Secret Scotland

If it's secret, and in Scotland…

Inside a concrete lamppost

After yesterday’s post about a repaired concrete lamppost I thought I’d dig out a view of what’s inside one.

If you know anything about concrete, then you know it has no strength in tension, only compression, so these poles are not simply solid concrete – which would break the first time a decent gust of wind hit – but are full of rebar (steel reinforcing bar) to prevent any tension causing the concrete to crack and fail, as it would do on its own.

Concrete Lamppost

Concrete Lamppost

But they’re getting old now, and the thinnest surface concrete is failing and allowing water to creep below.

This become a self-compounding problem as it allow air and moisture to reach the steel.

Two problems follow, firstly the water can freeze and the expansion of the ice causes further cracks in the concrete, so more moisture gets in.

Then the steel starts to rust as it combines with oxygen, this also expands and cause more damage, eventually blowing the concrete off the surface, exposing the material below.

Concrete Lamppost Reinforcement

Concrete Lamppost Reinforcement

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September 19, 2017 - Posted by | council, photography, Transport | ,

6 Comments »

  1. The not so technical trade term is – “concrete cancer” , when detected on motorway bridges etc. they bolt-on some electronic counter-measure , been around 30 + years.

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    Comment by Tony | September 19, 2017

  2. Please re-read my description, which relates to environmental effects and the seasonal effects of weather causing damage to the concrete, and allowing air/oxygen to reach the steel which would otherwise be isolated if the concrete barrier was not compromised.

    Concrete cancer is a completely different issue and refers to an alkali-related reaction within the concrete itself, has no relation to oxidisation of the steel rebar, and can take place in plain concrete with on rebar or steel present.

    As far as I’m aware, cathodic protection only applies to corrosion (where metals are involved), to extend the lives of related structures, and cannot be used for the unrelated chemically based failure from the alkali effect of concrete cancer.

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    Comment by Apollo | September 19, 2017

  3. Many years ago since I read about it , ISTR the cathodic protection on motorway bridges -experements were on the A1M (Darlington by-pass) , problem started as cancer (may have been road salt entry) then progression of spalling of concrete off the re-bar. You can see the patch repairs and IIRC the protection boxes are painted green and look similar to Telecom boxes.

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    Comment by Tony | September 19, 2017

  4. Intriguing. I honestly hadn’t thought of road/motorway bridges, but then again, it doesn’t matter what the initial damage mechanism is so long as it get water down to the steel to start oxidisation and expansion. Now that you’ve planted the seed, I will have to look into cathodic protection of rebar. I know the theory as regards exposed metal, but will have to read up on how it can be made to work on ‘protected’ steel (protected by being isolated from air/water by undamaged concrete). The worst example of a ‘Bad Design’ I saw of this effect was on the Isle of Bute, where some rather nice new blocks of flats were completed, but I suggest they were built to a standard pattern, and with no consideration for the salt sea spray the shoreside estate occupied. Although only a few years old, the gleaming white exteriors were marred by weeping brown streams that had burst through the rendering at various places, and ran from the eaves all the way to the ground.

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    Comment by Apollo | September 19, 2017

  5. It was about the time I was involved with J bolts held by flat concrete roof steel mesh , these bolts were mild steel and secured window cleaning equipment tracks. Ultrasound was used as well as smashing open plinths that I marked with a % at random – thus to allow thorough examination. These foundation bolts were in tension , some tracks were modified to compression bolts by extra counterweight on the davits. To me the whole business was a mares nest and I did not want to be involved – earn a crust.
    Then a person was killed when a street lamp-standard (galvanised m.s. I seem to recall) felled itself at a early age, all depends on coat-wrap procedure and soil acid level. Somehow – I became invollved but on on huge mast-type standards that had a circular chandelier , the galv. standard had internal ropes for raise / lower. – powered by a portable industrial 1″ square drive pneumatic nut-runner. I was more interested in the base plate foundation bolts. You may still see these in supermarket car-parks, odd motorway locatiuons but in the main on approach roads to bridges. Again , like amusement park ride inspections – a bit out of my specialist field and I did not wish to be involved , reputation for being versatile an a basic £ salary only with s.f.a. for overtime , just out-of-pocket for out-of area..

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    Comment by Tony | September 20, 2017

  6. WOW! Such a parallel…

    I took on a life/limb-critical certification job when the only chap in the UK then was diagnosed with cancer, and was interviewing people to take over when his time ran out.

    I won’t go into detail, but it didn’t worry me or my staff while the systems were based on electronics, but when the manufacturers moved into programmable systems I baulked and told them to find someone else. The problem was not the use of programmable systems, but the lack of testing and verification standards for them. I even went to the body that checks software for nuclear power stations and drive-by-wire systems (then not on the market, but only a few years from introduction), but while I could point to relevant etalons (standards) as a defence if there was an incident with analogue systems, there was no official system in place for digital, so the can would have stopped with me.

    I still have the pics of a girl who had to have both hands reattached after a system approved by another test house removed them – and consider our withdrawal was done at the right tome to maintain our record (and that of the chap I inherited this job from) of zero incidents during out periods of tenure.

    You had me with mention of ‘compression bolts’ – until I remembered I had used one, and it’s really terminology rather than a bolt in compression. I MUST stay more awake and alert!

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    Comment by Apollo | September 20, 2017


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