Acoustic emissions evaluation of the dynamic splitting tensile properties of steel fiber reinforced concrete under freeze-thaw cycling

This study empirically investigated the influence of freeze-thaw cycling on the dynamic splitting tensile properties of steel fiber reinforced concrete(SFRC).Brazilian disc splitting tests were conducted using four loading rates(0.002,0.02,0.2,and 2 mm/s)on specimens with four steel fiber contents(0%,0.6%,1.2%,and 1.8%)subjected to 0 and 50 freeze-thaw cycles.The dynamic splitting tensile damage characteristics were evaluated using acoustic emission(AE)parameter analysis and Fourier transform spectral analysis.The results quantified using the freeze-thaw damage factor defined in this paper indicate that the degree of damage to SFRC caused by freeze-thaw cycling was aggravated with increasing loading rate but mitigated by increasing fiber content.The percentage of low-frequency AE signals produced by the SFRC specimens during loading decreased with increasing loading rate,whereas that of high-frequency AE signals increased.Freeze-thaw action had little effect on the crack types observed during the early and middle stages of the loading process;however,the primary crack type observed during the later stage of loading changed from shear to tensile after the SFRC specimens were subjected to freeze-thaw cycling.Notably,the results of this study indicate that the freeze-thaw damage to SFRC reduces AE signal activity at low frequencies.

Stability of Fe-As composites formed with As(Ⅴ)and aged ferrihydrite

During the aging process,ferrihydrite was transformed into mineral mixtures composed of different proportions of ferrihydrite,goethite,lepidocrocite and hematite.Such a transformation may affect the fixed ability of arsenic.In this study,the stability of Fe-As composites formed with As(Ⅴ)and the minerals aged for 0,1,4,10 and 30 days of ferrihydrite were systematically examined,and the effects of molar of ratios Fe/As were also clarified using kinetic methods combined with multiple spectroscopic techniques.The results indicated that As(Ⅴ)was rapidly adsorbed on minerals during the initial polymerization process,which delayed both the ferrihydrite conversion and the hematite formation.When the Fe/As molar ratio was 1.875 and 5.66,the As(Ⅴ)adsorbed by ferrihydrite began to release after 6 hr and 12 hr,respectively.The corresponding release amounts of As(Ⅴ)were 0.55 g/L and 0.07 g/L,and the adsorption rates were 92.43%and 97.50%at 60 days,respectively.However,the As(Ⅴ)adsorbed by the transformation products aged for 30 days of ferrihydrite began to release after adsorbed 30 days.The corresponding release amounts of As(Ⅴ)were 0.25 g/L and 0.03 g/L,and the adsorption rates were 84.23%and 92.18%after adsorbed 60 days,for the Fe/As=1.875 and 5.66,respectively.Overall,the combination of As(Ⅴ)with ferrihydrite and aged products transformed from a thermodynamically metastable phase to a dynamically stable state within a certain duration.Moreover,the aging process of ferrihydrite reduced the sorption ability of arsenate by iron(hydr)oxide but enhanced the stability of the Fe-As composites.