Dried concrete on rebar

[N3M4N]

Is it practical to ask contractor to clean up dried concrete from previous pours, The contractor I am working with thinks this is not practical that he has to clean up old concrete from rebar. I tried to explain to him that the old concrete if not cleaned will reduce bonding. I like to hear you thoughts on this?

 

[SlideRuleEra]

...old concrete if not cleaned will reduce bonding. I like to hear you thoughts on this?

You are correct, old concrete will reduce bonding. In fact, the photo shown is probably the worst that I have seen. This type problem is a construction management issue, it should not be allowed to happen. Short of sandblasting the rebar or taking the caged apart to "try" to clean the rebar, I've got only one suggestion. Don't allow the next concrete placement to take place until the problem is solved (by the Contractor, at his expense). Otherwise you, the EOR I presume, just inherited what was the construction manager's issue.

http://www.SlideRuleEra.net

 

[flight7]

I'm going to reluctantly play devil's advocate. CRSI has a free publication titled "Rust, Mill Scale, and Other Surface Contaminants on Steel Reinforcing Bars", which states that dried cement paste or spatter does not affect bond strength, even when the entire bar surface is coated with paste. With that being said, when I look at your photo I wonder whether there is so much hardened concrete that the clear spacing between bars is reduced. So I would agree with SRE's recommendation.

 

[JoelTXCive]

I wonder if the contractor could materially improve the situation by just hitting the cage with a hammer a few times? Maybe a good portion of the excess concrete would fall off?

I agree with flight7 that it appears clear spacing might be impacted. That is a lot of excess concrete on those bars.

 

[epoxybot]

See this thread from 2006 Concrete 'splatter' on reinforcing steel Link

 

[Ron247]

The research on splatter does not appear to indicate how thick it was. I see in the picture, one rebar has the "valleys" between the lugs filled with splatter. I wonder what splatter like that with bond-breaker doused on it would do. To me, that rebar needs to be cleaned. The CRSI article states the deformations appear to be the major contributor to bond. Well, if the splatter is thicker than the deformations, you have more of smooth bar than a deformed bar.

 

[TehMightyEngineer]

I'll join flight7 in playing devil's advocate; if it's well bonded (taking some effort to chip it off) then I'd say it can remain.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[MIStructE_IRE]

I had this recently on a deep transfer beam with large diameter bars. I had to get the guys to chip it off the top bars with a vibrating hammer. They got it clean, within reason, and its fine.

 

[TehMightyEngineer]

If you do get them to chip it off; try a needle scaler.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[N3M4N]

Thank you everyone for your responses the majority of you agreed that the concrete should be cleaned, the question is how do you pour a column or a wall and avoid concrete splatter on reinforcing steel? The contractor I am working with uses a large concrete bucket.

 

[SlideRuleEra]

...how do you pour a column or a wall and avoid concrete splatter on reinforcing steel? The contractor I am working with uses a large concrete bucket.

Use a "tremie" (large steel funnel with a long pipe attached to it). Keep the bottom of the pipe near (or even below) the surface of the fresh concrete as it is placed.

http://www.SlideRuleEra.net

 

[KootK]

One additional factor to consider in your decision making here, I think, is the function of the bars. In many cases, I don't feel that column tie bars actually need to bond to the concrete in any significant way. More of a reach around situation.

 

[GC_Hopi]

I have seen contractors build their own funnel using plywood and 2x4's if they are in a pinch. And when they get real desperate there is the old upside down traffic cone trick. lol

 

[MIStructE_IRE]

I agree Koot. These bars aren’t doing a lot for me anyway, so lube them up for the reacharound for all I care...

The only time I was really concerned about bond was when I arrived on site to see the contractor spraying shuttering oil on the forms with the cages inside, effectively greasing all the rebar!! It was promptly whipped out!

 

[KootK]

Yeah, I think that there are two parts to it really:

1) What must be done, if anything, to correct this specific case and ones like it and;

2) What must be done going forward regarding overall expectations of quality and quality control.

 

[struct_eeyore]

The more you let them escape proper construction practices, the more errors you will see - and the contractor will be ever more reluctant to be fix them. I've grown incredibly tired of s#!t jobs, and have made it a point to be the most "hated" inspector. Make them do some "punishment" cleaning if anything - it will be a big incentive to be more careful when placing concrete. As was suggested, a needle scaler should get at least 50% off of whats there.

 

[MIStructE_IRE]

I agree with that structee. I often set the tone at the start of a job in that sense. Its a bit like Mariah Carey asking for 1000 red M&M’s...

 

[Settingsun]

How do they prevent segregation? Looks like the fall is 2m before the concrete even reaches the top of the form, so around 5m to the bottom.

 

[SlideRuleEra]

How do they prevent segregation?

The pipe of the tremie does that in addition to keeping the rebar from being contaminated with concrete.

http://www.SlideRuleEra.net