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We had an 8000 gallon concrete water tank installed. 12" thick foundation and 6" thick walls.

The forms came off during a wet and rainy week - it rained on and off and was cool (45F) and cloudy the first week of curing. We filled the very bottom of the tank IMMEDIATELY with an inch or two of water - so the foundation has never been without standing water on top of it.

HOWEVER, we don't have a water source yet so we could not fill the tank more than about 150 gallons per day.

It has now been two weeks, water level is about 1500 gallons out of 8000 gallons, and the builder/contractor says we need to fill the tank with water ASAP to support the curing process.

So my questions are:

  1. Is this too little, too late ? Those tank walls have been without water on them for two weeks since the forms came off - can we put water back onto them and slow down this cure in any meaningful way ?

  2. Isn't this all-or-nothing - either we fill the tank to the brim or we don't ? He is suggesting fill it up halfway but what about the concrete above the water line ? If less than a full tank is still useful (wicking ?), why isn't the two feet of water we have now just fine ?

  3. Just how badly have we botched this up ? Or did the cool temps and intermittent rain save us ?

Thanks.

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  • unfortunately you will find in few weeks if it was to late, when the micro cracks appear
    – Traveler
    Commented Nov 22, 2022 at 22:57
  • The good news, it might have sucked up some water from the bottom part. I had a 10,000 Gallon pool build as in-ground concrete, all I had to do is keep the walls wet for a week, but not fill up with 10,000 gallons, now. 4 years later it still looks good.
    – Traveler
    Commented Nov 22, 2022 at 23:01
  • @ruskes when you say keep the walls wet, do you mean, continuously wet with a mister or sprinkler or do you mean just spray it down with water 3-4x per day as I hear a LOT of people say when they discuss managing a concrete cure ...
    – user227963
    Commented Nov 22, 2022 at 23:05
  • @ruskes as for the micro cracks - aren't those just surface/shrink cracks ? I am not concerned with aesthetics at all - I just need the tank to not leak for 50 years ...
    – user227963
    Commented Nov 22, 2022 at 23:06
  • I sprayed it down 2 times a day (mind you I live in hot+dry Arizona), in case you have some "cosmetic" cracks you can repair those if they leak
    – Traveler
    Commented Nov 22, 2022 at 23:09

2 Answers 2

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Can we put water back onto them and slow down this cure?

You misunderstand utterly. Concrete does not "dry" and cure - concrete sets and needs moisture to cure - it's a chemical reaction. Drying interferes with the curing process, being wet helps it proceed, and the concrete becomes stronger as it cures (by a very large factor.) So you are not trying to slow the cure, you are trying to keep it going, as you are slowing it by allowing the concrete to dry out.

The tank does not need to be full, but the walls need to be wet. You could put a sprayer, mister, or fogger in the tank for that purpose - you could put soaker hoses on the top of it and run them from a pump in the tank, or any number of ways to keep it wet. Covering with wet burlap is one method that helps to wick water up and prevent surface drying. Covering the wet fabric with plastic sheeting helps to keep it wet if water is not being continuously applied.

The following is quoted from Page 220 of this (book on the web, I guess, though it's just one chapter linked) Design and Control of Concrete Mixtures x EB001 http://www.ce.memphis.edu/1101/notes/concrete/PCA_manual/Chap12.pdf

It covers the fact that curing will resume, but you may not get the full strength of the concrete if delayed too long.

When moist curing is interrupted, the development of strength continues for a short period and then stops after the concrete’s internal relative humidity drops to about 80%. However, if moist curing is resumed, strength development will be reactivated, but the original potential strength may not be achieved. Although it can be done in a laboratory, it is difficult to resaturate concrete in the field. Thus, it is best to moist-cure the concrete continuously from the time it is placed and finished until it has gained sufficient strength, impermeability, and durability. Loss of water will also cause the concrete to shrink, thus creating tensile stresses within the concrete. If these stresses develop before the concrete has attained adequate tensile strength, surface cracking can result. All exposed surfaces, including exposed edges and joints, must be protected against moisture evaporation.

Which is pretty much a direct answer to the title question: yes, it's better to manage the cure after mismanaging it than to compound the problem by continuing to mismanage it.

Page 221 notes:

If sprinkling is done at intervals, the concrete must be prevented from drying between applications of water by using burlap or similar materials; otherwise alternate cycles of wetting and drying can cause surface crazing or cracking.

Page 221 clarifies:

Alternate cycles of wetting and drying during the early curing period may cause crazing of the surface.

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  • Thank you. How much is our 1/5 tank level helping us ? Is it only helping the tank interior that is actually submerged or does the entire structure get some benefit from that water through wicking ?
    – user227963
    Commented Nov 23, 2022 at 3:00
  • Some benefit - sure, it's better than if the tank was bone dry. But dubious that water is wicking up the face of the concrete any significant distance, so you really do want to address getting water onto the surface of the concrete, ASAP, by any means available. A semitruck trailer tanker could probably fill most of the rest of the tank in one go, for instance.
    – Ecnerwal
    Commented Nov 23, 2022 at 3:29
  • Thank you. These two weeks post-pouring were cold (lows of 38F highs of 55F) ... if this was, as your book describes, an interruption of moist curing, was the impact of the interruption less serious than if it had been warm ? That is what I meant by "slowing" the cure - I meant slowing the bad effects of dryness during the cure ... were these negative effects "slowed" because it was cold and humid/raining ?
    – user227963
    Commented Nov 23, 2022 at 4:04
  • 2
    humid/raining obviously helps, and cold on the one hand slows the rate of curing, but also limits the rate of drying .vs. hot - but fundamentally, your concrete contractor is there, knows what the weather has been, and is telling you to get water on it - so make that happen.
    – Ecnerwal
    Commented Nov 23, 2022 at 13:00
  • Think of it as investing a month or two out of the 600 months lifetime you'd like to get, to ensure that you get the 598 months after the first two. Alternatively, consider what you have invested in the project $-wise, and view the cost of the tanker-full of water, or pump & sprayers/misters/foggers/soaker hoses as ensuring that investment isn't wasted.
    – Ecnerwal
    Commented Nov 23, 2022 at 17:09
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When the contractor says "fill with water immediately" they probably mean "keep the concrete wet". They don't want the concrete to dry out. You could do that just with sprayers, fed from circulation pumps that lift out of the water in the tank. If you aim them right, 90% of the water will fall back into the tank, and you won't need much make-up water to keep level.

Keep in mind this is not the same thing as watering plants. When you water plants you're really replenishing the soil the plants sit in. Your contractor wants the surface of the concrete actually kept wet so you should be running lawn sprinklers and the like 24x7. So hitting it with a hose a couple times a day is not the same thing at all.

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