To infinity and beyond

Chloride ions are highly mobile, which can cause problems for reinforced concrete. The chloride ions can penetrate into concrete over time and when their concentration around the steel reaches a critical mass, the passive protection layer formed by the concrete can be broken down leading to rusting of the steel and spalling of the concrete.

BS 8500 defines chloride exposure conditions as either XD for deicing salts applied to roads or XS in a marine environment. Both these classes are divided into 3 cases, with the most onerous being XD3 and XS3 (where the concrete is cyclically wet and dry).

The XD3 and XS3 areas are clearly defined e.g. XD3 is for structures within 10m horizontally of a carriageway or for bridge soffits within 5m vertically. However, it appears that Buzz Lightyear was on the drafting panel when XD1 zone was defined. XD1 is for structures greater than 10m horizontally and 5m vertically from a carriageway; i.e. to infinity and beyond, as no limit is specified.

Generally, this doesn’t have too big an impact on specifications as the limiting values (strength, water cement ratio, minimum cement content and cover to reinforcement) are not too onerous for XD1. However, sometimes specifications don’t permit certain materials to be used in a chloride environment, e.g. weathering steel, so if we are building a bridge near a motorway we need to have an idea what the likely spray zone is to know if these material restrictions apply.

For a recent project I’ve been working on, I came up with the following envelope using probabilistic modelling on the relationship empirically derived in Germany and used in fib bulletin 34 for the maximum content of chloride in a profile against distance from carriageway:

There is less research around to help evaluate the penetration of chlorides into the soil. However, considering the following facts:

  • Salt is usually applied in freezing conditions therefore the chloride contaminated water will tend to run-off the hard ground.
  • Chloride ions are mobile and will tend to flow away with groundwater
  • Research shows increased chloride in aquifers near roads with salt spreading because the chlorides have been transported away (supporting the first two points above)
  • High concentrations of chloride are not normally found at depths >1m

On this basis I limited the buried chloride zone to 2m giving the following overall envelope.

Logic tells you that a concrete element close to a motorway which will have a lot of traffic and relatively frequent salt addition in cold weather will be at far greater risk than one alongside a quiet road that may only get an occasional gritting and far less spray

It seems reasonable to me that you could differentiate your specification between busy (e.g. ‘M’ or ‘A’ roads) and quiet (e.g. ‘B’ or ‘unclassified’ roads). So my suggestion would be:

  • Any element subject to direct application of chloride (quiet or busy road) design as XD3
  • Any element in the ICZ on a busy road design as XD3 and XD1 in the OCZ.
  • Any element in BZA on a busy road design as XD3 and XD2 in BZB
  • On quiet roads use XD1 for all zones (except where subject to direct application- see above). Note to comply with the current versions of BS 8500 concrete in the ICZ should be classified as XD3 exposure (on both quiet and busy roads).

What do you think?

Fast track concrete

I recently attended the kick-off meeting for a really interesting and challenging project. Mott MacDonald are working with Transport for London to help them develop a specification for concrete for their track repairs.

“Bread and butter to you”, I hear you thinking; but just a minute I’ve not told you everything yet. The concrete is being used as part of a Mechanised Renewal Vehicle process and needs to be batched in the depths of the London Underground network. It needs to be quality controlled, accurately batched, tested, placed, finished and one small final point……

make 15MPa strength at 1 hour.

If it doesn’t, a significant number of the up to 5 million journeys made each day on the London Underground network will be disrupted.

Typically, the workforce in the Track Delivery Units (TDU) need to get on site, smash out the concrete currently holding in the rails, clear up the rubble, fix the rails in place and then pour up to 9 cubic metres of this fast strength gain concrete. Quite a challenge in the small overnight engineering windows available on the network. To make the project even more challenging, wouldn’t it be great to reuse the concrete broken out as aggregate in that days, or probably more realistically, a future concrete pour.

I’ll let you know how the work progresses but if any material suppliers out there have any products they would like me to consider, then please do not hesitate to contact me through this blog or look me up on LinkedIn.

Jackhammer not Jackboot needed

Okay, so I have a reputation of defending all things concrete, but sometimes you wish that the
material was not so durable and capable of withstanding the ravages of mother nature, instead like a sandcastle on a beach, it would be nice if it could get easily washed away.

