Last
month we discussed the detrimental effects deicing chemicals can have on
concrete. This month, we will discuss ways to protect concrete from these harmful
effects
Air Entrainment, Air
Entrainment, Air Entrainment
Incorporating
the proper amount of entrained air into concrete is very effective in
mitigating the harmful effects of moisture freezing in concrete. Air
entrainment is the intentional incorporation of numerous tiny air voids in the
concrete paste. These tiny air voids provide areas for the freezing and
migrating moisture to enter before they produce expansive pressure on the
concrete. Air entrainment is even more critical to freezing concrete when
deicing chemicals are applied.
Low Permeability
Concrete
that is less permeable is less likely to become saturated, is more resistant to
sulfate and other chemical attack, and is more resistant to chloride-ion
penetration. Permeability can be lowered by using a low water-cement ratio
(preferably below 0.45) and an adequate moist-curing period (preferably 7
days).
Adequate Drying after
Curing
After
concrete is properly moist-cured, it should air dry for at least 30 days before
it is exposed to freezing or deicing chemicals. At the end of the moist-curing
process, the
concrete is still saturated. This moisture can still freeze and
expand, deteriorating the concrete. This is why adequate drying is the most
overlooked detail in protecting concrete from deicing chemicals.
The
drying rate of concrete cured with a membrane-forming compound is much slower
than the drying rate of concrete without a membrane-forming compound. When
concrete is scheduled for placement in the fall, especially late fall, consider
whether postponing the placement until spring is an option. If postponement is
not an option, which is often the case, using a moist-cure method instead of a membrane
cure will give the concrete a better chance of drying before being exposed to
freezing and deicing chemicals.
Durable Aggregates
Aggregates
that perform well under freeze/thaw cycles should be used. Low porosity, absorption
and permeability are aggregate qualities that help prevent harmful effects from
the freeze/thaw cycle. Durable aggregates can also help mitigate alkali-silica
reaction (ASR) between cement and aggregate. ASR causes excessive expansion
that deteriorates concrete.
Protect Reinforcing
Steel
The
chloride-ions in most deicing chemicals will aggressively corrode reinforcing
steel. Because rust expands to four times its original volume, the protection
of reinforcing steel from deicing chemicals is critical. This can be done in
the following ways:
-
The use of low permeability concrete will slow the rate of penetration of chloride-ions thru the concrete, thereby buying time before the steel starts to corrode. In addition to the suggestions above, silica-fume and latex can be added to concrete to make it less permeable.
- Epoxy-coat the reinforcing steel. The epoxy coating acts as a barrier between the reinforcing steel and the chloride-ions in the concrete.
- Increase the thickness of concrete cover over the reinforcing steel. This forces the chloride-ions to travel further thru the concrete to reach the reinforcing steel.
- Overlays. The concrete surface can be overlaid with a waterproof material to keep chlorides from penetrating the concrete and reaching the reinforcing steel. Overlays should be evaluated for water tightness, durability (especially when exposed to snow removal equipment) and longevity (especially UV rays).
- Provide cathodic protection. The first method of providing cathotic protection is with an impressed current cathodic system. This system induces a small electrical current into the reinforcing steel in the opposite direction of the electrical current found in reinforcing steel during the corrosion process. Cathodic protection is the only system that can actually reverse the corrosion process and stop corrosion. However, it is expensive and difficult to install and maintain.
The second form of cathodic protection is through the use of sacrificial galvanic anodes. After the galvanic anodes are connected to the reinforcing steel, they will sacrifice themselves and greatly reduce the rate of corrosion in the reinforcing steel. However, once the sacrificial anode is totally sacrificed, the corrosion of the reinforcing steel will once again accelerate.
When we are forced to use deicing chemicals for
public safety, we are also forced to find ways to minimize the costly
deterioration that deicing chemicals can cause. Hopefully, some of these
suggestions will help you provide concrete with a lower overall life-cycle
cost.
With winter in full swing and the return of the polar arctic
blast, now is a good time to review a few winter survival tips. Here are a few
things you should keep in your car:
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