Materials Performance

NOV 2014

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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the pH of an alkaline lime- or cement-based grout would have decreased sometime after installation because of carbonation. This occurs when carbon dioxide (CO 2 ) penetrates the material in the joint line and mixes with calcium hydroxide [Ca(OH) 2 ] to produce carbonic acid (H₂CO₃), which reduces the pH of the grout material. Since a high pH is required to ensure the stability of the protective passive film on the surface of the steel, a drop in the pH level would cause the passive film on the steel components to deteriorate and allow the steel to oxidize and corrode in the presence of moisture and oxygen, and create a weak point in the limestone masonry that is conducive to cracking. Throughout the years, the steel frame corroded along the angle edges, upper flanges of the C-channels, and at some lug anchor locations. An investigation of the soffit and entablature in 1992 noted that water was infiltrating the structure through open joints and failed sealants in the parapet. There was a lack of grout around the anchor bolts, as well as corrosion on the anchor bolts and the steel channels. The entablature stone was experiencing corrosion-related stress—repetitive crack- ing—and previously repaired hairline cracks were observed. New anchor bolts were recommended as well as general waterproof- ing, and repairs were made. Corrosion of the steel components continued, however, and the soffit stone failed in 2009 when it split at a crack at the wedge anchor location. The subse- quent corrosion investigation and analysis assessed the structure's existing corrosion conditions and clarified which building elements were susceptible to corrosion. Out of the 24 bays, it was noted that 50% of the soffit stones had visual signs of distress in the form of hairline cracking. Also, it was determined that corrosion of a wedge anchor had caused the failed soffit stone to crack and split apart. "We were able to study the construction details and test the corro- sion activity," says Crevello. "The results indicated that, due to the architectural features and geometries, this type of failure could potentially happen in the future." stone prior to setting to hold the lug anchors in place. C-cramps or anchors were used at vertical joints and transverse and wedge anchors were installed over the horizontal steel members to anchor the soffit stones in place for additional support. Crevello and Noyce note that anchor- ing was typically done on site by the masonry team, so the original construc- tion documents do not detail the size and location of anchors and cramps or the method used to anchor the stones. Additionally, the anchors and cramps were not painted with a protective coating. Although evidence indicates that grout was used to secure the lug anchors and shims, The red circle shows the steel support structure in this diagram of the rotunda's entablature. Image courtesy of Harboe Architects. The rotunda's entablature lost a soffit stone (left), which cracked and broke apart at the wedge anchor location. Photo courtesy of Gina Crevello, Echem Consultants. Parapet Roof Brick Back-Up Steel Structure Unit Above Soffit Metal Anchors Soffit Inner Colonnade Cornice Dentil Frieze Soffit Unit Above Soffit Column Capital FEATURE ARTICLE 28 NOVEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 11

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