In March of 2006, Holcim (US) Inc. began construction of what will eventually be the largest cement plant in the United States. Located in St. Genevieve County, Mo., the plant includes two clinker silos and two cement "four-pack" silos consisting of four silos each in a four-pack pattern. Measuring 151 feet in diameter and 207 feet tall, the two clinker silos will be the largest in the world at the time of construction. Each of the silos in the "four-packs" measure 79 feet in diameter and 275 feet tall.
The slip form method was chosen for constructing all silos. Slip form construction is common for tall structures such as silos and towers. The process involves a set of forms that are continuously moving, or slipping, upward at a rate of approximately one foot per hour by means of jack rods and hydraulic jacks. As the forms slip, rebar, post-tensioning ducts, post-tensioning bearing plates and concrete are continuously placed between the forms until the structure is completed.
After the construction for the overall project was awarded to a joint venture team, the post-tensioning contractor was selected to design the tendon layouts, provide systems, installation support, pushing, stressing and grouting equipment, and technical assistance. The technical assistance included preplanning, field staff training, full-time observation of the duct installation during slip forming and technicians on-site during the strand installation, stressing and grouting operations.
In total, the ten post-tensioned silos contain 3,900 anchorages; 300,000 feet of galvanized duct; 15,500 cubic feet of grout; and 5.7 million feet or 2,110 tons of strand.
The use of post-tensioning on this project proved very successful. There were several advantages of using this method of construction over conventionally reinforced systems:
Thinner wall sections with greatly reduced quantities of mild reinforcement improve the labor-intensive slip form process and placement precision.
Post- tensioning strand, being the main circumferential reinforcement, is primarily installed, stressed and grouted off the critical path of the slip because the strand is installed after the slip form process.
Post-tensioning steel is approximately four times the strength of the mild reinforcement resulting in a total reduced steel area and congestion.
Superior crack control is achieved with post-tensioning because it provides residual compression in the concrete under all loading conditions, increasing durability. Additionally, in the event of an unforeseen overload, the post-tensioning, which is active reinforcement, has the ability to close cracks.
The residual compression provided by the post-tensioning is an efficient means of improving the shear capacity of the concrete.
Each silo is constructed with pilasters or buttresses the full height of the silo where the post-tensioned bearing plates are cast-in during the slip form process. These pilasters provide access to the ends of the ducts from the outside of the silo for installing strand and performing the stressing and grouting operation from swing stages. Stressing forces and strand elongations are recorded for every tendon. Once elongations are approved, the tendon tails are cut and permanent grout caps are installed to prepare for injection grouting of the tendon.
One of the greatest challenges of this project is the quality control during the slip. For example, on the four-pack silos, all four silos were slipped at one time with more than 100 workers per shift working around the clock to install up to seven tons of rebar per hour as well as post-tensioning ducts and anchorages, concrete set-backs and embeds. Another challenge was logistics. All of the cast-in materials for each silo had to be onsite and carefully accounted for before the beginning of the slip because once the slip form process starts, it cannot stop until the silo is complete.
Work on all silos will be successfully completed in the spring of 2008 and overall project completion is expected in 2009. Once on-line, the plant will generate over four million tons of cement annually.