21 Dec What is the Freeze Thaw Cycle and How to Avoid Its Damage to Concrete?
Winter, especially farther away from the equator, equals white landscapes and that time of the year to cozy up to a fireplace. To us humans, winter is just another season, but to concrete structures, it’s when those buildings, roads, bridges, etc.… feel the worst type of distress: the freeze thaw cycle effect.
What is the freeze thaw cycle and why is it damaging to concrete?
In a nutshell, this phenomenon is a process of distress that concrete experiences through the cyclical freezing and melting of water that has seeped into it. Eventually, this cycle can cause the total cracking and permanent damage to the concrete structure.
You might wonder why a little water can do so much damage? Here is the long answer:
This phenomenon occurs when concrete is saturated with water and the temperature drops, freezing the H2O molecules. Since frozen water expands, it causes distress to the concrete structure. Once the warmer months come by, the H2O molecules melt away and reveal tiny cracks in the surface layer of the structure. The following winter when those tiny cracks are filled with water once again and the temperature drops, the H2O molecules expand, making more room for themselves and causing more distress in the concrete. The continuous freezing and thawing of water causes those tiny cracks to become larger over time and if not repaired, will permanently damage the structure.
How does it occur?
There are 2 ways this phenomenon happens in nature. The first is through the repeated cycle of melting and freezing of water on the natural cracks and grooves of rocks, such as in mountains, tundra and the like. The second is through the expansion and contraction of the surface layer of rocks that are backed by direct sunlight and heat, such as in desert regions. Can’t visualize the end result of years of distress and erosion? Example: think The Grand Canyon – mighty beautiful, but we want our human-made structures to stand proud and strong for years.
Dry concrete can suffer from both types of distress, either by freezing and thawing of water or its expansion and contraction under extreme heat. The distress cannot be avoided due to the geographic location of certain cities. However, concrete can better withstand the effects of the freeze thaw cycles with a little help from additives.
Why does the freeze thaw effect damage concrete?
Distress and damage to concrete from this phenomenon happens when concrete is heavily saturated with water, which happens when more than 90% of its pores are filled with H2O. Added to the fact that frozen water occupies 9% more volume than water at room temperature and there is a limited space for the volume increase within the concrete, the freezing of water causes microcracks. This damage begins from the first cycle of the freezing and thawing of water and with continuous exposure to winter seasons, will result in repeated loss of the concrete surface.
What are the signs of concrete damage caused by the freeze thaw cycle?
Often the diagnosis of the effects of this phenomenon is complicated, since other mechanisms may be involved, such as the Alkali-Silica reaction (ASR). But if all other mechanisms can be excluded, the typical signs of the freeze thaw cycle could be:
- Spalling and scaling of the surface
- Surface parallel cracking
- Large chunks are coming off the surface of concrete (cm size)
- Exposure of aggregate
- Usually, the exposed aggregate is uncracked
- Gaps around the aggregate
How to avoid the freeze thaw damage to concrete?
The short answer is: air. It sounds so simple, but if there are built in “expansion chambers” in the dry concrete, the structure would have room to accommodate the 9% increase in volume of frozen water. Thus, the distress, damage and cracks to the concrete surface would be minimized.
There are 2 kinds of air in concrete, entrapped and entrained. Entrapped air results naturally from the mixing process, with approximately 1.5% of air becoming trapped when concrete is being made. The entrapped air bubbles are easily seen with the naked eye and irregularly shaped. Due to their shape and large size, entrapped bubbles would not help against the freeze thaw cycle. Entrained air bubbles are the opposite. They are innumerous, spherical and microscopic, creating the expansion chambers to accommodate the freeze thaw action of water.
So, in order to protect concrete from damage, concrete should be air-entrained by adding a surface-active agent to its mix, like Carbofen 6060 or Carbofen 5055. Once added to the concrete mixture, Carbofen creates thousands of closely spaced, microscopic air bubbles in the hardened concrete. These air bubbles are what helps concrete withstand the distress from the freeze thaw cycle, since they act as expansion chambers for the frozen water to occupy.
Usually, the bubbles should be well distributed, with a distance of 0.25 mm or more between each other in the cement paste. Thus, approximately 4% to 6% air by volume is needed in concrete to achieve the necessary air bubbles to better withstand the strain from the freeze thaw cycle.
As a rule of thumb, concrete with high water content and high water to cement ratio is less frost resistant than concrete with lower water content.
Why invest in an air entraining agent for concrete?
Other than enjoying the lowered maintenance costs of repairing less cracks in concrete caused by the freeze thaw cycle and increasing the overall longevity of the structure, air entraining agents are used for 3 primary reasons. The first and foremost reason is to entrain air in concrete and make it more resistant to the effects of repeated freezing and melting of water in colder climates. Secondly, air entrainers like Carbofen 6060 and Carbofen 5055 are used to prevent bleeding – water coming to the surface of freshly placed concrete (remember, water is required to make air bubbles and entrain the concrete). Lastly, Carbofen is used to reduce the unit weight and the water/cement ratio of concrete.
If those weren’t good enough reasons to invest on an air entraining agent like Carbofen, here are some other benefits:
- Grant concrete better resistance to sulfate and chloride
- Reduces the amount of cement and aggregates needed to make the concrete mixture
- Concrete gains more plasticity
- The water/cement ratio is reduced, thus balancing the loss of mechanical strength.