Fiber Mesh Issues

[Krausen]

Wanted to get the forum's thoughts on a recent construction issue I've had with a reinforced concrete foundation for a large pump skid. The foundation is roughly a large rectangular block consisting of ~25 cu yards of reinforced concrete. Our project manager insisted we specify fiber mesh to be added in the concrete mix design for this foundation. The idea being the fiber mesh would help in controlling cracks/shrinkage on this foundation. A number of engineers working under the PM questioned the need for fiber mesh, especially in a large section block foundation like this. My understanding is fiber mesh is typically only used in thin section concrete mix designs (slabs, panels, etc). The client & contractor also had some push back on adding fiber mesh for this foundation since they'd never encountered this before. In the end, they went ahead with it.

The foundation was poured using a pump truck on a 50 deg F day. Based on site photos, the contractor appeared to cover the top the foundation with an insulating blanket after the pour. I have not been able to determine if the fiber mesh was added at the plant or on site yet.

About 10 days after the pour, the contractor returned to begin chipping the top surface of the foundation down ~1" deep to prepare for grouting the pump skid baseplate to it. Upon chipping, the contractor encountered extremely weak & crumbling concrete on the top surface. Concrete was chipping off in large chunks & the large chunks could be broken apart by hand. Looking at site photos, it looks to me that there is way too much fiber mesh in the top few inches of this foundation block. Also, the color of the chipped concrete did not look uniform like it does when most concrete cures properly, but I'm not an expert on judging this.

There is no indication that excess water was added to the mix on site. The w/c ratio appears to be fine based on the tickets. The lab test breaks show no signs of bad concrete either. The design strength was 4500 psi. The three (3) 7-day breaks we had came back at ~4000 psi, which was a little higher than expected.

Any ideas what went wrong here?

Obviously there are a lot of things can wrong in something like this, but my initial thought is with the contractor being unfamiliar with the fiber mesh mixing & placement. Could there have been issues with how the fiber mesh was added, mixed, & ultimately placed & finished? One thought was the lighter fiber mesh material may have floated up toward the top few inches of this foundation block and prevented the concrete in this area from properly binding/chemically reacting like it should have?



Tolerance is the virtue of the man without convictions -GK Chesterton

 

[TehMightyEngineer]

Was this poly fibers? Got any pictures you can share of the chipped areas?

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

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[Krausen]

Yes, synthetic/poly fibers I believe. Both of what they call "macro" & "micro". I guess that has to do with their lengths. I believe the "macro" fibers were added at 3 lbs per cu yard & the "micro" fibers at 1 lbs per cu yard. Im still in process of getting more info on this. I will try to load some photos too

Tolerance is the virtue of the man without convictions -GK Chesterton

 

[TehMightyEngineer]

That's a decent dosage of fibers. If they didn't mix that properly then I could see issues occurring. I could also see issues if they didn't vibrate the concrete correctly. Sounds like your defect isn't related to finishing or mix design. Pictures would definitely help. Getting a few cores of the concrete might be useful as well.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

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[dik]

Polyfibres, if that's what you used, is no substitute for a well designed and constructed reinforced concrete pad... They do little for reinforcement and other than controlling 'bleed water' do very little at all.

Dik

 

[TehMightyEngineer]

I think I understand what you're saying dik as fibers are not a substitute for reinforcement, but in my experience they do provide a measurable improvement to plain concrete. We use them a lot in unreinforced items or minimally reinforced items (pads, covers, manholes, lightpole bases, ballast blocks, etc.) They definitely improve resistance to damage and the plain concrete flexural properties. Crack control is improved but we use it more as a backup to proper crack control reinforcement. The biggest benefit we see is improved durability of the concrete but you're entirely right that it's not a replacement for reinforcement.

For a large concrete placement I agree with you and Krausen that they weren't required and can't replace the rebar for crack control. It shouldn't have been a waste though and the contractor should have seen their durability improvement effect during their chipping operation. Something definitely went wrong here.

P.S. Nice usename Krausen. You home brew I take it?

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[dik]

TME: the benefit, as I understand it, is due to the control of bleed water.

Dik

 

[SlideRuleEra]

...there is way too much fiber mesh in the top few inches of this foundation block.
Any ideas what went wrong here?

Inadequate or ineffective field oversight... unless you have positive proof otherwise.

I'll bet a dollar to a doughnut this is a construction management, no a technical issue. The Contractor "forgot" or did not care, and dumped in all the fibers near the end of the concrete placement.

What are the three dimensions of the block?

