Concrete Surface Defect - Reinforce Pattern Shows on Surface

[dalliso]

We have a project in which the reinforcing pattern is showing in the surface of the concrete after it has set (photo attached). The clear cover over the reinforcing is 2 inches. Does anyone have any idea why this is occurring and how to prevent it?

 

[Ron]

This is slumping or settlement over the rebar. Usually caused by segregating mix or segregation caused by over-vibration during placement. I would also bet that your cover is not as required.

This can be prevented by using a lower water-cement ratio and controlling the slump of the mix. Over-vibration is also a common cause.

Check the actual cover on the rebar.

 

[darthsoilsguy2]

If I were on the gameshow "The Price Is Right", I would put in a bid of 3/4" cover.

 

[dalliso]

Thanks for the feedback! The clear cover over the reinforcing is definitely not the issue here (photo attached). The top mat of reinforcment is set to elevation with a laser and the top of concrete is at elevation. Slump is just under 4" prior to plasticizer being added on site. Only half the maximum dosage of plasicizer is being used. Our resident engineer doesn't think over-vibration is the issue, but he will watch more closely during subsequent placements and finishing operations.

 

[Ron]

What size is your coarse aggregate?
What is the slump after Super-P addition?
Why are you adding plasticizer at site? What brand?
How is it being added?

 

[hokie66]

How thick is the slab? Is there cracking along the bars? I doubt that over vibration is the issue, as plastic settlement problems are normally associated with under vibration.

 

[dalliso]

Answers to questions:

Q: What size is your coarse aggregate?
A: ASTM C33, No. 57 (from 1” to No.4).

Q: What is the slump after Super-P addition?
A: Slump not taken after the addition of Super-P. Half the maximum recommended dose is being used.

Q: Why are you adding plasticizer at site?
A: Monitor slump prior to the addition of Super P.

Q: What brand?
A: DARACEM 55 by Grace.

Q: How is it being added?
A: Measured in a calibrated container and then added to the drum and thoroughly mixed.

Q: Is there cracking along the bars?
A: No.

Q: Slab thickness?
A: 20 inches.

 

[hokie66]

Another question...hard to tell from the photos, are there ridges or valleys over the bars, or is there just a variation in colour?

 

[dalliso]

There are valleys over the bars.

 

[hokie66]

This fellow had a similar problem, albeit in a slab on grade. Ron diagnosed segregation, and I agree that the mix is at fault. I disagree that overvibration caused it, mainly because I have never experienced an instance of overvibration. It is always undervibration that causes differential plastic settlement in "normal" concrete.
thread507-293677

 

[csd72]

1" is quite a large aggregate size and 4" is quite a high slump these could have contributed along with the possibly inadequate vibration.

 

[hokie66]

csd72,
I doubt those are the reasons. You could use much larger aggregate in a 20" slab, and 4" slump is pretty standard stuff. I would look more to the plasticizer...not sure why it was required.

 

[dik]

I typically spec 3" slump for slabs... and with 20", if 1-1/2" agg is readily available, would have spec'd that, too... 1" is not large.

Dik

 

[csd72]

Yes, forgot to mention the plasticiser, definately our main suspect, particularly if it has increased the slump above the 4 inches.

 

[conceng]

I vote for segregation over the rebar. Starting with a 4" slump and 1/2 the maximum dose of high-range water reducer, I'd suspect the slump is in the 8-10" range. At that kind of slump you really need to police the guy on the vibrator in proper use. Only vertical penetration of the vibrating head into the concrete with no contact to the reinforcing steel or formwork. If he hits the steel, it energizes the whole mat, contributing to the problem you are seeing. Vibrator guys normally like to drag the head through the concrete, over the rebar and whatever. This haphazard approach will not work well in this application with the high slump and a deep cross section.

Greg

 

[wiktor]

I agree with Ron that the problem is slumping over the rebar.
Overvibration or vibration of the rebar mat and too high slump of concrete are the main reasons.
Consider dropping the plasticizer and use the coarse and fine aggregates from the same source.

 

[Shadenem3]

Just want to toss this out there to see where it goes.....

My opinion....temperature. Particularly of the steel before placement, but also of the mix. We generally use 1 1/2" aggregate in a slab of this thickness to reduce heat.

 

[dalliso]

It was definitely not related to the temperature of the concrete or steel. Concrete placement began at 5:00 a.m. one morning and ice was added to the mix to keep the temperature within spec.