Strength vs. Bonding for Masonry Components

Brief 

An excess of specified masonry strength may come at the cost of the ability to form a strong bond, John Chrysler writes. Chrysler explores how individual masonry materials work together to achieve the best combination of strength and bonding and avoid the danger of cracking and water intrusion.

Insight

How Much is Enough?

Structural engineers often get a greater level of comfort by specifying masonry component strengths higher than are needed.

However, this can be counterproductive since some masonry components, such as mortar, may sacrifice bond in lieu of strength, leading to cracking and water intrusion into structural masonry walls. Masonry grout may also be considered a form of concrete, which it is not.

A better approach is understanding how the individual materials work together based on code requirements supported by extensive research. The ultimate goal should be to specify masonry components that are economical with maximum structural integrity.

The Masonry System

Concrete block masonry is designed as a homogeneous system that assumes the compression components of masonry units, mortar, and grout are equal. Code design parameters are conservative, which takes into account that there will be some variation in material properties.

Joint reinforcement or structural deformed reinforcement, if present, are sole tension components and not subject to multiple material variations.

Based on the Unit Strength Table 2 of The Masonry Society’s TMS 602, Specification for Masonry Structures, and minimum strength requirement of 2,000 psi for concrete masonry units as required by ASTM C90, Standard Specification for Loadbearing Concrete Masonry Units, the natural design threshold for concrete block masonry is 2,000 psi.

When one looks at TMS 602, Table 2, the 2,000 psi strength verification is based on Type M or S mortar, but the strength verification is reduced to 1,750 psi when using Type N mortar. The natural instinct is that more strength yields better performance, but this is not always true.

Mortar with less cement content provides a better bond to the masonry unit, mitigating or eliminating moisture intrusion into the masonry system. Moisture intrusion can lead to walls that leak which is never desirable.