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.
Effect of shrinkage reducing admixture on tensile and flexural behaviors of UHPFRC considering fiber distribution characteristics
Abstract This study investigated the effect of shrinkage reducing admixture (SRA) on the properties of ultra high performance fiber reinforced concrete (UHPFRC) including fluidity, compressive, single fiber pullout, tensile and flexural behaviors. In addition, the influence of fiber distribution characteristics such as fiber orientation, fiber dispersion, number of fibers, and packing density on the flexural behavior of UHPFRC was evaluated according to the amount of SRA, using an image processing technique that was developed. Three different SRA to cement weight ratios (0%, 1%, and 2%) were investigated on UHPFRC with 2 vol.% of steel fibers. The specimen without SRA exhibited the best performance in compressive, single fiber pullout, and tensile behaviors including load carrying capacity, strain capacity, and energy absorption capacity and had a highly densified interfacial transition zone between fiber and matrix. In particular, the flexural strength of UHPFRC varied with the fiber distribution characteristics, rather than the amount of SRA.
Preparation of hydrophilic poly(vinyl butyral) hollow fiber membrane via thermally induced phase separation
Abstract Hydrophilic microporous hollow fiber membranes were prepared via thermally induced phase separation (TIPS). Poly(vinyl butyral) (PVB) and three kinds of polyethylene glycols (PEG) with the molecular weight of 200, 400 and 600 were used as polymer and diluents. In all cases, liquid–liquid (L–L) phase separation occurred and asymmetric cellular structures with the smaller pores at the outer surface were obtained. The properties of membranes such as pore size, water permeability, solute rejection and intensity were investigated by changing molecular weight of PEG, concentration of PVB, composition of the bath and air gap distance. The smaller molecular weight of PEG, the more PEG in the bath and the shorter air gap distance resulted in the higher water permeability, the lower solute rejection property and the lower mechanical strength.