Late last week, I wrote about the techniques for doing building layout with batter boards and the 3-4-5 triangle rule, in preparation for building a concrete pier foundation. (Actually, that method works well regardless of the foundation type.)
In part two, I’ll talk about how to work with concrete pour tubes and actually install the concrete piers. Check it out below!
Preparing and Cutting Concrete Pour Tubes
Before even getting our holes dug, I prepared the concrete pour tubes for the foundation a day in advance. Concrete pour tubes are basically glorified toilet paper rolls — long recycled cardboard cylinders, extremely convenient to work with, and easy to cut to length. (These tubes were a little expensive at $50 for 12 feet, but I later learned you can get them cheaper. Oh well.)
With height measurements in hand, I proceeded to cut the tubes to length, and carefully labeled them to make sure I would remember where each of the 15 sections would go. (I cut them all about 5″ longer than they would ultimately stand.) To cut them, I simply measured along the tube and made a number of marks around the tube. To connect the individual measure marks, I used a flexible 24″ stainless steel ruler — very handy for these large 12″ diameter tubes. Cutting them was very easy with a typical hand saw — I simply kerfed along my pencil mark, and then cut all the way through. (Of course you would use a power saw, but I avoid such things as much as possible.) Log pups were useful for keeping the tubes stable on a pair of sawhorses.
With the pour tubes fully prepped, it was time to start digging.
Digging Holes for a Concrete Pier Foundation
We chose to have our Mennonite neighbor do the work of digging the holes for our concrete piers. Our soil type is extremely clay-heavy, and holes will fill up with water very quickly with even small amounts of moisture in the ground. That was part of our justification for bringing in a tractor. Not only that, but a tractor with an auger can go a full four feet deep (definitely past the frost line), whereas a hand-powered auger can only go three feet. Oh, and hand augers are usually only 8″ in diameter (at least all that I have seen). So. Given the speed, depth, and diameter of a tractor-mounted auger, we bit the bullet and chose to use fossil fuels to help get our holes dug for the pour that afternoon. It look less than an hour to dig 15 4″ deep holes. (And it only cost $50, too. Thanks, Mennonite friends!)
Setting Concrete Pour Tubes
Now that we had our holes dug, the rush to set up the pour tubes was on. The first step was placing them on the ground in their respective locations, leveling them, and stabilizing them. There are a number of methods for stabilizing pour tubes, including surrounding them with sand bags, bracing them with wood, or the method we chose — simply piling enough dirt around them to keep ’em steady.
It was quite labor-intensive, as it took a lot more dirt than I expected (of course). Thankfully we had some help moving all of that soil, but… it was kind of annoying to wheelbarrow all of that material, I’ll admit. The tubes were not stable unless they were at least half-way mounded up with dirt. They were also tricky to keep plumb in both directions, but we managed. It took at least a two or three hours to get everything plumb and stable. (Our idea was to then wet the soil to help compact it better than we could do so by simply stomping it, but we ran out of time… it turned out alright anyway.) It is critical that the tubes are absolutely stable, so they do not shift during the concrete pour!
The pour tubes surrounded by loads of soil was an amusing sight, something like a cross between small volcanoes and ant hills.
The Final Foundation Level Check
Remember that we intentionally left the concrete pour tubes a little longer than they ultimately needed. This was so that we could do our final heights with the builder’s level. Once again, I set up the scope and had help with making our level marks, four marks per tube. No there could be no doubt that the tubes would be level. Once again, it was time to cut the tubes, now standing in place — not the most pleasant task, but it was manageable. (Sawing vertical things kinda sucks.) Using that same flexible 24″ ruler, we connected the four marks around each pour tube, and then I went around with a hand saw and lopped off the tops.
Before the concrete truck arrived, I went around to make sure the tubes were still plumb.
Cutting Rebar for the Piers
Now it was a mad dash to get the rebar cut for each pier — three long lengths, plus a short central piece. The long lengths were all 8″ shorter than the full height of each pier — meaning, the hole plus the tube itself. That way they could be pushed down into the concrete, but not so far down that they made contact with the ground — you want to avoid the rebar making contact with the soil.
The short 12″ piece was to be inserted when the concrete was poured, sticking up about 2-3″, a kind of pin for the eventual sill beam to sit on and keep steady when working.
Time for the Concrete Pier Pour
The concrete truck arrived, and with a team of 4-5 helpers, we had plenty of hands to shuffle around and complete the pour in less than 1.5 hours. One person scooped the concrete off the slide into each pier, while another made sure that the tube was secure and did not shift. Once the concrete was up about halfway, another person used rebar to “tamp” the concrete, and work out any air that might have been introduced. The three long rebar pieces were put in place, and the rest of the concrete was poured in.
Someone took the task of lightly tapping the tube to once again eliminate any extra air that may have been introduced, and once that was completed, the top of the tube was smoothed with a screed — nothing more than a straight 2×4. A trowel was used to get a nice flat finish, and then the 12″ rebar piece was stuck in.
Occasionally, the concrete would settle, and we’d have to go back, pull the pin, and add another scoop of concrete mix. Not a big deal, but it was important that the concrete be completely level with the top of the tube.
Fifteen tubes later, we were done! The total quantity of concrete we used for 15 12″ diameter piers was about 3 yards.
Extra Concrete Pier Tips
Here are a couple of tips for a successful concrete pier foundation. To determine the quantity of concrete you will need, use the equation for volume of a cylinder. In our case, we had 15 piers, averaging about 6 feet in length each. All were 12″ in diameter. That means about 100′ of 1′ piers.
The volume of a cylinder is:
So, again, in our case — r=6″, and h=1200″. You can do the rest.
Also, always order a little extra concrete. You don’t want to find that you have run out during the pour! Concrete cures very quickly and needs to be worked fast.
And, on the other hand, if you have too much concrete left once your piers are finished, have some forms ready to be filled with concrete. Think about using the tube offcuts, or making forms for patio pieces, etc. Use as much as you can!
That’s it! Good luck.