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Mass concrete cold joints
3

Mass concrete cold joints

Mass concrete cold joints

(OP)
I have some 9 ft X 9 ft X 9 ft underground blocks that I am considering having poured in 3 lifts creating 2 cold joints instead of applying mass concrete requirements and having the blocks poured all at one time. The blocks are being used to resist thrust loads from piping, so I will put vertical reinforcing across each joint and design each lift for the shear friction required to develop the soil passive pressure to resist the thrust.

How much time should they wait between pours to make sure the joints mitigate cracking from thermal differential? I would be using some 25% or more fly ash replacement with about a 0.5 w/cm ratio, but nothing special to mitigate thermal effects.

RE: Mass concrete cold joints

haynewp....it depends a lot on the concrete temperature at the time of placement and the ambient temperature. Assuming you are placing this in an excavation, the ambient temperature will be relatively consistent, given the depth. This can create a relatively larger temperature differential between the core of the concrete and the outer edge.

Your thickness of 36" lifts is borderline mass concrete. Considering that, determine the equivalent cement content as follows:

Count any portland cement as 1 lb. per lb./cy of the mix.
Count fly ash as 0.5 lb per lb./cy of the mix

Multiply the result by 0.14 and you'll get the approximate core temperature rise in deg F. So....

Let's assume you have a mix with 450 lb/cy of portland cement and 150 lb/cy of fly ash, your temperature rise would be.....(450 + (0.5 x 150)) x 0.14 = 73.5 F

If your ground temp is 72F and your concrete temp is 85F at time of placement, the ground will cool off the outer shell and the inner part will likely rise to as much as 160F....that's borderline to delaying ettringite formation which can cause delayed cracking of the concrete. The rise in temp will take as much as a day or so to occur.

Given all that, I would cut the lifts down a little to get four lifts instead of three and control the temperature at the time of placement to as low as practicable for your location, then wait 48 hours between placements.

RE: Mass concrete cold joints

For thrust blocks, I would tend to take the opposite approach. Several lifts in small excavations is a nuisance. Reduce the amount of cement, and reduce the temperature of the concrete by using shaded aggregates and chilled mixing water.

RE: Mass concrete cold joints

I agree with Hokie. We typically used 3000 psi concrete for thrust bocks for large piping at electric generating stations. That was very conservative.

This reference "Pumping Station Design", the applicable part of which can be read online, even recommends a minimum of 2000 psi concrete, all of it placed at one time. The way a thrust block distributes forces, I would be comfortable with 2000 psi concrete. Here is the link:
https://books.google.com/books?id=biWHfrpd9gsC&...

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Mass concrete cold joints

Agree...if you don't need the strength, drop the cement content. Also, if you don't need to gain strength in 28 days, increase the fly ash content to 50%. Use the largest possible aggregate. All will help mitigate the issue. If you need the strength as implied by the OP (w/c=0.5), then my previous post would apply.

RE: Mass concrete cold joints

(OP)
I don't need that much strength, they are only resisting around 100 kips. What would be a good w/cm ratio for a 2000 psi mix? I've never used that low of a psi for structural concrete.

Other than the ash and aggregate suggestions in the above posts, any other suggestions? I have the UFGS mass concrete spec but it has a lot more requirements than I think is needed for this case.

RE: Mass concrete cold joints

haynewp....for 2000 psi concrete, a w/c ratio of 0.75 to 0.80 should get you there. Considering that you'd probably like to place the concrete at a 5 or 6 inch slump and no admixtures, you'll need about 350 lbs of water/cy. With a 0.8 w/c ratio, that would yield 437.5 lbs of cement. Using 50% fly ash, you could calculate the approximate temperature rise as follows:

(219 + 109) x 0.14 = 46F

For a placement temp of 85F, the temperature could be expected to be around 130F at the end of the second day. That's a bit warm still; however, you can get the placement temp down by doing as hokie66 suggested with the aggregates and cooler mix water.

As I recall you are not that far north of me, so hot weather placement is not uncommon. You might consider a night-time placement. I would expect also that the ready-mix suppliers in your area have chipped ice capability as well.

RE: Mass concrete cold joints

(OP)
Thanks!

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