Carbon Fiber Reinforce Polymer (CFRP) to reinforce the top of a drilled pier

[dauwerda]

I had an interesting call today and I am looking for some help and opinions.

Background:
This is in regard to the construction of a substation, so lots of cantilever columns with 4+ anchor rods. These structures are installed without any grout under the baseplate, instead they are installed with the baseplate clamped between a lower leveling nut and the upper nut, with no more than two anchor diameters space between the top of concrete and the bottom of the base plate. The foundation reactions range from 2 kips of shear and about 10 kip-ft of moment to 50 kips of shear and 2500kip-ft of moment. Foundations are drilled piers that range from 2ft diameter (lower loads) to 8ft in diameter (higher loads).

Issue:
Apparently when the contractor installed the rebar cages for the drilled piers they were allowed to drop to the bottom of the drilled piers, so where the first hoop should only be 2" below the top of the concrete it is instead anywhere from 1ft to ??? below the top of the concrete (they have started scanning as well as drilling holes to locate the rebar for each pier). They are now looking for a way to remedy this issue.

My thoughts:
Most if not all of the anchors are pretty long (minimum of 25x anchor diameter embed depth and more in most cases), I haven't run any numbers yet but I'm thinking tension capacity of the anchors will be less of an issue (still plenty of lap with the vertical rebar) and the shear will be more of an issue. So, the first thing to do will be to check anchor capacities as if there is no reinforcing and if it checks out for some of the structures, probably no further action will need to be taken. For the structures/anchors that don't check out, my first thought was to reinforce the top of the piers with CFRP. The problem is, I don't have any experience with that.

What do you guys think? Is CFRP a valid solution? What other ideas or solutions do you have? Has anyone ever run into this before?
If CFRP is a good solution, can anyone give me any pointers/background on how that is typically coordinated - is this a specialty design that the provider/installer performs or would they be looking to me for design info?

Thanks!

 

[KootK]

This has been very informative, and I've learned alot about CFRP wraps.

If anyone would like to learn more about how GFRP is used to strengthen columns, the president of Quakewrap recently authored a Concrete International article on the topic which can be downloaded for free here: Link. It's both informative and very brief, which is key for me these days.

 

[driftLimiter]

@KootK I am curious about the radial confinement pressure acting in towards the center of the pad. I don't see why the wrap cannot preclude concrete breakout... if you had enough pressure acting over the outside area of the frustum The concrete cant displace in the way needed to make the breakout happen.

Of course there is a relationship between the confinement pressure and the 'hoop' stress of the wrap that would need to be balanced. Maybe its too much load to develop with wrap. But in principle I am thinking that the wrap restrains and confines the concrete with some equivalent pressure.

Its been a while since I had advanced concrete course. I suppose you can't develop the confinement pressure until you have sufficient axial load to cause Poison's ratio to kick into gear and make the confinement due to the wrap possible. But I see some similarity with the Poison's effect, and the outward pressure due to concrete breakout.

By all means set me straight if you think I'm way off course here.

 

[KootK]

@KootK I am curious about the radial confinement pressure acting in towards the center of the pad. I don't see why the wrap cannot preclude concrete breakout... if you had enough pressure acting over the outside area of the frustum The concrete cant displace in the way needed to make the breakout happen.

If that radial pressure acting inwards is present before the concrete breakout frustum forms then I agree, that would do the job. This is basically exactly what I was getting at when I proposed the prestressed steel collar above. The key to it, however, is that it prevents the shear breakout frustum from ever forming. Thus, the spalling potential is eliminated.

With the rebar/GFRP hoops, the character of the prestressed system is changed from that an active system to that of a passive system where no hoop stress is developed until after the shear breakout frustum forms. At which point the spalling issues likely neuter the thing per the sketch below. You simply have to move the failure frustum a fair bit before you develop meaningful hoop stress in a non-prestressed, passive system.

But I see some similarity with the Poison's effect, and the outward pressure due to concrete breakout.

Certainly, there is some similarity. However, there is also a critical difference in my opinion. And that difference is that true confinement situations, ala Poisson, create a uniform hoop stress around the entire hoop such that the hoop is in complete equilibrium and nowhere subject to local shear or bending. It's this local shear and bending that ultimately doom the shear breakout condition.

As I've mentioned previously, the effectiveness of the hoop stress is intimately related to the angle at which the hoop exists the potential shear breakout frustum. And that's a function of of the curvature of the hoop. A tighter radius of curvature will yield better results. In the extreme, imagine the sketch below in your mind's eye when the radius of curvature is so large that the hoop exits the shear failure frustum effectively orthogonal to the applied shear load. That's basically analogous to an anchor trying to pop out laterally through the top of a wall, right? And in that case, I think that we can all agree that the reinforcement would be ineffective.

The effectiveness of the hoop steel in any real situation lies somewhere along the continuum between awesomee ties parallel to the load and the useless "hoop" of the wall example.

 

[KootK]

Its been a while since I had advanced concrete course.

If you learned any of this stuff in school then you clearly went to a much better school than I did. I learned about beams, columns, development length (incorrectly), and antiquated methods for estimating deflection. And I got a C- because there was a new gal in my life that semester and I decided that sleep was extraneous for a spell.

 

[azcats]

Anecdote: One of my classes in college was instructed by Dr Ehsani from Quakewrap. Think he was just getting that started when I was down there.

 

[BridgeSmith]

You make a good point about the possible buckling of the unconfined anchor bolts if it cracks significantly.

If analysis shows that they will crack from the shear, I'd strongly consider busting the concrete down far enough to get mechanical splices onto the existing vertical reinforcing, and chip out for a shear key (a square key with plan dimensions of 40% of the shaft diameter is optimal), add a hefty reinforcing hoop near the top of it, and repour the top.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[BridgeSmith]

1) Per ACI, if you're going to assume an equal distribution of load to the front and rear an anchors, that needs to be justified somehow. Weld washers, non-over sided holes, bolt clamping friction... something.

If the base plates are installed like we do our high mast light towers, tightened by turn-of-nut so they are fully tensioned and checked with hydraulic torque wrench at 6000 ft-lbs (for 1-3/4" HS anchor rods), I think the clamping friction will be plenty sufficient.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[KootK]

If the base plates are installed like we do our high mast light towers, tightened by turn-of-nut so they are fully tensioned and checked with hydraulic torque wrench at 6000 ft-lbs (for 1-3/4" HS anchor rods), I think the clamping friction will be plenty sufficient.

Perfect, I'll take it if present.