BALTIMORE, MD - (Oct. 30, 2007) - VStructural, LLC has been selected to furnish a bonded post-tensioning system for the construction of ten silos being built at a new production plant for Holcim Cement.

Once completed, the new plant, located in St. Genevieve County, Mo., is expected to be the largest cement plant in the United States. Measuring 150-feet in diameter and 207-feet tall, the two clinker silos will be the largest in the world. These clinker silos and two cement "four-pack" silos consisting of four silos each in a four-pack pattern, each 79-feet in diameter and 275-feet tall, will support the new plant, expected to go online in January 2008.

After the project was awarded to a joint venture team comprised of MC Industrial and T.E. Ibberson Co., VSL was selected to provide the post-tensioning systems, installation support, stressing and grouting equipment and technical assistance. The technical assistance provided by VSL includes preplanning, full-time observation of the tendon installation during slip forming and having technicians on-site during the subsequent post-tensioning placement.

The system chosen for this project was the ECI 6-19. In total, each clinker silo involves 769,737-feet of strand; 42,441-feet of duct and 344 anchorages. Each four-pack silo utilizes 2,074,248-feet of strand; 107,316-feet of duct and 1,584 anchorages.

According to Justin Anderson, VSL Project Manager, there are several advantages of post-tensioned silos over conventionally reinforced silos:

• Thinner wall sections with greatly reduced quantities of mild reinforcement speed the labor-intensive slip form process and improve placement precision.
• Post-tensioning, being the primary circumferential reinforcement is placed off the critical path of the slip.
• 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 as it can provide residual compression in the concrete under all loading conditions, increasing durability. Additionally, in the event of an unforeseen overload, the post-tensioning 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.

Anderson stated that one of the greatest challenges of this project is the quality control during the slip. For example, on the four-pack silos, there will be four silos slipped at one time with more than 100 workers per shift working around the clock to install seven tons of rebar per hour as well as post-tensioning ducts and anchorages, set-backs and embeds. Another challenge is sequencing. All of the cast-in materials for each silo must be onsite and accounted for before the beginning of the slip because it is very undesirable to stop and restart the process once it begins.