Minimum Reinforcement for Mass Concrete 1

[damitall]

Full Story - We need to build a 3 to 4 ft deep by 15 ft square concrete base slab to support a circular water tank.

For the mass concrete base slab, the minimum reinforcement equations tell us we need X sq-in of steel per foot. Is the steel req'd all through the concrete slab or is it just required to place the steel on the outside edges?

 

[JMorrison]

If the reinforcing is required to resist design loads, it needs to be located consistantly with your design assumptions. Take a look at ACI 318, chapter 22 (Structural Plain Concrete) - if your stresses are low enough reinforcing might not be required at all, unless you have a waterproofing / weathering issue.

Assuming that reinforcing is required, for something of this depth it would be common practice to have layers near the top and bottom faces.

 

[damitall]

Let me ask a more general question.

If I have a hunk of concrete which does not have significant stresses and I decide to reinforce it with temp & shrinkage steel only, would it be sufficient to determine the steel req'd and place steel in the outside foot width of concrete only and disregard the concrete in the interior? (Treat the interior portions of concrete as if it was a controlled density fill.) Would the expansion & shrinkage of the whole concrete block need to be resisted by this outside cage of steel??

 

[Ussuri]

Is your base slab going to be directly in contact with the water? In which case you may need more than the minimum reinforcement in order to make it water retaining.

To my mind reinforcing the outer foot will restrain the concrete around the perimeter but the concrete in the interior will shrink. You may find you get cracks at the interface between the reinforced and unreinforced section.

 

[JStephen]

Typically, a slab to support a tank (not part of a concrete tank) would have reinforcement at the top and bottom faces. On a large slab, the bending is primarily in the bottom mat, and the bulk of the steel would go there.

 

[rapt]

damitall,

In answer to your second question, you only need to base the minimum reinforcement on the outside 200-250mm of the mass concrete.

 

[damitall]

rapt (or others),
Is there a code section which states your last response?

 

[miecz]

damitall

I don't see the provision for 200-250mm in ACI318. There is such a provision for Environmental Structures in ACI 350. However, ACI350s requirement for minimum shrinkage and temperature reinforcement is completely different (and more conservative) than ACI318.

 

[rapt]

There is more to engineering than codes. They set limits. They are not text books or bibles. if you were involved with code committees you would understand that a lot of the decisions as to what the code says are not based on good theory or good practice, they are often political.

Mine was an "engineering response" to something not covered in most codes.

It is about time many engineers realised that codes do not and cannot cover all situations and also that they are sometimes WRONG when they do cover it.

 

[MWPC]

damitall,

I am not sure of which sections of 318 you are referring but please consider: 1. most minimums can be ignored if you provide xx% more reinfircing than required by the design. 2. Do you care if it cracks? If so reinforce the face that you care about, probably the top, with 5@12 E.W. 3. In a 15' square unrestrained along the edges, the concrete will usually develope strength as it cures fast enough to resist the frictional forces resisting the shrinkage therefore no cracks. Hope this helps.