Creating a balanced
mix design for highstrength concrete
By Bryce
Simons
igh-strength concrete is used
more widely with each passing year. But what is highstrength concrete? According
to ACI 363 “High Strength Concrete,” any
concrete with a specified compressive
strength of 6000 psi or greater is highstrength concrete.
When this definition was written, it was
highly appropriate. However, in the years
since, a significant amount of experience has
been gained for concrete mixes with compressive strengths far exceeding 6000 psi. I
have personally designed many mixes with
compressive strengths exceeding 15,000 psi
and several exceeding 20,000 psi. So much
experience has been gained, in fact, that the
Canadian code currently defines highstrength concrete as any concrete with a
compressive strength exceeding 10,000 psi.
It is important to remember several things
about high-strength concrete. Initially, it is
just one of many types of concrete mixes
that have been adjusted to deliver a specific property or properties. In this case, it is
compressive strength. The same understanding which made it possible to develop
high-strength concrete is also applied when
developing concrete mixes with high flexural strengths, high durability benefits, high
skid resistance, high abrasion resistance,
and super-flat characteristics. In other
words, high-strength concrete is just one
type of high-performance concrete.
Secondarily, high-strength concrete is not
a large part of the production output of any
ready mix plant. However, the ability to produce this type of concrete without difficulty
requires a higher level of sophistication and
skill than is considered necessary for more
conventional 3000-psi and 4000-psi concrete
mixes. Having access to this level of sophistication automatically precludes many of the
problems that plague other facilities where
these skills are missing.
H
Defining high-strength concrete
My personal definition of high-strength
concrete is somewhat different than the formal definitions described earlier. For the
purposes of my clients, I normally define
high-strength concrete as any mix with a
compressive strength requirement at least
2000 psi higher than what they are accustomed to dealing with.
Based on this definition, there are many
parts of the country where 6000 psi is highstrength concrete. However, in those markets where 8000-psi to 10,000-psi concrete
has been available for many years, it takes
compressive strength requirements of
10,000 psi to 12,000 psi to be classified as
high-strength concrete.
It is also possible for two producers in the
same geographic region to have different definitions of high-strength concrete. A well-established central mix plant may have been
producing 10,000 psi concrete for years. However, a few miles away, another well-established truck mix plant may have experience
with concrete to only 5000 psi. For them,
7000 psi becomes high-strength concrete.
Why does a minor difference of only 2000
psi shift a concrete mix into the highstrength category? Keep in mind that to
comply with ACI 318 Chapter 5, a new mix
(one without any historical backup information) needs to have an average compressive
strength 1200 psi greater than specified for
strengths between 3000 psi and 5000 psi,
and 1400 psi greater than specified for
strengths greater than 5000 psi. These
overdesign factors are normally used for
high-strength concrete because it is not often that a ready mix plant can show historical data. This means that the specified
strength is not just 2000 psi greater than
what the supplier is accustomed to, but that
the supplier must deliver a mix that is 3400
psi (2000 psi plus 1400 psi) greater than
what the supplier is accustomed to.
Making the necessary revisions to the mix
proportions will significantly affect the
fresh and hardened properties of the concrete. Knowing what to expect when these
changes are made and how to react to unforeseen circumstances can make the difference between a successful program and
an unsuccessful program.
The sledgehammer method
I have had many people tell me that they
produced 8000-psi concrete simply by
adding two or three bags of cement to an
already existing mix, cutting back some of
the sand (or cutting back on the sand and
gravel) and adding superplasticizer (sometimes adding a lot of superplasticizer) to
create the mix.
Unfortunately, there are a lot of problems with this method. This is typically
what I refer to as the sledgehammer approach. This approach ignores the individual characteristics of the mix constituents and forces the mix to achieve a
certain fluidity without regard to the other properties which are also being affected.
Of course, this approach works. There
have been far too many examples of this
type of high-strength concrete to ignore
reality. However, these mixes tend to
have difficult handling characteristics, or
they may be harder to batch, or they may
be harder to place or pump, or they may
be more difficult to finish. Shrinkage and
retardation also may be issues. A reduction in the amount of bleedwater may
make it much more difficult to finish. In
short, there are a number of inherent difficulties with this approach.
