What is a U-block?
A U-block, also called a U-shell or bond beam block, is a masonry unit with a U-shaped cross-section that acts as permanent formwork: you lay the blocks in the wall, place reinforcement in the hollow and fill it with concrete. The result is a reinforced beam that has the same material and finish on the outside as the rest of the wall. U-blocks are commonly made of lightweight clay, calcium silicate or concrete.
How it works
- The U-blocks are laid or glued in the wall with the opening facing up.
- Over an opening they are temporarily supported with props or a board.
- The reinforcement is placed in the hollow on spacers, so it is fully surrounded by concrete.
- The hollow is filled with concrete and compacted.
- Once cured, the reinforced core carries the load; the shell itself contributes very little.
Application
- Lintels over window and door openings, made on site instead of using a precast lintel.
- Ring beams on top of a wall, tying the wall together and supporting a floor or roof.
- Stiffening long or tall walls, for example garden walls.
Practical tips
- Reinforcement and concrete from the calculation: the number of bars, their diameter and the concrete cover depend on span and load. Have a structural engineer specify them.
- Leave the props in place: remove them only when the concrete has gained enough strength. In cold weather this takes longer.
- Thermal bridge: a concrete core in an external wall insulates worse than the surrounding masonry. Some U-blocks have an insulation insert on the outside; otherwise the insulation layer must compensate.
- Stay within one system: use U-blocks from the same system as the rest of the masonry so sizes and course heights line up.
Related terms
- Lintel
- Ring beam
- Formwork
- Reinforcement
- Calcium silicate brick
- Thermal bridge
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