I’m gearing up to start another round of fencing. I do add concrete my fence post holes, because in the floodplain, the flotation forces of full-submersion flooding can cause whole fence lines to float- or so I’m told by a neighbor who learned this the hard way! So the question I’m pondering this year is, pour the concrete in the hole dry and let it cure on its own, or pre-mix the concrete first?
Last year, we did a few posts with dry concrete, then I started to worry about this, and did the rest of them with wet concrete. At the time, I convinced myself that the wet concrete flowed better around the posts. And I wanted it to cure as fast as possible, since I’d be stretching the fence sometimes within days of setting the posts.
The downside is, hand-mixing the concrete adds a tremendous amount of labor and time to post setting. Since I’d do the posts little by little, renting a concrete mixer wasn’t practical. Instead, I’d mix one bag at a time in a plastic bin, pour it in the hole, tamp it, then do another bag. I often did 3-4 sixty pound bags in each hole (and I’m now thinking maybe that was more than I really needed). It was back-breaking!
So this year, I have been feeling tempted to dump dry concrete in there, add a little water, mix and tamp a little, and call it good. This is apparently called “in situ” curing. For one, the concrete manufacturers even recommend doing this, so it can’t be all that horrible to do! I searched the web on the subject and found a lot of good debate on either side of the issue, especially here. Many people feel strongly that a wet pour is far superior.
The con’s: possible compromised concrete strength, possible pockets of uncured concrete in the hole, longer cure time (though we have such wet soil, that may not be an issue here).
The pro’s: I found the abstract of an actual study done by engineers, (Paper number 034003, 2003 ASAE Annual Meeting; authors David R Bohnhoff, Zachary D. Hartjes, David W. Kammel, Nathan P. Ryan) that says this:
Hydration of a dry concrete mix after the mix has been covered with soil is herein referred to as in-situ hydration. In this study, a series of dry concrete mix footings were hydrated in-situ by burying them in sand and subjecting them to different water treatments. Footings were removed and cored at 4, 12 and 24 weeks. Compression tests on these cores showed that in-situ hydration could produce concrete with strength comparable to a normally hydrated mix. Additional research is needed to determine how in-situ hydrated concrete strength is affected by aggregate properties, initial compaction, confinement pressure, dry mix uniformity after placement, as well as conditions related to water movement into the confined mix.
I’m liking this idea, I think it will really speed up my fence building, and save my back! As I’ve mentioned in other posts, I think in our region, the first and biggest failure point of fencing is posts rotting. So, as long as my posts have enough decently cured concrete to keep them in the ground during a flood, and to help resist wire sagging, dry-cure concrete may be good enough for me!