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Foundation Rebar


Rebar implementation

Tying rebar

There are two good options for tying the rebar.  Either plastic cable ties or galvanized wire.  I use both methods depending on the particular situation.  The argument for cable ties is that they will not rust, but in practice the wire is thin so if it rusts will not expand enough to crack the concrete, and anyway my concrete is kept dry.  There are some situations where wire is the only option eg when needing to use it through a small hole on the end of a piece of vertical rebar.  For horizontal rebar and simple joins in vertical rebar, cable ties are quicker to use than galvanized wire.  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 120 pound rated.  For really critical joins you can even use 175 pound rated, but that's probably overkill.  When using wire, it is best to use galvanized wire rather than regular rebar tying wire.

On balance I prefer to use galvanized wire, especially because it better allows multiple pieces of rebar to join at the same point without it all becoming a mass of cable ties.  I cut the 16 AWG galvanized wire ahead of time into 8" lengths.

Details on where to get cable ties can be found here .  Details on galvanized wire is here .

Tie wraps   Galvanized wire

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".  Many people use plastic rebar chairs, but I prefer to use chairs made out of concrete.  This is because they are made of the same material (concrete) as what will be surrounding them, and thus have the same thermal expansion/contraction characteristics.  That avoids there being a gap around the chair that water might try to get in.  You can make your own concrete chairs using yogurt pots of the required size.  If you make your own concrete chairs then you embed galvanized wire so they can be attached to the rebar.

Concrete chair inch and a half


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 concrete chairs horizontally to the wall of the footing trench.  Attaching the concrete chairs horizontally is a bit of a challenge, but if you drill a small hole in the end of the rebar you can wire-on a 1.5" concrete chair and then glue it with  some PL-Premium adhesive .

Concrete chair end 

If you are making your own concrete chairs then molding them onto the end of sticks of rebar is a good option (and my preferred option).

Cushion concrete chairs with EPS pads

It is necessary to cushion the bottom of the concrete chairs so there is no danger of them hurting the waterproof membrane.  This is done by using double sided tape to fix a 4" square piece of 1" thick EPS under where the concrete chair bottom will be, or 1/2" EPS where it is against the sides.  On top of the EPS, a 3.5" square of PVC sheet (with the sharp corners cut off) is also attached with double sided tape.  This gives the concrete chair something to rub on.

First cut up some PVC sheet into 3.5" squares.

PVC for pads

Apply 3 strips of double sided tape to the most shinny side.  Then cut off the corners so there are no sharp corners to damage the waterproof membrane.

Double sided tape on PVC 

Cut up some half inch and some one inch EPS into 4" squares. 

Half inch EPS for pad 

Using the double sided tape, attach the PVC to the EPS squares and push it well on so the tape takes properly. 

Assembled half inch EPS pad 


Assembled inch thick EPS pad 

In the case of the one inch pads, glue on 1.5" concrete chairs using PL-Premium adhesive

Rebar chair on pad 


Safety first

Cut rebar is very sharp and also will impale you if you fall on it, so building code 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.

Rebar safety caps

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 .

Horizontal rebar safety cap

Even when using safety caps, it is important to also wear protective eye goggles or glasses because the ends of the wire used to secure the rebar can also do you a lot of damage.

Safety goggles when working on rebar 


Foundation Design

It is worth looking closely at the overall foundation design to see where all the rebar goes.

Foundation design 


Rebar placement

Place cross bars in bottom of footing

These bits of rebar are 29" long.  Assuming the lower Form-a-drain is in the right place (ie the lined trench is 33" inside dimension at the bottom), then that should on each end leave 2" of clearance between the ends and the polystyrene.  If the trench width is not right then adjust the rebar length accordingly. 

Drill 1/8" diameter holes in the rebar at each end about 3/8" from the ends.

To hold them in the right place laterally, wire-on a 1.5" a concrete chair at both ends of the rebar and then a 1/2" EPS pad to hold it the required 2" from the walls of the footing trench.

Concrete chair end 

To hold it the right distance from the bottom of the trench use the chair on 1" EPS pad assemblies.  Use two of the pad assemblies per piece of rebar to support them off the footing floor.

Rebar chair on pad 

The two pad assemblies go at specific distances along the rebar.  Referring to the foundation drawing you will see that on the outside edge of the trench they are 9.5" from the trench wall and on the inside they are 5.5" from the inside trench wall (measured to the center of the pad assembly).  Given the 1/2" EPS pads, that means they are 5" and 9" from the outside ends of the horizontal concrete chairs.

