The Scotia Plaza is a 68-story building (275m high) in downtown Toronto. It was constructed in 1987 and is constructed entirely of 70 MPa (over 10,000 psi) concrete. The Scotia Plaza illustrated one of the first uses of slag in a high-strength concrete mixture. Silica fume was also used in the mixture. The concrete was placed between temperatures of –20C and 35C. In the summer, liquid nitrogen was used to keep the concrete below 25C. Quality control testing yielded an effective design strength of 85Mpa, 93.6MPa with a coefficient of variation of 7.3%. The extra strength helped the contractor obtain a new contract for a second high-rise building.
The Ile de Re Bridge in France is 3 km long but only took 20 months to build. The bridge is separated into six different viaducts to limit the effect of collision with large ships. Four 100m channels are available for navigation. To construct the bridge, precast box-girders were used and were placed by the cantilever method and external prestressing. The characteristic concrete strength was 59.5MPa.
The Two Union Square Building in Seattle, Washington is a 58-story building braced by an innovative composite core of four 3m diameter steel pipes and filled with 130MPa concrete. To stiffen the 216m building to limit swaying and to provide earthquake load resistance, concrete with an elastic modulus of 50GPa was used (Aitcin). 90MPa concrete would have been sufficient for the strength needs but the requirement for the elastic modulus was only met by concrete with a 130MPa compressive strength. Concrete was placed at night to avoid traffic. Up to 750m^3 of concrete was placed per night. Actually making the high strength concrete is not easy to achieve in practical applications. Seattle’s high quality raw materials enabled the success. A low-alkali Type I/II cement with low rheological reactivity to superplasticizer was used. The sand had sharp, angular particles and a fineness modulus of 2.8. The coarse aggregate was very clean and thus bonded well with the paste. The moisture content of both fine and coarse aggregates was monitored for each 40m^3 of concrete to avoid problems with the critical water content.
As the use of high-strength concrete is relatively young, many more structures will reap its benefits in the future.