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[Krausen]

General dimensions are 11'-6" long x 8'-0" wide x 5'-0" deep. There are also some pedestals designed to be formed & poured with this block that increase the concrete volume.

Tolerance is the virtue of the man without convictions -GK Chesterton

 

[SlideRuleEra]

So, 17 yd³ in the block, with fiber bunched up in the last 3 yd³ of the placement (top 10", or so, of the 5' thick block)... IMHO, construction management failure.

Since you mention that there will be pedestals on the block, suggest removing the entire block and starting over. You could "twiddle" around trying to repair the upper portion of the block and anchoring the repair to the underlying 4' +. Even if this is successful, the pedestals are largely anchored in the repair, not in the core of the block.

We had the pedestal on an induced draft fan at a coal fired electric generating station break free from the foundation mat. The fan was securely anchored to the pedestal, but the combination fan / pedestal were "hopping" around during plant operations... not a pretty picture. Temporary tie-downs kept things together long enough to do a pedestal replacement at a scheduled plant outage.

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[Archie264]

Some things go perfectly well together and are hard to improve upon. Concrete and reinforcing steel fall into that category (for main load-bearing structural members, at least). The fiber mesh was out of place here, in my opinion.

Also, the PM should not be making that call. That falls exclusively within the purview of the structural engineer of record. It seems your PM felt the need to insert himself into the process and now look what you've got.

SRE's probably correct about the oversight but the fiber didn't belong in that mix in the first place. My $.02.

 

[Ron]

Agree with SRE....this is a construction issue, not a mix issue, although the fiber dosage is way too high!

Fiber is only a mix enhancement....not reinforcing. It offers some property enhancement to a properly designed concrete mix but is not a substitute for appropriate good placement procedures.

In thick concrete sections, the bleed water and over-vibration can tend to float the fibers. It doesn't happen with steel fibers, but can happen with poly fibers, particularly in high dosages.

 

[dik]

forget the fibre and throw in some thermocouples in appropriate places...

Dik

 

[Krausen]

Since you mention that there will be pedestals on the block, suggest removing the entire block and starting over.

Yes, we will be hauling in an excavator to pull this poor foundation block & will be starting from scratch.

Thank you all for your helpful responses on this. The over-vibration aspect did not occur to me, but that could be a major factor here as well.

I agree with this being a construction management/field oversight issue. Another recent finding is the materials testing field & lab documents on this concrete ... they show the pour took place in 28 F temperature last month (outside St Louis, MO). Looking at the wunderground.com history here, the temperature on the day of the pour only got down to 42 F. The credibility of the materials testing company, who also claimed the test cylinders for this poor concrete all broke with adequate high strengths, is now in question.

Tolerance is the virtue of the man without convictions -GK Chesterton

 

[Archie264]

>>>I agree with this being a construction management/field oversight issue.<<<

Actually, to me it still sounds like this problem started when your PM interjected himself into the design process. You have to admit that had he not done so you wouldn’t be facing this problem. We’re told fibers alleviate bleed water issues. Was that his reason for using them? And, if so, did he specify or review the amount to be added? Or did he do it in order to have to input to the project and/or assert his authority? If the former then yes, it becomes a CM issue. If the later, though, then you have a larger problem in that it will likely reoccur. If not with fibers then with some other admixture, specification or process.

And maybe the problem is with terminology. To me PM implies the overall project manager; not a technical designer. If he’s a technical person operating within his discipline then that’s one thing. If not then he should have left the design decisions to those charged with making them. He probably outranks you or is your boss and if so you’re stuck with the situation. If not, though, you ought not let him off the hook until the lesson his learned.

There’s an old saying that if something is stupid, but it works, then it’s not stupid. Well how about a corollary? If something’s smart, but it doesn’t work, then it’s not smart. With each added complication comes more opportunities for failure. Especially when the person specifying it doesn’t understand it.

 

[SlideRuleEra]

Archie264 - One solution to vague "management" interference that I have used is to do the absolute minimum requested (as long as that does not create a technical problem). For this case, that could mean using one small "bag" for fiber and, if asked, tell "management" the truth: you complied with the request.

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[Archie264]

SlideRuleEra,

That sounds like a superb application of "human engineering." Excellent solution!

 

[SlideRuleEra]

Archie - That strategy is really just for casual comments by a manager who has only passing interest in the project.

A more diplomatic and useful approach is to agree (in this case, add fibers). After a day or two of "looking into the issue" point out why only a small quantity is justified. Negotiate with facts, things like limited technical usefulness of fibers, extra construction management duties, Owner/Contractor objections, and additional cost. Hopefully you can get the request reduced to a level where the chances of a technical problem are limited.

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