Classy cements

We just love to classify things.

  • Our social status (upper class, middle class , working class)
  • Our cars (MPV, saloon, mini, hybrid, estate etc)
  • Our bread (granary, wholemeal, sourdough etc)

I quite liked this classification of science.

The classification of cement in various standards has caused some confusion amongst colleagues so I thought I’d share my conclusions with you.  Eurocode 2 uses a simple classification system when calculating early age strengths, creep coefficients or drying shrinkage strain.  Depending on the rate of strength gain, concrete is classified as either Class R, N or S. 

The European cement standard, EN 197-1 uses a similar classification system.  Until the November 2011 amendment of EN197-1, the standard only had two classes of early strength for each standard strength class – cement with ordinary strength gain indicated by N and high early strength by R.  The 2011 revision introduced a third category “L” (I suppose “S” would have been too obvious!). 

The introduction of Class L at least brought EN 197-1 in line with the complementary British Standard to EN 206, BS 8500, which also uses the designations Class R, N and L.

I hope you’re following this as it’s about to get more confusing.  The table below reproduces the cement classes given in clause 3.1.2(6) from EN 1992-1-1.  The list of cements does not cover the full range of cements in EN197-1 or the range typically used in the UK.  Furthermore, it is immediately clear from Table 1 that the classes used in EN 1992-1-1 do not correlate with the designations used in EN 197-1 (e.g. CEM 52,5N is class R while CEM 32,5R is class N).

I need to add another standard in here, BS EN 14216 which covers the specification and performance of very low heat cements.  There is actually close correlation between the definitions of R, N and L in  EN 197-1, EN 14216 and BS 8500 but  irritatingly there are some differences as shown in the table below (figures in brackets show the discrepancies between the standards).

From the Table above, it can be seen that a 32,5R has the same minimum 2 day strength as a 42,5N which helps to understand the logic of the classes used in EN 1992-1-1.

The question I often get asked is what class should be used for cements not covered in clause 3.1.2 of EN 1992-1-1.  In the table below, I have extended the range of cements with my additions shown in bold.

Often full descriptions of the cement are not available and we may only know the generic type.  I use the assumptions in the following table in those circumstances.

These are my own personal view and you are welcome to use them, but their use comes with no guarantees. Any comments?

New Year Resolutions

Do you make New Year’s Resolutions? I must confess to having mixed success with mine. I made one three years ago that has become much easier to maintain as time has gone on. In an effort to improve my fitness I pledged to walk up the stairs to my desk rather than take the lift. My desk is on the top (6th) floor of Mott MacDonald House in Croydon, 124 steps, so not an insignificant undertaking. Given that I also cycled to the station on my Brompton, I also had to lug my bike up all those steps, which was hard work.

I did pretty well and most days I kept to my resolution. It became easier when I decided to start walking to the station rather than cycling and I was generally only using the lift when I was collecting a visitor from reception. I have achieved 100% compliance with my resolution over the last nine months. Not because it has become an established habit but thanks to Covid, I have been working at home where there is no lift and just one flight of stairs to negotiate.

This year I am setting myself some more challenging targets around being more organised:

1) Keep my to do list up to date.

2) Complete my timesheet at the end of each day rather than each week.

3) Do my project management tasks on time

4) Regularly update this blog (at least once per month and preferably more often).

You will know in due course how successful I have been on the last one. The others will largely be known only to me but I must admit I don’t start off with a great deal of confidence in my ability to keep them up as I usually find project deliverables get in the way.

Aggregation of Sustainable Gains

It’s a cold day.  You’re out for a walk, hands thrust deep in your pockets, protecting them against the bitter wind. On the path ahead of you something glittering catches your attention.  On closer inspection you see that it’s only a penny coin.  Do you pick it up and pocket it? More likely you’ll carry on walking, thinking you don’t know where that coin has been, it’s so cold you don’t want to take your hands out of your pockets and anyway, it’s virtually worthless, you can’t buy anything with it.

