In Situ Monitoring of Water-to-Cement Ratio of 3D-Printed Concrete

Precise control of the water-to-cement (w/c) ratio is critical in three-dimensional concrete printing (3DCP), for which minor deviations compromise extrudability, buildability, and structural integrity. Existing non-destructive w/c (moisture) sensors have not been validated for real-time, in situ monitoring during the concrete extrusion process. This study develops and validates a four-electrode Wenner-array probe, interfaced with an LCR meter, for in situ electrical resistance measurements of fresh concrete. The system was calibrated against NaCl and KCl electrolytic solutions at resistivities of 1, 5, and 10 Ω·m and then tested on ordinary Portland cement pastes at w/c ratios of 0.40–0.60 and on a proprietary 3D-printable blend at 0.40–0.65. The results show that resistance drift remained below 1.2% under ambient temperature fluctuations, with sub-second response times. Strong nonlinear correlations (R2 > 0.99) were established between electrical resistance and w/c ratio for both formulations, with resistance increasing with higher w/c ratios due to ionic dilution and decreasing over time as early-age hydration progressed. Proprietary mixtures below w/c = 0.51 exhibited insufficient pore connectivity (CV > 30%) for reliable measurement, whereas mixtures at w/c ≥ 0.51 demonstrated excellent repeatability (CV < 2.80%). Electrical resistance measurement provides a viable real-time technique for w/c determination in fresh concrete, enabling automated closed-loop control in 3DCP operations.