One of my final fall projects is to pour a small concrete slab for my trashcans to sit on. Until now I have been placing them in the same spot that the slab will go but once winter comes I will need something more substantial to place them on and I will also need some type of fencing to hold them in place during the harsh winter winds.
Probably the hardest thing about this project has been finding time to do it. I purchased the concrete back in the spring when I bought mulch for the garden but I just never had time this summer due to other things that life brings you. I don’t recommend doing that because if concrete gets wet it could cure in the bag and that is not very pretty.
The first thing I did was to build a small form for the slab. It is pretty simple and just required two pieces. One six foot piece is the center of the form and then the two sides were made by cutting one piece to length and then cutting corner to corner to form triangles. Screws were used to join it but nails would work just as well. The form is held in place by 1×2 stakes that are driven in on the outside of the form at least 6 inches deep and driven below the top of the form so I can later use a screed board to level the concrete once it is poured.
After the form is in place I trenched around the perimeter of the form using the flat side of a pick. You want the footer for the slab to be about 6 inches underground. You aren’t pouring a footer for your home it is just for trash cans but since its the garden waste can it can get to over 200 pounds easy so the form needs a decent footer.
The slab also needs to be supported in the center so I dig two small footers under the slab in the center of the opening to support the weight of the cans so the slab won’t crack.
Using rebar is really up to you and I actually had some but because the concrete slab will be thick and the footers pretty deep I didn’t use rebar for this project. If you were building something more important or substantial like a slab for in front of your front door then I would strongly suggest the use of rebar and rebar mesh if you can get it and cut it to size.
Finally I placed about an inch of stone on the dirt both below the slab and under the footers. This allows the ground below the footer to move a bit and it is a stronger material than the sand/clay that the slab will sit on. Never build a slab on top of topsoil. I had already removed all the topsoil and then in my vegetable garden I dug down below the good soil and got some hard clay sandy material that is about as hard to dig through as concrete. Well maybe not that hard but it has a high compaction rate and nothing will grow in it not even weeds.
Mixing the Concrete
Now that the footing is ready its time to mix the concrete. This is a job where you really should have help. Unfortunately I didn’t have any help so I had to work as fast and as hard as I could to continuously add concrete so it wouldn’t cure where I first laid it before the entire form was filled.
This slab may look small but it took 15 bags of concrete and a bag and a half of 1/2 inch stone to fill it. Never underestimate how much concrete you need because the last thing you want to do is try to run to the store to get more to complete the job. It is better to have a couple more bags than you need and they are pretty inexpensive so it won’t hurt your budget. Not to mention its always good to have a bag around for some type of small job.
When adding water to the concrete you want to be very conservative. You only want enough water that the entire mix is completely wet. You don’t want soupy concrete and the dryer you make it the stronger it will be.
The reason that using less water makes your concrete stronger is because the water in the mix will evaporate and leave behind openings like swiss cheese. The less of that in your completed slab or footing means the concrete is denser and it will not only be stronger but it will last much longer.
Use a shovel and keep turning the concrete as you mix it. Chopping into it with the shovel also works. Make a series of chops into the concrete about an inch apart starting away from you and working in or back to yourself and water will get down to the bottom.
While the form is being filled it is important to use a 2×4 to compact it. This is very important around your footer. Just use it like a ram rod and pack it into the form.
Once the form is filled you want to screed it level as you can with a long 2×4 that reaches across your form. Tap down on the concrete and pull it towards you then go back and fill any voids or low areas using a shovel or two of concrete.
You should then trawl the surface to push the very top layer of stone down about a 1/4 inch and to level it better. The top layer of cement that remains will give the surface appearance of finished concrete and pressing down the stone means stone won’t be showing through. If you were not to do this the slab would still be fine it just wouldn’t look very good and may not be level or without bumps.
Continue using your trawl for as long as you think is necessary. Stroke in straight lines and in half circles. Eventually you will know when its good enough.
In my case I also had to match the driveway with the slab. Now this is not the method most people would use but I just removed the stone near the edge so I could spread the cement even with the driveway. Later on if I can I will go back with a diamond blade saw and cut an expansion joint between the driveway and the concrete but in this situation its probably not necessary. Concrete and asphalt will expand and contract at different rates which is not good but hopefully there won’t be problems with this small slab. I didn’t have any with our walkway.
Finishing the Slab With Anchor Bolts
Since I will be installing a small fence to keep the trashcans in place I need a way to attach them to the slab. I probably could have just inserted the whole steel post in the concrete but that would mean they would probably rust faster in the concrete than the rest of the post even though they are galvanized. Instead I opted for inserting some half inch anchor bolts and I will have to find a way to secure the posts to them. I have been considering many methods such as welding on a bracket or filling the post with construction adhesive or epoxy but probably what I will end up doing is just using some Automotive hose clamps until I figure out something better.
This really is a 2 or more person project even with such a small slab because after digging the and installing the form there was a lot of concrete to mix. By the end of it you can expect to be pretty tired if you are doing it yourself.