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

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 in properties and microstructure of ECC exposed to temperatures up to 800 degrees C (1472 degrees F). 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 fire-deteriorated 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 400 degrees C (752 degrees F) 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 400 degrees C (392 to 752 degrees F) temperature range. After a 1-hour exposure to temperatures of 600 and 800 degrees C (1112 and 1472 degrees F), 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 spoiling 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 spoiling. This introduces additional channels for vaporized moisture in ECC to escape without creating high internal pressure in the material.