I have heard that if you use brick with a very low IRA in masonry walls, they will float on the mortar during construction, which results in later water leakage problems.

Is this a common problem? If so, which IRA brick should not be used in masonry walls?

Masonry walls built using brick units with a low Initial Rate of Absorption (IRA) often have lower bond strength than walls built with moderate IRA units because very little water is available to be absorbed into the unit during installation into the wall. absorption There is little mechanical bonding without water absorption, which often results in lower bond strength. Because little water is absorbed, little paste is absorbed, which limits chemical bonding.

Using mortars with higher cement content often significantly increases bond strength. However, as the cement content of mortar goes up, there is an increase in the rate of mortar shrinkage. High shrinkage can result in cracking of the mortar, which increases water penetration.

When it pertains to low IRA brick, if a particular combination of mortar and brick has relatively low bond strength, it does not necessarily mean that the rate of water penetration is correspondingly increased. In fact, some studies suggest that low IRA brick may actually out perform brick of a mid-range IRA in resisting water penetration.

In research performed by J. Gregg Borchelt and J. A. Tann, which is summarized in a paper published in the proceedings for the Seventh North American Masonry Conference, walls built with low IRA brick units had lower water penetration rates than those constructed with moderate IRA values. In this study, walls built with Type O mortar and brick units with IRA values of 1 had less water penetration than walls built with Type N mortar and brick with IRA values of 15. The bond strength in this study, however, was measurably higher using Type S mortar with the low IRA brick units than it was with Type N and O mortars with low IRA brick units.

In projects where high bond strength is required, I recommend using a Type S portland cement/lime mortar with low IRA bricks. Where bond strength is not important, Type N mortar could be used to reduce the potential for shrinkage cracking. Type M mortar; however, should not be used in walls above grade since this mortar typically has a very high shrinkage.