May 9, 1997
Special concrete may give steel stiff competition
By CHRISTOPHE DAURIAC
Interest in advanced cement-based materials is not solely because of their increased brute strength. They possess other high-performance properties as well, such as low permeability, limited shrinkage and increased corrosion resistance, all valuable characteristics for the construction industry.
HDR Engineering Inc.'s former parent corporation, the French firm Bouygues, has used High Performance Concrete (HPC) with characteristic strengths up to 14,500 psi -- approximately four times the strength of conventional sidewalk concrete -- in more than a dozen bridges and other structures.
Through this work, new frontiers have been opened in the development of new materials and led to a breakthrough product: Reactive Powder Concrete (RPC).
RPC rivals steel
The RPC concept is based on the principle that a material with a minimum of defects such as micro-cracks and inside voids, will be able to realize a greater load-carrying capacity and greater durability.
RPC can, in some specific areas, compete with steel. The refinements involved in RPC technology make it possible to create a more homogeneous cement-based material by reducing the differences between the cement and aggregate. This results in a concrete product with properties which, in the past, have been reserved for metals and allows for the design of new products and structures using concrete.
All these improvements, however, result in a substantial cost increase over conventional and even high-performance concrete. Because of its cost, RPC will not replace concrete in applications where conventional mixes can economically meet the performance criteria. However, with some of its performances nearing those of metals and at a minor cost compared to steel, RPC becomes truly competitive in areas where steel is predominant.
Transcending current limitations
Historically, concrete and cement-based composite materials have been incorporated into structures as compression-only systems. RPC transcends this traditional limitation and brings a new approach to prestressed concrete design because it can be used to resist all but direct primary tensile stresses. This eliminates the need for shear and other auxiliary reinforcing steel in precast members, which results in cost reduction for the items themselves but, more important, for the labor they usually generate.
The high-performance properties of RPC provide many enhancements compared to conventional concrete structures:
Characteristics and comparisons
RPC 200 with compressive strengths in the range of 30,000 psi can be achieved with a mixture which incorporates regular concrete components and can be placed in the field like conventional concrete.
The main inconvenience with high-performance concrete materials was their brittle response and lack of flexural performance. Adding steel fibers allows RPC material with flexural strengths in the 7,000 psi range, up to eight times that of HPC. The material's ductility and ability to absorb energy is also improved with typical values 300 times greater than HPC, making it comparable to that of some metals.
RPC also has ultra-high durability characteristics resulting from its extremely low porosity. Penetrations of liquid and/or gas become nearly non-existent in comparison to HPC.
Sherbrooke pedestrian bridge
An international team has been formed to construct a lightweight space-truss pedestrian/bikeway using this ultra-high performance concrete in Sherbrooke, Canada. To demonstrate the unique characteristics of RPC and minimize weight, a design concept was chosen which differs significantly from those used for traditional concrete structures.
The superstructure of the bridge will be a three-dimensional prestressed space truss spanning 200 feet. The walkway deck, which also serves as the top chord of the truss, is only 1 inches thick. The web members will be a composite design using RPC confined in thin-wall stainless steel tubing. No conventional reinforcing steel is planned for the entire superstructure. The footbridge's effective thickness is six inches. A comparative study showed that the same structure made of HPC would have required a thickness of 15 inches.
RPC technology will allow the footbridge builders to optimize material use, realize economic benefits and build a structure that is environmentally friendly. Applied RPC technology allows the design of more mechanically intelligent structures and a more rational use of construction materials.
X-shaped prestressed beams
The current structural precast shapes used for prestressed beams in bridges and buildings have been proportioned for concrete with much lower strength properties. These proportions would not take maximum advantages of the capabilities of RPC.
There is now an opportunity to introduce new shapes into prestressed beam design. An RPC prestressed beam, with an hourglass cross section (referred to as X-shaped) and without any kind of secondary steel bar reinforcement, was configured and tested to carry equal moment capacity when compared to its steel wide-flange counterpart and to have the same depth and very close weight. By using RPC, the depth of the member can be reduced by 50 percent and its weight by 75 percent compared to conventional prestressed concrete.
An agreement was signed by Bouygues and the French equivalent of the U.S. Nuclear Regulatory Commission aiming at fabricating High Integrity Containers (HIC) for long-term interim storage of medium-level nuclear wastes.
Current technology involves steel or cement-based multiple-walled containers in which wastes are immobilized by the injection of concrete or grout. This process does not comply with the new policy for waste volume reduction.
Containers made of RPC are currently being developed which would make it possible to "bulk" package the wastes. In this case, the container is the only barrier for waste confinement. This specification makes RPC the only available cement-based product for this application. RPC is a perfect match because it provides low porosity, excellent mechanical characteristics and streamlines the fabrication process of the barrel. It also results in a container that can be upgraded to meet future standards and is more durable, economical and more easily maintained.
Security for banks, computer centers
RPC is being investigated for security applications which require extremely high penetration resistance as well as fire recovery properties and weight reduction.
Current security technology, using either metal alloys or concrete composite/steel panel structure, does not satisfy new security and weight requirements. To be globally resistant, enclosures now require complicated layers which generate high costs and weight. Regular or even high-performance concrete cannot provide the toughness and mechanical resistance often needed.
A new generation of security products is being developed which will be more economical, especially for small enclosures which must be protected but are not worthy of very high-priced alloys. Such enclosures also have to be lightweight because of transport costs and weight restrictions in existing buildings.
RPC provides an economical alternative. It also permits monolithic pre-casting which reduces the numbers of parts to assemble and also reduces the chances for the enclosure to be fractured.
Other potential applications include a cooperative research agreement with the U.S. Army Corps of Engineers focusing on pipes for water, sewage and other liquids under pressure or gravity flow. High-strength liners for tunnels and mining shafts as well as architectural and technical floor panels are also being developed.
For additional information, or to discuss ideas or production partnering proposals for RPC, contact the Seattle office of HDR. More information is also available at the HDR web site under Hot Topics/Research & Development.
Christophe Dauriac is a transportation engineer with HDR Engineering Inc. at the Seattle office. The Omaha, Neb.-based firm is a full-service engineering and architectural design consultant and is responsible for development of reactive powder concrete in North America.
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