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Question for those concrete guru's out there.

Don Baer

Don Baer

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somewhere east of Queen Creek, AZ - South East of
LOML and I were out at the new house today and she noticed a crack iiin the floating slab in the house. The slab is 60 ft wide by 50 feet from to back. The crack apears to go all the way from front to back. I am not to concerned about it since I figure that a floating slab that big would be bound to crack anyhow but LOML is realy excited about it. Is this normal on a floating slab?
 
Don, I either pour the concrete myself or sometimes on bigger pours, sub it out, but either way there should have been some control joints in your floor...do they use those where you are located?
 
It figures...the only crack in the floor and it isn't in the control joint :rofl:

Old boy told me once he guaranteed all his concrete to not crack....until he got out to the road.

In my opinion, other than the crack being unsightly, it will not have any effect on the strength of the floor. Doug Sinjem(sp) pours a ton (literally) more concrete than I do, maybe he'll offer his opinion.

I have a 32 x 48 shop with 3 control joints and it cracked....right in the control joint, I've poured others that cracked, just like yours, away from the control joint....go figure :huh:
 
Did you gents install post-tension cables? Did you have piers drilled to support the slab?

Add these two features and you can forget about watering around the foundation in the summer.
 
Ken Garlock said:
Did you gents install post-tension cables? Did you have piers drilled to support the slab?

Add these two features and you can forget about watering around the foundation in the summer.
Click to expand...

Ken
Here is a thread that you can see how the foundation was poured.

http://familywoodworking.org/forums/showthread.php?t=42

There are no pillars and no post tensioning. This is basically how my house in California was built.

The foundation was done first then the outside footers then the floating slab. I would think the footer would contain the slab
 
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Don, Slabs are going to crack, How much depends on alot of different things .
1. how much fill is under the slab.
2. What kind of fill is under the slab.
3. how much moisture is in the fill at the time of the pour.
4.how fast the slab dries.
And I could go on and on but there is no exact science on what a slab will do. When the slab is poured the mason makes a guess where the joints should be and how many there should be. Sometimes they get lucky and sometime they don't. When the rebar steel or steel wire placed where it should be the crack will not be a problem.Tell your wife not to worry and enjoy the house.

Hope this helps Donald Wood

On a lighter note, you could soak it in DNA and wait a few weeks:rofl:
 
Don, my dad (a concrete engineer, among other things) has always said there are two rules about concrete: It gets hard, and it cracks. From the sounds of things, this crack is nothing to worry about, just as you suspect. The fact that it's 18" away from a control joint indicates the masons didn't get the joint in the place where the concrete wanted to crack. In general, concrete slabs want to be in squares. If you pour a 10' x 20' slab 4" thick, in most cases it will crack into two 10' x 10' slabs.

And not to be contrary or anything, but I disagree a bit with some of Donald's points, based on my experience in concrete testing and foundation inspection. My comments are in red below...

Donald Wood said:
Don, Slabs are going to crack, How much depends on alot of different things .
1. how much fill is under the slab.
2. What kind of fill is under the slab.
The quantity and type of fill is not nearly as important as how well the fill was placed. Nearly any engineered fill can be made to be stable, but it's a matter if placing and compacting it correctly, and at the right moisture content.

As an aside, I've seen a case where a house was built on 20' of engineered fill, and when it settled 8" from one end of the den to the other, we determined that it wasn't the fill that was moving, but the loose native sands 20' to 30' below the fill. The native soil was stable until the homeowner started overwatering her lawn. She was watering the equivalent of 8" of rainfall per day -- our drill rig found saturated sand at least 30' down.

3. how much moisture is in the fill at the time of the pour.
Other than right at the surface, the fill moisture has very little to do with surface cracks in the slab. If the fill's bone dry on the surface, it can cause cracks as it sucks the water out of the bottom of the freshly-placed slab, but once you get an inch or two below the surface, the fill moisture doesn't play much of a role. Unless, of course, the fill was not properly placed, but then it's more of a compaction problem than a moisture problem.

4.how fast the slab dries.
This can account for hairline cracks running different directions in the surface of a slab, but generally not continuous cracks that run through the slab like Don described. You sound like you have experience with concrete, Donald, but for anyone who's interested, here's a pretty good overview of the different types of concrete cracks.

http://www.cement.org/tech/faq_cracking.asp

And I could go on and on but there is no exact science on what a slab will do. When the slab is poured the mason makes a guess where the joints should be and how many there should be. Sometimes they get lucky and sometime they don't. When the rebar steel or steel wire placed where it should be the crack will not be a problem.Tell your wife not to worry and enjoy the house.
I fully agree.

Hope this helps Donald Wood

On a lighter note, you could soak it in DNA and wait a few weeks:rofl:
But where would you find a bucket big enough to put it in?
Click to expand...
 
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