Masonry Magazine April 1977 Page. 11
Technical Notes
on Brick Construction
Brick Institute of America 1750 Old Meadow Road, Mclean, Virginia 22101
INTRODUCTION
SALVAGED BRICK
Selecting a building material requires considering three factors: esthetics, engineering, and economics. Salvaged brick are occasionally selected, sometimes for their "rugged appearance", sometimes for their low initial cost. Rare is the case when salvaged brick are chosen for their engineering properties, however; for in general they produce weaker, less durable masonry than new brick.
FIG. 1
When existing walls are demolished, hard-burned brick and salmons are hopelessly mixed. It is virtually impossible to distinguish between durable and non-durable units.
MANUFACTURING METHODS
Most salvaged brick are obtained from demolished buildings which had stood 40 to 50 yr, or more. In fact, it may be next to impossible to salvage brick from modern structures. Recently, a wrecking contractor said:
"Brick laid in the last 40 years are useless to us. We can't salvage brick set in (portland) cement (mortar). It's too hard. The only thing we can do is to [sic] knock such walls down with a pear or dynamite them."
Fifty years ago, manufacturing methods were markedly different from those of today. De-aired brick were unknown; coal-and wood-fired periodic and scove kilns were commonplace. The modern gas-fired tunnel kilns with accurate temperature controls throughout were also unknown. Manufacturing conditions then generally were such that large volumes of ware were fired under greater kiln-temperature variations than could be tolerated today. These conditions resulted in a wide variance in finished products. Brick from the high-temperature zones were hard-burned, high-strength, durable products; those from low-temperature zones were under-burned, low-strength products of low durability. These temperature variations also resulted in a wide range in absorption and color. The under-burned brick were more porous, slightly larger, and lighter colored than the harder-burned brick. Their usual pinkish-orange color resulted in the name salmon brick.
During these bygone years, prevalent methods of construction made production of both hard-burned and salmon brick economically feasible. Most buildings had loadbearing brick walls which were 12 in. or more in thickness. The hardest most durable units were used in exterior wythes; the salmons (and others) were used as backup. Much sorting and grading of brick was performed at the construction site by the mason, although the brick manufacturers eventually assumed this responsibility.
The advent of skeleton frames marked the beginning of high-rise construction and the gradual demise of thick loadbearing masonry. Architects and engineers began to design non-loadbearing walls, and gradually decreased wall thicknesses. This evolution in construction procedures necessitated a change in brick manufacturing procedures. Slowly but surely, the demand for salmon brick dwindled. And, after the use of hollow backup units became prevalent, the need for salmon brick became practically nil. At the same time, having invented the thinner, lighter weight panel wall, designers focused their attention on wall strength which they equated to compressive strength in individual brick.
Because the principal demand was for high compressive strength and durability, manufacturers had to produce a high proportion of well-burned brick. To do so necessitated a change in manufacturing methods. Thus, an evolution in construction procedures wrought a significant and beneficial evolution in the production of brick.
Danny Goodman, Ace Wrecking Co., quoted by George Kennedy in "The Rambler", The Evening Star, Washington, D. C., May 18, 1960,
It is the nature of ceramic products to shrink during firing. Generally, for a given raw clay, the greater the firing temperature, the greater the shrinkage and the darker the color.
Despite the reduction in its use, loadbearing masonry remains a very economical method for constructing low-rise buildings.
For a synopsis of present day manufacturing methods, see Technical Notes No. 9. "Manufacturing Structural Clay Products."