Effect of dispersion method of copper-plated steel fiber on piezoresistivity of cement mortar with waste ceramic powder

Waste ceramic powder can enhance the electrical properties of cement mortar, while copper-plated steel fibers possess excellent mechanical, electrical and corrosion-resistant properties. Introducing both materials into cement mortar can effectively improve cement mortar’s electrical conductivity and endow it with piezoresistivity. The dispersion states of copper-plated steel fibers in the cement mortar undoubtedly play a significant role in determining its electrical conductivity and piezoresistivity. Therefore, this study prepared copper-plated steel fiber reinforced cement mortar with waste ceramic powder (SFRCM) and investigated the effect of two dispersion methods of copper-plated steel fibers, namely direct pouring method and uniform sieving method, on the electrical resistivity and piezoresistivity of SFRCM.

(1) Electrical properties testing: At 28 days, a digital multimeter was employed to test the DC resistivity of each SFRCM specimen by using two-electrode method.(2) Piezoresistivity testing: A testing system consisting of a universal testing machine, strain acquisition instrument, and digital multimeter was employed to evaluate piezoresistive properties. For the piezoresistivity tests under cyclic load, the loading rate was set to 0.4 mm/min, and the loading amplitude was set to 15 MPa. For the piezoresistivity tests under different loading amplitudes, the loading amplitudes were 10 MPa, 15 MPa, and 20 MPa, with loading rate of 0.4 mm/min. For the piezoresistivity test under different loading rates, the loading rates were 0.2 mm/min, 0.4 mm/min, and 0.6 mm/min and loading amplitude was fixed at 15 MPa. For the piezoresistivity test under failure loading, the loading rates of flexural and compressive strengths were 0.1 mm/min and 1.2 mm/min, respectively.(3) Characterization of copper-plated steel fiber dispersion: Cross-sections from two different position within the specimens (parallel to the electrode direction) were obtained by using a cutting machine. The fiber distribution was quantitively analyzed with the help of image processing techniques of Image J. Each cross-section was divided into 6 regions, and the number of fibers in the 12 regions was counted. Fiber dispersion coefficients were calculated to quantify dispersion uniformity, with lower coefficients indicating more uniform fiber distribution.

(1) Dispersion state of copper-plated steel fibers in SFRCM by using different dispersing methods: When the content of copper-plated steel fibers is 0.50 vol.%, the dispersion coefficients of SFRCM prepared by the uniform sieving method and the direct pouring method are 0.452 and 0.629, respectively. When the content of copper-plated steel fibers is 2.50 vol.%, the dispersion coefficients of SFRCM prepared by the uniform sieving method and the direct pouring method are 0.493 and 0.699, respectively.(2) Electrical properties of SFRCM produced by different dispersing methods: The DC resistivities of SFRCM with 0.50 vol.% copper-plated steel fiber prepared by the uniform sieving method and the direct pouring method are 128.27 kΩ·cm and 135.92 kΩ·cm, respectively. At 2.50 vol.% copper-plated steel fiber content, the DC resistivities of SFRCM prepared by the uniform sieving method and the direct pouring method are 94.07 kΩ·cm and 109.24 kΩ·cm, respectively.(3) Piezoresisitivity of SFRCM prepared by two different dispersion methods: Under cyclic load, the strain sensitivity growth rate of SFRCM with 0.50 vol.% and 2.50 vol.% copper-plated steel fibers prepared by the uniform sieving method is 801.40% and 49.70% compared with that of the direct pouring method, respectively. The strain sensitivity of SFRCM increases with the increase of loading amplitude and increases with the decrease of loading rate. Under the loading amplitude of 20 MPa, the strain sensitivity of SFRCM with 0.50 vol.% and 2.50 vol.% copper-plated steel fibers prepared by the uniform sieving method increases 192.20% and 107.00%, respectively. Under the loading rate of 0.2 mm/min, the strain sensitivity growth rate of SFRCM prepared by the uniform sieving method is 1026.30% and 85.80%, respectively, when the copper-plated steel fiber content is 0.50 vol.% and 2.50 vol.%. Under the ultimate flexural load, when the copper-plated steel fiber content is 0.50 vol.% and 2.50 vol.%, the strain sensitivity growth rate of SFRCM prepared by the uniform sieving method is 32.60% and 268.20%, respectively, compared with the direct pouring method. Under the ultimate compressive load, when the content of copper-plated steel fiber is 0.50 vol.% and 2.50 vol.%, the strain sensitivity of the uniform sieving method increases 21.30% and 4.80%, respectively, compared with the direct pouring method.Conclusions：(1) The uniform sieving method produces SFRCM with lower DC resistivity compared to the direct pouring method. Specifically, the uniform sieving method reduces DC resistivity by up to 5.60% at 0.50 vol.% copper-plated steel fiber content and 13.90% at 2.50 vol.% copper-plated steel fiber content, compared to directing pouring method.(2) The uniform sieving method achieved a 28.14% and 29.47% reduction in fiber dispersion coefficients at 0.50 vol.% and 2.50 vol.% copper-plated steel fiber content, respectively, compared to the direct pouring method. This indicates that the uniform sieving method results in more uniform fiber distribution, minimizing fiber agglomeration and facilitating the formation of conductive pathways, especially at higher fiber contents.(3) Across different loading amplitudes, rates, and flexural and compressive failure conditions, the uniform sieving method significantly enhanced the piezoresistive performance of SFRCM. The maximum strain sensitivity improvement rates for SFRCM with 0.50 vol.% and 2.50 vol.% copper-plated steel fiber content reach 1432.20% and 268.20%, respectively, compared to the direct pouring method.