New slab cracking concerns

[SarcasticDwarf]

Unless he has a contract specifying compaction, comp strength of the mix, w/c ratio, slump, crack control, flatness, etc.... he really doesn't have much of a claim.

That is the risk of being your own contractor, not really knowing the basics and not overseeing the subs.

He may have some success in pointing out that there are industry standards. For example, I believe ACI 318 specifies a certain level of flatness for a garage (which is not particularly flat).

 

[Supergumby5000]

I didnt read every single post in this thread so my apologies if this post is just repeat information.

I'm a concrete guy. That is definitely shrinkage cracking.

Some people get lucky with decent sized slabs, typically over some type of slip surface (nice quality vapor barrier) so when the slab cures and shrinks, it slides and does not crack.

More commonly, a slab of that magnitude without any type of crack control WILL crack like that. Either the friction against the ground underneath OR the size of the thickened perimeter and/or grade beams did not allow the slab edges to move with the shrinkage. Thus, you have a nice crack right around the midline of your slab. Yes, that crack will wander to follow the weakest points in the slab (could be concrete quality, thickness of concrete, etc etc).

For a slab that size there should have been some type of crack control. Most commonly you would either have 'soffcut' joints (more commonly referred to as saw joints) in the field of the slab and sometimes edge relief (such as a zip strip).

Fortuntely for you it looks like the cracking is in one direction. You could cut the slab in the opposite direction to avoid more cracking but its a pain if you dont cut it when the concrete is fresh. We typically cut our slabs within 24 hours of pouring with soff cut machines.

Considering the ignorance of the contractor with sawcutting, settlement could also be a contributing factor but based on the pictures I saw in this thread, I think it is primarily just lack of relief joints.

For commercial slabs on grade, we typically cut our slabs at a 15-20ft grid (for slabs without any interference, such as concrete columns or walls)

EDIT: If you were to go ahead and get that slab cut, a good dry finisher should be able to hide those cracks. They wont go away 100% but they can be blended quite well. Most shrinkage happens within the first 8 weeks. Dont do any remedial/dry finish work before then. the longer you wait, the better.

 

[walrus]

The last post is why you need to find a concrete guy to come and give you a second opinion. Maybe there is some way to salvage the pour.???

 

[Supergumby5000]

And for the record, there are a ton of variables here. Shrinkage, settlement, concrete quality, rebar and/or mesh, how the rebar was tied, temperature.....

The sawcut joints are a concrete contractors way of accounting for all those issues that create cracking. By putting in sawcuts, you create weakpoints, so when something isnt right in the concrete, the crack will be hidden in the cut.

 

[BadMannerz]

These are all excellent points that have been brought up. I went with a contractor that did the entire job for me that came highly recommended, I even went and personally viewed other buildings in the area that he constructed. I was impressed by his work, and attention to detail in the two buildings I saw.

I have a concrete professional with 35 years on the job coming over this weekend to give me his opinion.

I can deal with just a crack in the floor..... What I cant tolerate is a void in the floor with a step in it. That's what scares me, how far this might go.

If a builder was confident with his work than he shouldn't be scared to come and look at a problem. At least that's my opinion.

 

[daveindenver]

Hi! I know this thread is several years old but I am very curious about how this ended up. Can you provide an update?

 

[couch67]

OP hasn't been active for a few years...