This tutorial shows how to lay a strong 6' x 3' x 4" concrete slab a foundation for a small structure such as a toolshed or children's playhouse. What's different from this foundation and a simple concrete slab is the inclusion of steel bolts embedded in the concrete that a 2x4 frame plate can attach to. If you just want a concrete slab for other uses (grill, trash cans, etc.) just leave the bolts out in the last step.
I made it in TechShop. You just need to take the WOD101: Woodshop SBU class in order to be qualified to use all the tools in the TechShop to make the concrete mold frame and stakes. The really nice thing about TechShop is that you can buy the lumber next door at Lowes and walk right into the woodshop and cut it before taking it home.
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Step 1: Tools & Materials
Tools you'll need:
- flat head shovel
- float or cement trowel
- curved cement trowel for creating a smooth rounded edge around the slab
- large flat bucket for mixing the mortar
- 2x4 about 2 or 3 feet long for tamping the concrete
- (2) 6' 2x4s and
- (2) 3' 4" 2x4s for the concrete mold frame
- several 1x2 stakes to secure the mold frame to the ground while pouring
- gallon bucket
- long (3' ) level
- (optional) bolt cutter to cut the wire mesh to fit inside the frame
Materials you'll need
- (6) 80 lb bags of Maximizer quick setting concrete mix (or the equivalent that makes a 3' x 6' x 4" slab)
- 14 gallons of water (2.33 galllons of water for each 80 lb bag) (or the equivalent for your concrete mix)
- (optional) 3' x 6' piece of steel wire mesh
- (11) 3" long 1/4" wide hex bolts
- (not needed right now for the foundation but for when you attach your frame) (11) 1/4" washers
- (not needed right now for the foundation but for when you attach your frame) (11) 1/4" nuts
Step 2: Prep the Site
First you'll need to scrape a 3' x 6' section of ground with the flat shovel to level the ground while leaving the ground firmly packed. If you need to build up the ground rather than scraping down, you'll need to use a tamper to pack the ground before proceeding. Use the level to check your progress.
Then build a frame with internal dimensions of 3' x6' (i.e. the 6' 2x4's screwed or nailed to the 3'4" 2x4s).
Level the frame by digging down or using stones and dirt to raise the frame so it's level in both X and Y directions. Pack dirt under any gaps under the frame so the concrete doesn't leak out while pouring.
The concrete is very heavy and will force the frame to move sideways and even straight up as you pour it, so you'll need to stake the frame securely to the ground. I just cut the ends of some leftover 1x2s into stake-like shapes. After the frame is leveled, pound the stakes into the ground up against the frame then fasten them to the frame with screws or nails.
Finally, use a bolt cutter to cut the wire mesh to fit inside the frame. Lay the wire mesh propped up on some rocks to keep it about 1" above the ground.
Step 3: Mix and Pour the Concrete
Finally, we get to mix the concrete and pour it into the frame. You'll have to move quickly as the quickrete sets in an hour and we need this to be a monolithic slab meaning you have to mix and pour all 6 bags within an hour. Be mindful of your back. Tighten up your abs and lift with your legs...remember, it's easier to replace knees than spine disks.
Mix one bag at time in the large flat mixing trough. I cut open the bag in the trough, spread it out with the shovel, and added all 2.33 gallons at once, spreading it out over the whole surface with the shovel. After letting it soak in for a few seconds, I mixed it by shoveling it over and over within the trough.
After it looks like chunky ice cream, I pushed the trough up against the frame and shoveled the concrete into the frame.
After it's all in, pull up on the wire mesh a little in a few places to make sure it didn't fall off the rock props and so the wire mesh sits up in the concrete rather than on the ground.
Step 4: Level and Tamp the Slab
Level the slab with the scrap piece of 2x4 and the level.
Then, using the long side of the scrap 2x4, tamp down the entire surface of the concrete to knock down the aggregate (i.e. the gravel) so you have a nice shiny surface of cement. This will leave a smooth rather than rough surface on the slab after it dries.
Smooth the surface with the float/trowel.
Make nice rounded edges around the slab by sliding the rounding tool along the edges.
Step 5: Add the Frame Bolts
After the concrete has stiffened (about 1/2 hour to an hour), tap the hex bolts, thread side up, down into the concrete about 1" in from the edges every 1.5'. Leave enough thread on top to allow it to pass through a 2x4 and bolt to a washer and nut (about 2.5" above the surface of the slab). Use your fingers to squeeze the concrete back flush up around the bolt.
I am planning on having a single door to this tool shed, so I only added 2 bolts on the right front besides the bolts on each corner. These 2 bolts will fasten a short piece of 2x4 to the floor to attach the frame for the hinge side of the door.
Step 6: Cover the Slab (if Necessary) and Cleanup!
If there's a downpour, the pounding rain could wash off the cement from the surface of the concrete, leaving a rough surface, so if there's danger of rain, cover the slab with a tarp. The tarp also keeps off leaves, stray dogs, bicycle traffic, marauding children, etc. I just tied strings through the tarp grommets to the fence and draped the tarp over some stakes.
Hose off all your tools (and your clothes) and you're done! The concrete should set strong enough to start framing within a few days, then it's safe to pull up the stakes and remove the slab frame.
In the last two photos you can see where I worked ahead and attached the bottom foundation frame over the embedded hex bolts. I'll cover more in a future Instructable, but be sure to use water resistant lumber (I used pressure-treated chemical infused 2'x4's).