The Life of Mortar
Column Mailbox mortars should be assessed yearly so that water doesn't permeate the bricks or stones the mortar encases. Mortar's longevity is based on several key factors:
1. How well mortar is matched to the porosity of the stone or brick.
2. Whether drainage channels have been created along seams to prevent water from pooling.
3. Whether regular point-and-tuck maintenance is performed using the proper mortar.
4. That permeability is assessed prior to applying sealants.
Brick & Stone Porosity
When we speak about a brick or stone's porosity, we are referring to the amount of moisture they absorb from the air. High porosity bricks or stone absorb the greatest amount of moisture causing them to swell and contract during the absorption and release cycle.
Experienced masons know how to determine the porosity of brick or stone and can couple them with appropriate mortars.This is extremely important because:
If the mortar is too unyeilding, bricks and stone will crack and crumble because they cannot expand and contract.
The caveat to this situation is that few homeowners have the necessary tools to determine the age and/or porosity of the brick or stone used on their walls, chimneys, or column mailboxes. The image above shows brick that appears to be vintage, but is actually modern. (Photo by Hanson Brick - *Las Cruces *Mineral Wells *Western King, 2014 Catalog). One advantage of using modern bricks is that they are completely compatible with today's unyielding mortars.
Correctly Channeled Mortar Joints
If mortar channels are left flush with brick or stone, OR, if they portrude beyond their face, water will quickly pool at those junctures and wear into both the mortar and the stone. Decay and deterioration soon follow.
Mortar should be indented approximatly 1/4 to 1/2 inch deep, across the entire length of a row and between each stone or brick. These smoothly-rounded pathways help provide suitable channeling for water drainage.
Point & Tuck Mortar Maintenance
Experienced masons not only have the ability to measure the porosity of brick and stone, most are also schooled in the ratios of lime and sand mixtures that match these varying porosities.
It is important to note that brick porosity increases with each preceeding decade. Bricks made between 1950 and 1980 are far more porous than modern brick, and bricks made prior to 1950 are the most porous of all.
When mortars are mixed with too much cement and not enough lime and sand, they absorb moisture at a different ratio than the surrounding brick or stone. At that point, either the mortar or the brick (or stone) can crack, warp, crumble, or deface.
The image above shows a stone wall point-and-tuck repair using modern mortar against permeable rock. Within a year the wall warped. The chore of chiseling away at this mortar may damage the face of the stones, because modern mortar is extremely difficult to remove from porous stone.
The Controversy of Sealants
Although sealants and repellants differ, they have one thing in common; both prevent moisture from exiting stones and brick. For that reason, The Technical Preservation Services at the Department of the Interior suggests that neither be applied to porous bricks. (Article here).
Simply put, moisture that cannot escape pools internally, and eventually wears at the sealant (or repellant), causing it to disengage from the surface, carrying the face of the brick or stone away as it sloughs off.
Modern brick do not seem to be affected by this issue because their porosity is extremely low. Nonetheless, when considering whether to apply a sealant, it is best to wait until weather paterns have emitted less than 2% humidy for more than a week, to ensure that no moisture remains within.
An Overview of Mortar Considerations
In an effort to help our patrons understand the challenges of brick and stone repair, we wrote an extensive treatise on the subject at Plano Brick & Stone Repair, (link).
This webpage not only describes in greater detail the dangers of mis-matching mortars, but also addresses the varying properties of both brick and mortar that professionals consider prior to beginning construction or repair. These include:
1) The Coefficient of Expansion:
How much does a brick (or stone) move as a result of temperature change?
2) Compressive Strength:
The ability of mortar to withstand a compressive load.
The ability of brick to deform under pressure without failing.
The amount of moisture that will enter a brick, causing it to expand
5) ) Bond Strength:
The ability of mortar to adhere to the surrounding brick (or stone).
6) Modulus of Elasticity:
The ability of mortar to accomodate minor brick movement, including expanding and contracting, without cracking or crushing the brick or stone.
7) Tensile Strength:
Mortar's capacity to witstand tensile pulling against it.
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