Constructing Historic Masonry Test Walls
My name is Elle Farias and I am a Materials Research Associate at the National Center for Preservation Technology and Training. Together with my project partner, Silvia Lob, we are constructing historic masonry test walls to determine the effectiveness of commercially available anti-efflorescence coatings.
We are sharing our process in hopes this will be a tool for anyone looking to create historic masonry test walls for conducting research.
Preparing the Materials
Reclaimed Chicago historic bricks were purchased for construction of the walls due to their low-fire high-porosity composition which encourages development of efflorescence when subjected to the right conditions.
The bricks were first sorted, so as to use intact, whole bricks that were free of deformations. Then the bricks were physically cleaned of old mortar fragments, however, some tar remnants remained on the bricks. The bricks were then soaked in a water bath for 24 hours to leach out any potential existing salts in the bricks. After a drying period of 48 hours the bricks were ready to be used in construction.
Type O mortar was selected to construct the walls, for its high lime and sand content because it mimics a softer historical mortar and allows for greater moisture penetration into the walls. It is a medium low strength mortar, ideal for situations where the compressive strength and exposures are relatively low. According to ASMT C270, it requires a 9 parts sand, 2 parts lime, and 1 part cement ratio.
When mixing the mortar avoid using damp or wet sand which could lead to ‘balling’ of the concrete and lead to poor dispersion in the mortar mix. The cement was sifted to remove large pieces from the mortar mix. Water should be added only after the dry mix ingredients have been evenly dispersed. A good way to check that the water ratio is correct is to place a thin layer of mortar on the trowel and turn it over, the mortar should not fall away easily.
The walls were constructed in plastic tubs in order to be able to subject the walls to simulated groundwater flooding conditions and capture runoff water from simulated driving rain cycles. The walls were built as a double wythe running bond walls with 6th course headers and 3/8th inch joint size. However, there were small deviations in wall construction and variations in joint size. As the walls are being constructed, it is a good idea to pause occasionally to make sure the walls are level, plumb, and straight.
Loxon XP is a waterproof latex masonry coating that will create a moisture barrier. For our purposes, this coating was applied to the back surface, top surface, and sides of the walls. This effectively isolated the drying front to one test wall surface, establishing the primary test surface.
According to The Secretary of Interior Standards for the Treatment of Historic Properties and Guidelines for Preserving, Rehabilitating, Restoring, and Reconstructing Historic Buildings, it is recommended to remove the existing masonry to a depth of three times the size of the joint when preparing to repoint a wall.
The new mortar should match the old mortar in color, texture, and composition. A deviation was made from the recommendations in this respect. For our purposes, we were aiming to imitate existing conditions on some existing historical building stock. Often times in the past, historic brick masonry walls constructed with lime based mortars were repointed with a harder Portland cement based mortar. In attempting to replicate this problematic condition we repointed with a pre-mixed type N mortar, QUIKRETE® Mortar Mix (No. 1102). This mortar is readily available in premixed form and is a medium strength mortar which is ideal for general use. It is especially useful in projects like exterior masonry walls when exposure is a concern. According to ASTM C 270 type N mortar is comprised of 1 part Portland cement, 1 part hydrated lime, and 6 parts loose aggregate.
Before repointing the wall surface the joints should be brushed to avoid loose dust from causing the new mortar to adhere poorly.
The joints should also be moistened to let the new mortar adhere to the old mortar.
Finally, using a pointing trowel and hawk, new mortar should be applied to the joints in thin layers, filling in the deepest parts of the joint first, building up to create even layers that fill the joint. Avoid putting any mortar on front of the brick.
After curing, the walls are now ready to begin the study.