A complex, quaternary high performance concrete (HPC) mix designed for Chicago's $200 million Revive Wacker Drive urban viaduct reconstruction project is providing concrete supplier Prairie Material Sales, Inc. both challenges and compliments. Through 2001, the company has been producing quaternary mix for Wacker Drive daytime column and footing placements and critical nighttime mass deck pours as well as flowable fill for utility ducts and new fiberoptic cable placements.

The challenges come from meeting the tight HPC specs day after day, night after night. The compliments come from folks like Chicago Department of Transportation Commissioner Miguel d'Escoto, who congratulated Prairie following the crucial first mass pour in May 2001. “It is a testimony to [your] quality control team that we were able to place over 1,100 cubic yards of a new and relatively untried high performance concrete mix without rejecting a single truck,” d'Escoto noted. “We especially appreciate Prairie's attention to detail and willingness to work with the city to provide the highest quality concrete possible.”

“We had [mix designer] Wiss Janney Elstner checking the concrete, the feds checking the concrete, and the contractor checking the concrete,” said Denise Casalino, P.E., project manager for City of Chicago. “There was not a single truckload rejected for that pour. Prairie stepped up to the plate and delivered.”

“Of seven bridge deck pours through summer 2001, we've only lost one truck load that could be attributed to a production problem,” adds Prairie's Gary Hall, certified QC technician. “These pours have ranged from 960 to 1,200 yards.”

Unique mix for special project

Working with the Illinois DOT and the Federal Highway Administration, the City of Chicago Department of Transportation fixed on HPC to give new Wacker Drive a long service life. “We're planning for a 100-year structure,” notes Stan Kaderbek, S.E., P.E., Chicago DOT deputy commissioner and Bureau of Bridges and Transit chief engineer. “And we're making sure it happens. The city was clear that we didn't need to disrupt downtown Chicago more than once in a hundred years.”

The actual cast-in-place, post-tensioned roadway slab has a relatively thin profile at 13 inches, and is heavily post-tensioned. Mockup tests of the raw materials and mixes culminated in a trial placement of a full-scale HPC mockup deck at a Chicago DOT maintenance facility. Moreover, the upper-and-lower roadway design had to have good crack control, and fit within the existing structure's envelope. Structural designer J. Muller came up with a plan that met requirements.

For the lead-off phase of the project — known as Contract A — some 17,000 yards of HPC are being produced. That quantity is part of a full project schedule that includes 200,000-plus yards of conventional, high performance and flowable fill mixes; 4,000 tons of reinforcing steel; and 1,100 tons of post-tensioning strand.

Durable concrete sought

In Chicago, snow and ice control is a priority for city government, and citizens expect promptly cleared pavements. Given Chicago's liberal use of de-icers, the new Wacker Drive achieves durability by resisting chloride damage in every way. The HPC deck, roadway and column mix is extremely impermeable to water. After placement, specifications call for the deck to be covered with a latex-modified concrete (LMC) driving course which serves as an impervious, sacrificial friction layer. “We figure that if 25 years from now we have to mill it off and place a new course, that will be the least of our problems,” Kaderbek says.

“A mix design was developed that included the use of a moderately low portland cement content, a moderately low water/cement ratio, and supplementary cementitious materials such as fly ash, ground granulated blast furnace slag, and silica fume at moderate levels,” reports Paul D. Krauss, P.E., project manager and senior consultant at Wiss Janney Elstner Associates, Inc., Northbrook, Ill. “The intent was to realize the benefits of these additives without adversely affecting workability, set time, salt scaling or cracking resistance.”

This included features such as:

• Strength of 6,000 psi in 28 days, with 4,200 psi at post-tensioning in three to four days
• Good workability and placeability
• Durability with long life, up to 100 years, in a high-salt exposure and freeze/thaw environment
• Controlled shrinkage and thermal contraction to minimize cracking
• Long-term resistance to attack from alkali-silica reactivity (ASR), alkali-aggregate reactivity (AAR) and delayed ettringite formation (DEF).

