Abstract
Fresh concrete filling height is difficult to monitor in formwork where direct observation is limited. This study develops a contact-activated U-shaped carbon nanotube/epoxy (CNT/EP) composite sensor for real-time monitoring of this height. Its height-dependent response is interpreted using a parallel conduction model. A CNT content of 1.5 wt.% and a 5 × 15 cm geometry were selected from the conductivity transition and tests of nine geometries. Three independently fabricated sensors per condition were tested in water, cement paste, and concretes with slumps of 20 and 15 cm, against levels observed at 1 cm intervals. The water calibration applied to individual concrete responses gave pooled root mean square height errors of 0.41 and 0.37 cm and maximum absolute errors of 0.96 and 0.74 cm. Tests without liquid contact gave resistance changes of 1.9, 6.0, and 0.7 percentage points under compression to 150 kPa, temperatures from −20 to 80 °C, and relative humidity from 10 to 90%. In a full-scale mock-up, 13 sensors in three arrays monitored five placement lifts, and the reconstructed heights at Array 2 agreed with manual lift-end measurements within 2 cm. The arrays also captured horizontal differences in concrete arrival. These findings support concrete placement monitoring under the tested conditions.