SP_1102_lafarge_en.qxp:Layout 1 - Ductal

Concrete Plant International
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APRIL
20112
CONCRETE TECHNOLOGY
Equipment and Production Techniques with UHPC
SPECIAL PRINT
CPI 11/02

CONCRETE TECHNOLOGY
Lafarge North America, T2W 4Y1, Calgary, Alberta, Canada
Equipment and Production Techniques with UHPC
In the last issue of CPI, Lafarge provided an introduction to “Working with Ductal Ultra-High Performance Concrete” (UHPC). The following is
the second part of a series of related articles on this topic. In this installment – Lafarge will provide details covering the various batching, forming,
placing and curing techniques that are required for successful manufacturing of precast elements using UHPC.
Ductal is a family of Ultra-High Performance Concrete (UHPC) products. The appropriate formulation is selected based on the application and
performance requirements by its end use and by the customer. When selecting a Ductal product, determining factors include strength, ductility,
exposure to corrosive agents, desired aesthetics and exposure to human contact. Two different types of fibers are typically used to manufacture
end products using Ductal; high carbon metallic or poly-vinyl alcohol (PVA) fibers. The fiber and UHPC formulation are customized to best
fit the application requirements. Various rheologies (from dry cast to self-leveling) and curing techniques facilitate different product capabilities
which are suitable for a range of precast applications.
Vic H. Perry, Peter J. Seibert,
Lafarge North America, North America
With its unique combination of ductility,
strength, durability and aesthetics, precast
elements can have thinner sections and longer
spans that are lighter, more graceful
and innovative compared to conventional
concrete. Highly moldable, it replicates
form materials with enormous precision. Com -
pressive strengths reach up to 200 MPa
(29,000 psi) and flexural strengths reach
up to 40 MPa (5,800 psi), providing improv -
ed durability and impermeability against
corrosion, abrasion and impact. Due to the
material’s inherent strength properties,
rebars are often not necessary, there by facilitating
the ability to produce thin, curved,
complex shapes. Furthermore, UHPC structures
are sustainable and expected to have
an extended service life, resulting in less
environmental impact over time.
Manufacturing with Ductal UHPC
The manufacture and sale of Ductal UHPC
products began in the mid-90’s. Conse -
quen t ly, many precasters worldwide have
heard about UHPC and may be wondering
what type of equipment modifications and
capital investments are required to add
UHPC to their current product offerings.
This question strongly depends on the precaster’s
current production facility and their
new product application.
Manufacturing of UHPC precast elements
presents the industry with new challenges
and opportunities. Recognizing that old
production methods must be reassessed for
UHPC production is a fundamental change
to conventional manufacturing processes.
For instance, precasters are required to
review their current batching methods, casting
techniques, moulding expertise and
handling techniques.
Before commencing any major production
facility investments, we recommend that
manufacturers develop a business plan for
their proposed UHPC solution. This requires
the precaster’s review of their distribution
and promotion strategies – since they are
the experts with respect to their local markets,
accessibility and market tolerance. In
addition, they will develop their own value
proposition for any new, innovative UHPC
solutions. Once the business plan has been
completed, prototypes of the proposed
solution must be tested, optimized and certified.
These procedures will result in a clear
understanding of the required equipment
and capital investments. Ultimately, successful
UHPC commercialization relies heavily
on these business strategies, in order to
create continuous sales and efficient production.
Batching with Ductal UHPC
To date, the various Ductal product formulations
have been successfully batched in
many different mixers, ranging from a small
two-bag mixer (see fig. 1) to a fully automated
batching plant (see fig. 2). The mixing
efficiency and mixing performance de -
pends on the type and speed of the mixer,
requested mixing time by the precaster and,
the required UHPC volume for precast production.
When setting up the batch plant for
UHPC at a precast plant, the raw material
Fig. 1: In-field batching with pan mixers for smaller volumes
Fig. 2: Fully automized batching facility for Ductal UHPC
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CONCRETE TECHNOLOGY
Fig. 3: High Shear Pan mixer
introduction into the mixer must be consider -
ed. The key to producing high quality
UHPC products is very precise proportion
control of raw materials, temperature control
and optimization of the mixer’s performance
requirements.
In order to successfully produce any formulation
of the Ductal range of products, the
specialized Ductal premix, superplasticizers,
fibers (poly-vinyl alcohol [PVA] or high
carbon metallic) and water are required.
Lafarge arranges for delivery of each these
materials (except water) to their customers
worldwide. In North America, Ductal premix
is available in 50 lb (22.7 kg) bags,
2,000 lb (907 kg) super-sacks or bulk delivery.
