Rusting of Rebars inside Concrete

[thewmat]

Hi,

If there is a small crack in the concrete with rebars exposed inside.. and supposed it is not exposed to water.. can the natural moister from humidity corrode the rebars? how many percentage relative humidity before any rusts can start to occur and how long before that can happen?

If there is crack no matter how small (like 1 to 3mm).. how do you plug the crack? what materials to inject into it? Thank you.

 

[rapt]

1 to 3mm is NOT a small crack.

Most design codes would limit it to about .3mm maximum.

If you have cracks that big you need to find out why first and whether the cause is ongoing. Simply filling it will not necessarily fix anything.

Find a structural engineer who knows something about concrete and cracking and employ him to sort it out.

 

[thewmat]

Oh.. our senior structural engineer has only seen it.. it's tension cracks.. so in any tension cracks where rebars are exposed (even how tiny).. can it get rusted?

 

[TehMightyEngineer]

Crack width is only a part of the reason why concrete may corrode. If you need specific questions answered I can help but overall you're asking a very broad question that has many answers and details (as rapt alluded to).

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[thewmat]

I'm just asking if the rebars will corrode inside (note: not the concrete corroding) if there is any crack width of any size (assuming it's not exposed to the weather or not exposed to any water source).. and how do you fill up the crack say tension cracks?

 

[Ingenuity]

We have injected cracks widths less than 0.01" or 0.25mm using very low viscosity (less than 200 cp) 2-component resin. The injection is done with a plural-component pump at pressures typically less than 150psi.

Before any repair is undertaken it should be evaluated first and the cause established.

See the attached ACI info: Link

 

[thewmat]

I live in a country where such 2 component resin are non-existant and we have to import the technology.. but first I need to know first the dynamics of rust formations in the rebars.. how fast do rebars corrode inside concrete with a crack opening? Remember there are many tension cracks in old beams that don't have resin injections.. do the rebars inside all have rusts?

What I'm asking is.. without direct water exposure or weather exposure.. supposed the beams are inside the buildings.. can humidity itself cause rusts in the rebars.. i'm simply asking this. Please address my questions.. thanks.

 

[Ingenuity]

thewmat,

Check out this Link to obtain some basic understanding of the corrosion of rebar process.

Depending on the exposure conditions, the quality and properties of the concrete, concrete cover to rebar, etc etc, the corrosion process is measured in years.

For an interior condition, without direct exposure to wet/dry cycles, and assuming there are no coastal chlorides, the corrosion of rebar to beams/slabs/columns/walls that have cracks that are fine in magnitude will take significant years to manifest.

Please understand that RC beams and slabs, as flexural elements, have to crack in a controlled and distributed way to activate the rebar as tensile reinforcement. Fine, well distributed flexural cracks in RC beams at midpsan, for example, are common, so unless there is specific reason to suspect that there is a corrosion problem you are probably OK. Usually concrete spalls to cover concrete occur indicating that you have active corrosion.

If in doubt, undertake some small investigation by chipping and remove the cover concrete to bottom rebar and do a visual inspection.

What is the age of the subject structure?

 

[thewmat]

Age of the structure is 3 years... at midspan there are flexural cracks. Well.. for any structure and with flexural cracks at midspan.. if rusts develop.. it's like the bars are cut in half and the beam would collapse.. doesn't this occur for those old buildings that are decades old?

Anyway. Do you know the concept of humidity and water vapors in the air. This water vapor can cause molds to develop at 60% relative humidity.. isn't there similar process in rust formation where a certain relative humidity percentage can cause more rusts or quicker rusts formation even if the beams is not exposed to any direct water or wet/dry cycles or weather?

 

[rapt]

And how wide are the cracks? If they are less than about .5mm, there is probably no problem if it is an interior environment. This would normally be allowed by the design codes without any treatment.

If they are 3mm as asked at the top, then they are a real problem, not just because of the high likelihood of rusting and subsequent failure over time. They are probably very over-stressed as well.

 

[Ingenuity]

Based upon your Member Profile you are based in the Philippines. Certainly a hot and humid environment.

I lived and worked in Guam for many years (hot and humid there too) so I understand your weather environment.

For beams in a building of only 3 years of age I would not expect to see any corrosion to rebar (except mill scale during construction, that is not a problem), assuming it was designed in accordance with modern building codes, was constructed by an experienced crew and built with durable concrete. Assuming cracks width of nominal width I would not expect there to be any long-term problems related to corrosion to your beams exposed to your humid, interior exposure. Carbonation of the cover concrete will occur (via diffusion of carbon dioxide), but this will take many years.

