WHY is SCC Used?
Some of the advantages of using SCC are:
- Can be placed at a faster rate with no mechanical vibration
and less screeding, resulting in savings in placement
costs.
- Improved and more uniform architectural surface finish
with little to no remedial surface work.
- Ease of filling restricted sections and hard-to-reach
areas. Opportunities to create structural and architectural
shapes and surface finishes not achievable with
conventional concrete.
- Improved consolidation around reinforcement and
bond with reinforcement
- Improved pumpability.
- Improved uniformity of in-place concrete by eliminating
variable operator-related effort of consolidation.
- Labor Savings
- Shorter construction periods and resulting cost savings.
- Quicker concrete truck turn-around times enabling the
producer to service the project more efficiently.
- Reduction or elimination of vibrator noise potentially
increasing construction hours in urban areas.
- Minimizes movement of ready mixed trucks and pumps
during placement.
- Increased jobsite safety by eliminating the need for
consolidation.
HOW is SCC Achieved?
Two important properties specific to SCC in its plastic state
are its flowability and stability. The high flowability of SCC
is generally attained by using high-range-water-reducing
(HRWR) admixtures and not by adding extra mixing water.
The stability or resistance to segregation of the plastic concrete
mixture is attained by increasing the total quantity of
fines in the concrete and/or by using admixtures that modify
the viscosity of the mixture. Increased fines contents can
be achieved by increasing the content of cementititious
materials or by incorporating mineral fines. Admixtures that
affect the viscosity of the mixture are especially helpful
when grading of available aggregate sources cannot be optimized
for cohesive mixtures or with large source variations.
A well distributed aggregate grading helps achieve
SCC at reduced cementitious materials content and/or reduced
admixture dosage. While SCC mixtures have been
successfully produced with 1½ inch (38 mm) aggregate, it
is easier to design and control with smaller size aggregate.
Control of aggregate moisture content is also critical to producing
a good mixture. SCC mixtures typically have a higher
paste volume, less coarse aggregate and higher sand-coarse
aggregate ratio than typical concrete mixtures.
Retention of flowability of SCC at the point of discharge at
the jobsite is an important issue. Hot weather, long haul
distances and delays on the jobsite can result in the reduction
of flowability whereby the benefits of using SCC are
reduced. Job site water addition to SCC may not always
yield the expected increase in flowability and could cause
stability problems
Full capacity mixer truck loads may not be feasible with
SCCs of very high flowability due to potential spillage. In
such situations it is prudent to transport SCC at a lower
flowability and adjust the mixture with HRWR admixtures
at the job site. Care should be taken to maintain the stability
of the mixture and minimize blocking during pumping
and placement of SCC through restricted spaces. Formwork
may have to be designed to withstand fluid concrete pressure
and conservatively should be designed for full head
pressure. SCC may have to be placed in lifts in taller elements.
Once the concrete is in place it should not display
segregation or bleeding/settlement.
Full capacity mixer truck loads may not be feasible with
SCCs of very high flowability due to potential spillage. In
such situations it is prudent to transport SCC at a lower
flowability and adjust the mixture with HRWR admixtures
at the job site. Care should be taken to maintain the stability
of the mixture and minimize blocking during pumping
and placement of SCC through restricted spaces. Formwork
may have to be designed to withstand fluid concrete pressure
and conservatively should be designed for full head
pressure. SCC may have to be placed in lifts in taller elements.
Once the concrete is in place it should not display
segregation or bleeding/settlement.
HOW to Test SCC?
Several test procedures have been successfully employed
to measure the plastic properties of SCC. The slump flow
test (see Figure 1), using the traditional slump cone, is the
most common field test and is in the process of being standardized
by ASTM. The slump cone is completely filled
without consolidation, the cone lifted and the spread of the
concrete is measured. The spread can range from 18 to 32
inches (455 to 810 mm). The resistance to segregation is
observed through a visual stability index (VSI). The VSI is
established based on whether bleed water is observed at
the leading edge of the spreading concrete or if aggregates
pile at the center. VSI values range from 0 for “highly stable”
to 3 for unacceptable stability.
During the slump flow test the viscosity of the SCC mixture
can be estimated by measuring the time taken for the
concrete to reach a spread diameter of 20 inches (500 mm)
from the moment the slump cone is lifted up. This is called
the T20 (T50) measurement and typically varies between 2
and 10 seconds for SCC. A higher T20 (T50) value indicates
a more viscous mix which is more appropriate for concrete
in applications with congested reinforcement or in deep sections.
A lower T20 (T50) value may be appropriate for concrete
that has to travel long horizontal distances without
much obstruction.
The U-Box and L-Box tests are used for product development
or prequalification and involve filling concrete on one
side of the box and then opening a gate to allow the concrete
to flow through the opening containing rebar. The Jring
test is a variation to the slump flow, where a simulated
rebar cage is placed around the slump cone and the ability
of the SCC mix to spread past the cage without segregation
is evaluated. The U-box, L-box and J-ring tests measure
the passing ability of concrete in congested reinforcement.
Another test being standardized is a column test which
measures the coarse aggregate content of concrete at different
heights in a placed columnar specimen as an indication
of stability or resistance to segregation.
HOW to Order or Specify SCC?
When ordering and/or specifying SCC, consideration must
be given to the end use of the concrete. Ready mixed concrete
producers will generally have developed mixture proportions
based on performance and applications. The required
spread (slump flow) is based on the type of construction,
selected placement method, complexity of the
formwork shape and the configuration of the reinforcement.
ACI Committee 237 is completing a guidance document
that will provide guidelines to select the appropriate slump
flow for various conditions. The lowest slump flow required
for the job conditions must be specified. This will ensure
SCC can be attained easily with required stability and at
the lowest possible cost. The hardened concrete properties
should be specified by the design professional based on
structural and service requirements of the structure. For the
most part, hardened concrete properties of SCC are similar
to conventional concrete mixtures. Based on the requirements
of each project, SCC concrete designs can be submitted
by the producer only after specification provisions
regarding the performance of the concrete in its plastic and
hardened state are clearly defined.
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