Case Study #4
Testing Critical With Stone
Chicago’s fourth tallest building and the
world’s tallest marble-clad structure is the Aon (Amoco) Building.
Completed in 1973, this structure is 1136' high with 80
stories and 2.3 million square feet. The tower measures 194' x 194' and was
covered with 43,000, 275 lb. Carrera marble panels. The surface portions of these rectangular panels are
expanding at all four corners, a phenomenon known as “dishing”.
Dishing can occur when panels of the marble are sufficiently thin, yet adequately thick, to undergo permanent expansion of their faces while there is sufficient restraint to that expansion on the backs of these panels. Dishing is a result of thermal hysteresis, a phenomenon caused by an accumulative, infinitely small expansion of the marble during cyclic heating. Because of the peculiar property of the mineral calcite and the high calcite content of Carrara white marble, thin panels used as façade material are susceptible to the dishing problem.
Freeze/thaw cycles cause marble to undergo cyclic heating and cooling. The marble experiences a permanent volume change, progressively expanding and never returning to its original size. The calcite crystals interlock and prevent the stone from returning to its original size.
How do you prevent thermal hysteresis? Change the thickness of the panels. With a thicker panel, the dishing would not occur because of restraint by the backs of the panels, which does not undergo the same number of thermal cycles as the face. If the panels are thinner than an optimum thickness, they will expand relatively uniformly throughout, and if there is no side or end restraint, they will simply become longer, wider, and thicker.
When specifying stone, testing is important to prevent any problems, including thermal hysteresis. Lab tests can be performed to determine how stone will perform. More importantly, cyclic tests should be performed to see how a stone will perform after a given number of years. Test results will indicate any loss of strength or property changes than can be expected after the stone has been exposed. From those results, an anchoring system can be designed that will accommodate the stone initially as well as any loss of strength or volume change over time.
Footnote: The marble on the Aon/Amoco building was replaced with White Mount Airy Granite, making it this tallest granite clad building in the world. Portions of the original Carrera marble can be found in the landscaping near the BP refinery in Whiting, Indiana.