Sensitivity of fire resistance of 19th century fireproof flooring systems to thermal and mechanical properties of masonry

Masonry jack arches are commonly encountered in fire proof flooring systems constructed during the 19th century in Britain and central Europe. Masonry arches provide insulation to the steel girders and the load bearing elements. Because brick and mortar types/classes varied considerably in such historical construction, determining the thermal and mechanical properties of masonry at elevated temperatures is a difficult task. This paper presents the results of a research to resolve the difficulties to researchers when analysing such structures under fire conditions. Firstly, this paper will present collated experimental data reported in the literature for the variations of the thermal (thermal conductivity and specific heat) and mechanical (compressive strength and thermal expansion) properties of burnt clay masonry units and mortars at elevated temperatures. Lower and upper bound temperature dependent curves will be proposed to account for the variability of the material properties based on the collected data. This makes use of the boundary curves of the constituent materials according to expressions found in current design codes (for determination of the compressive strength), the rules of mechanics (applicable for calculating the thermal expansion) or the laws of physics (i.e. the additivity theorem for determining the specific heat of solid multi-component systems). Finally, the boundary curves are used in a series of sensitivity studies to examine the influences of these variations on the fire resistance time of the flooring systems. Results from this sensitivity study will show that even though the thermal and mechanical properties of masonry at elevated temperatures change considerably, with the exception of one case, the variability has little effect on the calculated fire resistance time. As a general rule, the elevated temperature properties of modern brick, given in literature for a similar density, can be used to approximately (but with sufficient accuracy) model masonry in elevated temperature analysis of 19th century jack-arch flooring systems.