I hope someone knows the answer, because I've wondered about it, too. It seems to me the main thing is keeping overlapping or crossing rebar together while the concrete is placed. In other words, once the concrete sets up, what difference would it make if the rebar was tied with yarn instead of steel tie wire?

All I've found on the web so far is this blurb from the CRSI manual, (which costs $145.00), from another forum, http://www.eng-tips.com/viewthread.cfm?qid=116986&page=1.

I'll report back with what our local inspector says, for what it's worth. You'd think there'd be tensile strength requirement for various pours, applications, (columns, foundations, etc.), and not just this reference to a wire gauge. The blurb follows:::

CRSI Manual of Standard Practice

Chapter 4 "Suggested Specifications for Reinforcing Steel"

E. Tie Wire:
1. Tie wire shale be 16.5 gauge or heaver, black annealed.
2. Epoxy-coated reinforcing bars shall be tied with plastic-coated or epoxy-coated steel wire; or other acceptable material.
3. Zinc-coated (galvanized) reinforcing bars shall be tied with zinc-coated steel wire, non-metallic-coated tie steel wire, or other acceptable material.

The CRSI book Placing Reinforcing Bars page 10-27 to 10-32 gives examples of different types of ties and why and when they are used. There is also lots of other information concerning ties and wire. There is no mention of the automatic tying machines. In the field I have observed that experienced rod-busters can out tie a tying machine any day of week.

Fine Homebuilding magazine has shown articles where a contractor use nylon ties to secure rebar (looped around another bar and the end sticking through an ICF form) and no outcry rose up. My only recall is the minimum overlap lengths required when bars meet up end-to-end.

Interesting. Thank you both. And I'll ask my structural and framing inspector, too, when he gives me a final.

Our rebar cages passed, with about half the ties made with plastic zip ties. Note that this is a residential application and the inspector seemed correspondingly lenient. He hadn't seen zip ties used before nor heard of the practice.

Sure enough, I found the Fine Homebuilding article mentioned, Issue 170, April/May 2005, page 83. Quote: "Plastic zip ties and internal webbing hold vertical and horozontal rebar in place." It's describing how to use interlocking foam forms for a foundation pour.

Bill

Yup, the local HfH chapter uses plastic zip ties in a design provided by the ICF manufacturer to tie the rebar to the plastic ribs built into the ICF forms. Personally, since my hands are large enough for getting into the tighter places---it requires both hands to apply the zip ties----I think is is a pain. But, it is code compliant.

The ties only need to hold everything in place during the pour.
They contribute very little strngth to the final assembly.
If additional strength is required, the bars are welded together.

One of the first jobs I had in the construction industry was working for a major concrete foundation contractor in South Texas. During that 3 year stint I learned more about rebar than what I ever wanted to know.

While residential slabs are pretty much commonplace nonetheless somewhere in your foundation specs or local code they will define the type of reinforcement required.

Typically a residential slab will call for #4bar 12"OC w/ 50% tie awg 16 or greater wire.

Rebar diameters graduate by 1/8ths of an inch so a #4 bar is 4/8" or 1/2" diameter.

The 12"OC is specifying the spacing between bars in the grid, in this case 12" center to center.

A 50% tie indicates that every other cross must be tied. A 100% tie would indicte tying every cross section and in some instances in commercial work when using large bar such as #12 they may call for 200% tie which is two wires at each intersection. When making 200% tie the wires must run opposite each other so one wire cross the X one way and the second wire crosses it the opposite way.

When making the tie the wire is to be passed under or behind the joint then bent back so the two ends of the wire are parallel then it is to be twisted while keeping a firm pull on the wire to insure a tight connection.

Generally they use precut, preformed wires that have a loop on each end. The wire is passed under or behind the joint then a twisting hook (Pigtail tool) is inserted in the loops and the handle pulled forward while twisting to insure a tight joint. An experienced Rod Buster using preformed wires and a pigtail could make a dozen ties in the time you could set one plastic tie and when done the wire tie would be much tighter than you can pull the plastic tie. Of course you could invest $32 in a "Panduit Gun" to pull the plastic ties but that would nearly triple the time required to make each joint and even then it is doubtful that you would gain any tensile strength in the tie.

When properly tied with wire the pads should be strong enough that they could be tied on the side of the job and lifted and set into place by a crane without any loss of structural integrity, and once the concrete hardens the wires serve no purpose as the rebar is totally encapulated in the concrete and cannot move.

What is of much more importance is the placement of the chairs under the rebar to insure the rebar will remain elevated into the concrete until the concrete hardens.

While I doubt if you would ever see it on a residential spec sheet, often in commercial work they even specify the manufacturer of the bar to be used. (Every rebar manufacturer uses a different shape for the ridges on the bar and even a short section of bar can be traced to the manufacturer by the shape of the ridges.)

If you use the plastic ties, cut off the excess length after they are tight. Having those ends in the concrete seems to me to be undesirable.

Thanks, Lazypup, for the info -I'll copy that into my files. And, bus_driver, we did cut the excess off - for aesthetics, if nothing else.