There was a time when you could get a Ford or a telephone in any color you wanted--as long as it was Black. Similarly, there
was a time when concrete repair materials were available in either Grey or Gray. At some point, for some applications, function alone becomes insufficient. Perhaps it is simply human nature that as we grow
accustomed to reliable, maturing technologies, we eventually turn our attentions toward attaining higher levels of aesthetics in the functional things which have become part of everyday life.
Over the past 15 years, prepackaged repair mortars which provide both functional performance and positive aesthetics have become increasingly available
and popular in North America and Europe. By closely matching the repair material to the color and texture of the host substrate, we can achieve repairs which do not noticeably alter the visual character of a
building. And having opened the door to an era when aesthetics may be becoming a basic component in a product's list of expected features, we find that increasingly better fulfillment of aesthetic objectives
In no specialty niche in the repair industry is it more evident that the aesthetic "bar" is constantly being raised than in the Historic Restoration/Preservation industry.
Frequently Historic Restoration involves buildings and monuments originally built to an exceptional level of aesthetic quality, and preserving those aesthetics is considered basic to maintaining the value of
these structures. The challenge of achieving repairs of aesthetic quality comparable to the original construction is even further heightened by the widespread acceptance of Portland cement-based repair material
technologies for repair of non-cementitious substrates, including both natural building stone and manufactured masonry, such as terra cotta.
Decorative masonry elements often display a level of artistry that is without parallel in concrete construction. The flow and detail in the shapes and colors just aren't found in
balcony and bridge deck repair. And in masonry repair applications, basic differences between the physical properties of cement-based mortars and masonry substrates compel consideration of compatibility
requirements which may be taken more or less for granted when repairing concrete substrates.
Terra cotta, for example, is a fired clay composite, consisting of a relatively soft, porous body or bisque, with a relatively hard and impermeable vitreous fireskin or glaze. It has a linear coefficient of thermal expansion only about half that of concrete and it may typically absorb up to two or three times as much water. Affix a Portland cement-based repair material to this composite substrate and there emerge some interesting performance differences to resolve if successful and durable aesthetic repair is to be realized.
How is success in aesthetic repair appraised? That's not easy to specify, as there is a highly subjective element in all of this. Furthermore, a job that "looks good" when
it is completed, or even a few years later, has only cleared the first hurdle to becoming a true success. In the world of aesthetic repair, and particularly in Historic Restoration, perspectives deal in decades
and centuries, not years. We may look back on a number of projects completed 10 or more years ago and conclude that they have achieved "interim success" status. But the bar has long since been raised
by clients who instantly expected increasingly greater aesthetic results that retain their aesthetic excellence for ever-increasing periods of time.
So how do
we achieve success in aesthetic repair? Almost paradoxically, the best aesthetic results are generally the unremarkable ones, the ones we hardly notice, the ones that don't look like a repair at all.
Why is one restoration such a disaster while another is such a success? It isn't just a matter of workmanship, or even just a matter of color. It is
to a great extent, a matter of material properties and selection.
A Performance Property Based Approach to Material Selection
The task of determining which combination of material properties is best suited to achieving the goals of aesthetic repair is formidable. Many conflicting properties compete for our
attention, and the perfect material which combines everything we would like to include simply cannot exist. Indeed, some properties are mutually exclusive. Emphasize one and we may compromise another.
There is little objective help to be had in wading through all of this either. There are no consensus standards guiding the aesthetic repair material selection process at present, and
none is immediately forthcoming.
Part of the problem is that each party to an aesthetic repair project has his own set of priorities and objectives. The Owner's concerns are cost, appearance and durability. The
Contractor is likely to focus on constructability issues--how the repairs will be achieved efficiently, effectively and profitably. Engineers tend to concentrate on mechanical performance properties.
Preservationists represent an entirely different set of concerns, based on their knowledge of disastrous past interventions which did irreversible harm to the irreplaceable structures they were trying to
protect. Consequently, reversibility and "breathability" (moisture vapor permeability) of repairs are preservation priorities.
So how do these differing points of view get resolved into a mutually agreeable course of action? How do we build a consensus?
One of the strategies suggested for resolving conflict is to begin by building on those things on which there can
be broad agreement. Perhaps the best way to find that common ground with regard to Aesthetic Repair is to set aside the issue of Aesthetics, for a moment, to focus first on the issue of Repair.
Fortunately, somebody has already been doing the consensus building on the Repair end of the equation. If we limit our technological domain to cement-based repair materials, ICRI
Guideline #03733 gives us a place to begin our material selection process.
The first step is to determine performance requirements, and the first question to ask is whether the repair is to be structural, or load-bearing. For most aesthetic repair situations we
can proceed from the assumption that we are dealing with nonstructural repairs, as it is rare that masonry elements will be effectively unloaded prior to repair and then structurally reloaded. Note that
while traffic bearing surfaces, such as stairway treads, may have requirements above and beyond those expected for vertical surface repairs (e.g., abrasion resistance), these, too, are still generally
Having made that assumption, we can straightforwardly produce a priority list of basic and special properties for a nonstructural facade repair.
High Tensile Bond Strength
Low Drying Shrinkage
Special Properties: Low Modulus of Elasticity
Similar Coefficient of Thermal Expansion
Positive Moisture Vapor Permeability
Tensile Bond Strength is given the highest priority, because if you don't have good adhesion, none of the other properties matter.
Low Drying Shrinkage is also basic, because higher shrinkage and the shrinkage cracking which tends to occur as a result, allow moisture to gain direct access to the adhesive interface,
which engenders disaster for bond strength over the long haul. Low drying shrinkage is one of several important factors which can reduce the likelihood of crack development.
Other volume changes occur as materials and substrates exposed to natural weathering go through wet/dry cycling or temperature cycling. If differences between the responses of the repair
material and substrate to these changes are too great, sufficient stress may develop between patch and host to induce cracking or distress in the weaker material. This is particularly disturbing when the
substrate is softer than the repair material, because the priority in historic preservation is to protect the original, valuable historic fabric, even at the expense of repair material failure, if necessary.
Now that we've addressed the basics of Repair, let's go back to the Aesthetic side of the equation. There are some special constructability issues which directly relate to the
requirement that colors, textures and complex shapes must be produced. Some repairs may best be accomplished using mold-making and casting techniques; others will require that fine hand work or sculpting be
performed. Materials have to facilitate the realization of a wide range of finishes using a variety of techniques.
As previously stated, we also have more complex substrate compatibility issues to deal with. Masonry substrates vary widely in composition, and they exhibit differences in coefficient of
thermal expansion, wet/dry volume change and even wet/dry appearance change.
Finally, there is the challenge of achieving close color matches and of retaining those colors for an extended period of time.
This brownstone row house (photo, below) in Albany, New York, exemplifies some of these special considerations. Approximately 75% of the scroll work has been
recreated using hand-carved custom brownstone patching compound, working from a set of original photographs.