Holdfast Training Services Trial Blog

A really juicy one here on concrete for the E&M’ers to get their teeth stuck into.

Here is something to watch out for if anyone is involved in slipforming.

1. 1. The four shear cores in my area at Battersea are being built using slipforms. The main benefit of this method is that it can be constructed very quickly (if carried out by a competent contractor with adequate round-the-clock quality assurance).
   2. Quick recap on Slipform: Shuttering (the formwork) is attached to yolks, which are held up by climbing tube. The slipform rig is held up entirely by the climbing tubes which run to the base of the slipformed wall, and the jacks are almost continuously lifting the slipform rig. The concrete that is exposed below the shuttering only needs to support it’s own weight, which is continuously checked as the slipform is lifted.
   3. The axial force produced by the jacks must overcome the friction between the shuttering and the concrete to allow the shuttering to move up. However, if the shear force due to friction between concrete and the shuttering overcomes the shear strength of concrete then the concrete can fail at the surface, resulting in poor finish. If the concrete has cured and the shear force overcomes the tensile capacity of the concrete then whole sections of wall can be dragged upwards. This happened on one of the cores in Battersea, resulting in large tear-outs and long delays.
   4. Sub-contractor’s reasons: The high strength C75/85 concrete that was used in this particular section of wall was going off too quickly, resulting in it adhering to the shuttering. A possible but flaky argument trying to transfer the blame to the concrete mixers.
   5. Weapon fires, weapon fires, weapon stops… The concrete was repaired using traditional formwork on the hanging deck, and after 8 days of repairs slipforming continued as normal… for a few days until the same thing happened again in an area where the normal C50/60 concrete was being used.
   6. What was the reason for failure? Slipforming is supposed to be a continuous process, so that the concrete is given time to harden in order to support it’s own weight, but not given too much time because it can adhere to the shuttering. Also, this means that there will not be weak ‘construction’ joints with the concrete below which will give the concrete poor tensile strength.
   7. First incident (photos below): There had been a long pause in order to fix some large steel embedment plates before the concrete was poured. The concrete was poured on the night shift, where as it later emerged, the subcontractor had no QA managers working and an inexperienced concrete foreman/supervisor. It is possible that the high strength concrete was going off quicker, but this should have been anticipated and either arranged to be poured quicker or designed with admixtures to slow the curing process. An operative on site also let slip that the concrete looked as if it had gone off when it was poured.
      

      
   8. Second incident: (photo below) After the first incident, the exposed shuttering was cleaned, however the shuttering overlaps the concrete at the bottom by about 0.4 m to allow a good seal when pouring concrete. It is likely that in this area there is a film of concrete that has stuck to the shuttering, which makes it much easier for fresh concrete to bind to the shuttering. The failure occurred after a very hot day, which is likely to have compounded the problem due to increased speed of curing.
   9. After some investigation it seems as if the most likely problem is that the sub-contractor has not been managing their logistics well, allowing concrete to stand on site, or having large quantities turn up at once causing queuing. It seems as if there were two failings here; in logistics managemnt and in Quality Assurance. A real Sliphopopotamus.