ENGINEERING PROPERTIES OF HISTORIC BRICK: VARIABILITY CONSIDERATIONS AS A FUNCTION OF STATIONARY VERSUS NONSTATIONARY KILN TYPES
DEBRA F. LAEFER, JUSTIN BOGGS, & NICOLE COOPER
Performance characteristics of an individual brick are fundamentally important both when attempting to predict the behavior of existing buildings and to select replacement units for such structures. For the modern design professional working with historic structures, performance variability in materials is a largely unfa-miliar concept. That unfamiliarity makes behavioral prediction extremely difficult, yet prediction is critical because of limitations and prohibitions on the extensive destructive testing that would be necessary to fully assess the masonry performance of an existing structure. For historic structures, variability in masonry performance tends to be extremely high compared to expectations for modern materials. Increasingly, the critical issue is picking input parameters to generate accurate computer modeling. By overpredicting capabilities, the risk to a structure from man-made or natural phenomena, especially ground movements such as tunneling, excavation, and earthquakes, may not be fully assessed; the resulting recommendation for no intervention can unnecessarily endanger the building. By underpredicting capabilities, limited resources may be unnecessarily expended to protect a structure that is competent to withstand potential ground movements without any intervention around the site or to the structures themselves.
Even without taking into consideration the additional complications of age-and weather-based deterioration, the prediction of masonry performance is highly complex. Masonry variability is a function of brick and mortar properties and the bonding between them. Even by considering only the historic brick (ignoring mortar-and bonding-related issues), extensive
differences can be seen in physical appearance (e.g., color, geometry, surface texture) and engineering properties (e.g., compressive strength, hardness, and absorption), and these differences can significantly impact the performance of masonry structures. Variability in historic brick is owing to the raw materials used as well as to the methods by which the brick were produced. Brick performance is a function of the clay type and processing as well as the molding, drying, and firing of the brick. This article concentrates on initial brick variability as a function of differences in firing technology. The firing process can profoundly influence the nature and performance of a masonry unit. To highlight performance differences as a function of the firing process, historic brick and brick replicated to match historic material are compared to modern material both through the presentation of historic data and through recent laboratory tests.