Make up the complete rebar assembly (minus the 1/2" EPS side pads) on a bench before placing in the footing trench.  Don't cut the tails off the cable ties until the assemblies are installed in the trench, just in case you want to tighten one a little bit more once installed.

Rebar in bottom of trench 
(Picture shows the old out-of-date way of doing horizontal chairs)

Place the rebar rod assemblies at 12" on center.  When you built the walkway with the wooden blocks and planks you will have marked out the grid of rebar over the slab area and these lines should correspond with where you are placing the rebar in the footing trench.

Start by placing the first rebar assembly in the center of the footing trench (ie half way along) and then work outwards (at 12" intervals) either side.  As you get to a corner, put a rebar assembly right at the edge before the corner, adjusting the last spacing to less than 12" to achieve this.

Note that the position of these cross pieces of rebar along the footing trench will also set the positions of the vertical rebar in the walls, so it is worth being accurate.

Slide the rebar assemblies in at an angle with the 1/2" EPS pads held in the right place with your hands.  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.

Rebar in bottom of footing trench 

Site with walkway and first rebar 

At the corners put diagonal horizontal bars

These are the same as the previous bars but need to be about 40" long.  They are horizontal at 45 degrees.  The same lateral EPS pad arrangement will work ok in the corners because in practice there is lots of bunched up polyethylene sheet in the corners to make a relatively flat surface.  It is  however worth using extra PL-Premium to make sure the EPS pad does not slip from under the horizontal chair.  The positions of the concrete chair pads is 12" and 7" in from the ends of the concrete chairs.

Corner rebar 

Lay 3 parallel lengths of rebar resting on cross rebar

These rebar lengths go all the way along the footing trench all the way round.  At the corners use angle pieces.  Overlap by at least 15" (ie at least 30 times the half inch rebar diameter).  Use the cable ties or galvanized wire joins that are on the cross rebar to make some of the joins to the angle pieces and then on both legs put an additional cable tie or galvanized wire join just before the radius part of the angle pieces to complete the job.

First horizontal rebar 

Install angle pieces from inner footing to on top of slab area

This step is in practice done in parallel with the following step ("Install lower rebar across slab and over footing area").

The angled rebar has one leg of 28", other leg of 20.5", and a bend radius of 6".  The long leg goes vertical and the short leg goes over the slab area.  You need to trim the length of the vertical long leg by about 3.5 inches.  For the cut long leg you also need to drill an 1/8" hole in the rebar about 3/8" from the end.  This will be used for galvanized wire to secure the end.

On the slab area the angle pieces rest on 1.5" concrete chairs.  The concrete chairs can go directly onto the 1" EPS that covers the slab area.  Note that the cable ties or galvanized wire that attach the angle pieces to the chair are shared with the rebar over the slab in the step that follows.

At the bottom they tie using galvanized wire to the inner horizontal rebar at the bottom of the trench.  The galvanized wire goes through the hole you drilled in the rebar.  These angle pieces of rebar are 24" on center (ie every alternate position relative to the previously added rebar cross bars) all the way round the outside of the slab.  Start with one at the center of each trench and work outwards.

Vertical rebar wired 

Install lower rebar across slab and over footing area

This step is in practice done in parallel with the previous step ("Install angle pieces from inner footing to top of slab area").

This is on 24" centers (it will alternate with the upper slab rebar).  It corresponds with the bent rebar that comes out of the footing trench also on 24" center.  It juts out over the footing trench, stopping 2" from the outer polystyrene of the trench lining.  Attach 1.5" concrete chairs horizontally to the ends using galvanized wire through an 1/8" hole drilled 3/8" from the end of the rebar.  At some convenient time add some PL Premium adhesive.

Concrete chair on end of rebar 

Where the end rubs on the polyethylene covered outer wall of the trench, use a half inch EPS pad.

Lower rebar over slab 

Install 35" vertical footing rebar

This 35" length of vertical rebar ties to the bottom to the 29" horizontals and at the top to the horizontal rebar that goes across the slab area.  The connection at the bottom is done by drilling an 1/8" hole 3/8" from the end and then using galvanized wire.

The verticals go on 12" center.  At this stage there is only horizontal rebar at 24" intervals so for the moment the alternate verticals can just flap around.

As you add each vertical piece of rebar, make sure you add safety caps on the top.