If it was a pound coin you might have picked it up.  If it wasn’t glinting but a rustling £20 note you almost certainly would have bent down and grabbed it and thought today was your lucky day.  In 2003, one man who did metaphorically pickup that 1p coin was Dave Brailsford and he used it to transform British cycling.  Since the start of the modern Olympics in 1908, Britain had won one solitary gold medal in cycling and our record was even worse in the most prestigious cycling race in the world, The Tour de France, which had never been won by a British cyclist in over 100 years of trying.

One penny is 1% of one pound.  Dave Brailsford the newly appointed coach of British Cycling and his team broke down everything you could think of that goes into riding a bike, and then set out to improve it by 1 percent.  Bike seats were redesigned, fabrics that were used for cycling jerseys were tested in wind tunnels even different massage oils were evaluated to see which gave the best performance. Within 5 years of starting this process that Brailsford called the “Aggregation of Marginal Gains”, the British team were dominating cycling.  In the 2008 Olympics the British team won 60% of the gold medals available, in London 2012 the did even better winning 16 gold medals across the Olympic and Paralympic Games, setting seven world records in the process.  The following year Bradley Wiggins won Britain’s first Tour de France and with Chris Froome and their Sky Team colleagues went on to dominate, winning 5 out of 6 Tours.

We are trying to harness this concept at work and apply it to sustainability of our projects.  Throughout the design process we will be looking for those small marginal gains (as well as big ones).  Whether it’s making a saving in the size of an element or reusing some existing foundations instead of building new ones, or increasing the slag content in concrete, or reducing the reinforcement by reviewing crack widths.  Imagine what a difference we could make if we can be as successful as the cyclists.  To help embed this practice in our work, we are developing a dashboard to help quantify and share some of these improvements.  Initiatives will be peer reviewed within the app and prizes will be awarded.

By aggregating our sustainable gains we may not win any gold medals, but our eyes are set on a bigger prize – net zero.  If we hit that target, we will all be winners.

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?

Happy birthday BS 8500

Last week BS8500, the complementary British Standard to EN 206 (the European Standard on Concrete) came of age. Although I live reasonably near the BSI HQ in Chiswick, I was not disturbed by any wild parties, in fact I’m pretty sure the 18th birthday passed by unnoticed by all. In the UK, becoming 18 marks the point when you are legally entitled to, amongst other things:

  • Serve on a jury
  • Get a tattoo
  • Buy an alcoholic drink in a pub

BS 8500 cannot partake of these new rights, but I bet that grappling with BS 8500 has driven more than a few engineers to drink over the years. You would have thought that after 18 years we would know how to use the Standard, but I keep coming across examples of it being incorrectly applied. Back in 2011 when BS 8500 was still young, I was motivated to write an article for the Structural Engineer magazine highlighting a common error that engineers made when using the Standard, and it’s still being made.

So, as my birthday present to this fundamental Standard of our industry, I’m going to go over the issue again and see if we can prevent a few more of you from making this error.


Technical jargon warning

The article gets a bit techy from here


Those of you familiar with the Standard will know that Tables A.4 and A.5 are the key tables in which for a given exposure environment, you determine your cover to reinforcement, concrete limiting values (i.e. strength, minimum cement content and maximum water cement ratio) and cementitious material type to achieve your required design life. Table A.4 gives the requirements for a 50 year design life and Table A.5 for a 100 years.

The two Tables give the specifier options. To achieve the design life, they can either specify a:

  • higher cover with a lower quality concrete
  • higher quality concrete with a lower cover

The quality of concrete can be improved by using a better performing cement and/or a lower water cement ratio (and associated higher minimum cement content and strength).

So, for each different exposure condition the specifier has a range of options to choose from. For example, consider a concrete element exposed to a marine splash zone (exposure class XS3) and a 50 year design life. Table A.4 gives the specifier 23 options with strengths varying from C20/25 to C40/50 (and all grades in between), 4 different groups of cements and minimum cover to reinforcement from 45mm to 80mm inclusive. In terms of the Standard, all these options are equally valid.

