What is tunnel formwork?
Tunnel formwork is a steel system formwork shaped like an upside-down U, used to cast the load-bearing walls and the floor slab above them in a single pour. Two half-tunnels are pushed together to form the mould for a complete room or bay. Once the concrete has been placed and has gained strength, the formwork is rolled out sideways and set up in the next position. In the Netherlands this method is a core part of “gietbouw”, the cast-in-situ way of building homes.
How it works
- The slab of the floor below is ready, with starter bars for the walls.
- The half-tunnels are lifted into place by crane and aligned.
- Reinforcement, openings, ducts and cast-in sleeves are fitted.
- Walls and slab are poured in one go.
- The concrete hardens, often accelerated by heating the covered tunnels overnight.
- The tunnels are struck through the open facade side and moved on.
This creates a rhythm in which one or more bays are cast each day: the typical daily cycle of tunnel formwork.
Application
- Repetitive housing: terraced houses and apartment blocks with identical bay widths.
- Hotels, student housing and care buildings with many similar rooms.
- Projects where walls must carry load, block sound and resist fire at the same time.
Tunnel formwork is a poor fit for highly varied floor plans: every deviating dimension means adapting the formwork, which costs time.
Practical tips
- Design for the tunnel: keep bay widths and storey heights consistent. That decides whether the system pays off.
- Facades come later: the tunnel has to come out on one side, so that facade is closed afterwards with precast panels or timber frame elements.
- Strike on strength, not on the clock: the structural engineer or concrete technologist decides when the formwork can come out, often based on maturity measurements.
- Fix services early: cutting chases in cast walls afterwards is expensive and can weaken the structure.
Related terms
- Cast-in-situ construction
- Climbing formwork
- Slip formwork
- Column formwork
- Concrete maturity
- Cast-in socket
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