Add Delay to Every 2nd Load of Concrete?

[MikeManning]

I'm wondering if anyone has come across this idea before.

I'm involved with a project doing a very high strength Mass Concrete pour. We did an insulated mockup to study the heat rise through hydration, and we are pushing the limits of temperatures in the range of DEF. So we are now exploring all options to decrease temperature gain. Obviously we've talked about using more SCAs, lowering the heat of the mix at time of pour, and supplemental cooling.

But an idea came up that I'm curious about - what about adding a retarder to every 2nd batch of concrete? We'll need about 40 loads of concrete overall.
I know that if we added retarder to every truck, that doesn't help the total heat rise, it just delays the total heat rise. But if we added it to every 2nd truck, theoretically, wouldn't that split up some of the heat gain and perhaps lower the peak temperatures?

Are there any other negative consequences that we might get if the concrete was not uniformly reaching strength?

Thanks

 

[jayrod12]

I think you're running into an area where it likely hasn't been tested. Our go to in these scenarios is using ice in the mix as part of the water content.

 

[flight7]

I have used mass concrete only once, on a 3000 psi mix, so take this with a grain of salt. Our peak temperature was reached after about 3 days, and stayed at a plateau for about 1.5 days before starting to slowly drop. This occurred in each of three pours. Based on my specific mix design, weather, formwork, etc., I would speculate that your idea would not have changed our peak temperature, but might have resulted in a slightly slower temp gain, briefer plateau, and slower subsequent drop.

 

[structSU10]

I assume this concrete gets vibrated in some way? That one truck batch will effectively mix with another? Are you envisioning pockets of concrete that begin curing at different times - different enough that it reduced heat production? I would guess that if that were the case you would be asking for cold joints at the least.I don't see how this is any different than just a 50% dose of retarder.

 

[SlideRuleEra]

Mass concrete is exactly what it says, a mass. Any given truckload is unlikely to end up at planned location. Placement sequence is modified because of field conditions or unexpected events... say a truck breaks down. Contractor performing placement needs the flexibility to put any concrete where needed, not just a predetermined location.

Use ice, per Jayrod's suggestion; we would use 100% ice on occasions.
Mix design that includes flyash.
Start placement at sunrise.
Wet curing, including pooled water on slabs when possible.

 

[TLHS]

I don't know if your idea is great. As I understand it, most retarders prevent hydration for a period of time. They don't slow down the reaction, they coat things and stop it from starting. It seems to me that you're risking a cold joint between the pours if that's the case. If the design can work with those cold joints, then maybe there's some merit to it, because you'd be staggering your heat peaks. But now you'll have shrinkage at different rates so you've created a different cracking issue.

What kind of fun adventure are you having where you need both high strength concrete and a mass pour?

 

[RWW0002]

I agree that retarder will be of limited use. Both due to the placement logistics mentioned above and due to the cement chemistry. By delaying one mixture you might actually shift the peak temperature of the mass during the driving hydration reactions... I don't want to delve too deep into the cement chemistry, but sulfate attack and DEF are complicated mechanisms. I would not mess with varying mixtures across the placement..

Use of Type II or V Cement and limiting fineness should also help lower the mass temperature. There is an added benefit of these cement types that they have limited C3A content, one of the main drivers of DEF and sulfate attack in general.

 

[MikeManning]

@TLHS - It's a "strong floor" for a university, which will be used for load testing. It's about 6' thick, slammed full of bar, 65MPA concrete, also needs to be FF100... going to be an interesting pour!
We did a mockup to see the temperature gain of the mix, and we are approaching 70degrees C at peak, into the area where we need to start worrying about DEF.
It's a self-consolidating mix, because of all the bar in the pour, which means we can't use ice in the mix and our mix can't be too cold for workability.

To give some context - that's a bit of why we are trying to think outside of the box a bit

 

[SwinnyGG]

There's always the 'nuclear' option... to go to active cooling.

400 yard pour ought to be in the 4-6 hour range. Starting your pour a few hours after sunset would get your concrete placed with an hour or two of darkness and cold night temps remaining during the last of the initial heat of hydration peak.. if you're using multiple pumps and your crew has their poop in a group, you may be able to get it finished and pooled prior to any early sun hitting it.

I would shy away from unconventional retarder/SCA strategies, as others have noted, because these sounds like a pretty important pour and I wouldn't want to be operating in the realm of the unknown.

 

[SlideRuleEra]

We did an insulated mockup to study the heat rise through hydration...
It's a "strong floor"...

Did the study account for heat transfer that occurs in a full-size slab, such as thermal conduction (down) to the subgrade "heat sink"?
Or was the mockup insulated everywhere except the top where there would be only convection heat loss to air?