Thread by @russiankiwi3

Lots of angry people in Nottingham currently due to delays caused by the #A52 #CliftonBridge emergency closure over the River Trent, so a quick thread to help explain why these sorts of things happen (and why we can expect more of them) 1/

There are two bridges over the Trent at Clifton; the west bridge (on left in photo) opened in 1958. The east bridge (closed) opened in 1972. It is an unusual arrangement in that on the northbound A52, the lanes split with lane 1 using the west bridge & lane 2 using the east! 2/

The west bridge is made of post-tensioned concrete and due to its age was strengthened in the late 90s due to over 25% of the tendons being corroded. Put simply, tendons are long steel strands fixed under tension to pull the bridge together 3/

The east bridge, I believe, is made from in-situ reinforced concrete, which is steel reinforcement bars cast into concrete as per pic below (without concrete). Due to cost, stainless steel isn't used which means the steel is susceptible to corrosion 4/

The corrosion takes two forms:

1) Carbonation, a reaction between cement and cabron dioxide in the air
2) Chloride penetration, a reaction caused by chloride ions, mainly found in de-icing salts deposited on the road by gritting & winter maintenance 5/

Corrosion also leads to rust, an oxide layer forms on the steel, the pressure can blow concrete out exposing the steel (called spalling). Pic below shows spalling on an bridge over the M1. Once exposed the corrosion can speed up and 'chase along' the bar behind the concrete 6/

As the bar corrodes its cross-sectional area is reduced, and the bars deal with tensile loads in reinforced concrete - so corroded bars have an impact on the ultimate load the structure is able to carry...so you can see why when corrosion is detected it can be a major issue 7/

So how do we detect? Firstly, as part of inspection schedule. Two main types of inspections:
1) General - every 12-24 months typically, a quick check to make sure nothing has changed
2) Principal - a detailed, close-up, touching-distance thorough examination of the structure. 8/

A principal inspection is generally every 5 years although this can vary depending on the age, type & location of the structure (as well as other types of inspections can also be done). More guidance on bridge inspections here standardsforhighways.co.uk/ha/standards/d… 9/

Earlier I said corrosion can go behind the concrete which makes it difficult to detect. One way we can look through the concrete is using sonic logging - this sends an ultrasound signal through concrete, voids caused by corrosion will create a 'blip' in the signal 10/

But it is interpretive data, and hard-to-access areas can be missed. It is not usually done unless there is evidence of corrosion or other structural defects. The best way to examine corrosion is to break the concrete out and have a look 🧐 11/

Concrete corrosion is a well-known issue, a lot of research is going into it and there are all sorts of clever ways to solve it. The M6 Bromford Viaduct / Spaghetti Jct / M5 Oldbury Viaduct works have all been recently trying to repair corrosion of the bridge decks and piers 12/

To investigate & repair, the concrete must be broken out to expose the rebar, this involves mechanical breakers (jackhammers) or hydrodemolition (blasting high-pressure water jets) - both are noisy & dusty, neither is a quick option 13/

So in the context of Clifton Bridge: A defect discovered on Thursday, most likely to be corrosion of some kind with the concrete - this could be safety-critical so the bridge is emergency-closed as a precautionary measure 14/

Friday & the weekend will be spent assessing the defect, and deciding on a solution. If it's a small area you can break out the concrete, chop the corroded bars, splice & couple new bars on, recast the concrete (or grout) and hey presto is is fixed 15/

Example of repair in progress; concrete broken out, damaged bars cut, couplers(things with bolts on) connect the bars back together. All this would take a week minimum and rely on no further defects/corrosion being found! If so, we could be in for disruption for a long time! 16/

To all motorists sat in traffic, my advice is leave earlier/later, switch to train or tram, or even try cycling. The disruption is a pain but not a decision taken lightly, not something "known about for months", or that can be fixed in 2 days unfortunately! 17/

I hope this helps explain things, any professionals feel free to add/correct me, and hopefully it is all resolved soon! /18 ENDS
@HighwaysEMIDS @Transport_Nottm #A52cliftonbridge #nottingham #highways #civilengineering

PS: @kevjs1982 has pointed out that there's scaffolding beneath the bridge but not much evidence or work being carried out. This suggests maintenance/inspection work was already planned and is likely how the defect was detected in the first place 19/

It could be the best way to access defect is now from top of deck rather than soffit. Or, Highways England are waiting on test results or decision from structures engineers/designers on best way to proceed prior to carrying out any remedial works. Likely to see work soon! 20/END

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