Architects and owners, preservation societies and landmark commissions have come to terms with the need to match not only brick but also mortar in doing restorations. They recognize that mortar joints can make up 20 to 30 percent of a masonry wall and that the color and texture of those joints are major factors in influencing the wall's overall appearance.
With a minimal impact on overall cost, colored mortar can be used to match the masonry units and the weathered mortar of the original or adjacent walls. The cement industry has capitalized on this trend toward the use of color and texture in building design by increasing both the range in color and the availability of custom colored cements for masonry for new buildings. This technology can be applied to restoration as well.
According to James L. Nicholos, technical manager Cements for Masonry at Lafarge North America in Atlanta, "A contractor must do a thorough review and have full understanding of the job specifications and the use of premixed colored cements incorporating inert, color fast iron oxide pigments.
Understanding the importance of sand color and gradation on finished joint color and proper detailing to prevent water related problems especially efflorescence with the walls is also critical."
If you haven't done much coloring of mortar to match existing joints, Nicholos suggests that, when ordering cement for colored mortar, a good rule of thumb is nine bags per yard of sand or 1,000 brick, or with concrete masonry units one bag per 35 block. Additionally, on high profile jobs, a pre-construction meeting with the specifier, owner, general contractor, testing agencies and material suppliers, along with erecting and obtaining approval of a sample panel, will greatly reduce the potential for misunderstandings as the job progresses.
He adds, "When using a mechanical paddle or spiral mortar mixer, batch at the rate of one bag of masonry cement to three cubic feet of damp, loose masons sand (use of a cubic foot box to measure sand will ensure batch-to-batch uniformity), and clean, potable water. First add two-thirds of the water followed by half of the sand, all the cement, then the remainder of the sand and sufficient water to produce the desired workability."
Continue mixing the mortar for five minutes after all materials have been charged. If a Portland cement and hydrated lime mix is used, mortar should initially be mixed four minutes after charging. The mixer drive should then be disengaged for two minutes to allow enough time for the hydrated lime to satisfy its initial need for water. After the wait, re-engage the mixer drive and mix for an additional two minutes, adding water to bring the mix back to the desired workability. The resultant mortar should exceed the property requirements of ASTM C270.
"Try to avoid re-tempering colored mortar by batching only enough to be used in a reasonable amount of time, as the addition of water can degrade the finished color of the joints," he cautions. "Also, it is important to be consistent in the time of tooling of the joint, as early tooling will result in a lighter shade, while those tooled late will be darker. The proper time is when the joints are thumbprint hard."
Care should be taken to minimize cleaning of the finished walls by utilizing techniques that reduce mortar droppings and smears on the face of the walls. Protection from mud and mortar droppings at the base of walls with straw or sand, brushing down as they are tooled, and turning up scaffold boards and covering the top of the wall at days end will reduce or eliminate the need for cleaning.
"When cleaning is required," Nicholos says, "it is important to use the products and techniques specified or recommended by the manufacture of the units and the cleaner. Masonry walls should be allowed to cure for a minimum of seven days before the use of acidic cleaners."
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