In Europe, Ductal premix is available
in 25 kg (55.1 lb) buckets, 1,000 kg
(2,205 lb) super-sacks or bulk delivery. The
raw materials consist of components commonly
found in concrete products such as
cement, silica fume, and silica sand – plus
other materials unique to Ductal such as
ground quartz and fibers, as mentioned
above. Prior to delivery, all raw materials
and the blended premix undergo a strict
quality assurance process to ensure consis -
tency and the required performance.
Vic H. Perry received his Bachelor of
Civil Engineering with Distinction in 1978
and Master of Applied Science in Structural
Engineering in 1984 from Dalhousie
University in Nova Scotia, Canada. In 1999,
he was bestowed a Fellow of the Canadian
Society for Civil Engineering (CSCE) and elected President of
the CSCE in 2010. Since 1997, Vic has been involved in the
development of Ductal (UHPC), initially as Director of Marke -
ting (Ductal) for the Lafarge Group in Paris, France and since
2001, as Vice-President & General Manager Ductal, Lafarge
North America, in Calgary, Canada.
Vic.Perry@Lafarge-na.com
Peter J. Seibert obtained his Bachelor of
Civil Engineering from the University of
Toronto in 1996, a Masters in Civil
Engineering from Queen’s University,
Kingston, Ontario in 1998 and an MBA from
Wilfrid Laurier University, Waterloo, Ontario
in 2003. Based in Calgary, Canada, Peter is the Technical
Director for Ductal (UHPC), responsible for the manufacturing,
supply chain, quality control and technical aspects of Ductal
throughout North America since 2006.
Peter.Seibert@Lafarge-na.com
For the most efficient and consistent mixing
of UHPC, high shear mixers have been
used successfully; especially counter-current
pan mixers which can provide accelerated
mixing times (see fig. 3). These high shear
mixers disperse water and admixtures onto
the cement particles without heating the mix
through kinetic energy generated by the
mixing process.
Others, such as mortar, horizontal shaft or
pan mixers have also been used however
they are generally slower. Therefore, the
precaster should consider the trade-offs between
mixing time, batch volume and material
placement. Prior to dedicating a mixer
for UHPC production, it is recommended to
calibrate the mixer by measuring mixing
time and flow characteris tics of UHPC and
comparing the compressive strength to a
reference strength. For projects requiring
larger volumes, mixing procedures have
been perfected to allow batching of Ductal
UHPC in ready mix concrete trucks (see
figure 4). When setting up the batching
facility for UHPC manufacturing, precasters
must consider how each of the materials
are weighed and proportioned prior to
mixing. Accurate proportioning is essential
in successful batching of UHPC. The premix
is typically supplied in paper bags or supersacks
and is added first to the mixer. Then,
the water and liquid superplasticizers are
introduced.
Fig. 4: Batching of Ductal UHPC in a ready mix concrete truck
Once the batch is fluid, the weighed fibers
are manually or semi-automatically added
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CONCRETE TECHNOLOGY
to the mixer. When turning the UHPC formulation
from the dry to liquid stage, significant
energy is required by the mixer.
Therefore, batch sizes are often reduced,
from 40% to 75% of the recommended size
by the mixer manufacturer. Prior to placing
of UHPC, the mixer may be slowed down in
order to allow entrapped air to escape due
to the high speed mixing. Entrapped air
may lead to a weaker matrix and poor surface
finish of the precast element.
Depending on the casting technique and
performance requirements, the various
Ductal products are available as self-levelling
or dry-casting formulations.
Forming Ductal UHPC
Successful execution of the precast project
depends upon design of the moulds and
the procedures developed to use them.
Traditional hand screeding and finishing of
UHPC is not normally used due to its high
flow and high fiber content of the plastic
matrix. Also, the self-levelling UHPC formulations
have no internal shear in the plastic
state and behave similar to self-consolidat -
ing concrete.
This creates challenges to develop formworks
that are completely en closed with high tolerances
but it also creates opportunities for
the precaster to offer a new range of unique
and innovative end products with slender
elements and almost any surface texture on
all sides of the element.
For accurate mould design, any potential
mould deflections and the accommodation
of initial UHPC shrinkage must be consider -
ed. Attention to specific moulding details
including: release agents; type of moulding
materials; methods of release during initial
shrinkage; orientation and; product support
– are all critical factors to consider when
designing and using formworks. Since
Ductal replicates surfaces with great precision,
the selection of the moulding material
is based on the expected surface outcome.
The following moulding materials have
been used successfully in the manufacturing
of various precast UHPC elements: steel,
silicone, lexan, polyurethane, Teflon, glass
and wood (with epoxy painted surfaces).
