Concrete was TOO dry?

Question

I am building a tree house. One side of it will be supported by the tree. The other with two 6x6 8' pressure treated posts.

I wanted to support those posts with concrete footings. So I dug 24" down (minimum required here), and used tube forms.

Inside the tube forms are rebars - 4 flat in a # shape, then 4 coming up from the bottom. All rebars are surrounded by concrete so they don't get exposure to air/soil.

For one post, I surrounded and tube with more concrete. With the other I did too, but less concrete.

When I prepared the concrete, I used exactly the mid-range of water that the bag said (range was 3.5-4.5 QT, I used 4).

It was a bit dry but seemed ok. I poured it in and kept compacting it as I went.

Since then (36 hours ago) I've been wetting the footings regularly.

Now, I begun stripping away the forms, and I see what's in the attached.

Does this look right? There's a lot of little "gaps" there and it's quite clumpy.

Answer 1

One thing is for sure: It could have benefited from better consolidation. This is usually accomplished with vibration. Vibration causes the mix to liquefy, allowing trapped air to escape and allowing the fines in the mix to make a smooth uniform surface against the form.

Professionals often use a power tool that actually vibrates; they immerse it into the wet concrete and move it about. Professionals, and DIY too, can also vibrate a post form like this externally. Do it by tapping/striking the outside of the form with something. Maybe a hammer/mallet, maybe a stick, short piece of lumber, a stake, etc. Hit it just hard enough that you see the concrete inside shake a little. Moving a stick through the concrete in a reciprocating up-and-down motion also helps.

My eye for concrete isn't quite good enough to tell whether your mix was truly too dry, was set up too much before it was placed in the form, or was practically perfect in every way but simply didn't get enough attention to consolidation.

I'd be a little concerned about whether water freezing in those voids might eventually cause the concrete to crack. Given that it's for a treehouse, and that there's a generous helping of reinforcing steel cast into the concrete, maybe it doesn't matter too much.

If you do re-pour these, you could consider placing a post base or an anchor bolt into the concrete before it sets. Sometimes that's a little more convenient than drilling holes and bolting a base to it after the concrete has hardened.

Answer 2

Honeycombing is due to poor consolidation. Vibration helps the consolidation during the pour. I doubt that compacting is a substitute for vibrating.

Answer 3

There’s two issues: 1) strength, and 2) durability.

1. Strength is based on several things, including adequately mixing the concrete to a paste that can adequately hydrate the cement. Also, in order for the cement to bond to the rebar and aggregate, there must be adequate moisture to allow the cement paste to coat the rebar and aggregates.

When the cement is not adequately mixed with an adequate amount of water, it will reduce the concrete strength.

Because your construction is a tree house, it has minimal loads and this reduction in load capacity is immaterial. (A good way of checking this is to wait 14 days (70% strength) and whack part of it with a hammer. If it falls apart, you’ve got a problem.)

2. Durability is a bigger issue. Depending where you live, water can penetrate your footing and freeze. Because of the large amount of voids, lots of water will penetrate deeper into the footing. When it freezes, it will cause the footings to spall. If enough of the concrete cracks and falls off, the footing will fail.

Summary:

The footings don’t need to be at maximum strength. The footings are huge compared to the load they’re carrying. Likewise, the treehouse won’t be there forever. If it hangs together for 5-6 years, maybe that’s fine for your family. (To extend its life, mix up a paste and coat the exterior with a 1/2” layer and make sure water can escape from around the footings with drainrock and a pipe leading away.)