“It was a quaternary mix, as opposed to a tertiary mix,” Kaderbek states. “Most HPCs include cement plus substitution of blast furnace slag, silica fume or fly ash. But we've gone and included all three with the cement. The mix has worked well in terms of placement and pumpability.”

Class F fly ash was selected due to its ability to fight ASR. “We're convinced that Class F fly ash is the way to go, because it has a better overall capability of fighting reactivity,” Kaderbek explains.

Plant just minutes from site

Prairie Material's Yard 32 plant was selected to supply the HPC mix. Located near the intersection of Halsted St. and Chicago Ave. along the Chicago River, the plant produces large amounts of mix from a surprisingly small footprint of 5.5 acres in a rapidly gentrifying area of inner city Chicago.

The Cor-Ten steel-clad facility is 27 years old and consists of two, twin central mixed plants “sistered” to draw from the same cementitious products and six types of aggregate supplied from 17 1,600-ton storage silos, notes Keith Vopelak, yard manager. “Each silo is served by four lower discharge gates serving two conveyor belts. We can feed sand into one plant and stone into another simultaneously. Each plant can yield 250 to 300 yards per hour,” he says.

Rigorous prequalification

Potential concrete suppliers for the project underwent a rigorous prequalification process. “Following a prequalification meeting we had to submit samples of proposed raw materials to Wiss Janney Elstner for testing,” Gary Hall recounts.

“We also had to run four proposed mix designs based on information supplied by the mix designer, which involved extensive prequalification testing. We made cylinders, beams, shrinkage prisms, and slabs for chloride ponding. They ran hardened air content tests and a host of other prequalification tests,” he adds.

The four mixes varied in percentages and composition of cementitious materials and included binary and tertiary designs. After contractor and concrete supplier were selected, another meeting ensued. “At that point it was determined which mix would be used, based on prequalification of all the mixes,” Hall recalls. “It was a quaternary mix consisting of cement, slag, fly ash and microsilica. We had only done a quaternary mix when experimenting with trial batches one winter. We did it not really thinking it would go anywhere.”

Mix optimized for durability

The quaternary mix is unique for its application, Hall affirms, adding, “They're looking to attain the optimum mix for durability, and the proportions and composition of the mix complement one another. They're optimizing the benefits of each material in the mix in the right proportions to get a 75- to 100-year life cycle on this project.”

The mix is a 6,000 psi at 28 days design. “Typically they get well above 6,000, probably in the 7,000 psi range,” Hall explains. “There's also a maximum strength specified, because the designers want to avoid brittleness. With all those materials, and a low water/cement ratio, we expressed a concern that we could exceed the maximum strength set at 9,000 psi at 28 days. We've come under that, but when you have that many pozzolans working with a low water/cement ratio and a superplasticizer, it could be easy to go over 9,000.”

There also is a hardened air spec. “Every so often cylinders and beams will be cast and checked for hardened air,” Hall says. “We have to meet those specs as well.”

Production under microscope

Prairie found itself under the spotlight for the first mass pour in May and continues to produce HPC under a good degree of self-discipline. “The mix design requires constant vigilance on our part to meet expectations,” Hall notes. “There's virtually 100 percent inspection on the job site. Just about every load we ship out is tested for air and slump.”

“It's such a high profile project that we're always under the microscope to ensure service and quality,” Vopelak tells Concrete Products. “Since nothing like this has been done in this area in this volume, we're on the cutting edge and don't want to fall.”

Working under these conditions can be exhausting for the staff, adds colleague Keith Vopelak. “Anytime you have to produce 1,100 yards of a quaternary mix at a certain rate per hour can take its toll. Every load has to be perfect, or else it's gone. This has required the most effort to produce, and keep consistent, than any other mix I've shipped.”

Quality assurance is provided by CTE Engineers, Chicago, resident engineer for the first Wacker Drive contract, and construction engineer Alfred Benesch & Co., Chicago. The general contractor, Walsh Construction Co., is casting and breaking its own cylinders for quality control as the lab of record.

The mix needed an alteration in mixing procedure, Hall reports. “It required changes in sequencing and length of time to stabilize the mix,” he explains. “With so many ingredients, we have so many sources of variation in the mix, so we have to keep a tight control on everything we do.

