{ "id": 2796, "date": "2024-12-27T18:41:33", "date_gmt": "2024-12-27T18:41:33", "guid": { "rendered": "https:\/\/prairieprod.vcnastaging.com\/?p=2796" }, "modified": "2024-12-27T18:43:30", "modified_gmt": "2024-12-27T18:43:30", "slug": "cool-solution", "status": "publish", "type": "post", "link": "https:\/\/www.prairie.com\/fr\/cool-solution\/", "title": { "rendered": "Cool solution" }, "content": { "rendered": "

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About 8 miles southwest of Chicago near Interstate 55, the\u00a0Metropolitan Water Reclamation District of Greater Chicago\u00a0<\/strong>is working on the latest phase of a massive project to expand its deep tunnel system. The ultimate goal: keeping up to 17 billion gallons of waste and storm water from flowing into Lake Michigan and nearby rivers when heavy storms hit.<\/p>\n

The agency is building a series of reservoirs to hold storm runoff, the largest of which lies in south suburban McCook. This summer, crews from\u00a0Kiewit Infrastructure\u00a0<\/strong>continue their work to connect the reservoir with the 109-mile tunnel network, focusing on construction of the main gate and shaft that will regulate flow between the network and the McCook reservoir.<\/p>\n

The gate assembly consists of 6 concrete shafts that hold 3 sets of metal-wheel gates, creating a system of 6 wet wells that serve as a holding tank between tunnel and reservoir, says Joe Ungari, Prairie Senior Account Manager working with Kiewit.<\/p>\n

This structure, 88 feet in diameter and extending 280 feet below grade, will be built over a 2-year schedule with a concrete liner placed in 3-foot wide, 8-foot by 10-foot lifts. A challenge in itself \u2013 but as plans moved forward, mix temperature became the main issue, Ungari explains.<\/p>\n

Specs demand a new approach<\/h2>\n

\u201cPreventing thermal cracking is critical with these mass pours,\u201d says Prairie Technical Services Director\u00a0\u00a0Steve Fleming<\/strong>. \u201cCurrent mass concrete specs dictate that the maximum heat of hydration cannot exceed 160 degrees but in this case, the Army Corps of Engineers went a step further, specifying a slag-rich mix to be delivered at less than 70 degrees.\u201d<\/p>\n

“This requirement couldn’t be met by chilling the mix with with cooled water or ice,” Fleming says. \u201cWe knew liquid nitrogen would be the only approach.”<\/p>\n

Fleming and his team called on experts at Canada Building Materials to learn more about LN cooling. Prairie and CBM are member companies of Votorantim Cement North America, or VCNA.<\/p>\n

\u201cWe\u2019ve been using LN technology for more than 25 years now,\u201d says\u00a0Phil Zacarias\u00a0<\/strong>, CBM\u2019s Quality Control Director. \u201cIt\u2019s the best solution for mass pours like this one where low heat of hydration is crucial.\u201d<\/p>\n

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Bringing LN to Yard 33<\/h2>\n

Five Prairie staffers from QC, operations and maintenance toured CBM batch plants, where workers use 7 permanent LN stations to chill concrete inside truck barrels. A motorized and electrically actuated lance is used to inject LN directly into the hopper as the barrel rotates at a specified speed. Cooling takes just 2 to 10 minutes, depending on target temperature.<\/p>\n

Prairie staff used CBM\u2019s platform design to direct the installation of 2 cooling stations at Yard 33, about 5 miles from the Kiewit job site. Yard workers underwent special training following CBM\u2019s dispensing and safety protocols.<\/p>\n

In June the stations went into operation. \u201cNow we\u2019re supporting 3 weekly pours of about 350 yards each, or 40 continuously dispatched loads of cooled concrete,\u201d says\u00a0Mike Urlacher\u00a0<\/strong>, Yard 33\u2019s quality control expert.<\/p>\n

Cooling without changing the mix<\/h2>\n

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Gearing up for this level of production involved careful testing. \u201cWe\u2019ve found that a 5-minute blast of LN cools the mix from 88 to 63 degrees,\u201d Urlacher says. Tests performed after cooling confirm zero change in slump or air entrainment. That\u2019s the big benefit with LN, says Fleming: \u201cIt\u2019s inert, so it cools the concrete but leaves all other mix characteristics unchanged.\u201d<\/p>\n

Cooling depends on the length of time the concrete is exposed to LN rather than the dosage used. Exposure time is determined by load size, starting concrete temperature, the target temperature and head pressure in the LN tank, since cooling slows as the tank empties and pressure drops. For this job, target mix temperature at the yard was set at 63 degrees to meet the placement spec of 70 degrees or cooler.<\/p>\n

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\u201cOn hot days it can take up to 9 minutes to cool each load,\u201d says Urlacher. Tests are run every 5 to 10 loads with more frequent monitoring in variable weather.<\/p>\n

Safe for workers and the environment<\/h2>\n

Our atmosphere is about 78% nitrogen, so the nitrogen and water vapor clouds that surround concrete trucks during LN dosing don\u2019t harm the environment. But since LN can freeze skin instantly and vapor displaces oxygen in the area surrounding the truck, safety procedures are key.<\/p>\n

\u201cThe main danger is being engulfed in the vapor and not being able to see,\u201d says CBM\u2019s Zacarias, \u201cso a worker who fell might be incapacitated by low oxygen levels at ground level. But we haven\u2019t had a safety issue with LN in the last 25 years.\u201d<\/p>\n

Moving technology forward<\/h2>\n

The deep tunnel project marks Prairie\u2019s first use of LN in the region and, based on its success, the technique will be available for future mass pours.<\/p>\n

\u201cWe\u2019re committed to applying new technologies in partnership with leading contractors like Kiewit,\u201d says\u00a0Jim Munro\u00a0<\/strong>, President and General Manager U.S. Ready Mix for VCNA. \u201cThis is an example of shared expertise that solves real challenges in the field.\u201d<\/p>\n

\"\"<\/p>", "protected": false }, "excerpt": { "rendered": "

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