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Abstract: Darkening of the surface within alkali‐silica reaction context – Root cause analysis with non‐destructive measurement methods The damage evolution in concrete pavements due to alkali‐silica reaction is characterized in the initial phase by a darkening of the surface along joint regions. It is generally assumed that an increased moisture content of the concrete boundary zone is responsible for this visual appearance. Against this background, a concrete pavement slab of a representative German motorway section was systematically investigated in‐situ using different non‐destructive measurement methods. The results of the high‐resolution, planar radar showed an increased time of flight of the radar pulse between the pavement surface and the base of the concrete slab in the darkened areas. This indicated that the overall moisture content is increased near joints in the slab. Additional NMR moisture measurements in the concrete boundary zone confirmed the correlation between increased moisture content and the dark‐colored surface of the concrete pavement. The results of both non‐destructive measurement methods indicated that the moisture content near the intersection of two joints was higher than the moisture content near the area around the transverse joint. The lowest moisture content was measured in the center of the concrete pavement slab. The results obtained with the two non‐destructive testing methods were in overall good agreement with moisture measurements of boring dust analyzed using the gravimetric method. [ABSTRACT FROM AUTHOR]

The launch of ERS-1, the first operational radar satellite, in 1993, has led to considerable interest in images acquired at radar wavelengths (1-50 cm) as means of gathering information about agricultural crops and their condition. The main advantage of radar systems with respect to optical satellites is that they can acquire images of the earth's surface irrespective of cloud cover, since clouds are transparent at these wavelengths. This is especially important for agricultural applications, where reliably timed images are needed throughout the growing season in order to follow continously the status of the crops as they develop. Multi-frequency polarimetric radar systems represent a convincing evolution of radar systems and their capabilities. The X-SAR/SIR-C radar-instrument, which was flown twice on the space shuttle in April and October 1994, allowed many researchers access to high quality radar images aquired from a space-borne system for the first time. Thus far, the full potential of radar has not been realized, mostly because existing radar satellites such as ERS-1 only aquire information at a single-wavelength and polarization. By restricting the collection of information to a single channel means such images often lack the extra information needed to characterize crops. However, recent studies using multifrequency-radar have shown that vegetation type, structure and condition can be better characterized when data is aquired at several wavelength and polarizations. In order to obtain significant differences in growth stages for identical crop types during the first 10-day long shuttle mission, winter barley fields were chosen along a gradient from southern Italy to southern Germany. These test fields were surveyed coincidentally with shuttle passes over them. One of the main results was that the spread in fresh biomass measured in (g m-2) between winter barley and maize fields as a function of their different growth stages, could be detected with great precision from the backscatter data. The best parameter for the measurement of winter barley and maize biomass was the ratio HH/VV at L-band. For the extraction of dry biomass in winter barley fields, the cross-polar polarization HV at C-bard showed the best results. [ABSTRACT FROM AUTHOR]