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Fiber optic sensors for an in-situ monitoring of moisture and pH value in reinforced concrete
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2002
Year
EngineeringFiber OpticsChemistryAnalytical ChemistryAnion SensingCation SensingChemical SensorPhotochemistryConcrete TechnologyReinforced ConcreteStructural Health MonitoringFiber Optic SensingFiber Optic SensorsPh ValueColor ChangeCement-based Construction MaterialConcrete StructuresUv-vis SpectroscopyOptical SensorsSensorsEnvironmental EngineeringCivil EngineeringOptical Sensor
Concrete structures such as social buildings and bridges are important economic goods. Thus, maintenance and preservation of these structures are of major interest. Buildings of reinforced concrete are exposed to a variety of damaging influences. In particular, moisture has an important influence on the lifetime of concrete structures. This is caused by the involvement of free water in corrosion of the steel, and the fact that water acts as transport medium for damaging ions such as chloride, sulfate, carbonate and ammonium. Thus, we designed and developed an integrated fiberoptical sensor system, which allows in-situ non- destructive long-term monitoring of concrete structures. As moisture indicator we use a pyridinium-N-phenolat betainital dye, which shows a strong solvatochromic behavior in the ultraviolet-visible spectral range (UV-VIS). The dye is embedded in a polymer matrix, whose moderate polarity is enhanced by free water diffusing into the sensor. This leads to a continuous hypsochromic shift of the absorption spectrum according to the water concetration. Another appropriate dye is 4-amino-N-methylphthalimid, which shows a similar behavior in its fluorescent spectra, and presently we are developing its derivatives and suitable polymer matrices. The determination of the pH-value of concrete is of major importance for the assessment of acidic attacks which may lead to serious damage in reinforced concrete, as the embedded steel structures exhibit long-term stability (i.e. resistance to corrosion) only at pH-values of 9 or higher. Therefore we have developed a fiberoptical sensor system for the measurement of pH-values in concrete consisting of pH- indicator dyes immobilized in a highly immobilized in a highly hydrophilic polymer matrix. Any change in pH-value of the wet concrete material is indicated by a color change of the dye/polymer system. The sensor system displays long term stability even in aggressive media of pH12 - 13.