Crack growth and closure in cementitious composites: monitoring using piezoceramic sensors

Coupling piezoceramics with civil-structures represents an important approach for the development of cost-effective, fully automated damage detection systems. Such systems are particularly suited for integration with autonomic technologies for self-sensing and self-healing of cementitious members. Here we present a novel study investigating both the effect of damage (cracking) and crack-closure, on the electromechanical response of mortars, with surface attached Lead Zirconate Titanate (PZT) sensors. Transducer signals were interpreted using a new analytical procedure which effectively increases the sensitivity of the electromechanical technique for both damage and repair detection compared to conventional methods of analysing the PZT response. The frequency ranges most sensitive to the presence of damage, were demonstrated to be influenced by the distance to the sensors. Furthermore, for pre-cracked specimens, it was shown that the electromechanical impedance can act as an indirect method to detect crack-closures by detecting the increase in the efficacy of load transmission in the sample. This findings of this research will enable the cost-effective monitoring and initiation of repair for self-healing cementitious infrastructure, which is expected to pave the way for self-sensing / self-healing cementitious structures.</p

Electrochemical impedance spectroscopy to monitor the hydration of cementitious materials

The electrical properties of Portland cement, and cements containing supplementary cementitious materials (SCM), were obtained over the frequency range 1Hz-10MHz during both the initial 24-hours after gauging with water and up to 1 year. During the initial 24-hours period, the response was measured in terms of conductivity and permittivity with both parameters exhibiting significant temporal changes. It was also evident that whilst the conductivity increased only marginally with increasing frequency of applied electrical field, the permittivity decreased by several orders of magnitude over this range. Moreover, certain features of the permittivity response – which are related to bulk polarization processes – only revealed themselves in the higher frequency range (100kHz-1MHz), and went undetected at lower frequencies. The detailed frequency- and time- domain measurements allowed identification of several stages in the early hydration of cement-based materials and the response can be interpreted in terms of hydration kinetics, physico-chemical processes and microstructural development. It is shown that the methodology can be equally applied to cement-pastes and concretes. In the hardening stage, the conductivity response showed a clear influence of the SCM type, the age of the samples and the used water binder ratio (w/b) in the mixes. The pore solution conductivity has been shown to have a significant effect on the conductivity values particularly at the high replaced mixes. The electrical permittivity showed two different polarization signals depending on the frequency range used, as at frequencies in the range of 100kHz-1MHz, the permittivity response is more related to the samples electrical conductivity, however at higher frequencies (1MHz-10MHz) the permittivity is influenced more by the SCM type and the replacement level in the mixes when w/b is constant. The durability ranking which was obtained from the non-steady-state migration coefficient and the electrical conductivity, showed a strong linear relationship which is in contrast to the relationship between the ranking obtained from the formation factor. This would suggest that both the non-steady-state migration coefficient and the conductivity are affected by the pore solution conductivity of the mixes which, consequently, would give a false indication with regard to the real ranking of the mixes

Insights into the piezoceramic electromechanical impedance response for monitoring cement mortars during water saturation curing

Lead Zirconate Titanate (PZT) based electromechanical impedance (EMI) sensors were used to monitor the mechanical properties development of different water to cement ratios (w/c) cementitious mortar mixes, during the first 28 days of curing under water. Through using the analytical procedure proposed in this study to analyse the EMI data, the different mixes mechanical properties development through the curing period were detected, and the EMI response was able to provide a more detailed interpretation regarding the difference between the surface and the bulk material mechanical properties development. Both the peaks from the impedance signature (Z) and the first difference of the impedance signature (dZ) showed shifts to higher frequency ranges as the age of the samples increased, indicating an increase in the material stiffness. Furthermore, the compressive and the flexural stresses showed an R2 &gt; 0.8 and &gt; 0.9, respectively in relation to the frequency shifts. The relationship between the PZT-EMI response through the curing period and the sample’s mechanical properties was shown to be frequency-dependent; hence a numerical analysis using ANSYS Workbench 18.1 was undertaken to understand this frequency-dependence phenomenon. From the numerical model, the impedance signature response at higher frequency ranges was shown to be dominated by the response from the surface of the hosting material, whereas the response from the specimen's interior dominated the lower frequencies EMI response. The analytical approach proposed in this study is expected to assist in differentiating between internal cementitious materials processes, such as internal curing, and those originating at the surface, such as aggressive chemical agents’ penetratio

Self-healing potential of supplementary cementitious materials in cement mortars: sorptivity and pore structure

This paper presents the autogenic self-healing potential of Portland cement (PC) blends made with conventional supplementary cementitious materials (SCMs), to improve the water-tightness by reducing the overall pore size. Mortar samples were prepared by mixing PC, sand and water, and partially replacing PC by either silica fume (SF), pulverised fuel ash (PFA), or ground granulated blast-furnace slag (GGBS). Damaged samples were subjected to a water bath to heal microcracks and recover the water-tightness, by further hydration of the starting minerals. Water absorption and density measurements in undamaged, damaged and healed conditions were used to determine the autogenous healing potential of SCMs mixes, showing a post-healing absorption recovery of up to 68% compared to the mix with PC only. Thermal analysis, XRD and MIP measurements confirmed the capability of SCMs to promote the formation of hydrated phases, and reduce the overall pore size by more than 88% compared to PC mixes