Holdfast Training Services Trial Blog

Following the successful pour of South Block’s Upper Basement Zone 2 slab last week I continued the motion to start on Zone 3 this week with a programmed pour date of Wed 10^th. This involved the coordination of concrete cylinder testing and post tensioning to ensure that the pour could be completed on time after Zone 2 had been tensioned. The stated criteria for tensioning on a 32MPA concrete suspended slab beam is 9MPa or 24hrs for the initial stress to 25% of the maximum, and 5days and 22MPa for full tensioning. In this case the structural consultant allowed the period to be reduced to 3days and 22MPa for speed of construction. The RFI to use a stronger early age mix to assist in this process has now been approved by the structural consultant but has been held up at commercial the commercial level. In my view, with a liquidated damages rate of A$180,000 per day, the increased cost of a 40MPa early strength mix will be negligible if the project runs over.

The reinforcement tying went in without major issue except a couple more clashed with PT ducts and a lack of reinforcement design for a stair core that was partially encased by the slab. The design was for finally released on the Thursday evening before the Wednesday pour placing an increased pressure on the steel scheduler and the steel provider to get the couplers to site in time to be fixed. This didn’t happen, they arrived at 1745 the day before the 0530 pour and still sit idly next to the stairs. (I can potentially use them for a penetration that has been missed from the initial steel schedule but will require structural sign off first.) The solution to this issue is to drill and epoxy starter bars into the core wall prior to construction of the internal landing. This was already the solution for the other half of the stair wall that missed the inclusion of couplers when initially cast prior to my arrival so is not seen as a major issue. In my eyes it is galling that the parts were on site prior to the pour but there was nobody there to fix them. If I could have done it myself I would have.

The clients representative made a visit to the site the evening prior to the pour and made several observations to the structural consultant about finishes on column tops (not rough enough) and splashed concrete on reinforcement. Both easily fixed, one by me with a spare piece of reo smashing into the top of the column to scabble, the other after a fair degree of effort coercing the contractor to do it and in the end having to do it the morning of the pour.

The pour itself began well. The initial concrete mix was a 32MPa with Xypex and Eclipse as the surface was to be subject to external weathering and hence would require these additives to increase its durability. This mix flowed into a normal 32MPa mix. I placed myself at the concrete pump to check the mix prior to pouring and ensure quality of the batch. About halfway through the pour the formwork contractors spotters below the deck who move with the pour monitoring any deflections alerted the site manager that one prop had settled 30mm and that the pour should move to another area while they fixed the issue. This was done but the concreters deemed the deck to move so were immediately removed to a safer area whilst an inspection of the underside was conducted. The area was re-propped and initial props placed ahead of the pour to ensure stability. The pour continued with the concrete just on the limit of its use-by time (90mins post batching). The slump had decreased but was still in range.

 The concreters then claimed the formwork moved again and were once again removed from the slab. The H&S coordinator barricaded the slab and called in the Construction Director and Project manager to manage the initial investigation. It was found that a prop was not sat on the roadbase surface and had been undermined by water flowing from the deck above and had consequently settled 30mm. The subsequent additional propping had been a little over zealous and had lifted one of the deck units 15mm give the concreters the impression that the whole deck had dropped. The slab was deemed safe and after a brief H&S representative meeting the concreters returned to work, working a nearly cold joint. This continued whilst I turned away 5 trucks that were out of range and waited for new concrete to arrive whilst the concrete pump cleaned its pipes. The remainder of the pour was relatively uneventful.

My pour card was removed for evidence in the H&S investigation. Comcare (equivalent of the H&S executive was informed but as it was not a complete of partial collapse, and the incident caused no injuries, it was not notifiable. The unions were also informed and were reluctantly granted access to site – they found nothing and were satisfied with procedures. That said the rumour mill amongst tradies is similar to soldiers and pretty soon news that the deck had collapsed and dropped 180mm were rife. I hear it also made local news and Glenn Palin, MD of JHG was also informed. I am informed that there is often a small degree of settlement that occurs in falsework but to call it a collapse is an exaggeration. The clients representative has now issued a suspension notice to prevent any further suspended slab pours until it is satisfied by a JHG report that the correct processes were and are in place to manage such issues and ensure quality of the product. My pour card formed a substantial part of this report and included density tests of the roadbase, falsework plans and sign off from a 3^rd party certified engineer of its compliance and structural stability, and sign off from all contractors that their work was to the required standard. I’m glad I had all the ducks in a line and the only issue that my PM had to answer for was that of quality management.

I was subsequently involved in the H&S investigation to establish the root causes of the problem, not to issue blame, though it was blatantly evident that everybody in the room was posturing to remove themselves from the circle of blame.

I think the incident, although it had the potential to be catastrophic, was managed well. The systems were in place to monitor the pour and have been proved to be effective. One of the bigger issues I think is one of information release. Even the contractor referring to the incident as a partial collapse sends out the wrong message from the start and required stamping out early. In the event it wasn’t and spread across the site and indeed country rapidly which only served to increase pressure to perform on an already pressurised build. The additional work to compile reports took a day from all the South engineers and PM which was time that was not spent making an impact on site.