Rusting of Rebars inside Concrete
Rusting of Rebars inside Concrete
(OP)
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.
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.






RE: Rusting of Rebars inside Concrete
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.
RE: Rusting of Rebars inside Concrete
RE: Rusting of Rebars inside Concrete
Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com
RE: Rusting of Rebars inside Concrete
RE: Rusting of Rebars inside Concrete
Before any repair is undertaken it should be evaluated first and the cause established.
See the attached ACI info: Link
RE: Rusting of Rebars inside Concrete
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.
RE: Rusting of Rebars inside Concrete
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?
RE: Rusting of Rebars inside Concrete
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?
RE: Rusting of Rebars inside Concrete
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.
RE: Rusting of Rebars inside Concrete
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%.
RE: Rusting of Rebars inside Concrete
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
www.americanconcrete.com
RE: Rusting of Rebars inside Concrete
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.
RE: Rusting of Rebars inside Concrete
RE: Rusting of Rebars inside Concrete
RE: Rusting of Rebars inside 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.
RE: Rusting of Rebars inside Concrete
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".