Investigating the self-monitoring potentials of an engineered cementitious composite

Abstract

Cement-based materials are an important group of structural materials, and the ability of such materials to respond to internal and external changes could provide an added feature which could further enhance their range of application. One area of development that has received increasing attention within the research community is making use of the self-monitoring features of concrete with respect to deformation and damage. Ordinary concrete is, however, a poor conductor of electricity, particularly after cracking and under dry conditions, and attention is therefore directed towards a highly damage-tolerant family of concrete types with superior tensile strain capacities and controllable crack widths, generally known as the Engineered Cementitious Composite (ECC). This thesis explores the self-monitoring capabilities of the ECC under mechanical and non-mechanical loading and presents the a.c. electrical properties of ECC over the frequency range 1 Hz–10 MHz. The project was developed on three general fronts, focusing on key factors affecting the electrical properties of ECC: (i) investigation of the influence of cement hydration and temperature; (ii) evaluation of the influence of tensile straining and cracking; and (iii) investigation of the influence of wetting and drying. Laboratory samples of different geometries were fabricated and tested under various curing regimes and test conditions. Results are presented from each of the sub-themes listed above, with measured data presented in a range of formats to provide insights into features that could potentially be exploited for self-monitoring. This includes the Nyquist format, which has been generally used in a.c. electrical property measurements, and the permittivity and conductivity, which were de-embedded from the measured impedance and presented in the frequency domain to elucidate the nature of the conduction and polarization processes. Equivalent circuit models were also developed to simulate the measured response and offer a phenomenological interpretation of the origin of some of the features observed in the electrical response. It was found that, over a curing period of 180 days, the ECC displayed a classic impedance response comprising an electrode spur, a weak intermediate "plateau" region and a single bulk arc. Both conductivity and relative permittivity were found to be frequency dependent due to bulk relaxation processes operating within the composite. It was found that cement hydration has a negligible effect on the relative permittivity at high frequencies (i.e., > 1 MHz), as evidenced by the merging of the relative permittivity at different curing ages when presented in a logarithmic format. Moreover, the knowledge regarding the temperature effects on the electrical properties (through the activation energy approach) will have direct practical significance for removing the effect of natural temperature fluctuations. Tensile straining was shown to result in a detectable change in the impedance response but retained a similar overall profile. When presented in the frequency domain, a downward displacement in relative permittivity at high frequency (i.e., 1 MHz) was evident with increasing tensile strain for ECC with average crack widths in the range 50 μm–65 μm. In the ECC with larger average crack widths (i.e., >100 μm), a downward displacement in relative permittivity profiles together with an enhancement of the relative permittivity within the frequency range (> 10 kHz to ~low MHz) was observed. Overall, it is shown that the relative permittivity at the high-frequency end could be exploited as a potentially useful indicator for strain/damage detection. The electrical properties of ECC display significant increases in the impedance response when the material is subjected to drying. When presented in the frequency domain, an enhancement of the relative permittivity within the frequency range > 1 kHz to ~low MHz was observed. Within the low-frequencies range of ~1Hz to < 1 kHz, the relative permittivity of the un-cracked ECC curves showed a slight decrease, while the cracked-ECC was sensitive to drying. When subjected to wetting, a reduction of the impedance response was observed, and the enhancement of the relative permittivity at high frequencies disappeared, due to the presence of water in the micro-cracks. This thesis demonstrates the use of multi-frequency measurements to characterise the electrical properties of ECC under mechanical and non-mechanical loading.