On the residential part of our practice, we do a lot of pier and beam foundations. There are other options available to us -some include post-tensioned slab on grade and plain ol’ slab on grade, and all of these options are fine under the right circumstances but if I had my pick (which I normally do) we recommend the pier and beam structural foundation. The soil in my area of practice tends to be expansive because it contains a healthy dose of clay, which expands and contracts as moisture enters the equation. Add water, through rain and irrigation, and the soil expands. Remove water due to hot and dry conditions, and the soil contracts. All this soil movement adds up to cracks in the house. Because we like to help eliminate the movement issues that come along with expansive soil, we like designing with a pier and beam foundation.
Most people have heard of a pier and beam foundation at some point in their lives but more times than not, they don’t really understand how it works – even the “experts” get it wrong. I had a home inspector tell me, as he was inspecting a house I was in the process of buying, that the house had structural issues because there wasn’t dirt under the grade beam where he had checked.
Home Inspector: This house has structural issues.
Bob: Really? What did you find?
Home Inspector: I noticed when I was in the crawl space that there were gaps under the grade beam where the dirt was missing.
Bob: Right … and exactly what was the problem?
Home Inspector: Your beam will eventually sag if it isn’t supported properly. You’ll need to get someone down in the crawl space and put some dirt in.
Bob: What?!? There isn’t supposed to be dirt under the grade beam … you do know that dirt doesn’t hold the beam up, that’s the job of the piers.
Home Inspector: Son, I’ve been doing this a long time.
Bob: Uh-huh … right.
For the record, if it wasn’t already clear, dirt DOES NOT support the grade beam in a pier and beam foundation.
Since a number of people don’t seem to understand this concept, I thought I would whip together some sketches to help illustrate my point. If you can’t read drawings, hopefully my explanation will be good enough for you to understand.
Here is a typical wall section through a pier and beam foundation. The grey part is the house and for our purposes, it doesn’t matter in this conversation. The important thing to note is that a pier (think of it as a concrete column) is drilled down through the dirt until it can bear on rock. That’s important because rock doesn’t expand or contract based on the water content of the surrounding areas.
The grade beam is a concrete beam that spans from pier to pier – just like a beam that’s probably in the ceiling above your head spanning from wall to wall.
Underneath the grade beam are “void cartons” or “void forms” (it would have helped if I had labeled them in the sketch above, as it is, it is shown as a gray rectangle with two diagonal lines in it.) Believe it or not, void forms are made out of cardboard, and the concrete gets poured on top of them. They are strong enough to support the weight of wet concrete but over time, long after the concrete has cured and become hard, the cardboard will rot away and leave a void.
The last thing to note in the sketch above are the “soil retainer boards”. These are basically plastic boards that keep the surrounding dirt out of the space the void carton creates when it rots away.
This is a look at a typical grade beam when the void form has rotted away. There is now an airspace between the bottom of the grade beam and the dirt below. This is a good thing … it’s an important thing, you want this gap.
Why would you want a gap? You want it so that when the surrounding dirt absorbs water and expands, the dirt doesn’t push up against the underside of the grade beam. Having this gap is what allows you to not worry about cracks in your brickwork, or having doors stick seasonally. Piers, bear on bedrock, grade beams sit on piers, and no dirt under your grade beam means your house doesn’t move.
#scoreboard
I told you all that so that maybe these pictures would make a little more sense. We poured the grade beams at the KHouse recently and it was quite the show for a residential project. 13 concrete trucks, spaced 30 minutes apart, helped get this foundation pour done and we are now about 80% complete. All that’s left are the basement walls, driveways, sidewalks, and exterior stairs.
In order to reach all points on the site without damaging the trees, the contractor had to use a concrete pump truck with a giant arm on it. This machine takes the concrete in at the rear and pushes it through a bunch of pipes connected to the arm.
All the concrete goes into the hopper here … easy.
This was one of the trees we had to avoid messing with – it was a main consideration in deciding how the concrete would be placed. I don’t get to use these pump trucks too often on residential jobs, but they are extremely common on commercial jobs.
This is the pump arm expanding out – you can start to get a good feel for just how far it can reach. Pouring concrete is incredibly difficult and back-breaking work, I frequently think that other than being a roofer in the summer, this is the worst job on a construction site … except for the guy who controls the pump arm – that’s the sweetest gig ever on a construction site.
