Sudden, localised, settlement 6

[CELinOttawa]

Hello All,

I'm stumped. Just got called to site where we've been working on a condo with a single storey basement garage which needs midlife repair due to delaminations typical of a garage where tires are constantly tracking in salts and water, etc. We did a preliminary report some weeks ago about the main bars and confinement steel being corroded and in need of replacement in a number of columns. Pretty typical and ho-hum.

This evening and tonight are where this gets interesting. I got a call stating that an owner in one of the condos was complaining of cracks in their walls and that their door was sticking. I dropped everything and reported to site, fearful of seeing distinct pattern cracking in the slabs, evidence of rotation, or other evidence of structural issues.

What I have found looked for all the world like differential settlement, the type I'd expect in a timber structure due to bad frost heave or terrible compaction of the granular subgrade. Doors are titled and jambing, cracks have formed where gypsum infill (non-load bearing) wall meet structural beams, some minor pattern cracking in the CMU walls, all pointing at settlement of a discrete number of INTERNAL columns along two grid lines.

The clients who have been affected stated that the troubles with doors was worse on the week-end and has improved.

Here's the rub: In forty years, this settlement is new, with evidence appearing in just the last two to three weeks. I'm stumped, and so is my Geotech consultant. He agrees it is differential settlement, and has no idea why it would have started suddenly. Where do we go from here? I was so tempted to order temporary propping, but there is simply nowhere I would think to prop. The columns and slabs above look fine; The settlement is happening far enough apart that the slabs are accommodating the change, it is the gypsum and CMU infill walls that can't take the delta...

The plan, so far:

- Get original drawings from the city and review load paths in detail.
- Do a preliminary information search based geotech assessment (by others)
- Tape ends of all cracks and observe for change.
- Re-review on Thursday to see if any changes can be noted.

I don't mind telling you all that this one gives me the heebee geebees. I don't like *really* not knowing what's going on....

 

[CELinOttawa]

Still getting worse... *sigh*

 

[Ingenuity]

From the photo, is that a recently installed post shore and new asphalt patch? are those vertical column (wall?) cracks new?

 

[hokie66]

I would start looking at failure of the column itself, with resulting deflection, rather than footing settlement. If that is the case, the building will need to be shored quickly.

 

[CELinOttawa]

The left hand crack is old - The right hand crack is new. When I saw this today I immediately ordered that the column be further propped.

This column is nowhere near the areas where the cracking has occurred above, and the areas that have cracks are on columns which show no deterioration. It is perplexing, but I continue to believe this is corrosion of the steel popping the concrete, and the cracking is likely geotechnical in origin.

The plot has thickened as to cracking: I have found other condo owners who have had similar cracking in the past, may over a good number of years. It seems that we just got a batch all at once, so again this indicates geotechnical as likely. Cracking between the concrete structural elements and linings is an old symptom, the cracking in the columns is very new.

I think the confinement ties may have rusted through, and the column may in fact be failing. It may also be the supporting thickened concrete slab which is failing. Either way, first thing tomorrow is the fastest anyone can prop the column.

I've never seen one this bad, and I am not impressed. It is not a good situation. I'm specifying shoring for six of the columns, six props each. It will make the repairs a touch harder to do without re-shoring, but it will also make me feel a great deal better.

 

[Ingenuity]

It should be quick and easy to determine if the new/existing delaminations are due to column tie corrosion - take a masons hammer and break off an already delamed chunk of concrete and observe what is behind it.

Your masons hammer is on the top shelf, right next to your slide rule No engineer should be without one - if not to at least hit sense into the occasional architect.

 

[CELinOttawa]

I've already had a good look under the cover on two other columns. The main bars are badly corroded and the ties are terrible... Without questions there is severe rust jacking occurring at this property. That said I am still not discounting the possibility of other problem(s), particularly given the cracking above.

I'll keep observing and recording changes. This column is going to get heavily propped this morning, and repaired as quickly as possible.

 

[Canuck67]

I do not want to take away from any further structural investigation that is required as I am not familiar with the project specifics, but, the cracking in the photo is very, very typical for corrosion induced concrete delamination at column bases. It also looks as if someone in the past has applied a concrete sealer to the base of the column. These areas of cover concrete are cracked and detached from the main column already, no harm in knocking them off with a hammer to investigate the steel. If the depth of concrete deterioration does not go past the vertical steel, the column capacity should still be sufficient. I bet the ties and verticals are corroded. I would consider this repair an add-on to the real investigation to determine why the structure is moving. Perhaps as the structure is moving, load is being shifted around based on stiffness.

 

[CELinOttawa]

Absolutely Canuck... I agree wholeheartedly. I'd go so far as to call the corrosion of the main bars and confinement steel "textbook".

The propping has been ordered because of the number of unknowns, the 1970s flat slab amongst them, and because I want to be able to safeguard the residents while figuring this out.

I called in another engineer for a consult by phone and he agrees the props are prudent (thanks Hokkie for confirming my pull of the trigger there) and that he couldn't think of anything else that should be done. Now I just hope the city gets back to me with the original plans so I can do a good check of the columns. With the framing of the post-explosion steel beams, I suspect they may be near their Max and the deterioration is the straw on the proverbial back.

 

[CELinOttawa]

Just to keep this up to date, we had a job showing on Friday past. The contractors practically laughed themselves to death... The columns look fine, for the most part, but are heavily delaminated. A repair program was done in 1996 which has managed to keep most of the cover looking good, but not arrest the corrosion at all. What a frustrating job!

I can count my blessings:
- The Owner is taking it seriously.
- The City is giving us the time to address the issue.
- Shoring is in place and either this, or coincidence, has arrested the propagation of cracks.
- Cathodic protection is in. Actually I had advised this be ordered to minimize lead time, and they got it to us in two days.
- Ground water analysis has shown that there is *not* a deleterious element to the native location (at least not when my Geotech took samples)

To do/Remaining:
- We have found reference to the original Geotechnical Report. Despite having been done by one of the "big boys" who are still around (Patterson), I've not been able to find just *who* at that company I need to speak with to get an old report.
- The client now wants us to issue and administer the CCDC-2 contract. I don't mind, but it wasn't originally in our scope, so I want to try and get this done without causing any delay(s).
- Find a source of Galvanized Welded Wire Mesh. I don't want to use black bars when this structure has already been repaired on two occasions. I feel it would be a disservice to the client to omit the galvanizing.

Hope that interests someone... I just hate it when we put all this time into helping someone out and the thread simply goes cold and silent. At least this way the thread may be of some "lessons learned" value.

 

[TehMightyEngineer]

Definitely interesting, I too like to see the "what happened after" part of the story.

Maine EIT, Civil/Structural.

 

[Ingenuity]

CELinOttawa

Interesting, indeed.

Curious, what is the intended use of the Galvanized Welded Wire Mesh? For slab repairs?

What type of catholic protection are you specifying?

 

[CELinOttawa]

The detail I've specified is four corner bars, with stirrups around to replace the lost confinement. The midline cannot get new stirrps, and I don't believe we could get bars along the long sides of the columns without cutting existing steel. The galv WWM will wrap the existing column inside the new main bars and stirrups, giving us both additional steel to spread out the cracking (SLS) and more conduction for the cathodic protection.

In this case I'm specifying Vector XPT anodes.