Innovation in
Architectural Design and Construction
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Strength and ductility test
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HIGH
PERFORMANCE CRC PRE-CAST REINFORCED CEMENT
COMPOSITES
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Strength and ductility
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It is possible to achieve
a compressive strength higher than 400 MPa in CRC, but in
order to utilize this kind of strength, it is necessary
to provide ductility. Otherwise, the use of reinforcement
corresponding to the compressive strength of CRC would
result in large cracks even at moderate loads in bending.
In CRC this ductility is achieved by the use of small and
strong steel fibres. As the strength of CRC is
considerably higher than the strength of conventional
concrete, the content of fibres is also considerably
larger. This enables CRC to behave in a very ductile
manner. An example is the CRC beam shown in picture,
which achieved a center deflection of 70 mm in 4-point
bending. The beam was cycled to full load - a bending
stress of more than 300 MPa - 3 times, yet the amount of
cracking was minor.
All the cracks appeared at the transverse reinforcement.
This means that a strength and ductility similar to that
of steel can be achieved in CRC but, as the density of
CRC is less than half that of steel, CRC has the more
favorablestrength/weight ratio. This can in some cases
make CRC better suited for long spans, moving structures
or cantilevered structures. |
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Anchorage The bond properties of CRC are of special
interest, when the designer looks at load-transferring
joints and connections. CRC has some advantages compared
to conventional concrete. Basically due to the high
amount of silica fume in the binder, which provides a
large contact surface between the concrete and the
reinforcing bars. Also
due to the steel fibres, which provide reinforcement
against cracks in the concrete around the reinforcing
bars thus making it possible to achieve a very high
stress level in the concrete around the reinforcing bars.
Based on the results of tests, an empirical model to
estimate the anchorage strength of a reinforcing bar in a
CRC matrix with 6 vol.% of fibres has been developed.
Where: tu = shear strength (MPa)
fc = compressive strength of CRC (MPa)
c = cover to reinforcing bar
d = diameter of reinforcing bar
L = embedment length of reinforcing bar
t = nAst/dL 0.1
Ast = cross section area of the reinforcing bar
n = number of cross bars
The model is based on trials carried
out with pullout specimens, but a number of bending tests
have also been carried out. The CRC-matrix used for
joints is called CRC Joint-Cast.Besides the applications
in slab connections, the bond properties of
CRC are used in other applications, such as frame
connections, beam connections and for structural concrete
repair, where additional reinforcing bars are
incorporated into the existing structure and covered with
CRC.
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Fire resistance Structures made of very dense concrete exposed
to fire can exhibit a rather severe sometimes even
explosive spalling. The main reason for this is that the
water vapor in the structure produced by heating cannot
escape readily enough through the dense matrix. In time
the vapor pressure may reach the level of the tensile
strength in the matrix, and explosive spalling or
delimitation may occur. In conventional concrete spalling
will often occur as a partial delimitation, but
especially in materials with high tensile capacity
combined with a dense structure, explosive spalling may
occur, as the vapor pressure is allowed to build up for
longer time before it is released.
This problem can occur, also for CRC, and evaluation of
the behavior of CRC structures exposed to fire have been
carried out in several research projects, When drying is
achieved, the behavior of CRC is even better than for
conventional concrete as there is no free portlandite in
the CRC matrix.
The heat transferring ability of CRC is not very
different from conventional concrete, and therefore the
designer can use the same rules for calculations. This
has been observed with cone-shaped beams at DBI) where
the beams were instrumented with thermo-couples, and it
has also been confirmed in tests at VTT, Finland, and
CSTB, France. These tests were part of a Brite/EuRam
project, carried out to investigate the behavior of high
performance concretes exposed to fire. This project was
concluded in April 1999.
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