Towards the Establishment of a Continuous Nondestructive Monitoring Technique for Fresh Concrete

Despite the fact that concrete is the most widely used construction material, conventional techniques for the estimation of the mechanical properties are only applicable to the hardened material. Moreover, concerning the fresh concrete examination, the already existing techniques provide only qualitative information, are not representative over the full time period of curing, setting and hardening or have limited accuracy and repeatability. In this study, the acoustic emission (AE) technique is employed in order to investigate the activity within fresh concrete as early as a few minutes after mixing. Several processes like settlement, segregation, formation and migration of air bubbles, as well as formation of hydrates produce AEs which so far have not been properly examined in literature. The first and foremost goal is to check the capacity of the new AE setup to record emissions earlier than previous studies (just after casting). The next goal concerns the possibility to characterize the different types of emissions relatively to their original source mechanism while the final goal would be the investigation of possible correlations between the early age acoustic activity and the final mechanical properties. For this reason, an AE setup is applied utilizing the favorable wave transmission properties of the metal casting molds, which act as a sort of waveguide. After validating its sensitivity, several laboratory tests are performed in concrete with varying water and aggregate content. It is found that the rate of AE activity follows trends similar to the typical hydration curves known from literature and is also indicative of the mix parameters. Finally, it should be mentioned that it is the first study that exploits the acoustic activity as early as the moment of mixing while apart from the number of emissions which is usually examined, the monitoring includes waveform parameters that show strong characterization potential.     

Electron transfer mediator PCN secreted by aerobic marine Pseudomonas aeruginosa accelerates microbiologically influenced corrosion of TC4 titanium alloy

Titanium alloys possess excellent corrosion resistance in marine environments,thus the possibility of their corrosion caused by marine microorganisms is neglected.In this work,microbiologically influenced corrosion (MIC) of TC4 titanium alloy caused by marine Pseudomonas aeruginosa was investigated through electrochemical and surface characterizations during a 14-day immersion test.Results revealed that the unstable surface caused by P.aeruginosa resulted in exposure of Ti2O3 and severe pitting corrosion with maximum pit depth of 5.7 μm after 14 days of incubation.Phenazine-1-carboxy[ate (PCN),secreted by P.aeruginosa,promoted extracellular electron transfer (EET) and accelerated corrosion.Deletion of the phzH gene,which codes for the enzyme that catalyzes PCN production,from the P.aeruginosa genome,resulted in significantly decreased rates of corrosion.These results demonstrate that TC4 titanium alloy is not immune to marine MIC,and EET contributes to the corrosion of TC4 titanium alloy caused by P.aeruginosa.     

Mechanical properties,moisture absorption,and dyeability of polyamide 5,6 fibers

Polyamide 5,6 (PA56) and polyamide 6 (PA6) fibers were spun by melt spinning process and drawn by a thermal drawing in order to improve their mechanical properties. The effects of drawing conditions on mechanical properties, moisture absorption, and dyeing behavior were investigated by means of tensile testing measurements, moisture absorption test, as well as color strength measurements. It was found that the tenacity and Young’s modulus were significantly increased, while the elongation at break decreased as the draw ratio (DR) and temperature increased due to the increase in molecular orientation along the fiber axis and the crystallinity. PA56 fibers showed a little lower mechanical properties and much higher moisture regain than PA6 fibers at different DRs. This can be attributed to the amide group content in the polymer backbone, which yields higher moisture regain in PA56 fibers than PA6 fibers. The dyeing behaviors of PA56 fiber, in terms of dyebath exhaustion, color strength, and wash fastness are similar as those of PA6. Those results indicated that PA56 is a competitive novel fiber for textile material.     

Determination of the cutoff frequency of an acoustic circumferential wave using a time–frequency analysis

In this article, time–frequency representation of Wigner–Ville is used to analyse the acoustic signal backscattered by a thin elastic tube of radii ratio b/a. This analysis allows to determine the reduced cutoff frequency of the circumferential antisymmetric wave A1 propagating around the tube. The evolution of this reduced cutoff frequency in function of b/a is reported. The values obtained of reduced cutoff frequency are compared to the values computed from the proper modes method.