What is the initial rate of absorption (IRA) of bricks?
The initial rate of absorption (IRA), also called initial rate of water absorption or suction, is the amount of water a pre-dried masonry unit absorbs in the first minute of contact with water. It is expressed in kg/m²·min and applies to clay bricks, calcium silicate bricks and concrete blocks. It is tested to EN 772-11: the unit is stood bed face down a few millimetres deep in water and weighed after one minute. In the Netherlands bricks are grouped into classes IW1 (very low suction, below 0.5) to IW4 (high suction, above 4.0 kg/m²·min).
Why does IRA matter?
The suction of the unit controls how the mortar cures and bonds:
- High-suction bricks draw water out of the mortar too fast. The cement gets too little water, bond strength drops and fine cracks form along the joint faces, letting driving rain into the wall.
- Low-suction bricks absorb hardly any water. The mortar stays wet for a long time, the bricks “float” and joints can slump or smear.
Mortar manufacturers therefore use the IRA as the starting point for their mortar recommendation.
Practical tips
- Match the mortar to the brick. Ask the supplier for a suitable mortar; water-retentive mortars are available for high-suction bricks.
- Wet bricks only when advised. High-suction bricks may need wetting, low-suction bricks should not be wetted. Follow the brick and mortar manufacturer’s guidance.
- Watch the weather. Wind, sun and dry air amplify suction; cover fresh brickwork.
- Store bricks dry and covered, so suction stays consistent throughout the job.
Related terms
- Wetting bricks
- Capillarity
- Absorption water
- Mixing water
- Brick
- Efflorescence
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