A Croydon office being recycled

Clearly, you can recycle concrete and I’ve worked on a number of projects where we have looked to
turn old concrete structures or elements into crushed concrete aggregate (as it is called in BS 8500,
the UK British Standard for concrete). CCA can be reused in concrete as a replacement for natural aggregate. When concrete is part of general demolition waste and may be contaminated with other materials like bricks or plasterboard (the latter being a particular problem because excess gypsum disagrees with concrete) then its reuse is probably best left to low-tech applications like fill or hardcore.

However, having read of a recent landscaping scheme by a Mr Steven Johnson of El Sobrante, California, I cannot wait to see this particular use of concrete be subjected to a pneumatic drill and be smashed into oblivion or better still into CCA so that something positive can be crafted out of Mr Johnson’s monstrosity.

I am sure that like me, those of you that have seen or heard the coverage of the 75th anniversary of the D-Day landings in northern France will have found it moving and poignant. Yet, while we were marvelling at the tales of bravery of those once young men storming the Normandy beaches, Mr Johnson
decided that this was a good time to unveil a 3x3m concrete swastika in his front garden.

Mr Johnson’s garden “improvements”

Apparently, he thinks it looks “cool”; he likes swastikas and to him they symbolise “peace tranquillity and harmony”. Try telling that to those surviving veterans that saw their mates cut down in a hail of bullets.
Let’s hope that Mr Johnson soon sees sense and removes this offensive symbol. I’m sure he’ll find many volunteers to help him turn it into a pile of CCA. Now where did I leave my grading sieves……

The Concrete and the Divine

I changed my routine this morning. As usual on a Saturday I fleetingly toyed with the idea of joining my local park run for a refreshing 5K jog around my local park, but as usual lethargy overcame my momentary enthusiasm. Instead as usual, I went for my morning bath, a leisurely soak while catching up on the latest edition of “That Peter Crouch podcast”. Except Peter Crouch didn’t join me in the bath, I got distracted by a podcast about another Peter – St Peters Seminary in Cardross (link below).

The Seminary is an amazing building, built in 1966 for the Archdiocese of Glasgow. Archbishop Scanlon told the architects from Gillespie, Kidd & Coia, that he didn’t want “any of that modern stuff; I want a traditional Seminary”. What he ended up with was a brutalist concrete building and one of the most impressive post-war modernist structures.

Unfortunately, 14 years after its construction, it closed; the sweeping reforms of Vatican 2 making the Seminary redundant.

Today, the building lies in ruins, despite recent attempts to use lottery funding to start its resurrection.

St Peter’s Seminary today

However, anybody that’s visited Fountains Abbey in Yorkshire knows that being a derelict religous building doesn’t have to mean the end of your useful life and I quite like the idea of it lying in ruins, wood components rotting away, a modern monument to the durability of concrete.

Fountains Abbey – beautifully derelict

https://www.bbc.co.uk/sounds/play/b01rqnf3

Wood you believe it?

Oh dear, the New Scientist is the latest publication to fall under the spell of ‘timber can save the
world’ mantra by replacing the evil that is concrete (The New Age of Wood, 16th March 2019).

The article argues that we live in the “hydrocarbon age” which makes possible the materials that ‘define our
civilisation: steel, concrete and plastic’. It goes on to claim that “everything that is made from fossil-based materials today, can be made from a tree tomorrow’. While some examples are obvious, e.g. timber buildings, others require new technologies. Apparently, a timber ‘tougher and stronger’ than
high performance steel can be made from soft wood. Obviously, it must be processed first, which involves ‘chemically removing half of the lignin then brutally compressing what is left at high temperature’. No mention is made of how much carbon is emitted in this process. I wonder what
solution they will claim can replace concrete paving or asphalt roads. Perhaps they’ll transform
timber decking into something that can survive being run over by all the articulated lorries that will
be needed to haul all that imported timber and timber products around the country. Better hope it doesn’t rain, it might get a bit slippery!

I have rehearsed the arguments about the carbon content of concrete before and how it is a victim
of its own success and talked about the disadvantages of CLT, e.g. the poor acoustic qualities that required a school to ban pupils from talking in the corridors and the peeling layers that
means it can add fuel to a fire.

Let’s consider some of the other issues that get glossed over.
Where are you going to put all these trees? Has anybody worked out how many trees would need
to be planted to replace all concrete, steel and plastic and would there be any land left over to
provide food for the world’s growing population or house them?

