Engineering properties of historic brick: Variability considerations as a function of stationary versus nonstationary kiln types

Brick produced prior to the mid 20th century exhibit high levels of variability in appearance, geometry, and mechanical properties. Understanding historic brick variability is important for accurate performance prediction of existing structures and for the selection of appropriate replacement units. A lack of uniformity among histotic brick can be caused by molding and firing methods as well as the composition of their raw materials. This article focuses on one aspect of production-induced disparities: kiln type. Specifically, the distinctions between products from stationary and nonstationary kilns are compared through historic data and recent laboratory tests. Tremendous variance can be shown between both kiln types and even within a single firing for stationary kilns. Although kilns of past eras were constantly improved for energy efficiency, their products continued to be highly inconsistent in appearance and performance until the introduction of nonstationary kilns. This article presents the mechanics of heat distribution in kilns and demonstrates its direct impact on the variability of engineering properties due to inconsistent heat application. Laboratory results of pre-Civil War brick and modern brick are presented to illustrate the range of results and the level of variability for unit weight, compressive strength, and absorption.