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 103 in 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.
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.