Holdfast Training Services Trial Blog

The last post finished off at the point of the base plug being poured and the Waterproof sheet membrane being fitted to the shaft bottom. This week has seen 75T of steel being fitted into the base prior to the “2nd” base slab being poured.

Base Plug pour

Waterproof sheeting installed

Whilst the job of installing the rebar has been taking place, the new Sub Agent (Filipe) and I have been trying to coordinate the next two phases. The next phase in the construction of the shaft is to pour the base slab (430m3). However there have been a number of issues. The Concrete mix design that was submitted by the Sub Contractors (Barhale) failed to reach the spec required by KF10 ( Materials and Workmanship Spec – In Situ Concrete). This is because it had not been tested for  ‘Dry Shrinkage’ . The result of this has meant that I have been going through legacy concrete mixes, approved for other component ie Piling, Ring Beam to see if the mix complies with KF10. However there is no mix that has been tested for Dry Shrinkage. As a result the construction could be placed on hold for 90 days whilst a new concrete mix is designed and trialed/tested (requiring 90 day results) or continue with the pour and hope that the samples taken today reach the ‘dry shrinkage’ requirements in 90 days. THerefore I have just sent them this recorded Email:

‘1. They have failed to provide a mix with the appropriate testing information to confirm compliance with the relevant specifications (KF10) 2. It is their decision to proceed with pouring the base slab with a non compliant mix, and therefore is their risk 3. Any remedial works that are required to be carried out as a result of excessive shrinkage occurring will be covered entirely by Barhale 4. They will need to take appropriate samples at the time of the concrete pour and organise testing to confirm compliance of the placed material with relevant standards (KF10).’

Fast forward 28 days and hopefully the base slab will be poured, it will pass the dry shrinkage tests and the remainder of the shaft is waterproofed with sheeting. At this stage the In Situ lining will be poured. Amazingly again Barhale have failed to provide a concrete mix that reached the  KF10 requirement. Again myself and Filipe have tried to recover the situation. THe only mix that complies with the In Situ lining requirement is the Segment mix. This mix is designed with a 30min life cycle. THe idea being that 250 pre cast segments are produced each day at Old Oak common. These segments are then delivered to site. However in our case we need to get the wet mix to Fisher street and let it cure in situ. Clearly the life cycle needs to be extended to approx 4hrs. This requires approx 4% retarder, however this is a change in the Mix design. AS a result we need to carry out trials and testing, hence I am going to Old Oak Common on Monday to pour 4.2m3 of “Segment Mix” and 5.5ltrs of retarder and probably an extra 50ltrs of water (the water is required as our mix will be pumped and the segment mix is too dry for pumping). Hopefully I will be able to record the correct slump (S4) and the correct life cycle (approx 4 hrs). If I can do this I will pour it into 3 x panels, then Test Consult (concrete laboratory) can collect the panels and carry out the required testing (Compressive strength, water penetration, dry shrinkage etc). CRL have agreed that if the 7 day results are OK then we can use the mix for the insitu lining, as adding retarder should not effect the qualities of the already approved “segment” mix. As you can see, how it has got to this position I do not know. I would have thought that before the shaft construction went ahead, someone would have double checked that we had all the required mixes in place and ready to go. Im being a little harsh, as the detail of this saga is a little more complex but unless Filipe and I can produce a concrete mix with 4 hrs life cycle and a S4 slump, the shaft project will go on hold….again.

However on a positive point, Im off to the ICE presentation on the London Super Sewer in an hour and Donaldson Associates have accepted me for my 3rd phase placement, meaning I will be located at the base of the Shard by Jan next year.