Concrete lintel cracked

[russell210581]

Hi

I was wondering if someone might be able to provide some advice with regards to a cracked concrete lintel. I recently uncovered the lintel above a ground floor kitchen window and noticed a crack running vertically up the centre of the lintel, the crack goes about 2 thirds of the way up the lintel. I have included 3 photos in my album //www.diynot.com/network/russell210581/albums/ which show the overall lintel and close up shot of the crack. In terms of whats above the lintel, there are no windows on the first floor directly above it - solid brickwork up to the eaves of the house. Should this be cause for concern?

The property was built in 1933. Would this lintel likely be reinforced?

Thanks in advance

Russell

 

[stuart45]

The lintel should be re-inforced. In the 30's the lintels were sometimes cast insitu.

 

[alittlerespect]

Humm! There's only one photo shown, which does not tell a lot, but I would assume that the original window/s have been replaced with uPVC type, in which case the uPVC does not have the same load bearing characteristics as wood and as concrete/stone is relatively hopeless in tension that there has been some deflection in the lintel resulting in the hairline fracture that has occurred.

No immediate problems, but you will need to monitor conditions and if the cracking gets worse the best course of action would be to prop up the brickwork above the existing lintel and replace the lintel. This is not an unknown situation, numerous timber framed windows have been replaced with uPVC and in a minority of cases there has been a catastrophic failure of the lintel and the wall above or at least a section of it has come crashing down!

Regards

 

[Alastairreid]

View media item 10449 View media item 10448 View media item 10447

 

[^woody^]

Even if not reinforced, a lintel such as this is effectively wedged in and can not tip or bow at the crack due to the masonry around it preventing any rotation.

 

[Shytalkz]

You're assuming that it will stay as a homogenous mass though, Woods. But, if it wanted to go, if there was no rebar in it, it would, as elements along the length would fracture into disparate elements.

Fixity at the supports could be relied upon to keep an unreinfoced element in place, as that would generate as much stress, albeit on the top, as will be happening there in bottom in the middle of the span.

 

[^woody^]

But what is going to cause a lintel to go when there is just a wall above it?

These damn things will stay in place even when you are trying to get them out with a hammer or a kango

 

[Shytalkz]

Things stay up because they arch, or otherwise redistribute stresses. If that lintel had no rebar in it and was actually called upon to take a load in isolation, it would go when the tensile stresses got too much for it: building in at the bearings would make not a jot of difference.

Imagine if the bricks were not bonded together on top of the same lintel, what would happen then?

 

[^woody^]

it would go when the tensile stresses got too much for it:

Yes, that's what my guide book says. lol

But what I meant was ... what is in the wall above which is going to increase the load on this particular lintel which is then going to increase the tensile stress from what it is now?

That crack looks historic, and is either a common shrinkage crack or an early settlement crack as the load was added.

There will have to be some other occurance/movement in that wall to cause the lintel to go, otherwise it will stay wedged up there for a long time

 

[Shytalkz]

There doesn't have to be a change in anything.

Once the unreinforced beam has fractured, that means there is zero tensile resistance at the soffit and a significant increase in strain - thus a crack propagates upwards, which pushes the position of max tension further up the beam, increases levels of strain further up the beam, which causes further propagation upwards and so on. Add in creep deflection too, which will increase strain levels even further.

That's why concrete beams have rebar in them, to take the tensile force. The beam is designed as a cracked section ie the assumption is that there is no tensile resistance in the concrete below the level of the rebar; in reality, there is a smidgin of capacity within the concrete itself, but 3pofa, so it's ignored in the design.

 

[stuart45]

When lintels are built insitu, the brickwork is often started on top too soon before the concrete has hardened properly causing minor cracking like this.

 

[^woody^]

But the forces acting on this lintel are not changing, or will not change sufficiently to overcome the friction at the crack, the force holding each end down at the bearing, or the fact that it is wedged in.

So, whilst there may be zero tensile resistance at the soffit, there preumably must be close to zero force there too or the lintel would have already gone?

So the lintel is able to counter any force acting on it - ie it is performing satisfatorily.

Yes, there may be a point where it will fail if loads alter, but that applies to any lintel

I bet that there are thousands of these lintels with cracks in them, and stll up. There would have to be other visible factors evident before this lintel needed replacement

 

[Shytalkz]

Even if it hadn't cracked, you'd get creep in the lintel, which would increase the soffit curvature and further propagate the crack.

If there's no rebar in it, it's not doing much at all, effectively whatever is above it is spanning across the opening.

When you get cracks, there will be redistribution of forces; it might even be trying to act as two cantilevers off the support, or as a shorter span off two stubby cantilevers, there are all sorts of things that could go on to distribute the stresses.

But, if there's no rebar in there and it carries on gradually deflecting as concrete does, then at some point it would go and there would be nothing to keep it together. And that would happen with no change in load.

If it's got rebar in it, I wouldn't worry about replacing it. If none, I would.

 

[russell210581]

Thanks for all the info.

Can I ask - how could I tell if the lintel has got rebar in it? There doesn't seem to be any easy way??

Russ

 

[Shytalkz]

It can be tested with a thing called a covermeter. It's a variation on a metal detector; come to think of it, if you've got one of those, that should work!