A major mass pour for a bridge pier spread footing is the largest ground, granulated blast furnace (GGBF) slag cement application in the Midwest so far.

In May, Aucem GGBF slag cement -in a 70 percent substitution for Type II portland cement - was being used to lower the heat of hydration in a 2,600-yd. pour for a pier footing for the Phase 1 extension of Page Avenue (S.R. D) in St. Louis County, Mo. And this pour was only part of a larger application. "This is the biggest slag cement project in the Midwest," says Mark Langley, technical service manager, Aucem slag cement, Lone Star Industries, Inc., Chesterfield, Mo. "We have projected that over 100,000 yards of this mix - representing about 10,000 tons of slag cement - will go on this job over a year and a half."

The $325 million Phase 1 involves the construction of a multilane expressway from west of I-270 in St. Louis County, across the Missouri River to S.R. 94 in St. Charles County. Going west, the extension will descend the bluffs above the Missouri, cross river wetlands and bottomland, and span the Missouri with a new bridge. The total project will approach $600 million.

In addition to the use of GGBF slag and extensive wetlands mitigation, the project represents the first use of post-tensioned, cast-in-place segmental concrete box girders by the Missouri Department of Transportation (MoDOT). That's significant because rivers and river crossings are abundant in Missouri, and this design represents a noteworthy market penetration.

Controlling heat of hydration Control of heat of hydration was the prime purpose for GGBF slag cement being used. The mass pour concrete mix design calls for 50 calories per gram heat of hydration.

Breckenridge Material Company, the ready-mix producer, was trying to achieve crack-free concrete, notes Langley. "Breckenridge devised a 70 percent slag cement/30 percent Type II low-heat cement mix that did meet the criteria, and we've been pouring since August," he says. For this project, MoDOT has a crack specification of 0.03 mm.

These pours are averaging 4,100 psi in 28 days, and 4,500 in 56 days. In addition to the 70/30 slag/Type II cement content, the mix design includes MoDOT-approved river sand and No. 67 (31/44-inch) MoDOT gradation D limestone, plus a Grace air entrainment admixture. The mix contains 6.72-sack/yd. total cementitious material. Four additional mass pours of this size lie ahead.

"We're placing 240 yards per hour," says Russ Wolf, quality control manager for Breckenridge. "It's been a great experience. The mix has elements that haven't been done down here, such as the high dosage rate of slag cement. We like to be out on the edge, trying the new things that everyone else is afraid of." This placement began at 4 a.m. and we're expecting a 12-hour schedule. Each truck would be making 10 to 12 trips.

The same mix design as the mass footings is being used for the columns atop the footings, with the exception that a high-range water reducer is being added to enable placement of a concrete with a 5- to 6-inch slump around the steel, notes Breckenridge's Nathan McKean, director of sales.

The 24-hour strength requirement for the mass footings is just opposite of that needed for the cast-in-place concrete box girders of the viaduct's superstructure, which is not a Breckenridge job.

"When designing the mix for the footings, our challenge was 'how low can we go'," McKean says. "On the superstructure of the segmental bridge, it was the exac t opposite, requiring 4,000 psi in 18-24 hours. But in order to achieve the low heat specification in the footing mix design, we had to slow down the set time, down to 250 to 500 psi in 24 hours."

Slag cement is approved by MoDOT for up to 25 percent replacement of portland cement. "They did have to make a special provision to allow 70 percent slag cement," Langley said. "Lone Star does make a Type II low-heat cement that is good for mass concrete, but has a higher heat of hydration. And Type IV cement is a rarity in this area."

Lone Star is tracking Aucem performance on this project, generating data using heat couplers and establishing an on-site, semi-matched curing system demonstrating strength development mimicking in-situ strengths better than conventionally cured test cylinders, and enabling crews to work more efficiently.

"In the initial stages of the project we had some questionable early strengths pertaining to form removal," Langley says. "We set up a heated water bath, made cylinders, and put a thermocouple in a test cylinder. We then would take readings on the site, take a reading on the thermocouple in the cylinder in the water bath, and adjust the water bath temperature to create a semi-matched curing situation for the test samples. We realized a much higher - over a thousand psi - five-day strength gain compared with normally cured test cylinders. This enabled the contractor to pull its forms anywhere from three to five days earlier."

Selling the performance spec This project heralds another first: The first concrete mix performance spec for a Missouri bridge footing. "The spec was written as a performance spec, not a cookie-cutter recipe," McKean says. "We wanted to enable the contractor to meet or exceed the specification of the heat spec, which specified 160 degrees F maximum internal temperature."

"That's extremely hard to do conventionally, without going to extremes such as insulated forms and liquid nitrogen," McKean explains. "With this mix design we can actually take the mix up to 85 degrees F or 90 degrees F in-place and still achieve this 160 degrees F internal heat spec. It gave the contractor a cost-effective way of achieving the specification."

The mix design originated with Breckenridge, he says. "We started research 10 months before the project came up to bid," McKean adds. "We worked with vendors, testing many, many mix designs and blends of Type I and II cements, with different percentage replacements with fly ash and slag cement. We kept testing until we found the right heat gain that would enable us to take concrete temperatures up to 85, 90 degrees and still perform within the spec."

Getting the mix approved by the state took some jawboning. Prime contractor for this $73.3 million bridge project is Walter Construction, which selected Fred Weber Inc. as substructure contractor. "Breckenridge has had a good working relationship with Fred Weber," McKean says. "Our company has been in business for over 70 years, and we've had a good relationship with MoDOT.

