Why use a tube for post footings instead of directly pouring concrete into the hole?

Question

What are the reasons for using something like Quikrete's Quik-tube for pouring footings for posts rather than just pouring directly into the ground with a galvanized anchor set into the top of the concrete?

One thing I read is that the tube holds moisture to allow for stronger curing, but are there other reasons aside from that or perhaps if you wanted the concrete to be above grade?

It seems like it'd be nice to have the concrete fill all voids in the dirt to provide more lean strength.

Answer 1

There are many different reasons.

1. You can have it above ground. Code often requires 6" above ground before wood contacts the concrete.
2. You can calculate the volume a lot easier.
3. You don't waste concrete on the irregular shape
4. You can put the tube forms into an open excavation, fill around them and then pour your concrete. I did a 16' tall one.
5. It prevents loose dirt from mixing with the concrete as the concrete is poured into the hole. If a dirt clump falls into concrete as it is poured you can have a cold join.
6. It allows the curing of the concrete to be more controlled and result in a stronger end product.
7. You can vibrate the concrete without it flowing into all kinds of voids and areas that aren't giving extra strength, you don't have to worry about vibrating dirt into your mix.
8. You can lower the grade later and the concrete column would still be aesthetically pleasing.
9. There is less wicking of moisture into the concrete
10. You can guarantee a plumb load path to the bearing soil

Answer 2

If the area you are in freezes, a smooth column will allow the frost to heave around it, sliding by, while a rough mess poured into a dug hole will be lifted or at least stressed by frost action as the frozen soil locks into the irregularities of the surface and then expands.

You can use reusable wooden forms rather than sonotubes for the same effect, but with a bit more labor. The tubes win because they cost less if labor isn't free.

In most cases the actual footing (below frost line) is (or should be) larger than the column on top of it, that the framing connects to, and given the price of concrete, using a form or tube to pour a smaller column on the larger footing is cost-efficient as well.

Answer 3

With a tube, you know the exact volume of concrete you need. There's less chance of making too little or too much.

Answer 4

A tube also controls the drying rate of the concrete and gives it time to cure, where a dirt hole might drain it too quickly.Having said that, my rain-barrel platform sits on concrete poured into holes and that's up to 640 pounds per leg.