Holdfast Training Services Trial Blog

Ok, so the last post took us to the point of trials taking place, in order to ascertain the correct construction method. As a result the number of reinforcement bars, from the piles into the ringbeam reinforcement, was reduced. THe designers conceded that there was only a requirement for 3 x bars/male pile to protrude into the ringbeam. These 3 bars would provide sufficient shear reinforcement and bond length (in order for the next phase of excavation, where the ring beam will be temporarily suspended). The designers did specify that 2 of the bars had to be on the near face and the other on the far face of the shaft.
With the reduced amount of bars requiring coupling, the 200mm SCL profile was easily achieved and the time required to couple the bars greatly reduced (from 8 per pile to 3 per pile). The ring beam reinforcement cage was put in place and the shuttering positioned ready for the concrete pour.

However, before the concrete pour can be started I had to do a pre pour check. This includes checking the reinforcement of the ring beam, ensuring the connections between the piles and ringbeam, checking the shuttering is as per the shuttering design etc. After reading Rich Farmers comment about the couplers on my last post, I had done some investigation and found that the Lenton Lock couplers ‘bolt heads will shear off when proper installation tightness has been reached, which allows for completely visual inspection’. AS you can see from the photo below, they had not!!!

In fact the sub contractors, nor the CRL supervisors had realised the huge oversight. I then checked the bolts and not only had they not been tightened sufficiently to shear off the bolt heads, they weren’t even finger tight. The result was the concrete pour was cancelled, the shuttering taken down and the couplers all tightened untill the bolt heads sheared off. I was as popular as a pork chop in a synagogue.  However the cancelled concrete pour was a fraction of the price it would have been to rip out the concrete ring beam and start again, as there was no way of proving to CRL that the couplers would have held.

You would like to think the fun stopped there but it didn’t. The next saga was the concrete mix. The idea was to spray the concrete (C32/40) in behind the shuttering using the SCL machinery all ready in place. This apparatus had already sprayed the shaft piles with a regulating layer of 75mm and would be used later to do the 400mm layer once we got below the ring beam. However SCL concrete mix is very much different to the C32/40 mix that the ring beam is constructed from. THe first batch arrived, loaded into the pump and sent down the steel tube and then into the flexible rubber hose. However within 30 sec the dryer C32/40 mix had blocked the hose and steel tubes. THey then tried dropping the mix in behind the shuttering with the skip, it didn’t work. Because the concrete had by now been on site for so long we took another slump test and it failed so the entire load had to be dumped. THis then lead onto the next issue, because Barhale had cancelled two concrete deliveries and then dumped the third the concrete supplier couldn’t deliver another batch till the morning. THis meant that the small amount of concrete that had got in behind the shuttering would have been curing to long before the next batch arrived. This would have led to a cold joint forming, as a result bthe shuttering had to come down for a second time. The small amount of concrete removed and the ring beam reinforcement cleaned.

We are now in the position to pour the concrete again. The spraying of the C32/40 mix has been scrapped and they are using a skip with a tremi on it. This should result in the first quarter of the ring beam being  poured within the next few hours. In reality it is about an 8th because either end of the ring beam reinforcement cage can not have concrete poured onto it., as it must be left open in order to tie in the next quater of the ring beam reinforcement cage.