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ckhersey
Did I mix the concrete wrong?

This is my first post here and my first ever attempt at mixing concrete and sinking posts! Today I started work on my pergola project. My holes were already dug (thanks to my teenage nephew and his need to earn money) so I spent a lot of time trying my best to get the posts braced straight and plumb. I'm pretty sure I did ok there, but when it got to the concrete mixing, I am not sure I did it very well. The instructions on the bags said to fill the holes 1/3 of the way with water and just dump the bag of concrete into the hole but I had heard that this method doesn't do a great job so I put a tarp in the wheelbarrow and mixed it by hand according to the instructions. The bag said to use 2.5 quarts of water per bag, which didn't seem like a lot, and definitely didn't look like a slurry when it was done, but I put it in the holes anyway. I checked after a few hours and they seemed a little wiggly. Is this normal? Should I give them more time to set up before I say that the job didn't work well? I want some really sturdy posts because I want this to support a hammock, wisteria, and g****vines, and just generally be sturdy and safe with my wild children acting like children do in the back yard.

A. Spruce
Re: Did I mix the concrete wrong?

First, there are different grades of cement mix, post mix is mostly gravel with a touch of portland cement. With this type you can "dry set" the post and add water or visa-versa. The other common is concrete, which has a higher portland cement content and sand. This is a stronger/harder setting type, and should be premixed for optimal performance. As for water content, the less water the better, but you still want to get to, or almost to, a thick slurry.

For fences, the fastest/easiest way to go is to use post mix, dry set the post and either let the cement wick its own moisture from the ground or add water to the top. If you dry set the post, you can completely build the fence, with minimal wobble, then give it one last plumb before you toss water at the posts and walk away.

For structural things, like your pergola posts, wet setting is the better way to go, to assure no voids and have maximum stability in minimal time. It takes time for the concrete to set, which will vary depending on the amount of water used and the ambient temperature. I would leave it for at least 24 hours before messing with the posts, even then you want to be extremely careful not to bang or jar the posts around because until the concrete cures it is fragile and can be cracked/broken, which will affect how well it can hold the post.

Ideally, you don't set the post in concrete at all because wood rots, even pressure treated. Better is to use a wet set post base that will keep the post from touching the concrete. You will get your structural stability from the structure itself in the way of knee braces or attaching it to the house.

dj1
Re: Did I mix the concrete wrong?

Cement is a powder bonding agent. Together with crushed rock, sand and water, it creates concrete. There are different ratios of these 4 ingredients to create different concretes for different uses.

The post mix you buy at the stores has the minimal amount of cement required. I don't use this stuff, I rather mix my own "six sack" concrete. This formula requires a 3 minute mixing.

Google "concrete" to learn more and read about this phenomenal invention that changed the world centuries ago.

dj1
Re: Did I mix the concrete wrong?

I forgot to answer your question: If a day or two later your post wobbles, you mixed it wrong.

ed21
Re: Did I mix the concrete wrong?

All good answers above. You shouldn't mess with it for at least a day. If you wiggled only once after a couple hours the concrete probably settled back against the post. Full design strength takes 30 days, but it will have hardened overnight.

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