Settlement or Shrinkage Cracks in Silo

[Ron]

Have an issue with a heavily loaded mat foundation and the silos supported by it. About 1 year after construction of the silos (4 silos in cloverleaf group on 1 mat...each silo about 40 feet in diameter by 180+/- feet tall)"caking" problem developed with cement near top of silo (upper 50 feet). Problem obviously from water intrusion, so inspection of silo done and cracks found. Mostly vertical to steep diagonal, with at least one circumferential horizontal crack. Cracks are tight (0.006 inches,max).

Silo Design engineer says cracks are from differential settlement. I disagree and say cracks are shrinkage, since no evidence of any significant settlement(max measured at construction of 1/4-inch), and no evidence of differential settlement. Concrete at crack level is 10 or 12 inches thick and was slip-formed. Mat foundation is 8 feet thick and extends only about 5 feet beyond the silo walls in size. I contend this is too small for any significant bending to occur in the mat, particularly reverse bending as is claimed by the designer.

Does anyone have experience with cracks in cement silos and have any comments relevant to the claimed bending/differential settlement?

Posting this in Structural and Geotechnical also.

 

[Gourile]

Hi Ron!
I faced cracked water Tanks and went through all assumptions as differential settlement, shrinkage and even bacterial corrosion on the reinforcing steel. It is obviously very difficult to reach the real reason. But you should look at clues. If the cracks are neatly ordered i.e. horizontal circumferential or vertical, there would be no mechanical or material reason, as they cause most of the time randomly directed cracks. The main reason would be of bad manufacturing when dealing with construction joints. I mean in places where you pour new concrete on an old one. You also should check the place that reinforcing bars are overlaped. If they are overlaped over a vertical line cracking is very likely to happen, especially when the overlapping length is not enough. If the cracks are so separated, you should beworried about lack of reinforcement. You may do soundage or even a radiography to see if there is really reinforcing bars up there! or the contractor just didn't put them there!!
Good luck.

 

[Ron]

Gourile,
Thanks for the input. We went through a similar thought process in developing our probable cause. Our major task was to convince the design engineer that the cracks were unrelated to settlement because of their orientation, spacing, and location. Given that premise, we offered that through further investigation we could likely give them a good reason for the appearance and size of the cracks.

We were convinced that the cracks were not settlement related, based on field observation and some preliminary evaluation. We floated this issue to a couple of forums in Eng-Tips to get a "blind" peer review.

As usual, you and our other colleagues came through as expected.

Thanks,
Ron