Tying rebar is a hard job, a dirty job and if you're not in good shape can be a painful job. That is why I let my workers do the tying! Just kidding! So, we built the form to hold the concrete and we know the form is built strong and it's perfectly square, level and lined up.
Before we start placing and tying rebar we have to lay the vapor barrier! The vapor barrier serves two purposes, keeps moisture from coming up into the slab for the longterm and helps keep moisture in the concrete during the curing stage.
When you go to the building material store you're looking for 10mm grade vapor barrier. A lot of stores don't carry 10mm so you might have to settle for 6mm which is very common.
Pick up a roll of tuck tape, (red rolls) this will be used for taping the joints and any tears that may occur during the rebar installation. Roll the plastic out and remove all of the wrinkles, lay a few pieces of rebar on the plastic if the wind is blowing hard enough to lift the plastic.
One person can lay the vapor barrier if they use rebar as there second and third hand. As you take the wrinkles out, drag a corner of the vapor barrier to one corner of the form. Fit the vapor barrier in the corners so it fits nicely and staple the edge of the vapor barrier to the top edge of the form without going out over the top of the form. The plastic also serves as a protector that keeps your forms clean which makes things easy to clean up.
If the plastic is allowed over the top edge of the form it will get in the way of the concrete finish work. Make sure the plastic is pushed into all the corners and against the bottom of the form.
As the concrete fills the deeper part of the slab it will pull the plastic down away from the top of the form if there isn't enough slack to play with. That is something you'll have to watch as you pour concrete. Tape all the joints and make sure there are few or no wrinkles.
Placing and tying rebar
Be careful when placing the bar because we don't want to tear the plastic. To start, we are placing 4 runs of 15mm all the way around the thickened part of the form. Don't even try to put bar in place during the pour! You're thinking, who places rebar during a pour? It happens.
If all else fails, follow the drawings and install the rebar exactly the way the drawing says to. Check the building codes if you have to! My information is general, it's simply a step by step procedure to make things easier for you.
We need to cut 10mm bar into 16 inch lengths, enough to go every four feet all the way around the form twice. These bars hold the horizontal bar in place and evenly spaced. Start in one corner, using a small mallet, drive one rebar three inches out from the corner diagonally.
Standing in one corner, look at the cross corner and line the bar up. Then set out 17 more inches from the first stake and drive another bar, keeping them lined up with the cross corner. This doesn't have to be bang on exact but try to get them close, makes tying rebar easier.
Drive the two stakes in all four corners. There are four runs of 15mm rebar around the outside, two runs are three inches off the bottom and two runs are three inches down from the top of the form.
This spacing puts the rebar about halfway into the concrete or a little higher is better. This formula works if you're pouring a six inch slab. Following the two inside lines below, drive two rebar every four feet directly across from each other about 14 inches apart.
Wear gloves when handling or tying rebar, it's very hard on the hands! Ready for the top run of 15mm? Same procedure as the bottom only three inches down from the top of the form. Tie the rebar to every stake! Now, lets move on and build the grid with 10mm.
The photo below gives you an idea of what a rebar grid would look like. I didn't finish the lines all the way to the corner because I wanted you to see what the corners look like when you were done tying the 15mm bar.
This particular grid has a 16 inch spacing either way you look at it. Place the ends of the rebar about one half inch away from the form. Overlap the rebar joints by at least 16 inches, depending on the size of course and tie it in two places.
Rebar is delivered in 20 foot lengths, so you will have to cut it accordingly. You can have more than one splice in a single run if you have to but the least amount of splices is better.
Once you place a full run of rebar on the vapor barrier, use a black marker and mark off 16 inch spacings on the bar. Do the same thing all the way around the outside and down the middle if you're pouring a big concrete slab.
Place the rebar on every marking, building a grid. We use chimney brick to place under the rebar, this puts the bar a little higher than half way up into the concrete slab and makes tying rebar a whole lot easier too.
You need goggles for this job! Lay a brick on it's flat and hit the middle with a hammer, this will break the brick in half if you're careful. A half a brick or smaller is all we need to hold the bar off the vapor barrier.
Make sure the rebar is still on their marks. Starting in one corner, place a brick in a four foot square grid. The rebar will spring up and down when you walk on it so don't be alarmed.
After all the brick are placed and the rebar is off the ground, you can start tying rebar. Place the rebar on the marks if they moved when placing brick. Tie every other joint!
After you're done tying rebar, check your work to be sure. Make sure all bar is off the vapor barrier! We don't want the bar too high, at least 2-3 inches below the top of the form. If the bar is too high it can interfere with the finishing of the slab.
To check this, run a string line from one side to the other and hold it tight so it doesn't sag in the middle. Walk along the form and keep an eye on the string everywhere to make sure there is enough space between the string and the rebar.
Check the forms dimensions. Check the rebar joints and overlaps. Check the braces and all the stakes to make sure everything is screwed properly.
When you're checking to see if the rebar is the right height, measure down from the string to the plastic. You need to have an idea of how thick the slab will be, it should be close to six inches if the ground was prepared properly and the form likewise.
Take an average depth but then round your numbers up so you don't run out of concrete at the end of the pour. When we build our pads, we make them laser level.
Then, when it comes time to order concrete....the calculations will be more accurate. Running short of concrete on a pour isn't an option. Ordering too much concrete is an expensive waste at an average of $160 a meter for concrete.