“We went through growing pains,” Hall says. “HPC also is being used for columns, hammerheads and the piers they sit on. From the time we started producing HPC for those elements to the first mass deck pour, we had time to experience different problems with the mix.” These batches would be shipped 18 to 60 yards at a time, spaced further apart, compared to 1,161 yards for the first mass pour involving 240-ft.-long by 110-ft.-wide deck slab.

Control the air

“The control of entrained air is among the bigger issues with this project,” Hall reports. “All the different materials in the mix can cause variations, and there have been instances in which we have not let trucks leave the plant. I have to be vigilant in checking and adjusting the air. For deck pours I'll stay at the plant, and we'll have one QC technician at each pump with extra air and superplasticizer agents, just in case a load gets to the site outside the accepted parameters.”

Prior to the first deck pour Prairie was able to minimize air variations by preconditioning the mix with a small portion of superplasticizer, rather than adding it all at the site. “We even proposed the plant adding all the superplasticizer, but there were concerns about the mix if a delay or breakdown took place,” Hall attests.

It's an ongoing concern. Prairie could have materials that fall within a specification range, but vary widely within the range. “Certain conditions exist that if any one, or all, of the materials are very slightly off what the plant is used to, but still within spec, might cause variations in the air content,” Hall explains.

“It could be the microsilica, the slag, the fly ash,” he observes. “Even the configuration of the pump can affect point-of-placement air content. By the time we could track down the source of the variation, we could have gone through several loads of concrete.”

Continuing analysis of materials

“From a materials standpoint, every day HPC mix is produced we have to run gradations, moistures and absorptions on the aggregate to ensure that the material's in spec,” Hall affirms. “We have to do this all the time and provide the contractor with the documentation. We're also required to ship out a ‘water sheet’ with the first load for any HPC, which is a breakdown of moisture in the aggregates, the admixture, and the total water content, so they have verification that the water/cement is being observed.”

Hall takes aggregate samples to run gradations and moisture contents, but uses electronic probes for sand. “They're located above the weight scales and are of microwave design so we get instantaneous readings,” he notes. “Whenever I obtain sand samples I get them as close to the sand discharge gate as I can, to confirm that the probes are functioning properly within allotted tolerances.”

These probes take 20 readings each second, and based on those readings, water content is automatically adjusted. “We don't have probes for coarse aggregate because we haven't found one that can survive the daily pounding,” Hall notes. “So I'll obtain a sample of stone and obtain moisture as part of my daily gradations.”

Ready for first mass pour

The days leading up to the first mass pour (1,161 yards in May 2001) were busy ones. “There was excitement, because it is a high profile job,” Hall recalls. “We had the closure of a major artery [State St.] and a lot of eyes were on us.”

Supervisors went to a pre-pour meeting and obtained maps and instructions on where testing stations were located and pumps stationed. “Not too much was left to chance in terms of routing the trucks for each pour, and that's nice,” Vopelak states.

The entire Yard 32 facility was dedicated to the mass pour. Once all materials were ready they were discharged to the mixer for a 75-second tumble. To go from weighing to discharge to truck took about five minutes per mixing plant. Some 22 trucks were scheduled for the first pour, which provided 11 trucks for each pump. “Once we got the first run out, everything fell into place,” Vopelak says. One of the reasons for the smooth pour is that everybody here — the dispatchers, plant operators, QC, the drivers — works well with each other.” It's a good crew of people all around, he adds, with good communications.

“Not to say that we have not been conscientious up to this project, but a job like this just makes all of us better,” Hall notes. “It requires constant vigilance on the part of everyone involved. We're trying to achieve a long life cycle for this structure. And based on the project's success, we may be seeing the beginning of a trend for this type of mix.”

Both are quick to point out that the project would not have been a success without the hard work of Prairie's Chicago area quality control manager Al Cygan, who died two weeks before the first pour. “He's the one who attended the meetings and met with all parties,” Hall relates. “I like to think he was watching over us on the first pour.”

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Tom Kuennen is Principal of Expressways Publishing, Wheeling, Ill., specializing in coverage of public works construction.