Archive for category Cleaning
An Interview with Lauren McCroskey, U.S. Army Corps of Engineers – Revised UFGS Historic Masonry Specification
I had the unique privilege to interview one of the leading historic preservation experts, Lauren McCroskey, Program Manager, for the U.S. Army Corps of Engineers, at the Seattle District, on the recent revision to the UFGS for the Restoration and Cleaning of Masonry in Historic Structures.
USACE Official Announcement:
Technical Center of Expertise (TCX), Preservation of Historic Structures and Buildings Technology Update
As part of its mission to provide leadership in historic buildings technology, the TCX announces a major revision of its specification, “Restoration and Cleaning of Masonry in Historic Structures.” The spec now reflects state-of-the-industry guidance for the treatment of historic masonry and mortar, and surpasses existing preservation guidance provided by other federal agencies.
Property managers and cultural resource specialists are encouraged to use the spec in contract documents to ensure that masonry work is performed appropriately to prolong the life of historic buildings. See Unified Facilities Guide Specification 04 01 00.91
Speweik: What is the official title of the specification?
McCroskey: The title is the UNIFIED FACILITIES GUIDE SPECIFICATIONS
DIVISION 04 – MASONRY SECTION 04 01 00.91
RESTORATION AND CLEANING OF MASONRY IN HISTORIC STRUCTURES
Speweik: Who originally authored it?
McCroskey: The Corps was the preparing agent and performed the processing. The exact author(s) are unknown, though Corps staff would have prepared it.
Speweik: How long has it been in use/circulation?
McCroskey: It’s been available since 1991.
Speweik: Who is authorized to use it?
McCroskey: The Guide is to be used by the Military Departments (Army, Navy, Air Force, etc.), the Defense Agencies and the DoD Field Activities for planning, design, construction, sustainment, restoration and modernization of facilities, regardless of funding source. But anyone can use the guide to adapt to a particular masonry project.
Speweik: What government agency owns it?
McCroskey: The Architectural Discipline Working Group are the owners of the Section; Scott Wick is the Corps representative of that group.
Speweik: What is your position with the USACE and what specific responsibilities do you have regarding historic preservation?
McCroskey: I manage the Technical Center of Expertise for the Preservation of Historic Structures and Buildings, a program of nationwide service. The program provides technical assistance and preservation planning for Corps Districts, DoD, and other federal agencies to ensure facility and property managers apply the best practices to historic structures. We try to set the highest standards of preservation practice through quality project work, training, and by developing technical information.
Speweik: What prompted you to request an update to the Historic Masonry Division Section this past year?
McCroskey: For several years I’ve had an awareness that the Corps’ existing standards and guidance for the treatment of historic masonry has lagged behind newer developments and technological advancements for treating historic brick, stone, and mortar. I receive inquiries from Corps Districts and other agencies asking for specific guidance to address deteriorated stone or brick. Property managers rarely know how to approach these issues from a historic preservation perspective, and often have maintenance and field crews tackle masonry problems. While their intent is good, the methods, materials, and applications are often not appropriate for historic structures, and can lead to further harm and long-term costly repairs. That’s why it’s essential for us to be able to pass along the most appropriate, state-of-the-industry techniques and standards.
Speweik: What do you believe to be one of the most significant changes to the specification?
McCroskey: There are many improvements, but one of the most important is the depth of information, which is far more educational for the user than the old spec. There is much to be learned from this document. Another key improvement is that materials application is not just described, but preceded by a thoughtful examination of building and masonry conditions. There is extensive information about how to investigate existing conditions so that the best decisions can be made about materials, conditions, and methods.
Speweik: How do you see this change making a positive difference for the quality-level of Historic Preservation Projects in the United States?
McCroskey: I believe the TCX is obligated to provide the best guidance regarding the treatment of historic structures and buildings. By encouraging the use of this guide, the rehabilitation of historic masonry should be performed in a manner that is appropriate, efficient, and prolongs the life of historic materials.