The common mistake I keep coming across is that the specifier who may have an element that requires a design characteristic strength of say C32/40, believes that they must look up that strength in the XS3 exposure class row in Table A.4 and then use one of the two combinations they find i.e.


C32/40 mcc 360 w/c 0.45 with IIB-V or IIIA cement and 60mm cover


C32/40 mcc 360 w/c 0.45 with IIB-V (min 25% fly ash) or IIIA (min 46% slag) cement and 60mm cover

Worse still, the specifier will often compound this error by restricting the cement type, e.g. there is no C32/40 option in Table A.4 for cements with a high supplementary cementitious materials content, so the specifier will exclude IIIB or IVB-V cement.

While this is a solution that meets the requirements of the Standard, it is unnecessarily restrictive and could be technically poor, e.g. if it is a large element that requires a low-heat cement to minimise the risk of thermal cracking.

Instead it should be noted that the limiting values in Tables A.4 and A.5 are a minimum. If the specifier wants to use IIIB cement at 50mm cover then Table A.4 says you need a minimum specification of C28/35 mcc 360 w/c 0.45 to meet durability requirements. If you need C32/40 for structural reasons then specify C32/40 IIIB cement mcc 360 w/c 0.45 with 50mm cover and your concrete will comply with both structural and durability requirements. Four of the 23 options have durability limiting values with a strength greater than C32/40. The specifier can still use these options but they will have to increase the limiting values, including strength, to match the requirements in Table A.4.

Simple?

#concrete #BS8500 #EN206

Stuff matters

My son is the black sheep of the family. Unlike his sister, his mother or me (all of whom studied/is studying science or engineering at university) he graduated with a Batchelor of Arts degree.

Although he got top grades in all his science GCSEs, he showed no interest in studying what my dad would call “a proper subject”. I blame myself and I worry what he will make of his life saddled with a wishy washy arts degree.

I mean apart from umpteen Politicians, a few Prime Ministers, media figures, business leaders etc. etc. what ever becomes of Oxford University politics, philosophy and economics graduates?

My son drew to my attention the following post on website linked to his old college by a student studying a “proper subject”, Materials Science.

I had never heard of Mark Miodownik, but I wanted to find out more about this book that has clearly had such an impact on an impressionable young undergraduate. So £5.99 later I was the proud owner of an audio version of “Stuff Matters” and, I must say, it’s very good. I particularly liked his analogy to describe the feeling that many people share who prefer their concrete hidden away behind steel and glass.

Like bone we prefer it on the inside, when it sticks out we are repulsed

Mark Miodownik

Apart from concrete, Miodownik writes in an accessible and engaging way, about a number of different materials , including steel, wood, ceramics, carbon and even chocolate. Perhaps, if I’d given my son a copy of this to read instead of Harry Potter he might have seen the light and maybe studied Materials Science or Engineering. Mind you, if you choose the 3 year degree option, you will graduate with a BA in Materials Science (only at Oxford!).

Unsurprisingly, like the perceptive undergraduate, concrete tops my favourite material list, but after listening to Miodownik’s book, carbon has shot up the rankings to 2nd. The world of diamonds, graphite and graphene sounds fascinating, but not as fascinating as concrete.

#Miodownik #concrete

What are you giving up for lent?

My apologies for the deafening silence emanating from my blog recently. I’ve no excuse except that life has been busy. I have been surprised (and very flattered) that a number of you have lamented the lack of blogging and even came across someone who said he’d used some of the content in client meetings. So, this year instead of giving up beer or wine or chocolate for lent, I’m giving up silence and undertaking to revitalise my blog with a weekly post. So enjoy your pancakes on Tuesday and
hopefully I will see you back here on Ash Wednesday, unless this lent pledge goes the same way that many of my others have!

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……