Loss of all Cohesion in Masonry Walls

[countervail]

Bit of a long one.

I have a 14x story historic brick tower (about 11mx11m in plan and 52m high) where sulphates in the brick have eaten all the cement within the mortar beds, it’s basically just a very large stack bricks bedded on sand bands now.

Water has got into the structure and caused significant voiding as it percolates; in, down and out of the tower.

I have had a look at overturning (does the weight of the tower prevent tension in the sand beds, and yes it does) and sliding forces (does the frictional component of the weight in the sand beds provide sufficient friction resistance to overcome applied wind loads, and with a coefficient of friction of 0.4 yes its does).

The compression force generated in the masonry is about 1N/mm² and I recon the confined sand between the bricks will be ok to carry this force (although it may settle as we squeeze down the voids in the sand over time, and I will probably monitor settlement / movement.)

It looks to me all I need to do is prevent water getting in and sand getting out. So repoint the mortar and job done.

This all seemed much simpler and easier to assess and deal with than I expected; so I’m suspicious I’m missing something!!!

Does anyone have any thoughts on the subject or has anyone come across guidance on assessing capacity of thin layers of sand sandwiched between masonry; I would love to see some guidance or testing that has been doe to double check my approach isn’t far to simplistic.

 

[GeorgeTheCivilEngineer]

Unreinforced masonry are designed as stacks of bricks anyway, really. You can't rely on the tension capacity of mortar because one crack and it's reduced to zero.

However.... the consequences of failure here are immense so I'd be looking at repointing then grouting to fill those voids. Or demolition.

Confirm whether it is cement or lime mortar. Older lime mortar reducing to sand is not uncommon.

I'll have a think about guidance.

 

[JedClampett]

I'd second George's advice. For the consequences of poor execution of CMU/Brick repairs see thread815-507372.

 

[WesternJeb]

I agree that the calcs can be very straight forward. My only word of caution is that calculations for buildings like this seem to be a fool's errand. There are so many unknowns and rash assumptions made with brick construction and deteriorating mortar that the calculations should only be used to look for giant red flags.

I would spec some pretty extensive Simpson Heli-Ties just to CYA across the areas of large concern. Maybe not because they are necessary, but it would help me sleep well at night knowing that my name and liability are on a multiwythe brick building now.

 

[dik]

There is no easy way to fix if the mortar 'cement' is gone. I've encountered this often with historic brick. Not caused by the sulphates, in general, but a poor choice of lime. In early days, masons had to source the lime they used for mortar to make sure it had hydraulic properties. If not, with time the 'cement' leaches out leaving sand. The walls are seriously compromised.

How important is the structure. You may have to design a 'supporting structure' to support the 'loose' bricks.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[countervail]

In case anyone wants an update.

We did our calcs. These were by design broad brush to give a feel, I don’t believe the numbers can do more than support a qualative assessment given the size of the structure and the variability in what’s it’s become. They were there to tell me to PANIC or not.

So it all looks fine, I was worried I would have to put a whole new RC core down the middle and keep the brick tower as a facade. It’s historically nationally significant here so I couldn’t replace it!

My only real concern is that the voids are so significant they may properly compromise the compression resistance of the tower but I’m also thinking that as the mortar is now sand they are / will just squeeze out under load causing the whole structure to settle (which there are signs of), which may be fine if we can stop more voids forming in future.

So it’s fine on my assumptions but now I have decided to test them by getting in somone to carry out some Insitu compression tests and shear tests to see how some of it at least really reacts to loads. If thats good then we repoint and monitor for settlement and vertically and if things look good then smashing, and if not then we go for drastic measures whatever we decide those should be.

 

[Greenalleycat]

I'm assuming that this is a non-seismic region...if it is seismic, I'd be running from the hills, or at the least staying at least 1.5x52m = 78m away from this thing!
Historic significance is always tricky...just because something may be worth keeping from a heritage perspective does not obligate you to lower your standards to keep it using historic structural systems
You're only one big wind or heavy rainfall away from the front page of the newspaper with a structure like this
If money is being spent on this thing to keep it alive long term then is now the time to spend big boy money on making it actually last - not just throwing small change at it to repoint and make it look nice? It seems that ongoing degradation will be a feature of this construction, and at some point those chickens will come home to roost

Could you do something like tie Helifix ties into the back of the bricks and spray a 150mm shotcrete core around the inside of it onto a new footing?

 

[XR250]

I would have a very tight contract on this where your customer takes most of the risk.

 

[Aesur]

Sand has no cohesion, so there is no real friction here, that must be some integrity left in some of the mortar. If it has truly deteriorated to just sand beds as you say, I am of the opinion the ONLY option is to tear down. Can you imagine repointing all the mortar for 14 stories of bricks.. that's a heck of a job. How many wythes of brick are we talking here? How can you say there is no tension in the beds, near the top, I would suspect there isn't enough weight to counteract wind uplift, nor shear wall overturning for this height of a structure without some kind of mechanism to transfer loading down grabbing a sufficient area of weight.

 

[Aesur]

My only real concern is that the voids are so significant they may properly compromise the compression resistance of the tower but I’m also thinking that as the mortar is now sand they are / will just squeeze out under load causing the whole structure to settle (which there are signs of), which may be fine if we can stop more voids forming in future.

If you are seeing signs of settling and the sand squeezing out of the joints, I'm not sure its salvageable without reconstruction, how could you even shore to put a joint back?