Cracks in suspended slab due to magnesite

[tngv752]

The building was built in 1960s. The suspended slab is 140 thick and the areas near to the balcony was damaged becuase of magnesite flooring. About 50-80mm top of slab were excavated to eliminate magnesite. Top steel bars were exposed in living room and top steel bars in the kitchen were cut.
Tempory props were put under the damaged areas and between the cracks. And the balconies also were temporary props. The crack width is about 1 to 1.5mm.

The slab were poured in one piece with the length shown in the sketch is 12m by 9m.

All the wall is load bearing wall. We scan the reinforcement and I've done some draft calculation and it seem that there is enough reinforcement.


My question is:

- The steel bars must have yield strength < 350MPa ?
- The cracks were formed due to the damage magnesite areas in which reduce the slab stiffness near the supports and combined with shrinkage crackings to joint all cracks. The long bottom crack in the middle is about 1.5mm and is due to the damage magnesite area ?
- The top crack ruuning along the wall is flexural crack or shrinkage crack ?

 

[tngv752]

Bottom Cracks

 

[hokie66]

Are you the engineer evaluating the slab? What is your location in the world?

At first, I had no idea what you were talking about, as magnesite is a mineral, magnesium carbonate. Google told me that magnesite is also a misnomer for a type of leveling compound, full of chlorides, which was used years ago, and has led to chloride contamination, corrosion, and spalling.

I wouldn't think the "magnesite" would be causing the cracking in the areas remote from where it was used. Because the cracks seem to go all the way through the slab, they are most likely direct tension shrinkage cracks caused by restraint of the walls, likely combined with inadequate curing during construction.

If you don't know the yield strength of the reinforcement, it is best to assume 250 MPa. That would be true of most countries at that time.

 

[tngv752]

We are engineers to fix the slab which is damaged by magnesite flooring used to leveling the floor in 1960s. I just one to know exactly how the cracks were formed.

As I think there were already some shrinkage cracks. And then the areas were damaged by magnesite flooring (concrete cancer), it make the slab form some crack from there and joint to the shrinkage cracks.

 

[Ron]

tngv....you still have not told us where you are in the world...that might help to evaluate; although hokie66 has offered some very good insight.

Colloquial terminology only helps us if you define it. You have noted magnesite as "concrete cancer". Have you done any testing to determine the chemistry of the topping and the existing slab? Have you done petrography on either to characterize the concrete or the topping?

You must define the probable cause of an issue before you can adequately address its correction.

 

[tngv752]

We are in Australia. We are doing test of a core drill and waiting for the result how much chloride affect the concrete.
The whole topping of magnesite flooring must be removed.

 

[Ron]

Chlorides will have no effect on the concrete, but significant effect on reinforcement or essentially any metal in contact with the concrete (railing posts, embedments, anchor plates, etc.)

Further, in wet environment or intermittent wetting and drying, the chlorides will migrate, such as moving from the topping to contaminate the concrete below, and thus, affect the rebar.

 

[BAretired]

I have never run across magnesite flooring, but here is a link to a few articles on the subject:

http://www.google.com/#sclient=psy&..._gc.r_pw.&fp=165bfefa6b75e1a2&biw=853&bih=563

Ron, it appears that magnesite does have an effect on the concrete as well as the reinforcement.

BA

 

[hokie66]

BA's link only took me to Google, so I looked myself. Lots of hits, but not much definitive for engineering applications. As the OP is in Australia as I am, thought it might be specific to one part of Australia, perhaps NSW. However, according to this site, the material is still in use in California.

http://www.westernmagnesite.com/services/magnesite.asp

The mechanism of deterioration is probably that the porous material acts as a sponge, stores acidic moisture at the concrete surface, which gradually reduces the concrete alkalinity, leading to corrosion of the reinforcement, spalling the concrete. Not a good idea. Interesting that one of the sites said that Frank Lloyd Wright promoted the material as a topping over unlevel wood floors. He has a lot to answer for!

tngv, this type problem is impossible to evaluate from afar. Suggest you contact a consultant in your area.

 

[tngv752]

The core sample near the damage area is good and no sign of steel corrosion and chloride affected (a little bit at top 10mm thick). We have to excavate the top layer to eliminate the affected concrete and corroded steel and then lay the new reinforcement and new concrete on top.

 

[dik]

tngv752... which is it? "...no sign of steel corrosion..." or "...to eliminate the affected concrete and corroded steel..."

Can do a quick Phenolphthalein test for chloride intrusion...

Dik

 

[apsix]

According to the ARC Reinforcement handbook the yield strength could be within a range of 230 to 410 MPa depending on bar type, I suggest you download the handbook for more info.

Dik; you're thinking of carbonation not chloride intrusion.

 

[dik]

I was thinking of both... If corrosion, then there is likely carbonation... caused by chloride...

Statement is correct...

Dik

 

[hokie66]

Maybe it's just semantics, but I agree with apsix about the phenolphthalein. It is a test for carbonation, which is the lowering of pH in the concrete due to reaction with airborne CO2. This leaves the concrete more receptive to chloride attack, but the chloride is not what causes the carbonation.

http://www.theconstructioncivil.com/2010/02/concrete-carbonation.html

 

[dik]

Sorry hokie... I agree... semantics and wording are wrong... gives the impression that the carbonation is caused by chloride. You are correct... The depth of carbonation reduces the corrosion protection of the rebar and permits the chloride to corrode it. It is the depth of carbonation that is measured with the phenolphthalein. My apologies for confusing the matter... I knew it, just didn't say it...

Dik

 

[tngv752]

Thanks Apsix,

I've done some calculation. The steel stress is not more than 250MPa. But top steel mesh (6mm diameter at 75mm to 80mm spacing)cover is from 20mm to 60mm. With 60mm top steel cover, the slab was a litle low of moment capacity.

It is clearly that some cracks were formed at the damaged areas. (Note that the damaged areas were removed of 50-80mm top cocnrete affected by magnesite flooring and top steel).
These cracks were propagated and jointed to the other shrinkage cracks.

Three short cracks running in the short direction of the living room are likely direct tension cracks ?
Two long cracks running along the wall were flexual cracks joining o the cracks at the damaged area ?
One long bottom crack at mid span was a flexual crack originated from the damaged area and propagated through the whole legnth of the slab ?

Any ideas ?

 

[TXStructural]

From the sketches, I would not believe that the topping had anything to do with most, if any, of the cracking. Most of the cracks appear to be too remote to be related.

As far as the repair goes, be sure that you develop shear capacity across the interface from old concrete to new, by use of both surface roughness and mechanical connection. The old and new concrete must act compositely. Use a low shrinkage concrete mix. Depending upon size and thickness of the repair, a repair material might be better for this use than normal concrete.

 

[tngv752]

Guys,

Magnesite flooring was used to level the concrete in 1960s and 1970s. The material is like cork in which is soft and porous, therefore it is easy to keep and absord water. When it react with water, it release a Chloride which is harmful to concrete and in turn to attack the steel bar and make steel corroded.