Holdfast Training Services Trial Blog

The Wastewater Treatment Plant project I am working on has quite a lot of concrete tanks. Amongst these is the ‘Plant Water Vault (PWV)’ which for the first month I assumed was just another tank, but couldn’t place it in the treatment process. Turns out its not a tank, but a pump lift station. Confusion rectified it still looks like a tank and part of my role has been doing rebar inspections. In the grand scheme of things they are probably pretty simple as it is a box with a few, but not that many, penetrations.

Below shows the form work and rebar for the ceiling.

Rebar in the vault ceiling with the penetration for the service hatch.

And the ceiling is where the problem lies; it’s not meeting the design strength. The pours for the base and walls have been comfortably meeting the 4500psi strength in 28 days as one might expect. However, at 28 days the contractor had to do some rounding up of the percentage strength up in order to meet the 85% threshold for removing the formwork. Now at the 56 day point the last two breaks have come in at 4400 and 4320 psi.

So what… The short and simple answer is strip it out and try again it as it doesn’t meet the spec. At the 28 day point when we were looking down the barrel of this as a possibility that is exactly what my boss said cueing the squirming; not least from the concrete supplier who I believe will be footing any bill. However in reality all parties are aware that a compromise will probably be reached. The options on the table on Thursday were:

• Feasibility of replacing slab. This really is the last resort for all:
  • Quality: It will definitely meet the quality.
  • Cost: Nil for the Government; expensive for the contractor and concrete supplier.
  • Time: 2 weeks for breaking, 2 for forming, fixing and pouring, 4 for curing. The PWV isn’t on the critical path but the steel fixers have weeks of work in front of them and winter is coming fast.
• Destructive testing of cores. Will assess the difference between the cylinders and the real condition; they are as likely to be worse as better.
  • Quality: Difficult to find a gap in all the rebar to core.
  • Cost: Cheap for contractor
  • Time: Results back in a week before next decision is made.
• Structural Modifications. Adding more steel to bring to strength. This requires a design analysis of which element of the concrete requires to be 4500psi.
  • Quality: Unknown as we don’t know where steel will have to be placed. The PWV is hardly a work of art already so a bit of steel won’t hurt.
  • Cost: Less expensive than replacing the slab.
  • Time: Overall it would be concurrent activity so wouldn’t add to the programme.
• Analyse the slab to check the required strength. Apparently concrete is specified in 500psi increments so there is a chance that it is strong enough already. USACE could ask our design consultants to do it but that wold cost us money so we won’t. Instead the contractor can do it independently and our consultant can check it.
  • Quality: We wouldn’t get what we asked for, but we would get what we need if the calcs showed that the concrete was strong enough.
  • Cost: $500 – $1000 for a Professional Engineer for a day.
  • Time: A week or two and if all is good then no knock on to the programme time.

So we are going down the analysis option, which would be required for the structural modifications anyway. From there on we will see what happens.

The PMV in the background. The actual tanks are currently being poured and will keep the steel fixers busy until the end of November.

And finally…

This is now the second car crash I have seen in 6 months of being out here. The woman in the blue car turned in to get gas, but tried to go through another car to get there. I had been planning on using that pump but it was blocked when I had turned up. Fortunately no one was hurt but all of the blue car’s airbags deployed and her groceries met her in the front of the car.

I now put a couple of fuel pumps between me and the road.

PS. I don’t know what mix the concrete is Damo so don’t ask.