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Dynamic compressive and flexural behaviors of ultra-rapid-hardening mortar containing polyethylene fibers

Chun Booki, Shin Wonsik, Oh Taekgeun, Yoo Doo-Yeol

Archives of Civil and Mechanical Engineering

2021

Vol. 21, no. 2

576--592

In this study, the dynamic, compressive, and flexural behaviors of ultra-rapid-hardening mortar (URHM) containing 2% polyethylene fiber are investigated. The results confirm the robust strain-hardening behavior of URHM at an early age of 4 h. Its tensile strength, strain capacity, and g value at 4 h were found to be 7.3 MPa, 5.1%, and 297.5 kJ/m3, respectively. The compressive and flexural strength and toughness of URHM increased with the strain rate. A higher loading rate led to a greater increase in the strength; the rate sensitivity was higher during flexure compared to that during compression. The highest dynamic increase factor (DIF) of the compressive strength was 1.75 up to a strain rate of 115/s; the highest DIF of the flexural strength was 3.34 up to a strain rate of 96/s. Its deflection-hardening behavior was converted to deflection-softening behavior under impact loads having a potential energy of 392 J or greater. Furthermore, the greater potential energy led to a lower energy dissipation rate, and more energy remained in the system. The rate sensitivity of the URHM under compression was similar to that of other fiber-reinforced concretes; however, its flexural strength was less sensitive to the strain rate than that of the others.

Influence of chemically treated carbon fibers on the electromagnetic shielding of ultra-high-performance fiber-reinforced concrete

Yoo Doo-Yeol, Kang Min-Chang, Choi Hong-Joon, Shin Wonsik, Kim Soonho

Archives of Civil and Mechanical Engineering

2020

Vol. 20, no. 4

367--381

The effects of carbon fiber and its surface treatment through chemical solutions on the mechanical properties and electromagnetic (EM) shielding of ultra-high-performance fiber-reinforced concrete (UHPFRC) were analyzed. Three types of carbon fibers chemically treated with sodium hydroxide, nitric acid, and ammonia solutions were evaluated, along with a plain carbon fiber control sample, at two different concentrations of 0.1% and 0.3% by weight. The surface of carbon fiber was oxidized by chemical solutions. The conductivity of UHPFRC increased with increasing the carbon fiber content, and slightly better conductivity was obtained using the chemically treated carbon fibers than plain fibers at the lower content of 0.1 wt%. Both steel and carbon fibers were effective at improving the shielding effectiveness of ultra-high-performance concrete, and a higher shielding effectiveness was achieved for higher carbon fiber content. Surface treatment using the nitric acid solution was the most effective at enhancing the tensile performance and EM shielding effectiveness, and the best shielding effectiveness (49.0 dB at 1 GHz) was achieved for UHPFRC with 0.1 wt% nitric acid treated carbon fibers. The shielding effectiveness was found to be generally proportional to the electrical conductivity, although its increase was minor relative to that of the conductivity.