"So we took all of our months of research and asked for a meeting to get approval of the mix to achieve the performance spec. This was a dramatic change from everyday specifications for concrete, and after three or four meetings we got approval. We had enough backup data that they realized it was the best way to go. We're about 50 percent through the project and have met every spec."

"It was left up to the contractor to provide a mix that would work, and this seemed to do the trick," notes MoDOT's Doug Franks, construction inspector and project manager. "Because of the extreme mass of the footings, we would be generating an extreme amount of heat. The contract calls for that heat to be controlled within a maximum core temperature, and maximum differential temperature that can be allowed.

"It was a stretch for MoDOT to accept this mix," Franks concedes. "We've done quite a bit of testing since we've started using it to see if it's something we want to use on other projects. The jury's still out, but it seems to be working well on this project. It's kept our mass pours as crack-free as can be expected with a placement of this size."

Slag cement makes inroads While this is MoDOT's first use of slag cement in this proportion, slag cement has surfaced in other Missouri markets. "Introduced to this market four years ago, Aucem has been used on a variety of residential and street paving projects," Mark Langley affirms. In St. Paul, Minn., 70 percent slag cement was used in the Wabasha Bridge project over the Mississippi. "And it has been used in higher percentages in Europe for years," Langley adds.

Lone Star's Aucem is a Grade 120 slag cement, an ASTM 989 designation. "We are grinding and producing it in New Orleans and barging it to terminals serving our markets," Langley says. "It's a processed, tightly controlled, granulated coproduct of the steel mills, ground to achieve the correct particle size distribution. We travel all over the world, checking the raw product's chemistry and hardness."

In addition to higher strengths, other benefits of the Grade 120 GGBF slag cement are sulfate resistance, alkali-silica reactivity (ASR) suppression, reduced permeability to chloride ions, almost to microsilica levels, Langley notes. "You're turning out a higher quality product using a recycled material," he adds.

One Aucem customer - a precast statuary producer - is sold on slag cement. "They cast very elaborate statues for a lot of the churches," notes Michael Baker, Aucem sales specialist. "They have commented on how slag cement closes pores and makes painting and finishing much easier than conventional mixes."

But slag cement's value-added features stand on their own, Baker adds. "We're anticipating that over the next 10 to 15 years in markets where it is available, slag cement will be used in 15 percent of the concrete."

But that doesn't mean everywhere. "Slag cement has been widely available in the Northeast, where the steel mills are," Langley says, "and it's followed the transportation systems, such as in Missouri, where the Mississippi runs. It is prevalent all through the Eastern Seaboard, it has taken over Florida, is being introduced with success in Texas. State approvals keep coming in, projects like this keep being spec'ed, and percentages keep going up."

Slag challenges producers GGBF slag cement also makes demands on the ready mix producer. "He has to have a separate silo, and a computer capable of handling three binders in his batching sequence," Langley says. "Most computer systems now are geared toward two powders for the cement and ash mixes." In a ternary application, he adds, the producer must have a three-silo configuration as well as a batching system able to weigh out and batch three binders into the weigh hopper. Furthermore, he has to rework his mixes knowing the different properties of those mixes, such as setting times, finishing characteristics, and admixture compatibility.

"It can be a lot of work for a producer," Langley notes. "We provide assistance - that's my job. If a ready mix producer is serious about going with slag cement, I provide the technical backup to deliver mixes, and offer field advice."

"It's not just a five- and six-sack mix anymore," he adds. "Our industry is being asked to turn out higher strengths, meet new permeability requirements, and it all requires producers to learn what these new products can do."

One such producer is Breckenridge. "They added two silos at two different locations for this project," Langley says. "They have other locations where they are going with three and four silo configurations, able to carry different cements to adjust to different job requirements and specifications. They're the first in this market to go this route."

Breckenridge has plans to market slag cement mixes to other customers as well. "We certainly think there's an opportunity in the heavy/highway market, as well as the commercial market," Nathan McKean says. "We've just introduced it to a parking garage in downtown St. Louis. It's an opportunity for us as a concrete producer to offer value-added, value-engineered mix designs to enable contractors to more cost-effectively achieve specifications."

Recently Breckenridge upgraded its nearby plants to be able to serve the Page Avenue Extension, as well as the coming billion-dollar airfield expansion of Lambert-St. Louis International Airport.

"We expanded this [Maryland Heights, Mo.] plant in 1996," McKean said. A Ross transit mix plant had been on this location since 1986, and in 1996 a central mix plant was constructed there. "The total facility will produce in excess of 400 yd. per hour and handle up to seven different cementitious products and seven different aggregates. We also have the capacity to chill 70,000 gallons of water."

GGBF slag cement is not a new product, having been used for years in private sector construction, such as the high-profile Rock & Roll Hall of Fame in Cleveland, but still is undergoing the acceptance process among state departments of transportation (see "Waste" works wonders, Concrete Products, June 1996, page 50).

However, in 1996, use of GGBF slag cement in highway projects was encouraged by the Environmental Protection Agency's classification of it as a "recovered" product under the Resource Conservation and Recovery Act. That move put it on the same level as fly ash as a target product for federally funded construction jobs of $10,000+.

More than just broken-up, cooled slag used as aggregate, GGBF slag cement is a value-added, manufactured "glassy, granular material formed when molten blast-furnace slag is rapidly chilled as by immersion in water," to quote ASTM.

The benefit of the rapid, controlled cooling is that the product becomes amorphous in structure, that state being more reactive than the crystalline structure resulting from slow cooling. GGBF slag cement has attributes such as a low heat of hydration, increased compressive and flexural strengths, inhibition of alkali-silica reactivity (ASR), resistance to sulfate attack, and reduced permeability to protect rebar from chloride penetration.