Self-Consolidating Concrete
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Show: Self-Consolidating Concrete Offers New Opportunities for Architectural
Concrete
Architectural concrete is a hot topic. Defined by the American
Concrete Institute as “concrete which will be permanently
exposed to view and which therefore requires special care in selection
of the concrete materials, forming, placing and finishing to obtain
the desired architectural appearance,” it’s showing
its face everywhere. Examples of applications are buildings and
transportation structures like bridges and highway sound/barrier
walls. Delivery methods include cast-in-place, precast, and tilt-up,
and in every case, good appearance is essential because the concrete
is to be left exposed. Achieving good surface characteristics requires
thorough consolidation of the fresh concrete and thoroughly filled
forms. Fortunately, this also leads to a durable material because
water, precipitation, and aggressive chemicals/ions are shed by
the surface skin.
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| Slim and accurate architectural
constructions are much easier to realize with SCC, such as this
83-m high Stockholm Airport tower, which is decorated with excerpts
from the work of Antoine de Saint-Exupéry. Reduced noise
levels made construction during nighttime hours possible.
Photo courtesy: HeidelbergCement |
Conventional concrete is placed using vibrators to fill the forms.
SCC is a big step forward in fool-proofing that procedure. SCC technology
has the potential to revolutionize the concrete industry, and architectural
concrete stands to benefit from easier construction techniques and
improved surfaces.
Self-Consolidating Concrete
The construction industry has always longed for a high-performance
concrete that can flow easily into tight and constricted spaces
without requiring vibration. The need for this technology has grown
over the years as designers specify more heavily reinforced concrete
members and ever more complex formwork. Honeycombing or exposed
reinforcement in structural walls or columns is a constant concern
in these structures.
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| Placement of white cement SCC. Photo courtesy
of Aalborg White. |
Until recently, the industry used superplasticizing admixtures
(also known as high-range water reducers) in conventional mixes
in an attempt to achieve flowable concrete and duplicate the advantages
of a true “self-consolidating concrete.” This allowed
the use of concrete having an 8-in. or greater slump; however, some
vibration was still required for adequate consolidation. While high
doses of superplasticizers can create a very fluid concrete that
flows readily, the concrete will segregate if the mortar is too
thin to support the weight of the coarse aggregate. The key to creating
self-consolidating concrete (SCC), also referred to as self-compacting,
self-leveling, or self-placing concrete, is to produce a very flowable
mortar that retains a viscosity great enough to support the coarse
aggregate. Today, advances in admixtures and mix proportioning are
making SCC a practical reality around the world.
SCC Formulated with White Cement
Architectural expression in concrete means different things to
different people. Some designers appreciate concrete for its unadorned
appearance. So gray portland cement concrete can receive architectural
treatment. Some designers want to use color to express aesthetics,
and white portland cement is available to create white or colored
concrete. Much of the experimental work to date on SCC has focused
on gray cement, but there are opportunities for white cement formulations.
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Slump flow test of white SCC.
Photo courtesy of Aalborg White. |
Concrete producers know that there is quite a bit of development
associated with any mix design. When SCC is brought into the equation,
the fresh properties are the most important characteristic of the
mix. The concrete has to flow easily (slump flow) including passing
through obstructions like rebar (measured by the J-ring test), but
has to do so without bleeding or segregating. See “Testing
SCC” below or click
here for a description of the ASTM tests for assessing fresh
SCC properties.
If a mix is designed with gray portland cement, it can be modified
to use white portland cement, but the mix will most likely change
somewhat. Testing done by Aalborg Portland A/S identified several
suggestions for converting a gray SCC formulation to white cement
SCC. (1, 2) They take into account the difference in chemistry of
white cement and gray cement, dosages of cement, water and admixtures,
etc. Alternately, a mix can be initially designed in white portland
cement.
 Testing
SCC: New ASTM Standards Are Approved
Flowability, passing ability, and stability are three of the most
important characteristics of SCC. In 2001, ASTM
started development of test methods that would assess these characteristics.
In 2005, the first SCC standard was published: ASTM C 1611, Test
Method for Slump Flow of Self-Consolidating Concrete, which
assesses the flowability of SCC. This was
followed in March of 2006 by ASTM C 1621, Test Method for Passing
Ability of Self-Consolidating Concrete by J-Ring. Test methods
to assess static and dynamic stability are currently
under development. These standards will be used in the mix design
process of SCC as well as a quality control tool.
Click here for more information.
References
(1) Thrysøe, Jacob, and Hansen, Tommy Bæk, “Self
compacting concrete based on white portland cement,” Concrete
Plant International, #1, Concrete Plant International Worldwide,
February 2006, pages 84 to 93.
(2) Self
Compacting Concrete Based on AALBORG WHITE®
Resources
Self-Compacting
Concrete: Bibliography of Resources
(LB06)
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