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Minimum reinforcement for mass concrete 2

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AlexWong122

AlexWong122

Structural
Apr 23, 2018
28
Hi there,

I have a question regarding minimum reinforcement for mass concrete (4m W x 4m H x 24m L). As shown in the picture, this is a tunnel plug design and my calculation (based on tunnel plug design guidelines) shows that mass concrete will be adequate for withholding the water pressure upstream and no tension reinforcement is required. Now I need skin reinforcement for both upstream and downstream faces of the concrete to prevent cracking of temperature and shrinkage. The code gives equation of minimum steel area as 0.2% x gross area. Which gross area should I use? Is that the gross area of the upstream / downstream face (i.e. 4m x 4m) or is it the gross area of side face (i.e. 4m x 24m). If it is the side face then my rebar will be extremely heavy.

I am new to this so thanks for the helps.
1_gkkt40.jpg

Alex
 
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It should be the total area, you can split the rebar over both faces.
 
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When you say "the code says" what do you mean? ACI 318, ACI 350, or what?

I no longer have a copy of ACI 350, but I could swear it used to define the Ag for temp / shrinkage reinforcement based on outer 12 or 18 inches of the concrete.
 
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Heat is going to try and escape via the shortest path. So from the middle of the pour where the most heat is generated, heat will travel toward the tunnel walls as opposed to parallel to the tunnel. Therefore, use the area of the long face (4m x 24m). Also, for larger pours I try and distribute the shrinkage steel more evenly by using multiple layers of steel instead of just 2 layers (one at each face). The temperature at the middle of the pour is not the same as the temperature at the face of the pour. On a side note, have you given any thought as to how this will actually be poured? It may need to be done it several lifts which may require shrinkage steel in each lift.
 
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AlexWong122 said:
Now I need skin reinforcement for both upstream and downstream faces of the concrete to prevent cracking of temperature and shrinkage.
Click to expand...

I don't think any reinforcing steel is needed... anywhere. The concrete is underground in direct contact with tunnel walls (a very large thermal mass). After heat of hydration has dissipated by conduction into the tunnel wall thermal sink, what is going to cause temperature to change?

As for shrinkage... select a concrete mix design to minimize shrinkage. Need to do this anyway to make the plug as watertight as possible. Then let is "shrink"... any cracks won't be deep enough to cause problems on a 24 meter long plug.

Agree that placing the concrete is the real challenge. Including (unneeded, IMHO) rebar make it more so.

[idea]
[r2d2]
 
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I'd use the 0.2%, but, place it in the outer foot... if TS is the norm, then the heat of hydration will be much greater than the thermal cycling the concrete will go through during its life... You need to install thermal couples and maybe use icecubes for water... it will radiate heat for more than 2 weeks.

Dik
 
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ACI 350-06 Has the following table for minimum steel for sections thicker than 24 inches. I'm not sure where the rule is located in ACI 318-14.

If you have a 3ft thick section, then you can just look at top 1ft and bottom 1ft individually for minimum T & S steel. We use this provision for lift station slabs that can often be 3ft+ thick. ACI even lets you cut the steel in the base slab portion that is in contact with the soil by 50%. We usually do not do that though, we just match the top and bottom reinforcing.


ACI_350_Min_Steel_iitmhi.jpg
 
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Thank you Joel, that's exactly the section of ACI 350 that I was thinking about.

Note: ACI 318 does NOT have a similar provision. In fact, it's minimum reinforcement provisions (at least for slabs) would generally have you put in double the about of reinforcement of a 24" thick slab if you have to have reinforcement in both faces of the slab.
 
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Agree with SRE. Reinforcement will do nothing except corrode in the long run and deteriorate the plug.
 
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I would make sure you have a detailed curing plan to keep the temps down and within allowed range. A curing plan is more important here than min reinforcement.
I agree that pouring in sections is appropriate. The temp of hydration would be exceeded significantly if you pour such thick mass at one time (using your standard mix).
 
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