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.
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
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
RE: Mass concrete cold joints
RE: Mass concrete cold joints
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
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RE: Mass concrete cold joints
RE: Mass concrete cold joints
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
(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