Early-age concrete undergoes displacements and volume changes due to ongoing processes such as settlement, hydration, shrinkage, and cracking, which can strongly affect its durability and long-term performance. In this paper, fresh concrete is monitored by the non-destructive techniques of Acoustic Emission (AE) and Digital Image Correlation (DIC). Elastic waves released by the physical processes taking place while concrete is in a fresh state can be well-recorded by AE, while the three-dimensional strain and displacement evolution on the surface can be measured by DIC. Monitoring fresh concrete is of paramount importance to ensure the desired final mechanical properties, especially when novel admixtures for internal curing such as SuperAbsorbent Polymers (SAPs) are added to the mixture. SAPs are particles that can swell by absorbing water when exposed to it, and later release it back to the cementitious matrix when the internal relative humidity linked to the capillary pressure decreases, mitigating autogenous shrinkage. These admixtures strongly interact with the microstructure, resulting in an increased amount of AE activity. The motivation of this study is to obtain real-time information on the different ongoing processes in fresh concrete using AE and compare the results to concrete containing SAPs. Specimens are subjected to different environmental conditions, to monitor the changes in the SAP activity. Results are complemented by DIC to confirm the mitigation of shrinkage by the SAPs. The DIC results showed that SAPs mitigate settlement and shrinkage in early-age concrete, while AE showed SAP concrete exposed to windy conditions demonstrated a delay in the SAP activation, lower amplitude values and higher peak frequency values than the ambient SAP concrete.
KEYWORDS: Chemical elements, Structural health monitoring, Cements, Inspection, Ultrasonography, System integration, Optical tracking, Integrated optics, Digital imaging, Acoustics
The damage evolution on thin-wall large-scale textile-reinforced cement composite structural elements loaded under bending is tracked in this study using acoustic emission. The monitoring technique has been extensively applied in recent decades for the structural health assessment of voluminous and bulk concrete structures, still its performance and efficacy on thin elements is not extensively investigated. In a well-established approach today, the AE waveform is analyzed based on a series of wave features. Indicatively, the damage progress is tracked based on AE energy and correlated to the mechanical properties (load, displacement). AE source localization detects and pinpoints the zones where cracks and internal debonding occur indicating the damage shift from the middle bending span to the edges of the beam. Based on this observation, the rise time and frequency analyses pinpoint the effect of source-sensor distance on the wave shape. This latter result is discussed considering a preliminary attenuation analysis at intact and damage state. The RA-value and frequency trends are also reported in an attempt to relate the research outcome to the Rilem TC 212-ACD protocol. It is shown that the AE inspection accuracy does not diminish due to thin-wall geometry, still taking into account of the attenuation effect at both intact and damage state is recommended.
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