Impact of Moisture on Calcination Depth

Gypsum wallboard (GWB), also known as drywall, is commonly used in North American construction to line the interior walls and ceilings of structures. During a structural fire, as the GWB is heated, the paper facing decomposes (and potentially burns away) while the core material undergoes an endothermic reaction known as calcination. Calcination involves the release of chemically bound water from the crystalline structure of the core material, converting it to a softer, calcined state. Being a thermally-driven process, the calcination of GWB is an indicator of the heat exposure sustained by the material (NFPA 921). 

Measuring Calcination Depth on Fire-Affected GWB

During fire investigations, two identifiable characteristics of fire damage patterns on GWB are discoloration and mass loss fire effects. Discoloration of GWB is observable to the naked eye and areas of demarcation may be identified visually; examples include charring of the paper facing and color change in the core material. Mass loss occurs as the core material becomes calcined, but mass loss is not measured directly as this would require destructive (and impractical) measurements. Instead, a calcination depth instrument is used to measure the depth to which a small probe will penetrate into the calcined material. The probe is expected to penetrate further into GWB that has undergone a greater amount of calcination (and thus experienced greater mass loss). Calcination depth measurements can help to identify GWB that has experienced longer or more intense heating. A calcination depth survey may be used to identify areas of demarcation on fire-affected GWB that may not otherwise be identified visually. These measurements may aid a fire investigator in establishing areas of greater or lesser fire exposure.

Investigating the Impact of Moisture on the Utility of Calcination Depth Surveys

Calcination depth measurements are known to be affected by parameters such as the ambient temperature, the type of GWB and paint or coating, and the moisture absorbed into the material due to fire protection (e.g. sprinklers), suppression, and humidity (e.g., atmospheric or standing water). Although moisture exposure is known to affect the softness of the material, and thus affect individual measurements of calcination depth, the degree to which moisture affects the utility of calcination depth surveys as a fire investigation tool is not well understood.

For calcination depth surveys to be considered a viable tool to support fire investigators, it is necessary to understand the degree to which moisture exposures impact the reliability and accuracy of inferences supported by those measurements (e.g., establishing areas of greater or lesser fire exposure). Additionally, there is a need to better understand how the calcination depth instrument design (e.g., probe shape, application force, precision, and repeatability) and measurement methodology (e.g., procedure, operator bias, outliers, and data recording) affect the accuracy and reliability of measurements. 

Researching the Impact of Moisture on Calcination

In this study, the Fire Safety Research Institute (FSRI), a part of UL Research Institutes, will conduct experiments in both laboratory and large-scale settings to better understand the impact of moisture on fire damage patterns derived from calcination depth surveys.  FSRI aims to provide practical guidance to the fire investigation community on the utility of calcination depth surveys in cases where moisture exposure is present. Experiments will focus on several objectives:

• Investigate the relationships between fire exposures, mass loss, depth of calcination, and moisture exposures. 
• Investigate the impact of moisture exposures on the reliability and accuracy of inferences made from depth of calcination measurements.
• Develop guidance on the impact of moisture on the utility of calcination depth surveys.