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B7 / P7 Water Absorption

Water Absorption is used for classification of clay bricks in BS3921: 1985 and is useful when products are used externally as facing materials. 

·         Engineering A class requires a compressive strength greater than 70 N/mm2 and water absorption less than 4.5%.

·         Engineering B class requires a compressive strength greater than 50 N/mm2 and water absorption less than 7.0%.

·         Damp-proof course 1 class requires a compressive strength greater than 5 N/mm2 and water absorption less than 4.5%.

·         Damp-proof course 2 class requires a compressive strength greater than 5 N/mm2 and water absorption less than 7.0%.

All others require a compressive strength greater than 5 N/mm2 but no specification on water absorption.

The main test to ensure product conformity testing is described in BS3921: 1985 as the 5-hour boil method.  Which consists of drying the product to constant mass, weighing dry, boiling in water for approximately 5 hours and allowing to cool for a further 16 to 19 hours before re-weighing wet. The results are expressed as a percentage.

Other methods commonly used in quality control are 24hr soak at room temperature and 7 day soak at room temperature which give slightly different results that are comparable.

There is a loose link between water absorption and durability and for specific clay bodies the water absorption value give a good indication to whether the product will be frost resistant.  Typically an extruded brick or paver made out of etruria marl with a water absorption of less than 7% tend to be very frost resistant, a soft mud product made from a similar body may be frost resistant with a water absorption of 12%. It would be extremely unlikely for an extruded product made out of etruria marl to be durable at a water absorption of 12% without the use of body additives.

 

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Last modified: October 05, 2000