Auger Cast Pile - Rebar Cover Tolerance

[ThStructuresGuy]

I have a project with 16" diameter auger cast piles and a cage with 7-#8 bars with #3 ties. We have specified a 3" clear for the cage, however some of the pile cages have come up with a 1 3/4" cover (project is under construction). To me this is out of specification per ACI 117 2.2.2 "-1/2 when member is over 12in" and/or ACI 318 7.5.2.1.

Are there any code requirements where we can reduce the required cover to less than 3"? Two of the piles out of tolerance are gravity only, the third is at a P5 lateral pile cap.

Is the solution to abandon the pile and construct a new one, I can't think of another repair solution.

 

[phamENG]

It's a stretch, but could the piles be considered to have been "manufactured under plant control conditions" for everything but the rebar? (i.e. The proportioning, curing, etc. was held to a higher standard than typically seen in the field.) I doubt it, given the rebar placement, but if so you could fall into 7.7.3 - which puts you much closer to being ok. Not sure I'd hang my hat on it, but if your project particulars fall in line it may be an option. Of course if you're in highly corrosive soils, you're in trouble, and if you're anywhere near the ocean with a high water table you'll probably be in trouble too.

At the end of the day, if the piles need the reinforcement and it's not adequately covered, the service life of the pile and thereby the entire structure is pretty significantly reduced. I'll add the caveat that I don't have much experience with augercast piles, so take my opinions as you will.

 

[JAE]

For drilled piers we used to show a 2" clearance on our plans. This was not because of the reasoning phamODU suggests (plant controlled conditions) but rather due to the fact that the actual construction of a drilled pier resulted in a shaft diameter that was actually larger than the bit size.

As the drilling commenced, the Kelly bar and bit would have some degree of wobble in them, and the crane operator would have only limited ability to position the bit EXACTLY at the pier centerline.

So if you used an 18" diameter bit, you'd usually end up with a 20" or larger earth shaft. So a cage fabricated to 14" diameter (2" clear on an 18" shaft) would actually have 3 to 4 inches of clearance.

With a flight auger you may not have that degree of wobble or over-cut on the shaft but you could investigate that with some other subsequent shafts.

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[SlideRuleEra]

The U.S. Federal Highway Administration (FHWA) interprets (see page 120) required minimum cover (3") being measured from the longitudinal steel. Transverse reinforcement minimum cover is 2.5", so there is still a problem.

As a former bridge contractor, suggest not being to "harsh" on this spec deviation. The key to what is "harsh" may depend on who designed the rebar cage, your firm or the contractor?
1 3/4" cover may be pretty good for this type real world construction, unless corrosion issues which phamODU pointed out are a known concern.

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[JedClampett]

I'm a softy, but if they were taking measures to give the specified cover and this one got away from them, I'd accept it. Give them a stern look and ask them to do better or the next time, you'll make them dig it up. Cover is pretty arbitrary (why not 2 1/2"? or 3 1/2"?). It seems like a code decision made in a smoke filled room.
The auger cast piles I've seen use these centralizers. They seem fool proof. Allow the one bad one and make them use them in the future.

 

[hokie66]

Jed, those centralizers work well with bored piles, but I don't see how they would stay in place with auger cast piles, in which the reinforcement is pushed down into the fluid concrete.

It sounds like the OP is just saying the cages are too big. I don't know how you actually determine how far the reinforcement ends up from the edges of the excavation.

 

[ThStructuresGuy]

Just an update, and a thank you for the responses. They are continuing to uncover piles and the amount out of spec has increased to 7 piles. We are allowing up to a min of 2" clear cover as IBC 1808.8.2 allows for 2.5" cover on deep foundation elements not enclosed by a steel pipe and ACI 117 allows for a 1/2" tolerance.

The pile contractors engineer has requested the loads for the piles in question and will redesign the piles ignoring the rebar that is outside the 2" cover. However, we have a concern about the possibility of these bars corroding and spalling the face of the pile below grade. Has anyone seen this suggested repair before where the bar is ignored for design?

 

[dik]

We often ignore the reinforcing steel; the concrete capacity if generally far greater than the load, unless the pile is taking a moment loading. and just throw rebar in 'for the heck of it', or to 'tie' the pile into whatever is above.

As long as the pile is laterally restrained (by the soil, even), there should be no issues.

Dik

 

[dcarr82775]

I would let 1.75" go. My thinking on ACI318 cover (even though 318 doesn't apply to drilled piers) is that the 3" for cast against soil acknowledges that the soil profile will be rough and some locations of less than 3" is acceptable. 1.5" is good for a formed surface exposed to earth.

Are cages too big, or inadequate/no centralizers?

 

[ThStructuresGuy]

dcarr82775, the cages weren't too big, they used inadequate centralizers and/or didn't stabilize the cage after they dropped it into the grouted hole so it shifted.

 

[SlideRuleEra]

ThStructuresGuy - What is the actural design outside diameter (not the nominal diameter) of the rebar cages?

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[Lomarandil]

What are these piles supporting? (Sorry, I don't know what a "P5 lateral cap" is). How large are the lateral/bending loads compared to axial?

Where are you measuring the achieved cover? Top of pile, or can you get that somehow along your pile length?

Certainly I'd be OK with the gravity piles being out of alignment (assuming your soil isn't total swampy junk). Like dik mentioned, the rebar is only in those to help us feel good, maybe some ductility/durability, and connection to your structure.

If you were talking about a flagpole (large moments), I'd feel differently.

----
The name is a long story -- just call me Lo.

 

[ATSE]

"...the cages weren't too big, they used inadequate centralizers and/or didn't stabilize the cage after they dropped it into the grouted hole so it shifted."
Or the auger flights are worn
Or the hole is squeezing or sloughing
Or the superintendent has no idea what he is doing
Or the crew is using the wrong type of centralizers
ACIP is very dynamic, requires a much higher level of training and inspection

 

[JLNJ]

If the one or two bars which don't have the required cover didn't exist, would the pile still be OK?

It's not like the piles are scantily reinforced. Seven #8s is something like 2.8%. That's a lot of reinforcement in a gravity pile.