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.
Elastic waves are commonly used for the evaluation of concrete structural health. Wave speed is firmly connected to the stiffness and is indicative of strength and damage condition. When access to multiple sides is limited, the evaluation takes place solely from the open surface where all sensors are placed. In this case, the size of the sensor is crucial because of the “aperture effect”. This is basically the phenomenon of wavelengths shorter than the sensor size cancelling each other since both their positive and negative phases act simultaneously on the sensor’s surface. Although this effect has been studied relatively to the amplitude and the frequency content of the surface wave pulses, its influence on velocity has not been similarly studied, even though the velocity value is connected to concrete stiffness, porosity, damage degree and is even empirically used to evaluate the compressive strength. In this study, numerical simulations are conducted with virtual sensors of different sizes to measure the surface wave velocity as well as the dispersion (or its dependence on frequency) in relation to the sensor size on homogeneous and heterogeneous material. The strong effect of sensor size is indicated and suggestions towards rules for reliable measurements on a concrete surface are made. Experimental measurements on cementitious media by sensors of different sizes are also conducted validating the numerical results.
Ultrasonic monitoring of fresh cement-based materials is important as pulse speed and attenuation are indicative of the increasing stiffness of the medium, and enable characterization of the curing stage and projections to the mechanical strength from an early age. Despite its importance, practical application is not straightforward due to severe heterogeneity and inherent damping. One crucial parameter in the ultrasonic behavior of fresh cement is the air bubbles, which impose a frequency dependent phase velocity and attenuation, as also observed in all bubbly liquids. In this study, ultrasonic experiments take place in fresh mortar as well as in reference media like water and shampoo. Results show that both shampoo and mortar exhibit strong dispersion relatively to water, seen by the dependence of phase velocity on frequency. Gradually and as bubbles are released due to gravitational settlement (in shampoo) or constrained (hardening of cement) the dispersive trend weakens reaching towards a nearly flat dispersion curve like water. The results highlight the influence of cavities which are considered one of the strongest types of scatterers, while quantification of cement ultrasonic dispersion opens the way for more accurate characterization of the curing behavior.
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