What is a reinforcement continuity box?
A reinforcement continuity box (also called a continuity strip or rebend box) is a steel casing containing pre-bent reinforcing bars, fixed against the formwork so that no bars need to pass through it. After the concrete has hardened and the formwork is struck, the lid is removed and the bars are straightened out, ready to lap with the reinforcement of the next pour. The Dutch call it a stekkenbak, literally a “starter bar box”.
They are used wherever a wall is cast first and a slab, stair landing or balcony is connected to it later. Without one, the formwork would need a hole for every bar.
How it works
The unit is made off site from galvanised steel sheet with the bars already bent inside. On site it is nailed to the formwork or wired to the reinforcement. The casing stays in the concrete and its profiled face forms a key for the next pour. Only smaller bar diameters are suitable, because bending a bar and straightening it again stresses the steel. Manufacturers’ literature in the UK has given 16 mm as a practical upper limit; in Germany the DBV guidance allows up to 14 mm. Always use the limits in the product’s own approval.
Application
- Let the engineer specify the system. Bar size, spacing and projection length follow from the design and the product approval.
- Fix it firmly and accurately. A box that shifts during the pour puts bars in the wrong place.
- Set the level carefully. The slab that follows must line up exactly with the bars.
- Straighten with the proper tool. Use the supplier’s bending tube and bend in one smooth movement.
- Clean the joint. Remove lid and debris before casting the next element.
Common mistake: hammering the bars straight or bending them back and forth. The steel can crack at the bend, and the connection is no longer as strong as designed.
Related terms
- Starter bar
- Reinforcement
- Reinforcing steel
- Formwork
- Striking formwork
- Construction joint
Want to know more about building terms? Visit our knowledge base at fredsdiyplans.com.