Vertical outer rebar 

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 none of the EPS pads can slip from under the concrete chairs.  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 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 footing top center horizontal rebar

This gets tied to the lower over slab rebar and will be used to support the center footing angle pieces.  Start by cutting rebar straights to form the full length of the wall center dimensions (joining lengths with at least a 15" overlap if necessary to achieve the wall length).  Drill 1/8" holes a 3/8" from the ends.  Position the center wall horizontal rebar and connect the ends with galvanized wire to form a rectangle (or whatever composite rectangle-like shape represents the positions of all the wall centers).

Measure half way along the straights of the rectangle and put a mark (with a red sharpie).  Measure 10.5" in from the waterproof membrane on the outer inner face of the footing trench (see foundation diagram).  This should be the center of the wall.  Make a mark on the over slab rebar to show this position.  Tie the center wall horizontal rebar such that it is just to the outside of this mark.  Tie it on the center over slab rebar first and of course this tie position should correspond with the half way along the wall mark on the horizontal center of wall rebar.  Tie the horizontal rebar to all the over-slab rebar in the same way.

Use angle pieces at the corners to strengthen the rectangle that constitutes the center of the wall.

Install angle pieces up the footing center

Even the short leg of the angled rebar is too long for the footing width, so rather than cutting the short leg, it is angled  to get it to fit.  Having more rebar is best so there is no sense in cutting off rebar and throwing away the offcuts.  Alternate left left and then right right at every 12" position.  The pattern repeats every 4 feet along the wall.  Tie the vertical long legs of the angled rebar to the center top ring of horizontal rebar that is the wall center,  At the bottom tie the short legs horizontally to the original cross trench pieces of rebar.

As you add each vertical piece of rebar, make sure you add safety caps on the top.

Angled rebar to wall center 

Vertical rebar that goes into walls

Because the angled rebar does not have long enough legs to form a proper overlap join above basement floor level, it is necessary to set pieces of straight rebar up from the footings. These rebar straights should protrude 36" above the surface of the slab.

The straight rebar lengths are 72" long so that they go down to where the original cross trench rebar bars are.  They attach to the cross rebar using galvanized wire through an 1/8" hole that is drilled 3/8" from the end.

Particularly at the top connection as they pass the center of wall horizontals, you may need to one at a time undo the galvanized wire and remake the join to now include the extra vertical rebar.

As you add each vertical piece of rebar, make sure you add safety caps on the top.

Vertical rebar that goes up into walls 

At the corners you also need the vertical rebar on 12" centers.

Vertical rebar at corners 

Install cross rebar that's half way up the footing

These go on 12" center across the footing trench half way up.   They tie to the vertical rebar in the middle and at the ends.  These pieces of rebar are 20" long (although if your trench walls are a bit bowed in then you might have to make then slightly less (possibly even 18") to avoid the possibility of them touching the waterproof membrane).  Drill a 1/8" hole about 3/8" from the end as this is used for the connection on the end nearest the slab.  They go 15" above the original cross pieces.  On the end without the wire hole, use a rubber thimble on the end of the rebar so there is no danger of a sharp end hurting the waterproof lining.

Half way up cross bars 

Install the upper rebar over the slab area

This alternates with the lower rebar over the slab area.  The rebar is supported 6" off the EPS that covers the slab area by using rebar chairs that have rebar extension pieces.

Rebar chair wired 

Rebar chair wired and glued 

Alternatively these can be made by molding your own concrete block on the end of an appropriate length of rebar.

Extended concrete chair 

On the outside end on the horizontal over slab rebar, wire-on 1.5" rebar chairs horizontally and use 1/2" EPS pads to the outer side wall of the footing trench.  Note that the ends of the rebar droop down a bit (to the height of the outside verticals) in order to avoid the keyway for the wall.


Install angle pieces from footing to upper slab rebar

These angle pieces hang down into the footing.  The short leg goes into the footing.  The short legs tie to the cross rebar that is half way up the footing trench.

Upper over-slab rebar with chair on the end


Add two extra angle pieces at corners

Install two extra pieces of angled rebar at the corners as shown in the picture.

Angled corner rebar 


Install top two rings of horizontal rebar

These inner and outer rings of horizontal rebar use angle pieces at the corners. 

Horizontal upper rebar rings 


Implement 2x4 sticks to form top groove

There needs to be a groove in the footing wall to key to the ICF wall concrete.  This stops the walls from moving laterally on the foundation.  The first thing to do is to know exactly where the center of the wall will be.  This is necessary in order to get the 3.5" key notch in the center of the 8" of concrete in the wall.

