The Haller number is a measure of the water absorption (suction) of a completely dry masonry unit, expressed as the amount of water the unit takes up in a set period of time. It indicates how “thirsty” a brick, sand-lime block, or concrete block is, and is therefore important for the workability of masonry mortar.
Why does the Haller number matter?
The water absorption of a unit determines how the mortar bonds to it. When laying masonry, this plays a central role:
- Suction too high — a dry, strongly absorbing brick draws the mixing water out of the mortar too quickly. The mortar “burns”, cures insufficiently, and bonds poorly. Such bricks are often pre-wetted.
- Suction too low — a saturated or dense unit takes up no water, so the mortar stays wet too long and the masonry can “float” (sink).
- Optimum suction — moderate water absorption gives a good, durable bond between unit and joint.
Applications
The Haller number is used to match masonry components properly:
- Choice of unit and mortar type — the supplier matches the mortar to the suction of the chosen brick
- Pre-wetting — strongly absorbing bricks are dampened before use, especially in warm or dry weather
- Preventing efflorescence — a poor match can lead to white salt deposits and weak joints
The Haller number is determined in the laboratory by placing a dry brick in contact with water for a set time and measuring the difference in weight.
Related terms
- Brick
- Masonry
- Mortar
- Sand-lime brick
- Mixing water
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