Elastic Dynamic Young’s Modulus and Permeability of Concrete in Fire Damaged Structural Members

This article reviews the results of two studies examining the effects of exposure to elevated temperatures on the dynamic elastic (Young’s) modulus $\left(E_d\right)$ and air permeability index (API) of concrete. The relationship between $E_d$ and API in fire damaged concrete, and the use of $E_d$ and API to determine both the extent of damage due to exposure to fire and the presence and extent of damage gradients are reviewed. Cores were removed from a precast, prestressed concrete beam containing lightweight aggregate and from a reinforced concrete structural wall containing conventional mineral aggregates. Both of these structures were exposed to intense fires and the laboratory investigation of cores was part of a forensic engineering evaluation to determine the extent of damage to the structure and the necessary repairs prior to returning to service. The $E_d$ and API were determined using $25\mathrm{mm}$ thick disks sawn from the cores. The findings of the study suggest that $E_d$ and API are linearly related for a given set of raw materials until damage is significant and permeability increases nonlinearly when significant damage is sustained for both types of concrete that were evaluated. The findings also suggest that permeability of lightweight aggregate concrete increases more rapidly than conventional mineral aggregate concrete per unit change in Young’s modulus, under exposure to fire. Analyzing $E_d$ and API of concrete disks at $25\mathrm{mm}$ $\left(1\mathrm{in.}\right)$ depths provided insight into damage gradients and was found to be an effective tool in the assessment of damage gradients. API was found to be particularly sensitive to fire damage.