Re: REBARS inside poured concrete !

Sometimes it does rust. Especially when water (which contains dissoved osygen, as balarila explained in another thread) seeps into the concrete and gets in contact with the rebars.

When this happens, the rebars "swell" because of the rust and the concrete cracks. Much like the growing roots of a tree pushing concrete.

Re: REBARS inside poured concrete !

Kaya importante na walang voids when pouring concrete. If you observed when concrete is being poured, someone always stirs or has a concrete vibrator to eliminate voids not only to ensure structural integrity but also so there's no excess air and water.

We recently demolished portions of some pre-war structures and were surprised to find that re-bars were pristine. Excellent concrete.

Re: REBARS inside poured concrete !

G15--There is no so much interest in surface oxidation but more of it results to serious rust -cross sectional reduction. Reduced in the cross sectional area of steel reduces its strength capacity therefore it was a serious matter specially in building construction. There were 2 condition why steel bar became rusted.. Chloride penetration and carbonation - prevention is how to prevent penetrating into the concrete lies on the concrete you are using and he rate of carbonation in concrete is directly dependent on the water to cement ratio of the concrete. As for chloride penetration - this was considered seriously in bridge thats why they put the best concrete on it with very permeability properties..

Re: REBARS inside poured concrete !

Thanks guys for the replies. I will post pics later of my discoveries of my water reservoir beam and terrace slab above the garage. Yup, what joey81 said seems to make sense but of course all your replies as well, bro Ding and pyth.

Again Thanks, :groupwave:

Re: REBARS inside poured concrete !

There is a minimum requirement for concrete cover for reinforcing steel as per ACI codes.
Suspended slab - 3/4in or 19mm
slab on grade 1-1/2 in (38mm)
walls above grade - 1" (25mm)
Beam stirrups and column ties - 1-1/2 (38mm)
Exposed to earth concrete but poured against forms - 2" (50mm)
concrete is deposited directly against earth 3" (75mm)

The concrete cover must have a minimum thickness for three main reasons:

to protect the steel reinforcement bars (rebars) from environmental effects to prevent their corrosion;
to protect the reinforcement bars from fire, and;
to give reinforcing bars sufficient embedding to enable them to be stressed without slipping.

Re: REBARS inside poured concrete !

To add to Fort and Pyth's comments (and make things more confusing) 😉 cement used in concrete is also varied depending on how aggressive the environment is; e.g. Type I, Type II...

Re: REBARS inside poured concrete !

OK guys, I have here some pics I took yesterday and did some editing just to sharpen and brighten up some portions. These are what I was talking about regarding rebars oxidizing inside poured concrete. 😮

For the Water Reservoir under beam :

This is my concrete water reservoir. Do you see the lower edge of the supporting beam ?

It seems the rebars has been contaminated, oxidized, rusted and has expanded pushing out a portion of the lower edge of the of the beam, similar to what bro joey81 said in one of his reply.

What I plan to do here is to set up my scaffolding way up then do some manual chieseling ( tik-tik ) of the concrete surface up to where portions of the rebars have no rust, cut it and replace with new piece of the same size corrugated steel bar by welding and connecting the ends, after which I will apply a coat metal epoxy primer then plaster it with concrete. Will that be a sound solution or approach in repairing the said beam ? Any suggestion guys ?

Re: REBARS inside poured concrete !

And here are another set of pic with s similar problem though its not the beam but the slab.

For the under Slab Flooring of the terrace above the garage area :

This is the underside of the concrete slab of my terrace.

And here is the damage and I have no idea the extent of this rusting on the corrugated steel bars. 🙁 I guess I will be doing the same repair as similar with the water reservoir beam. What do you thing guys again ? Anymore sound suggestions ? :confused:

Re: REBARS inside poured concrete !

... up to where portions of the rebars have no rust,[COLOR="Red"] cut it and replace with new piece of the same size corrugated steel bar by welding and connecting the ends, after which I will apply a coat metal epoxy primer then plaster it with concrete. Will that be a sound solution or approach in repairing the said beam ? Any suggestion guys ?

That doesn't sound right. The rebars are already carrying some load. You can't just cut and reconnect them.

