It is about time we publish a hands-on, how-to, nuts & bolts article. Last week we poured a new concrete slab that is part of a larger remodeling project. The pad was made for a Jacuzzi hot tub that needed to be relocated.
Ready? Here we go…
1. Figure out the dimensions of the slab you need. I like to make a simple drawing to make sure I am not missing anything — see below. (In the bottom right-hand corner the client’s identity was redacted)
2. Determine precisely where you want it, and keep in mind that most municipalities require a clearance from the property line. This distance can be four feet or more. Check with your local building or planning department. While on that topic, most building departments do not care about so-called flatwork — a different term for concrete pads, driveways, and such, as long as you do not put something underneath, such as plumbing, electrical conduit, and so forth.
3. Figure that the slab should be at least five inches thick. Why? You need at least two inches from the top of the rebar to the top of the slab, and that same distance from the bottom of rebar to the bottom of the slab. The area where two pieces of rebar intersect produces a dimension of one inch, assuming you are using 1/2″ rebar. Plus two to the top and two to the bottom = 2 + 1 + 2 = 5. If you plan to install something very heavy on the slab, or something with a high point load, do yourself a favor and check with an engineer. It is better to spend a few hundred bucks on an expert than far more on a demo crew to take out your concrete.
You may already know about my disdain and distrust of articles written by e-how. They publish info that is often incorrect, in part because many of their so-called ‘contributors’ have zero experience with the topics they write about. Stay far away from these folks — all they want is for you to click on ads.
4. Excavate your site to make room for the forms and slab, and be sure to allow for at least three inches of gravel underneath. Figure out what you will do with the dirt. Can it go elsewhere on the property, or does it have to be hauled away? Is there anything below the surface that you are excavating? In our case there was a lawn sprinkler, so we asked the client’s landscape to disconnect it. Otherwise, if a sprinkler pipe ever needed repair you would not be able to get to it.
Should your excavation be more than 5 or 6 inches, be sure to alert Underground Service Alert before you start work. Read more about them in this earlier post.
5. Once your form is in place (remember to install braces so the form will not fall apart once concrete is poured), compact the gravel with a Whacker compactor or similar. This equipment is heavy, so having a helper on hand is a good idea.
6. Install the necessary rebar, and be sure to use ample 2″ dobies (shown above) to create the necessary clearance between the gravel and the underside of the rebar. Be sure to tie everything together, and please do not use pieces of rock or other loose material that could dislodge during the pour.
7. Figure out how much concrete you will need. There are calculators on the internet that can help you figure this out. When you enter the dimensions, keep in mind that the grade of your location may not be level. At our job site, the yard sloped toward the house, so one side of the pad required more concrete than the opposite side. For example, using the dimension shown in the sketch, the calculator linked to above says I need 0.9 cubic yards. Yet I ordered 1-1/4 cu. yd. and still did not have enough. We happened to have two left-over bags sitting around, and those saved our bacon.
I recommend ordering about ten percent extra. Also consider if you would like to have a color added. Something like ‘lampblack’ yields a slight darker color, which is often nicer than the standard color of cement and it does not cost much. Will you want an accelerant to speed up the drying process?
8. Depending on the size and location of your project, you can either carry the concrete by wheelbarrow from the deliver truck to the site, or you can hire a pump truck which uses hoses to pump the cement from the truck to your project. An so-called gondola can also be used. Given that this was a small project, concrete went straight from the truck in a wheelbarrow. Concrete companies usually include a brief standby time in their charges to allow for this. Apply form oil to the inside of the form material to help concrete not stick to it. Be sure to not get this oil on the rebar.
9. Once the pour is complete, used a screed to level the concrete from side-to-side. Then use a wide float attached to a pole to smooth out the surface. Let it sit for awile so it can start to set. Then with hand-held float start the finish, round over the edges, and so forth. The final finish can be a ‘broom’ finish or a trowel finish. Depending on what you intend to use the slab for, many other finishes are available.
10. Remove the forms, finish the sides of the concrete pad if necessary, and you are good to go!
In our particular case, we installed 3/4 inch rigid electrical conduit underneath, had it inspected by the local building inspector, and then did the pour. You can see the conduit stubbed out of the pad; it has black electrician’s tape at the top of it. The disconnect for the tub was installed in the right rear of the tub in the photo below (not shown). Be sure to check the National Electrical Code, as there are specific distance and line-of-sight requirements.