Concrete cover on ends of bars

[DTS419]

The standard definition for concrete cover seems to be "the least distance between the surface of embedded reinforcement and the surface of the concrete" (ACI ITG-7-2)

In cross section, this is typically specified from the edge of the bars to the face of the concrete.

Is anyone aware of any exceptions for the ends of the bars, say at the end of a beam or the edge of a slab? Is the concrete cover requirement the same on the ends of the bar, or can a lesser amount of cover be justified at the bar ends?

 

[BAretired]

Minimum required cover applies to any part of the bar.

 

[Tomfh]

BARetired is correct, however in practice reduced cover at the end of a bar is typically less critical than reduced cover along a bar. A way you can justify it is epoxy coating ends of bars, or epoxy coating cut faces, so the that pathway to the bar meets the code rules.

A place where reduced cover to bar ends happens all the time is where pavement slabs are saw cut. Zero cover remains. It generally doesn’t matter, despite completely failing the code rules.

 

[lexpatrie]

slabs really aren't under ACI 318. They aren't buildings.

I believe highway joints (and expansion joints in buildings) are cut/separated full depth with dowels to provide shear transfer for the various induced wheel and impact loads.

On, for example, a footing, you would want 3" side cover for the footing flexural bars, (this is going to provoke a debate, but "slop footings" without formwork on the sides should have 3" cover). As well as 3" cover to the bottom (cast against earth) (this will be less debated....), 3" side cover is perhaps debatable on a footing, but it's not all that critical in those last 3 inches, unless there is something odd going on with development length a dn you need that extra inch.

When it comes to a pier, the side cover is more debatable, as there's formwork (for sure) on the sides of it, and reduced cover is possible (less than 3").

"Clear" cover, i.e. edge of bar to face of concrete, yes.

I would not try to justify reduced cover by dipping the ends of the cut bars into epoxy. The intent here is to prevent corrosion, in the galvanic sense, the rebar doesn't know it's covered in epoxy at the carbonation front. While I'm not a corrosion engineer, (I may not have passed the bar but I know a little bit) that sounds like it wouldn't accomplish much. Unless somebody has seen that kind of approach used successfully, say, in a New York State DOT project? (road salt), or a research report?

 

[Tomfh]

The intent here is to prevent corrosion, in the galvanic sense, the rebar doesn't know it's covered in epoxy at the carbonation front

If the end of the bar is covered in epoxy there is no electrical pathway that can occur. You need a current and carbonated concrete for the corrosion to occur.

This is the principle behind plastic tipped metal bar chairs:

 

[hokie66]

Prestressed precast elements like hollow core slabs generally have reinforcement exposed at the ends.

 

[Trippelsewe]

@Tomfh the issue that I see with epoxy coating bar ends is that you still have close to zero cover around the bar. You could likely epoxy coat an area of the slab as well to mitigate that? Personally I spec repair mortar to suspended slab edges.

 

[Tomfh]

Yes I agree. By coating the bar ends, I am referring to a length of bar, like the bar chair pic above. Alternatively you would need to coat a wider area around the bar as you suggest, in order to increase the effective cover to the steel.

 

[MHobbs]

As Hookie mentioned it is normal for prestressed elements to have exposed ends.
I believe this is accounted for by the bearing lengths required and often by insitu pours or mortar beds at the ends of precast units.
I guess the cover to the reinforcement that is presumed to be effective is maintained as the first 25mm of strand or wire is presumed not to be effective.

 

[lexpatrie]

Metal bar chairs with plastic tips are basically formwork, they hold the rebar before the concrete hardens, corrosion at the tip is concealed by the plastic tip, not prevented. This is not analogous to coating the ends of structural rebar.

While the coating would seem to give some cathodic protection I'm not convinced it's been demonstrated to actually do this.

Epoxy coated rebar is most typically coated the entire length. That would act as an electrical break, for lack of a more technically correct phrase from the corrosion engineering folks.

 

[Tomfh]

This is not analogous to coating the ends of structural rebar.

When I mentioned that coating the ends of the rebar can inhibit corrosion, I was referring to the same concept as using metal bar chairs with plastic-coated ends.

Why wouldn’t it also work at bar ends? What the actual difference.

 

[dold]

In what case is doing any of this epoxy coating business going to be more cost effective than just providing an extra inch (or however much) of cover? Unless OP is asking because of an as-built condition.

 

[Tomfh]

Yes it’s preferable to increase the cover, but there are situations where you can’t, or situations where the something is already huilt, where some extra coating can provide an additional barrier.

 

[lexpatrie]

Why wouldn’t it also work at bar ends? What the actual difference.

Not to be abrasive, but try reading what I wrote.

 

[TLHS]

The cover is the cover and applies everywhere. If you have specific needs, it might make sense to reduce cover in areas, but you'd need to show that on the drawing.

If you have a field situation, remember that the CRSI placing manual has tolerances on the cover even though everything calls them "minimum clear cover." So the 'minimum' isn't actually minimum. How much you're okay with eating into your cover for a field condition is going to be pretty specific for a given situation though.