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A study of mineralogical failures in the construction industry

Black Granite
Steven J. Stokowski, PG
Geology Building Room 200A

In the center of the Rhode Island Veterans Cemetery is a large Ceremonial and Commemorative area. This dramatic, horseshoe-shaped memorial has sweeping cast-in-place concrete walls with fifty-eight, inlaid, 6-foot high, black granite panels having engraved veterans' names. During the initial 1998-99 construction phase, the project was relatively trouble-free except that the availability of the large stone panels delayed the project for 7 months. The original granite specified was from Zimbabwe, but the architect actually wanted stone from South Africa, which was not available because of sanctions. Of the "black granites" available, the architect then approved a very dark and coarse-grained “granite” from Canada. Soon after installation, most of the stone panels progressively developed popouts and cracks. The only realistic solution to the pervasive failure was to replace all the panels, although the manufacturer would not admit to any materials problem and supply replacement panels free-of-charge. Ultimately, there was no inexpensive and amicable solution for any of the participants in this project. The state declared non-performance and suspended the project engineer, the design architect, and the contractor from bidding on state projects for two years. This resulted in many lawsuits that were resolved to nobody's satisfaction, but with the vindication of some of the parties. The state ultimately slightly redesigned the memorial, demolished the 1999 construction, and caused a new memorial to be constructed with stone from another source. The stone panels deteriorated because the dimension stone product was inherently defective at the microscopic level. Saponite, a water-sensitive, swelling clay, is present in the stone. It did not swell in the 1999 Rhode Island drought that followed the installation, but, as soon as it began to rain, the iron-rich saponite absorbed water, causing popouts and stressing the stone until it cracked. Black granite is a stone industry term for black, hard, crystalline rocks such as norite, gabbro, and anorthosite, as compared to fine-grained crystalline rocks such as basalt, or the black varieties of softer calcitic rocks such as limestone or marble. In addition to saponite alteration, the Peribonka® anorthosite contains deleterious ore minerals such as chalcopyrite, younger veins of secondary calcite, and later antigorite veins. The calcite and antigorite veins are weak planes in the stone that cracked when the panels became stressed by the swollen saponite. These natural weak points are not defects, but the natural locus of cracks when the stone was stressed to failure

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