Historic buildings and archaeological monuments are periodically subjected to aggressive environmental conditions such as storms, high winds and freezing winter conditions. Alterations in local environmental conditions can impact the microstructure of historic and modern building materials, eventually leading to macro-scale damage such as spalling (the detachment of fragments of material), granular disintegration (powdering) and fragmentation. Durability testing of new mortars, stones and conservation treatments attempt to predict the stresses of decades of aggressive environmental conditions through simulated accelerated weather conditions. In real-world condition, the concerns surrounding this seasons cold temperatures and snowfalls are more usually focused on what may happen now in the short-term. However, if sudden building failures in the winter months are wrongly attributed to freeze-thaw action simple because snow is present, larger, more serious and costlier problems can be overlooked.
‘Freeze-thaw’ damage is a common form of moisture-related decay of building materials, and a sub-set of how buildings handle precipitation. Along with drizzle, rain, heavy downpours and storms, in winter historic buildings and structures must also handle sleet and show, dew and ice. Freeze-thaw cycles see the transformation of water into ice, and this can exert two types of pressure on historic building materials such as stone, brick, mortar and render.
The first process is the most widely known, where liquid moisture penetrates the stone, brick or mortar, and as ice has a volume 9% higher than liquid water, the stresses of expansion can lead to fracturing and other forms of deterioration. The second type of pressure is often overlooked – as unfrozen water migrates within the material to cause overpressure in the pores. In both cases, the porous network is a critical parameter, as it determines the amount and distribution of moisture. There is no consensus on what moisture content is acceptable, as this depends on the porosity and other properties of the material (such as elastic properties, mechanical strength, water transport, pre-existing defects) and its position and use within the building and the prevailing conditions to which it is exposed. Cementitious mortars, impermeable renders, coatings and paints can also play a significant role in freeze-thaw cycles, ‘trapping’ or re-directing moisture to where it can cause damage.
The mechanism of freezing within a building stone is complex. During initial heat loss, the temperature drops below freezing but ice has not yet formed as the moisture within the pores become supercooled. As the temperature continues to fall, ice begins to form within the pores (solidification), releasing heat energy as a result of the latent heat of fusion of water. The solidification phase can be instantaneous or take more than an hour depending on the material. When ice first forms, the temperature rises rapidly and then stabilises until all pore moisture is frozen, then heat release ceases and the temperature begins to fall again. In a temperate oceanic climate like Ireland, the sudden arrival of snow and ice and temperatures dipping below 0°C can give rise to many freeze/thaw events in a day, rather than the longer, colder winters conditions experienced in other parts of Europe. As a result, studies from other countries are useful in describing how freeze-thaw causes damage, but the solutions from other parts of Europe cannot be transplanted to Ireland without taking the local environmental conditions into account.
In addition to the micro-level and macro-level stresses associated with freezing conditions, the sudden melting of snow cover can saturate a building and its surrounding grounds. Salts spread on roads and pavements are also absorbed by adjacent buildings, and ‘tide marks’ of salt effloresence could commonly be seen on many brick buildings after the last snow, and will be seen in the coming weeks after the current snow has melted into memory.