Re-inventing the Wheel

Back in the 70's I was working for a rather large masonry contractor - 150+/- masons and laborers. Over the course of three years we worked on numerous schools, prisons and other institutional buildings. These buildings were always solid masonry throughout.

Most of the government work back then was using Type "S" and some Type "M" mortars - both have very high percentages of cement and they were both pretty new on the scene.

Within a couple of years of completion of these schools, they were suffering from severe cracking in the masonry walls. It looked as if they had been through an earthquake!

Unfortunately, the system the architects developed was completely void of any real flexibility. The brick were highly ceramic - so brittle that it was nearly impossible to cut them with a hammer, so most cuts were sent to a table saw. These brick teamed up with Type-S mortar made for a system that was as hard and inflexible as tile.

If you've ever studied a very old masonry building in the city, you may notice that brick coursing may no longer be straight and brick surfaces may no longer be in a flat consistent plane, and yet the mortar seems to be in pretty fair shape - adhering to the brick. This phenomenon exemplifies the opposite extreme and the beauty of the older softer mortars. The lack of Portland cement and the presence of the lime made the softer mortars self-healing - a term known as "Autogeneous healing". As small separations developed in the system through settlement, the lime and calcium in the mortars would slowly re-fill the gap.

I suppose that in the case of the schools, architects thought they were designing a building that would be indestructible. But, the lesson learned was quickly confirmed in the next generation of schools we constructed as evidenced by a retreat to Type "N" mortar.

The main reason for the Type "S" and "M" mortars fall into two categories:

1. PSI Rating - the ability to bear weight
2. The ability to repel water.

The Type "S "and "M' mortars have increasingly high levels of Portland cement, which allows them to bear more weight and, when properly tooled, produce a mortar joint that is very dense and repels water well.

The Type "N" and "O" mortars, have less Portland cement and more lime and other plasticizers, which make them more trowel-able.

One of the main lessons learned was to not make the system any more dense and brittle than was necessary, and equally important, to match the mortar strength to the masonry unit strength.

Matching the materials became evident when masons began to ignorantly re-point old historic buildings with Type N mortar. The mortar was much stronger and denser than the original mortar, which actually tended to wick the moisture out of the brick and promote evaporation. The more cementitious mortar impeded evaporation and kept the brick damp, which in turn caused the bricks to freeze and expand. Whole faces of brick would slowly pop off (spall).

In Summary, the whole architectural experiment with mortar strengths taught the designers an age old lesson - "If it ain't broke, don't fix it."

Even in today's houses we see a great example of the flexibility of brick and type "N" mortar compared to a more brittle cementitious material. That is why we see, almost every day, cracks in the concrete block backing of a foundation wall and no cracks in the outer brick withe. There are six bricks to one block, which means there is a lot more mortar in the outer withe. The smaller units and greater percentage of mortar give the outer brick withe increased flexibility.