Masonry Magazine October 2004 Page. 23
The Voice of the Mason Contractor
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October 2004 Masonry 21
The Test of Time
AS THE WALL PASSES half a century in age, interesting degradation features have been observed, including differences in the relative durability of stone and in the way stone (even of the same lithology) perform.
The limestones and marbles generally have faired well, though the surfaces have roughened a bit due to dissolution of the stone. In some cases, this apparently enhanced the stone appearance, as with the Knoxville marbles. In other cases, the surface roughening has increased its susceptibility to trap dirt and stain as overlying stone erode. In some cases, there is a local history for stone application - the Potomac marble does fine indoors but bleaches out to a gray in the sunlight. In other cases, the relationship between durability and lithology is not so clear. This leads to the third question where some of the durability issues have been studied but others are not so well understood.
As the left side of the wall is set in lime mortar and the right side of the wall in Portland cement, another area of evaluation is available. Observers were concerned that lime mortars could cause discoloration on masonry, so they replaced lime in the Portland cement mortar with ground limestone. Although this will require a more direct comparison, over time the lime mortars have not held up as well as the Portland cement mortars. Discoloration appears to be affected by the coloration and weathering characteristics of the overlying stone.
Conclusion
THE BULK OF THE STUDY is just in the beginning stages for this project. A proposal with the Parks Service regarding evaluation data from an unpublished study containing weathering depth measurements and images will provide an interesting assessment of erosion rates in the future.