Water-cement ratios
The key to properly designing highstrength concrete mixes is successfully
lowering the water-cement ratio while
still maintaining workable, placeable concrete. Most high-strength concrete mixes
have water-cement ratios substantially below 0.40, while many that I have worked
on have gone as low as 0.20.
Clearly, there are only two ways to
lower the water-cement ratio: 1) reduce
the amount of water or 2) increase the
amount of cementitious material. Depending on the magnitude of strength being called for, it is often necessary to do
both.
Lowering the water content can be ac-
complished by two primary techniques: 1)
reducing the inherent water demand of the
mix and 2) replacing some of the water
with water reducers and superplasticizer.
Water demand
The water demand of the mix is a natural
property of any concrete mix. Essentially, it
is the amount of water required for the individual mix constituents to achieve the desired
degree of workability. The total surface area
of the constituents and particle shape have a
profound effect on the water demand.
The sand has the greatest impact on the
water demand of a concrete mix. This is
because the total surface area of the sand
particles is substantially greater than the
T he use of highstrength concrete allows
for the construction of
modern-day, high-rise
buildings such as 311
South Wacker Drive in
Chicago. This 70-story
building completed in
1991 used concrete with
compressive strengths to
12,000 psi.
other mix constituents (except for
the cementitious products which go
into solution). Additionally, a sand
with more fine particles will have
substantially more total surface area
than will a coarser sand. A sand
with a fineness modulus of 2.6 will
have a significantly greater water
demand than a similar sand with a
fineness modulus of 3.2
Particle shape also plays a significant role in water demand. A
crushed material has much sharper
edges than a rounded material. Consequently, the crushed aggregate
particles will require more water to
float them past adjacent particles
than will the rounded aggregate.
Crushed aggregates are also normally covered with residue from the
crushing process (crusher dust). This
dust has a tendency to increase water demand and impairs the ultimate
bonding between the cement paste
and the aggregate particle.
Gravel size and type
The type of gravel available must
be assessed. My experience has
shown that for high-strength concretes, well-washed, rounded gravels tend to work better than crushed
coarse aggregates. I believe this is
due to the fact that most crushed
aggregates are stockpiled without
being thoroughly washed. As discussed earlier, the crusher dust that
coats each rock particle provides a
bond breaker between the resultant
cement paste and the rock itself.
The proper sized gravel needs to
be adjusted to the application. It is
a published fact that in order to reduce shrinkage and increase the
modulus of elasticity of a mix, a
larger sized gravel should be used.
However, much of the work which
was done in Seattle in the mid1980s showed that the workability
and the resulting strength of the
mixes actually increased when 3⁄8inch minus gravel was used instead
of a 7⁄8-inch minus. Although the total surface area of the gravel will increase as the maximum sized particle is reduced from 3⁄4 inch to 3⁄8
inch, the magnitude of the change
is relatively small and, therefore, is
typically not a major concern.
The cleanliness of the gravel is
probably the single most important
factor in a successful high-strength
concrete program. It has been proclaimed repeatedly throughout the
country that the reason the Seattle
area could produce such highstrength concrete was because the
aggregate was so hard. In reality, the
aggregates used for the mixes in
Seattle were almost identical in hardness and strength to many of the aggregates in other parts of the country
where the perception is that the
gravel is not so good. The primary
reason the Seattle aggregates worked
so well is because they came from a
glacial outwash source that naturally
washed the aggregates so thoroughly
that there was virtually no minus 200
material in the pits. There was no
need to wash the aggregates because they were already clean.
It is typically necessary to adjust
the sand-gravel content to accommodate the higher cementitious
contents of a high-strength concrete
mix. Obviously, there are going to
be more fines in the mix from the
cementitious materials than is ordinary. It is not uncommon to have
mixes with seven to 10 bags of cement, 15% fly ash, and 5% to 10%
silica fume. The total cementitious
content of these mixes is unusually
high. Consequently, the need for
the fines from the sand contribution
is not as significant. By using a
coarser sand and allowing the fines
to come from the cementitious materials, high-strength mixes can be
very effectively balanced.
In addition to using a coarser sand,
it is important to use as little sand as
possible. Often, the sand portion of
the total aggregate fraction is several
percentage points lower than is typically used. In many areas, it is not
uncommon to find balanced highstrength concrete mix designs with
sand comprising only 39% of the total aggregate volume. In some
paving mixes, it has been as low as
34%. However, in other locations,
where the coarser aggregate is the
only kind available, or for several
other reasons, the balanced sand
content may be as high as 48%.