Speweik: How do you envision the revised specification affecting the work you do at the USACE?
McCroskey: The spec will be the only guidance we provide to customers, or when advising others on the best standards for masonry. Since this spec now surpasses all other historic masonry guidance, we now consider this document the “gold standard.” Of course, there are sub categories of masonry, such as terra cotta and concrete, which may require other technical information. But where brick, stone, and mortar are concerned, this is our “go to” standard.
Speweik: Did you consider the possible additional costs to Historic Preservation Projects as a result of some of the changes? And, if so, do you believe the additional cost is a significant percentage of overall project costs?
McCroskey: When good preservation practices are used, the life and performance of historic materials is extended. When improper practices are applied, greater costs can be incurred when the wrong treatment or method causes damage that requires repair. Taking short cuts by using commercial products that are not suited to historic stone or brick, or using techniques that are not consistent with historic methods can cost more long-term, and rarely satisfy the Secretary of the Interior’s Standards for Rehabilitation and treatment of historic structures that all federal agencies must follow.
Speweik: How do architects, owners, consultants and contractors find out more about this important specification document?
McCroskey: The guide is now available on-line at the TCX web page: http://www.nws.usace.army.mil/BusinessWithUs/HistoricPreservation.aspx
For additional information or clarification regarding the spec’s application, your readers may contact me at:
Technical Center of Expertise
Preservation of Historic Structures & Buildings
U.S. Army Corps of Engineers
Most people don’t realize that lime particles are said to be some 500 times smaller than portland cement particles. This might explain, in part, why some lime mortar applications for repointing get so messy with mortar on the faces of the stonework – and yes, very difficult to remove especially the longer the lime mortar sits on the stone. The key in delivering a clean project, one that needs minimal cleaning (just with a small brush and water at the edges of the stone) is water content of the repointing mortar when being applied.
The consistency should be like that of stiff compacted brown sugar like you find in the kitchen cabinet. Yep, just like that. The feeling, the moisture content, and the compaction power. Mortar made to this consistency will not stick to the surfaces of the stonework and cannot be dragged along the top causing a stain from the hawk being pressed against the wall. You get a cleaner wall and the chances of shrinkage cracks are reduced as the mortar cures.
But moisture is important when repointing a wall. Bond strength is delivered during the application when a thoroughly soaked wall (with water) is allowed to partially dry-out and become surface-saturated-dry or (SSD). The SSD condition gives the dry mortar a bond potential with the advancement of each masons pressurized push against the material to the back of the joint. This is why it is so important that the masons repointing tool be sized to fit within the joint to allow for this compression.
I have seen a trend in recent years to rely on washing down the repointed walls with a light solution of an acid-based cleaner to remove the mortar stains from the stone surfaces. Problem is that the cleaner also cleans the mortar and dissolves the binder paste from the surfaces. While some in the industry call this aging the mortar, because it exposes the aggregate and gives the appearance of an old mortar joint. Well, you would get that appearance anyway if you waited 10 years as the lime paste naturally wears off the surface of the aggregate particles.
So, in fact, what you are doing in washing the wall down is giving the customer a used wall – a clean wall, but a wall that has been exposed to accelerated weathering is how I look at it. I figure you rip off at least 10-15 years of life cycle performance from the face of a mortar joint by washing it down with an acid-based cleaner. Seems the evidence is clear that the lime mortars do not withstand a cleaning as well as portland cement-based mortar mixtures. What makes things much worse is that lime mortar is very absorbent to water by its natural ability to transfer water in and out through evaporation which often causes the cleaners to penetrate deeper into the joint surface weakening the material even further.
The story I tell in my masonry seminars is a fun one to illustrate the point. It would be like selling someone a brand new set of tires for their truck and make them pay full price for them, but just as they are ready to drive away, you tell them, “Let me use your tires for say 20,000 miles first, then I will give them back to you” – essentially selling them used tires for the price of brand new ones. Don’t sell used lime mortar.