Many conservation of heritage masonry specialists claim that if masonry cleaning cannot be undertaken without strong justification it should not take place at all. A major objection is generally that the products of natural weathering of masonry provide a degree of protection, which would be lost if removed during cleaning operations - the masonry would then be exposed to modern-day, potentially more harmful pollution. Other worthy concerns are that the often distinctive weathering of masonry - particularly for some dimension stones - can often define the character of a heritage structure, the latter also being lost when an 'as-new' appearance is produced by cleaning operations.
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However, I strongly believe there are occasions when cleaning is justified, and the most important consideration then surrounds the selection of a system which can effectively do the job without damaging the underlying fabric. The following are excellent examples of when the decision to clean was justified using equipment and techniques designed for use on heritage masonry.

​WHAT WAS THE PROBLEM?

The photographs on the left illustrate one of several carved natural stone wall plaques before and after cleaning. The masonry on an old school property was undergoing conservation work prior to it being re-purposed as an office complex. As illustrated by the “Before” photograph, much of the masonry had been covered with decades of atmospheric soiling and sandstone was being lost due to its reduced ability to naturally “breathe” after rainfall.
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WHAT WAS THE SOLUTION?

The solution was to remove the heavier deposits of soiling using a cleaning system that introduced fine particles of crushed glass into a stream of water directed at the substrate under very low pressure (<50-psi). As illustrated by the “After” photograph, the system was able to reveal most of the otherwise lost detail - without producing an “as-new” condition - nor did it cause damage to the vulnerable underlying sandstone.

The photographs on the right illustrates a similar issue - this time on a more than one-hundred year-old art gallery that was undergoing major restoration work. During the pre-conservation investigations, it had been observed that, in some areas, heavy soiling was causing damage to the natural stone masonry units.

WHAT WAS THE PROBLEM?
Similar to the previous project, the damage, in the form of scaling and flaking, was the result of the soiling reducing the stone's natural ability to transmit moisture vapour to the exterior. At the time of year when a rapid fall in temperature followed a mild wet day, the inability to dry quickly caused the stone to suffer the effects of freezing.
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WHAT WAS THE SOLUTION?
Therefore, part of the conservation strategy included cleaning to remove the heavy soiling and thereby improve the masonry's ability to dry rapidly. Trials were specified to be carried out in advance of cleaning to ensure that the selected cleaning method could achieve the objective without causing damage to the underlying substrate. The successful trials resulted in approval being given for the contractor to use a low-pressure cleaning system utilizing glass beads and water.
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The selected cleaning method proved to be very effective and the heavy soiling was efficiently removed without causing damage to the masonry.

​The Owner’s representative for a heritage brick masonry building wanted to repair the cracks highlighted within the photograph on the left - without a prior condition assessment. Retained as the consultant for the project, I observed that the cracks resembled those caused by differential movement within the natural stone masonry foundations. I also observed that previous attempts at repointing cracked mortared joints had failed.
A review of the damp and deteriorated conditions within the basement confirmed to me that the below grade walls could not have been waterproofed. There was also evidence of debris that could only have originated from the inner core rubble of the traditional mass masonry foundation walls. I therefore concluded that the cause of the problem was the destabilization of the foundations due to water leaching the lime binder from the mortar/grout that had been used during their construction. (Illustrated by the Figure) I also concluded that it would be unwise to repair the cracked brick masonry - without first addressing the problem that caused the cracks. (I also did not charge for the ad hoc condition assessment!)

​Stainless steel helical masonry ties were installed from within the basement to provide the required restraint during grouting, to stabilize the walls during excavation work, and to provide a better composite action across the masonry assembly. The foundation walls were then exposed and waterproofed. The voids within the inner core rubble were then grouted using very low-pressure techniques to inject a lightweight cellular foamed cement grout. Finally the excavations were backfilled and cracked masonry joints were cut out and repointed using a hydraulic lime heritage repointing mortar.

​This project is  one of several case studies discussed within a chapter devoted to “Addressing the Cause - As Well as the Result” within
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MASONRY PROBLEMS
Investigation, Identification & Restoration
Canadian Project Case Studies

The full technical reason for this phenomenon - often referred to as the ring or halo effect - is too involved to explain briefly within this blog. However, I’ve always found the Figure I have included below as very helpful in understanding what can happen.
Often the approach to restoration is to simply remove visibly deteriorated concrete while ignoring the fact that corrosion will continue within the surrounding concrete. However, the repaired areas will change from anodic to cathodic sites, thereby increasing the remaining cathode to anode ratio - this will increase the rate of corrosion of the remaining anodic sites. (as illustrated by the Figure) For this reason, I believe that the use of corrosion evaluation testing methods in conjunction with a delamination survey can be essential.

This and other restoration issue are included in
INVESTIGATING CONCRETE PROBLEMS
Learning from Those Who Learned the Hard Way!