affordable mix of cement, sand and aggregate, ideally suited for stabilising non-structural post jobs that can wait up to 24 hours before removing supports including fence posts, letter boxes and clothes lines. ... More info? STEP 1 Place the post in the hole and provide support so that it does not fall over. Note: The hole should be pre-soaked to limit the absorption of water from the Post Set Concrete mix. STEP 2 Place Post Set Concrete in a wheelbarrow, create a hollow and add 2.5 litres of water per bag and mix. Only if absolutely needed, add small amounts of additional water to achieve a workable mix.. Too much water ruins good concrete. STEP 3 Place mixed Post Set Concrete in the hole, compact around the post and leave to set. Supports should be left for 24 hours.