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.
The adoption of self-healing cementitious materials has gained attention as an alternative to costly and labour-intensive manual repairs. Cementitious blends possess an inherent ability to repair formed cracks through so-called autogenous healing. Whereas the efficiency of autogenous healing remains limited as moisture needs to access the cracks, the healing capacity can be improved through the inclusion of superabsorbent polymers (SAPs). To encourage the use of these self-healing blends within the construction industry, an assessment of the healed state is necessary to ensure a structure’s safety. The requirements for such evaluation method comprise the ability of assessing the regained mechanical performance, while maintaining the structural capacity of the member under study. A non-destructive method that has proven its potential is the application of ultrasonic waves, which are sensitive to the elastic properties of the material they travel through. Coupled ultrasound is currently most often used, while air-coupled ultrasonic measurements allow to reduce the occurring coupling variability. In this study, the self-healing evolution of cementitious mixtures with and without SAPs was assessed through coupled and air-coupled ultrasound. A comparison between both techniques confirmed the potential of air-coupled ultrasound, paving the way for automated self-healing evaluations.
Cracking of cementitious materials affects the durability of concrete structures and might lead to premature failure. As manual repairs are costly and labor-intensive, self-healing mixtures have been studied. The advantage of cementitious blends lies in the inherent ability of the material to repair damage through autogenous healing. As water is essential to be present to induce autogenous healing, the healing ability can be improved by adding water reservoirs in the form of superabsorbent polymers (SAPs). As a wide variety of SAPs with different characteristics exists, an assessment of their capacity to improve the self-healing ability is necessary to optimize the mix design. While most standardized evaluation techniques are limited in their characterization potential or due to their intrusive nature, ultrasonic measurements allow for a non-destructive material characterization. Due to their sensitivity to the obtained microstructure, the damage present and the elastic properties of the material under study, the self-healing evolution can be monitored, and the results provide information on the regained mechanical performance. In the present study, various set-ups are utilized to assess the self-healing capacity of mortars with and without SAPs. The experimental framework includes coupled ultrasonic evaluations through surface wave and transmission measurements. In addition, numerical simulations were performed to isolate the healing layer and simulate the effect of healing by increasing the stiffness of the material in the crack. A comparison between experiments and simulations allowed to assess the elastic modulus of the deposited healing products.
Self-healing cementitious composites provide a solution to the application of costly, manual repairs of construction elements. Additionally, as the healing mechanism is inherently present within the cementitious mixture, issues concerning the repair of structures with limited accessibility are omitted. However, the assessment of the regained mechanical performance as well as the monitoring of the evolution of the healed properties requires destructive tests, which cannot be applied in situ. For this reason, a non-destructive test set-up based on ultrasonic wave transmission was established. Thanks to the sensitivity of ultrasonic waves to the material properties, significant changes between the uncracked, cracked and the healed state of cementitious specimens can be verified, enabling the crack closure monitoring over time as well as the visualization of the interior. In this study, a comparison between the healing ability of a reference mortar and a mortar with superabsorbent polymers (SAPs) was performed and a correlation with the crack width evolution was demonstrated.
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