Self-Consolidating Concrete
Shedding light on a revolutionary advancement
in concrete technology
By
Adam Neuwald
Adam is NPCA’s Technical
Services Engineer.
If we could take a glimpse into
the future of the manufactured concrete products industry,
we may find that it is headed in the direction of
fully automated plants that can prep, cast, cure and
demold any product. Forms will be coated with release
agent and filled with self-consolidating concrete,
reducing or eliminating the need for vibration. Reinforcing
steel will be replaced by structural fibers already
dispersed throughout the mix. Wall thicknesses will
be reduced by using high-strength concretes that are
capable of achieving superior early release strengths
allowing for accelerated production schedules.
Sound far-fetched? It’s really
not, because the key component to the above scenario
– self-consolidating concrete – has already
been developed and is moving along the technology
curve at an accelerated pace. Self-consolidating concrete’s
ability to flow three-dimensionally greatly reduces
surface defects typically associated with conventional
concrete, requiring very little, if any, vibration.
In addition, a typical self-consolidating concrete
(SCC) mix design will develop higher early strengths
than conventional concrete used in precast applications.
Many within the industry believe
that SCC will revolutionize the manufactured concrete
products industry, while others feel that SCC will
find a niche market similar to zero-slump concrete
or high-strength concrete. Only time will tell how
SCC will ultimately affect the manufactured concrete
products industry, but one thing is for sure: Self-consolidating
concrete is here, and it is here to stay.
By now you have at least heard
the buzz about self-consolidating concrete and the
advantages associated with its use. A recent informal
survey conducted by the National Precast Concrete
Association found that approximately one-quarter of
NPCA producer members are currently using self-consolidating
concrete for 80 percent to 100 percent of their monthly
production. It was also found that 54 percent of the
survey respondents are using SCC in at least one of
their product lines, with a majority casting between
30 and 60 cubic yards of SCC per production day (see
Figure 1).
Figure
1: Cubic yards of SCC produced by NPCA members per
production day.
Having heard this, you may find
yourself asking the following questions: Why isn’t
everyone using SCC? What do I need to know to successfully
produce SCC at my plant?
Some producers who have tried SCC
could not achieve the desired characteristics, while
others have been turned away by the increase in material
cost or lack of DOT acceptance. If you are among these
groups, you may want to reconsider its use due to
new advances in synthetic chemical admixtures and
optimized mix designs. Both allow for the production
of SCC at the same cost as conventional concrete.
Understanding SCC
First of all, there is no such
thing as an SCC admixture. Self-consolidating concrete
is achieved by utilizing a highly engineered mix design
that incorporates a synthetic polycarboxylate (PC)
based admixture, which incidentally can also be used
in conventional concrete. But there is much more to
producing SCC than just adding a new type of admixture.
Each mix is specially designed based on available
materials, required performance specifications and
production practices. Nearly every aspect of a production
process must be evaluated to fully capitalize on the
advantages associated with the use of SCC. With time,
one should be able to develop an optimized mix design
comparable in cost to conventional concrete.
SCC can be defined as a highly
workable concrete that can flow through densely reinforced
or geometrically complex structural elements under
its own weight to adequately fill voids without segregation
or excessive bleeding and without the need for vibration.
This is achieved by designing a mix that has a low-yield
stress and an increased plastic viscosity (see Figure
2). In other words, the mix should require minimal
force to initiate flow, yet have adequate cohesion
friction between the mortar and coarse aggregate to
ensure uniform flow and resist aggregate segregation,
blocking and excess bleeding. The yield stress is
reduced by using an advanced synthetic high-range
water-reducing admixture (HRWR), while the viscosity
of the paste is increased by using a viscosity-modifying
admixture (VMA) or by increasing the percentage of
fines incorporated into the SCC mix design.
Figure
2: Yield stress vs. plastic viscosity.
