Assessing Mechnical Properties and Microstructure of Fire-Damaged Engineered Cementitious Composites

Abstract

In the last few years, a number of investigations of engineered cementitious composites (ECC) have been carried out, and the mechanical behavior and durability characteristics of this type of composite are now increasingly better understood. The fire-resistant behavior of this specialized concrete, however, has not yet been studied as extensively. This investigation develops important data on the mechanical properties and microstructure of ECC exposed to temperatures up to 800degC (1472degF). In this study, the mechanical properties (the residual compressive strength, stress-strain curve, and stiffness) and mass loss were determined after air cooling, subsequent to temperature exposure. Changes in the microstructure, porosity, and pore size distribution of the firedeteriorated ECC specimens were identified using scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) techniques. Test results revealed no significant changes in the mechanical properties for tested specimens exposed to temperatures up to 400degC (752degF) for an hour. Microstructural analysis showed the creation of supplementary pores and channels in the matrix due to polyvinyl alcohol (PVA) fibers melting in the 200 to 400degC (392 to 752degF) temperature range. After a 1-hour exposure to temperatures of 600 and 800degC (1112 and 1472degF), the mechanical performance of fire-deteriorated ECC mixture is similar to or better than that of conventional concrete incorporating polypropylene or steel fibers, despite a significant reduction in compressive strength and stiffness. Moreover, no explosive spalling occurred in any specimens during the fire test. The promising performance of ECC under fire exposure may be due to the presence of PVA fibers and high-volume fly ash (FA). The beneficial influence of FA can be ascribed to the pozzolanic reaction consuming calcium hydroxide in the hydrates. PVA fiber is also beneficial in that it prevents explosive spalling. This introduces additional channels for vaporized moisture in ECC to escape without creating high internal pressure in the material.