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Residential unreinforced concrete foundation cracks

Residential unreinforced concrete foundation cracks

(OP)
Hi all

I am working with a construction company that's doing a reno on a 20 year old ranch style home.  I was asked to to look at putting in a large beam in the basement when I noticed two cracks in the foundation wall.  My experience with concrete is in the original design and placing stages not so much with retro work. So I thought I'd run it by you first before I carry on.  The cracks are located at one corner, one is about 10' from the corner while the other is 15' from the corner on the other side.  Both originate at a window opening.  Crack 1 is about .4" wide and one side of the crack has move about .25" out laterally.  The crack is from top to bottom of the wall at about a 20 deg angle.  The homeowner had filled the crack (with expanding foam insulation) then painted it two years ago. There was no movement in the crack in that time period as the paint was still intact.  Crack 2 had been filled with hydraulic cement (assumed) more than six years ago and at that time the crack was about .25" wide, it is now .5" wide.  It goes from top to bottom and is also about 20 deg.  There is no sign of water getting inside. This crack is pulling apart length wise not laterally.

There is a slight slope to the grade.  Grade is near the top of the front of the house while there is four feet of foundation exposed at the back (where the cracks are).

I am concerned with the one crack that does not seem stable.

What are your thoughts?

Regards
Malcolm
 

RE: Residential unreinforced concrete foundation cracks

woodman1967,

These cracks sound like they could be either temperature and shrinkage cracks or due to settlement of one part of the structure.

For either option it is common for the cracks to start at discontinuities such as wall openings because these are the weakest point.

If they are partial settlement cracks then an idea of the direction of movement can be given by the fact that the cracks are normally perpendicular to the direction of the tensile stresses (think in terms of a tensile diagonal in a truss).

If they are temperature and shrinkage cracks then they are usually caused by some sort of restraint stopping the walls from shrinking and expanding freely.

At that width, these cracks are definately significant.

 

RE: Residential unreinforced concrete foundation cracks

Woodman:

With the four feet of exposure, could be temparature/shrinkage issue here, but I think something else is more likely.

If I understand the situation correctly, with the 20 degree cracks near the corners going from top to bottom, it sounds like a rotational failure of the wall to me.  Too much pressure against the wall, for whatever reason.  

My question is, with the house only 20 years old - say 1988 UBC - why is the foundation unreinforced?  Also, please clarify here, but I am reading a 9 foot high unreinforced concrete wall backfilled with 8 feet of soil that did not fail, and a 9 foot high unreinforced concrete wall backfilled with 4 feet of soil that did.  Is that correct?  If it is, it seems funny.  

Is is possible for vehicles fo pass by the 4 foot section close to the wall?  Is there a cutoff trench in the vicinity of the 4 foot wall that might be in the process of failing?   

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Residential unreinforced concrete foundation cracks

(OP)
Mike,

This is a Canadian home. It is not unusual for that era foundations to not be reinforced. You are understanding it correctly, the 8' foundation wall with 4' of soil failed while the 8' wall with 7' backfilled did not.

The wall that cracked is in the back of the home and there is a large deck wrapped around that corner.  The way the topography and fencing is there is no way a vehicle could get near that wall so that isn't the cause.

Also, the main floor of the home is showing no signs of foundational movement as of yet.  All ceiling to wall and wall to wall corners are solid without cracks.  All windows and doors open with ease.  Floor is level and true.  

Thanks for your help
Malcolm

RE: Residential unreinforced concrete foundation cracks

Woodman,

I have assumed from your description that the foundation wall retaining 4 ft. of soil is a full height concrete wall for the full 8ft. basement height. If it is partially concrete and partially wood framed, then it is a different issue.

A couple of things that I would consider. You don't mention the width of the window openings. Keep in mind that the foundation wall will be laterally unsupported at window openings and the wider the opening, the more problematic.

Also, with walkout basement situations I would check depth of frost cover at the footings. Likely not an issue if you have not noticed signs of foundation movement in the upper portion of the house, but would be good to verify. I have encountered a number houses where during renovations homeowners have regraded around their houses for a walkout basement and taken the frost cover away from the footings.

 

RE: Residential unreinforced concrete foundation cracks

Is there any chance that the back of the house is on fill. with the crack marking the transition between undisturbed bearing soils and fill soils? Often houses built on a slope are founded on a balanced cut-fill operation.

RE: Residential unreinforced concrete foundation cracks

The wall that has the crack with the 4 foot of backfill, is this a 5 foot high wall with a wood pony wall above?

And the other wall that has supports 8 feet of fill, does it run to the underside of the floor joists above?

If so, then that is the reason for the cracking of the 4 foot wall - rotational failure due to a lack of lateral support, either at the top of the wall, or with a moment connection to an enlarged strip footing, i.e. a designed retaining wall.  

The wall that supports 8 feet of fill has the lateral support if needs at the top of the wall and consequently shows no distress.  

Mike McCann
MMC Engineering

 

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