Construction of Clinker Silos

Holcim (US) Inc., located in St. Genevieve County, Missouri, has one of the largest cement plants in the United States. 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 were 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 construction since it is a common method 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.

STRUCTURAL TECHNOLOGIES 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, and full-time observation of the duct installation during slip forming and technicians on-site during the strand installation, stressing, and grouting operations.

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. The pilasters provided 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 were recorded for every tendon. Once elongations were approved, the tendon tails were cut and permanent grout caps were installed to prepare for injection grouting of the tendon.

One of the greatest challenges of this project was quality control during the slip. On the four-pack silos, all four silos were slipped at one time. More than 100 crewmembers per shift worked 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 planning the site logistics. All of the cast-in materials for each silo had to be onsite and accounted for before the beginning of the slip because once the slip form process had started, it could not stop until the silo is complete.