Horizontal Cold Joint in Unreinforced Underpinning

[JKJohn]

Underpinning a wall footing of a two story masonry building (not much loads but very very critical structure)
The underpinning (no reinforcement) is segmented at 4-feet each. The depth is about 5 feet down from the bottom of the footing.
For one of the segment, the contractor ran out of concrete 1 foot below bottom of footings. The top of the new concrete is fairly leveled.
The contractor is proposing the use of bonding agent and continue the pour.
My issue is that there is no justification that this would work. I have seen horizontal joints in footings with vertical reinforcement and had no issues with it.
I am not anticipating any issues with the vertical loads.
It is lateral what concerns me. I am expecting some seismic load but have no idea if it is anything worth discussing or even how to calculate it.
Any experience/ideas/brainstorming?
Thanks,

 

[KootK]

When done by the book, I believe that underpinning design involves some consideration of the lateral loads imposed by earth pressure etc. That produces moment about horizontal axes and, therefore, flexure in the underpinning concrete pours across the plane that you're concerned about. Presumably this is what has sparked your concern. If you can justify a flexural capacity without relying on concrete tension, based on stocky proportions etc, there might be some hope there. I'm pretty skeptical about the efficacy of the bonding agent as used in what is likely a difficult install application. As I understand it, improperly applied bonding agent can become bond breaker if used incorrectly. You might be able to install steel strap reinforcing on the tension side but I'd have a hard time getting behind that from a durability perspective. For a single section of underpinning, I think I'd be pushing for removal.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JKJohn]

The calculation includes soil pressure verification. I believe this is for construction only. Once the backfilling is complete the soil pressure is balanced.
I was talking earlier today to a friend engineer and his point of view was, there is no difference between the original footing to the new underpinning condition and the underpinning (new section to be added) to the underpinning section that is already in place.

 

[BridgeSmith]

For concrete repairs on bridges, we commonly use a bonding agent, but our notes include a requirement that if the bonding compound gels before the concrete is placed, they have to sandblast it off and reapply. However, we really only use it to seal the joint, since there is always reinforcing crossing the joint. If there's moment across this interface, I would recommend making them epoxy in some rebar dowels. If it's only shear, you could have them cut in a keyway. They screwed up by not forming a keyway at the joint when they had to stop. We include a detail for keyed joint for emergencies on our slab pours, and our construction specs include a requirement that they have to form a keyway at any unplanned joint locations.

 

[KootK]

Once the back-filling is complete the soil pressure is balanced.

I may not understand the situation then. In all of the underpinning situations that I can recall, there's a low side floor and a high side floor creating a permanent imbalance in solid pressure. Can you provide more detail regarding the situation. Is this retrofit frost protection or something?

The calculation includes soil pressure verification.

Does that verification specifically address moment in the underpinning concrete, particularly at this horizontal construction joint? Color me skeptical.

I was talking earlier today to a friend engineer and his point of view was, there is no difference between the original footing to the new underpinning condition and the underpinning (new section to be added) to the underpinning section that is already in place.

I agree, there is little difference. Both of the planes that you've described should be checked for stability and flexure. However, if one plane was not checked properly for these effects in the first place, then it's a rather egregious logical fallacy to use that as justification for neglecting the check at the unplanned construction joint.

The bonding agent business is something that I've always wondered about myself. I typically deal with it in the context of trying to make something behave compositely. I'm used to seeing it handled via pull off testing with direct tension stress is the 1.5 MPa range. It's unclear to me whether or not bonding agents can effectively restore 100% of concrete modulus of rupture strength which might be on the order of 300 psi. Pretty close I guess.







I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JKJohn]

I may not understand the situation then. In all of the underpinning situations that I can recall, there's a low side floor and a high side floor creating a permanent imbalance in solid pressure. Can you provide more detail regarding the situation. Is this retrofit frost protection or something?

The underpinning is to allow grade beams and auger cast piles to be constructed for an adjacent building.

Does that verification specifically address moment in the underpinning concrete, particularly at this horizontal construction joint? Color me skeptical.

The calculation does address rotation similar to retaining wall calculation. There is no moment verification at the location of the cold joint. There is one at the bottom of the original underpinning design.

 

[spieng89]

Have you explored using a carbon fiber wrap? It's works great for situations like this where you need to retrofit tension capacity into a concrete section

 

[KootK]

The calculation does address rotation similar to retaining wall calculation. There is no moment verification at the location of the cold joint. There is one at the bottom of the original underpinning design.

That does sound promising then. Should be a simple matter to adapt that analysis to the lower cross section, check the numbers, and thereby simply know with certainty.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.