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What is Concrete Spalling?

Choosing the Engineer

Florida is a great place to live.  First, you have the beautiful beaches. Second, you have great weather throughout the year.   Third, you have great architecture that includes art-deco to modern styles.  However, these positive traits come with their fair share of problems.  The high concentration of “salt air” can seep into the  concrete over time.

This is a sample of concrete spallingEventually, the salt air damages the steel inside the concrete and causes it to rust.  Ultimately, this rust causes the concrete to spall.   There are some complex chemical reactions that take place  for a spall to occur, but we will try to keep it simple for those non-engineering types.  As an introduction, the picture shown here is an actual spall.  The classical spalled concrete will show the reinforcing steel (or “rebar“) once the concrete comes loose.

Passivation Layer

Lets start at the very beginning.  When the original builder installs the rebar inside the concrete, it is typically in a pristine condition.  That rebar has a protective layer that is called the “passivation layer”.  Coastal areas have high concentration of salts (chlorides) that penetrate the concrete through microscopic hole in the concrete.  Next, these salts come into contact with the steel and damages the protective “passivation layer” on the rebar.  When this protective layer is gone, the rusting process will begin and will continue over time.  At this point there is very little one can do to stop the rusting.

Concrete Spall defined

Now that there is an expanding piece of steel inside the concrete, it will only be a matter of time until the concrete breaks.  This rupture causes both small and large pieces of concrete to become dislodged.  Those detached pieces of concrete is the spall itself.  Needless to say, concrete spalls are potentially dangerous and even life-threatening.   On high-rise buildings they can damage properties below or even make contact with a person.  Spalling concrete should be repaired as soon as possible since it will spread and get worse.  Eventually, the rusting rebar will render the structure unsafe.

There have been many instances where structural collapses have occurred and people have been severely hurt as a result. Spalling concrete is a sign that your structure is under attack and this sign should not be ignored. Many people kick the can down the road and leave the repairs for a future date.  The old adage of “a stitch in time saves nine” is most certainly true.  A concrete repair that costs $1,000 today, will cost $10,000 a few years from now.

I like to think of spalling as a type of cancer.  In the same way that a cancer can grow until you die, the same is true of a spall.  The exception is that there is no “chemo-therapy” that will cure the spall.  The truth is that the only way to take care of the spall is to remove it using special techniques.  These methods that are employed by contractors to fix spalling concrete is varied and a little complex.  The ICRI (or International Concrete Repair Institute) has very specific guidelines on how concrete spalls are to be repaired.

Structural Engineering 101

Pantheon Dome
This is the pantheon dome

Concrete has been used for thousands of years in various forms. The Romans built magnificent concrete structures that survive to this day, such as the Pantheon, with its impressive concrete dome. The Pantheon (see picture), however does not have any reinforcing steel in it because of its design. The dome’s concrete was always compressed under its own weight. Concrete by itself, without any steel rebar inside of it, can withstand a tremendous amount of this compressive force. On the other hand, concrete cannot withstand a large amount of tensile force, which is the force that wants to pull it apart. Because domes are hardly built these days, the structures that we build today are subjected to both tensile and compressive forces.  Around the turn of the century engineers noticed that if you include reinforcing steel inside the concrete,  it could  withstand  a  tremendous  amount of  tensile  forces in  addition  to compressive forces. This meant that one could now design and build all sorts of structures without being hindered by concrete’s structural limitations. This is how the era of building modern concrete structures began.  As such, without including steel inside your concrete to support the structure, you would not have a soundly-built building.  The reinforcing steel (rebar) inside the concrete literally keeps your entire structure from crumbling to the ground.

Conclusion

To conclude, concrete spalls are a fact of life in Florida where there is a lot of “Salt air” in the atmosphere.  In reality, there is very little one can do to avoid any spalling to happen in a structure.  However, if one follows a typical building maintenance routine, the chances that spalling can be found and identified increases.  Buildings that are painted frequently and are maintained suffer the least from this malady.

27 thoughts on “What is Concrete Spalling?

  1. Thanks, it is quite informative

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  5. […] cause the reinforcing steel inside the concrete to rust, expand, and then rupture the concrete.  Click here for our blog on the definition of a spall. This is of particular note in a parking structure that is not only open to the elements, but also […]

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