What does Freeze-Thaw mean and what happens to the stone?
Natural stone is an ideal material for exterior cladding and paving. A common concern when purchasing stone is the serviceability under weathering over the life of the project. Freeze-thaw cycles can cause damage overtime to porous materials and lead to adverse effects on the durability of stone.
Water enters the tiny pores of materials like stone and freezes when the outdoor temperature decreases to at least 0° Celsius. The transition of water to ice causes expansion of the stone and potential formation of microcracks. The microcracking is not visible until a number of cycles accumulate to cause noticeable damage. By then, the strength properties of the stone have been significantly reduced. The extent or rate at which freeze-thaw occurs, depends on the stone’s porosity, pore size and moisture content. A saturated stone that is susceptible to frost damage is likely to experience more cracking and at a faster rate than the same stone when dry. When the weather becomes warm, thawing process occurs and the ice transitions back to water. The water may stay, further accumulate or even travel to interconnecting pores that leads to further damage when subsequent freezing begins.
Where and when does Freeze-Thaw occur?
This problem is more prominent in colder regions like Canada and places in the United States. How many freeze-thaw cycles occurs within a region depends on which country and the severity of the winter. However, the cumulative effect over many years in even moderate climates can eventually be significant and should be considered.
How do they test a stone for Freeze-Thaw resistance?
Based on articles that detail research and results, the general conclusion seems to be that the majority of damage occurs between 50-125 cycles. The frost resistance is checked by standardised methods to assess the effect of freeze-thaw cycles on stones. It takes substantial resources to repeat a large number of freeze-thaw cycles. As a result, it can be expensive to carry out, which is why standard testing usually involves 30-50 cycles to show preliminary results. That being said, the stones that undergo testing are durable and able to withstand enough cycles to satisfy serviceability for both façade cladding and paving. If a stone fails the tests after just 50 cycles or less, it is safe to assume that it would not last more than a couple of years especially in colder regions.
How can Freeze-Thaw cycles and its effects be mitigated?
The installation process is just as important as the stone selection. An effective way to protect building cladding or pavers is to ensure that the stone and mortar interface is tightly sealed during installation. Doing this avoids moisture buildup and prevents water from easily passing through and accelerating the effects of the freeze-thaw cycle. A common misconception is that using the same stone but thicker will make it more frost resistant. However, a thin stone will dry out faster than a thick stone and because of moisture, wet stones are more susceptible to damage compared to dry stones. Keeping the issue of freeze-thaw in mind and taking steps to potentially avoid damage to your stone, will greatly increase the chances of a long-lasting project.