Testing masonry materials for durability and performance has been going on for some time. It is important to study and test materials prior to construction of an actual wall to prevent wasted material, time and labor. Some of the earliest recorded tests performed on masonry mortar ingredients were carried out by the U.S. Army Corps of Engineers in the construction of fortifications during the early part of the 19th century.
The durability of masonry buildings relied heavily upon the past performance of the actual structures and the master mason’s experience with the individual materials. Much of the heritage knowledge of making good mortar was passed down from generation to generation through the trades. Testing mortar ingredients historically involved the masons working with the architects in a team approach for common understanding. However, signs of change began to appear as early as the 1890s.
Uriah Cummings writes in his book, “American Cements,” which was first published in 1898. “With their former teaching and experience on the one hand, and the testing machine on the other, the question was not long in doubt. The machine was victorious, and henceforth all judgment founded on experience was laid aside and they became blind believers in the tensile strain tests. What matter though they were continually befogged by the frequent, unreasonable, and capricious pranks of the machine, they had found a god, and were determined to worship it. And so it came to be established as a fixed belief among engineers and architects that the best cement was the one which tested the highest, and the manufacturer had no alternative but to strive to make his product test as high as possible.”
Seems from the tone of Mr. Cummings writing that he knew the industry was going in the wrong direction toward high compression. Is it high compression that destroys historic masonry? Well indirectly it does. Most mortars that are very high in compressive strength are very low in vapor permeability. The ability a mortar has to capture and release water easily through evaporation. What tends to happen in a historic masonry wall is moisture infiltrates by various ways; rising damp, poor roof/parapet/flashing details; driving rain; capillary action through cracks among other ways. The water needs to escape from inside the walls through the mortar joints ideally keeping everything dry from the inside out. Hard, high strength mortar prevents water from escaping thus trapping it inside the wall potentially causing damage to the masonry units of brick and stone as well as terra cotta over the course of time. It’s always better to insist on a lower compressive strength lime mortar that readily breathes with the masonry allowing quick evaporation of water, and in addition, provides the natural flexibility needed for traditional load-bearing masonry walls to perform at their best.