Bondek Slab - Brick Failure

[Warramoos]

Hi All,
I was looking at some brick failure the other day, and thought I'd ask if anyone had seen similar and or know the cause of the failure. See photos below.
There is an old bondek slab at the front of a house forming a small veranda. The bricks have localized cracking where it interfaces with the veranda. It looks as if the slab has exerted pressure on the bricks and pushed them out of alignment. My theory is the steel edge beam supporting the edge of the slab is rusting between the slab and beam. The rust is causing the beam to expand and push away from the slab, exerting a force on the bricks and pushing them out of alignment. Just a theory, but not sure if there maybe other forces at work I'm not aware of. I don't think its a foundation issue as the movement is localized around the top wall around the slab. Any other theories?

Thanks again for your help.

 

[XR250]

I agree that corrosion jacking has its part here. However, the stair-stepped crack by the door seems too large and distant to have been caused by that phenomenon.

 

[bones206]

My first guess would be thermal expansion of the steel angles around the perimeter of the veranda. I would suspect that there are angles embedded in the walls similar to the ones on the exposed edges.

 

[masonrygeek]

I agree with XR250 and bones - rust jacking (corrosion) and expansion of steel. Maybe time to open up sections to see what's going on.

 

[JAE]

That diagonal crack looks totally like a footing settlement issue to the right of the opening.

 

[phamENG]

I agree with the corrosion at the outer face, but I'm not so sure about footing movement. Do you have a closer view of the stairstep crack in the last picture? As it looks from this view, all of the movement is lateral. The bed joints still look tight. Tight bed joints usually indicate there's no differential vertical movement, and therefore little to no foundation movement.

 

[Ingenuity]

...and Cal-Sil bricks. Worse bricks ever made, IMO.

 

[Warramoos]

Thanks all,

Do you have a closer view of the stairstep crack in the last picture?

See below a front on view of the "Staircase" crack. Not sure if my camera is playing games here. The top half of the photos bows down, but the bottom bows up. I'll have to put a straight edge on it. But it looks like the course lines are level - the photo is abit deceptive.

The lateral "push" seems to be occurring at every steel/brick interface only. The rest of the house has no visible movements or cracking - its only around this veranda.

corrosion jacking

Thanks for putting a name to it, had not heard of this before but its what I thought maybe going on.

 

[hokie66]

I've seen this numerous times. There was a period of time in Australia when builders thought that type framing was a good idea. It is not.

Corrosion is the primary issue there. That veranda needs to be demolished and rebuilt.

 

[bones206]

The rust looks rather superficial to have caused that large of a displacement though.

 

[Warramoos]

The rust looks rather superficial to have caused that large of a displacement though.

Yeah it'd be interesting to see if the rust is occurring on the inside. As the water has been tracking in and around the steel beam to rust out the bondek underneath.

 

[3DDave]

It may be a combination. When the steel contracts it leaves a small gap. Loose rust falls to the bottom of the gap. When the steel expands it shoves the bricks. Cools again, drop in more rust flakes and some airborne dirt. Repeat.

The total amount of rust depth could be small overall but if gravity is replenishing the supply at the bottom of the gap it would not take too much to rust jack an item out of place.

Edit - it could be what looks to be concrete beams doing the shoving from expansion and contraction. They are going to lead in temperature changes over the rest of the structure and the rust is accumulating between the steel and those beams.

 

[civeng80]

Looks to me as though it could be more a concentrated loading on the edge of the channel causing the bricks to rotate and the joint to crack. There would be a gap between the bondeck and the flange of the channel which seems as though it is bridging. Rust would make things worse.

 

[hokie66]

The Bondek (a type of composite deck common in Australia) is almost completely gone along the outside bearing edge, and the edge angle or channel which supported the Bondeck is showing a lot of corrosion as well. When corrosion develops in the narrow ribs of the deck, it has nowhere to go but laterally, as the ribs act as wedges to expand the deck perpendicular to the ribs. Parallel to the ribs, the movement could be a combination of corrosion and just instability of the supports. At the house, it is probably supported just on one wythe, and on the outside, those skinny brick piers, probably unreinforced, look dangerous. Those piers apparently extend up to support the roof. I suggest that this deck and the piers need urgent attention, with safety being the first objective.

 

[Tomfh]

Yes the bondek looks pretty bad. If that edge fails it could all come down.

 

[Retrograde]

Is the brick loadbearing or is it a predominately timber framed house with brick veneer?

 

[hokie66]

Oh, it's loadbearing, Retrograde. Where would you find any timber in those brick piers?

 

[Warramoos]

Yeah they're definitely load bearing. No timber on the bottom levels.

 

[bones206]

The deck is highly restrained at the supports (in the in-plane directions). Given the difference in thermal properties between the brick and the steel perimeter angle, damage from thermal expansion seems inevitable to me. Not to say that rust jacking or local bearing failure aren't also in play. I just don't see how this structure would avoid thermal damage given the detailing. If it was detailed with freedom to slide in-plane, that would probably avoid damage from corrosion as well, since the load action is basically the same as thermal expansion. I'm curious what the edge detail looks like within the wall.