Concrete mix problem

[karma134]

Dear fellow engineers

I have been having problems in my concrete for the couple of weeks in every grade of concrete.
Generally we keep the FA:CA=50%:50% which is as you know fixed after the all in aggregate gradation.
The fineness modulus varies from 2.7-3.5.
We have a good mix at the batching plant but as soon as it is compacted at the site the mix starts to bleed (oozing cement paste/slurry along with formation of froth/bubble on the surface). And also we never had problem with its compressive strength. Its only that while pouring it makes one hell of mess.

We use PC based admixtures(HWRA)and OPC 43 and Micro Silica (5 kg to reduce the bleeding). We had tried every thing with the mix like playing with the all in aggregate ratio and also with the w/c ratio and with the admixture dosage.

Currently we are using 4 different brands of cement but still there is problem in all the mix.We also tried to determine whether it was the cement type or not which is creating the problem. We did this by performing mix trials with every cement type and for every grade. There is still the problems which i stated earlier.

 

[Ron]

You might want to consider changing the fine aggregate ratio down to about 38 to 40 percent, with a corresponding increase in the coarse aggregate. This will reduce your water demand, and thus, your cement demand.

 

[Hoaokapohaku]

karma,

I notice your FM varies in a very wide range. Variation in a range of that scope will produce concrete with very different water demand and it's difficult to isolate some other problem variable when you have that much variation in your fine aggregate. That being said, I'm not surprised you would have the kind of problem that you have when the FM is around 3.5, but I'm surprised that you would have the same problem when the FM is 2.7. Is the 2.7 to 3.5 range typical only during the period you have had the problem? Or, is that the historical range and perhaps for the past couple weeks you've been in the upper end of that range?

Since the same problem occurs regardless of how you adjust the course/fine ratio, I'd suspect that the problem is in the fine aggregate. Either it is just too course (towards the 3.5 FM), or it is gap graded. Either way, the FA leaves wide open pathways for bleed water to escape. This is why the silica fume fixes the problem. Its ultra fine particle size plugs the pathways for the bleed water.

Unless you can get a more consistent, well-graded sand, you're bound to continue to have problems of one sort or another. You might try "duck-tape for concrete," viscosity modifying admixtures (VMA). It's pretty amazing how this inexpensive product can make up for various aggregate deficiencies. In fact, if I had your FM range I had to deal with, I'd put it in every nearly every mix produced.

Also, if finishing is performed before all that excessive bleed water is gone, the surface will have a high water/cement ratio and be susceptible to pop outs (a problem you mentioned in another thread) while finishing. Also, dusting, crazing, and blistering are potential problems with the hardened concrete.

 

[karma134]

We did try using VMA & industrial citric acid as per the manufacturers suggestion. We still had the same problem.Infact when I used VMA (TAM) the concrete kinda gave me light weight feeling(like putting my hand in a bubble bath mixed with clay).

As for the FM we still had the same problem when the FM was still 2.7. A little mistake when I said earlier regarding the FA:CA ratio its actually 60 %:40 %.
So I also had some hunch that it could be due to the FM of the sand. So if I assume this is the problem how can I verify it and what can i do to give me the required FM and what would be its limits?

KARMA

 

[Hoaokapohaku]

Look at the gradation graphs for the fine aggregate and look for flat areas in the percent finer than the various sieves. This indicates gap grading, a deficiency in a particular particle size, or maybe several sizes. I don't know any way to fix the problem except fix the gradation or "duct tape" with admixtures. You might try a different VMA, or up the dosage, or both. I've found some work well for me, others don't. I think whether one is effective or not depends on local conditions rather than one is good, the other bad. For my location, BASF VMA products work well.

A gradation problem can be fixed either by fixing the production methods, or by blending another FA that provides the needed particle sizes.

Also, you might analyze your mix using COMPASS, a concrete design optimization program developed by Transtec for the U.S. Federal Highway Administration and available for free download. See the links below. I haven't used this program, but I've heard it employs the Shilstone method, which I have used and know is effective for optimizing mixes.

http://www.fhwa.dot.gov/pavement/concrete/cptu604.cfm#s6

http://www.pccmix.com

If you don't have something similar already, you may find this spreadsheet from the Texas DOT useful for analyzing your gradation, ftp://ftp.dot.state.tx.us/pub/txdot-info/cst/forms/form470.xls

A little bird in the back of my head keeps telling me to ask if your cement supply has changed recently. Has it?

Can you post a picture of your "hell of a mess?" Also, gradation of your aggregates?

 

[karma134]

Thank you @Hoaokapohaku you are a life saver. I had been meaning to ask you about the spreadsheet of some kind. I thought it would be breach of professionalism or copy right law or something like that.

I am already feeling confident after reading the first few paragraphs.(No pun added)

KARMA

 

[Hoaokapohaku]

Glad to help karma! Many of the U.S. State DOT's have excellent spreadsheets for a variety of uses. The only problem with most them is they are usually protected in some fashion that prevents you from editing them (except for certain fields), or in some cases, even from seeing the formulas and/or VBA programming. But if they suit your needs "out of the box," they can be a huge time saver. If not, they are still great places to get ideas to design your own form. I've found that the DOT's from Texas, Georgia, and Minnesota are the places I'm most likely to find a quality form that is available for use to whoever downloads it.