After 56 years, the coal-fired power plant was worn out and obsolete. Corn Products International, Westchester, Ill., built the boilers and generators in late 1948 to provide steam and electricity for its manufacturing plant in Bedford Park, Ill. To replace this resource, Corn Products is now building a state-of-the-art power plant that will burn high-sulfur Illinois coal and meet all current emissions requirements. The State of Illinois provided funding for this purpose. The new plant will cost $97 million and be completed in 2006. Guy Buchner, the engineering manager for Corn Products, says the new facility will include a “1.1-million-pound-per-hour, 100-megawatt, circulating fluidized bed boiler.” A byproduct of the boiler will be high-grade fly ash, which Corn Products plans to sell—hopefully to the concrete industry.
Corn Products decided to serve as its own general contractor, signing the contracts for each subcontractor involved in the project. It retained ESI of Tennessee in Kennesaw, Ga., to engineer the project and to serve as the construction and start-up manager. Besides Lindblad Construction's ability to perform the concrete work, Corn Products wanted its expertise at setting anchor bolts with high tolerance requirements and the company's commitment to job-site safety. Lindblad's experience modification rate (EMR) of 0.73 and its previous year OSHA Recordable Frequency Rate of 0.0 was very important to Corn Products.
The boiler for the project will weigh more than 6 million pounds, and most of the weight will be suspended from the top steel so that its thermal expansion won't affect the slab. The total dead weight that will rest on this slab will be over 12 million pounds. To support this, engineers designed a 4½-foot-thick reinforced mat slab. Because of the complexity of the mat geometry and loadings, Corn products asked Ambitech Engineering Corporation of Downers Grove, Ill., to verify the design using PCAMats and STAAD software. Mark Stadalsky, Lindblad's vice president, notes that ironworkers placed 805,000 pounds of #10 steel reinforcing bars, generally spaced at 10 inches, but with the more highly stressed areas at 5-inch centers. The slab covered 22,800 square feet and required 3819 cubic yards of concrete coming from three different yards. It took 79 ready-mix trucks to keep up with the schedule.
ACI 116 defines mass concrete as “any large volume of cast-in-place concrete with dimensions large enough to require that measures be taken to cope with the generation of heat and attendant volume change to minimize cracking.” The heat generated by the hydration of cementitious materials causes temperatures in the center of a slab to be much higher than at its surfaces. This temperature gradient is the critical thing to remember about mass concrete since it can cause thermal cracking that would compromise the structural strength of the slab.
For practical purposes, according to PCA's Design and Control of Concrete Mixtures, (Kosmatka, Kerkhoff, and Panarese, Portland Cement Association, 2002) slabs and footings more than 3 feet thick qualify as mass concrete. Thermal cracking in mat slabs can be managed by the type and amount of cement used, by adding pozzolans, by increasing the amount of reinforcing steel, by reducing the delivered temperature of the ready-mixed concrete, by using a concrete with a low thermal expansion aggregate (such as limestone or granite), and by increasing the size of the aggregate. On the jobsite, covering a placement with insulated curing blankets reduces the rate of cooling at the concrete surface, helping to keep the temperature differential within acceptable limits. So, even with no danger of freezing, insulating blankets will often be used on a thick mat slab.
Given the 4½-foot thickness of the slab for this project, a mass concrete mix design with a low heat of hydration was needed in order to avoid thermal cracking. John Gajda, a principal engineer for Construction Technology Laboratories (CTL), Skokie, Ill., says that concrete temperatures for mass concrete placements shouldn't exceed 160° F. Additionally, based on a simple rule of thumb, the temperature differential between the center of a placement and the exterior surfaces shouldn't exceed 35° F to 56° F, depending on the aggregate used in the mix. In the development of the concrete mix design for this project, Gary Hall, who does quality control for Prairie Materials, Bridgeview, Ill., says it designed the mix not to exceed 130° F.
The mix used contained the following ingredients:
Anchor bolt placement
Lindblad Construction is known for its attention to detail and accurate placement of anchor bolts. The owner considered accurate anchor bolt placement to be a vital part of the contract. Before installing the reinforcement, workers marked the location of all the anchor bolts on the subgrade. Next, they cast “mud mats” over those spots and under the angle support locations (see photo, p. 33). Lindblad's two on-site superintendents then verified the precise location for each anchor bolt assembly on the mud mats.
Each anchor location included four anchor bolts mounted in a steel frame. Workers mounted the frames to the mud mats with concrete anchors. The angle supports were then attached to hold each assembly securely in place. Ironworkers created a clear space between the anchor assemblies and the reinforcement to insure that there would be no movement during concrete placement. A total of 264 bolts were set in this manner. After the anchor bolts were set and secured, surveyors checked each anchor bolt and verified that all of them were in the proper location.
Forming trench drains
To anchor trench drain forms, Lindblad used a procedure similar to that used to position the anchor bolts. Workers again placed concrete mud beds on the subgrade under the drains and attached angle iron to them with concrete anchors. Angled supports were also used. As shown in the photo, vertical form panels secured to the angle iron formed the walls of the drain, but the drain floors were left open. Stadalsky said that they were concerned that the buoyant forces of the concrete against the bottom forms would float the drains out of position. As a result, they planned to bush-hammer some concrete and place an overlay cement topping to provide pitch to the drains in each trench.