How do I determine the fiber content in cured FRC?

[JasonG]

My client suspects that the fibers are forming clumps when the fiber reinforced concrete is placed. I need to determine if this is the case. Are there any test methods available for determining the fiber content per area of concrete? Any suggestions or references are appreciated.

 

[Ginger]

What evidence does your client have in order to make this assumption? If you are certain that the fibres have been batched correctly and the concrete has been correctly mixed and placed the suggest that the client takes cores. If the cores show good distribution then he/she will pay for the cost of both the coring and repair.

You may find that the client decides that all is well !!!

 

[JasonG]

I'm not certain that the fibres have been batched correctly or that the concrete has been correctly mixed and placed. We are providing materials testing services, not the concrete.

What would I do with the cores to determine the fiber distribution?

 

[Abedkayrouz]

Go for petrographic analysis of concrete

 

[RolandMTL]

Alternatively, if you can clearly see the fibres, count the fibres per unit area. Use a sufficient number of sample areas. Each test area should be several fibre lengths distal from any other test area. Knowing the fibre geometry and considering any preferential fibre alignment (if not randomly aligned), you can then estimate the representative fibre content for each face. Expect variations.

 

[dirtdoctor]

Your client may have a good reason to suspect fiber concentration - I have seen it happen with both metal and traditional poly fibers. The easiest way to fix this is to set a maximum amount of concrete per truck to allow sufficient volume for mixing. If the truck has a capacity of 10 cy, only allow that 9 cy be batched per truck. This will ensure that the fibers are well mixed - we did this on one job where fiber began to clump. We also noticed that if the concrete temperature is above 85, the fibers tended to clump more readily.

As far as a test method for determining clumping, petrographic analysis cannot be beat. But, be prepared to spend some cash, as petrographic tests can run as high as $500 each, and you will need to run at least 3 to prove it.

 

[Rx4slump]

There are a few things you can do to feel better about the improper consolidation issue. One is to look at the distance of the batch plant to the job site. If it is close, look at the concrete tickets to see what time it was batched and what time it had begun its discharge. The driver is required to document this in most cases. It is also supposed to have revolution counter info as well. Find out how the batch plant loads the fibers. All the batch plants I have seen load it by hand. They are supposed to load the fiers prior to batching, like color, but many time the batchman forgets until after he has batched. If you had an ACI tech on the jobsite he also should have noted this information on his test specimen paperwork. Request the specimens be returned to look for visual inconsistancy. I know this won't help now for your situation but wanted to note this for the archives.

 

[dirtsqueezer]

Yeah, just to elaborate on the petrographic analysis. If clumping is a problem in your mix, it might be obvious if you're able to get a core sample. The sample can be analyzed by slices if you want to get a better idea of distribution inside the core. A thin section of the material would give you just a small representation, though if you wanted to emphasize a clump, it might be a good visual. In my experience, fiber is pretty easy to spot in fresh concrete, and I wouldn't be surprised if you could make conclusions from a simple core.

Good luck.

 

[dirtsqueezer]

an afterthought,

This is in lieu of a better established method for forensic analysis of your fiber content. You could order small amounts of concrete, one w/ no fiber at all, and one with an unacceptable low/high amount of fiber, and one with an acceptable amount. Have all other variables stay the same- company, mix design, ect. In this case, I would hire two inspectors- one at the batch plant to verify the mixes, and one on site, for sampling. You can either sample the mixes directly or take cores from a similiarly-finished slab. Take samples/cores, and make a visual analysis. Here a thin section may be more valuable- you might even be able to do a sort of fiber-count, or some similiar analysis. Thus you would have have a convincing visual model for the acceptability limits of your fiber, and whether the cores in question fall in that range. If the project is still ongoing, you could take samples from active pours if you're sure to have one inspector at the batch plant verifying batch weights. It would save the cost of ordering trucks special for the analysis, though ordering small amounts of concrete is relatively cheap, compared with the price of litigation. I know this analysis would be an easy task for my lab, though we'd have to outsource if we thought a thin sections were necessary. I think this would be a convincing method, for lack of an ASTM.

Anyway, just a though. Best of luck.