What is the best way to remix the gravel that has separated out of the concrete during placing?
Don't try to solve the problem at the wrong end. Care must be taken to see that there is no segregation of the coarse aggregate in the first place. It should be placed carefully from the wheelbarrow, buggy, chute or bucket so that segregation does not occur. After the concrete is in piles it should be carefully moved with a square-end shovel or a concrete rake devised for the purpose.