Anyone know concrete mix design?

[brainanator]

For the mix we tried 1.25:1:1 Cement:sand:plastic. But that came out to be 120g instead of the required 270g per cube. The molds were also really crappy. We didn't think it'd be so hard to make them.... So a couple days later we did a new batch of 1.25:1:0.5, cutting the plastic way down. I had spent 4 hours the previous night making new molds of cardboard and packing tape. These cubes look amazing. But they are still a little light at 220g but we didn't want to re-pour since time was short....and then he moved the testing date up a week!
We crush tomorrow. Will post up results.