New Foundation Drying Shrinkage Cracking 1

[Melky]

Sorry Guys re-post! I learned my lesson, never post in the middle of the night!
Just trying to resolve a debate about this and would like to get some opinions

Question/Questions....
New house foundation poured 6 or 7 months ago. House was 90% completed in mid June when construction stopped due to work restrictions during summer months.
Hammer Law in beach association. Everything was perfect at that point. During that time a dehumidifier was left running in the basement to try to keep hardwood flooring (above) from gathering moisture and cupping.When site was visited mid July a couple of small cracks where noticed. One under a window and one near an outside corner. Both small hairline vertical. No big deal. During the last couple of weeks more cracking pretty much all some what vertical. They seem to be popping up all around as a tiny crack at top of wall, some seem to be moving down the wall some don't. The first two cracks have made their way through wall, everything else is just interior surface so far. In June foundation appeared to be crack free. Hardened Dry Shrinkage that was delayed? Could the dehumidifier have caused this by pulling moisture out of the wall to fast? Could they have created an issue trying to resolve another? I know they put an additive in mix for curing and was told it hardened up pretty quick. Could that be the reason it is shrinking now? The dehumidifier has now been shut off. Walls cracking/hardwood cupping. Would epoxy injection of cracks slow them or create more?
House structure is fine, minus hardwood. Should this subside or is there more to come? Any Advice.............

 

[Hoaokapohaku]

This certainly drying shrinkage cracking, exacerbated by differential moisture on opposite sides of the wall. More shrinkage occurred on the inside than the outside. The concrete might curl, except for that it is restrained by the ground, the reinforcement, and the corners, so the concrete cracks an the side with the greater shrinkage, the inside, and near the top where the concrete is exposed to the dry environment on two surfaces. Dry shrinkage cracking can continue for years, but most of it occurs in the first 6 months. In this case, very favorable conditions were provided for this type of cracking to occur during the time the concrete was most vulnerable. Then, the conditions were changed to more normal, where the humidity on the inside and outside of the foundation are closer to the same. Now, as the concrete is returning to moisture equilibrium, the existing cracks will probably widen and new ones may appear. Once the foundation returns to moisture equilibrium, probably further shrinkage cracking will be minimal.

Other factors that likely play a part are below.

You do not mention anything about the design of the concrete, or whether it produced in compliance with the design. I have no way of knowing except what is typical when there is problem concrete, but I would bet the concrete had a high water/cement ratio as placed.

There is no such thing as an "additive in mix for curing." This, combined with your statement that the concrete "hardened up pretty quick," tells me the additive was most likely an accelerator. I don't know your climate, but you say the concrete was placed 6 or 7 months ago - January or February. This may have been cold weather concreting, and if so, calcium chloride may have been used as an accelerator. Calcium chloride can greatly increase drying shrinkage cracking.

Somebody thought the additive was for curing, but was mistaken. So, how was the concrete cured, if at all? Concrete with no early wet curing shrinks far more that concrete properly cured. If the contractor thought that an additive was providing curing, perhaps the concrete was not properly cured.

Epoxy injection should neither slow nor create more cracks, but the epoxy injection should not be performed until the concrete moisture has returned to equilibrium and cracking has all but stopped.

Your statement that the concrete hardened "pretty quick," combined with the fact of the cracks appearing, makes me more concerned about potential improper use of calcium chloride than I would be about the shrinkage cracks alone. Just how quick was "pretty quick?" The maximum dose for calcium chloride is 2% of cementatious materials. More than that will cause corrosion of the reinforcement. The cracks themselves may not be a structural issue, but corrosion of the rebar will be.

You can find a very good discussion of drying shrinkage cracking from a standard reference at

http://dc102.4shared.com/doc/5Ta79y2n/preview.html.

See page 260 under "Moisture Changes (Drying Shrinkage) of Hardened Concrete."

 

[Melky]

Wow Great Job! You confirmed some things for me.
Yes, 1% calcium was used. Connecticut weather. It was pretty mild in Feb. here, but the nights could get cold. They probably just felt better with a little something in there. Foundation hole was blasted scraped down to bedrock with a 6" stone base under footings. The footings were the only thing that had rebar in them with no calcium in mix. Pretty typical design up here. Most houses don't have any reinforcement up here. I should mention that the basement isn't a full one, walls are 10" thick and 5'-6".
I'm sure they added some water during pour. Witnesses told me it didn't seam like a lot. The mix was 3500psi and the mix had 50% 3/4" and 50% 1/2 aggregate, which probably didn't
help. Contractors some times use some 1/2", because they believe it counter act the damage of possible cold joint and not having to do vibrating. There probably didn't need that much extra water for pour, but smaller mix aggregate and some calcium probably weren't the best thing. These guys grind everything down after they strip panels, so your left with a very smooth foundation. At the time they were doing this they mentioned the foundation was pretty hard. The basement is not heated, only gas furnace down there. Should another bunch of new cracks be expected come winter or will this be the worse of it in your opinion? The foundation has nine different walls to it the longest being 32'. Each wall has aprox. 3 or 4 cracks on it with most being smaller top of wall. Will all these cracks end up being full height of wall and crack completely through the wall before they subsides? If all cracks are epoxyed will the foundation be pretty much restored? Is it possible to have total failure in these type situations? If monitored and repaired in the next 6 months will they be out of the woods or should it be watched for years to come? Interesting how fast a wall can curl, I thought it took much more time.Oh one last thing is it ok to run ac upstairs,they are still trying to save hardwood?
Can't thank you enough Hoaokapohaku

 

[hokie66]

What was wrong with your other thread? It is still there, with Ron's answer.

