Holdfast Training Services Trial Blog

A few weeks ago we completed the first of our substantial concrete pour operations in order to construct the main tower core pile cap in our basement. This is a big block of concrete. The pile cap is roughly 18m wide, 36m long and 3.5m metres deep giving a total C32/C40 concrete volume, with quite a few lift pits thrown in for good measure, of 2830.5m³. We are constructing it in four roughly equal  sections. During the first of these pours  we emplaced 610m³ of concrete in one go. The pile cap will eventually carry roughly half of the 40 storey towers structural load so getting this right in terms of quality is important.  This task was also on the critical path so ensuring it went  in error free and on programme was a major concern. As the blog title implies, all did not go to plan.

Whilst my ability to walk over re-bar mats without looking like an amateur construction worker is improving rapidly, even I felt reasonably uncomfortable being stood on top of the cage suspended 3m in the air with clear sight to ground. The top mat re-bar inspection is probably the most unsafe I have felt at any point on this project. The potential to fall through a hole in the mat or beak an ankle/leg was alarming, as such work was stopped on multiple occasions to address H&S concerns. Thereafter I took to doing re-bar checks from Ground level looking up. Despite its considerable size the re-bar cage is a surprisingly simple design made up, in general, of 32mm dia bars at 200mm spacing’s. We have 4 layers in the top and 4 layers in the bottom.

Unlike most concrete pours the interesting thing about our methodology is that once the formwork is stripped away it’s conveniently easy to inspect a complete face of the pile cap throughout its whole depth. This makes post pour quality assurance checks considerably easier for the main contractor whilst also making it more difficult for  the sub – contractor to hide any poor work underground.

The above picture shows the top 1.5m metre of our 3m deep concrete pile cap once the formwork had been removed. The structural engineers were initially quite concerned about the concrete quality. If you look closely you can see horizontal lines running in the concrete throughout its complete depth. Initial concerns were that the pour process had enabled horizontal cold joints to form between delivery loads of concrete.  Had that been the case, then we were concerned we had produced a concrete block with weakened shear planes running through it.  The question I tried to get answered was: why is this a problem? As I see it; If you simplify the pile cap in the permanent condition and consider it a large beam (the concrete block) carrying a large UDL (the tower) through numerous evenly distributed supports (the load bearing piles) then I think you would end up with a very simple FBD and BM/ SF diagram. With a few convenient assumptions and a quick check on google I think it would like something like the following

If indeed this pile cap behaves like a beam (And not a reinforced deep truss) then it sees greatest shear over the pile positions. Under the load of the completed building multiple horizontal cold joints caused by poor installation will undermine the capacity of the concrete pile cap, particularly in those positions. As it’s such a vital structural component stripping out this concrete and starting again was suggested by one overzealous member of the construction team. That was quickly put on hold because breaking out 610m³ of heavily reinforced concrete would be very unpleasant, not to mention crippling for our tight programme. As such an investigation was completed and after chipping away 30mm of concrete face the effect disappeared and with it the concern over the complete block.  Had it continued we would have cored into the concrete block in a desperate search for any shred of evidence that would have prevented the need to strip it out.

Interestingly, even had we found weakened horizontal shear planes running through this mass of concrete, automatically stripping it out would have been a hasty reaction. When you look at it simply, all we need to do is install a solution that allows shear to be effectively transferred between the layers. Conceptually this could also be achieved by post fixing shear links into the pile cap by drilling and installing vertical reinforcement steel rods through the concrete. If we emplaced enough steel, focusing on the areas where the shear is known to be greatest, it is likely we could provide enough shear resistance to mitigate the long term risk. This would be difficult but considerably easier, quicker and cheaper than stripping it all out and starting again.

We are still unsure what caused this effect so consistently. In general concrete work seems reasonably poor quality across site. At the time I asked the concrete sub-contractor what lessons they had learnt and what measures they were putting into place to improve this process, they couldn’t give a particularly good answer; concerning when you consider our next pour was 850m³ of the stuff.