I don't know how explain it... I'm not a structural engineer. 😀

Analogy na lang. Hehehe. Imagine its a guitar string. After you reconnect it there will be some slack (rebar no longer bearing the load). You have to tighten the guitar string to get the tension back. And there is no way you can tighten that rebar. 😀

I don't think you can repair that. I think all you can do now is to prevent further damage by sealing all possible entries of water.

Re: REBARS inside poured concrete !

Parang walang spacer na inilagay between forms & rebars bago nagbuhos bro ah. kung na maintain lang yong minimum concrete cover plus vibrator during concrete pouring hindi sana kalawangin iyan bro.

Re: REBARS inside poured concrete !

@ fortnapz,

Yeah it seems that way nga, when I read your sort of table on the minimum requirement for concrete with rebars, I guess hindi nga na follow yun, considering the fact that and gumawa nyan ay isang contractor/structural engineer so many years back specially the terrace slab flooring. Very bad indeed !!!

Re: REBARS inside poured concrete !

sir rosy,
considering the location of the exposed and rusted rebars, doesn't look good nga. I suppose tama ang sabi ni joey81, prevention (of further deterioration) might be the best approach.

on this note, kung gagawa ng similar structure, or any type of concrete structure which will be exposed to or submerged in water (e.g. footings, columns, etc.), would it be wise to mix waterproofing compound on the mixture? will it have any effect on the strength of the concrete when cured?

Re: REBARS inside poured concrete !

I recall from a post by Jorge that the underside of a beam carries the most stress.

Those rebars, therefore are a risk especially with the load that it carries. Remember, Bro Rosy, that one cubic meter of water weighs a ton. Suggest you talk to some structural experts on repair. I agree with Joey that you can't cut and reconnect.

I'm not sure if applicable but you may want to look at carbon fiber with epoxy.

Re: REBARS inside poured concrete !

Hi rosy.
You seem to have a simple RC beam suffering from some spalling, and then a 2-way RC slab
suffering similarly. Here's what I would do (and indeed have done in the past):

The RC slab first:
1. Shore up the slab with a few 2x4's, each topped w/ a wooden plate against the slab
underside. Based on your photo, this is likely superfluous, but it's a good habit to form,
especially if you will face a more serious spall-repair, such as that RC beam you posted
a photo of.

2. Expose the rusted steel further, 6" out in every direction from the edge of the spall,
and 1/2 an inch "behind/above the steel". IOW, tiktik. (Some folks just punch through
the whole slab to see what's up, fix the steel, then pour fresh concrete from above
--with a plyboard form under the slab, of course-- but if you have nice existing tilework
on top of that slab, you might not want to get so scorched-earth).

3. Clean the reinforcing steel of all rust and, assess how much steel was lost to rust.

4. As necessary, splice additional lengths of steel bar to the original, using #16 GI wire:
You literally place a new rebar beside the old, and tie tightly with wire throughout
the length of splice.

5. Paint all exposed steel with an epoxy (typically sold as primer) for moisture protection.

6. Patch and refill with repair-mortar ('tis why you have to chip even "behind" the steel,
so that the mortar can totally embrace the steel). Repair mortars are low-sag specialty
products, but in a pinch you an just use old school cement and sand.

7. Address the root cause of spalling, most likely moisture seeping in from cracks in the
slab from above. Sometimes it's as simple as better/faster drainage of rainwater.

Same general approach to the beam, except you want to shore it more diligently, with
steel shoring props, and carefully restrict all chipping to the lower half of the beam (the
beam in your photo is a simple beam, primarily dealing with a distributed dead load).
Again, expose and clean the steel. Splice additional bottom bars and stirrups as needed,
Finish the steel with epoxy rust-protetion and then refill what you chipped out with some
mortar if the void is small, or strong concrete if the void is large (w/ 1/2" crushed gravel).
In signifiant repairs, the shoring isn't taken down until 3-4 weeks after the repair, when
the concrete has properly cured. Obviously, you need to address the root cause of the
spalling, likely a new waterproofing of the cistern interior.

Here's a webpage with photos showing some of what I'm describing.
http://howardridley.com/Spalls.html

Spalling, indicative of further hidden damage:

Beam shored up, and concrete cover removed to expose rebars, which are cleaned
and given additional steel reinforcement as needed:

After repatching:

hth.