like any admixture, SAP requires
determination of compatibility with the concrete mixture ingredients.
As a ‘water-entraining’ admixture, SAP shows potential
for internal curing capability.
- SRA reduced the drying shrinkage significantly and prolonged
the net-time to cracking considerably but actually increased the
amount of autogenous shrinkage in mortar. In addition, by decreasing
the pore size of the capillary network, microcrack development resulted
in a decrease in compressive strength. This compressive strength
reduction was accompanied by an increase in the tensile capacity,
however.
- At an amount ranging from 40-60% of the sand replacement,
lightweight sand was incorporated into several mix designs. Shrinkage
values did not decrease when replacement levels were increased from
40 to 60%. Utilizing void space within the individual particles
to absorb mix water, lightweight aggregates can act as an internal
curing mechanisms. Lightweight sand was found to be more effective
at internal curing than lightweight stone because sand showed better
dispersion and less effect on compressive strength of the specimens.
In all cases, improved interfacial transition zone (ITZ) properties
were reported.
This project is a good example of how the continued development of
cement and concrete research is advancing concrete technology. Meeting
the need for a clearer understanding of shrinkage behavior and options
for reducing the shrinkage potential, this research will surely aid
in the development of high-strength concrete for the future of concrete
construction.
REFERENCES
Concrete Technology
Today, CT021, Vol 23, No. 1, Portland Cement Association,
Skokie, IL, 2002.
Lam, Hoa, Effects
of Internal Curing Methods on Restrained Shrinkage and Permeability,
SN2620, Portland Cement Association, Skokie, IL, 2005.
Neville A.M., Properties of Concrete, Fourth Edition,
Prentice Hall, 1995.
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