Cast-In Starter Bars vs Post Fixed Epoxy - Edge Breakout

[BacBac]

Hi All,

Does anyone know why the effect of edge breakout is more severe in post-fixed rebar with epoxy compared to cast-in rebar?

Say you're drilling a post-fixed rebar the same length as what the development length of the cast-in rebar, if you check it in Hilti or similar software, the capacity will not be even close to the yield capacity of the cast-in rebar due to edge breakout failure. While in cast-in rebar, there is no edge breakout if you embed the bar as development length. Please see snapshot below for clarity.

I'm asking this because there's a site issue with missing wall starter bars, and the wall is in tension at its edge. The post-fixed rebar needs to reach its yield strength to resist the tension demand.

Thanks in advance!

 

[ANE91]

Subject to pure tension, there is no edge breakout, because that is a shear limit state. You're thinking of splitting, which is prevented with minimum edge distances and/or confining rebar. Chapter 17 will limit your ability to develop the bar this close to the edge. You can design the post-installed rebar to behave just like cast-in-place rebar, if embedding beyond 20*d. Certain epoxies have this capability; refer to the ESR for RE-500-V3, for example.

I have no problem developing a #4 bar 1.5 inches from the edge. Good luck getting the contractor to drill a straight-enough hole at that distance; they're more likely to break the concrete just making the hole. I suggest specifying drill guides. (Yes, it's a means'n'methods thing, but that's the only way your design intent will be achieved.)

 

[BacBac]

@ANE91

Subject to pure tension, there is no edge breakout, because that is a shear limit state. You're thinking of splitting, which is prevented with minimum edge distances and/or confining rebar.

I meant the concrete breakout failure that is worse when your post-fixed rebar is close to the edge. Please see snapshot below for clarity.

Chapter 17 will limit your ability to develop the bar this close to the edge. You can design the post-installed rebar to behave just like cast-in-place rebar, if embedding beyond 20*d.

I'm assuming this is referring to ACI? Could you please guide me to which clause you are referring to?

Good luck getting the contractor to drill a straight-enough hole at that distance; they're more likely to break the concrete just making the hole.

Yea, we will be specifying this to be done and monitored by Hilti representative as they specify that they can drill up to 60db.

 

[ANE91]

Can’t speak to non-US codes.

Yea, we will be specifying this to be done and monitored by Hilti representative as they specify that they can drill up to 60db.

Exactly. You’ll find these provisions in Acceptance Criteria 308. ACI 318 relies on ACI 355.4, which itself establishes AC308 as the requisite test regime. Chapter 17 governs the design of anchors, be they cast-in or post-installed. By definition, these provisions are only applicable <20d embedment. There’s nothing special about rebar in a hole vs. all-thread in a hole. They behave more or less the same way at shallow embedments. Go deep enough, and your post-installed rebar starts to behave as if it were a cast-in reinforcing bar rather than an anchor, shunting you out of Chapter 17 and into Chapters 12 and 21.

ACI 318 17.1.3(d)
ACI 355.4 1.2.1
AC308 Table 3.8

This topic has been discussed ad nauseum on this forum.

 

[BacBac]

@ANE91: Thanks! Very helpful.

 

[EngDM]

Can’t speak to non-US codes.

Exactly. You’ll find these provisions in Acceptance Criteria 308. ACI 318 relies on ACI 355.4, which itself establishes AC308 as the requisite test regime. Chapter 17 governs the design of anchors, be they cast-in or post-installed. By definition, these provisions are only applicable <20d embedment. There’s nothing special about rebar in a hole vs. all-thread in a hole. They behave more or less the same way at shallow embedments. Go deep enough, and your post-installed rebar starts to behave as if it were a cast-in reinforcing bar rather than an anchor, shunting you out of Chapter 17 and into Chapters 12 and 21.

ACI 318 17.1.3(d)
ACI 355.4 1.2.1
AC308 Table 3.8

This topic has been discussed ad nauseum on this forum.

Interesting that you get thrown out of anchors when you go deeper. In CSA A23.3 Chapter D you just get thrown into a differen equation past a certain depth and your strength gets axed. I wonder if there is a similar provision to treat it as a cast reinforcing.

 

[ANE91]

Interesting that you get thrown out of anchors when you go deeper. In CSA A23.3 Chapter D you just get thrown into a differen equation past a certain depth and your strength gets axed. I wonder if there is a similar provision to treat it as a cast reinforcing.

The US approach is overcomplicated, but it makes sense. The adhesives are usually a two-part polymer or a cementitious material. They’re so strong — the bond formed between steel and existing concrete often greatly surpasses the concrete’s direct grip on cast-in steel. Mobilize enough concrete by volume and the only possible failure mode becomes yield. Grossly oversimplifying…gotta watch out for dusty holes and adhesive seepage. We’re still researching to improve Chapter 17…it doesn’t help that development length provisions are already sus.

Hijacking this thread: Why do you lose strength, with depth, in Canada?

 

[EngDM]

The US approach is overcomplicated, but it makes sense. The adhesives are usually a two-part polymer or a cementitious material. They’re so strong — the bond formed between steel and existing concrete often greatly surpasses the concrete’s direct grip on cast-in steel. Mobilize enough concrete by volume and the only possible failure mode becomes yield. Grossly oversimplifying…gotta watch out for dusty holes and adhesive seepage. We’re still researching to improve Chapter 17…it doesn’t help that development length provisions are already sus.

Hijacking this thread: Why do you lose strength, with depth, in Canada?

It only happens once your breakout cone hits edge of concrete, as the equation is based on the ratio of theoretical cone to actual cone at the edge. So with depth the theoretical denominator keeps increasing but the numerator doesn't. So for stuff like grade beams and pilasters, sometimes shallower is better (according to the code).

Now if you lap it to the grade beam steel to preclude breakout, that is a different story.

 

[CDLD]

Where are you seeing this clause of concrete breakout doesn’t apply to embedments deeper than 20d? Based on below snapshot, full bar development does not preclude breakout on cast-in reinforcement.

 

[ANE91]

Where are you seeing this clause of concrete breakout doesn’t apply to embedments deeper than 20d? Based on below snapshot, full bar development does not preclude breakout on cast-in reinforcement.
View attachment 6861

Follow the code path outlined above. It’s not an anchor in the eyes of the code beyond 20d, so 17 doesn’t apply. Agreed; bar development does not preclude breakout, and I might be able to dig up an old paper that proves just that. But that has nothing to do with whether the rebar is treated as an anchor (<20d) or not. Don’t think I said otherwise…apologies for any confusion. We covered this in a recent thread:

Thread 'Understanding ACI 318-19 - Rebar as Anchorage'
https://www.eng-tips.com/threads/understanding-aci-318-19-rebar-as-anchorage.526734/