Concrete Spalls: Carbonation vs. Chloride Intrusion

[bridgemaster]

Hi,

Is there a visual test an structural inspector can perform to determine if a concrete spall is caused by either carbonation (CARB) or chloride intrusion (CI)? I am not concerned with structural cracks such as flexure or shear cracks that eventually cause a spall. My understanding is that both CARB and CI spalls occur due to the expansion of corroding steel reinforcement, but the mechanisms are different for how the corrosion is initiated.

Essentially, I would like to know if there is a easy visual method to differentiating a CARB and a CI spall without lab testing of chemicals, etc. Maybe the shape of the spall gives it away? Maybe the reinforcement deteriorates in a unique fashion?

Type of concrete structures that pertain to my inspections: prestressed piles, prestressed slab beams, RC columns, RC beams, RC pile caps.

Thanks in advance for any help!

 

[Ron]

Neither carbonation nor chloride permeation actually cause spalling. Carbonation can have the effect of reducing the passivation protection of the rebar is it goes deep enough. Chlorides directly attack rebar.

Carbonation is rarely deep enough to cause a rebar corrosion issue. It can cause other durability issues, particularly when the concrete is exposed to abrasion or freeze-thaw cycling.

Chlorides can permeate almost any concrete and the effect is direct corrosion of the rebar, resulting in exfoliation and expansion of the rebar, thus causing spalling.

If there is spalling due to carbonation, it will be a near-surface condition, usually less than 1/2" inch deep. Spalling resulting from chlorides will be thicker....all the way to the depth of the rebar.

Other than the noted issues, there is no distinction between the sources of spalling that can be assessed visually.

 

[bridgebuster]

Take a look at this article, which mentions how to identify carbonation. However, it doesn't give any information for determining the cause of spalling.

http://www.concrete-experts.com/pages/carb.htm

I remembered reading about using a phenolphthalein spray some years ago to identify carbonation.

 

[DamsInc]

Like Ron said, the cause of spalling is rebar corrosion.

If you spray phenol on the surface of the concrete, that won't tell you anything about the DEPTH of carbonation which is the important factor. For that you need to take cores and have a petrographic analysis done. Ditto for chlorides.

 

[Ron]

When checking for depth of carbonation using phenolphthalien, you must use a freshly broken piece of concrete. We usually do this by taking a concrete core and then lay the core on its side as if doing a splitting tensile test, load to failure and you'll have a nice, freshly broken cross section from the surface downward. You can then apply the phenolphthalien and you'll see the color change where carbonation occurs. In the areas where the pH of the cement paste is greater than about 9, the phenolphthalien will show a dark pink/purple color. Where the pH is less than 9, the phenolphthalien will have no color effect....thus the carbonation area will appear uncolored.

 

[bridgemaster]

Thank you for the quick responses! From what I have gathered, there is no simple way to look at a corrosion spall and tell if the corrosion is caused by concrete carbonation or by chloride intrusion.