Anyone know concrete mix design?
 

Well I know this is really off, but you guys are mostly pretty smart with lots a diverse knowledge base, so I figured I'd ask for opinions before trying my ideas.

My reinforced concrete design professor just sprung a little project on us. We need to design a 7000psi concrete mix. We're making three 2" cubes that weigh 270g each (128pcf).
So it basically needs to be stronger than standard concrete (3-4000psi) and lighter (145pcf). We haven't done a lick of mix design in the class (we design beams and stuff).
So far my plan is to use plastic odd shaped beads as the coarse aggregate, sand as the fine aggregate, portland cement, and just enough water to make it workable (like a clay almost, little thinner). I'll also add a little extra cement to the mix for the extra strength. I'm not sure on the exact ratios, but I think we'll go with 1:1:1 (cement/coarse/fine) to keep the cement content high.

For a full list of rules see here: http://www.concrete.org/students/st_concretecube.htm

This is NOT for that competition, just a little one in class. Pretty sure if you do it and back your reasoning you'll get a good grade, I just want to maximize my performance.

Thanks for any advice you guys have.

Without the use of fiber my best ideas are useless :-)

I would say lots of various mixes are in order with a multitude of testing...
Do you have the capacity to test your mixes?

I suggest pozzolans such as fly ash and silica fume. I also recommend a superplasticizer and reduced water.

Yeah, the concrete canoe team was bummed they couldn't use fibers since that was a big part of their work.

I might be able to get some fly ash from local concrete supplier. I don't think they have any superplasticizer.

I've been considering fly ash, but I find all over that it increases long term strength. We're looking at a curing time of only 32 days. (after the first day the concrete will be submerged to add strength) I think just using extra cement would be more beneficial than adding fly ash, based on what I have read so far.

thanks for the inputs so far. I think we will be pouring this Friday. Sadly we cannot test the mixes...

Rice flour...worked for the Chinese!

Rules
 

Well according to the rules you really do not get much leaveway to be creative. It seems the professor has hamstringed you on additives or binders, so you pretty much have to use basic ingredients. From what I read the only variant is what precentage of each of the three parts you use in order to come up with the right consistancy for the poor and that all three blocks be poured at the same time. Kind of sucks if you ask me. If it were me I would probably get in touch with some of the pro's and ask for advice directly from one of the biggest and best concrete suppliers. Talk with someone in the know about special concrete mixes.

I have seen some pretty tough concrete in my day, but exactly how they managed to get it that way is pretty much a mystery. Hydraulic cement is pretty tough stuff, but I do not know if that would disqualify you considering the rules. It will be kind of interesting to hear how your team does and exactly what the winning mix turns out to be. Please share the results as we always enjoy learning something new from this forum. Good luck with that.

Yup, I'll come back and post pictures of the event and roughly what the mix that won was.
Pretty sure the rice flour would be considered a binder PBO, but thanks for the input. Interesting really.

I think I have a pretty good plan outlined already. I don't expect the other students to do well either :p. Pretty sure the project is just to get us thinking on alternative concrete mixes and the theory behind them.

Surely
 

Yea surely just about every combination known to man has been tried one way or another, but we never know if someone will come up with the magic mix that will surprize the world. I am sure that winning this competition is more luck than anything else by mixing up just the right amount of ingredients to make your mix just a little bit tougher than the rest of your guessing peers. Kind of a cool test just the same because everyones hands are tied just the same way.

Just like you said ordinary portland cement is better than flyash. I think your on the right track with extra cement to up the mpa. I think the trick to get even more mpa will be to have lots of varying sized aggregate. Brickies sand is pretty much yellow sand of the same grain size and colour, on the other hand plastering sand has more clay content and varying size grains which lock in better.

How big is your large aggregate? You might consider crusher dust (fine blue metal) it's about 5mm IIRC. Having jaggered edges it will lock in good too but might be on the large side for your application.

Soaking it in water while it goes off (sets) is great for it. The more moisture you can retain the stronger it will be. For this reason try to get all faces as smooth as possible so it is less porourus when it dries out.

Moisture content??
 

And of course make sure you vibrate it aggressively so you get every single air bubble out of the mix. Using any kind of aggrigate that will give you some kind of bite would be the ticket to good holding power. Keeping it cool in water will help with setting up, but I would believe that you want it to be fairly dry by testing day or it will surely crumble. Getting the right moisture content will be crucial in the final day of testing preperations. Sounds like a great way to get everyone thinking on something that seems so simple on the outside. One would never imagine that it takes so much thinking power to mix concrete the right way. Most of us have been spoiled with bags of Quickcrete most of our lives. The outcome will be most educational for all of us. Thanks for sharing the experience with us. We learn something new every day, right?

Oh yeah, compaction. Good thinking Jerry. Compaction is usually acheived by vibrating the bubbles out but if your going to make a thick clay type paste then take small lumps in your hand and press it firmly into the cast. Try to make it so it stays workable while through out this process, if it starts to go off and your still pressing lumps into it ...... actually nevermind about that, we're talking 2" cube.:lol:

Not allowed any metallic aggregate, so the crusher dust is out. Will have to take a look into plastering sand. Thanks for the tip.
Large aggregate is going to be some kind of plastic beads I'm pretty sure. I've instructed the member to get the most irregularly shaped ones he can find. We will also vary the size of them by ripping them apart with pliers and gouging them all over.

Dang, I thought crusher dust was masonry.
http://www.ehow.com/about_6648851_de...sher-dust.html

http://www.oliveaustralia.com.au/Oli...sher_dust.html

http://wiki.answers.com/Q/What_is_blue_metal

Only it doesn't help for your weight.

It appears you are very right....guess the name blue metal is misleading....haha. I will for sure look into it for out fine-med aggregate. Thanks for the info! Guess they would sell that stuff a hardware/home improvement store eh? Will have to call around, I hope they sell it in something smaller than 1000lb bags for these 3 2" cubes, haha.

haha...I don't think I'm going to find blue metal crusher dust locally....called around and the two hardware stores laughed at me when I said the name. After describing it they refereed me to the redimix plant in town. Waiting for them to call me back, but the receptionist had never heard of anything like it.