Post-Fire Nondestructive Evaluation of a Prestressed Concrete Double-Tee Joist Roof

Abstract

Prestressed concrete exhibits resistance to damage induced by fire and associated elevated temperatures, often permitting reuse of structural elements following exposure to accidental fire. However, prudent determination of suitability for reuse or rehabilitation of fire-damaged prestressed concrete requires a post-fire structural evaluation, which can be greatly aided by application of nondestructive testing for quantitative assessment of residual material properties, structural strength, and serviceability. In this study, a suite of nondestructive inspection and evaluation methods including visual documentation, camber surveying, penetration resistance testing, and impact-echo are applied to fire-damaged and unaffected double-tee joists of a precast, prestressed roof system. Qualitative and quantitative measures of fire damage obtained through the nondestructive tests are presented and strong correlations, in particular between longitudinal wave speed estimates obtained in impact-echo testing and depth of probe penetration, are exhibited in the test results. Furthermore, the test results are compared with empirical models of residual compressive strength and residual elastic modulus of concrete following exposure to high temperatures. These results support recent laboratory studies suggesting that nondestructive measurements of longitudinal wave speed or pulse velocity through fire-damaged concrete may be linearly related to relative reductions in compressive strength. Results from a survey of the camber in the fire damaged joists is also presented with conceivable interpretation of the observed profiles to highlight the shortcomings of this often recommended practice and emphasize the need for more definitive nondestructive test methods.