Cracking in a storage unit slab

[jlholt911969]

I have run into a problem that I have not had brought to me before. One of my projects has multiple 40x150 storage unit buildings with continuous slabs. Each slab has a step about midway. The slab is 4" thick with a thickened edge perimeter, and the step is 12" thick. I went out and inspected the first slab yesterday and it had a crack in it along the width of the slab about 18" from the step. The crack is about 1/2 the width of a dime or less. I just received an email that the owner has 5 more cracks in other parts of the slab that are all going across the 40' width of the slab. The project is in Minnesota near St. Cloud. There is rebar in the thickened part of the footing at the slab perimeter, and only fiber mesh was used in the slab part. All the soil compaction tests showed greater than 98% compaction. I am thinking that we are having temperature and shrinkage cracking, and I am wondering if it is because of the swing in temperatures between day and night. When I was there yesterday I did not see anything that would indicate the slab was covered at night, and night time temps have been in the mid-30s to mid-40s for the last 2 weeks. Daytime highs have been in the 50s to upper 60s. Control joints were cut at 12' on center in each direction. Where are the cracks coming from?

 

[jayrod12]

So you have reinforcing in the thickened edge that terminates some minor distance into the slab, and then the slab only has fibres?

When was the saw-cutting performed? How long after the pour?

 

[jlholt911969]

The thickened edge only has reinforcing in the bottom of the thickened edge. The slab only has fiber mesh in it. The cuts were made within a day of being finished.

 

[dauwerda]

Are all of the cracks in a similar location in relation to a step? My following thoughts are going to assume that they are.

How close to the steps are there control joints cut?
If the control joints are cut 12' on either side of the step, then you actually have a section of slab that is 24' rather than 12' - this would certainly lead to shrinkage cracking in the area of the step(s).

How was the subgrade graded at the step in the slab?
If the cracks are happening on the upper side of the step, it could also be due to a reentrant corner that is created in the subgrade next to/below the step

 

[jlholt911969]

Dauwerda, I have instructed the contractor to cut a control joint at the step on the future slabs to be poured, because I thought the same thing as you. The soil at the step is sloped down so no reentrant corner unless they didn't follow the plans. This is the only slab poured so far, and it only has one step so the other cracks are in other areas, I just don't know where yet. I am planning on going out there this afternoon and seeing where they are located, and hopefully see something that may indicate further what has happened.

 

[dik]

Your sawcut joints should be 10' to 12' on centre for 4" slab... and they have to be cut at the right time... within hours, not days. 6' on either side of the step as noted... also the step would provide an anchor point to further restrain shrinkage.

Dik

 

[jlholt911969]

Thanks everyone for the assistance.