How-To pour a concrete slab

by andrewodom on July 21, 2011 · 1 comment


While it is perfectly fine for a trailer to be parked directly on the ground (read: topsoil, dirt, gravel, etc) we decided very early on that since we were going to be “parked” for years at a time, even, that parking on a concrete slab would be best. It would allow for good drainage, keep us elevated from the dirt, allow our tires to last infinitely longer (without rotting), and also provide a steady foundation for our tiny house trailer. Afterall, if concrete slabs are good enough for most RV outfits, it should certainly be good enough for us.

After deciding on the size of slab needed, calling around to local concrete companies (thus continuing to fulfill our pledge to use 70% local/regional resources and labor), getting quotes, deciding on type of ‘crete, we set our date for Saturday July 16 and began the process of forming up and preparing the ground.

Learning how to lay a concrete slab-at-grade takes a lot of effort and attention to detail we found out. Until this point I had only assisted others and even that was minimal. Come to find out, it is more than just pouring the concrete into a form and letting it dry. Some very specific basics must be followed to successfully know how to lay a concrete slab-at-grade.

First, what is the difference between a regular slab of concrete and a slab-at-grade? A slab-at-grade is a concrete floor slab entirely supported directly by the ground (or recently, de-treed, disced, graded, cleared, and packed ground). For our particular need there is no established floors, columns, or beams for the concrete to be poured over that would be needed for a regular slab of concrete; just ground!

Preparing the Ground

Before the first linear foot could be poured we had to spend several hours on ground preparation. The subgrade, or the ground below your slab, has to be prepared to support the slab and whatever weight it will carry. In our case we knew we were looking at about 10,000 dry pounds. We had already decided on a 10′ x 35′ slab, 3″ thick of 3000psi, fiber-enhanced, concrete. We leveled the grounded, flattened out any bumps, removed any debris and leveled again.

The subgrade then be graded, moistened, and compacted to make it as firm as possible. As with a dirt lot, any spot in the subgrade that held water or showed problems with retention had to be scratched out, filled in, leveled, packed, and leveled again.

At this point most traditional builders or mason would lay down gravel or a waterproof membrane to act as a vapor barrier or to keep water from seeping up. Because of our specific need we opted for fiber in our concrete. Fibers are usually used in concrete to control cracking due to both plastic shrinkage and drying shrinkage. They also reduce the permeability of concrete and thus reduce bleeding of water.

Building a Wood Frame

The part of this project I felt most comfortable with and found to be most familiar was the building of the form. Little more than a system of 2x4wooden rails, known as screed rails, we took about 3 hours to build out the screeds on all four sides of our future pad, level them, add some stobs, and then level again.

If it hasn’t become evident yet the most important part of laying concrete is leveling. If your concrete leans one way you can have irrigation issues. If you have too little support you can have cracking. It is a delicate dance and often takes more time to properly level but it is what will literally make or break  your pad.

Pouring and Finishing

As Saturday morning came we welcomed the Concrete Cowboy (half man, half A&E reality show star) to the Tiny r(E)volution where he dropped some 5 yards of fiber-enriched concrete. Sliding off a 22 foot chute, the frame filled quickly. My work party was amazing. I had asked my 3 brother-in-laws, as well as father-in-law to help and as the concrete filled the form, two of us spread it, two of us screeded it, and one worked the slide. We were like a well-oiled machine!

As the concrete began to cure we used hand trowels and a large extended float to smooth out the imperfections, work off the extra water, and level the whole job. It was a meticulous finish but so important. We then left so the slab-at-grade could solidify.

Overall I would say framing up and pouring concrete is not all that hard. However, it is not a job that can be learned by reading about or watching a video. If you know you are going to have to pour concrete, ask someone else if you can assist them to see the whole process. And don’t be afraid to ask questions of your concrete company. Remember, you are paying them for a product and a few short answers are well justified!

 

You can see the photos of the day by visiting our Flickr page. Don’t forget to add us a contact on Flickr if you “live” there as well!

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