Before venturing into actual mix
design considerations, one must first take a comprehensive
approach to assess how the self-consolidating concrete
will be used. Consider all aspects of the product,
starting with the performance specifications. Specified
strength and durability requirements will dictate
the water-to-powder ratio and amount of entrained
air as well as the size, type and gradation of aggregates.
Next, consider the required fresh
properties needed to efficiently place the concrete.
The geometry of the formwork, density of the reinforcement
and specified surface finish will dictate the fresh
properties of the mix. Plant production practices
such as mixing, transportation and placement of the
concrete will also have an effect on a particular
mix design as far as aggregate segregation is concerned.
Once you have established the specified
performance requirements, assess the quality of materials
being used at the production plant. Understanding
the interaction between the aggregates and cement
paste will aid in successfully developing self-consolidating
concrete.
Excess Past Theory
The Excess Paste Theory helps
explain the mechanism of workability for fresh concrete.
The theory is based on the idea of minimizing friction
between aggregates to increase workability by increasing
the amount of paste around each aggregate, in turn
creating a dispersion effect as shown in Figure 3.
Figure
3: Excess Paste Theory
Professor Hajime Okamura of Kochi
University of Technology in Japan was the first to
successfully develop a self-consolidating concrete
mix design during the late 1980s. Okamura effectively
produced SCC using the Excess Paste Theory by increasing
the amount of fines to combat aggregate segregation
and bleeding while reducing the amount of coarse aggregates.
In doing so, he greatly reduced the frequency of aggregate
collisions, subsequently reducing internal friction
and stress while increasing overall concrete fluidity.
Unfortunately, many available aggregates
throughout North America are gap graded to produce
economical concrete. Gap graded aggregates have one
or more intermediate-size fractions omitted. However,
these intermediate-size particles are required for
suspending larger-size particles and resisting aggregate
segregation in SCC. Higher proportions of top-size
aggregates may also lead to aggregate blocking in
densely reinforced structures. To account for this,
aggregates should be blended to fill voids, as indicated
by a sieve analysis. In SCC mix designs, coarse aggregate
contents are typically reduced while increasing the
fine-to-total aggregate ratio to be between 0.45 and
0.55. Having a good working relationship with an aggregate
supplier is helpful to effectively achieve the required
optimized aggregate gradation for self-consolidating
concrete.
Although a well-graded aggregate
source is the key to successfully producing economical
self-consolidating concrete, it should be noted that
SCC can be produced with poorly graded aggregates
as well. Doing so will usually increase the overall
premium since other measures are necessary to achieve
the required fresh properties. Such measures include
increasing cement contents or using a viscosity-modifying
admixture to assure stability. Experimenting with
different types of aggregate such as rounded river
rock can increase workability, but may also increase
material cost. That’s why it is recommended
to consider local availability and market price of
aggregates during the mix-design process.
Mix design procedures
Once you achieve an optimized
aggregate gradation, you can determine the amount
of fines by using any number of mix design procedures.
At present, several SCC mix design procedures are
being used throughout the world. This is because of
the current lack of industry consensus on the design
of self-consolidating concrete. It is best to work
closely with your admixture supplier during the mix
design process, since he has the most experience in
the design of a truly self-consolidating concrete
mix.
It should be noted that self-consolidating
concrete can be achieved with the use of conventional
admixtures, but it is not recommended because of the
high dosage rates required. Using conventional-type
admixtures will also lead to increased set times,
requiring the use of an accelerating admixture to
meet specified production strengths. Both the high
dosage rates and use of an accelerating admixture
greatly increase the premium for SCC produced with
conventional-type admixtures.
The preferred admixture for self-consolidating
concrete is a polycarboxylate-based admixture due
to its superior water-reduction capabilities and high
early-strength gains at low dosing rates. This new
generation of synthetic admixture has been specially
designed to increase the dispersion of the cement
particles, preventing flocculation through electrostatic
repulsion and steric hindrance (see sidebar “Polycarboxylate-Based
Admixtures in Concrete”). Ultimately, higher
release strengths are achieved with the absence of
accelerated curing methods, in comparison to conventional
precast concrete products, which are often steam cured.
