Ultrasonic Monitoring of Setting and Strength Development of Ultra-High-Performance Concrete
In this study, the setting and tensile strength development of ultra-high-performance concrete (UHPC) at a very early age was investigated by performing the penetration resistance test (ASTM C403), as well as the direct tensile test using the newly developed test apparatus, and taking ultrasonic pulse velocity (UPV) measurements. In order to determine the optimum surface treatment method for preventing rapid surface drying of UHPC, four different methods were examined: plastic sheet, curing cover, membrane-forming compound, and paraffin oil. Based on the test results, the use of paraffin oil was found to be the best choice for measuring the penetration resistance and the UPV, and attaching the plastic sheet to the exposed surface was considered to be a simple method for preventing the rapid surface drying of UHPC elements. An S-shaped tensile strength development at a very early age (before 24 h) was experimentally obtained, and it was predicted by a power function of UPV. Lastly, the addition of shrinkage-reducing and expansive admixtures resulted in more rapid development of penetration resistance and UPV of UHPC.
Geometrical and boundary condition effects on restrained shrinkage behavior of UHPFRC slabs
Six large Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC) slabs were fabricated and tested to investigate the restrained shrinkage and cracking behaviors. The use of expanded polystyrene and Teflon sheets with two different slab thicknesses was considered to improve the shrinkage crack resistance. Free shrinkage was simultaneously measured to evaluate the degree of restraint according to the above test parameters. The test results showed that free shrinkage strains of –689 με to –723 με were obtained after 9 days, and prismatic specimens with a higher exposed surface area-to-volume ratio (S/V) had slightly higher free shrinkage strains than those with a lower S/V. Increasing the concrete slab thickness and using expanded polystyrene and Teflon sheets were effective at reducing the degree of restraint and improving the shrinkage crack resistance of the UHPFRC slabs. Among the various specimens, the slabs with the expanded polystyrene exhibited the lowest degree of restraint by 0.45 after 9 days.
Measuring Sheet Resistances of Dielectrics Using Co‐Planar Hydrogel Electrochemical Cells with Practical Applications to Characterize the Protective Quality of Paints on Sculptures
Electrical properties of thin dielectric films at the solid-liquid phase boundary are an important performance characteristic of many devices, coatings and sensors. In this paper, co-polymeric hydrogels of polyacrylic acid co-sulfonic acid, swollen with a salt solution to act as the solid electrolyte, were used to assess interfaces using electrochemical impedance spectroscopy (EIS) in a co-planar geometry. Silane-modified glasses were characterized by the co-planar hydrogel EIS cell and found to be distinguishable based on their surface monolayer chemistry. EIS measurements were also made on primed and painted metal substrates, using both test panels and an outdoor sculpture, Tony Smith's Stinger. The panels were then exposed to accelerated and outdoor weathering and showed degradation on the surface paint layers, which was observable electrochemically using EIS and confirmed visually by SEM. Electrochemical spectral features were compared between data from a standard paint-test cell versus this co-planar hydrogel cell; both cell types were able to measure coating capacitance, providing useful information about the condition of the bulk coating. Yet, sheet resistance (Rs) was a spectral feature seen only by the co-planar hydrogel cell. Thus, it can be concluded that the use of such co-planar hydrogel cells can provide an earlier warning sign to coating degradation and such cells provide a new type of spectral information that is not assessable by the standard geometry.