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Foundation pad on top of a very thick plain concrete block?
3

Foundation pad on top of a very thick plain concrete block?

Foundation pad on top of a very thick plain concrete block?

(OP)
I am in a troubled situation.

1) I have to put a foundation pad
2)The pad is subject to some 20tons uplift
3) I have very little room for it and very little soil mass to rely on
4) Dont want to rely on the soil fill below it, cause no guarantee it will be compacted without damage to neighboring structure.

So I dig into the narrow area and fill with plain concrete and later on cast the pad

QUESTION

1) Is the bond between plain concrete and my later-on-added pad reliable againt 22tons ?

2) IF not, will dowels placed in plain concrete be useful?

3) If not, should I specify some SIKA? (I am not very happy with this alternative though)

4) Whatever my experienced friends recommend.

Respects and Regards

IJR

RE: Foundation pad on top of a very thick plain concrete block?

2
Hi IJR...
To get adequate interaction with the plain concrete block and an overlying slab for uplift, you will need a positive connection between the block and the slab.  I would suggest "dowels" but not in the shape or configuration of typical dowels.  I would use deformed bar in a rectangular configuration similar to the ties used for a beam.  Allow this to be cast into the plain concrete with enough exposed to engage the overlying slab.

I would not rely on a "glue" bond for substantial uplift.  If failure occurs it will be fast and unforgiving.  A typical concrete-to-concrete bond without enhancement would likely not be adequate, though I don't know the areas involved.

One thing you might consider is to use helical anchors to enhance both the plain concrete's uplift and the attachment to the overlying slab.  The helical anchor could be placed, the plain concrete placed, and an extension of the helicals allowed to extend through the plain block into the overlying slab.  This ties everything together for good mass resistance as well as soil interaction resistance.

Good luck.

Ron

RE: Foundation pad on top of a very thick plain concrete block?

IJR

There is another reason that Ron's suggestion of helical piers is excellent.  If you do not trust the native/fill subgrade, some means of carrying/supporting the new construction load, to include the concrete underpad 'ballast' is justified. If you are concerned about the nearby structure, the potential disturbance to the native/fill subgrade must be minimized. The helical should be designed to carry the excess load which the native/fill subgrade may not be able to carry. Depending on the soil profile, the design of the helicals may be controlled by either hold down required or the expected structure weight due to subgrade consolidation.  

This is an interesting problem and would require good soil samples, complete soil consolidation testing and some thought.     Have Fun!

RE: Foundation pad on top of a very thick plain concrete block?

(OP)
Easter Greetings

AND A MILLION THANKS TO RON
A MILLION THANKS TO EMMGJLD

Now for the rest who might visit here: My current position allows me to use dowels as explained above by Ron.In the area I am helicals are not yet in the market though there are efforts to bring them in.And definitely no glue bond.

Respects

IJR

RE: Foundation pad on top of a very thick plain concrete block?

IJR

While thinking over your problem, I realized that I got tunnel vision, based on Ron's excellant suggestion of helicals. Not too long ago I had 2 projects which used small diameter (<10" or 25 cm)piles, which could be driven or pushed. Depending on the subsoil conditions, sufficient resistance for uplift could be achieved, along with desirable increased bearing.

As per my previous suggestion, the piling could take a small to large percentage of the load, both support and uplift, making part or all of the extra excavation and concrete ballast unnecessary. Unless very firm bearing is encountered during pile placement, the piles and the subgrade soils will probably be subject to some settlement. A balance is usually made between the amount of settlement (convenience) which can be tolerated and the length/number of piling (expense) to be inserted.  I am afraid that precise numbers will be hard to come by, a condition which is always worrisome for my structural brethren.

But never fear, the Romans used the same technique with wooden piles, some works which lasted for centuries!!

Good subsurface information, laboratory testing (consolidation) and interpretation will be required. My previous, similar projects were interesting and I am sure this will be for you.

RE: Foundation pad on top of a very thick plain concrete block?

(OP)
Thanks Emmgjld for keeping the faith and revisiting here.
And it so happened that your last comments turned in the right day where my concentration is at a max.

So thanks man

Respects
IJR

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