Pumps Handle "Special" Concrete Mix to Put Chicago Loop Back in Shape February 2002

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A fast-paced $200 million road renovation is underway in the heart of Chicago, and Putzmeister equipment is taking an active part by pumping a "special" high-performance concrete mix designed to keep Wacker Drive from crumbling for the next generation.

The two-mile-long double-deck viaduct section being rebuilt carries about 60,000 vehicles a day. Eight bascule bridges and two elevated transit lines intersect with the upper roadway — along with 60,000 pedestrians in the Loop. The original road was falling apart from freeze-thaw cycles, road salts and wear. Despite a patch-up in the 1970s, the deck continued to show failures.

Walsh Construction of Chicago was awarded all three contracts on the major Wacker Drive reconstruction, which was divided into sections of $75 million, $43 million and $55 million — all scheduled for completion within two short years.

To avoid another expensive and extensive undertaking for the next 75 years, the city project is using the first major high-performance concrete (HPC) mix in the state — tailor-made for Chicago's harsh environment. It was imperative that the mix reach 6000-psi in 28 days while attaining a maximum compressive strength of 9500-psi. Other requirements included air voids with maximum spacings, a moderately low water/cement ratio and consistency of aggregates, materials and admixtures over time.

Final mix is perfected After months of thorough research and testing among several involved parties, the final mix was perfected to include portland cement, 10 percent Class F fly ash, 5 percent silica fume and 15 percent ground-granulated blast furnace slag. Class F fly ash helps tie up alkalis in the cement that could cause future problems and increase impermeability. The HPC mix costs about twice as much as regular concrete, but it is hoped that life-cycle cost savings will more than make up for it.

Of course, the criteria also involved a highly durable mix that could be "placed" on the upper Wacker decks without a lot of problems. As a result, Walsh called upon the use of two powerful Putzmeister 52Z-Meter concrete pumps from Original Concrete Pumping in Bensenville, Ill. And these pumps handled the mix without fail. Utilizing a rugged S-valve to gulp down the tough mix and high performance .16H pump cells to push it out the end hose, the pumps were capable of handling both high outputs to 210 yards an hour and high pressures up to 1233 psi (85 bars).

As a result, they successfully pumped eight major upper deck pours, consisting of 1200 yards each for the first contract. The pours consisted of a 6- to 8-inch slump, and the unique mix was continually monitored by a host of on-site quality control personnel.

Ken Warrick, Walsh's general superintendent said, "The Putzmeister pumps did a good job handling the tough mix, and Original was very responsive in helping with job-site coordination and ensuring on-site service support when each pour started. We certainly plan to use them for the pumping involved with the two remaining contracts."

Setup coordination a challenge The biggest challenge for the pumps wasn't the mix, but the setup coordination involved during the 2065-foot -long middle section of the deck during the first phase.

Ron Kohn, operations manager of Original said, "Logistical meetings were held onsite a few weeks in advance to outline possible pump setup and movement patterns among the congestion. These scaled out plans were critical to maintaining an uninterrupted flow of concrete."

Due to the special mix design, only 90 minutes was allowed from the Bridgeview, Ill., Prairie Group's batch plant to onsite discharge before the concrete would be rejected. Combined with up to forty-minute haul times and slower than usual finishing rates, this allowed only a short wait time. Therefore, exact timing was required so that one 52Z-Meter unit would always be pumping when it was necessary to move the other. This avoided backups from the 25 mixer trucks continually being cycled throughout each pour.

A somewhat slower than expected 100-yard-per-hour pace was determined not by the pumps, but rather by the finishing crew. They had the labor-intensive task of constantly adjusting for crown sections and working around big sidewalks and medium strips in the center of the decks.

Only 14 feet of clearance under L tracks Even though the larger 52Z-Meter pumps provided a longer reach to better handle the 140-foot-wide by 220-foot-long deck sections, approximately 65 feet of slick line was still needed on selected pours. This was especially the case when placing concrete under the L tracks where there was only 14 feet of clearance. Working from the farthest point toward the pump, line was then gradually removed to make the process less demanding on the crew.

To avoid closing access to businesses along Wacker Drive, prevent traffic congestion and maintain passage to weekend summer events, pumping normally occurred on a Friday night when streets could be blocked off. The pours would start at 9 p.m. and finish around 7 a.m. the following morning, utilizing a large crew of 40 to 50 personnel to ensure adherence to this tight schedule.

Walsh had a backup pump onsite along with 24-hour access to Original's support personnel. Fortunately, due to the reliable performance of the Putzmeister equipment, these backup measures were never needed.

The "A" part of the total contract required an unusually fast-paced schedule and 11,000 yards of concrete to meet a final completion date of November 2002. Ninety percent of the pumping is already done, with Original having handled 80 percent of it, and Walsh using their smaller-sized fleet pumps as a backup for minor pours. Work has already begun on the two remaining B and C contracts, which will consume 10,000 and 11,000 yards of concrete respectively, and be completed in spring 2003.

The city is planning further post-Wacker Drive development of the riverfront along with reconfiguring the interchange to include Interstate 290. They are also studying the idea of a row of shops and a floating river walk on the lower roadway. Ideally, construction would begin in 2003.

JOB SPECS: General contractor: Walsh Construction - Chicago Pumping contractor: Original Concrete Pumping - Bensenville, Ill. Ready-mix supplier: Prairie Group - Bridgeview, Ill. Equipment: Putzmeister 52Z-Meter truck-mounted concrete boom pumps (2)

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