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Replacing the nearly 100-year-old Wabasha Street Bridge in St. Paul, Minn., in the early 1990s was a project not to be taken lightly. The weight of history, not just a new bridge, would bear upon piers for a new cast-in-place concrete segmental box girder structure. Project consultant MnDOT agreed with Kevin Nelson, project engineer for St. Paul, that replacing 70% of the portland cement by weight with ground granulated blast-furnace (GGBF) slag would control thermal cracking in the piers and meet specified concrete compressive strengths.

To Nelson, the only option for slowing the hydration rate was using a high GGBF slag content, but MnDOT to that point allowed only 35% in paving mixes. That wouldn't maintain the required temperature differential using the Schmidt Model found in ACI 207.1R-99, ""Mass Concrete,"" which measures gradients between the core and outer surfaces of a mass concrete structure. However, MnDOT was wary about increasing the slag content.

Gary Brenno of Cemstone's quality control department showed MnDOT test data revealing the effects of GGBF slag on heat of hydration and later strengths in concrete.

Nelson had agreed that the 4000-psi strength specified for the pier footings and stems by designer Figg Engineering was to be reached at 56 instead of 28 days because loading wouldn't occur quickly. Brenno designed a mix with 60% GGBF slag, or 354 pounds of slag and 236 pounds of Type I cement per yard. Brenno based the mix on a state mix that was to reach a 4300 psi minimum anticipated strength according to MnDOT's prescriptive approach.

Thermocouples recorded and collected data at nine points on the first footing placed, for Pier No. 3, in summer 1996. The footing turned out to be a good test of the mix.

Brenno designed a mix with 413 pounds of GGBF slag or a 70% replacement of cement weight based on a MnDOT mix with the next lower minimum anticipated 28-day strength, 3900 psi. The new mix also used 177 pounds of Type II cement, 3% air entrainment, and a low-range water-reducer for pumpability.

The pier stem was poured in 5-foot lifts. After concrete temperature spiked at 72 hours and then leveled off after 9 days, workers raised the pump intakes to flood the cofferdam, first to 1 foot above the footing, then every 3 feet up to 15. During this controlled cooling process, no thermal cracking was visible. Fifteen days after pier stem concrete placement, workers refilled the cofferdam so superstructure construction could begin.

""We decided that once we reached strength and the core temperature was dropping, we'd waive the differential spec,"" Nelson says. Cylinder testing on the footing revealed 28-day strengths of 4680 to 5090 psi, 28-day strengths of 4010 to 4630 for the stem, and 56-day strengths of 4770 to 5550 psi for the stem. Construction of Piers 2 and 3 would continue the following autumn and spring with similar strength and temperature differentials.

When it opened in summer 1998, the new bridge, noted for its curved twin traffic lanes and accompanying pedestrian paths, linked downtown St. Paul to the city's redeveloping riverfront. And the piers supporting the main structure are a product of careful study of the benefits of GGBF slag when used as a high percentage of the total cementitious material for concrete in massive structures.

That's good news to Nelson, who feels that MnDOT's calculated risk-taking has opened some new doors for similar future work.

The article also discusses how GGBF slag improves concrete.