In 1987, the Norwegian concrete industry organized an international symposium on the subject of high-strength concrete. At that time, the Norwegians were actively involved with the design and construction of high-strength concrete structures for use in the offshore oil industry. In the United States, the use of high-strength concrete was largely restricted to columns of high rise buildings. Subsequent symposiums in the same series were held in the U.S. (1990), Norway (1993), France (1996), Norway (1999), and Germany (2002). Organized by the American Concrete Institute, the seventh symposium will be held in the United States in 2005.

In the evolution of these symposiums, the scope has expanded from considering high-strength concrete alone to further encompassing high performance concrete (HPC). Simultaneously, in many countries, the focus has increasingly shifted from one of primarily academic interest to practical and economic viability. The academic community, of course, continues to research how and why the material works and to extend the boundaries of the technology. Based on papers presented at the Sixth International Symposium on High Strength/High Performance Concrete held in Leipzig, Germany, June 2002, this edition of Technical Talk provides an update on applications in two European countries.

Germany

In Germany, high-strength concrete was first used in 1990 in the construction of a 610-ft. (186-m)-tall office building in Frankfurt. The concrete had a specified design strength of 12,300 psi (85 MPa) as determined by using 6-in. (150-mm) cubes. When measured by means of a cube, concrete strength tends to be 10 to 15 percent higher than that measured using a cylinder. Since 1990, concrete with design compressive strengths as high as 16,700 psi (115 MPa) has been used in columns of buildings.

Recently, in Frankfurt, concrete with a strength of 12,300 psi (85 MPa) was used in two construction projects. Frankfurter Welle is a 15-story complex of office and residential buildings with a height of only 177 ft. (54 m), demonstrating the advantages of high-strength concrete for lower buildings as well. The Gallileo project at 449 ft. (137 m) in height was constructed with concrete of specified design strengths of 12,300 psi (85 MPa) in the columns and shear walls of the lower stories and 13,800 psi (95 MPa) in some columns on the upper stories. High-strength concrete was selected to increase the load-carrying capacity and to maximize the gross floor area.

Most impressive is the application of high-strength concrete in combination with high-strength reinforcement in the precast concrete columns of the Herriots complex in Frankfurt. The columns contain concrete with a design compressive strength of 18,100 psi (125 MPa) and steel reinforcement with a yield strength of 109 ksi (750 MPa). The concrete compressive strength was specified as a 6- ∞ 12-in. (150- ∞ 300-mm) cylinder strength. Polypropylene fibers were included in the mix to provide the required fire resistance. The average 56-day cylinder compressive strength during early production was 20,450 psi (141 MPa), which rivals the compressive strength of any concrete produced commercially in the U.S. The difference in strength measured using cubes and cylinders was reportedly negligible.

The use of HPC in German bridges has also been growing. Since 1998, seven small pilot bridges have been built, mostly in the southern part of Germany. These pilot projects utilized different design concepts and construction methods. All superstructures were prestressed either with conventional bonded tendons or with unbonded tendons. Cast-in-place HPC as well as precast HPC elements were used to demonstrate the range of applications. In addition, two larger bridges were built on the autobahn network as pilot projects prior to the use of HPC in bridges. In this regard, bridge applications in Germany have followed a course similar to that in the U.S. — start small and then extend to larger structures as confidence in design and construction develops.

The first large-scale pilot application was a five-span continuous cast-in-place, post-tensioned concrete bridge across the Zwickauer Mulde River near Glauchen. The longest span was 128 ft. (39.0 m). The specified concrete cube strength for the design was 10,000 psi (70 MPa). Actual strengths at 28 days were about 14,500 psi (100 MPa).

The second application in a pilot project was the Luckenberger Bridge in the city of Brandenburg across the River Havel. A main span of 131 ft. (40 m) was required to keep the river clear of any piers. At the same time, the geometry of the roadway had to conform to the old bridge consisting of 39-ft. (12-m) spans that was being replaced. Using high-strength concrete made the construction of such a structure possible. The specified concrete strength was the same as that used for the bridge near Glauchen.

France

Beginning in the 1980s, HPC was used in France primarily in the construction of bridges due to the favorable attitude of the owners. Today, over 100 bridges have been built with HPC. Much of this development may be attributed to a national program known as BHP 2000. Involving representatives from all segments of the industry, this program has focused on HPC and removal of obstacles to its implementation.

Using high-strength concrete to reduce superstructure weight or facilitate more efficient pylon design has been effective in cable-stayed bridges such as the Beaucair-Tarascon Bridge over the Rhone and the Normandy Bridge over the Seine. Also used in standard overpass bridges, HPC has been successfully applied in “ordinary” French countryside projects. The majority of applications have incorporated concrete with a design compressive strength of 11,600 psi (80 MPa). The use of HPC in precast elements has resulted in members that are lighter to ship.

A new bridge across the Rhine from France to Germany was recently completed near Strasburg. The approach spans on the French side and the main bridge are French-owned and, accordingly, were designed and built using French rules and codes. On the German side, the approach spans were constructed on the basis of German specifications. HPC with a strength of 9,400 psi (65 MPa) was used for the main span structure consisting of a cast-in-place haunched box girder built by the balanced cantilever method. The center span has a length of 673 ft. (205 m). The required strength of the concrete was 6,200 psi (43 MPa) at 2½ days to facilitate a one-week construction cycle.

The French have also made significant progress in modifying their codes and standards to address the use of HPC. In Europe, most codes limit their applicability to a maximum concrete compressive strength. This is similar to the AASHTO LRFD Bridge Design Specifications, which have an upper limit for the concrete compressive strength of 10,000 psi (70 MPa), unless tests are made to establish the relationships between the concrete strength and other properties. However, for buildings designed using the ACI Building Code Requirements for Structural Concrete, no general upper limit is specified, although some provisions are limited to a concrete compressive strength of 10,000 psi (70 MPa).

As illustrated by the various projects cited and concomitant standards development, HPC technology is no longer limited to North America. Today, it enjoys international acceptance.

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Henry Russell is an engineering consultant based in Glenview, Ill. He is a member of American Concrete Institute, American Society for Testing and Materials, Precast/Prestressed Concrete Institute, and American Segmental Bridge Institute. He is a former chairman of ACI's subcommittee on High-Performance Concrete.