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High-performance concrete
(HPC) is a term used to describe concrete with special
properties not attributed to normal concrete. HPC was
first known to be concrete with high strengths for
structural purposes. However, advances in concrete
technology have generated a new definition for HPC.
High-performance means that the concrete has one or more
of the following properties: low shrinkage, low
permeability, a high modulus of elasticity, or high
strength. As a consequence HPC is referred as concrete
with better durability or higher strength compared to
normal and moderate strength concrete.
High-strength concrete is typically recognized as
concrete with a 28-day compressive strength greater than
6000 psi (42 Mpa). More generally, concrete with a
uniaxial compressive strength and flexural strength
greater than that of moderate strength concrete.
Strengths of up to 20,000 psi (140 Mpa) have been used in
different site applications; for instance, Seattle's
58-story Two Union Square Building was called to have
concrete with a compressive strength of 14,000 psi (96.5
Mpa), although testing revealed it to be near19,000 psi
(131 Mpa). The most recognizable building with high
strength concrete is the Twins Petronas Towers Kuala
Lumpur, Malaysia; which has concrete with strengths
around 20,000 psi (138 Mpa). Laboratories
have produced strengths approaching 60,000 psi (800 Mpa).
High strength concrete can resist loads that
normal-strength concrete cannot. Several distinct
advantages and disadvantages can be analyzed. It is
important to consider all peripheral results of selecting
high strength concrete since special considerations must
be addressed beyond strength properties.
High early strength concrete can also be produced as a
high quality product. Early age properties in concrete
might be very important for construction loads, speed of
construction, and they can significantly affect long term
performance. Early age strength concrete can be obtained
using different approaches, and the desired strength can
be reached within hours or days. This type of concrete
may be useful in a variety of situations; for instance,
bridge decks or overlay replacements can be performed
without affecting the normal traffic flow of a bridge if
construction work is done at night. The concrete deck
could reach its design strength and the bridge could be
open to traffic in a matter of hours. Another application
occurs during construction of high rise buildings; time
of construction is an important driven factor for
contractors and owners alike, therefore during high rise
building construction the early strength concrete
provides quick floor to floor construction.
Once it is decided to use high strength
concrete, the mix design and production process can
begin. Materials used and concepts involved in increasing
the strength of concrete must be clearly understood by
the mix designer in order to obtain the desired
properties. Testing is an integral step during the
production process, since quality control studies show
that slight changes in mixture proportions can lead to
large changes in the compressive strength of concrete.
After laboratory design phase is finished, the mix should
be tested under simulated field conditions prior to the
real casting. It is often found that an excellent mix
design under laboratory conditions does not behave with
the same properties in the field. If field conditions are
not tested prior to construction, the desire strength or
workability may never be reached causing future
structural problems within the structure. When the design
proportioning is complete, mixing can commence with extra
consideration for workability and related properties of
the concrete mix and its application.
In the design and production process cementitious
materials include Portland cement, fly ash, silica fume,
ground granulated blast furnace slag, or natural
pozzolans. Typically, fly ash or ground granulated blast
furnace slag substituted for some of the Portland cement
is an effective method to increase the long-term strength
of a mixture. Ground granulated blast-furnace slag can be
use in the production of high strength concrete.
Microsilica (condensed silica fume) are used to meet high
strength and low permeability requirements. Benefits
include reduced permeability, increased compressive and
flexural strengths, and increased durability. Silica fume
can be used in concrete to produce compressive strengths
approaching 20,000 psi (131 Mpa) under jobsite
conditions. This pozzolan can be added in slurry or in a
dry form, whichever meets the batching equipment needs,
in either case, performance is the same. Silica fume can
make a significant contribution to early-age strength of
concrete. One pound of silica fume produces about the
same amount of heat as a pound of portland cement, and
yields about three to five times as much compressive
strength.
Derived from burning coal, fly ash is a valuable additive
that makes concrete stronger, more durable and easier to
work with. Fly ash aids the formation of cementitious
compounds to enhance the strength, impermeability and
durability of concrete. Two main classes of fly ash are
used in concrete: Class F and Class C. Class F fly ash
reduces bleeding and segregation in plastic concrete. In
hardened concrete, increases ultimate strength, reduces
drying shrinkage and permeability, lowers heat of
hydration and reduces creep. Class C provides unique
self-hardening characteristics and improves permeability.
Especially useful in pre-stressed concrete and other
applications where high early strengths are required.
Also useful in soil stabilization.
Concrete strength enhancement can be
achieved through use of admixtures to produce a low
water/cement ratio giving high performance concrete.
These admixtures promote a high slump, extremely flowable
concrete that achieves high strengths while providing
superior workability and pumpability. High range
water-reducing admixtures can also be used for
precast/prestressed structures where it is desirable to
keep the water/cement ratio to a minimum for low
permeability and high early strengths without set
retardation. They are also used for concrete requiring
high-early stripping strengths. Factors of compatability
between the cementitious materials and the admixtures
should be evaluated during the testing period.
