A hop-dobber is a special, adapted tremie pipe with a float, used to place underwater concrete accurately. The device ensures that the underwater floor slab is placed at the right level and with the right composition.
Why underwater concrete?
In deep excavations that fill with groundwater, a layer of underwater concrete is often placed first. That layer seals the bottom and resists the upward water pressure, so the pit can then be pumped dry (dewatering) and the final floor built. The concrete is placed under water, which makes accurate work difficult.
How does it work?
- The hop-dobber works on the balance between the water column and the concrete column in the pipe
- A float lets the discharge opening rest in the concrete already placed, so no water reaches the fresh concrete and it is not washed out
- The pipe “floats up like a float” as the concrete rises; graduation marks help measure the level
- A rolling bridge moves the device for even distribution
The hop-dobber is named after the pour-float used since 1980 by engineer G.P. Hop.
Related terms
- Excavation pit
- Concrete mortar
- Fresh concrete
- Concrete pump
- Dewatering
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