Red card moments!!
One of the constituent parts of the plethora of concrete mixes used on the Crossrail project, is Pulverised Fuel Ash, or PFA. A waste product from coal fired power stations, the material is ground down to a fine powder, mixed with heated air and burned. This forms a proportion of the material (about 20%) into fine glass particles which are separated from the remaining material.
This material has a number of uses in the construction industry;predominantly as a construction fill; as a raw material in cement manufacture, and most pertinent to me as a partial replacement to cement in concrete. This has a number of benefits:
Economic -reduction in the overall quanitity of cement purchased, and consequently a reduction in cost by using a waste product (relatively very cheap to transport from the power stations)
Environmental – green credentials in recycling waste product without the need to dispose of the stuff.
Technical. A number of technical benefits occur from the use of PFA as a constituent of the concrete mix, including:
– improved long term strength performance
– improved durability through reduction in mixing water
- – improved cohesion and workability for a given water content
– improved surface finish
– improved resistance to sulphate attack(ground water levels are on the rise in London, and in my little corner of the Lambeth Beds, there is an increased risk of oxidised pyrite leading to corrosive acidic groundwater)
– reduced heat of hydration (important in our specification as the concrete musnt remain below 60degrees during curing…difficult in summer…our tunnel reached 42degrees yesterday!)
– reduced shrinkage and cracking (see above!)
– reduced bleeding
– resistance to alkali-silica reaction
All in all, its good stuff to have on board…
So why do I tell you this? Keen to hear from all you concreters out there about reduced stocks of PFA in the UK. Our supplier CEMEX has withdrawn all supplies without notice. Without all the facts, it seems that it is a direct result of many of the coal fired power stations switching to gas. Hansons, the concrete supplier, run out of PFA as at today, and having explored all other options with no joy, have advised that we revise our mix designs sharpish! This affects all their customers, not just Crossrail, and I would imagine other suppliers are feeling the pinch too. Anyone else had this buzz? (When I asked Steve whether he’d had the email about the PFA, he point blank refused to return to Chatham to run a mile and a half! Thats not what I mean….)
So what?
Hansons are contracted to supply us with concrete to specification. They ultimatley are not achieving that, so contractually it seems very simple. However, the pressure of the program mean that the blame game is slightly moot. This apportionment will no doubt will be thrashed out later, but right now it’s in our interests to review these mixes and return to production
In terms of where the blame does sit and who ultimately wil pay for this…does this count as a ‘force majeure’, being as it was, unforeseen?
Holdfast Training Services Trial Blog 
Without dwelling for too long: 1) Consider specification as an output specification not a mix/method specification i.e. don’t say what the mix must contain, say what properties it must possess. That gives the supplier total flexibility to design with whatever consituent materials/products they wish to or can obtain and they can change to suit availbility as long as they meet spec (surprised it wasn’t set up that way from the start). 2) See GGBS. 3) I belive the Australians are presently shipping in PFA from China which seems to be generating a significant quatity! (Ask Nik West).
I’m not sure you can give the supplier ‘total flexibility to design’ since the mix used on site has to be agreed by the Structural Engineers. Therefore having something that is ever changing is not manageable. On the slipform here we have 5 different mixes approved which gives the contractor some flexibility to choose one depending on environmental conditions.
Incidentally none have PFA. They are GGBS.
See below for reply. These boxes are ridiculous!
Shipping in pfa from other countries with all the associated surely has dubious sustainability credentials even if it is a waste product.
One man’s poison is another man’s medicine. What might be a caustic waste to one party might just be cement replacement material for another i.e. it’s a valuable commercial commodity not a waste product; it’s manufacture involves high energy processess which are most cost effectively completed at point of creation and it’s use, in addition to the concrete propertiy gains, allows a reduced anmount of cement to be manufacturered (and imported?). You might even argue that manufacture of PFA generates power as a by product which can be sold locally to the production plant whilst the PFA product can be processed anmd sold around the globe?
