CMU Cracking from top of wal at corner

[stevenspm]

Hello,
I have an issue that has come up on a building under construction. At one of the corners there is a hairline crack starting at the top of the wall and extending down about 12', see attached photo taken by contractor. The wall is 8" CMU reinforced with #5 @ 48" oc. I do call for a bar in the corner cell and joint reinforcing every 16". The max roof height is about 17'-4" with the top of wall @ 18'-0". There is a bond beam at the roof level / top of wall. The crack goes through the head joint and through the face of block in a almost strait line. What are possible causes for this and what can we tell them to do to remedy this?
Thanks.

 

[manstrom]

Out of plane movement of the wall before the roof was on?

My guess is the wall was up and unbraced / poorly braced. Wind pressure put some bending moment right at that end joint.

It doesn't look concerning, there is horiz reinforcement.

 

[JAE]

Do you have control joints in the wall?

With a crack like that near a corner I would look into the possibility that it is due to thermal shrinking/expansion where the orthogonal block is resisting the
wall movement near the top (parapet areas where thermal movement is higher).



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[stevenspm]

JAE,
on the front wall the wall ends 9'-0" from the corner and along the wall with the crack the control joint is 24'-0" away.

 

[JLNJ]

I'm with JAE in that it seems like shrinkage, maybe in combination with wall stresses in the corner due to minor settlement or diaphragm restraint. God put in a control joint near the corner for you.

I've done my share of schools and have seen more than a few cracks which I could not really explain. Unfortunately, most owners don't want to hear the "concrete masonry shrinks and therefore sometimes cracks" defense. Owners do, however, seem to buy into the engineer's statement that such a crack is not a structural strength issue. One the building is up and at room temperature, the crack can be filled, painted, and be made to disappear (by a good contractor).

 

[hokie66]

The "joint reinforcement" used in the US is not worth a cracker, in my opinion. And it probably isn't continuous at the corner. The wall just decided it needed a control joint there.

 

[CELinOttawa]

Fully detailed bent bars and special inverted "saddle" CMUs are the norm in Aussie I take it, eh Hokkie?

 

[JAE]

One other aspect that I've seen in the past is that one of the two orthogonal walls may have spanning roof elements (such as beams or joists) that also expand and contract thermally and push and pull on one wall while the orthogonal wall resists.

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[hokie66]

CEL,
Yes, that is correct, in a nutshell. The wire ladders to fit concrete masonry construction don't exist here, to my knowledge.

 

[CELinOttawa]

Yup, same as NZ, but I had thought that was because of the seismic. I suppose it is the standard in the region...

Do you fill the cells with standard concrete using cleanouts throughout like the Kiwis as well?

 

[hokie66]

We typically use "pea gravel" concrete, usually called grout. Structural walls here are commonly reinforced in both directions and all cores are filled. We use "H" blocks or knockout blocks to facilitate the flow of the grout. Cleanouts are normally specified, but sometimes you have to fight to get them.