Gypsum plasterboard, commonly known as drywall, is a widely used construction material in the United States and plays a crucial role in forensic fire investigations. Fire investigators often rely on fire patterns, which are visible or measurable physical changes to materials, to determine the origin and cause of a fire. Scientific studies with in-depth analysis of the physical and chemical processes of gypsum calcination are limited despite the widespread use of gypsum plasterboards and their significance in forensic fire investigations. This project addresses these gaps through comprehensive experimental and numerical investigations to develop a reliable predictive tool for analyzing gypsum calcination and estimating the depth of calcination.
The research was conducted by integrating laboratory experiments, material characterization, and computational modeling. Commercially available gypsum boards were characterized using Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), and Fourier Transform Infrared (FTIR) Spectroscopy. These analyses helped to understand the multi-stage endothermic dehydration process of gypsum at various heating rates (10°C/min to 100°C/min) and to develop an improved chemistry model that accounts for variable heating rates. The variable heating rate chemistry model was then incorporated into a one-dimensional (1D) unsteady computational model that solves for mass, species, momentum, and energy conservation. This was further expanded into a three-dimensional (3D) model capable of simulating non-uniform heat flux and resulting lateral heat and mass transfer. To validate the numerical predictions, controlled experiments were conducted using three different heat sources: a radiant burner (delivering uniform heat flux), a premixed burner (providing high heat flux), and a diffusion burner (simulating realistic, turbulent fire conditions). Heat fluxes ranged from 10 kW/m² to 100 kW/m². During these tests, thermocouples measured internal temperature profiles across the material, while depth probes tracked the progression of the calcination front. The numerical models were validated against the experimental data, showing strong agreement in temperature profiles and the propagation of the dehydration front.
The study revealed that the propagation of the dehydration front is highly non-linear. TGA data showed that mass loss rates depend significantly on heating rate, necessitating the development of Arrhenius-type reaction rate equations in the study. Both local and global sensitivity analyses showed that porosity and initial density are the dominant parameters influencing the depth of calcination. Experimental measurements and numerical predictions showed that higher heat fluxes speed up calcination. The velocity of the dehydration front's propagation is strongly non-linear even under uniform heat flux due to the complex interactions of heat transfer, endothermic reactions, and vapor transport within the porous medium. The study also highlighted that local irregularities, such as fiberglass reinforcement and voids within the gypsum core, introduce significant measurement uncertainty. 3D analysis indicated that gypsum calcination can generally be considered as a one-dimensional process, except in cases involving abrupt changes or drastic gradients in heat flux.
Mathematical correlations linking the depth of calcination to incident heat flux and exposure duration were developed and validated. These correlations were further verified by re-evaluating historical full-scale compartment fire tests and Fire Dynamics Simulator (FDS) predictions. To make findings from the study practical for fire investigators, two standalone applications were created. The "Depth of Calcination Predictor" allows investigators to input heat flux and duration and estimate the depth of calcination. The "Comprehensive Gypsum Thermo-Chemistry Solver" provides visualizations of transient temperature and vapor density profiles inside the gypsum board. This study offers fire investigators a more scientifically grounded method for interpreting calcination depth, enhancing the reliability and accuracy of determining fire origin and cause.
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