Options for repairing shear cracks in concrete beams

[StrEng007]

Trying to get an idea of what can be done to fix shear cracks in existing beams.

This request has come through on a 40 year old structure, but I'm not sure yet if this is was due to lack of stirrup detailing, insufficient grade beam design, or the incorrect placement of a piles support.

1. If the analysis AND field investigation proves that sufficient stirrups were used and cracks still occurred, what is the protocol?
2. If the analysis OR field investigation proves an insufficient design, detailing, or construction, can we remove existing concrete to expose longitudinal bars and wrap new stirrups?

Suggested reading?

 

[KootK]

Your path is likely to be very dependent on what these cracks actually look like and what the context of the members involved is.

Less subtle: give us some photos and details of the situation so that we have a modest chance of being able to help.

Some things to keep in mind with shear cracks:

1) Stirrups aren't doing anything until after shear cracks develop. Sometimes the cracks that freak occupants out are just the members performing as designed.

2) Many shear cracks are actually flexural shear cracks. It's tension face flexural cracking that gets the shear cracking started. This understanding, in combination with context, can aid diagnosis.

3) You mentioned piles so I'm guessing that we're talking about grade beams. The original purpose of grade beams was really to iron out differential deflection between piles. So, again, this could be an example of the member doing exactly what it was designed to do. Additionally, in a settlement scenario, both the moment and shear will be at peak values at the supports. This ties back into the flexural shear cracking thing that I mentioned in #2.

4) The kind of suggested reading that might be useful will be a function of the kind of repair strategy you use. And the repair strategy you use will be dependent on the size of the cracking that we're talking about and it's context. For small cracks, you might just inject them with repair mortar etc for durability. For very large cracks where a true shear deficiency is identified, there are a myriad of interesting details hat can be used. Most amount to installing external stirrups: through bolts, grouted rods, FRP wrap... Of course, such things may not play well with your durability environment, whatever that is. Sure, you can remove and replace the cracked concrete and add some new stirrups. That would normally be at the extreme end of the repair spectrum from my experience. Among other things, if the grade beams are restraining differential pile restraint at present, you may lose that once you disrupt the existing beam.

 

[StrEng007]

Initial review of the cracks was via a live screen share, I haven't had the chance to get some nice photos yet.

The cracks appear to be initiated as flexural cracks that followed a typical 45° web-shear crack through the web. What caught my attention the most is the crack has traveled right through the entire compression block, so it's a web-shear crack that made it all the way to the compression fiber.

You mentioned piles so I'm guessing that we're talking about grade beams. The original purpose of grade beams was really to iron out differential deflection between piles.

As usu, you're spot on. Very interesting what you've mentioned here though. I'm not sure why I never really put a whole lot of focus on the differential deflections between piles. I just always focused on grade beams supporting the overall gravity load transfer to the deep foundations. These soils (south beach, Miami) can't muster much compression and are prone to erosion... but at least they don't heave/swell.

Most amount to installing external stirrups

Say what! I'll have to find some detailing for something like that.

 

[KootK]

What caught my attention the most is the crack has traveled right through the entire compression block, so it's a web-shear crack that made it all the way to the compression fiber

That would catch my eye too. That said, I believe that a crack must extend through the entire depth of the cross section in order for the shear reinforcement to be engaged. Otherwise, the meaningful slip transverse to the beam required to strain the stirrups will not occur. I think that the cracking across the compression block is simply not visible to the naked eye in many cases precisely because the crack is compressed over the compression block.

If we're talking about a big, gaping crack through the compression block, that suggests some other possibilities. One common source of cracking in grade beams is just the axial shrinkage stress developed in the grade beams from the axial restraint provided by the piles. We tend to stretch grade beam control joint spacing pretty far and/or often allow it to be omitted altogether. That usually works out okay but sometimes we get unlucky. Really stiff piles, high shrinkage concrete, etc.

Say what! I'll have to find some detailing for something like that.

I suspect that I have details somewhere. I tried to find them for you on my Dropbox but couldn't do it in five minutes so I gave up. I suspect that durability and constructability will neuter most of the solutions used for elevated, interior beams anyhow. Most versions of the reinforcement will have exposed steel / frp on the exterior of the beam and require work under the beam that would be tough. If this progresses to the point where you truly feel this kind of work is needed, let me know and I'll take a 15 minute swing at chasing down those details.

 

[TLHS]

I've done shear reinforcement from the top once with epoxied bars. There's a bit of literature in the past decade about epoxied in shear reinforcement and you can get a reasonable amount of reinforcement out of epoxied in bars intercepting the shear failure planes. The anchorage issue is the thing you have to get comfortable with and your spacing needs to get pretty tight to even try to justify that. I was definitely surprised that definitive anchorage was less critical than I expected for ultimate strength when I started looking through the literature. Throwing steel in there, regardless of how it gets anchored at the bottom seems to help a bunch. That being said, I'm pretty sure the shear cracking of all of these weird alternates is worse than conventionally anchored bars. You're also going to spend weeks trying to decide if you're comfortable with it based on your situation and the available research. Mine was in a situation where there was a very large factor of safety to the point where I knew I didn't need to rely on this reinforcement until well after service loads, and I saw some other alternate load paths. It was an extra layer of comfort rather than a primary load path. You also need to have a pretty good idea of how the existing bars are installed, because you're likely drilling pretty blind.

It's not an ideal option. If you can get some kind of bearing plate or other anchorage on the underside you'll be a lot more comfortable with the situation.

Seismic reinforcement literature is pretty good for this stuff, because stopping the non-ductile mechanisms is pretty important. Your preferences are likely:

-External Stirrups with plate/steel member under the existing beam (pretty typical seismic upgrade)
-Tensioned through bolts with bearing plates on the underside. You can epoxy the holes as well if you like (pretty typical seismic upgrade)

The two above are good for already cracked situations because you can tension them. May have exposure issues.

-FRP wrap / adhesives
-Cast a new layer outside of the beam with additional reinforcement and stirrups (this also repairs the cosmetics)
-Retrofit stirrups where you notch the concrete and put full U-Bars in and then cast new concrete or epoxy over them
-Epoxied bolts with nuts on one end for tensioning

The above have pretty good literature

-Epoxied bars without any sort of tensioning
-Straight bar retrofit stirrups on the beam sides without great anchorage

These ones are fiddly, but appear to have reasonable efficacy. You need to be comfortable for your application though and you're going to need to do a bunch of research

Personally I'd be trying to avoid this if possible and would be looking at all the available load paths. What's the failure mechanism here and how scary is it (i.e. is this life safety, potential structure failure but not life safety, serviceability, or cosmetic)? What is the grade beam really doing and what is the system on top of it? Is it potentially just cracking to release some sort of restraint that it wasn't designed for and that will dissipate with the cracking? Will the superstructure redistribute things? Is the cracking displacement based and self-limiting? If it is displacement based, are you comfortable enough with the current state of the structure that you can monitor the cracks for a year or two and see if there's changes? Can you apply whatever's happening as a displacement to the structure and see if you're actually in any sort of trouble? Are the grade beams deep enough that you can find a deep beam load path that's stronger than you expect using compression struts or the increased shear strength of members close to support points? Can you pull in some bearing capacity of the soil under the grade beams that gives you ultimate strength capacity?

 

[TLHS]

I don't have it in front of me at the moment to see how much it talks about shear reinforcement, but this book by Newman covers a lot about repair and retrofit concepts. All of his books are really good. I know there's at least a handful of pages with conceptual sketches about a few of the options I mentioned.

https://www.amazon.ca/Structural-Re...p-1260458334/dp/1260458334/ref=dp_ob_image_bk