Anything less, and you run the risk of having to set the fence posts all over again in only a few. In fact, CTS Rapid Set Concrete Mix sets in only 15 minutes.
Set the post into the hole and attach 2×4 braces to adjacent sides of the post. Use a level to position the post perfectly vertical. Fill the hole with Fast-Setting Concrete up to 3 to 4 inches below ground level. Pour about a gallon of water per 50 lb bag into the hole and allow the water to saturate the concrete mix.
Dig the holes the same way as you would to set posts in concrete. When setting fence posts in gravel, you should bury at least one-third of the fence post. Use crushed gravel to set the posts. Pour 5 in.
This will seal the gap between the concrete and post that's caused by. Have the necessary tools for this DIY wooden fence post project lined up before you .