It is the outside face of the ICF that is the reference and will form walls with an outside dimension that is an integer number of feet.  Using my chosen ICF wall system, the vertical rebar will be exactly 1 foot on center with the exception of the last one before the corner which will be 11-3/8".  The diagram can be easily extracted from your AutoCAD house drawings.

Rebar wall placement diagram 

The diagram shows the center of the 2x4 lumber that will form the notch in the top of the concrete.  Cut (and as needed join) some 2x4s to form a rectangle with a center as per the drawing.

The planks hold the rebar in the right place.  Drill 5/8" holes in 2x4 long planks to correspond to the rebar pitch, which is exactly 12" on center, except for the last before the corner which is 11-3/8".  Given that there will be short joining pieces of 2x4 to join the corners, you will also need to continue the hole through this double thickness.  Don't actually join the rectangle at the corners until each straight section has been fitted over the vertical wall rebar.

Cover the 2x4s with 7.5" wide 1/4" gasket foam.  This will make it easier to remove the 2x4s from the concrete later.  You can use tape to hold it in place on the top side of the 2x4s.  Do the wrapping before installing the 2x4s over the vertical rebar.  After wrapping, cut the gasket foam away at the holes.

As you gradually work the 2x4 straights down the vertical rebar you will likely need to make a few small adjustments on the positioning of the rebar and you may need to do some loosening and re-tying in a few places.  Sometimes a few light hits with a hammer on the rebar will do the necessary to get everything to line up.

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.  You can use a spirit level between the top of the Form-a-drain and the key notch board to get the height about right.

Need the grove sticks on the internal ICF walls too.

Key notch fitted 

Key notch level 

First key notch 

Implement support lumber

In order to hold the key notch sticks at the right height and in the right position, implement 2x4 assemblies as shown in the photos below.  They are 4' lengths of 2x4.  At one end add two pieces of 2x2 to form a 3" spacer.  That same 3" of spacing is done on the key notch planks using two 4" blocks of 2x4 that are screwed to the top of the key notch planks.  The 4' long pieces are screwed to the blocks on the key notch lumber from above when in place thanks to holes drilled at 19" and 21" from the outside end.  You will also need to relief drill the holes with a 1/2" drill to allow 3" wood screws to be used.  At the inside end (2" from the end) you also need a relief drilled hole.  This will be used to screw to the walkways at the support cube locations.  On the outside end screw two 1' pieces of 2x4 perpendicularly to provide a cradle for holding a large rock.  Leave a 3.5" gap between the cradle pieces of 2x4.

Support for key notch lumber 

Two pieces of support lumber are needed at the corners and typically half way along walls to keep the key notch lumber in the right place.

Measure diagonal key notch rectangle 

The outside ends rest on the outer Form-a-drain.  Place a large rock (or some bricks) on the cradle at the outside end to weigh it down so that it properly rests on the Form-a-drain. 

Wire to adjust key notch rectangle 

Before screwing down the inner ends of the support lumber it is necessary to get the key notch lumber rectangle 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 diagram 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.

Wire tensioning diagonal adjust 

You also need to ensure the sides of the rectangle are exactly straight.  You can do this by shining a laser along the edge of the 2x4 key notch lumber.

Where the walkway jogs around pipes etc, you may need a longer piece of 2x4 to span the greater gap. 

Bracing creativity for key notch lumber 

After the key notch rectangle is exactly square and straight. and a spirit level off the outer Form-a-drain has been used to get it exactly the right height, then the inner ends of the support lumber can be screwed to the walkway (preferably above he support cubes).  You will typically need to put a few shims under the inner end to set the height to get the support lumber level.  Having the support lumber exactly level should mean that the top of the key notch lumber is at the same height as the top of the outer Form-a-drain.

Site with key notch planks  


Check the levelness

The final step is to make sure the key notch boards (and the top of the outer Form-a-drain) are accurately level.  They should be unless there has been some uneven settling.  It's worth checking just to be certain.  Use a self leveling laser to check the levelness.

Self leveling laser  

The best way is to place the laser level on the slab area and arrange for the height of the beam to be at finished concrete height plus about an 1/8".  This can be done using the following box arrangement.  This box arrangement is designed to be left in place even while pouring the concrete.  The sides of the box are exactly 8" high which corresponds to the slab thickness and the inset allows the body of the laser level to sit down into the concrete so the beam comes out at about 1/8" above the surface of the concrete.  The box is an 8" cube and is wrapped in foam gasket to make it easier to remove once the slab concrete has set.

Laser box stand 

You are now ready for the concrete!

Site ready for the concrete pour