bond-breaker vs tilt-wall rebar ?? 7

[boffintech]

Am an inspector (not EOR) on some work on a tilt-wall project. I have not asked the EOR about this yet but will if needed. No tilt panels have been poured yet. Thirty to 40 are under construction. The contractor is using Maxi-Tilt as the bond-breaker between SOG and tilt-panels. It says right in the Dayton instructions "Do not spray on reinforcing steel." But regardless I'm apparently the only one on the project that thinks it a bad idea to spray the reinforcing steel with the bond-breaker. Do I have it all wrong?

 

[BAretired]

Here is another article on the subject.

BA

 

[BAretired]

Another article.

BA

 

[SteelPE]

Is there any insulation in these panels?

On a project that I am currently involved in calls for a 3" concrete face 3" of insulation and a structural back up. I don't think there is any reinforcing in the face but I may be wrong (I'm not designing these panels). The panels are cast face down against the SOG with the lifting lugs on the back side of the panel. Once the bond breaker is placed and the face case they should need to use bond breaker again. So, on my projects I would think it would be difficult to spray the structural reinforcing with bond breaker.

 

[BAretired]

sandman21 wrote: "The EOR should have some very kind words for the GC regarding this issue."

And he did: OK as-placed - spraying rebar with bond breaker not an issue.

The question is: was the EOR justified in this opinion? There seems to be some literature indicating that he was.

BA

 

[hokie66]

BA's second article indicates that while the ultimate bond strength is not affected by the bond breakers, there is more initial slip, which could mean more deflection in bending elements. The contaminents affect the bond, but the deformations still mechanically develop the bars.

 

[BAretired]

So, can we conclude that standard practice is to spray bond-breaker with reinforcement in place? I am beginning to believe we can.

BA

 

[spats]

BA,

I didn't get a chance to read all your articles... doesn't it matter if the bondbreaker is petroleum-based vs. water-based?

 

[hokie66]

BA,
I don't think we can conclude that this is standard practice, and it is definitely not allowed by most engineers where I am. Actually, I doubt that anything about tilt wall construction can be said to be "standard practice", as there are lots of different ideas about best practice in tilt wall construction.

 

[BAretired]

spats,
Petroleum based and water based bondbreakers were included in the tests.

hokie,
I'm not sure that "standard practice" was the right choice of words. Perhaps "acceptable practice" would have been better.

Prior to seeing this thread, I would never have even considered allowing form oil to come in contact with reinforcement and I believe that most engineers in my area feel the same way. However, the cost of removing the reinforcement to spray the forms, then replacing it is being questioned in the above named literature and I am simply wondering if we have been overly cautious in the past.

BA

 

[hokie66]

I don't understand why anyone would want to do it in that order. It is much easier to spray before placing the reinforcement.

 

[BAretired]

Hokie, I suspect that nobody intended to do it in that order, but rainfall occurred after the reinforcement had been placed and before the concrete was poured, so the bondbreaker had to be applied again.

BA

 

[hokie66]

I see. Yes, boffintech did explain that. I don't think that is a valid reason. Rain creates havoc on job sites in a lot of ways, but should not be allowed to compromise quality.

 

[TXStructural]

I will bring this to the Tilt-up Concrete Association when we meet with them at World of Concrete in February.
The reinforcing industry obviously needs to clarify what is required and why. When it comes to how much effect there is from "materials deleterious to bond", it seems that we are talking in terms of fractions rather than orders of magnitude. There is a good bit of research going on right now into straight and hooked bar development length, and we expect results on these projects within the next year.

As far as development of deformed bars goes, only about 15-30% or bond strength is generally considered to rely on surface bond between steel and concrete. One of the previously-linked articles noted that "initial slip" was affected. This could very well be the critical part of the equation, since once slip starts, the only resistance remaining is provided by resistance to splitting (that is, concrete tension strength and confining reinforcement and geometry.) Splitting failures of unconfined splices can be brittle and sudden. In the case of a thin wall reinforced in one plane only, reduction of adhesion between bars and concrete could decrease the force required in the bar to induce splitting along the plane of reinforcement.

 

[hokie66]

And therein lies the problem. Thanks, TX. Look forward to clarity on this.

 

[SteelPE]

Yeah more codes, brought to you by the SOB's who don't want to do their job.

 

[OHIOMatt]

I ran through this exact scenario a few years ago. I came to same conclusion that the EOR for this project did. I still did not like the idea, and knew that I would be working with the GC and subcontractor again in the future. My main concern was cracking due to the reinforcing slip during lifting. It was in an enviroment of moderate freeze thaw. I suggested to the owner that they require the GC to purchase a performance bond that would cover repair of the panels if the need arose in the future. This way the owner was protected.

 

[dik]

My initial reaction was that this was not a good practice and I'm still of that opinion. It appears that it is not as detrimental as I would have originally anticipated.

Three items that would be of concern are the initial slippage, and a greater tendency to cracking due to lack of bond and the consequences of this. OHIOMatt has, somewhat, addressed this item. The second item would be the possible interaction of the chemical with the reinforcing steel and the promotion of corrosion. The third item that Ron alludes to, is that in accepting this 'silliness', it's only moved the 'goalposts'... and permits 'greater silliness' down the road...

Dik

 

[Archie264]

I'll be interested in seeing what, if any, similarities there are in the discussion of this issue as there might be with epoxy-coated rebar. I know ACI provides a reduction factor when epoxy-coated rebar is used and I know from personal observation that the surface of the concrete that's exposed to it appears quite slick in the core samples I've seen. Just something to think about...

 

[TXStructural]

Official word from the Tilt-up Concrete Association (TCA), regarding bond breaker on reinforcement:

Always remove the reinforcement and apply bond breaker to the form slab. This is for a couple of reasons:
1) Bond breaker applied through placed reinforcement will have irregularities in thickness and surface that will cause shadowing of reinforcement patterns onto exposed surfaces. And
2) Thick applications of bond breaker can result in adhesion of tilt slabs to form slabs, resulting in pop-outs and other defects in the tilt panels.

This is what TCA tells their member contractors and designers.

They tell us that they have seen drips and such cause both of these problems, as well as recognizing that ACI prohibits materials deleterious to bond on the reinforcement (and a bond breaker is by definition going to interfere with bond.)

I will be reviewing ACI 551 (best practices for tilt-up concrete) and will get back to the forum soon.

John Turner CSP PE
CRSI Greater Southwestern Regional Manager

 

[dik]

I'm not sure that item 1 is not something that can be easily overcome... I would think that if there was a drip on the reinforcing this would form a 'soft' spot on the bar and a drip on the form... I'm not sure that 'shadowing' would be a big issue...

Also, it's interesting that there is no reference to bond... one of my bigger concerns.

Overall, I don't think spraying with the rebar in place is a good practice... I've not encountered this problem, but, wouldn't permit the contractor to do it...

Dik