Patch Repairing Chloride Induced Corrosion.
The problem with chloride induced corrosion is that the corrosion mechanism is often not fully understood by Engineers and Clients. Whereas carbonation induced corrosion causes relatively large areas of the reinforcement to corrode, chloride contamination usually causes very localised areas of the steel to corrode, which is known as pitting corrosion. This can result in deep "pits" in the steel surface and considerable localised loss of section. Often the corrosion deposits are black and hard and there appears to be little loss of section at first, until the deposits are either dug out with a penknife, or water jetted clear, when the full extent of damage can be seen.
Repairing chloride contaminated concrete is similar to that previously written for carbonation induced corrosion except for several very important points:-
||All corrosion deposits must be removed by grit blasting or water jetting to bright steel. Failure to do so leaves chloride contamination behind in the corrosion deposits and failure will ensue.|
||All of the chloride contaminated concrete must be removed, not just the areas which have spalled or delaminated. See comments below on the "incipient anode effect."|
||A criterion often employed is to repair all areas where chloride exceeds, say, 0.4% or 0.5% of cement. Strictly, for chloride added at the time of mixing, the criterion should be 0.4% or for chloride ingressing post hardening, 0.2%. For pre-stressed, post tensioned or heat cured concrete, the maximum tolerable chloride level is only 0.1% by mass of cement.|
When an area of steel is corroding under the influence of chloride contamination, steel is dissolving causing the formation of iron "ions", tiny charged particles of iron. Simultaneously, electrons are released which flow along the bar and react at some point remote from the corrosion with both air and oxygen. Cathodic protection, which will be discussed in the next section involves providing a small electric current to the steel which prevents further corrosion. The corroding areas are supplying electrons to surrounding areas of steel effectively providing localised cathodic protection to the adjacent steel.
If you now break out a corroding area and apply a patch repair, without dealing with chloride contamination in adjacent areas, the cathodic protection system has been removed!!
New corrosion cells will rapidly spring up on either side of the repair and early failure will often ensue. One manufacturer uses a sacrificial anode tied to the rebar on either side of the patch repair. We have recommended this system on several occasions and it is, for example, installed at a car park in Colchester. We have published a paper on this project, copies of which are available free of charge.
The sacrificial anode system can only deal with limited levels of chloride, up to about 1-1.5%. It can cope with higher levels, but the lifetime would be reduced. In such cases, chloride removal (desalination) or impressed current cathodic protection may be viable alternatives.
These repair recommendations are only a guide and apply only to conventionally reinforced structures. Repair of pre-stressed or post tensioned structures with tendon damage is a specialist task and beyond the scope of these notes.
More detailed recommendations, together with specimen bills of quantities can be found in a Concrete Society Technical Report.
Regards, Harold Sprague
The only testing so far is tapping the concrete to detect detached and spalled material. The building is a block away from the beach.
I cannot picture how sealing the exposed rebar can initiate galvanic action. The steel would have a moisture barrier and I would think no longer exposed to the chemicals in the concrete, or to moisture or oxygen, similar to epoxy coated rebar. I thought galvanic action required dissimilar metals with some connecting element like water to occur.
Sika has products that claim to have the sealing and mortar protection characteristic that I believe would help here, and a CALTANS engineer I spoke to has recommended some products that they use to seal and reconstruct deteriorated concrete and no mention of concrete material testing.
Fortunately the deteriorated concrete is at the edges on the underside so this is a low stress region even though I would certainly like to halt the deterioration before the decks become unsafe.