Concrete Slab Reinforcement Rippling

[imagine18]

I have a slab on grade that has experienced reinforcement rippling (the slab is higher over the reinforcement than between the reinforcement). Two questions: 1) How does this occur? Air voids below the bars? Bad vibration technique? 2) how can this be prevented? Thanks for any advice!

Rob

 

[ToadJones]

Is this a new pour?

Did you look very close or throw some water on the slab to see if there are cracks in the raised portion?

This could be the result of the result of plastic settlement in/around the reinforcing bars.

 

[ToadJones]

oops....too many "results" in above post....
Wish I could get that many "results" at work

 

[hokie66]

Toad's suggestion sounds right to me. So yes, that would be due to inadequate consolidation and finishing, which is usually a problem only in thick slabs or footings. How thick is your slab?

 

[Ron]

This can also result from inadequate cover on the rebar and/or incompatible coarse aggregate size relative to the amount of cover.

Look first at Toad's suggestion...it is the more likely.

 

[ToadJones]

I have only personally seen plastic settlement on thick footings with top reinforcement and a total lack of consolidation...When I do large footing pours (with top reinforcement, that I prefer) I make the guys vibrate. I haven't seen the problem on any of our jobs.
It guys it is possible that the concrete got a little hot during placement and prevented good consolidation.
I could be wrong, but I also thought this was only a problem on thick pours.
How think is the slab?
How big are the bars?

 

[imagine18]

Thanks everyone for your quick responses. This is a slab on grade on the bottom level of a parking garage. The slab is a 5" thick with #4 @ 24"o.c 2" clear from the top. I am attaching a photo so you can better see what I am talking about. I think I miss addressed the issue in my first question...It appears (in the picture) that the slab has sunk at the rebar locations.

 

[imagine18]

I tried to upload a photo, but let me know if you guys don't see it...

 

[a2mfk]

No pic, it will appear as a link along with your post

 

[imagine18]

ok here it is. I think it worked this time.

 

[Ron]

Most likely overvibration causing segregation of the coarse aggregate over the rebar. This usually happens when a strong vibrator contacts the rebar itself. Check the cover...you might not have the required cover.

I would cut a core directly over one of the rebars. The diameter of the core should be at least 4 inches (you're not using it for compressive strength, so L/D doesn't matter). Cross-section the core and you'll be able to see the issues.

 

[imagine18]

Thanks again for all the responses. Ron - I agree that a core sample will be a good way to check what is going on with the concrete. From what I saw today on site, one guy is using a hand stinger to vibrate the concrete.

 

[hokie66]

Very strange. My eyesight is not too good, but those looks like valleys rather than ridges. There are also some lines, maybe cracks, running in the other direction. And there is absolutely no indication that any attempt was made to finish the slab.

 

[imagine18]

yeah, hokie you are correct. they are valleys rather than ridges, my original question was posted before getting all the info...I am wondering if this could be caused by the concrete having a high slump and then the aggregates consolidating away from the rebar, leaving a "paste" that settles lower...

 

[Ron]

imagine18...yes, that's exactly what it is...segregation of the aggregate over the rebar.

 

[imagine18]

Ron, thanks! Do you know of any literature that would discuss this? for documentation purposes?