Multiple Hairline Cracks in Newer Foundation

[MiketheEngineer]

Residential concrete foundation 8'' x 8' tall pour. Has more than enough steel in it - both horizontally and vertically. Poured about a year ago.

In the last 2-3 months it has developed many hairline cracks all over the place. Seem to be getting more numerous - but not wider.

No signs of settlement anywhere. Exterior brick, drywall, doors etc. are all fine.

Any ideas??

 

[born2drill]

Sulfate attack?

 

[Ron]

Drying shrinkage.

What is the crack pattern? Any faulting? Consistent width? Horizontal or vertical?

 

[molerat2210]

Concrete cracks due to shrinkage. That is the nature of concrete. Not a big deal.

 

[fattdad]

all concrete cracks. to expect otherwise would be a problem.

f-d

¡papá gordo ain’t no madre flaca!

 

[born2drill]

I assumed the OP was familiar with concrete work and was describing the cracking as outside of what's expected for normal shrinkage. Perhaps this was not a good assumption.

 

[hokie66]

It is not correct to say that all concrete cracks. Unless the cracks occur while the concrete is plastic, cracking is due to stress caused by loading. The most common types of hardened concrete cracks are flexural cracks and direct tension cracks. For determination of why a concrete element cracks, look for how it is loaded. Walls commonly crack more or less vertically because they are in direct tension from restraint of drying shrinkage.

 

[Ron]

Unless the cracks occur while the concrete is plastic, cracking is due to stress caused by loading.

Most drying shrinkage cracks do not occur in the plastic state. You are right that they are tension induced, but that tension is often internally induced, not caused by external load application. Drying shrinkage cracks typically occur within a few days after placement, not while the concrete is plastic.

Plastic shrinkage cracks, of course, do occur in the plastic state. They are rare in vertical placement because the conditions that cause plastic shrinkage cracks are abated by the formwork.

 

[MiketheEngineer]

born2drill -

While I am quite familiar with concrete - I am not real good at concrete forensics. Hence my inquiry.

ALL concrete will ALWAYS crack - that I know and advise as such.

But these just really started in the last few months - almost a year after the pour. And there is no obvious reason. Soil test was good. Back fill was good. No cracking in any drywall or brick. Steel was more than required.

Just a bunch of hair line cracks popping up all around?? Some vertical, some horizontal, some at 45.??

No water intrusion - well graded and sumped.

Just weird!!

Bad concrete??

 

[hokie66]

Ron,
Agreed that internal restraint is a component of the force causing cracking. But I don't agree that most drying shrinkage cracks occur within a few days. It is highly dependent on the w/c ratio. For low w/c ratios, most of the shrinkage occurs early, within the first 2 months, but let the w/c ratio get up to say 0.65, and there will still be a lot of drying shrinkage occurring after a year. In these high w/c concretes, less than half the ultimate shrinkage occurs in the first month.

 

[cvg]

never seen structural concrete with that high of a w/c ratio. In fact, never seen any concrete with that high. Also, regardless of how long the shrinkage lasts, within less than a month the concrete has achieved sufficient strength to resist cracking. further longterm shrinkage should not cause any more cracking than what occurred early on

 

[hokie66]

You might be surprised how much water is in soupy mixes used for house construction, particularly in a thin element like an 8" wall. Anyway, my point is that the shrinkage varies a lot with w/c ratio, and drying shrinkage does occur over a long period, and if it is still shrinking and restrained, cracking can still occur.

 

[Ron]

hokie66...agree completely with both of your last two posts.

MikeTheEngineer...can you give a timeline of construction and occupancy? If walls were constructed a year ago, cracks probably developed soon after, but have gotten worse as house is used and "dehumidified" by AC system.

Also, as hokie66 pointed out, residential concrete is often "soupy", resulting in lots of long-term shrinkage.

 

[hokie66]

If these cracks really are in the form of random map cracking as Mike reported, alkali-aggregate reaction could be a possibility. Let's hope not, as that is a much more serious issue than some drying shrinkage cracking.

 

[MiketheEngineer]

Ron -

Poured about 12-14 months ago.

Just occupied in the last 6 months or so. So yes, we did go through a summer in St. Louis of high heat and humidity where I know the AC was probably running full time - mine was.

 

[MiketheEngineer]

Hokie66 -

Can you give me the Readers Digest version of an alkali-aggregate reaction.

While the builder said he used 6 sack concrete, only God knows what was supplied. And if it was far from the plant and I think it might have been and weather hot - it may have been watered down quite a bit.

Seen that before!!

 

[cvg]

see chapter 3:

http://140.194.76.129/publications/eng-manuals/em1110-2-2002/toc.htm

 

[hokie66]

Mike,
The following page gives a lot of references. I have never had the displeasure of having to deal with it.

http://www.concrete.org/technical/ckc/troubleshooting/articles/059.htm

 

[MiketheEngineer]

THANKS FOR ALL THE LEADS!!!

These forums are just flat incredible

 

[Ron]

MiketheEngineer...I've done petrography on ASR concrete...not pretty. I don't know that there is much reactive aggregate in your area, but check with the local ready-mix suppliers...they can tell you if any particular aggregates in your area are suspect.

The typical pattern, as hokie66 noted, would be tightly spaced cracks and some popouts where reactive aggregate is near the surface.

I'm still leaning toward reduction in humidity causing shrinkage cracking, based on your description.

Any chance of a photo or two, or a sketch of the crack pattern?