High-performance concrete today: nothing routine: initially used in bridge decks, today's HPC applications are diverse—requiring demanding performance from materials and construction

Concrete Construction, June, 2002 by Susan C. McCraven

Ternary mixes: opportunities

Why must we mix portland cement with other cementitious materials? Ternary cements, blends of portland cement with silica fume and either fly ash or slag cement, are required for high performance. It is this combination of cementitious materials that gives HPC its innovation, durability, and strength. For the contractor, HPC with silica fume alone can lead to constructibility problems not encountered with conventional mixes. Finishability is improved by adding other cementitious materials, such as fly ash. "The spherical shape of fly ash particles," explains Trent Renfro of Lafarge NA and the American Coal Ash Association (ACAA), "is well-known to facilitate the pumping of concrete, which is important in light of the rigorous demands for low w/cm in HPC construction."

Environmental concerns and efforts to recycle industrial byproducts encourage specifications that increasingly incorporate supplementary cementitious materials. While these materials allow a greater flexibility in the types and amount of cement used, the advantages of ternary cements have not been fully exploited by all segments of the industry. Spreading the news of the success of HPC with blended cements must continue so designers and builders will seek out the many advantages offered by supplementary cementitious materials.

Three innovative HPC projects

Braddock Lock and Dam--in-the-wet technology

In Braddock, Pa., the U.S. Army Corps of Engineers (USACE) is using HPC to construct a replacement dam, "An innovative design involving float-in and in-the-wet technologies can save time and money when compared with conventional cofferdam construction," according to Carol L. Tasillo, USACE, Pittsburgh District. The dam is being built with two precast concrete segments set down onto a drilled shaft foundation, then filled with concrete.

"This is the first application of this type of construction on an inland waterway," explains Tasillo. "Three HPC mixes for high flowability and washout resistance are a challenge for the concrete producer."

A consistent, cohesive mix that meets government specifications "demands the use of various combinations of admixtures for the underbase, tremie infill, and dry infill portions, says John E. Weisbarth, Euclid Chemical Co. The tremie infill concrete was the most demanding HPC used on the project due to the need to control washout, laitance, and heat, claims Tasillo (see Table 1, p. 32).

Floated from a casting site 26 miles downstream, the segments were filled with water and positioned onto the drilled shaft foundation. An HPC grout was used to fill the void between the segments and the river bottom. The HPC tremie infill concrete was then used to fill the first 10 feet inside the precast segments, replacing the ballast of the water. Weisbarth states that the mix "was designed to be highly flowable and self-leveling with minimum washout and bleed to control laitance. To achieve the washout resistance and maintain cohesiveness, four cementitious materials were used in combination with an anti-washout admixture." Flowability was achieved through good mixture proportioning and the use of mid-range and high-range water-reducing admixtures. The remaining portion of the precast segments will be dewatered and infilled using a third flowable HPC.