Once the high strength concrete is placed, the hardened
concrete properties can be predicted in addition to other
special characteristics. Some of the properties slightly
differ from concrete with lower strength while some vary
more significantly. In order to examine the performance
of high strength concrete in practice, several case
studies can be investigated. However, during the last
decades high strength concrete has become more popular
and researchers continue to develop high strength
concrete with better durability to harmful agents.
"In general the primary characteristics of high
performance concrete an be summarized as workability,
high early-age strength, toughness, superior long-term
mechanical properties, and prolonged life in severe
environment." (Nawy)
Questions
- High Performance concrete has better durability and
higher loading capacity. High strength concrete has
higher strength than normal concrete.
- Normal strength concrete is used in low to mid-rise
buildings and short span bridges.
- The Petronas Towers are the tallest building in the
world, and it was made out of (HPC) concrete.
Advantages / Disadvantages
High strength concrete resists loads
that cannot be resisted by normal strength concrete. Not
only does high strength concrete allow for more
applications, it also increases the strength per unit
cost, per unit weight, and per unit volume as well. These
concrete mixes typically have an increased modulus of
elasticity, which increases stability and reduces
deflections producing concretes with higher compressive
and flexural strengths.
Along with the inherent advantages of high strength
concrete, several less clearly defined disadvantages can
materialize. Most of these disadvantages are due to a
lack of adequate research under field conditions,
although many of the issues are currently being
alleviated through the use of improved admixtures and
during the last decade concrete technology has evolve so
much more. First, increased quality control is needed in
order to maintain the special properties desired. High
strength concrete must meet high-performance standards
consistently in order for it to be effective. Inspection
in the field should be of high standards because if the
contractor should decide to change the mix design to
improve workability, adding water for instance, the
change will diminish the properties of the concrete.
High quality materials must be used. These materials may
cost more than materials of lower quality; but "the
economic benefits that can accrue from the use of high
strength concrete need not be overemphasized. The ACI
committee 439 concluded that the use of high strength
concrete outweighs the additional expense. Higher economy
can be obtained with high strength concrete rather than
high strength steel." (Shondeep L. Sarkar) High
strength concrete may require special curing and
placement requirements. Delays in delivery and placing
must be eliminated and sometimes it may be necessary to
reduce batch sizes if placing procedures are slower than
anticipated. Consolidation is very important, high
strength concrete needs to be compacted well. Therefore
high frequency vibrators are required. High frequency
vibrators may cause segregation, loss of entrained air,
or both in normal strength concrete. But with high
strength concrete, commonly undervibration is of major
concern because these types of concretes usually are
relatively stiff and contain little air.
Case Studies
īThe Scotia Plaza is a 68-story
building (275m high) in downtown Toronto. It was
constructed in 1987 and is constructed entirely of just
over 10,000 psi (70 Mpa) concrete. The Scotia Plaza
illustrated one of the first uses of slag in a
high-strength concrete mixture. Silica fume was also used
in the mixture. The concrete was placed between
temperatures of -20C and 35C. In the summer, liquid
nitrogen was used to keep the concrete below 25C. Quality
control testing yielded effective design strength of over
13,500 psi (93.6Mpa), and with a coefficient of variation
of 7.3%. The extra strength helped the contractor obtain
a new contract for a second high-rise building.
The Two Union Square Building in Seattle, Washington is a
58-story building braced by an innovative composite core
of four 3m diameter steel pipes and filled with 19,000
psi (131 Mpa) concrete. To stiffen the 216m building to
limit swaying and to provide earthquake load resistance,
concrete with an elastic modulus of 50GPa was used.
13,000 psi (90Mpa) concrete would have been sufficient
for the strength needs but the requirement for the
elastic modulus was only met by concrete with a 19,000
psi (131 Mpa) compressive strength. Concrete was placed
at night to avoid traffic. Up to 26,500 ft3 (750 m3) of
concrete was placed per night. Actually making the high
strength concrete is not easy to achieve in practical
applications. Seattle's high quality raw materials
enabled the success. A low-alkali Type I/II cement with
low rheological reactivity to superplasticizer was used.
The sand had sharp, angular particles and a fineness
modulus of 2.8. The coarse aggregate was very clean and
thus bonded well with the paste. The moisture content of
both fine and coarse aggregates was monitored for each
1,400 ft3 (40 m3) of concrete to avoid problems with the
critical water content.
Other structures with high strength concrete are:
- Twins Petronas Towers, Kuala Lumpur, Malaysia (~ 20,000
psi (138 Mpa))
- Mid-Continental Plaza Building, Chicago (9,000 psi (62
Mpa))
- Texas Commerce Plaza, Houston (7,500 psi (52 Mpa)
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