Apparently, one cubic metre of timber stores one tonne of CO2, which contrasts positively to
cement where one tonne of cement creates getting on for one tonne of CO2. However, concrete is a
low carbon material because not much cement is used in its production (and that cement is often partly replaced by low CO2 products like slag and fly ash). What happens at the end of life? The timber will probably be burnt to produce energy also known as releasing all that stored CO2 back into the atmosphere. So, when the New Scientist claim “switching to timber would immediately wipe a billion tonnes off global carbon emissions”, what they fail to add is that in 50-100 years time much of it will still end up in the atmosphere. Concrete by contrast, reabsorbs CO2 throughout its life by a
process of carbonation. At the end of its life, if it is crushed up to produce recycled aggregate, the
increase in surface area of the particles will accelerate the carbonation, increasing the amount of
reabsorbed CO2.

Don’t get me wrong, I’m not saying don’t use timber, or develop new technologies. What I am
saying is that concrete is a wonderfully adaptable, durable, cheap and locally available product. Let’s
look at ways to improve further the sustainability of concrete, e.g. by sequestration of CO2, rather
than trying to create a concrete-free fantasy land.

THE SPY WHO LOVED ceMEnt

I’ve arrived in Dubai on the next leg of what my son calls my mini Gap Year. The change of view from my Bangladesh Hotel to my Dubai Hotel is welcome.

Room with no view (Dhaka)

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Room with a view (Dubai)

Somebody once told me that during the cold war British spies operated under cover as cement salesman. The fact that cement goes everywhere in the world apparently offered the perfect cover for their clandestine activities. I don’t know if this is true, but I rather like the thought of a smooth talking cement salesman suggesting your concrete should be “shaken not stirred”. I tried googling to see if I could find any evidence and I did find some interesting facts that suggest it might not be a million miles from the truth.

For example the connection is made by Lord Selborne, who was Director of Cement in the Ministry of Works during WWII and left this role to become Minister of Economic Warfare, which put him in political control of the Special Operations Executive, the organisation responsible for organising clandestine activities in occupied Europe. Atlantic Wall, a musical based on a true storey about the French Resistance has a character, Henri Giraud, an ex- First World War artillery officer, recruited to develop a fledgling spy network in Normandy. He uses his job as a cement salesman – which allows him to travel freely between Paris and his home town Caen – as cover for his intelligence work.

Why this interest in espionage and cement? Well it’s partly to add a bit of glamour and excitement to the “grey” world I work in and partly to put into context the second part of this post. I’m taking the opportunity of being in the UAE, to promote the activities of the small part of Mott MacDonald I belong to. While our, Bridges, Tunnels, Ports, Coastal and Offshore operations have no problem selling their skills – “they do what it says on the tin” as the advert goes, our function is not so clear.

I work for Special Services! Our team has nothing to do with clandestine operations (although I would say that wouldn’t I) but we are the repository that ends up with projects that nobody else wants to and/or can deal with. We were formed when alkali-silica reaction was dominating the concrete world back in the mid-1980s and there was a belief that they needed to bring together structural and materials expertise to address the problem. We’ve developed a wide-range of skills over the ensuing years as problems have come our way.

Some of our areas of expertise are shown below. Do not hesitate to contact me if you’d like to find out more about any of these special services we provide.

I apologise if you can’t get the image of me wielding a Schmidt Hammer with a licence to kill out of your mind. In that case I’m just glad I didn’t mention the other kind of special services that are offered by certain kinds of ladies in the personal ads of your local newspaper. Oh dear, too late!

List of some of our Special Services

  • Dynamic Analysis,
  • Forensic engineering,
  • Slab track analysis and design,
  • Human induced vibration,
  • Durability modelling and design,
  • Cathodic protection,
  • Wind engineering,
  • Blast analysis and design,
  • Noise and vibration measurement, analysis and mitigation,
  • Short-circuit analysis
  • Fire analysis
  • Fatigue analysis
  • Aerodynamics,
  • Finite element modelling
  • Seismic design of structures
  • Non-linear buckling analysis
  • Analysis of masonry vaulted structures,
  • Flood containing retaining walls,
  • Expert witness
  • Alkali-activated slags
  • Stress corrosion cracking,
  • Welding investigations,
  • FRP design
  • Tribology,
  • Through life asset management,