 

[Melky]

Brand new to site! Thought I was in the wrong forum and then like a dope I re posted in the same forum as the first. Was having a debate about this and Ron's answer was more in line with the opposition. Not being super similar with this shrinkage, I just felt the dehumidifier probably made the conditions totally different on opposite sides of the wall.
Sorry about double post

 

[Hoaokapohaku]

Glad to help!

Regarding your further questions and concerns, see below.

Yes, 1% calcium was used. That's ok. If that is all that was used, you don't need to worry about it, except that it probably has made the problem somewhat worse than it would be if it was not used. No need to worry about corrosion.


I'm sure they added some water during pour. Witnesses told me it didn't seam like a lot. What do the witnesses know about concrete? What may not seem like a lot to them could be enough to significantly weaken the concrete and make it more susceptible to shrinkage cracking.

The mix ... had 50% 3/4" and 50% 1/2 aggregate.... Course aggregate size used not a concern, unless they didn't consolidate the mix or they constructed cold joints.

Should another bunch of new cracks be expected come winter or will this be the worse of it in your opinion? My opinion is that you have already seen, or are presently seeing, the worst of it. I think if it has not stabilized already, it will very soon. The period of maximum vulnerability has already passed, and the condition that created unusual drying probably already stressed the concrete as much or more than normal seasonal moisture variations.

The foundation has nine different walls to it the longest being 32'. Each wall has aprox. 3 or 4 cracks on it with most being smaller top of wall. Will all these cracks end up being full height of wall and crack completely through the wall before they subsides? I doubt it.

If all cracks are epoxyed will the foundation be pretty much restored? Opinions may vary, but my opinion is yes, as long as it is done properly.

Is it possible to have total failure in these type situations? Ummm...well anything is possible, but I don't think it is at all likely.

If monitored and repaired in the next 6 months will they be out of the woods or should it be watched for years to come? If it were my house I'd keep an eye on it, but I think that once the repair is done it shouldn't be an issue. Also if it were my house, if the contractor failed in any way that contributes to the problem (too much water, improper or no curing (this is the main one), using dehumidifier in space where concrete is curing, etc., I would insist on an extended warranty against cracks in the foundation and their consequences. Five years probably wouldn't be necessary, but that's what I would ask for.

Interesting how fast a wall can curl, I thought it took much more time. In this case, the cracking on the inside occurred because the concrete was mostly restrained from curling rather than from actual curling. If the concrete could have freely curled, most or all of the cracking would appear first on the outside of the foundation. Curling and shrinkage cracking are both caused by volume changes in the concrete, and volume changes begin almost immediate after placement and are the largest during early the early curing period. Therefore, curling and cracking happens most in early curing stages. For some idea of where you are in the process, see Figure 15-15 in the reference I provided in the earlier post. In your case, the curve is probably steeper because of the artificially induced environmental conditions.

Oh one last thing is it ok to run ac upstairs, they are still trying to save hardwood? I think so, but minimize or eliminate air exchange between upstairs and down. You don't want to dry out the concrete again. Something like a few "kiddie pools" partially filled with water might help maintain the humidity in the basement. I doubt whether that will save the hardwood though. Is the wood cupped with edges up or edges down? If with edges up, I think the AC will make it worse because the wood is dry at the surface, more moist at the bottom. The AC will dry the surface in constant contact with the air even more causing more cupping. What is needed now for both the concrete and the floor is to allow everything to get back to normal equilibrium.

 

[Melky]

Life Saver!!!
I happened to have another guy with me when I did the inspection. Very different ideas of what we each were seeing and how to proceed.
I did a lot of reading on it since. I just needed someone who has had experience with it to tell me if what I am reading and seeing are true.

The Builder...who does really nice work in my opinion, is more upset than the owner. He will probably stop by there weekly for the next 5 years to check on things.

I got a couple of more things in my head and I will stop bothering you!
The curling makes a ton of sense now. The biggest crack, which comes down near an outside corner. 1 to 2 ft away. Looked like a settling crack. Wider at top to hairline
at bottom. Top width of crack slightly smaller than an 1/8 " wide. The crack started at an anchor bolt over a panel tie (weak spot) Slightly angled to vertical running, but what was interesting is the 2 surfaces are slightly off set. Cracked at the weakest point and opened off set because of curling. Its all starting to make sense.
I knew it wasn't settlement, that place sits on one big rock. When I looked at one of the other outside corner, it shows signs of multiply hairline with similar directional forces..
Clarification...Would epoxying some of the smaller cracks now keep them from getting any longer or will it just create something new near it? I did read about timing of the dry shrinkage and most agree with you. Some say 1 or 2 years a few say longer for heavy cracking period. I'm putting my money on you! Almost forgot, section of slab showed signs of curling too. I think you are right about hardwood too. It is cupped with moisture 13% in sub floor and 8% or 9% top of flooring. I was wondering about running the heat, but everything stable with surrounding will probably be best. What about moving dehumidifier up stairs in house or do you think it would still pull from basement? I think they would rather replace floors than a foundation at this point. Good idea on pools. Could open the 3 windows down there. Their dealing with two completely different animals one needs moisture and the other doesn't. During the fall, winter and early spring the relative humidity level would probably remain pretty favorably for the foundation situation I would assume. Its walking distance to the ocean, so they have no short supply of moisture. Is there any additional treatment or favorably moisture % that should be maintained especially during heating season for basement. Thanks for the great advice! everything makes total sense, just hope the end is near!