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Triboluminescence (TL) of the methyltriphenylphosphonium tetrahalomanganate(II) complexes such as bis(methyltriphenylphosphonium) tetrabromomanganate (PMBB) and bis(methyltriphenylphosphonium) dibromodichloromanganate (PMBC) was switched ON and OFF reversibly by vapors of aprotic and protic solvents, respectively, for the first time. Detailed analyses indicate that solids of the PMBB and the PMBC undergo phase transitions depending on the environment, which regulate the TL activity of these compounds. The combined results of luminescence, powder X-ray diffraction, differential scanning calorimetry, and electron paramagnetic resonance were used to demonstrate crystal dynamism as well as the TL emission of PMBB and PMBC.

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Corrosion of rebar in concrete structures is one among the various causes impairing its long-term durability. Precise assessment of corrosion rate (CR) is of prime importance to evaluate the structural safety as well as for estimation of service life of concrete structures. Among the electrochemical techniques, Galvanostatic Pulse Technique (GPT) is very promising for field mapping due to its rapidity. The reliability of GPT in determining the CR under passive and active state of rebar has been carried out using small size laboratory specimens and large scale aged concrete structures. The CR determined by the GPT is compared with the CR obtained by Electrochemical Impedance Spectroscopy Technique (EIST) and weight-loss method. The study reveals that an anodic pulse of 100 µA with a pulse duration of 10 seconds is able to determine the CR from 1-663 µm/y (from negligible to higher corrosion activity) on the rebar network more precisely even up to 65 mm of cover concrete. For instance the rebar corroding at higher rate, the CR predicted by GPT is very close to the CR by weight-loss method whereas it is 20 times less by EIST. In the case of passive state of rebar, the CR predicted by EIST is very close to weight-loss method whereas GPT predicts 10 times higher. In aged structures, the change in microstructure of concrete and loss of moisture from the concrete make the potential of rebar and resistivity of concrete more unpredictable and mislead the status of rebar embedded inside the concrete.

The corrosion rate of rebar during the various stages where it occurs, such as passivation, initiation of corrosion and severe corrosion, needs to be determined non-destructively for the maintenance, restoration and replacement of concrete structures. The double layer capacitance (C dl ) and the charge transfer resistance or polarisation resistance (R p ) of the corrosion processes have been associated with the slope of the low-frequency arc in the Nyquist plot, and this can be related to the electrochemical phenomenon that occurs at the steel-concrete interface. The present studies, based on electrochemical impedance spectroscopy (EIS) conducted on three different densities of concrete with addition of 0 . 5 and 1% chloride over a period of 1765 days, reveal that the capacitive behaviour of a lowfrequency arc with a slope more than À1 indicates the passive condition of rebar. Warburg diffusion behaviour with a slope exactly equal to À1 denotes the initiation of corrosion on the rebar. A slope of less than À1 is obtained when corrosion spreads uniformly on the rebar. Other electrochemical parameters such as R p , C dl and phase angle are correlated with the phenomenon occurring at the steel-concrete interface. An R p value greater than 250 kÙ cm 2 indicates the passive condition of rebar, whereas values of less than 230 and 14 kÙ cm 2 indicate initiation and severe corrosion of the rebar respectively. Similarly, a C dl value greater than 1000 ìF/cm 2 indicates the severe corrosion of rebar, whereas less than 100 ìF/cm 2 denotes the passive condition of rebar. If the rebar is in the passive condition, the phase angle is more than 308, whereas it is less than 208 under severe corrosion. The reduction of intrinsic chloride diffusivity owing to pore restructuring by pozzolanic reaction and adsorption of a greater amount of chloride ions into the interlayer of additional calcium silicate hydrate content are responsible for delayed initiation of corrosion in Portland pozzolana cement (PPC) and Portland slag cement (PSC) concretes, when compared with ordinary Portland cement (OPC) concrete.

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