Concrete Beam Design Strength After Repairs

[DayRooster]

Hypothetical situation after Reading ACI Repair CEUs: A reinforced fixed-fixed concrete beam (1'-6" wide x 3' tall) has been designed with (3)-#8 rebar top and bottom. In this situation, let's say there are no stir ups since the beam shear was sufficient with concrete only. Now let's say 1'-0" from the beam end the concrete has spalled out around the bottom of the rebar and a (1'-6" wide x 9" deep x 1'-0" long) section of concrete is lost.

My question is this: if the concrete is repaired back with conventional concrete repair mortar (sika, eucluid, etc.) would this section of the concrete beam have a reduced shear depth and, therefore, reduced shear strength? Or would you still be able to account for this "patch" to act the same as would be required of monolithic concrete (specifically ACI shear strength sections)?

I am trying to visualize if the crack would start to form diagonally and what it would do once it hit the cold joint repair patch? Would the patch provide extra strength? Or would the concrete patch just split and there we would be left with a typical concrete crack down to the repair and then the crack would follow the shape of the repair cold joint.

Also, I am assuming no epoxy joint compound either. Just conventional cement based concrete repair mortar only.

 

[Just Some Nerd]

I tend to have little faith that repair mortars end up being correctly applied in practice even if in theory you could get it to behave as a perfectly monolithic section. I'm not sure what the answer to your hypothetical is, but I certainly know what I'd work with in a real scenario.

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Why yes, I do in fact have no idea what I'm talking about

 

[DayRooster]

Yeah I’m not overly confident in repair mortar either. I just see fixes like this suggested without any talk from ACI about potential strength implications. Like RAP-4 has all of these fancy images showing a person pouring back half a wall and half of a slab like this like it’s a piece of cake. Just slap a bit of frosting on it and it’s one big cake now. But nowhere does it discuss the effect on the ACI 318 design parameters like effective shear depth.

 

[KootK]

Or would you still be able to account for this "patch" to act the same as would be required of monolithic concrete (specifically ACI shear strength sections)?

I would say yes. If you can't count on the repair concrete in this way, then I would say that the whole philosophy of the repair would be bunk.

...would this section of the concrete beam have a reduced shear depth and

I don't feel that a reduced shear depth would be theoretically tenable. Vertical and horizontal shear demand within the beam will be complementary (albeit in a complex way for concrete). The effective shear depth of a beam has to extend to the tension reinforcing, pretty much by definition.

I share your and JSN's concerns over the terrible things that might happen if the repair quality is not what we would hope. But, then, that is kind of a different question.

Hypothetical situation after Reading ACI Repair CEUs

Would you be able to direct me to those?

 

[DayRooster]

KootK: I don’t disagree with you because I have no information pointing me either way on it. Kind of amazed all of this concrete repair documentation exists without even a brief discussion about strength.

Also, my the references are below. Maybe I overlooked something but I was just surprised it doesn’t talk about it. I’ve only done basic repairs in the past (luckily) so I’m not fully up to speed with the larger ones.

https://www.concrete.org/store/prod...LINE_LEARNING&Language=English&Units=US_Units

ACI 546 and RAP reports

 

[KootK]

I know what you mean. I spent some time at a firm where the folks who restored stuff were a different group than the folks that designed new stuff (me at that time). It always seemed to me that the restoration folks had developed some kind of selective blindness to exactly these kinds of issues. They just had this kick ass repertoire of "procedures" and that was pretty much it. If it was repaired, it was considered restored to it's original state or better. And that's probably true to a large extent. I still think that it's worth noodling on this stuff though.

 

[dik]

Ya, Koot... choose another manner of repair... BTW, they have 1.5x more bottom reinforcing than they need!

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[DayRooster]

KootK: I was reading more into the ACI RAP Reports and stumbled upon this paragraph.

"How do I check the repairs?
After stripping of forms various tests can be performed to confirm the placement of repair material has achieved
complete consolidation and intimate contact with the substrate to achieve bond. A uniaxial bond test can be performed by drilling through the repair into the substrate. A bonded plate attached to the core is pulled until rupture occurs. Bond values should exceed 100 psi (0.7 MPa), and in most cases exceed 150 psi (1 MPa). These tests are performed in accordance with ACI 503R Appendix (see Fig. 3)."

I wonder if this is how they ensure the repair is acting like bonded concrete. By doing a core drill of repairs section to check it. Even though that bond value seems low since I think the tensile stress of concrete is closer to 10-12% of the compressive strength...or maybe none of this matters...still not sure.

 

[Ingenuity]

I wonder if this is how they ensure the repair is acting like bonded concrete. By doing a core drill of repairs section to check it. Even though that bond value seems low since I think the tensile stress of concrete is closer to 10-12% of the compressive strength...or maybe none of this matters...still not sure.

In my experience, bond testing is commonly done to concrete repairs, most especially bonded concrete overlays.

The integrity of concrete beam repairs is also subject to the method used to do the repair: e.g overhead manual trowel applied vs form-and-pump, etc.

ACI 562 Code Requirements for Assessment, Repair, and Rehabilitation of Existing Concrete Structures and Commentary has some good info on this subject generally.

 

[DayRooster]

Ingenuity - I think this is the missing link that I was looking for. I will go back and search ACI 562 for more information the bond characteristics and tests. Thank you.

 

[dik]

... a link:

https://vdot.virginia.gov/business/...RTHACI_562_Repair_and_Rehabilitation_Code.pdf

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[DayRooster]

dik - That looks like a great resource. I noticed that some of the bigger repairs have anchors between the existing and new. I’m going to have to dig deeper into it. Thank you.

 

[MotorCity]

I am of the belief that any type of cementitious patch cannot be relied upon to homogeneously act as if the spall or break never happened. Like it or not, any sort of patch creates a cold joint. While there is clearly a bond strength between new and existing material, I am not convinced that it has the same bond strength akin to the internal "bonds" of the original construction. The patch repair also does not have the benefit of confinement. If it were somehow confined by surrounding concrete or wrapped with FRP or cover plated, then it would be less likely to pop off. If a concrete member was tested to failure, I would bet that the repair would pop off prior to any observable failure of the beam.

Now, there are bonding products that claim to be stronger than the concrete itself. If such products are in fact capable of that, and mockups and testing were performed to demonstrate this, then I would be comfortable saying that the repair is as good or better than the original member and would rely upon traditional methods of calculating there strength with no reductions. I would put the burden of proof on the contractor and product manufacturer. That said, I think in most cases the debate of a spall repair is a moot point when it comes to strength. Most of the spalls I have seen are spalls that simply expose rebar and effectively reduce or remove a local portion of the concrete cover. Concrete cover on the tension face of a beam does not contribute to its strength, nor does side cover unless it happens to occur in an area that reduces the compression block.

 

[dik]

Patching works for repair to strength, too. Toronto did a massive repair to their Lakeshore route where the repair was for strength as well as serviceability a while back...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik