Mike you may be able to just fill the pipe with some sand,gravel or brick pieces and sink them after filling them with what concrete you have already used. After they hit the bottom they should stay there. Now this is only a guess as to how much concrete you did get in the buckets. Trial and error is the best that you could do at this time. Just take one and place in some water about 4' deep and see if it is still buoyant or not then you will know what and how much to make others.
Be safe and good luck fishing
Most PVC drainage pipe is slightly buoyant by itself but most as far as know or have seen so far very little weight will sink it but if there is either current or wave action....