Preheater tower after completion of strengthening showing end anchorages.
Precision coring rig.
View of preheater tower from the northeast, with scaffolding.
Anchorage reinforcement.

Preheater Tower Structural Strengthening

Project Description 

The Holcim Portland Precalciner Cement Plant near Florence, Colorado is one of the largest in the United States. It has an annual production capacity of 1.9 million tons of cement and its preheater tower, a 328-ft-tall, reinforced concrete frame structure, houses equipment that reaches 390 feet in height. This equipment thermally processes raw material prior to being fed into a dry-process cement kiln… making the tower vital to plant operations. About two weeks after the kiln’s initial start-up, cracks were noticed in the concrete columns at three levels. The cracks coincided with the terminations of flexural reinforcement in the concrete beams. Plant management stopped kiln operations to assess the tower’s condition.

After stopping operations, the key goal for Holcim was to restore its production to full capacity as quickly as possible by bringing the tower back on line. The solution - create a fast track plan that would ensure a stronger, more durable tower.

Restoration consultants were engaged to assist locally with engineering and construction administration. STRUCTURAL was engaged to review the constructability of several alternate repair schemes and maintain the fast-track schedule. After considering structural capacity and serviceability requirements, durability issues, the high-temperature operating environment, constructability, and an aggressive construction schedule, the team recommended a retrofit consisting of bonded post-tensioning within internal holes drilled in the beams.

This solution was quite extraordinary, as it required precision-drilling horizontal holes up to 87 feet long in the beams of the elevated frame structure, without cutting existing embedded reinforcement. Nondestructive impulse radar testing was used to locate existing embedded reinforcing steel, as well as to monitor the trajectory of the drilled hole. This process helped ensure proper tendon alignment and prevent damage to embedded steel.

The structural strengthening repairs were executed quickly and under challenging circumstances, including working high on the exposed structure through a cold winter with severe wind conditions. The unique retrofit resulted in a structure that is stronger, more serviceable, and more durable than the original tower – ultimately restoring full production and revenue capacity to the Holcim facility.