Freeze/Thaw Cycling in Natural Stone
As Spring has arrived and protective matting gets removed from the many entranceways of buildings, we can’t help but notice the effects that Winter has left behind. On the exterior of the buildings, you’ll notice damage caused by freezing. Interior damage is caused from the tracking in of salts, sand and de-icing agents.
We live in a freeze/thaw climate and are faced with numerous challenges regarding the effects it has on natural stone and concrete. To keep it simple, the root cause of the problem is water. With freezing comes expansion; notably when the water has penetrated beneath the surface of the stone, which is porous to varying degrees. Many stones have such low porosity, varying degrees of absorption, that they just don’t take on enough water to be significantly affected, and a few stones have such open porosity that they effectively evacuate the water before it has a chance to freeze. A very important factor to consider, is the orientation of the stone. Vertical work is far less vulnerable than horizontal. Of course the face of a vertical panel will get wet, but the depth of the penetration is a fraction of what happens to a horizontal stone. When the water, in any form, is allowed to dwell on a horizontal surface, the saturation level of the stone is far greater than what is achieved in a vertical orientation.
Mills and quarries tend not to handle frozen stone because it tends to fracture and spall away as a result of minute cracks. Freeze-damaged limestone, for example, is quite friable (easily crumbled or pulverized). Its grains tend to separate and can be rubbed off. It may split easily into sheet- like fragments. Thus, stone damaged by freezing is seldom shipped. When installed stone fails due to freezing, its often found at parapets and copings when leaking joints allow more moisture to accumulate than the stone can exude or breathe away. Such examples occur in areas protected from the sun by trees or nearby buildings. Lower areas in the building, even those constantly soaked by grade moisture, suffer little damage because of heat absorbed by the building and the earth.
Many stones damaged by freezing may not be repaired and must be replaced. However, specific conditions often may be treated by maintenance through professional and qualified and knowledgeable personnel. Unfortunately, many decisions to clean buildings are made without adequate preparatory steps and without investigation of alternative methods, their costs and results.
At Marble Renewal, we give consideration first to the easiest and cleanest method which will produce desired results. Not all buildings will clean up so easily, and different areas of the same building may require different treatments. A general rule should be to use the most conservative treatment or material which will achieve the required degree of cleanliness. Cleaning methods are categorized as wet, dry and chemical. Each has its place, and sometimes all must be used to achieve desired results.
The surface should be protected after cleaning. Once sealed with a premium quality impregnating sealer, the treated surface becomes water repellent. The sealer works by repelling water and oils, and leaves the pores open to breathe so moisture can escape by evaporation. Because the pores remain open, spills should be cleaned up as soon as possible as, given enough time, they can penetrate. The end result will be protection against problems caused by water and water borne salts, including efflorescence, salt spalling, freeze-thaw spalling and picture framing. This treatment will keep the surface looking newer for longer, making it easier to clean while controlling mold and mildew growth.
