What is Initial Rate of Absorption (IRA)?

Initial Rate of Absorption (IRA) measures how quickly a brick absorbs water from mortar, expressed in kg/m²/min. It is tested by placing the bedding face of a dry brick in 3mm of water for 60 seconds and measuring the weight gain. The IRA determines how the brick interacts with mortar — too high draws water out too fast (poor bond), too low prevents suction adhesion. The ideal IRA range is 0.5-1.5 kg/m²/min.

What happens if brick suction is too high?

Bricks with very high suction (IRA above 2.0 kg/m²/min) draw water from the mortar too rapidly, preventing proper cement hydration at the brick-mortar interface. This results in: weak bond strength, dry and crumbly mortar joints, difficulty laying to line and level (mortar grabs too fast), and increased risk of rain penetration through poorly bonded joints. High-suction bricks should be wetted before laying.

Do I need to wet bricks before laying?

Bricks with an IRA above 1.5 kg/m²/min should be wetted by dipping in water or spraying with a hose, then allowed to surface-dry before laying. The aim is to reduce the IRA to the 0.5-1.5 range, not to saturate the brick. Never lay saturated bricks as they will slide on the mortar and not bond properly. Engineering bricks and low-absorption bricks (IRA below 0.5) should never be wetted.

How does suction affect mortar choice?

High-suction bricks need a wetter (more workable) mortar mix and faster working. Low-suction bricks (engineering bricks, some clay stocks) may need a stiffer mix to prevent sliding. Lime mortar is more forgiving of varying suction rates than pure cement mortar because it retains water better and has autogenous healing properties. The mortar designation should still follow BS EN 998-2 for structural requirements.

Mortar Suction Rates — IRA Testing Reference

Initial Rate of Absorption (IRA) targets for brickwork, testing methods, wetting requirements and effects of suction on bond strength per BS EN 772-11.

IRA Classification by Brick Type

Brick Type	Typical IRA (kg/m²/min)	Suction Level	Wetting Required?
Engineering Class A	0.1 – 0.3	Very low	No — never wet
Engineering Class B	0.2 – 0.5	Low	No
Flettons (London bricks)	1.5 – 3.0	High	Usually yes
Wirecut facing bricks	0.5 – 1.5	Medium	Check — usually no
Handmade / stock bricks	1.0 – 3.5	Medium to high	Often yes
Calcium silicate (sand-lime)	0.5 – 2.0	Medium	Check on site
Concrete bricks	0.5 – 1.5	Medium	Rarely

Optimal IRA Ranges

IRA Range (kg/m²/min)	Classification	Effect on Bond	Action
< 0.5	Low suction	Weak initial adhesion, sliding risk	Use stiffer mortar, lay fewer courses per lift
0.5 – 1.5	Ideal	Good bond, easy to work	No wetting needed
1.5 – 2.0	Moderately high	Mortar may dry too fast in hot weather	Consider wetting in warm/windy conditions
> 2.0	High suction	Poor bond, dry joints	Must wet bricks before laying