Abstract
Ultra-high performance concrete (UHPC) has a high self-healing capacity and is prone to bursting after exposure to high temperatures due to its characteristics. This work evaluates the damage and improvement of UHPC with coarse aggregates through mechanical properties (compressive strength and ultrasonic pulse velocity), transport properties (water absorption and a chloride diffusion test), and micro-properties such as X-ray diffraction (XRD), Mercury intrusion porosimetry (MIP), and Scanning electronic microscopy (SEM). The result demonstrates that polypropylene (PP) fibers are more suitable for high temperature tests than polyacrylonitrile (PAN) fibers. The result shows that 400 °C is the critical temperature point. With the increase in temperature, the hydration becomes significant, and the internal material phase changes accordingly. Although the total pore volume increased, the percentage of various types of pores was optimized within 400 °C. The mass loss gradually increased and the ultrasonic pulse velocity gradually decreased. While the compressive strength first increased and then decreased, and the increase occurred within 25–400 °C. As for the transport properties, the chloride migration coefficient and capillary absorption coefficient both increased dramatically due to the higher sensitivity to temperature changes. The results of the property improvement test showed that at temperatures above 800 °C, the compressive strength recovered by more than 65% and the ultrasonic pulse velocity recovered by more than 75%. In terms of transport properties, compared to the results before self-healing, the chloride migration coefficient decreased by up to 59%, compared with 89% for the capillary absorption coefficient, after self-healing at 800 °C. With respect to the enhancement effect after exposure to high temperatures, the environment of a 5% Na2SO4 solution was not as good as the clean water environment. The corresponding changes in microstructure during the high temperatures and the self-healing process can explain the change in the pattern of macroscopic properties more precisely.
Highlights
IntroductionLi et al [15,16] investigated the change in the pattern of the pore pressure and pore structure after exposure to high temperatures and found that the combination of PP fiber with steel fiber or PP fiber with coarser aggregates could effectively improve the permeability and inhibit the occurrence of burst
This study provides a feasible way to improve the resistance and mechanical properties of ultra-high performance concrete (UHPC) after exposure to elevated temperatures
Was determined based on the average value of threeThe samples using the microcomputer-controlled automatic pressure testing three samples using the microcomputer-controlled automatic pressure testing machine, according to the GB/T 31387-2015.The ultrasonic pulse velocity of UHPC was determined by utilizing the average value of three samples through the NM-4A non-metallic ultrasonic detection analyzer, according to the standard CECS 02: 2005; three test points were taken for each surface of the test specimen
Summary
Li et al [15,16] investigated the change in the pattern of the pore pressure and pore structure after exposure to high temperatures and found that the combination of PP fiber with steel fiber or PP fiber with coarser aggregates could effectively improve the permeability and inhibit the occurrence of burst. The transport property after exposure to elevated temperatures is one of the most critical indicators to evaluate phenomena such as internal moisture migration and pore pressure accumulation, which are related to the microstructure of cementitious composites and invasive solution ions [27,28]. The effect of post-fire curing as a Buildings 2021, 11, 416 self-healing method on the mechanical and transport properties of fire-damaged UHPC remains to be explored. The present work investigates the deterioration and recovery of the transport properties and mechanical properties of UHPC containing coarse aggregates after exposure to high temperatures.
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