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Iowa Boasts First UHPC
Highway Bridge in U.S. (Click on title for story)
First Use of Ultra-High Performance Concrete for an Innovative Train Station Canopy
By V. H. Perry and D. Zakariasen, Lafarge Canada Inc.
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| Figure 1. Shawnessy Light Rail Transit Station, Calgary, Canada. |
The Shawnessy Light Rail Transit (LRT) Station, constructed during
fall 2003 and winter 2004, forms part of a southern expansion to
Calgary's LRT system and is the world's first LRT system to be constructed
with ultra-high performance concrete (UHPC). The innovative project,
designed by Enzo Vicenzino of CPV Group Architects Ltd., is owned
by the City of Calgary, managed by the Transportation Project Office
(TPO), and constructed by general contractor, Walter Construction.
The Design
The station's twenty-four thin-shelled canopies, 5.1 m by
6 m (16.7 ft by 19.7 ft), and just 20 mm (0.79 in.) thick, supported
on single columns, protect commuters from the elements. UHPC technology
has a unique combination of superior technical characteristics including
ductility, strength, and durability, while providing highly moldable
products with a high quality surface aspect. The contract document
specified a minimum requirement of 130 MPa (19,000 psi). In addition
to the canopies, the components include struts, columns, beams,
and gutters. The volume of material used totaled 80 m3
(105 yd3).
Manufacturing and Installation
The precast canopy components were individually cast and consist of half-shells, columns, tie beams, struts, and troughs. Table 1 summarizes test data from production of the twenty-four canopies.
Table 1. Test Results — LRT Canopies
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Mean value after 72 hours thermal treatment |
Standard development |
Property |
MPa (psi) |
MPa (psi) |
Compressive strength
Flexural strength |
152 (22,000)
18 (2,600) |
6.2 (900)
3.4 (500) |
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| Figure 2. Half-canopy in steel form. |
The columns and half-shells were injection cast in closed steel forms (Figure 2). Troughs were cast through displacement molding, while struts and tie beams were produced using conventional gravity two-stage castings.
The columns were installed on the concrete platform first. Then, the right and left half-shells, along with the tie beams, were pre-assembled in the plant and transported to the site where they were lifted (by crane) over the railway tracks, for placement on the columns (Figure 3). Upon arrival at the site, the canopies were set on temporary scaffolding, and struts were attached to the shells and previously installed columns with welded connections.
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| Figure 3. Canopies ready for transportation. |
Conclusion
The material's unique combination of superior properties and
design flexibility facilitated the architect's ability to create the
attractive, off-white, curved canopies. Overall, this material offers
solutions with advantages such as speed of construction, improved
aesthetics, superior durability, and impermeability against corrosion,
abrasion and impact—which translates to reduced maintenance
and a longer life span for the structure.
This project was the first of its type in the world using this mix for thin, architectural, curved canopies. While this solution demonstrates many of the benefits of the material technology, it is apparent that the true benefits are not fully recognized. Furthermore, the material is still in its infancy, and, in the next few years, much progress is anticipated in the area of optimized solutions.
Ultra-High Performance Concrete (UHPC), also known as reactive powder concrete (RPC), is a high-strength, ductile material formulated by combining portland cement, silica fume, quartz flour, fine silica sand, high-range water reducer, water, and steel or organic fibers. The material provides compressive strengths up to 200 MPa (29000 psi) and flexural strengths up to 50 MPa (7000 psi).
The materials are usually supplied in a three-component premix: powders (portland cement, silica fume, quartz flour, and fine silica sand) pre-blended in bulk-bags; superplasticizers; and organic fibers. The ductile behavior of this material is a first for concrete, with the capacity to deform and support flexural and tensile loads, even after initial cracking. The use of this material for construction is simplified by the elimination of reinforcing steel and the ability of the material to be virtually self placing or dry cast.
The superior durability characteristics are due to a combination of fine powders selected for their grain size (maximum 600 micrometer) and chemical reactivity. The net effect is a maximum compactness and a small, disconnected pore structure.
The following is an example of the range of material characteristics for UHPC:
|
STRENGTH |
|
| Compressive |
120 to 150 MPa
(17000 to 22000 psi) |
| Flexural |
15 to 25 MPa
(2200 to 3600 psi) |
| Modulus of Elasticity |
45 to 50 GPa
(6500 to 7300 ksi) |
| DURABILITY |
|
Freeze/thaw
(after 300 cycles) |
100% |
Salt-scaling
(loss of residue) |
< 60 g/m2 (< 0.013 lb/ft3) |
Abrasion
(relative volume loss index) |
1.7 |
| Oxygen permeability |
<10-20 m2 (< 10-19 ft2) |
Cl- permeability
(total load) |
< 10 C |
| Carbonation depth |
< 0.5 mm (< 0.02 in.) |
References
Lafarge North America Inc., Technical Characteristics: UHPC
with Organic Fibres, National Building Code of Canada, 1995.
Perry, V.H., "Q&A: What Is Reactive Powder Concrete?"
HPC
Bridge Views, No. 16, July/August 2001.
Lafarge North America Inc. Ductal® Website: http://www.imagineductal.com.
For a PDF of the Concrete Technology Today Newsletter
containing this article, click here.
Iowa Boasts First UHPC
Highway Bridge in U.S.
Iowa’s Wapello County boasts the first UHPC highway bridge
in the United States completed in May 2006. Although a simple, single-span
bridge with a 3-beam cross section, the Mars Hill Bridge is a significant
step toward “The Bridge of the Future” – utilizing
110-feet UHPC girders that do not have any rebar for shear stirrups.
To read more about this award-winning bridge, click
here. This paper was one of 96 presented at the 2006 Concrete
Bridge Conference held in May in Reno, Nev. Conference proceedings
are available on a CD available at the special price of $15 for
a limited time. Click
here for more information or to order.
Click here
for more information on ultra-high performance concrete.
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