A wire loop box is a thin galvanised steel box, cast flush into the edge of a precast wall, holding a folded high-strength wire rope loop. After de-moulding the tape is peeled off and the loops fold out; the loops of two adjacent elements overlap inside the joint recess, a vertical lock bar is dropped through the overlapping loops, and the recess is grouted — the joint becomes load-bearing only once that grout gains strength. It transfers longitudinal shear, transverse shear and tension (combined loading checked with VEd/VRd + NEd/NRd + FEd/FRd ≤ 1), but it cannot transfer bending moment — and it must never be used to lift an element.
1. What a wire loop box is
A wire loop box — Lastschlaufenbox in German, loop box or wire rope connection box in English — is a small, thin-walled galvanised steel box containing a folded loop of high-strength wire rope, whose free ends are pressed into a steel sleeve. The box is nailed, magneted or glued to the formwork at the edge of a precast element, cast in, and closed with a strong flexible tape so no concrete can get inside.
Its purpose is to replace protruding bent rebar starter bars at element joints. Bent bars have to be straightened on site, they are a snag and impalement hazard on the stacking yard, and they make elements awkward to transport and store. A loop box keeps the connection flush inside the element until it is needed.
2. The three forces it carries — and the one it can't
Once the joint is grouted and the lock bar is in, the connection behaves as an overlapping-loop splice. The overlapping wire loops from both elements clamp onto the central lock bar; the grout transfers force into the loops by bearing and bond. That mechanism can carry:
| Action | Carried? | Notes |
|---|---|---|
| Longitudinal (vertical) shear, V | ✓ Yes | The classic wall-to-wall shear splice; usually the governing action. |
| Transverse shear, N | ✓ Yes | Out-of-plane; resistance is typically a small fraction of the tensile resistance, so check it — it is easy to under-estimate. |
| Tension, F | ✓ Yes | Carried through the lock bar. No lock bar, no tensile capacity. |
| Bending moment, M | ✕ No | The connection cannot transfer moment. If the joint must be moment-resisting, this is the wrong product. |
| Lifting the element | ✕ Never | Not a lifting anchor. See the warning at the top of this page. |
Combined loading: the interaction check
Real joints rarely see one action alone. Loop-box joints are verified with a linear interaction rule, in the spirit of the EN 1992 design framework:
3. The lock bar and the grout rule
Two site details decide whether the joint you designed is the joint you built.
The lock bar
A vertical reinforcing bar — typically B500B, around Ø12 mm for mid-size boxes and Ø16 mm for the largest — is dropped from above, down through every overlapping wire loop in the joint. It is the component that closes the tensile load path: the loops from both elements clamp onto it, and the tension across the joint is carried by the bar sitting in the middle of the grouted recess.
Consequences that matter on site: the loops must all be folded out into the same plane, perpendicular to the box face, so the bar can pass cleanly through all of them; and the loops on both sides must have been installed at the same heights, from the base of the element up, or they will not line up.
The grout
Use a high-strength, free-flowing, non-shrink grout or self-compacting mortar whose compressive strength is at least equal to that of the precast element. The grout must reach into the steel boxes and fully surround the loops — voids there mean the loops are not gripped and the tested capacity is fiction. Self-compacting mixes remove the need to vibrate the joint, which is why they are preferred.
4. Sizing: wall thickness first, load second
Engineers instinctively start from load. For loop boxes, geometry usually decides the family before load does, because three dimensions have to co-exist inside a thin wall:
- Wall thickness. The box, plus a minimum 20 mm concrete cover on each side, has to fit. This alone rules out the larger boxes in thin walls.
- Joint geometry. The recess must be deep and wide enough for the loops from both sides to unfold fully and overlap without hitting each other. A longer loop needs a deeper recess — and a longer loop is exactly what a bigger box gives you.
