Holdfast Training Services Trial Blog

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 Our supplier CEMEX has withdrawn all supplies without notice.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?