How long should concrete set for a post before I perform additional work?

Question

I'm installing a gate for a fence around the house and have set wooden posts in concrete foundations for support. I'm using Quikcrete and the bag states that I need to wait at least 24 hours before doing additional work, but a friend of mine was saying that concrete doesn't achieve full strength for 28 days.

The pour was done per the manufacturer's instructions and was as follows:

1. The post was approximately 4" diameter and 8' tall.
2. The hole dug was triple the post's diameter, so 12". Depth was 18".
3. Per the manufacturer's instructions, the hole was backfilled 6" dry concrete.
4. Post was set and concrete mix based on the manufacturer's recommendation for water. This was 3 quarts per 80 lb bag. A little water was added during mixing because of workability issues, but not much.
5. Hole was backfilled with concrete mix to grade and concrete was tamped in with a shovel during the entire process.
6. Per manufacturer's recommendation, the top was graded to promote standing water on top of the foundation and left to set.

I will need to drill holes in these posts to set hooks for the fence and am concerned that the vibrations might compromise the concrete. In addition, I may or may not cut the excess to height depending on if my wife wants to place climbing plants on it. If I cut, the exposure will be 4' above grade plus a few inches for gate swing clearance.

It's been 4 days since I set the foundations and while the posts wobble when pulled hard, it appears that the wobble is because wood isn't absolutely solid, not because they aren't firmly in the foundation.

Is it safe to drill the holes, or should I wait longer? Could I have just drilled the holes after the first day?

Answer 1

The 28 days mantra is a bit misleading. Concrete cure times vary widely depending on environmental factors and mix. It's often at 80% or more by a week.

That said, we don't know much about your pour. If you have 4" of concrete around the post, it's fairly fragile and drilling could crack it. If you have 8 or 12", it's much more robust and would probably tolerate drilling just fine, if you keep pressure light. All this depends on adequate moisture having been provided and temperature maintained.

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