Hygric Swelling of Portland Brownstone
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Hygric Swelling of Portland Brownstone Inmaculada Jimenez Gonzalez, Megan Higgins, and George W. Scherer Princeton University, Eng. Quad. E-319, Princeton, NJ 08544 USA
ABSTRACT Portland Brownstone (PB) is a widely used building material, particularly in New York City, that is susceptible to severe deterioration from weathering. This stone contains clays that may cause damage by dilating during wetting/drying cycles. We have measured the swelling strain of PB in aqueous solutions to determine the effect of ionic strength (of KCl and CaCl2) and water activity (in isopropanol solutions). We have also measured the stress exerted during swelling when the expansion is constrained. These data permit calculation of the stresses produced during wetting/drying cycles, which are found to be comparable to the tensile strength of the stone. INTRODUCTION Many types of stone contain inclusions of clay that dilate with changes in humidity. In some cases this leads to deterioration of the stone1; it can also result in destruction of consolidants2. This paper presents the results of preliminary investigations of the effect of swelling in a particular stone, Portland Brownstone, that is widely used as a building material in the eastern United States. This is a complex sandstone containing grains of quartz and albite in a matrix including kaolinite, mica, iron oxides, illites and smectites. The clay flakes around the quartz grains are evident in Figure 1.
Figure 1. Scanning electron micrograph of Portland Brownstone used in this study.
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This stone is subject to relatively rapid deterioration from weathering, and swelling of the clay components has been suggested to contribute to the damage.3 We will quantify the swelling stresses and show that they can exceed the strength of the stone. EXPERIMENTAL PROCEDURE Plates of Portland Brownstone (PB) were purchased from Pasvalco Co. [Closter, NJ] and samples were prepared by cutting with a diamond saw or core drill (2 cm I.D.). The pore structure was characterized by nitrogen sorption using a Micromeritics ASAP 2010 and by mercury porosimetry using a Micromeritics 9410. The dynamic elastic modulus was calculated from acoustic pulse delay, which was measured using a PUNDIT instrument. The static modulus for a fully saturated sample was determined by a 3-point bending test on a rod roughly 3 x 10 x 210 mm. Tensile strength was measured by the Brazil (or splitting) test on cylindrical samples 2 cm in diameter and 5 cm long. Sorptivity was measured by suspending samples of stone from an electronic balance so that the bottom of the stone touched a large pool of water, and recording the output from the balance continuously; the sides of the sample were coated with grease to prevent evaporation. Swelling of the stone was quantified using a Perkin-Elmer differential mechanical analyzer (DMA). Samples with a maximum height of 18 mm and diameter no more than 10 mm were placed on the sample holder that was then immersed in liquid; the length of the sample was continuously monitored
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