The steel and concrete industry both calim to be greener than each other on the grounds that one generates a cement replacement by product(GGBS) that can be utilised by the other or the other consumes the unfriendly by products of the former – who wins the prize?
There is a sensible debate to be had around the globalisation aspects of manufacture and the efficiencies it can deliver in process versus the inefficiencies and imbalances in transport and market economy.
@ Rich Hall,
That’s an interesting observation. I recognise the concept of approved mixes. Do you think that the contractor sees that as a transfer of risk for perfomance to the ‘approving’ consultant or does the risk remain with the supplier? My understanding is that the strucutral engineer is merely confirming that they have no objection to the proposed mix i.e. it’s stated properties are not an issue as far as their design is concerned. I conceed that in practical terms this does restirct the supplier to using a mix design that they have proposed in advnace albeit it does not preculde proposal of other mixes and leaves the risk firmly with the supplier.
The risk sits firmly with the supplier for performance of the concrete they supply, but the Structural Engineer should always have the last say when approving the mix. Since the Structural Engineers own the risk on the performance of the global structure, and as the mix is quite crucial to this performance, both the supplier and designers are linked together.
Yup. I buy that, although I think it is more a case that the design team have a time risk that needs to be managed which is why you wouldn’t want to pemit any old mix to go in on the say so of a supplier and then suffer the delays that failure to meet specification would entail. Supplier recieves performance specification and then proposes his means of delivering so that the design team can manage the materials quality risks and therfore project time risks. All of which says that, if it has been written correctly, Ryan can fall back on the specification and simply ask the supplier how he proposes to meet the performance spec. It’s up to the supplier to propose a suitable mix for review (imported PFS, change to GGBS or use other options…).
All useful stuff here…
As it stands, the specification, dictates by performance requirements. We submit to the supplier, and the reality is that in 90% of cases he takes a mix of the shelf. The contractors’ Materials Engineer, trials the mix, to satisfy ourselves that it meets the specification. This has been used as a process to allow him to try to make some efficiency savings with reduced cement content (how the PFA came about) or overlay some benefit by utilising various combiations of additives; really he is titivating with it. The supplier will provide certification as to the provenance of all the constituents, along with some laboratory test results. It is then up to us as the contractor, to demostrate compliance to the client through trialling and reporting.
This issue has arisen as a result of Cemex, the PFA supplier, pulling the rug from underneath Hansons, the concrete supplier, who in turn have notified us that they are unable to meet to the demand. Far from us holding their feet to the fire, we have submitted an Early Warning Notice to the client. The Clients representative has recognised a project wide issue, and has agreed to identify alternate mixes currently being used by other contractors across the wider project, such that we can tap in to that until a more sustainable solution is identified.
Reflections
1. Contractually, I agree that it is incumbent on Hansons to find a resolution. The program imperatives however, put the ball back in our court, like a Djokovic backhand. An alternate mix would require laboratory tests, on site trialling and approval by the client. This process has been taking anything from 4-8 weeks, which we can ill afford as some of these mixes are already in permanent works and/or be about to be put in the ground. The cost of a cease works across 4 sites runs to approx £650k per day. Parking the responsiblity on Hansons in the hope that you can recoup this in disallowed cost later on is a very big gamble.
2. Ironically, the scale of this project has afforded everyone a potential escape route, by looking at other contractors. I we can poach another approved mix, then work continues saving us program delay. Hansons lose the business potentially (unless they supply someone elses approved mix), but they potentially avoid disallowed costs. Had we been on a smaller project, without this get out, then we would currently be at an impasse likely with a smaller contractor and supplier much less able the soak up these losses. Too big to fail?????? I feel like Royal Bank of Scotland!
PS. On GGBS…this wasn’t discussed as an option. I am yet to get a decent answer as to why, but I suspect given the longer curing time, it is precluded for use in sprayed concrete as we’re seeking a rapid early age strengh gain? I’ll do some digging……
I guessing the hours it takes our GGBS slip-form mix to gain strength is not quite quick enough for you. It is pretty speedy though.
Do you have the mix design handy? My Bluf o meter was ringing when I asked around the office earlier