HP Concrete Flexes Its Muscles

Concrete Roads Article December 28, 2000
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As the use of high-performance (HP)
concrete with microsilica increases on bridge decks, engineers
and contractors need to understand that the material requires a
keener eye to the batching sequence, better mixing, and
equipment capable of meeting ASTM C 94 uniformity standards. If
basic, good concrete practices are followed with no shortcuts
taken, the resulting concrete will live up to its
high-performance name.

While there have been a few isolated
problems with HP concretes, these can be traced back to improper
batching, mixing or equipment problems. As DOTs loosen bidding
guidelines for approved ready-mix contractors, the burden falls
on the engineers and contractors to ensure that the concrete
producer uses the most up-to-date procedures and equipment to
avoid such problems.

The microsilica advantage

Microsilica first started being used in concrete bridge decking
in 1984 to help extend the life of the decks, which through the
years have faced increased traffic volumes and heavier truck
weights. Some states, such as New York, now require all bridge
decks to be constructed with HP concrete. This forward thinking
is supported by a recent 10-year study on an Illinois bridge
that showed the microsilica concrete used in the replacement
bridge deck overlay is expected to last more than 50 years
compared to 13 years for the first full-depth conventional
concrete deck.

One reason for the longer life is that
microsilica acts as a pozzolan, which reacts with water and
cement to form compounds with cementitious properties. Without
microsilica, cement/water hydration produces 75% to 80%
strength-generating calcium silicate hydrate (CSH) products. The
remaining is a weak by-product called calcium hydroxide.
Microsilica reacts with the calcium hydroxide by-product,
converting it into good strength-producing products such as CSH.

Another benefit of microsilica concrete is that it helps
prevent chloride-induced corrosion of reinforcing steel. It does
this by reducing chloride permeability and increasing the
electrical resistivity of the surrounding concrete.

Microsilica has an extremely small average particle size of less
than 0.50 micron. This is up to 100 times finer than cement
grains and 50 times finer than fly ash. This fine size allows
microsilica to fill in the gaps between cement particles, like
marbles would fill in the gaps between a volume of bowling
balls. This particle packing reduces the ability of deicing
chemicals to penetrate the bridge deck concrete.

But
chloride permeability is only half of the electrochemical
corrosion process. The "electro" part concerns the transport of
electrical currents, which determine the rate of corrosion. If
corrosion has initiated in a bridge deck, corrosion currents
will flow from one steel rebar to another. The rate of corrosion
is proportional to the current flow. Corrosion of the
reinforcing steel will cause swelling to about four times its
regular size. The resulting tensile forces will cause cracks in
the concrete.

Microsilica increases the concrete's
electrical resistance and thus limits this damaging electrical
flow. Conventional concrete has an electrical resistivity of
about 4,000 ohms-cm. Microsilica concrete has demonstrated more
than seven-fold increases in resistance, beyond 30,000 ohms-cm.
By reducing the electrical current flow with high- resistive
microsilica concrete, the potential for macro cell corrosion is
reduced. The result is a bridge deck that can last many times
longer than conventional concrete decks.

Proper batching
critical

A main factor in how long a HP-concrete bridge deck
will last is how the concrete materials were batched. Before HP
concrete, concrete producers had mix designs that were routine.
Most of the designs had water-to-cementitious material (w/cm)
ratios that were above 0.40. HP concretes have mix designs that
consistently are below the 0.40 w/cm ratio, making the batching
and mixing steps much more critical. Any shortcuts in this area
of batching and mixing will result in significantly magnified
problems.

The challenge the industry is facing is that the
material technology is ahead of the experience of some ready-mix
contractors and material suppliers, many of which have not
recognized that HP concrete is not as forgiving as conventional,
higher w/cm-ratio concretes. When the industry first started
making HP concrete, there were many people baby-sitting the
process to make sure there were no problems. Now that HP
concrete is being used extensively throughout the world, some
producers may forget to use proper HP concrete batching and
mixing procedures.

When making HP concrete, the material
batching sequence must be controlled. The materials, especially
the fine cementitious powders, need to be fed into the mixer at
a slower rate for better mixing efficiency with the aggregate
materials.

In powder form, microsilica should always be
treated and batched as any other cementitious material. It
should be accurately weighed and slowly fed into the mixing
vessel at the same time as the cement. You should never feed dry
or slurry microsilica products into the mixing vessel without
aggregate and water already in it. If you do, the microsilica
may ball and not disperse throughout the concrete, just as
cement would if batched first.

