Anyone know concrete mix design?

[pinkpanda3310]

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.

[JERRY2KONE]

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?

[pinkpanda3310]

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.