Concrete Water Retaining Reservoir - Pour Sequence

[Everynameistaken]

Hi All,

I have a question about the pour sequence of a reinforced concrete reservoir tank.

We have completed quite a few of these over the recent past and I am looking for ways and opinions from people in industry to try and make sure we are state of the art and efficient in our designs and construction sequencing.

Typically we design these structure per the CSA A23.3 (Canada) for the strength requirements and then employ the Eurocode rational approach for checking the reinforcing for the a service case of crack control. This approach almost always adds flexural bars to reduce the bar stresses and the size and spacing of the calculated crack widths per Eurocode. The we are now almost always adding Xypex or the like as a self healing additive.

My question comes relating to the pour sequence and potential timing delays between adjacent pours, since we try and use a state-of-the-art design procedure I want to make sure our construction methodology side is also meets the same standards.

In the past we used to use a checkerboard approach with adjacent pours having to be min 14 days apart. We have moved away from this to a strip model where our foundations slabs are usually in 3 strips with again adjacent pours having 14 days between. Then we build the reservoir in these three strips including the walls and the roof slab, all with 14 days between adjacent pours. This often leads to the two outside strips being poured on day 1 and the middle strip being poured on day 15.

I am looking for advice on this methodology, do people agree?
is 14 days to long?
Should we be pouring the base slab all fun one shot and letting it shrink as one entity?
should we pour strip1, then 2 then 3, but with less or no restriction on timing?

Another thoughts from people out there would be much appreciated.

 

[msquared48]

I assume that you meant 14 days rather than 145 days at the bottom of the 5th paragraph?

Mike McCann, PE, SE (WA)

 

[Everynameistaken]

Yes Sorry typo!

 

[SlideRuleEra]

Should we be pouring the base slab all in one shot and letting it shrink as one entity?

What is the size of base slab (both the volume and the surface area)? As the volume goes up, a more sophisticated (expensive) contractor may be required to manage the placement. Same thing for surface area, more concrete finishers and improved project coordination.

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[Everynameistaken]

Hi,

The base slab is in total 275 M^3 (360 yd^3) of concrete. A perimeter and central strip of thickness 700mm (27") and inner slab 300mm (12") thick.

The reservoir has two cells each with a slab surface area of 238m^2 (2500ft^2). The base slab has total plan dimensions of about 20m x 28m.

The slab is split into three pours that are the full width of the tank.

 

[SlideRuleEra]

360 yd³ of that thickness is enough concrete (in one placement) to require a fair amount of planning and coordination by a contractor. Spitting the work into 3 placements does bring the volume of each placement down to a range where more (qualified) contractors can handle it without stretching resources. Concrete finishing area does not sound excessive.

As part of the decision on any changes, I would include discussion with potential bidders on how they will handle one placement. Also, in general terms, how would that change affect their costs (per cubic yard).

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[Everynameistaken]

Hi,

Thanks for the thoughts, I agree it is a fair pour size in one but not unmanageable.

My question is more with regards to the effects of shrinkage on the final water tight structure. If we split it up, should we be waiting a certain amount of time (14 days) between adjacent pours to allow for shrinkage, kind of like the old checker board slab theory, or should we allow them to pour the adjacent slab section with little or no time between or should we pour the whole slab in one shot, and let it shrink as a single unit and crack as it cracks?

 

[SlideRuleEra]

Based on just the technical issues, I would make it one pour. 20m x 28m is not that big.

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[JAE]

I assume you include a waterstop across the joints correct?

Also - per ACI 350, a couple of snippets:

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faq731-376

 

[hokie66]

My approach would be to insist on casting a slab of that size in one go. Not large in plan, and not an excessive volume of concrete for any substantial contractor to handle. Minimising joints is my philosophy. As to shrinkage cracking, casting in strips helps in one direction, but makes things worse in the other.

 

[Everynameistaken]

Hi All,

Yes, full PVC waterstops for all joints below the waterline and Volclay stops at the wall to roof connection.

 

[Everynameistaken]

Also, what about the walls?

Aprox. 7.0m tall, we have them split into the three pours above the slab, so two "C" on the ends and an "I" in the middle, again with two weeks between.

 

[SlideRuleEra]

A contractor qualified to make one pour for the base will be able to handle the walls in less than 3 pours. I would make it 2 pours, however. This will allow reuse of the wall forms for the second wall pour (cutting in half the square footage of wall forms needed for the project).

My concern on the commercial issues has to do with the total concrete volume for the project. A well staffed contractor, who can easily handle a larger pour, may be competitive on an 8000+ yd³ project. He probably would not bother to bid if this is an 800+ yd³ project.

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[BUGGAR]

When we did Bayview Reservoir in California, we went through the full concrete shrinkage process (ACI). We had to get a concrete supplier on board early to start the shrinkage tests on the proposed mix. Then follow up testing during construction. This gave us concrete cure time numbers to strip by. It was an expensive process and it made scheduling impossible but this was justified because it served La Jolla in California. The pours were successful, however we used pour strips, form vibration, etc., so I can't say how much the shrinkage dance helped.