PSI - Issue 64

Shaofeng Qin et al. / Procedia Structural Integrity 64 (2024) 168–174 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

170

3

2.2. Preparation of the fiber-cement composite The mix design of the fiber-cement composite is shown in Table 2. The water-to-cement ratio was 0.26, while the cement/binders (cement and fly ash) ratio was 0.26. And superplasticizer was used to control the rheology of the cementitious material for better dispersion of CFs and printability. Cement and fly ash were first mixed for about 3 mins. At the same time, the superplasticizer was mixed in the water before they were added to the mixed raw materials for 5 mins. After that, CFs were added to the cement paste and mixed for 5 mins. A part of the as-prepared fiber cement composite was then placed in an injector, as presented in Fig. 1a, for the extrusion-based 3D printing with the specific route presented in Fig. 1b. The extrusion-based 3D printing used in the study was designed based on a commercialized 3D printer (MOORE 1, TRONXY Co., Ltd, Shenzhen, China) equipped with a 100 ml injector. It is reported that the 3D printing technique was applied to control the fiber orientation to a large degree [Matthias et al. (2021)]. It should be noted that the composite was under continuous stirring when the 3D printing was proceeding, avoiding the initial setting of the composite. Besides, the extrusion speed was set as the same to the printing moving speed, reducing the adverse effect on the fiber distribution and alignment. When the printing moving speed is larger, the extruded composite will be elongated, while the lower will result in the compaction of the composite and the mold.

Table 2. Mix design of the carbon fiber-cement composite.

Cement (g/L) Fly ash (g/L) Water (g/L) Superplasticizer (g/L) CF (vol.%)

Mass

402

1127

397

4.6

0.5

In addition to the designed longitudinal region with CFs, red regions were filled with the composite, in which the fiber orientation is random (Fig.1b). The extruded composite and random part were overlayed and then mixed again by using a mini stirring stick. Slight vibration was allowed to ensure the densification of the sample, so that the electrical properties will not be affected by the difference in porosity rather than fiber alignment. The difference groups were named LCF, MCF, and RCF, respectively, depending on the various forms of designed fiber alignment (L: longitudinal; R: random; M: mixed). After 28 days of hydration under the standard condition (95% humidity, 25°C), the upper surface of the as-prepared samples was polished till the thickness of the sample was around 3.4 ± 0.1 mm. After cleaning and drying under room conditions, the specified surface of the samples was surrounded by conductive silver paint before the copper tape was attached to the position, as shown in Fig. 2b. Four-probe measurement was performed to obtain the electrical resistance in the following section.

2.3. Fiber orientation analysis

Optical microscopy with a minimum magnification (5x) was applied to capture the fiber distribution on the upper surface of the fiber-cement composite, as presented in Fig. 1c. About 28 surface images of each sample (4 x 7 matrix) were collected, and the fiber orientation angle ( θ ) of each fiber was then measured via ImageJ Fuji software [Hambach et al. (2016)]. Three parallel samples for each group were recorded, and the standard Gaussian curve was then used to fit the sum of the angles in each group to analyze fiber orientation with/without using extrusion-based 3D printing.