Here is a look at the controller for the pump arm. It’s really like playing video games all day – at least that’s what the guy told me, and he ought to know. He stands in the shade fiddling with the joy sticks while everyone else is breaking their back pushing wet concrete around.
These were some of the guys locating the concrete into the plywood forms. As I took this picture, the guy in the front wearing the plaid shirt said “you really don’t want to stand there”. I know, but I wanted to take the picture …. and about 5 seconds later, an air bubble came through the line, the concrete coming out kind of spluttered a little bit and then *BOOM* I was splattered head to toe in concrete (I didn’t take a picture of that). The only saving grace, as all the concrete guys were laughing, was that the contractor was standing right next to me and got covered as well.
Speaking of the contractor – that’s him in the white polo shirt sitting atop the concrete mixing truck. That’s called “doing your job.”
This is looking across the top of the grade beams – the dark gray stripe in the middle is the wet concrete. From the top of the grade beam, you can get a sense of how deep the crawl space will be – just under 4′ total. In this project, we are installing our air-handling units below the floor and this is where they will go.
Just because no post where pouring concrete is discussed would be complete without talking about a vibrator, I feel it is incumbent on me to point it out. As the concrete is poured into the formwork, the vibrator is inserted into the concrete to make sure that the aggregate is uniformly distributed and that there aren’t any air pockets. Pretty simple job but there is some skill required … too little vibration and you don’t get the concrete distributed correctly, too much and you can over mix the concrete and settle the aggregate towards the bottom rather than uniformly.
The last thing I wanted to point out is the concrete testing. This doesn’t happen on many residential projects but this isn’t just any residential project. An independent testing company came out and check the concrete from two concrete trucks (selected at random) to perform slump tests and to take compression core samples. They measure the temperature of the concrete to make sure that the concrete hasn’t been sitting in the truck too long since it came from the batching plant and is still suitable for use.
Here is a collection of images showing a slump test being performed. They way it works is that the metal cone is filled in three loads, each load being tamped down 20 times with the metal rod. Then the cone is slowly lifted off and the cone placed next to the pile of concrete that it just uncovered. The metal rod is placed across the top and a measurement is taken from the rod down to the top of the “slumping” concrete. The amount of slump that takes place is an indicator of the strength of this load of concrete. I believe the structural engineer called for 3,500 psi concrete so we were looking for a 4″ slump – which is what we got. Too much slump and the concrete would have been rejected.
Just a few days later, all the plywood formwork gets removed and you can see the finished concrete product. Considering I know just how much work went into getting to this point, I am always amazed at how simple every looks … but looks can be deceiving. Getting something wrong here means everything after will be wrong. All the embed plates, drop heights, brick ledges, inverted brick ledges, joist pockets, etc. have to be coordinated and resolved. It looks simple because that’s the point.
Here is a detailed look at what soil retainer boards look like … corrugated plastic. Surprised?
In this picture you can see the three things that we’ve talked about in today’s post; the concrete grade beam, the soil retainer boards (the black panels above) and the cardboard void form (look to the left of the guy above – you can still see the void forms before they get covered up by the soil retainer boards).
And here is a close up finished look at a corner – see that notch cut out of the soil retainer boards? That part of concrete is just a tiny bit of the pier that the grade beams sit on (if you want to see more about the piers on this project, read this).
Finally, this is the detached garage on the right and the main house on the left with a walkway in between. Soil retainer boards have been put in place on the house and you can see the pier at the bottom left-hand corner of the house. In the trenches, that you see to either sir of the walkway space, will be perimeter drainage pipes … but that’s a different post for a different day.
I’ve tried to walk the line between keeping things simple, but include enough information as to be of some benefit. On the day the concrete was poured at the KHouse, we brought a bunch of the staff out to see how things worked. We even met the owner on site and walked the concrete pour with him and explained what was happening. This sort of thing can seem really dry and boring but as I’ve mentioned on many occasions, you have to understand something in order to appreciate it. This is not an inexpensive foundation or project so all indications would tell me that the time we spend explaining what is going on at the site with the owner, the more excited he is becoming to what he will eventually receive.
I have no doubt that the stories our client will tell folks about his house will include more than just “this is white oak flooring” or “this is a pier and beam foundation.” He’ll be able to talk about the cut of the flooring and tell people “this is white oak flooring and this pattern is called “cathedraling” and that it took 13 loads of concrete to pour his grade beams … and I think that’s pretty cool.
Cheers,
