Dr. Hongjian Du Profile

Dr. Hongjian Du

at National Univ of Singapore

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Proceedings Article | 10 April 2014 Paper

Strain and damage self-sensing cement composites with conductive graphene nanoplatelet

Sze Dai Pang, Hongchen Gao, Chunying Xu, Ser Tong Quek, Hongjian Du

Proceedings Volume 9061, 906126 (2014) https://doi.org/10.1117/12.2045329

KEYWORDS: Resistance, Cements, Composites, Graphene, Sensors, Electrodes, Numerical simulations, Environmental sensing, Smart materials, Carbon

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A novel cement composite containing graphene nanoplatelet (GNP) which can sense its own strain and damage is introduced in this paper. Piezoresistive strain sensing was investigated for mortar specimens with GNP under both cyclic and monotonically increasing compressive and tensile strain. Under compression, the electrical resistance decreased with increasing strain and the normalized resistance can be described by a bilinear curve with a kink at about 400 microstrain. At low strain, a high gauge factor exceeding 10³in magnitude was obtained and it increased almost linearly with the GNP content. This can be attributed primarily to the reducing interfacial distance and forming of better contacts between GNP and cement paste when the composite was initially loaded. At higher compressive strain beyond 400 microstrain, the gauge factor is consistently about 10² for GNP content exceeding the percolation threshold. A different response was observed for specimens under tension due to the formation and propagation of microcracks even at low tensile strain due to the brittleness of the material. The initial gauge factor is of the order 10² for tensile strain up to 100 microstrain and it increases exponentially beyond that. The damage self-sensing capability of this conductive cement composites is explored using electric potential method. Closed form expression for the assessment of damage are derived based on the mathematical analogy between the electrostatic field and the elastostatic field under anti-plane shear loading. The derived expression provide a quick and accurate assessment of the damage of this conductive material which is characterized by its change in compliance.

Proceedings Article | 19 April 2013 Paper

Smart multifunctional cement mortar containing graphite nanoplatelet

Hongjian Du, Ser Tong Quek, Sze Dai Pang

Proceedings Volume 8692, 869238 (2013) https://doi.org/10.1117/12.2009005

KEYWORDS: Cements, Resistance, Composites, Interfaces, Electrodes, Carbon, Polarization, Particles, Scanning electron microscopy, Carbon nanotubes

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The piezoresistivity-based strain sensing ability of cementitious composites containing graphite nanoplatelet (GNP) is investigated in this paper. GNP offers the advantages of ease of processing, excellent mechanical and electrical properties at a very low cost compared to carbon nanotubes and carbon nano-fibers. Cement mortar with 0%, 1.2%, 2.4%, 3.6% and 4.8% of GNP (by volume of composite) were cast. The electrical resistance of the specimens was measured by both the two- and four-probe methods using direct current (DC). The effect of polarization was characterized and the percolation threshold was experimentally found to be between 2.4% and 3.6% of GNP based on both accelerated and normal drying specimens. The assumption of Ohmic material was tested with varying current and found to be valid for current < 0.01mA and 0.5mA for four- and two-probe methods respectively. The piezoresistive effect was demonstrated by comparing the gage factors of mortars with GNP vs plain mortar under cyclic loading in compression at 3 strain levels. At low strains, the high gage factor is believed to stem from both the effect of the imperfect interfaces around the GNP and the piezoresistivity of the GNP; at higher strains, the gage factor is likely to be attributed to the piezoresistivity of the GNP and it is still 1-2 orders of magnitude larger than the gage factor arising from geometric changes.

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