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Q & A
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Q & A
Answers to questions
about HPC and its applications. (Click on question for answer.)
Q: Is
there a standard mix for HPC?
Q: What is service life and how is
it predicted?
Q:
Are there quantitative measurements
for HPC?
Q: What
is match curing and can I use it to determine specified release
strengths and design strengths?
Q:
What is reactive powder concrete?
Q: Does
HPC need to be air entrained for frost resistance?
Q: With
HPC, should I specify compressive strength at 56 days rather than
the traditional age of 28 days?
 A:
The use of a 56-day age for the measurement of concrete compressive
strength was intro¬duced into the building industry many years
ago. The primary application was cast-in-place columns for high-rise
buildings. For structural purposes, columns only receive their full
design load after the building is finished and occupied. Since it
takes many months to build a high-rise building, the concrete design
strength is not needed until an age much later than 28 days. An
age of 56 days is selected for most projects with an age of 90 days
being used occasionally. From an economic viewpoint, this means
that the same concrete mix can be used for a higher design strength
because of the strength gain that occurs between 28 and 56 days.
This is particularly important when a mineral admixture such as
fly ash is used.
For precast, prestressed concrete bridge components such as beams
and piles, the engineer generally specifies both a minimum strength
at release of the prestressing strands and a design strength. For
conventional strength concretes, mix proportions are then selected
to achieve the release strength while the specified design strength
at 28 days is easily exceeded. With high strength concrete, the
design strength is higher and the release strength is correspondingly
higher. To achieve the higher strength, it is necessary to increase
the cementitious material content. As a result, the heat of hydration
is higher. The resulting higher curing temperature facilitates development
of the release strength but makes it more difficult to achieve the
design strength because of the slower strength gain at later ages.
The use of a specified design strength at 56 days, therefore, makes
it easier to achieve the design strength. Many concrete specifications
for high strength, high performance concrete for prestressed bridge
beams are now using 56 days.
For cast-in-place high performance concrete, as used in bridge decks
or substructures, durability criteria rather than strength often
control the selection of concrete mix propor¬tions. When high
compressive strengths are not needed or specified for cast-in-place
concrete, the strength can usually be achieved at 28 days even with
the use of mineral admixtures. Consequently, there is no need to
change from the traditional age of 28 days.
Q: What
is match curing and can I use it to determine specified release
strengths and design strengths?
A: Match curing is
a system in which a standard concrete cylinder— usually 4x8
in. (102 x 203 mm)—is cured at the same temperature as that
measured in a concrete member. The system includes a temperature
sensor in the member, a controller, a special insulated cylinder
mold with a built-in heating system, and a temperature sensor in
the mold.
A reference sensor is located in the member to obtain the temperature
of the freshly placed concrete. The reference sensor and the sensor
from the cylinder mold are connected to the controller. The controller
continuously compares the reference temperature with the temperature
of the cylinder mold. When the reference sensor temperature exceeds
the cylinder temperature, the controller activates the heater on
the cylinder until the cylinder temperature and reference temperature
are equal. One controller can be used with several molds. The controller
can be replaced with a personal computer that can also record temperature
versus time.
Curing temperature can have a significant effect on measured concrete
compressive strength at release and a lesser effect on later-age
strengths. This effect is more significant with HPC because the
higher amounts of cementitious materials produce more heat of hydration
and higher temperatures.
Whether or not the match-curing technique can be used to determine
specified strengths will depend on the specifications. Several state
DOTs now allow its use to determine release strengths but still
use member curing or moist curing for design strengths. Some states
use the technique for HPC bridges only. Other states are experimenting
with the technique. As research data indicate, a match-cured cylinder
produces a compressive strength that more closely matches the strength
of the concrete in the member than the strength measured using other
curing methods. This is particularly true at early ages. If your
specifications do not currently permit the use of match curing and
you are producing high strength HPC, it is time to consider a change
to your specifications.
Q:
Are there quantitative
measurements for HPC?
A:
The most common measurement
for HPC is concrete compressive strength. The American Concrete
Institute recognizes that concrete with a specified compressive
strength of 6000 psi (41 MPa) or greater is a high strength concrete
and, therefore, a high performance concrete.
However, there are many other measurements
that can be used to specify HPC. For structural properties, these
may include tensile strength, modulus of elast i c i t y, shrinkage,
or creep. For durability, performance can be measured using freeze-thaw
resistance, de-icer scaling, abrasion resistance, or chloride permeability.
Concretes may also require a special density or low heat of hydration.
The goal is to specify quantifiable performance to match the intended
application. In many cases, this will mean that performance requirements
other than strength will be specified.
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