One of the most brutal winters in recent memory is now officially behind us, but the headaches from property damage caused by the extreme weather conditions are far from over. Many issues often are not apparent until the weather warms and the freeze-thaw cycle is complete. Although the problems caused by the changing weather are not necessarily a reflection on the quality of design or construction, there are some things that owners and property managers can do to minimize the effects of extreme winter conditions on properties in the future.
The U.S. Weather Service’s Winter Severity Index, which considers snowfall, temperatures and other factors, ranks 2014 among the five worst Chicago winters in recorded history. The damage to property could have been worse—there have been years, such as the winter of 1978-79, when excessive snow damaged or collapsed many roofs, but this year’s heavy snowfall was spread throughout the season rather than concentrated over short periods. The problem in 2014 has been the cycle of extreme, extended cold spells worsened by occasional above-freezing temperatures and rain.
When it comes to how weather impacts site conditions, the concepts are usually rather simple. The cold causes the ground to freeze. The lower the temperatures and the longer the duration, the deeper the frost penetrates. All soils contain a percentage of moisture, and when they freeze the water fraction expands in volume and can cause related heaving, or upward swelling of soils. Additional and potentially worse heaving may also occur due to the formation of ice levels in the frozen soils with associated water pulled up by capillary action or fed from the surface.
Expanding soils and ice can exert extreme pressures on adjacent and buried structures, and such expansion causes movement and damage. Below the surface, these extreme pressures can rupture sewers and water mains, causing all kinds of problems, especially during very cold winters when soils freeze deeper. On the surface, frost heaving can cause significant displacement to asphalt pavements and concrete sidewalks. Normal and expected “heaving” usually is not problematic, but multiple and extreme freeze-thaw cycles can cause severe damage. Chicagoans are very familiar with this phenomenon with potholes in streets throughout the city and suburbs every spring. But the winter of 2014 has been worse than most, and the fallout may be more costly.
Snow tends to be an insulator, reflecting sunlight and acting like a blanket as it protects the ground from the cold and deep frost penetration. In snow covered areas, frost depths may have reached two feet or less, even with the extreme cold. However, the insulating effect of snow cover is lost on sidewalks, parking lots or other areas where it is removed for traffic, parking and pedestrians. Also, a quick thaw and snow reduction followed by another quick deep freeze renders the once insulated ground susceptible to deeper frost penetration.
In the Chicago area, architects, engineers, and contractors plan building foundations for a maximum “frost” depth of 42 inches. New water mains are typically installed with a minimum cover of 66 inches. However, existing and aging utilities in Chicago and surrounding suburbs (some of these structures being 100 years old) may not benefit from adequate soil cover. This year, temperatures remained below freezing for a majority of the winter, and plummeted and stayed well below zero for extended times. The frost depth pushed well past the 42-inch code requirement, and even reached 60 inches or more below pavements throughout the area.
The Effects
From November through March, major cities like Chicago witnessed the effects of the freeze-thaw cycles with broken water mains and local flooding. Many businesses experienced concrete slabs heaving, cracking and spalling, or flaking around their entrances, which required walks and entries to be ground down in order to open doors. Others observed parking lots, pavers and concrete aprons heaving several inches above surrounding curbs and walks, showing cracking and distress even in new pavement and concrete.
And the arrival of warmer weather does not mean the end of property woes. A great deal of pavement damage may occur once the season changes. Concrete walks and asphalt pavement will settle once the frozen soils thaw and move back towards their original elevations, but cracked concrete walks may create tripping hazards and present problems next winter when the freeze-thaw cycle occurs again. The deterioration of pavements and sidewalks that have heaved and cracked will be accelerated if it is not repaired. Making this worse are the new cracks that increase the amount of water that will penetrate into the underlying soils and freeze the next winter. Parking lots that already needed help before the winter may be in desperate need of repair. Even new asphalt may show distress and require filling and sealing.
Owners also may see more water seepage than usual this year because of the amount of snowmelt and more/new cracks opening up in foundations and masonry systems than usual. After a severe winter “we often see ratcheting, where masonry moves a fraction of an inch in the cold weather, and slides back a different way when it warms up again,” said Ernest Rogalla, Associate Principal of Wiss Janney Elstner. “If an entire structure settles two inches, you may not see damage because it all moves together. But when one corner moves differently than the rest of the building, that’s when there can be problems.” This is also a function of how abrupt the shift is. “A difference of a half-inch of movement over a distance of 30 feet is not as bad as if it occurs over 10 feet,” he said.
The Fixes and Tradeoffs
This resettling is fairly common even in newer buildings, and like other weather related damage to buildings, is not necessarily an indication of poor design or construction. Owners and developers can mitigate risks relating to frost and ice damage, but some measures tend to be viewed as not cost-effective. After a winter like this, it may seem logical to install water mains deeper than 60 inches, but for most owners, the additional protection against frozen utilities is not worth the extra cost of digging the trench a foot or two deeper. It would be much more cost effective to replace old utilities before they break, but many municipalities are budget strapped. Unfortunately, an emergency replacement costs far more than one that is planned.
Sidewalks and pavements are more prone to heaving and deterioration when the underlying soil is not conducive to water drainage, as is the case with northern Illinois’ clay-rich soil. Standard practice is to replace the clay beneath pavements with granular materials that often are structural and not free draining, so they hold moisture in winter that can freeze and promote heaving. There are new base materials on the market that are both structural and enable free draining. While these products tend to cost more, the initial investment may prove to be a cost savings over time as maintenance costs and insurance claims may be tremendously reduced. As a bonus, free-draining base products generally promote environmental sustainability.
Other potential moisture and extreme cold problems can be headed off with relatively simple fixes, and some problems can be mitigated with good property management. Be aware that letting snow pile up next to a building increases the chance that water will seep inside when the snow melts. It also can damage the walls that absorb the moisture and undergo freeze-thaw cycles. Using the wrong products can accelerate the deterioration of concrete surfaces. And salts should never be used on pavements that are less than 12 months old.
When it comes to building structures, owners can prevent water infiltration from ice dams by properly insulating roof lines and removing potential dams before they occur. Owners and property managers should make sure that brick buildings are inspected in the spring to repair cracks in the mortar, a process known as tuckpointing or repointing. “It’s good to do tuckpointing well in advance of the next winter to give the mortar plenty of time to cure before freezing,” Rogalla said. “But doing it too soon after the thaw can be a problem if the building has not finished resettling. Using the proper pointing mortars and techniques are also important. Parapets should get special attention, because they can deteriorate easily and pieces can fall and potentially injure people.”
If you are building a new facility, be aware that some types of brick are more absorptive than others, and when water gets into the pores and freezes, the result is spalling where pieces of the brick flake off. It is difficult to address spalling effectively once it starts. Also, the type of mortar used on new brick veneer buildings is important. Type “S” mortars are used for structural masonry and provide additional strength to a structural wall, but type “S” mortars are not necessarily good for brick veneer applications. Type “N” or normal mortar tends to be more flexible and forgiving in hostile environments and is more appropriate for brick veneer. In this case, stronger doesn’t necessarily mean better.
Although it’s too late to do anything for the winter of 2014, it’s not too late to think about how to mitigate future problems before they occur. Climate experts suggest that the next two or three Chicago winters could be just as brutal as 2014.
