Six silos, a cluster of four and a cluster of two, constructed of reinforced concrete and standing 150 feet high and 23 feet in diameter were being utilized as load-out cement silos. An initial inspection revealed radial and circumferential cracking that could have been structural in nature. Subsequent analysis of the silos determined that cracks were caused by a 30% lack of required vertical and circumferential steel. To avoid a structural failure, the owner was limited to partially filling the silos (approximately ½ of their height). Because of market conditions and the immediate need for storage, replacing the silos was not an option. To develop a plan to strengthen the silo, a true design-build relationship was formed between the owner, engineer, contractor, and material supplier. The design-build team considered several repair options including enlarging the interior walls with a reinforced concrete liner, external post-tensioning, and various interior and exterior FRP composite strengthening techniques.

The final solution consisted of near surface mounted FRP bars epoxy bonded in pre-cut grooves. This plan offered the durability benefits of FRP while proving constructable and cost-effective. The other benefit of FRP rods is the ability to dowel them into the common wall to a depth that ensures complete development of the rod. Vertical FRP bars, some 150 feet long, could be handled in single pieces due to their lightweight properties. Upon completion of the vertical installation, the circumferential rods were installed. The final step was to coat the entire silo with a breathable, acrylic topcoat for both durability and aesthetic purposes. This repair strategy removed only one silo from service at a time. This project, utilizing over 10 miles of FRP rod, saved the owner approximately $4,000,000 and the 18 months of construction for replacement.