Use of electrical methods for detecting cracks, monitoring cracks, and monitoring self-healing in cementitious materials

: (Alternative Format Thesis)

Abstract

Electrical techniques integrated with smart cement-based composites incorporating self-sensing and self-healing technologies show great potential for the development of resilient concrete structures. There have been extensive trials using self-sensing smart cement-based composites (or cement-based sensors, self-sensing concretes) as promising alternatives to the off-the-shelf sensors for automated monitoring and alert systems of reinforced beam/slab, driveways/pavements, smart parking, room occupancy, and so on. Self-healing smart cement-based composite (or self-healing concrete) has also been proved to be promising for the replacement of traditional structure repair technique by its crack/damage healing functionality. Good examples of the trials are the HEALCON project by Ghent University and the self-healing concrete walls built by Cardiff University in the EPSRC funded RM4L project. Although the feasibility of self-sensing smart cement-based composite have been proved under laboratory conditions, studies on the environmental effects are quite scant hence the limited knowledge of in-situ performance. Also, the self-healing smart cement-based composite is in urgent need of a more efficient validation technique that allows for real-time evaluation of the healing condition. This doctoral study investigates methodologies for progressing electrical techniques with self-sensing smart cement-based composites towards the in-situ monitoring of moisture, stress, cracking, and crack healing. Electrochemical impedance spectroscopy (EIS) was used as the primary electrical technique for the material characterization, signal excitation, data acquisition, and data management. Macro-scale, PAN-based chopped carbon fibre was used as the conductive filler in self-sensing smart cement-based composites (or cement-based sensors). The finite element method (FEM) was used for the computation of complex geometries.

While increasing the fibre loading in smart cement-based composites, a series of transitional process takes place. Microstructurally, the porosity and the fibre-fibre contact points (i.e., continuity of the electronically conductive pathway) increased with the increasing fibre contents, but the degree of fibre dispersion and the homogeneity of cementitious matrix decreased. Mechanically, the compressive and flexural strength firstly increased to peak at fibre contents of 0.2 – 0.3 vol.%, followed by a decrease thereafter as porosity increased. The frequency-dependent impedance behaviour of smart cement-based composites was determined under two-point EIS at frequencies 1 Hz – 10 MH, where the Maxwell-Wagner type interfacial polarization (MWP) was identified as the main bulk behaviour, and a special type of electrical double layer polarization (EDLP) was identified as a strong local surface effect in the interface of carbon fibre-pore fluid. While increasing the fibre content, both the relative permittivity and conductivity were enhanced at all frequencies.

Self-sensing under variations in water saturation was investigated. A stage where the fractional change in bulk conductivity (FCS) is linearly correlated to the saturation degree was discovered at saturations of 20 – 80%. The bulk conductivity of carbon fibre incorporated smart cement-based composites with fibre loadings 0.1 – 0.3 vol.% was susceptible to the saturation degree, while the influence was reduced with fibre loadings 0.5 – 1.5 vol.%. While changing the saturation degree from 80% to 30%, the stress sensing sensitivity of the smart cement-based composites with 1.2 vol.% of carbon fibres fluctuated but maintained its effectiveness. The stress sensing sensitivity of smart cement-based composites with 0.5 vol.% of carbon fibres, although having lower values, was the least susceptible to varying water saturation.

The self-sensing of cracking and autogenous healing was successfully achieved by using EIS on a multifunctional smart cement-based composite incorporating carbon fibres and ground granulated blast furnace slag (GGBS). An innovative equivalent circuit model was constructed manifesting the impedance responses of the bulk hydration (Zm), the crack opening (Zc), and the autogenous healing in the crack (∆Z). While increasing the same crack mouth opening displacement (CMOD), the impedance of the smart cement-based composites increased less than that of the mortars thanks to the fibre bridging and touching in the major crack. Smart cement-based composite had stable and strong signal output for the monitoring of cracking and healing. At healing time 14 – 77 days, the fractional change in ∆R (FC∆R), of smart cement-based composites incorporating GGBS was higher than that of smart cement-based composite using only OPC.

This work has, for the first time, presented that it is possible to manufacture a multifunctional smart cement-based composite that can sense stress, moisture changes, cracking and the healing of the crack under in-situ conditions without the need for extrinsic measures and over-engineering. The outcomes showed great potential for the development of sustainable and intelligent concrete structures by the employment of a multifunctional system integrating EIS and smart cement-based composites.

Date of Award	24 May 2023
Original language	English
Awarding Institution	University of Bath
Supervisor	Andrew Heath (Supervisor), Richard Ball (Supervisor) & Kevin Paine (Supervisor)