OT: Concrete Cracking

[TomMoorehouse]

ok so they poured the concrete for my garage last week. Today I noticed two sizeable cracks developing. How bad is this? They have not even built the garage yet!

 

[jeff baham]

cement will crack, what is the depth of the slab, 2'',4'' 6'' and did they dig a footing around the form boards and across the middle of the slab. did they put rebar rods and wire mesh in the cement.

 

[wswatzell]

Concrete will naturally crack, because it shrinks when it cures, & due to ground shifting. It will also crack due to the freeze/thaw cycle in the colder climate. But you really shouldn't see "sizable" cracks in the first couple of weeks. Did they compact the ground under the concrete, & did they add at least 4" of sand on top of the compacted soil before pouring the concrete? This might cause early cracking, & it will only get worse if they didn't compact the soil 1st. If you have had a ton of rain like we have had here, the ground will swell, & as it dries out, it could cause the concrete to crack.

Another possibility is that the concrete company made the mixture wrong.

Either way, you should only see very minor cracking in the 1st couple of years.

 

[TomMoorehouse]

they did put down a crushed stone base. they did use a wire reinforcement..... i would not call it a mesh kinda like 4" X 6" squares or so. no rebar. no compacting was done. it should be 4" of concrete. the crack is right at the footing at the side and goes through the entire footing and about 4-5 feet into the slab. we have been getting a lot of rain. I also saw another crack through the footing on the other side

 

[wswatzell]

That wire is called welded wire mesh, or re-screen. That is very typical for flat work like driveways, & floors. In my area, sand is code for the base for concrete, because it remains more stable during freeze/thaw. This might be the culprit.... too much rain. it is possible that you got so much rain, that the slab floated while it was still green (1st day or 2), or the rock since it was not compacted was washed a way in a spot causing a void under the slab. Any void under the green concrete, could cause premature cracking. I would tend to believe this more that the 1st thought. Concrete has hardly any strength in tension, that is why that add steel to it to make it stronger in tension.

 

[wswatzell]

Ohh, & for advice, call you concrete guy & get him out there to see it, & if you don't like his answer, call someone else for a second opinion, like a building inspector. Around here, the floor usually isn’t poured until after the roof in on, that the flat work doesn't get messed up.

 

[Tony Jacuzzi]

In Ontario the floor would be poured after the the roof is on. Did they cut the floor while it was green?

 

[Mike G]

I would get them to fix (replace) it or it will only get worse in your area.

 

[nitrotoys]

Did they put any expanchion joints in your slab?

I would call the guy out and have him look at it.

I know if you pull a chain over the concrete, and you hear a HOLLOW Sound, TO call the guy back, You will hear a difference in sound when you pull a chain across it, and like a hollow sound you have a air pocket.

Tom

 

[Don Ferrette]

The floor of my old shop (30 x35) was poured AFTER walls & roof were up. We used a mix of sand & 21A grade gravel base and used a 2 man tamping machine to thoroughly pack it down. There were no expansion joints in middle but they were 3/8" on all outside floor to wall joints. Floor was poured 4" thick & never cracked until the day I moved out (4+ years) & I'm sure it won't. My guess is they also may have added too much calcium to accelerate the drying time plus you should ALWAYS tamp the gravel base so it doesn't shift under the weight of the concrete. Don't accept this cracking, call the county inspector out if they won't completely re-do it (you can't "patch it"). <_<

 

[CySlyde]

When we built my buddies uncles garge at thier weekend cottage, the flat cracked about a week after pouring.. very simular conditions to yours... we called the guy and he said it was ok. when we went to put the door on at teh end, we noticed teh corner where it had cracked was about 3" lower tehn the rest of the corners... ended up having to have it all ripped out and starting almost back at square one. really sucked. while we were building the crack got worse and teh base got washed out due to high levels of rain that seeped through the crack

 

[Gordon Dawson]

If concrete cracks after pouring it is because it cured to fast. The best way for concrete to cure, is very slowly. Keep it watered down for a day or two and pur it when it's cool. The best method is to water down the concrete and then put plastic sheeting over it for a couple weeks or longer. The concrete will be way stronger and won't have any cracks from curing.

 

[izitbrokeyet?]

Tom,

This is one of my professional areas of specialization.

Without knowing too much about your particular situation:

you really don’t have a problem unless there is a sign of vertical displacement or if your cracks are wider than 1/8"

Please feel free to contact me directly if I can be of assistance.

KB

 

[aaron alberico]

Tom

im a concreter all concrete will crack, did they put any expansion joints in the slab? the reason why it cracked at the footing is usually the concrete is thicker at the footing then the rest of the slab. the two different thickness cause a week piont. but would'nt worry bout it.

 

[not2bright]

of it is more of a cosmetic problem then structual then you could always use jetset to fill in the cracks. i have done a few pours and know a some about concrete because my dad does highend concrete work and he could comment on this but he is in alaska for 2 more weeks