Masonry Magazine August 1988 Page. 29
Alternative Materials
Alternative materials are attractive relative to in-kind replacements due to lower initial expense. They include cut stone, precast concrete, polymer concrete, glass fibre reinforced concrete and fiberglass.
There are specific advantages and disadvantages to the use of each of these materials as replacements for terra cotta.
These vary with their technical characteristics, but in general a replacement material should only be considered if it is physically, functionally and visually compatible with the surrounding terra cotta units.
The strength of the substitute material should be the same or less than that of the terra cotta with which it is being integrated. If it is significantly stronger, it may point-load the surrounding terra cotta, causing sufficient pressure to crack and damage it. The substitute material must thermally expand and contract at a similar rate to the original for the same reason. It must also absorb water at the same rate to ensure that it will not create stress on surrounding units through swelling at a greater rate.
It is also essential that the chemical composition of the replacement material be compatible with the terra cotta. The presence of even minute quantities of some acids salts or alkaline substances in the substitute material may cause corrosion in the surrounding terra cotta.
The new material must be as durable as the old. It must wear evenly. Frequent replacements will add up to a very expensive proposition in the long run. It must be a close match visually in form, color and texture, and it must maintain that similarity over time.
In-Kind Replacements
Replacing failed terra cotta units with new ones is a straightforward process that begins with the preparation of materials by the consultant or architect from which manufacturers can bid. The manufacturer will require a visual representation (photograph or isometric sketch) of each unit that is to be replaced, with supporting dimensional information. A performance specification for the terra cotta must also be provided. Thes rovided. These are normally based on an assessment of the performance of the original materials and of the conditions prevailing at the project site.
The bid should specify the quantity of each piece desired. Multiple units will vill cost less, on a per unit basis, than individual pieces, as a great deal of the expense in the manufacturing process comes from the cost of creating a mold for each piece.
Several sample pieces, taken from different parts of the building, should be submitted as well. These will be used for color matching and to determine the compatibility of the clay body and glaze.
The manufacturer will use the information to develop shop drawings. He may submit suggested changes in design to the owner and architect if he feels that they are necessary to assure the successful manufacture of the units.
He will also prepare setting drawings, to demonstrate how new work will fit with the old, and prepare samples of the glaze and body material for the approval of the client and architect.
Installation, Repair and Preventive Maintenance
The architect, often in conjunction with a masonry consultant, will write installation specifications and solicit bids for the project from masonry contractors.
Anchoring materials will also be designed and selected at this time. Nonferrous materials must be used. Their size and shape will depend on the original design, the work to be installed and the state of the backup masonry.