- Load capacity. Only once the first two are satisfied do you check whether the interaction equation passes; if it doesn't, add loop pairs up the joint rather than jumping to a bigger box that no longer fits.
| Design input | What it constrains | Practical rule |
|---|---|---|
| Wall thickness | Maximum box size | Box width + 2 × 20 mm cover ≤ wall thickness |
| Loop length | Recess depth & overlap | Longer loop → deeper recess → thicker wall. Loops must overlap, not collide. |
| Edge distance | Distance to top/bottom of element | Respect the supplier's minimum; loops too near an edge split the concrete. |
| Loop spacing (same side) | Whether capacities are additive | Keep adjacent boxes far enough apart that they do not interact — then capacity simply adds up. |
| Concrete & grout class | Resistance values | ≥ C25/30 element; grout ≥ element strength. Higher class → higher resistance. |
Element reinforcement around the box
The box needs the element's own reinforcement to develop its capacity. In practice that means two layers of mesh, stirrups local to each loop (a U-shaped stirrup in the area of every wire loop is good practice) and longitudinal bars carried through to the panel edge so the edge doesn't spall at de-moulding. National codes take precedence over any manufacturer recommendation here.
5. Single loop box or double loop box?
A single box holds one wire rope loop; a double holds two. Because resistance is additive when boxes don't interact, a double box roughly doubles the force transferable at that point in the joint.
| Single loop box | Double loop box | |
|---|---|---|
| Capacity per box | Baseline | ✓ ≈ 2 × baseline |
| Boxes needed up the joint | More | ✓ Fewer |
| Grout flow into the box | ✓ Easier | Needs a properly flowable mix |
| Cost per box | ✓ Lower | Higher |
| Best when | Modest shear demand, plenty of joint height to distribute boxes | High shear demand, or limited joint height / restricted box positions |
The practical decision: if you can distribute enough single boxes up the joint to satisfy the interaction check, do that — they are cheaper and grout more reliably. Reach for doubles when the joint geometry limits how many boxes you can place.
6. Nine-step installation
- Fix the box to the formwork, rigidly. Nails through the punched holes on timber formwork; magnets or adhesive on steel or plastic formwork — with the contact face clean and degreased, or it will let go during the pour.
- Start from the bottom. Set boxes working up from the lowest point of the element, and use the same arrangement on both sides of the joint, or the loops will not line up on site.
- Check the loop lies straight. The rope should sit as straight as possible between the mesh layers, in a plane perpendicular to the box face.
- Confirm the tape seal. The flexible tape is the only thing keeping concrete out of the box. A torn or lifted tape is a dead loop.
- Pour and compact — but never vibrate the box. Compact the concrete carefully near the loops; keep the poker off the box itself.
- De-mould, then peel. Once the concrete has hardened, strip the formwork, remove the cover tape, and fold the wire loop out.
- Straighten the loops into plane. Loops must end up perpendicular to the box face so the opposite loops overlap in a controlled way. Do not hammer a kinked rope straight.
- Erect, brace, and drop the lock bar. Place the element, secure it on temporary bracing, then insert the lock bar from above through all overlapping loops.
- Grout, cure, then unprop. Fill the recess completely with a flowable grout of at least element strength, ensure no voids inside the boxes, and only release the temporary bracing once the grout has reached its required strength.
7. The defects that quietly destroy capacity
A loop-box joint fails at the detail level long before it fails structurally. These are the ones inspectors actually find:
| Defect | Root cause | Consequence |
|---|---|---|
| Concrete inside the box | Tape torn, badly sealed, or box vibrated directly | Loop cannot be folded out — the connection is gone. Do not hammer it free. |
| Box moved during the pour | Weak fixing; greasy formwork face under a magnet/adhesive | Loops on the two elements no longer align; lock bar won't pass through. |
| Kinked or nicked wire | Forcing a fouled loop straight | Local loss of rope capacity — invisible, and not recoverable. |
| Rust / broken wire | Outdoor storage, humidity, salt, ice | Reduced breaking load. Store boxes dry, under cover. |
| Lock bar missing or short | Bar not passed through every loop | Joint has no tensile capacity, whatever the drawing says. |
| Voids in the grout | Stiff mix; grout can't reach inside the boxes | Loops not gripped; the tested resistance no longer applies. |
8. Frequently asked questions
What is a wire loop box used for in precast concrete?
