Bridgeview, IL (PRWEB) November 30, 2005
The Big Pour, as everyone referred to it, took 22 hours and placed 4,600 yards of self-consolidating concrete in a 198 ft x 60 ft rectangle that will support the 92 story tower at the center of the development. This is the single largest SCC pour to date in North America. Thirty trucks and two shifts of drivers worked continuously to deliver over 500 loads of concrete from Prairie’s Yard 32 on Chicago Avenue. This unique ready mix yard has dual central mix plants on site. Three conveyors transferred a truckload every 2 ½ minutes into the reinforced opening.
Unlike other Chicago landmarks Sears Tower, Aon Center and Hancock Building, which are all steel-reinforced structures, the Trump Tower is using reinforced concrete as its structural skeleton so it will fit into the limited space available. Without the concrete mat, say architects, the structure could never climb as high and still stay as thin. The 92 story structure will be the tallest concrete reinforced building in the world when it is completed in 2009.
"This mat is the heart of the building," said McHugh Construction's Dale Hendrix, a 45-year senior vice president -- and veteran in concrete -- who coordinated the pour. "This was a really unique challenge, something you're going to look back on some day and be proud of."
The site sits adjacent to the Chicago River just north of Chicago’s Loop and is one of the more congested areas of the city. The route through the city was five miles round trip and drivers made more than 500 trips back and forth. Prairie and McHugh planned the route carefully to avoid nearby restaurants and clubs. “We worked closely with McHugh to plan the traffic patterns on the jobsite and determine the best way for our trucks to leave the site and return for another load,” said Paul Blatner, Prairie’s vice-president of operations. "It would've been closer, and probably easier to maneuver, if they'd used Rush Street instead of Lower Michigan," added Hendrix, "but that would've taken them within a few feet of an outdoor restaurant while everyone was eating.” And the pour itself turned out to be part of the Friday night scene. “We took pride in seeing the Friday night crowd watching the pour from the various bridge structures around the site. We put on quite a show,” remarked Blatner.
The specifications for the concrete were equally challenging. One of the major problems with mat pours is that the heat of hydration of cementitious materials creates temperatures in the center of a slab of this thickness that are much higher than at its surface. This temperature differential can cause thermal cracking that would compromise the slab’s structural strength. To address this problem, the specifications set a maximum temperature of 80 degrees (F) during the placement of the concrete and a maximum temperature of the concrete in place at 170 (F) degrees. Standard concrete mixes would reach temperatures of around 200 degrees at the center.
The specification also called for concrete with strength of 10,000 psi at 56 days to support the building's 360,000-ton bulk. The combination of requirements made for a challenging mix design that has never been done before.
And Prairie nailed it.
Using a Self-Consolidating Concrete mix design that featured super plasticizers, low water and special cement, the concrete temperature never rose above 77 degrees during placement and the concrete in place reached a maximum of 155 degrees. The compressive strength reached 9,950 psi at 7 days and over 12,000 psi at 28 days - well within the specifications. "We've had projects where we've taken more concrete to one location, but never with a specification like this," said Gerry Krozel, a Prairie vice president. "This [pour] involved things that have never been done in this business."
The building will require 180,000 cubic yards of concrete and 25,000 tons of reinforced steel by the time it’s finished and will take $130 million of the estimated $600 million construction budget. But at no point, will anything be as complex as the weekend's big pour.
“This pour, as difficult as it was, is representative of how Prairie does business,” stated Blatner. “Our dedication to our customers is evident in everything we do. This was a true collaborative effort between our company and the people at McHugh Construction.”
Architect: Skidmore, Owings and Merrill
General Contractor: Bovis Lend Lease
Concrete Contractor: McHugh Construction
Ready Mix Supplier: Prairie Materials Sales, Inc.