Cement type and source
Cement type and source is a concern. As the strength of the concrete
mix increases, the number of factors
which influence the ultimate strength
begin to have more impact. Cements
from the same source have a certain
amount of day-to-day variation.
However, using cements from different sources will provide a level of
variation far in excess of that which
would come from a single source.
That variation can create major difficulties during the course of a project.
The chemistry of the cement is
important. The heat of hydration in
high-strength concrete mixes can
be quite high. Therefore, a consistent Type II cement with a lower
C3A content is a good cement for
most high-strength concrete applications. On some projects where
variation was a concern and storage
space was not a problem, the total
amount of cement required for the
entire project was stored from a
single day’s run from the cement
plant. The same cement was then
used to develop the mix designs
and for all of the field testing.
Fly ash
Fly ash is a major component of
any high-strength concrete mix. Fly
ash works as a water reducer. It also works to keep the ultimate heat
of hydration down. Finally, fly ash
works to make the overall mix easier to batch, discharge, pump, and
place. Normally, fly ash which is
not interground with the cement
works better than that which is.
It is important to use a good quality fly ash. This is important to reduce mix variability and to ensure
that the fresh properties of the concrete remain constant. One index of
fly ash is loss on ignition, or LOI.
This is a measure of the carbon content in the ash. This value should be
quite low. If the carbon content gets
too high, then water demand of the
mix can get large, superplasticizer
dosages will vary significantly, and
problems with false setting and air
entrainments can become an issue.
Silica fume
In many instances, silica fume
may also be used in the mix. Typically, mixes that have strengths in
excess of 15,000 psi will have silica
fume. The type of silica fume used
will make a difference. I have had
direct experience which indicates
that the silica fume which comes in
the form of a slurry works more efficiently than does that which
comes in a dry, densified form.
Typically, this does not become a
major concern until the water-cement ratio drops below 0.30.
For high-strength concrete, typical
dosage rates for the slurry-type silica fume range between 7% and
10%, by weight of cement. Typical
dosage rates for the densified silica
fume range between 10% and 14%,
by weight of cement. However, if
the slurry form is used, the water
contained in that slurry should be
included when calculating the water-cement ratio of the mix.
Chemical admixtures
Finally, the chemical admixtures
need to be considered. It is typically customary for a given operation
to use admixtures from a single
supplier. Most often, as the admixtures come from the same supplier,
there will not be significant compatibility problems. However, the
compatibility of the chemical admixtures used should be checked
regardless of where they come
from. Occasionally, setting or
shrinkage problems, or other problems, may be encountered due to
the larger quantities of admixtures
being used, even when using products from the same supplier.
Mixing can be a problem as the
water-cement ratio drops. Although
many literature sources prescribe
designing the mix to a 1-inch slump
before adding superplasticizer, this
simply is not possible with some of
the higher-strength mixes. It is not
uncommon to run a laboratory program in which there is so little water that the mix looks like so many
marbles, then baseballs, and finally
concrete. By changing the order of
addition of the ingredients, so the
sand is held out until all else has
been added, and then added last,
this problem can be reduced. In the
larger commercial size mixers, this
tends to be less of a problem than
in smaller laboratory mixers.
To create the properly balanced
high-strength concrete mix design, it
will be necessary to work with the
changing parameters listed previously. Don’t be afraid to experiment.
The higher the strength, the more
this mix is not going to look, act, or
feel like ordinary concrete. Instead,
as the water-cement ratio goes
down, it will begin to look more
like peanut butter than concrete.
Don’t be afraid to exceed the maximum allowable mixing times prescribed in ASTM. Finally, don’t be in
a rush to add more water just because it looks like there is a chance
the mix will not come out. Give it
time. Don’t be afraid of slumps in
excess of 4 inches. High-strength
concrete typically has slumps as
high as 91⁄2 or 10 inches. This is allowable because the slump is created with the admixtures—not water.
When the mix is finally balanced,
there will be no doubt about it. A vibrator will be able to immediately
effect a very large radius, the mix
will batch with relative ease, and the
fresh and hardened properties will
meet or exceed all expectations. ✥
Bryce Simons P.E. is manager of special
materials testing, AGRA Earth & Environmental, Albuquerque, N.M.
PUBLICATION # J950788
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