A Practical Approach to Condition Assessment of Asphalt-Covered Concrete Bridge Decks on Korean Expressways by Dielectric Constant Measurements Using Air-Coupled GPR.

The main objectives of this study are to investigate the variations of the dielectric constant of concrete on Korean expressways by using a 1 GHz air-coupled Ground Penetrating Radar (GPR) system and to develop a practical approach to the condition assessment of concrete bridge decks with asphalt overlay on Korean expressways by dielectric constant measurements. A total of 684 GPR investigations of 601 actual concrete bridge decks, which are in service between 2 and 43 years, were carried out during the period between 1999 and 2013. Statistical analysis revealed that the dielectric constant of asphalt-covered concrete bridge decks reduced with service age and this trend continued until service age of over 40 years. As a result, this study provides a practical dielectric constant curve that could be used for condition evaluation of top concrete in asphalt-covered bridge decks with consideration of concrete age. Based on regression analyses of the GPR field survey data and experiences through the field survey, a double cut-off dielectric constant criterion was proposed for condition assessment of asphalt-covered concrete bridge decks on Korean expressways. In addition, a GPR field survey was performed at an actual bridge on the Yeongdong expressway in Korea to test the proposed GPR signal interpretation method. The field survey results provide fundamental data to better understand the variation of the dielectric constant of concrete in actual bridges with asphalt overlay and to develop a practical approach to condition assessment of asphalt-covered concrete bridge decks on Korean expressways by dielectric constant measurements using air-coupled GPR.

A Silyl-Nickel Moiety as a Metal-Ligand Cooperative Site.

Nickel(II) complexes supported by a diphosphinosilyl ligand, [PhSi(2-PiPr2C6H4)2]- (PhSiP2), reveal unusual metal-ligand cooperativity (MLC). While (PhSiP2)Ni(NHMes) (2a) was cleanly isolated at room temperature, a nickel triisopropylphenylamido species, (PhSiP2)Ni(NHTrip) (2b), slowly transformed into a nickel(II) phenyl species, [(ArNH)SiP2]Ni(Ph) (3b), where (ArNH)SiP2- = [(NHTrip)Si(2-PiPr2C6H4)2]-. The X-ray crystallographic data of 3b exhibit a Si-N bond generated from Si-N coupling between the silyl moiety and amino group, along with cleavage of a Si-C bond. Because substoichiometric amounts of π-acidic ligands, such as isocyanide, enhance the conversion rate of 2 to 3 (kobs = 0.28 vs 0.44 h-1), this reaction may involve reductive elimination (RE) and oxidative addition (OA) operating at the silyl-nickel moiety. This is further supported by the fact that the presence of excess π-acidic ligands results in the generation of demetalated ligands (ArNH)PhSiP2 (4) having both Si-N and Si-Ph bonds and the nickel(0) species. Theoretical evaluations also agree on such a pathway. Interestingly, the reaction of a nickel phenyl species (3) with gaseous carbon dioxide (CO2(g)) produces a nickel(II) carbamato complex, (PhSiP2)Ni(OC(O)NHAr) (6), which may involve a RE-OA process occurring at a nickel center. Although 2 and 3 might be in equilibrium, the reaction of 3 with CO2 does not follow this pathway. Instead, a CO2 interaction induces RE at the silyl-nickel moiety, followed by amide group transfer, to give a carbamato product, 6, based on our experimental and theoretical evaluations. These results highly support that group transfer involving MLC can be managed via a RE-OA pathway at the silyl-nickel(II) moiety.

The unusual hydridicity of a cobalt bound Si-H moiety.

A paramagnetic cobalt-SiH intermediate possessing the Co-(η(1)-H-Si) moiety was studied using various spectroscopic techniques. Frozen solution ENDOR experiments show a Co-H distance of ∼1.53 Å and a Si-Co-H angle of ∼15°, whereas XRD data suggest a TBP geometry about a Co(ii) centre. Its stepwise deprotonation with base followed by dehalogenation was identified experimentally and theoretically.