Clay engineering bricks are defined in § 6.4.51 of British Standard BS ISO 6707-1;2014 (buildings & civil engineering works - vocabulary - general terms) as "fire-clay brick that has a dense and strong semi-vitreous body and which conforms to defined limits for water absorption and compressive strength"
Stronger and less porous engineering bricks (UK Class A) are usually blue due to the higher firing temperature whilst class B bricks are usually red. Class A bricks have a strength of 125 N/mm² and water absorption of less than 4.5%; Class B bricks have a strength greater than 75 N/mm² and water absorption of less than 7%.
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