It is not reasonable to expect
to be producing self-consolidating concrete overnight.
It may take weeks or months before you can successfully
reproduce a consistent SCC mix. The concrete technician
should keep you involved in the entire testing process,
ensuring you have a full understanding of the mechanical
behavior of your specially tailored mix design. Once
you understand the mix design process, you can adjust
the SCC mix to fit each of your production applications.
Some plants have two or three different mix designs,
depending on the specified strength requirements for
each product line.
Varying cement contents will affect
the mix’s ability to flow, resist segregation
and develop specified strength requirements. Replacing
a percentage of the cement with supplementary materials
can further reduce premiums. Fly ash is the most commonly
used supplementary material for SCC in North America,
while many European countries incorporate limestone
meal into their SCC mixes. Fly ash has several advantages
over limestone meal, the biggest being higher 28-day
strength gains and its superior chloride resistance.
Limestone also has a higher initial water demand,
leading to potential early-age shrinkage cracking
problems.
Shrinkage
Shrinkage is a primary concern
that DOTs and other specifiers have with the use of
SCC in prestressed applications. Initial studies have
indicated that drying shrinkage for SCC is very close
to that of conventional concrete. However, it should
be noted that significant differences have been found
in plastic shrinkage during the first 24 hours when
comparing SCC to conventional concrete, which may
be attributed to self-desiccation. Self-desiccation
occurs in low water-to-cement mixes and is the result
of hydrating cement particles consuming the available
free water during the hydration process. As the water
is consumed by the hydration process, the capillary
pores within the concrete partially empty, causing
the internal relative humidity to drop considerably,
which could lead to bulk shrinkage. Bulk shrinkage
may cause internal microcracking, affecting the overall
strength and durability of the product. This is generally
a minor concern for non-prestressed products, but
you should be aware of the problems that may result
from using a lower water-to-cement ratio.
Initial plastic shrinkage can be
reduced by moist-curing products. Providing a continuous
water source to the concrete as it cures will help
ensure that the capillary pores are filled and the
hydration reaction continues to take place. Increasing
coarse aggregate contents will also reduce plastic
shrinkage, but again this may affect the fresh properties
of the SCC mix.
In addition to moist curing, a
number of additional production procedures may need
to be modified when using SCC. The most important
is increased quality control to monitor moisture content
of the constituent materials, especially fine aggregates.
Small fluctuations in moisture content may lead to
segregation or affect the mix’s ability to flow
through densely reinforced sections.
One way to account for variations
in moisture content and even aggregate gradations
is to use a viscosity-modifying admixture. By incorporating
a VMA into the mix design, a more robust mix is ultimately
created with an enhanced capacity to absorb fluctuations
in aggregate gradations and moisture contents. A VMA
may increase the overall premium for an SCC mix, which
is why it is important to consider all aspects of
the production process to develop an optimized, economical
mix design.
Trying different aggregate blends,
increasing the amount of fines or just maintaining
better control of moisture contents may improve the
quality and consistency of the mix without having
to use a VMA. Spending a few extra dollars on new
moisture probes may save you a few thousand dollars
in the long run. The importance of moisture control
should be expressed to production employees.
Training
Production employees must be
educated on proper placement techniques and finishing
of SCC. Mixing times may need to be slightly increased
as determined during the mix design process. When
possible, self-consolidating concrete should be placed
into itself at a constant pressure head from one end
of the form, allowing air to escape as the concrete
flows into and around densely reinforced sections.
Make sure to avoid placement practices that add additional
energy to the mix causing unwanted segregation, such
as increased pour heights or increased discharge rates.
A visual inspection of the concrete during placement
will yield valuable information on its fresh performance.