One batching sequence that
has successfully been used with dry bulk microsilica is
concurrently adding the course aggregate, fine aggregate, a
minimum of 75% of the batch water, water reducers and
air-entraining admixture. Next add the cement with microsilica
batched on top. Follow this with high-range water reduction
chemicals as needed and then the remaining batch water.

In
slurry form, microsilica should be added concurrently with the
water and aggregate. If using microsilica in repulpable bags,
add them first followed immediately by the coarse and fine
aggregates and a minimum of 75% of the batch water. These
special bags need the mixing energy provided by the aggregate
and water combination to help in their disintegration. Keep in
mind that repulpable bags may not be appropriate for some HP
concrete mixes. The typically smaller diameter course aggregate
and low water contents of some HP concretes may not fully repulp
the bags.

Mixing it up

According to the ASTM C 94
Standard Specification for Ready-mix Concrete, conventional
concrete should be truck mixed a minimum of 100 revolutions.
Some international standards currently being written for HP
concrete often require 200 revolutions or more.

Rather than
counting revolutions, mixing speed should be the key mixing
criteria for HP concrete. If the mixing speed is in the 16- to
20-rpm range, the mixing blades effectively pull the mix
ingredients into the truck and throw them back on themselves. If
the mixing speed is less, even by 1 rpm, the mixing energy and
action are off and the concrete mixture may not be optimized.

HP concrete also does not combine or mix as well at high
slumps since it needs to collide against itself to thoroughly
mix the constituents with the microsilica and cement. The
recommendation is to mix the concrete at a 2- to 4-in. slump for
a minimum of 100 revolutions. High-range water reducers can then
be added and mixed to produce whatever slump is desired or
specified. This chemically induced slump helps the concrete
maintain its high strength while making it easier to pump and
place.

It is important to note that slump can no longer be
used to monitor the water-to-cement ratio of HP concrete because
of the inherent water-reducing additives needed. The only fresh
concrete properties now determined by HP concrete slump is, at
best, how easy the concrete will be to pump, work and finish.

The longer mixing time on top of the slower batching process
will increase the time needed to make HP concrete. Keep in mind
that HP concrete is routinely more expensive than conventional
concrete. As such, most concrete producers should have no
problem slowing down the batching and mixing processes and
keeping a closer watch on both.

Advance work

There are
numerous steps that engineers and contractors can take to ensure
proper batching and mixing of HP concrete.

All HP-concrete
jobs should have a pre-placement test pour. This is an
absolutely critical step for an engineer to ensure that the
ready-mix producer demonstrates the ability to make the HP
concrete.

Your bridge deck pour should not be the first HP
concrete that the ready-mix contractor does. Keep scheduling
test pours until the concrete producer has proven its ability to
produce and control your HP concrete. You may even consider
having more than one successful pour to further test and
evaluate the consistency of the delivered and placed HP
concrete.

Keep in mind that the microsilica and chemical
admixture company representatives can be your best friends. Ask
for their involvement early on to help the ready-mix contractor
with proper batching and mixing. Also require the admix
representative to be on-site during the test pours and first
placements on the job.

A simple yet often overlooked
up-front step is to ask the concrete producer to provide a
written batching sequence to be followed on the job. This should
list the steps, material ratios and mixing parameters. It's an
easy way to catch a big problem.

When experiencing problems
producing uniform HP concrete that are not caused by improper
batching, usually the problem can be traced back to one or two
trucks on the project that do not have updated equipment. Make
sure to ask the ready-mix contractor to use only those trucks
that can meet ASTM C 94 concrete uniformity requirements. This
uniformity standard covers five components that indicate good,
uniform concrete: air content, slump variances, unit weight,
aggregate proportions and compressive strengths. With this
requirement enforced, ready-mix contractors will pick their best
trucks--not necessarily their newest trucks--for your
HP-concrete jobs.

Finally, schedule a pre-job conference at
least four weeks before the start of the job. At this
conference, you can review the batching sequence, put in your
request for trucks that meet uniform testing, determine how many
trucks will be needed to do the job and request the presence of
the admix representative during the critical test pours and job
start-up. Schedule the test pour for no less than three weeks
before the first job pour. This gives time to adjust the mix
design, batching sequence and mixing parameters to meet
everyone's expectations.

Once all of the up-front bugs are
worked out through the pre-job conference and test pours, the
project can commence on time with the learning curve already
completed.

The future of bridge decking

HP concrete for
bridge decking has become synonymous with microsilica concrete.
Other materials commonly used in addition to microsilica to
produce HP concretes are fly ash and ground granulated blast
furnace (GGBF) slag. Combinations of two or three of these
materials with portland cement often make good material sense.

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