A wire loop box connects precast elements to each other — wall to wall, wall to column, corner joints and wall-to-slab joints. A folded wire rope loop sits inside a thin steel box cast into the element edge. On site the loops of two elements overlap in the joint recess, a vertical lock bar is threaded down through the overlapping loops, and the recess is grouted. The connection transfers shear and tension across the joint once the grout has gained strength.
Can a wire loop box be used to lift a precast element?
No. A loop box is a connection component, not a lifting anchor, and manufacturers explicitly prohibit lifting with it. The wire rope loop is sized for in-service shear and tension across a grouted joint, not for the dynamic shock of a crane lift, and there is no clutch interface or certified safety factor for lifting. Use a purpose-made lifting anchor or lifting socket instead.
What forces can a loop box joint transfer — and what can it not?
It transfers longitudinal (vertical) shear, transverse shear and tension, and combinations of the three. It cannot transfer bending moment. For combined loading the joint is verified with the linear interaction check VEd/VRd + NEd/NRd + FEd/FRd ≤ 1, with the resistances taken from the manufacturer's tested or ETA-approved data for the relevant concrete and grout class.
What is the lock bar and why does it matter?
The lock bar is a vertical reinforcing bar (typically B500B, around Ø12 for mid-size boxes and Ø16 for the largest) dropped from above through all the overlapping wire loops in the joint. It closes the load path: the overlapping loops clamp onto the bar, so tension across the joint is carried by the bar in the middle of the grouted recess. Without a correctly sized, fully inserted lock bar the joint has no tensile capacity.
What grout should be used in a loop box joint?
Use a high-strength, free-flowing, non-shrink grout or self-compacting mortar whose compressive strength is at least equal to that of the precast element (typically C25/30 or higher). The grout must flow into the steel boxes and around the loops with no voids; self-compacting mixes avoid the need to vibrate. The joint may only be treated as load-bearing after the grout has reached its required strength.
How do I choose the loop box size?
Size is driven by three things in order: wall thickness (the box plus 20 mm minimum cover each side must fit), joint geometry (the loops must unfold and overlap fully inside the recess without hitting each other), and required load capacity (more loop pairs, or a larger loop, gives more capacity). Longer loops need deeper recesses and thicker walls, so wall thickness usually decides the family before load does.
Single loop box or double loop box?
A single loop box contains one wire rope loop; a double contains two. Because capacity is additive when the boxes are far enough apart not to interact, doubling the loops roughly doubles the transferable force at that joint location — useful where shear demand is high but the number of boxes is limited by geometry. Where demand is modest, single boxes spaced up the joint are cheaper and grout more reliably.
Why did my loop fail to unfold after de-moulding?
Almost always because grout or laitance got inside the box during the pour — the protective tape was damaged, not fully sealed, or the box was vibrated directly. Fix the process: seal the tape, fix the box rigidly to the formwork (nails on timber; magnets or adhesive on steel formwork with a clean, degreased face), and never put the poker on the box. Never hammer a fouled loop straight; a kinked or nicked wire has lost capacity.
What quality checks should the supplier document?
Ask for the wire rope grade and minimum breaking load (high-strength rope of the EN 12385 family), galvanising of both box and rope, the pressed-sleeve material, dimensional inspection of loop length and box geometry, and tested joint resistances (tensile, longitudinal shear, transverse shear) tabulated against concrete/grout class. In the EU, an ETA or equivalent test report is what lets a designer actually use those numbers.
References
- EN 1992 (Eurocode 2) — Design of concrete structures, the framework behind the joint resistance and interaction check.
- Steel wire rope standards (EN 12385 family) — grade and minimum breaking load of the loop rope.
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