Influence of chemical admixtures on buildability and deformation of concrete for additive manufacturing

In this study, the buildability of three-dimensionally (3D) printed (3DP) concrete that uses coarse aggregate was evaluated in the fresh state according to the contents of three chemical admixtures (high-range water-reducing agent (HRWRA), viscosity-modifying agent (VMA), and setting-time modifying agent), and the correlation between fluidity changes and buildability was analyzed. Polycarboxylate HRWRA and VMA contents were varied to control the flowability and static yield stress, which affect the workability. In addition, the SMA content was varied, and slump tests and table flow tests were performed at 30-minute intervals up to 6.5 h after mixing to measure flowability. Simultaneously, static yield stress was measured to analyze rheological changes, and penetration resistance and green strength tests were performed to analyze the main factors influencing buildability over time. The highest buildability was observed when the VMA content was 1.5 %. The static yield stress of fresh concrete could be ≤3000 Pa to guarantee adequate 3DP concrete transport performance while that at 1 h after mixing could be ≥2000 Pa to secure buildability. When the penetration resistance of fresh concrete is ≥80 kPa, the deflection of concrete may be <10 %. Through this, buildability can be approximately judged based on penetration resistance.