Yes, rebar has to be secured so it stays exactly where the drawings put it while the concrete is placed, and tying with wire is how that is normally done. The ties do not make the concrete stronger. Their only job is to hold spacing, height and cover until the concrete sets. You do not have to tie every intersection: the RSIC Manual of Standard Practice, which Canadian rebar detailers and placers follow, says a tie at every fourth or fifth intersection is sufficient, with any extra tying clearly specified by the engineer.
This post covers whether and how much to tie. For tie wire sizes and coatings, see rebar tie wire. For tie types and hand-tying technique, see how to tie rebar.
Why rebar is tied
Every reinforced concrete design assumes the steel is in a specific place: a certain depth in a slab, a certain spacing in a wall, a certain cover from the face of a beam. Between placing the bars and finishing the pour, a lot can move them:
- Foot traffic. Crews walk on slab mats, and loose bars roll and spread.
- Concrete placement. A pump hose or chute drops heavy, wet concrete onto the steel.
- Vibration. Internal vibrators shake everything nearby.
- Gravity. Horizontal bars in walls slide down the verticals if nothing holds them.
- Buoyancy. Light bars such as GFRP can float up in fresh concrete if they are not tied down.
The industry's suggested specification in the RSIC manual puts the requirement plainly: bars are to be wire tied together at enough intersections to keep them from being displaced during concreting. The manual's placing chapter adds that bars must be tied so they do not move outside the allowable tolerances.
Those tolerances come from CSA A23.1, which limits how far bars can stray from the specified cover, depth and spacing, and says cover must never be reduced by more than one third. A mat that shifts during the pour can end up out of tolerance even if it was placed perfectly. Our post on rebar placement tolerances under CSA A23.1 lists the limits.
Ties hold position, not load
A common belief is that tied intersections help the steel work together in the finished slab. They do not. Once the concrete hardens, each bar carries load through its bond with the surrounding concrete. A 16 gauge wire twist at a crossing has no meaningful structural role.
That is why:
- Lap splices work through the concrete, not the tie. CSA A23.3 even allows non-contact laps, where the two bars do not touch at all, as long as they are no farther apart than one fifth of the lap length or 150 mm.
- Tie quantity is about stability, not strength. More ties than needed add labour, not capacity.
- A missing tie is a placement risk, not a strength defect, until the bar moves. A bar that has moved is the defect.
There is one place where "tied" means something structural: column ties and stirrups are bars, not wire, and they are designed to confine and carry shear. That is a different subject. See rebar stirrups vs ties.
How many intersections to tie
The baseline in Canadian practice is the RSIC figure: a tie at about every fourth or fifth intersection, which works out to roughly 20% to 25% of crossings. RSIC's estimating chapter treats any tying beyond that as extra work that the engineer must specify and the estimator should note in the quote.
A quick example: a 6 m by 6 m slab with 10M bars at 400 mm both ways has 16 bars each way and 256 intersections. Tying every fourth crossing is about 64 ties; every fifth is about 51.
That baseline is spread across the mat, but some spots always get ties:
| Location | Why it is tied |
|---|---|
| Contact lap splices | RSIC practice: contact laps are to be securely fastened |
| Bundled bars | CSA A23.3 requires bars in a bundle to be tied or otherwise fastened together |
| Mat edges and corners | Edge bars are the first to be kicked or rolled out of place |
| Bent ends and corner bars in footings | Keeps hooks and L-bars from rotating during the pour |
| Dowels | Holds the projecting length and position the next pour depends on |
| Bars on supports with tie wires | Locks the mat to its chairs or blocks so cover is kept |
| Walkways and pump hose routes | Traffic loads the steel far more than the concrete does |
The footing tying guide walks through these in a footing.
When more tying is needed
Some elements need more than the baseline, and the engineer or the placer's judgment should add it:
- Walls. Vertical mats are tied more heavily than flat slabs, because gravity works on every horizontal bar. Wall ties are usually wrapped so the horizontals cannot slide.
- Top mats and high chairs. A raised mat carries crew traffic on fewer supports, so it is tied more.
- Pre-assembled cages and mats lifted by crane. Lifting puts real strain on the ties. Cages that will be picked need heavier ties and often stiffening bars. See prefabricated rebar cages for footings and piers.
- Lightweight bar. GFRP mats are light and buoyant, so they are tied and anchored more thoroughly. Our guide to installing fiberglass rebar covers this.
- Architectural or exposed concrete. More ties help hold tight cover tolerances, with tie ends turned away from the face.
If the specification calls for tying every intersection, that is a legitimate requirement, but it has to be in the contract documents. RSIC is explicit that tying beyond normal industry practice must be clearly specified, because it changes the labour in the bid.
Alternatives to hand tying
Wire ties are the default, but not the only way to secure bars:
- Battery-powered tying tools. Same result as hand ties, much faster on large mats. Use the wire the tool maker specifies, and check it meets the job's coating requirements.
- Plastic clips and connectors. Used for some GFRP and coated bar work, where steel wire is not wanted.
- Welded wire mesh. Factory-welded intersections, so only laps and edges need ties.
- Tack welding. CSA A23.1 requires tack welding of reinforcing bars to be done in accordance with CSA W186. Welding can damage ordinary bar, so it needs weldable bar and the engineer's approval. See weldable rebar in Canada.
Whatever the method, match the tie material to the bar: galvanized wire on galvanized bar, coated or plastic ties on epoxy-coated or GFRP bar, so the tie does not become the weak point for corrosion.
What happens when rebar is not tied enough
Under-tied steel rarely looks wrong until the pour starts. Then:
- Slab bars get pushed down toward the base, losing their position in the depth of the slab.
- Wall horizontals slide, opening gaps and closing others below the minimum clear spacing.
- Dowels tilt or drop, so the projecting length no longer matches the next pour.
- Cover is lost at faces, which shortens the life of the concrete in wet or salted exposures. See does rebar rust in concrete.
- Inspectors can stop a pour when bars are visibly out of place.
None of these show up as a "tie failure". They show up as cracking, rust staining or a failed inspection, which is why the small cost of enough ties is worth it.
Getting bars that are easy to place and tie
Placing goes faster when every bar arrives cut, bent and tagged to the placing drawings, so the crew is tying instead of sorting and trimming. We detail, cut, bend, tag and bundle 10M to 55M rebar, supply GFRP bar, and supply and install rebar across the GTA and Ontario.
Send us your drawings for a free take-off and quote, or see our rebar products.
Frequently asked questions
Does rebar have to be tied together?
It has to be secured so it cannot move out of position while the concrete is placed, and tying with wire is the normal way to do that. The ties hold the bars in place; they do not add strength to the finished concrete.
Do you have to tie every rebar intersection?
No. The RSIC Manual of Standard Practice says a tie at every fourth or fifth intersection is sufficient, and that any additional tying must be clearly specified by the engineer.
Does tying rebar make it stronger?
No. Once the concrete hardens, the bars work through their bond with the concrete. The wire ties only keep the bars at the right spacing, height and cover until then.
Do lap splices have to be tied?
Contact lap splices are tied so the two bars stay together, and RSIC practice says they are to be securely fastened. Non-contact laps are allowed if the bars are no more than one fifth of the lap length or 150 mm apart.
Can rebar be welded instead of tied?
Only where the engineer allows it, using weldable bar and welding to CSA W186. CSA A23.1 says tack welding of reinforcing bars must also follow CSA W186. Casual tack welds on ordinary bar can damage it.
