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Concrete masonry (except that containing combustible material) is classed as a non-combustible and has a Class 0 spread of flame rating. Fire resistance periods of up to 6 hours can be obtained with relatively thin walls. The exact time will depend on the block type, loading and finish applied. Guidance on the period of fire resistance can be found in BS 5628-3 Code of practice for the use of masonry - materials and components, design and workmanship
Joints and connections between elements should be detailed carefully to ensure the fire resistance provided by the masonry is not reduced.
Acknowledgement: Concrete Society