As a part of my PhD thesis, I am developing a UHFRC concrete mix based on locally available materials. After a few trials, I got 7-day compressive strength of concrete around 82 MPa. In the same cube specimens, we have observed many air voids of size around 6 mm. We are using a high-range water-reducing agent of 4%(HRWRA to cementitious ratio) having solid contents of 46%. If anyone phased such issues in your trials, kindly let me know how to avoid voids in the mix.
Thanks in advance.
Best regards,
Chandrashekhar Lakavath
For reducing the air voids, use of pozzolanic filler which is much finer than cement will be helpful. Supplementary cementitious materials such as silica fume, alkofines etc are useful for both the purpose of strength enhancement and reduction in voids of ultra high performance concrete.
I think higher dose (4%) of high range water reducer creates the problem. It will also create the problem on later age strength. So in stead of higher dose of HRWR, utilisations of alkofine is beneficial for strength achievements and reduction of voids at certain w/c ratio.
For more details please refer
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You may have a shortage of fine aggregate in the mixture. The solution could be to adjust the coarse/fine aggregate ratio.
Alternatively, the use of aggregates such as micro-silica, fly ash, fine slag and sand, etc. can be another option.
On large jobs various types of mechanical compactors are used. I have seen vibrating roller compactors used on roads and dams. For small domestic use eg a garage floor, hand held vibrators should be used to spread the concrete as it is poured. It is a big mistake to have too much water. I have specified 80mm slump and use of vibrators. To me there is no excuse for having cracks in concrete. Cracks are the result of too much water, bad aggregate mix and lack of compaction.
Dear,
I agree with the suggestion to optimize fines ratio, with reactive filler like suggested before.
You could also use a defoamer that works after the casting reducing the size and the total amount of the porosity (6mm are really big bubbles)
Best
Alex Reggiani
Reduction in the dosage of HRWRA around 1.8 to 2.5% may be solve your problem. Please share the mix proportion using for trials, which also need to look upon to fix the problem.
Thank you Dr. Shashikant, Dr.Abdugafur Mukhamedbaev, Dr. Peter Benkendorff, and Dr. Alex Reggiani for your inputs. In the current mix, I have used micro-silica as well. The mix proportions (kg/m3) of the current mix is:
Cement = 890
Silica Fume = 169
Fine Sand = 1059 (4mm max Size)
HRWRA = 40
Woww, your mix design has very very high cement content and Silicafume also. This is extremely damaging to concrete because of the amount of heat hydration it generates (crack) and it makes the concrete more sticky, as well as its very high cost.
One of the reasons for the high air content in your concrete is the high content of cement and silicafume, that made your concrete become sticky, then air bubble cannot rise up and they trapped inside the concrete cube.
I think you should reduce the cement content by replacing part of the cement content with an appropriate amount of pozzolanic filler (Fly ash or GGBFS), It makes your concrete less sticky, more flexible and the air bubbles are easier to escape.
We have already made 112MPa concrete with 550kg of cement and 200kg of pozzolanic filler (included FA, GGBFS, Silicafume). By the way, too much silicafume is not good for concrete, it should not be more than 8% of Cem, normally 5% of Cem.
I actually have such a formula. It's my proprietary mixture.
1st: it's self consolidating (runny, reduced voids with minimal additional effort -vibrator recommended)
2nd: it has early high strength, yet slow curing properties (aids bonding, much like concrete-clad high-rise building mixtures)
3rd: once cured it is water resistant
4th: one admixture creates a microcrystalline matrix that aids in heat conductivity (thermal battery)
It is NOT cheap. Last estimate was over $10,000 per 12-15 cubic yards and it requires a volumetric mixer truck. It starts setting rapidly upon mixing (somewhat slower below 65*F)
Mine is colorized, as well, to absorb maximum solar energy .
While mixing a sample batch in a plastic bucket, the mixer broke through the bucket, spilling a portion onto the old, worn concrete inside my 'shop' and it bonded (irritatingly so, it is a bump in my shop). The rest was rescued and poured outside my shop, to fill a dip in the soil where water pooled. That piece gets partially submerged every time it rains here. It gains heat so well that it is warmer than ambient on any clear day.
Being an amateur, it is not well documented. I borrowed from several papers and 2 books on concrete. The result is impressive, at least to me.
The formula is really astonishing.
The content of cement is really high - like twice as much, yet the content of fillers is to low. I have never seen such quantitative composition O_O
Btw, as Dan Le says - such content of silica fume is absolutely too high. It may "consume" too much od the calcium hydroxide created during the and hydration - ergo it can decrease the pH of the whole system which is really dangerous.
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