My 28 day was based on using concrete structurally and most people that use bag mix use too much water. For weight in a box you are good the next day
My blob of concrete stayed in the form for a bit over two weeks. Estimated weight - 480 pounds. There is a short lifting chain - center, top of the blob.
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Fried - the form was plywood with 2x4 edges to hold it all together. Vibrator - you gotta be kidding. This was strictly a slop and drop job. As you can see - over the years the edges have suffered somewhat. But it still works.
............Also..... what brand/model vibrator?