08-08-2012, 11:04 AM
Steel fibre reinforced concrete (SFRC) – Quality, performance and specification
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Synopsis
Steel fibre reinforcement is widely used as the main and unique reinforcing for industrial concrete floor slabs,
shotcrete and prefabricated concrete products. It is also considered for structural purposes in the
reinforcement of slabs on piles, tunnel segments, concrete cellars, foundation slabs and shear reinforcement
in prestressed elements. Ensuring the quality and performance of the steel fibres and ultimately the SFRC is
critical and the challenge faced by engineers involved in designing these projects is to unambiguously
specify the performance required by the SFRC so as to achieve in the finished structure the performance
that was assumed in design.
Since 2006, design methods for SFRC have been available in NZS 3101:2006 Concrete Structures1. The
documented test and design methods describe not only how the material properties are determined but
provide guidance on the design of SFRC used in structural applications, in the ultimate and serviceability
limit state, for different Performance Classes (SFRC grades). An identical approach to the use of strength
grades for the specification of both steel and concrete.
Background
The New Zealand building industry has had access
to a range of different fibre types used to reinforce
concrete for over 10 years and in recent years the
choice of proprietary products available has
increased significantly. Steel fibres, macro
synthetic fibres, micro synthetic fibres, cellulous
fibres are widespread within construction today. It
is a common misconception that all fibre types
reinforce concrete in the same way and that
substitution between fibres is possible. Different
combinations of fibre raw materials, dosage and
geometry will all produce a fibre reinforced
concrete element with quite different material
properties. This will influence the performance and
dictate their suitability in certain applications. Steel
fibre reinforced concrete (SFRC) is the most
common in NZ and is included within
NZS3101:2006, encouraging engineers to consider
its use under the framework of the Building Code.
History of SFRC
Steel fibre reinforced concrete (SFRC) was
introduced commercially into the European market
in the second half of the 1970’s. No standards or
recommendations were available at that time
which was a major obstacle for the acceptance of
this new technology. Initially steel fibres were
mostly used as a substitute for secondary
reinforcement or for crack control in less critical
parts of the construction. Today steel fibres are
widely used as the main and unique reinforcing for
industrial floor slabs, shotcrete and prefabricated
concrete products. They are also considered for
structural purposes in reinforcement of slabs on
piles, full replacement of the standard reinforcing
cage for tunnel segments, concrete cellars,
foundation slabs and shear reinforcement in
prestressed elements
Performance Classes
This evolution into structural applications was
mainly the result of the progress made in SFRC
technology, a steady build up of knowledge and
understanding of its use into a wide range of
applications as well as the research carried out at
different universities and technical institutes in
order to understand and quantify the material
properties; a time line is shown in Figure 1.
Generic design guides or standards could only be
developed with this understanding of material
properties of SFRC independent of fibre type.
In the early nineties, recommendations for design
rules for steel fibre reinforced concrete started to
be developed and since October 2003, RILEM TC
162-TDF 2 recommendations for design rules have
been available. These form the basis of the design
methods provided for SFRC in NZS 3101:2006
Concrete Structures Standard.
Quality control – Ready Mix
It has been shown that the type of test method,
sample size and a number of other factors can
greatly influence the results achieved in testing the
properties of steel fibre reinforced concrete. For
this reason tests on their own shouldn’t be used as
a quality control measure in the production of
SFRC, but rather a manufacturing process should
be established that in itself is under control, with
consideration to the following.
Conclusion
New Zealand currently has design
recommendations through the provisions in
NZS3101:2006, but is lacking the minimum quality
and performance requirements for steel fibres.
There is an opportunity here to take advantage of
the progress made in Europe and to create an
environment in NZ where engineers have the
confidence to consider the benefits of SFRC as a
material in their designs and to have a regulated
market where quality and performance of the
material are guaranteed.
Determining or confirming design properties for
SFRC should be carried out with an understanding
of test variability and its causes and care should be
taken when assessing post crack strengths on
limited test data particularly when comparing two
different steel fibre reinforced concretes. Also,
such tests should not be used as a measure of
quality control in their own right but rather should
form part of a quality controlled process at the
batching plant.