Many, many buildings that are many decades old (some many more than that), in more severe exposures, and are still in excellent service today. Corrosion to rebar will NOT cause "the bars are cut in half and the beam would collapse". The corrosion process is slow, the corrosion by-product causes significant cover concrete spalling due to expansion, and then the rebar will be visible. This occurs at levels significantly less than 50% of rebar sectional area loss.

Many modern repairs undertaken today to reinforced concrete with corrosion damage are undertaken whereby the rebar is replaced (or supplemented) only when the cross-sectional area loss is more than say 20%.

 

[TehMightyEngineer]

A beam in constant moisture will corrode less than a beam subject to wetting and drying cycles. Knowing the concrete quality control of the Philippines I suspect crack width is less important than the electrical resistance of the concrete. I suspect the crack will only slightly accelerate your normal rate of corrosion and such corrosion will likely be over a very long time.

If it were me, in your situation, I would fill the crack with an elastomeric sealant and then run a finger down the sealant to force more into the crack and strike it off at the surface. This will not be a full repair but will provide a more attractive finish and provide a small boost in protection.

As Ingenuity said, I'd suspect you'll have repairs in the far future but actual structural failure is a concern that is very far off.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[Ron]

Corrosion of rebar from ambient air moisture, unless laden with chlorides, is a slow process. Crack repair is necessary, but as others have noted, there is a reason for the cracks and that needs to be determined before you embark on a repair scheme. There are lots of variables in this process that you have not provided to us for comment. Each exposure of rebar has to be evaluated on its own merit.

Within the concrete, you will generally find the relative humidity to be in the 75% range and sometimes higher if exposed to the exterior. Just above the concrete, the relative humidity will be dictated by the temperature/humidity conditions of the surrounding air. The two can be vastly different. If the air humidity is less than the concrete humidity, moisture vapor will migrate from the concrete to the air. If the air humidity is higher, there will likely be no change in the concrete humidity and there is an insufficient vapor drive to force moisture back into the concrete.

 

[cliff234]

Concrete is like a sponge. So even if there are no visible cracks (or if the cracks are very narrow) water might still get to the reinforcing steel. The "sponginess" of the concrete varies depending on many things related to the various components of the concrete - cement, aggregate, admixtures, air voids, etc. Another variable affecting the rate of reinforcing steel corrosion is the depth of concrete cover over the steel. Corrosion of steel is a complex topic of which I know just enough to know that I have to rely on experts for solutions on corrosion problems.

 

[Ron]

cliff234...some moisture is almost always in contact with the rebar. Because of the alkalinity of the cementitious environment of the rebar, corrosion is inhibited by a process known as passivation. In short, water in concrete does not significantly affect the rebar unless there is a high level of chlorides or unless there is continuous wetting and drying.

 

[thewmat]

Within the concrete, you will generally find the relative humidity to be in the 75% range and sometimes higher if exposed to the exterior. Just above the concrete, the relative humidity will be dictated by the temperature/humidity conditions of the surrounding air. The two can be vastly different. If the air humidity is less than the concrete humidity, moisture vapor will migrate from the concrete to the air. If the air humidity is higher, there will likely be no change in the concrete humidity and there is an insufficient vapor drive to force moisture back into the concrete.

I bought a lab-grade Hygrometer to monitor fungus/molds growth in my optical lens.. and as I got familiarized about humidity and water vapor and how there is constant water vapor in the air at all times even in the middle of Sahara dessert.. then I got curious how this interacts with rebars inside the concrete. You said relative humidity in the concrete is 75% or higher.. where does the additional water vapor comes from? Are you saying it's trapped or absorbed from the air? My Hygrometer reads 54$ Relative Humidity in the morning and about 45% Relative Humidity in the afternoon.. from this.. what is the estimate of the relative humidity inside the concrete? You seemed to be saying the concrete can suck the water vapor from air like sponge.. what makes it do that (or physics concepts)? Why doesn't it just go into equilibrium. Also remember there is no longer any water vapor retained in the concrete as most have already reacted originally when concrete hardened after pouring.

 

[Ron]

Thewmat....read it again. I specifically said that concrete will lose moisture to the air but not usually gain it.

The moisture in concrete is inherent. Only about 1/2 the water used in a concrete mix design is needed for actual hydration of the cement. The remainder is contained within the interstices of the concrete matrix. That water usually remains in the concrete and depending on temperature variations, is maintained in either vapor state or liquid state.

You cannot estimate the relative humidity within the concrete from the relative humidity of the air. It can be tested using a specific procedure and special equipment built for that purpose. Read ASTM F2170 "Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes".