There are two options for tying the rebar. Either plastic cable ties or wire. I use cable ties. The argument for cable ties is that they will not rust. For horizontal rebar and simple joins in vertical rebar, I find cable ties are quicker to use than wire. You cannot undo cable ties so they are not good when you initially have two pieces of rebar and then you need to add a third. If using cable ties then it is sensible to use strong ones. Regular tie wraps are rated at 18 - 50 pounds, but for rebar tying I recommend using at least 60 pound rated. For really critical joins you can even use 120 or 175 pound rated, but that's overkill.
Details on where to get cable ties can be found here .
Use wire cutters (both side cutters and end cutters) to cut off the tails, but typically leave half an inch of tail in case you need further tightening later.
Chairs to support rebar
To support the rebar the right distance from the bottom of the footing trench and over the slab, it is necessary to use what are called "chairs". The easiest and most cost effective chair to use is a thin plastic one such as these plastic rebar chairs . You want ones with a wide base so that the force is better taken by the EPS polystyrene sheet underneath.
These plastic chairs can also be purchased in a version that is 1" shorter, ie 2-1/4".
When necessary I modify the plastic chairs to the height I want using a small band saw.
Spacing rebar off the walls
It is necessary for the rebar assembly to not move about in the footing trench. It would be particularly bad if a sharp end of rebar were to puncture the waterproof membrane. The solution is to use rebar chairs horizontally to the wall of the footing trench. On a stirrup they are attached using a cross formed by two cable ties and then some PL-Premium adhesive to stop the position rotating. A chair can also be tied onto the end of a rebar straight using a cable tie through a drilled hole.
Also use PL-Premium adhesive to make it very secure.
Cut rebar is very sharp and also will impale you if you fall on it, so OSHA (Occupational Safety and Health Administration ) requires safety caps. They are made of high impact plastic with a metal plate embedded in them. They need to be OSHA compliant to guard against impalement. Detail on purchasing is here . Put them on any pieces of vertical rebar as soon as they are placed.
Also put safety caps on any horizontal rebar that might poke you in the eye. On horizontal rebar some people use the lower cost non-OSHA rebar safety caps. These are available from here .
Even when using safety caps, it is important to also wear protective eye goggles or glasses because the cut ends of the cable ties used to secure the rebar can also do you a lot of damage.
It is worth looking closely at the AutoCAD drawing of the overall foundation design to see where all the rebar goes.
Here is the drawing version for the internal concrete walls...
Set batterboard strings at vertical rebar positions
Currently there are screws in the top of the batterboards to indicate the outer edges of the Form-a-drain. What is now needed is a batterboard string between the two current strings that indicates the position of the vertical rebar.
For external walls, add a batterboard screw that is 1' 9-3/4" from the outer form-a-drain edge screw and add strings along all the footing trenches.
The positions of the internal wall vertical rebar are slightly offset compared with external walls, but again the positions will be as per your AutoCAD drawing. The following drawing shows the internal wall vertical rebar positions relative to the existing batterboard strings...
To summarize the above drawing for internal walls, the vertical rebar string needs to be 2' 4-1/2" from the internal wall outside form-a-drain.
Where the strings cross at a corner, is the exact position of the corner vertical rebar. Other vertical positions along the trench can be found by measuring in 1 foot increments from the corner position.
Mark rebar grid positions
Measuring in from the string in the perpendicular footing allows marks to be put on the slab protection EPS and on the outer form-a-drain. To do this, use a plumb-bob and a wooden stick marked with 1' marks.
Put marks on slab edge
Use half inch colored tape to show the positions.
One edge of the half inch tape indicates the vertical rebar position and the other edge is the horizontal rebar position that is offset by half an inch towards the building center.
To avoid getting confused, mark with a circle the edge of the tape that is the vertical rebar position. Mark the other tape edge with a line to represent the horizontal rebar position. It is also worth numbering the marks starting from 0 and working towards the center.
At the center of the wall the spacing will be less than 1 foot because the 1' grid is from the corners working towards the center. The actual dimension in my case is 7-1/4".
Mark on outside of trench
Also put tape pieces around the outside of the footing trench to show the 12" grid positions. Again mark one tape edge with a 0 to represent the vertical rebar position and the other edge with a line to represent the offset horizontal rebar position. Obviously the markings need to be as per your AutoCAD drawing that details the positions of the vertical wall rebar. Use a plumb bob off your batterboard strings to find the positions to mark.
At the center of the wall the spacing will be less than 1 foot because the 1' grid is from the corners working towards the center. The actual dimension in my case is 7-1/4".
Mark cube positions
Eight inch wooden cubes are used later to support the groove planks and the walkway. It is good to mark out early the places where the cubes will go. The cubes are in the spaces between the crosshatch grid of rebar that will go over the slab. The positions of the cubes are as per your AutoCAD drawing. It is worth doing it in CAD because on a large foundation it can get a bit complicated figuring out how to avoid the rebar grid. Mark the cube positions on the slab protection EPS using tape.
Make Footing Stirrup Assemblies
The structural engineering for the foundation calls for stirrups manufactured from half inch basalt rebar.
The bottom of the stirrup is supported off the bottom of the footing trench by 3.25" plastic rebar chairs to the bottom of their U. Smaller rebar chairs are used on the sides to position it properly within the trench.
Two of the following are also used...
Finally this piece goes horizontally half way up the footing...
It is good to do as much rebar tying work as possible before going down into the footing trench. Use a jig to form an assembly from the different rebar stirrups. You need to make a few hundred of the foundation stirrup assemblies. Tie them together using cable ties.
Foundation stirrup assembly jig
Add small wood blocks on a plywood base to form the jig. Where there are going to be cable ties it is good to cut an access hole to make it easier. The dimensions for fixing the wood blocks will come from your foundation AutoCAD drawings.
External wall stirrup assembly...
Internal wall stirrup assembly...
Here is the finished external wall jig that also has cable tie access holes. It also has cut slots in the edges where the rebar chairs will go...
The internal wall stirrup assembly is different from the external wall case. Here is the internal wall jig (sitting on the external wall jig)...
Make foundation stirrup assemblies
Lay the various stirrups into the jig. For both external wall and internal wall cases there are two versions of the sequence of adding rebar stirrups. This makes two variants of the stirrup assemblies.
for the Green variant (and Cyan variant) is...
Middle Big L
Inner Big L
For the Red variant (and Magenta variant), reverse the sequence.
For my house I need the following number of pre-made stirrup
External wall Green variant 90
External wall Red variant 90
Internal wall Cyan variant 48
Internal wall Magenta variant 48
At the corners, individual stirrups are used rather than stirrup assemblies.
On the stirrup assembly jig tie the stirrups together with cable ties at the places where there is a jig hole. If a stirrup is a bit warped then you may occasionally decide to add an additional cable tie.
Also add rebar chairs to the assembly, both at the bottom and the sides. At the bottom use 3.25" chairs and at the sides 2.25" chairs. Put the rebar in the chair U and then add two cable ties in a cross. Then once the assembly is off the jig spread PL-Premium adhesive on the joint to stop the chair position rotating. Gluing on just one side is sufficient.
Here is the built assembly on the jig (complete with rebar chairs)...
Remove the assembly from the jig.
Here is the internal wall version...
The assemblies go every 1 foot along the external wall footings. They should all be shown on your CAD foundation drawings.
Footing External Corners
Make sure the ToughLiner and underlying waterproof membrane are in the right position. You want the slack to be at the sides of the trench and you want to ensure that there is slack both above and below where the horizontal side chairs will be.
Add assemblies either side of corners
Either side of the corners, add one completely orthogonal stirrup assembly and one very slightly angled. This is enough to be tied to the three inner corner pieces and a few more of the further in corner pieces.
Don't put horizontal chairs on the inside for the very slightly angled stirrup assemblies as the inside edges will be cable tied tightly together.
Work from one side of the corner to the other side in order to get good access for cable tying.
It is useful to use a straight wooden stick between the marker tape on each side of the trench. Adding an 8" piece of wood on the end takes out the differences in the slab level compared with the outer Form-a-drain.
Cut to make horizontal #4 corner pieces
At the corners use #4 angle pieces. They will overlap by at least 22.5" (ie at least 30 times the #6 rebar diameter). In practice the legs of my corner pieces are 2 (24") as they are the corners cut from my rebar straights that have a bend in the end.
The bend pieces are made by cutting off the corners from the straights that have a bend in the end. Each leg is 2' and the corner is 6" so it leaves a straight offcut piece that is 16 foot long.
Even though it is not a requirement of the structural engineering, it is good to provide continuity between the horizontal rebar in the footings round the corner. As there are 13 pieces of horizontal rebar that means each convex corner needs 13 corner pieces. It is not a concern that you will be joining #6 and #5 rebar straights using #4 diameter corner pieces (given that it is not a structural engineering requirement even to have corner pieces).
My main house has 8 convex corners so that means I need 104 corner pieces.
The bend pieces will not all be within the stirrup assemblies and even the ones that are will be in different places inside the stirrup assembly.
Internal wall footing corners do not require corner pieces because these are all crosses and the horizontal rebar can be continued across the cross, all be it with a slight joggle of their position between an internal wall footing and an external wall footing.
Horizontal straight rebar at corners
When at 45 degrees the Big U stirrups are not wide enough to go to the corner of the trench. A horizontal rebar straight with a chair on each end will work ok in the corners to hold the stirrup assembly in the right place laterally. In practice there is lots of bunched up polyethylene sheet in the corners to make a relatively flat surface for the chair to rest on. This also keeps the liners properly fitted into the corners. A sensible length of the rebar straight is 5'5". Fix rebar chairs horizontally on the end using a cable tie through a hole drilled in the chair. Also fix 3.25" chairs near each end and in the middle to hold the rebar the right distance off the bottom. In addition to crossed cable ties it is good to also use some PL-Premium adhesive on the chair joins.
It is useful to mark on the horizontal 45 degree rebar the positions where the bent corner pieces will cross the 5'5" long rebar. Measured from the inside end the cross points are...
5' 0-3/4" outside
Add angled stirrups at corners
These are the same stirrups but placed at an angle in the corners. They cannot be pre-formed into stirrup assemblies because the dimensions of the assembly need to be different at the corners. You need to form modified stirrup assemblies in situ at the corners.
At the corners, tie the stirrups to the 45 degree horizontal rebar using cable ties.
First put in place the Big U stirrups. Then tie in place the Big L stirrups.
Note that even at the corners it is the position of the vertical rebar (that will go up into the walls) that determines the position of the stirrup assemblies.
In practice you will do most of your work from one side of the corner and incrementally form the corners. The important thing is to ensure you never snooker yourself such that you cannot get access to do the tying.
Put in place bent corner pieces
Thread the corner pieces through the variants of the stirrup assemblies used at the corners.
On the inside of the curve use cable ties to attach them such that they are in the right places to mate up with the horizontal rebar that is not yet added. This uses a few extra cable ties but it is worth it to hold the corners together. The positions for the horizontal rebar can be found from the orthogonal stirrup assemblies on each side of the corner.
The corner pieces on the outside will not span the full complement of angled stirrup assemblies. You will find it easier to add these after the horizontal rebar has been added to the trench.
Add diagonal straight rebar across corner
There will be some offcut pieces of 3/4" and 5/8" rebar from the cutting process (see later) and it is an added bonus to fit these at 45 degrees across the corners. It all helps with the strength of the foundations.
Use #6 rebar of lengths 8', 6'2", 5', and 3' 2-1/4" at the bottom. Use #5 rebar of lengths 8', 6'2", 5', and 3' 2-1/4" at the middle. Lay them into the U tails of the stirrup assemblies and you only need to cable tie them in a couple of places to keep them in place.
Add Big Horizontal at 45 degrees
As a final step on the corner add a horizontal big horizontal stirrup at 45 degrees on the corner just for good measure.
First orthogonal stirrup assembly
The last piece of what it designated the corner assembly is the first stirrup assembly that is orthogonal in the trench that you will be doing next. It is important to get this stirrup assembly in exactly the right place as all other stirrup assemblies will be set relative to this.
The stirrup assemblies are positioned such that on the stirrup assembly the rebar center of the top edge rebar that goes against the vertical is on the "horizontal" edge of the 1/2" marker tape. This is to make sure the vertical rebar added later will be in the correct position. Use a straight wood stick with an 8" spacer on the end between the marker tape on the slab and the marker tape on the outside form-a-drain to see where the vertical rebar will be. Use a spirit level to get the bottom of the stirrup assembly in the right place. Once the first stirrup assembly is in the right place you can at least on the inside edge fix it by cable tying it to the horizontal corner bent pieces. Fixing the outside edge will have to wait until the first horizontal rebar has been fitted and tied to the corner.
The corner assemblies should now form a stable "corner-stone" that can help hold everything in place.
General notes on methodology
Once you have done the first corner and the first set of horizontal rebar then you can increment on from there along the trench and then other trenches. The important thing is to make sure you never snooker yourself.
You can add the complete set of stirrup assemblies along the trench before you implement the corner on the other end. To do this you will have to have the bottom horizontal rebar positioned relative to the 45 degrees corner straights so you have something to tie the bottom of the stirrup assemblies to. Note that you cannot permanently tie the horizontal rebar at the corners unless you have the corner pieces rested in place.
Cut horizontal rebar
Typically in a trench for each horizontal position will be one uncut length and one cut length. It is good to work out the cut lengths in AutoCAD so they don't have to cut them in situ.
Opinion and code varies, but at absolute minimum they must overlap by at least 22.5" (ie at least 30 times the #6 rebar diameter). I chose to make the overlap at least 3'2" (38") to be conservative. In the calculation for how long the cut piece should be, assume the 19'2" rebar straight only provides 16'.
After calculating what the cut lengths need to be, use some judgment to increase the overlaps to avoid waste given that there is no point in having a bunch of little offcuts. Sometimes increasing the cut length to 19'2 (and thus avoiding cutting) is the right thing to do. Sometimes increasing the cut length to 9'7" (ie half of 19'2") is the right thing. These rounding ups make particular sense where the extra can extend into internal wall areas.
For each footing trench make a drawing to show the lengths of horizontal rebar required in each trench. The red figures give the figures that were decided on.
Furthest west (and the furthest east) north-south footing (2 off)
East-west corners (4 off)
Portico north-south short edges (4 off)
Portico east-west long edges (2 off)
Internal north-south long walls (2 off)
Internal east-west short portico walls (2 off)
Overall cut list
If you want to you can do all the cutting in one go prior to moving the rebar to the appropriate places. You can form an overall cut list. It is also worth listing the uncut 19'2" lengths to help with moving logistics.
19' 2" (2 x 13) + (4 x 0) + (4 x 9) + (2 x 9) + (2 x 18) + (2 x 9) = 26 + 36 + 18 + 36 + 18 = 134
14' 2" (2 x 2) + (4 x 0) + (4 x 0) + (2 x 0) + (2 x 0) + (2 x 0) = 4 = 4
13' 0" (2 x 1) + (4 x 0) + (4 x 0) + (2 x 0) + (2 x 0) + (2 x 0) = 2 = 2
11' 0" (2 x 2) + (4 x 0) + (4 x 0) + (2 x 0) + (2 x 0) + (2 x 0) = 4 = 4
9' 7" (halving) (2 x 0) + (4 x 9) + (4 x 0) + (2 x 4) + (2 x 0) + (2 x 9) = 36 + 8 + 18 = 62
6' 2" (offcut) (2 x 0) + (4 x 0) + (4 x 0) + (2 x 1) + (2 x 0) + (2 x 0) = 2 = 2
5' 0" (offcut) (2 x 0) + (4 x 0) + (4 x 0) + (2 x 2) + (2 x 0) + (2 x 0) = 4 = 4
19' 2" (2 x 5) + (4 x 0) + (4 x 4) + (2 x 4) + (2 x 8) + (2 x 4) = 10 + 16 + 8 + 16 + 8 = 58
114' 2" (2 x 1) + (4 x 0) + (4 x 0) + (2 x 0) + (2 x 0) + (2 x 0) = 2 = 2
13' 0" (2 x 1) + (4 x 0) + (4 x 0) + (2 x 0) + (2 x 0) + (2 x 0) = 2 = 2
11' 0" (2 x 1) + (4 x 0) + (4 x 0) + (2 x 0) + (2 x 0) + (2 x 0) = 2 = 2
9' 7" (halving) (2 x 0) + (4 x 4) + (4 x 0) + (2 x 1) + (2 x 0) + (2 x 4) = 16 + 2 + 8 = 26
6' 2" (offcut) (2 x 0) + (4 x 0) + (4 x 0) + (2 x 1) + (2 x 0) + (2 x 0) = 2 = 2
5' 0" (offcut) (2 x 0) + (4 x 0) + (4 x 0) + (2 x 1) + (2 x 0) + (2 x 0) = 2 = 2
Personally I tend to just cut the rebar to the right lengths as I need it.
Add horizontal rebar
Add bottom horizontals
Start by adding the #6 rebar straights in the bottom locations for the current end of the trench. Cable tie the ends to the corner assemblies. They end with 1" on overlap over the corner 45 degree pieces.
Four of the five positions are obvious on the stirrup assembly. Use a wooden measuring stick to set the position of the fifth one.
Note that typically at each location the horizontal is actually made up of two pieces to get the necessary length. It will be one full length piece and one shorter cut piece. Alternate which end of the trench gets the long pieces and which the short. As there are 5 bottom locations, make the two outer locations (and the middle location) be the full length pieces.
Add stirrup assemblies (of the correct variant for that section of footing) into the trench one at a time by threading the bottom horizontals into the assembly and then work the assemblies along the trench until they are in the right locations as indicated by the colored tape positions.
You want some membrane slack above the side chairs and some below.
Slide the rebar assemblies in at an angle with the horizontal rebar chairs held in the right place with your hands so they don't break. Make sure you are not pulling the foundation linings out of their correct position and make sure there is equal slack in the linings on both sides of the trench (and above and below the horizontal chairs).
Set the correct position of particularly the bottom of the stirrup assembly using a 4' long wood jig with 1' notches (5/8" notches).
Cable tie the two outer two bottom horizontals to the bottom corners of each stirrup assembly. This will hold the stirrup assembly in its right location. Note that it is good that we are only tying the bottom of the stirrup assemblies because it allows some adjustment of the top when later adding the vertical rebar. For the moment the stirrup assemblies will stay approximately vertical because of friction with the side chairs against the footing sides.
If you find they are flopping around then you can use 8' long wooden notch sticks to temporarily hold the tops of the stirrup assemblies.
Note that many of the footings span across the building center cross and as such the variant of stirrup assembly used will change half way along the trench.
When you get about half way along the length of the footing, add the second lengths of bottom horizontal rebar in the right locations (alternating long and short). Initially you can push it well into the trench length to keep it out of the way.
When you get about three quarters of the way along the footing, cable tie the second lengths of horizontal bottom rebar in the right locations such that they will overlap by about 1" what will be the 45 degree corner piece at the forthcoming corner.
Also at about three quarters of the way along, add all the other horizontal rebar, both the full length and the cut shorter length pieces. It needs to all be in the right general locations relative to the stirrup assembly rods, but it does not matter if it drops down. Initially push it all as far in as it will go to keep it out of the way. Make sure the ends of the rebar do not risk puncturing the membranes. Also then feeding it in it is good to use a plank of wood to protect the edges of the footing at the end. It is ok to bend the rebar to get it in as it will happily spring back.
Once all the stirrup assemblies have been added to the footing trench the middle and upper horizontal rebar can be cable tied in the correct locations from above. Lay a plank across the trench and lay on it to allow you to reach down into the trench.
Internal wall footing
Internal cross corners
At the cross points where and external footing meets an internal footing there is lots of crossover for the horizontals from one trench with the horizontals from the other trenches. Because of this there is no need for angled horizontal bars.
Rebar over slab area
Lower layer on slab area
The over slab lower layer rebar alternates between straights with a bend and pure straights that go across the footing to a rebar chair on the outside (or right across in the case of an internal wall footing). That means the bent ended rebar pieces are every 24" at the lower level.
The angled rebar has a bend radius of 6". The bent leg does not need to be cut as a 2'6" bent leg is fine. The bent vertical leg goes down the inner edge of the footing trench. The vertical leg must be tied in multiple places (4 places) with cable ties to ensure it cannot slip down and puncture the waterproof membrane.
On the slab area the angle pieces rest on short rebar chairs. Regular 2-1/4" chairs are modified on a band saw to support the bottom edge of the rebar 1.5" from the deck. The concrete chairs can go directly onto the 1" EPS that covers the slab area. Use cable ties to attach the rebar to the chair.
Use wood jigs to help you quickly and consistently cut the rebar chairs.
Upper layer on slab area
As with the lower layer, the upper layer alternates between a rebar piece with a bend in the end and a purely straight piece. The ends of the straight rebar droop down a bitin order to avoid the groove plank for the wall.
The leg of the bent piece goes vertically down near the center of the footing. In the case of the purely straight rebar, it goes over the footing trench and terminates with a horizontal rebar chair tied on the end.
The center to center distance between the upper and lower slab rebar is 4". That equates to 3.5" from the top surface of the lower rebar to the bottom surface of the upper rebar, or in practice 3-3/8" given that it will sit on a cable tie. This spacing can be achieved with a 3.25" rebar chair and 1/8" of error (which can be ignored).
The bottom end of the rebar chair is attached to the lower rebar using a cable tie through a hole drilled near the bottom of the chair. A cable tie is also used at the top round the U.
The 12" on center grid needs to extend all the way out including over the external (and internal) footing trench areas.
Glue everything in place
When you get to a natural point where you will not be moving things around for a day or two, it is worth going round with a glue gun loaded with PL Premium to make sure nothing can move. You will also want to put glue around the base of the vertical bits of rebar to make sure they don't slip out of position. Remember that pumping wet concrete can exert quite a large force on the rebar and you don't want it to move. You especially don't want the sharp end of a piece of vertical rebar to come free and possibly puncture the membranes that line the footing trench.
Also put PL Premium on the horizontal chairs on the ends of the over slab rebar.
Install vertical footing rebar
This bent length of vertical rebar ties initially to the middle horizontal of the stirrup assemblies. Tying at the bottom is too difficult to reach and tying at the top is undesirable because you want some position flexibility for later adding the groove planks. At the bottom, the bent leg is constrained by the U tails on adjacent stirrup assemblies.
To get it in the right position at the top of the stirrup assembly use a wooden jig. The horizontal is a 2x4 and the vertical pieces are 2x6. The right edge (inner edge) of the tall vertical indicates the edge of the vertical rebar (which is 1/4" from the center of the vertical rebar).
It can be convenient to use the same jig to lie on to spread your weight when doing cable tying.
The verticals go 12" on center.
The bent legs are never shortened, ie they are always 2'6" long. They are angled to fit them in. On the external wall footings there is more space inwards so the legs go inwards. The legs go away from the building center.
The legs are constrained at the bottom by placing them in the U tail on the adjacent stirrup assembly. This avoids the need to cable tie them at the bottom (reaching to the bottom to cable tie is difficult). Just cable tie them to the middle horizontal bar of the stirrup. Don't tie them at the top because you will need some position flexibility when fitting the groove plank later.
For internal wall footings, alternate the legs in and out.
The length of the rebar above slab height is given in your AutoCAD drawings as it varies depending on the door and window openings. There are some cases where the full length of the #4 rebar could be used, but it was decided that having this flopping around too far above slab height would be unmanageable. Even when it could be longer, the maximum height above slab height will be 3' 6-1/2".
As you add each vertical piece of rebar, add safety rebar caps on the top.
Jigs to form top groove
Cross trench jigsAbout every about 6 feet there is a 2x4 assembly across the footing trench. It hooks over the outer upper form-a-drain. At the other end it is supported by an 8" wooden cube that sits within the rebar grid on the slab area. The cross trench plank supports the groove notch plank that has 1 foot pitch holes for the vertical rebar.
Here's the drawing for the most common jig type that is used on the outer footings. In my case I need 34 of these.
5' 4" Outer footings 34 off
Where possible it is good to stick to standard dimensions for the jigs that support the groove planks, but in some cases they need to be non standard to ensure they don't conflict with the rebar positions. Also making some non-standard allows the number of 8" cube boxes to be minimized. Plan it out on your AutoCAD drawings.
Here are the other jig drawings.
5'9" Internal footings 10 off
6'7" East west at corners 4 off
7'7" North south at corners 4 off
On the slab area there are 8" wooden cubes to hold the assemblies at the right height. The cubes are designed to be easy to remove after the concrete has set. Screws are only used at the top and they are covered with tape to stop the heads being filled by concrete. At the bottom, tape is used to stop the wood warping out.
The cube edge is 3' 3-1/4" from the vertical rebar string in the case of outside walls. For internal walls the distance is usually 2' 9-1/2", although it is a bit more at the corners.
If the slab area is a fraction high in some areas then it may be necessary to shave a little off the bottom of the cube sides to ensure the top of the cube is accurately at slab height. Use a laser and a length of white pipe to establish how much if any needs to be shaved off. (If the slab area is too low in a place then use tape to add shims under the sides.)
Key notch plank
The 3.5" groove is formed by lumber cut to 2.5" x 1" with half inch EPS glued on the bottom and sides. The EPS makes it easier to remove after the concrete has set. The planks hold the vertical rebar in the right place.
Cut the 2.5"x1" wood from lumber that is 8' long. You can get two pieces from a 2x6. Join the planks together using a couple of 1/4" wood dowels in the end and some PL-Premium. Arrange the bow of the planks such that the joins are the highest point and the bow is down in the middle. The joins will be held lower by the straightening pieces that will be added later.
Start by making the corners (as these are the starting point for measuring the rebar hole positions). Join by drilling holes for the dowels in the side of a second plank. Glue the corners with PL-Premium. Once the glue has set, drill 3/4" holes in the groove planks to correspond to the vertical rebar positions, which are 12" on center.
Need the grove plank on the internal concrete walls too. Note that there is an offset relative to the plank on the outer footing.
Having the key notch lumber in place while pouring the concrete is useful as it will help show the required concrete level. The top of the planks will be at the same height as the outer Form-a-drain.
In order to take out bow in the groove planks it is necessary to attach on edge 2x4s to the middle of the top side. These are attached with just 2 screws so they can be removed for final screeding after the concrete is starting to harden.
Add paving slabs
At the form-a-drain end of each cross trench jig add a 12" square paving slab to keep the end resting tightly on the top edge of the form-a-drain.
This walkway system will stay in place even while doing 90% of the concrete pour for the foundations. It consists of 8" wide planks between the 8" wood cubes. It is designed to not get in the way of the rebar that will be over the slab. The planks will be screwed to the wood cubes (2 screws each end) and thus ensure that the cubes do not move and thus the groove planks do not move. The lengths of the planks are as per your AutoCAD drawing. Put tape over the screw heads so the heads do not get clogged with concrete. The planks can be removed as needed during the concrete screeding.
The 8" high blocks will stay in place during the concrete pour and concrete setting. The screeding is done off the 8" high wooden cubes. The holes in the slab left will be filled with bagged concrete later.
Stiffening and adjusting
This is a piece of 'on-edge' 2x4 screwed on top of the cross trench jig. It stiffens the jig and provides height adjustment.
Use a self leveling laser and a length of white pipe to get the key notch plank height exactly right. Use wood shims under the stiffening 2x4 to adjust the height of the groove plank.
Squaring up groove planks
First ensure the sides of the grove rectangles are exactly straight. You can do this by shining a laser along the edge of the 2x4 key notch lumber.
It is necessary to get the key notch lumber rectangles exactly square, ie have exactly right angle corners. This is done by measuring the diagonals of the rectangle to ensure they are exactly the same length. The distance should also be the same as your AutoCAD drawing shows.
Initially one diagonal will be longer than the other. Attach some garden wire between the corners of the longest diagonal and with a spare bit of 2x4 gradually twist it to pull the long corners in. Keep measuring and stop twisting when both diagonals measure the same.