Placing Ductal UHPC
When placing self-levelling UHPC into formworks,
it is important to take advantage of
its fluid characteristics. When discharged
from a concrete bucket onto flat surface
moulds, Ductal will create a mass of material,
which will spread itself throughout the
form. By moving the discharge point, at a
rate such that it always stays behind the
“leading edge” of the flow (see figure 5),
the mold can be filled in one continuous
motion. This is important because, if UHPC
flows meet each other, there will be minimal
fibers bridging the junction; resulting in a
weak plane. Due to the material’s fluid characteristics
and fiber dispersion, it cannot
be finished like traditional concrete (such as
raking, trowling, or brooming). After placement,
any exposed surfaces must be covered
in order to prevent dehydration.
Ductal’s high flow properties also permits
special casting techniques such as injection
and displacement methods to create sophisticated
forms and shapes that cannot be
achieved with conventional casting techniques.
For instance, an innovative injection
technique (see fig. 6) was successfully used
to produce a complex canopy roof system
for the “Shawnessy LRT Station” in Calgary,
Cana da – winner of 6 industry awards
including the fib (fédération international
du béton)/ International Federation for Struc -
tural Concrete) Award for Out stan ding
Struc tures (see fig. 7). For this project, the
UHPC material was forced into a complex
steel mould through a piping system, by
applying air pressure above the plastic
material.
Displacement casting is another method
that offers new opportunities for precasters.
This process is done simply by depositing
the precise volume of material needed on
the shape of the final casting into the bottom
portion of the form and then introduc -
ing the top portion of the form.
This will displace the plastic material into
the shape of the casting. If the entry points
of the secondary form are controlled, it is
possible to move the plastic UHPC in directions
that will influence fiber orientation and
facilitate the release of entrapped air. For
this casting method, alignment guides to
control the exact positioning of the backform
and considerable force of displacement
are required. This casting method was
recently used for the production of full
depth waffle highway bridge deck panels,
to be installed in the summer of 2011 (see
fig. 8).
For a high quality end product surface
appearance, moulds must be filled slowly
to prevent entrapped air. Internal vibration
cannot be used however limited external
vibration may be used to aid de-airing. Do
not allow excessive external vibration
where PVA fibers will float to the surface or
steel fibers will sink to the bottom of the
mould. Control of the fluid rheology minimizes
segregation of the fibers. Filling of the
moulds must be one continuous casting by
following behind the leading edge of
UHPC.
Curing
The various available formulations of the
Ductal family range of products require different
curing methods depending on the
UHPC performance expectations. Architec -
tural UHPC precast elements are typically
removed from the mould after final set has
been reached, whereas, for structural applications,
the finished UHPC precast elements
may be thermally treated after setting
and demoulding.
This process requires the UHPC element to
be exposed to 90º Celsius (195º F) at 95%
relative humidity for 48 hours to allow the
hardened UHPC element to reach its ultimate
strength and durability characteristics
by hydrating all of the free water within the
matrix. Thermal treatment also provides
improved dimensional stability of the product.
Commercial and technical partnership
with precasters
Lafarge offers continuous product and technical
support by working with their customers
and forming ongoing partnerships.
Over the past decade, the company has
developed many standard operating procedures
which are available to guide and
assist precasters with the manufacturing of
high quality Ductal UHPC products.
Support and essential, start-up training is
also provided, which allows precasters to
gradually master the technology and commence
production of a profitable solution.
For many Ductal projects or product applications,
the experts at Lafarge may also
facilitate important liaisons between the precaster,
owners, engineers, architects and
academics worldwide. In addition, Lafarge
may have opportunity to promote the precaster’s
UHPC solution through various
media outlets such as publications, websites,
tradeshows, and industry awards.
For a successful partnership, Lafarge contributes
its materials exper-tise and commercial
know-how, while the precasters provide
their technical skills in the industrial process
and knowledge of their desired markets.
The success of innovative projects requires
regular study and development of original
processes. Hindsight, along with Lafarge's
own experiences with the many different
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CONCRETE TECHNOLOGY
Fig. 5: The “leading edge” of Ductal UHPC flow
Fig. 6: Injection casting equipment for the Shawnessy LRT Station
canopies, Calgary, Canada
Ductal applications and products, allows
for quality guidance and facilitates the ability
to recommend the most suitable or innovative
production methods.
· Manufacturing of UHPC
architectural products
· Manufacturing of UHPC
artisan products
FURTHER INFORMATION
Future Articles in CPI
Watch for more articles on Ductal UHPC in
future issues of CPI, including:
· Manufacturing of UHPC
structural products
Lafarge North America
10655 Southport Road SW., Suite 1200
T2W 4Y1, Calgary, Alberta, Canada
T +1 403 271 9110 · F +1 403 278 7420
ductal@lafarge-na.com · www.ductal-lafarge.com
Fig. 7: The Shawnessy LRT Station, Calgary, Canada: 24 ultra-thin
canopies (16.7’ x 19.7’ x ¾’) (5.1 m x 6 m x 2 cm), supported on
single columns – also made with Ductal.
Fig. 8: Full-depth waffle panel mould, displacement casting
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