Aggregate should be visible on the
surface of the pour front as the concrete is placed
in the form (see Figure 4). Collections of aggregate
behind and near reinforcement may indicate a blocking
problem and call for a change in the mix design for
that particular application.
It will take plant personnel some
time before they are comfortable with the new material,
so it is best to educate them on the specific properties
of SCC. Self-consolidating concrete has been described
as having thixotropic characteristics, meaning that
in the absence of energy (vibration) the concrete
will stiffen on its own. This phenomenon may lead
to problems with pour lines between consecutive lifts
and finishers getting on the concrete too early. Pour
lines can be eliminated by simply adding a short burst
of energy to the mix with a vibrator, causing the
two pours to blend together. With time, production
personnel will become accustomed to the material with
regard to placing and finishing. Production practices
may be further modified to fully utilize the advantages
of SCC, such as casting panels vertically instead
of horizontal, which greatly reduces the surface area
that must be trowel finished.
SCC Standards
Now that a general understanding
of the mix design and production practices has been
established, you are probably wondering what measures
can be taken within the plant to monitor the consistency
and quality of your product on a day-to-day basis.
Unfortunately there is currently a lack of SCC standards
among the industry, which is the main reason why government
agencies are reluctant to allow the use of SCC on
many projects. However, it should be noted that the
Precast/Prestressed Concrete Institute (PCI) has recently
released “Interim Guidelines for the Use of
Self-Consolidating Concrete.” This publication
covers general mix design considerations, production
applications and quality control tests that can be
run on a daily basis.
Also, groups within the American
Concrete Institute (ACI) and the American Society
for Testing Materials (ASTM) are currently developing
industry standards. ACI Committee 237 recently drafted
a document outlining SCC mix design procedures, which
will be balloted in early 2004 and is expected to
be complete within a year and a half. Concurrently,
ASTM subcommittees are hard at work modifying verbiage
in existing standards to allow for the testing of
SCC in addition to developing four separate quality
control standards. The four new standards currently
in draft form are the Slump Flow test used to measure
unconfined flow and stability of SCC; the L-Box and
J-Ring tests used to determine the passing ability
of SCC; and the Column Segregation test, which quantifies
course aggregate settlement. Until these standards
are available, it is best to consult your admixture
supplier on quality control tests and obtain a copy
of PCI’s “Interim Guidelines” or
the European equivalent “Specification and Guidelines
for Self-Compacting Concrete” published by the
European Federation of Producers and Applicators of
Specialist Products for Structures (EFNARC).
Whenever new technology appears,
it tends to split people into two predominant groups.
The first group consists of the innovators who are
open to change and are quick to take advantage of
the new technology. The second group consists of the
skeptics who prefer to watch and wait until all tests,
research and standards have been completed before
embracing the new technology. Whichever group you
are in, it is important to stay educated on the development
of self-consolidating concrete, because SCC is here
and it is here to stay. It is just a matter of time
before its impact on the manufactured concrete products
industry can be fully measured.
References:
C.T. Kennedy, “The Design
of Concrete Mixes,” Proceedings of the American
Concrete Institute, Vol. 36, 1940, pages 373-400
H. Ludwig, N. Ehrlich, W. Hemrich
and F. Weise, “Self Compacting Concrete –
Principles and Practice,” Concrete Plant + Precast
Technology, June 2001, pages 58-67
S. Mindess, J.F. Young and D. Darwin,
“Concrete,” 2003, pages 85 and 429
H. Okamura and M. Ouchi, “Self-Compacting
Concrete,” Journal of Advanced Concrete Technology,
Vol. 1 No. 1, April 2003, pages 5-15
M.A. Bury and B.J. Christensen,
“The Role Of Innovative Chemical Admixtures
in Producing Self-Consolidating Concrete,” First
North American Conference on the Design and Use of
Self-Consolidating Concrete, ACBM Publication, 2002,
pages 137-140
Related
Articles
A
Rational SCC Mix Design Method
Polycarboxylate-Based Admixtures
in Concrete
