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Sink hole under my workshop!

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Re: Sink hole under my workshop!

Postby 9fingers » 22 Feb 2018, 17:19

RogerS wrote:
Could Bob use a concrete lintel aligned vertically to support his slab ?


Possibly could but still need to jack it up with some degree of force to raise the foundation by a couple of mm.
This is the degree to which I estimate the door frame has dropped over the past few years.

So a piece of concrete lintel and maybe a pair of folding wooden wedges to do the jacking up and then the final concrete. The wood would eventually rot but the concrete would take the load.

Bob

Having seen Mike reply, I would only be loading the lintel whilst the surrounding concrete goes off.
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Re: Sink hole under my workshop!

Postby 9fingers » 22 Feb 2018, 17:31

Rod wrote:Concrete is supposed to stop further rust but not always the case - shown by spalled concrete especially on exposed surfaces especially where insufficient cover has been created during construction.
To resist moisture, air entrained concrete is specified and highway structures are often sprayed with a special sealer after formwork is stripped to prevent salt action.

Rod


A number of council housing was built in Romsey after the 2nd WW using Reema PRC (prefabricated reinforced concrete). These were sold off in the 80s(?) to the tenants on very preferential terms under the govt policy of the day. However those who tried then to sell them to release the equity were found to be un mortgageable due to what the press called concrete cancer -cracks caused by the swelling of rusting re-bar in the precast panels.

A vast local expense the council offered to buy these back at full market price assuming no concrete cancer and those they still owned plus those that were bought back, had a brick outer skin built round them onto new foundations where unusually the outer skin was structural and the roof modified to also bear on the brick.

The roads with these houses in now look rather peculiar with a mix of slightly larger brick houses interleaved with a few painted concrete ones still in private hands that possibly can never be sold other than to cash buyers.

Bob
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Re: Sink hole under my workshop!

Postby RogerS » 22 Feb 2018, 18:27

I noticed on the M4 flyover exiting London near Chiswick roundabout that there was a hell of a lot of broken concrete and exposed steel on the underside of the overpass. I'm sure Highways are on top of things but I wouldn't like to be driving around that area when they have to close it to do the work!

This whole business of steels was one of the reasons why we didn't choose to remove the bay windows as they both had a lot of steel that would have had to come out and replaced with concrete lintels.
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Re: Sink hole under my workshop!

Postby DaveL » 22 Feb 2018, 23:54

I worked for STC in the 1970's at the Basildon site, the building was clad in big concrete squares that were hung on the frame of the building. In the 80's it was discovered that most of the steel eyes that were holding the squares on had broken out and the squares were just stood on the frame. Caused a major panic and they put up loads of scaffolding to stop any falling off.
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Re: Sink hole under my workshop!

Postby Rod » 23 Feb 2018, 00:30

A big problem with some older structures is ASR ( alkali-silica reaction) another form of concrete cancer caused by chemical reactions of the concrete’s constituents.
A visible defect, though not a structural one, is rust patches caused by the accidental incorporation of iron pyrites within the aggregates used in the mix. The IP rusts in contact with the moist atmosphere leaving ugly stains and the impression that the rebar is corroded.

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Re: Sink hole under my workshop!

Postby the bear » 23 Feb 2018, 01:13

Mike
With proper trench props (under pinning jacks), what stops these rusting and breaking the concrete when they are used during underpinning and get concreted in?
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Re: Sink hole under my workshop!

Postby Mike G » 23 Feb 2018, 08:38

Dunno. I didn't know they did that (I've never seen it happen). It doesn't sound like good practise to me, and I'd be interested in what a structural engineer had to say on the matter. All the underpinning I have ever been involved with has been specified in short (1m to 1.5m) non-adjacent sections which didn't require propping.
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Re: Sink hole under my workshop!

Postby Rod » 23 Feb 2018, 10:58

I’ve never seen that happen too, though if they filled the void with plenty of concrete to give the prop plenty of cover then it might not corrode.
One famous structure I worked on has its special supporting bearings (with a life of 25yrs) concreted in. To replace them they will have to break them out!!

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